Design and development.

Origins.

During the 1950s Douglas Aircraft studied a short- to medium-range airliner to complement their higher capacity, long range DC-8. (DC stands for Douglas Commercial.) A medium-range four-engine Model 2067 was studied but it did not receive enough interest from airlines and it was abandoned. In 1960, Douglas signed a two-year contract with Sud Aviation for technical cooperation. Douglas would market and support the Sud Aviation Caravelle and produce a licensed version if airlines ordered large numbers. None were ordered and Douglas returned to its design studies after the cooperation deal expired.  In 1962, design studies were underway. The first version seated 63 passengers and had a gross weight of 69,000 lb (31,300 kg). This design was changed into what would be the initial DC-9 variant. Douglas gave approval to produce the DC-9 on April 8, 1963. Unlike the competing but larger Boeing 727 trijet, which used as many 707 components as possible, the DC-9 was an all-new design. The DC-9 has two rear-mounted Pratt & Whitney JT8D turbofan engines, relatively small, efficient wings, and a T-tail. The DC-9's takeoff weight was limited to 80,000 lb (36,300 kg) for a two-person flight crew by Federal Aviation Agency regulations at the time. DC-9 aircraft have five seats across for economy seating. The airplane seats 80 to 135 passengers depending on version and seating arrangement.  The DC-9 was designed for short to medium routes, often to smaller airports with shorter runways and less ground infrastructure than the major airports being served by larger designs like the Boeing 707 and Douglas DC-8. Accessibility and short field characteristics were called for. Turnarounds were simplified by built-in airstairs, including one in the tail, which shortened boarding and deplaning times.  The tail-mounted engine design facilitated a clean wing without engine pods, which had numerous advantages. For example, flaps could be longer, unimpeded by pods on the leading edge and engine blast concerns on the trailing edge. This simplified design improved airflow at low speeds and enabled lower takeoff and approach speeds, thus lowering field length requirements and keeping wing structure light. The second advantage of the tail-mounted engines was the reduction in foreign object damage from ingested debris from runways and aprons. However, with this position, the engines could ingest ice streaming off the wing roots. Third, the absence of engines in underslung pods allowed a reduction in fuselage ground clearance, making the aircraft more accessible to baggage handlers and passengers.  The problem of deep stalling, revealed by the loss of the BAC One-Eleven prototype in 1963, was overcome through various changes, including the introduction of vortilons, small surfaces beneath the wing's leading edge used to control airflow and increase low speed lift.

Production.

The first DC-9, a production model, flew on February 25, 1965. The second DC-9 flew a few weeks later, with a test fleet of five aircraft flying by July. This allowed the initial Series 10 to gain airworthiness certification on November 23, 1965, and to enter service with Delta Air Lines on December 8. The DC-9 was always intended to be available in multiple versions to suit customer requirements, The first stretched version, the Series 30, with a longer fuselage and extended wing tips, flew on August 1, 1966, entering service with Eastern Air Lines in 1967. The initial Series 10 would be followed by the improved -20, -30, and -40 variants. The final DC-9 series was the -50, which first flew in 1974.  The DC-9 was a commercial success with 976 built when production ended in 1982. The DC-9 is one of the longest-lasting aircraft in operation. Its last successor, the Boeing 717, was produced until 2006.  The DC-9 family was produced in units: 976 DC-9s, 1191 MD-80s, 116 MD-90s and 155 Boeing 717s. This compares to 8,000 Airbus A320s delivered as of February 2018 and 10,000 Boeing 737s completed as of March 2018.  Studies aimed at further improving DC-9 fuel efficiency, by means of retrofitted wingtips of various types, were undertaken by McDonnell Douglas. However, these did not demonstrate significant benefits, especially with existing fleets shrinking. The wing design makes retrofitting difficult.

Legacy.

The DC-9 was followed by the introduction of the MD-80 series in 1980. This was originally called the DC-9-80 series. It was a lengthened DC-9-50 with a higher maximum takeoff weight (MTOW), a larger wing, new main landing gear, and higher fuel capacity. The MD-80 series features a number of variants of the Pratt & Whitney JT8D turbofan engine having higher thrust ratings than those available on the DC-9. The series includes the MD-81, MD-82, MD-83, MD-88, and shorter fuselage MD-87.  The MD-80 series was further developed into the McDonnell Douglas MD-90 in the early 1990s. It has yet another fuselage stretch, an electronic flight instrument system (first introduced on the MD-88), and completely new International Aero V2500 high-bypass turbofan engines. In comparison to the very successful MD-80, relatively few MD-90s were built.  The final variant was the MD-95, which was renamed the Boeing 717-200 after McDonnell Douglas's merger with Boeing in 1997 and before aircraft deliveries began. The fuselage length and wing are very similar to those of the DC-9-30, but much use was made of lighter, modern materials. Power is supplied by two BMW/Rolls-Royce BR715 high-bypass turbofan engines.  China's Comac ARJ21 is derived from the DC-9 family. The ARJ21 is built with manufacturing tooling from the MD-90 Trunkliner program. As a consequence, it has the same fuselage cross-section, nose profile, and tail.

Variants.

Series 10.

The original DC-9 (later designated the Series 10) was the smallest DC-9 variant. The -10 was long and had a maximum weight of. The Series 10 was similar in size and configuration to the BAC One-Eleven and featured a T-tail and rear-mounted engines. Power was provided by a pair of Pratt & Whitney JT8D-5 or JT8D-7 engines. A total of 137 were built. Delta Air Lines was the initial operator.  The Series 10 was produced in two main subvariants, the Series 14 and 15, although, of the first four aircraft, three were built as Series 11s and one as Series 12. These were later converted to Series 14 standard. No Series 13 were produced. A passenger/cargo version of the aircraft, with a side cargo door forward of the wing and a reinforced cabin floor, was certificated on March 1, 1967. Cargo versions included the Series 15MC (Minimum Change) with folding seats that can be carried in the rear of the aircraft, and the Series 15RC (Rapid Change) with seats removable on pallets. These differences disappeared over the years as new interiors were installed.  The Series 10 was unique in the DC-9 family in not having leading-edge slats. The Series 10 was designed to have short takeoff and landing distances without the use of leading-edge high-lift devices. Therefore, the wing design of the Series 10 featured airfoils with extremely high maximum-lift capability in order to obtain the low stalling speeds necessary for short-field performance.

Series 10 features.

The Series 10 has an overall length of, a fuselage length of, a passenger-cabin length of, and a wingspan of.  The Series 10 was offered with the -thrust JT8D-1 and JT8D-7. All versions of the DC-9 are equipped with an AlliedSignal (Garrett) GTCP85 APU, located in the aft fuselage. The Series 10, as with all later versions of the DC-9, is equipped with a two-crew analog flightdeck.  The Series 14 was originally certificated with an MTOW of, but subsequent options offered increases to 86,300 and. The aircraft's MLW in all cases is. The Series 14 has a fuel capacity of 3,693 US gallons (with the 907 US gal centre section fuel). The Series 15, certificated on January 21, 1966, is physically identical to the Series 14 but has an increased MTOW of. Typical range with 50 passengers and baggage is, increasing to at long-range cruise. Range with maximum payload is, increasing to with full fuel.  The aircraft is fitted with a passenger door in the port forward fuselage, and a service door/emergency exit is installed opposite. An airstair installed below the front passenger door was available as an option as was an airstair in the tailcone. This also doubled as an emergency exit. Available with either two or four overwing exits, the DC-9-10 can seat up to a maximum certified exit limit of 109 passengers. Typical all-economy layout is 90 passengers, and 72 passengers in a more typical mixed-class layout with 12 first and 60 economy-class passengers.  All versions of the DC-9 are equipped with a tricycle undercarriage, featuring a twin nose unit and twin main units.

Series 20.

The Series 20 was designed to satisfy a Scandinavian Airlines request for improved short-field performance by using the more-powerful engines and improved wings of the -30 combined with the shorter fuselage used in the -10. Ten Series 20 aircraft were produced, all of them the Model -21.  In 1969, a DC-9 Series 20 at Long Beach was fitted with an Elliott Flight Automation Head-up display by McDonnell Douglas and used for successful three-month-long trials with pilots from various airlines, the Federal Aviation Administration, and the US Air Force.

Series 20 features.

The Series 20 has an overall length of, a fuselage length of, a passenger-cabin length of, and a wingspan of. The DC-9 Series 20 is powered by the thrust JT8D-11 engine.  The Series 20 was originally certificated at an MTOW of but this was increased to, eight percent more than on the higher weight Series 14s and 15s. The aircraft's MLW is and MZFW is. Typical range with maximum payload is, increasing to with maximum fuel. The Series 20, using the same wing as the Series 30, 40 and 50, has a slightly lower basic fuel capacity than the Series 10 (3,679 US gallons).

Series 30.

The Series 30 was produced to counter Boeing's 737 twinjet; 662 were built, about 60% of the total. The -30 entered service with Eastern Airlines in February 1967 with a fuselage stretch, wingspan increased by just over and full-span leading edge slats, improving takeoff and landing performance. Maximum takeoff weight was typically. Engines for Models -31, -32, -33, and -34 included the P&W JT8D-7 and JT8D-9 rated at of thrust, or JT8D-11 with.  Unlike the Series 10, the Series 30 had leading-edge devices to reduce the landing speeds at higher landing weights; full-span slats reduced approach speeds by six knots despite 5,000 lb greater weight. The slats were lighter than slotted Krueger flaps, since the structure associated with the slat is a more efficient torque box than the structure associated with the slotted Krueger. The wing had a six-percent increase in chord, all ahead of the front spar, allowing the 15 percent chord slat to be incorporated.

Series 30 versions.

The Series 30 was built in four main sub-variants.

Series 30 features.

The DC-9-30 was offered with a selection of variants of JT8D including the -1, -7, -9, -11, -15. and -17. The most common on the Series 31 is the JT8D-7 ( thrust), although it was also available with the −9 and -17 engines. On the Series 32 the JT8D-9 ( thrust) was standard, with the -11 also offered. The Series 33 was offered with the JT8D-9 or -11 ( thrust) engines and the heavyweight -34 with the JT8D-9, -15 ( thrust) or -17 ( thrust) engines.

Series 40.

The DC-9-40 is a further lengthened version. With a longer fuselage, accommodation was up to 125 passengers. The Series 40 was fitted with Pratt & Whitney engines with thrust of. A total of 71 were produced. The variant first entered service with Scandinavian Airlines System (SAS) in March 1968.

Series 50.

The Series 50 was the largest version of the DC-9 to enter airline service. It features an fuselage stretch and seats up to 139 passengers. It entered revenue service in August 1975 with Eastern Airlines and included a number of detail improvements, a new cabin interior, and more powerful JT8D-15 or -17 engines in the class. McDonnell Douglas delivered 96, all as the Model -51. Some visual cues to distinguish this version from other DC-9 variants include side strakes or fins below the side cockpit windows, spray deflectors on the nose gear, and thrust reversers angled inward 17 degrees as compared to the original configuration. The thrust reverser modification was developed by Air Canada for its earlier aircraft, and adopted by McDonnell Douglas as a standard feature on the series 50. It was also applied to many earlier DC-9s in the course of regular maintenance.

Operators.

A total of 30 DC-9 aircraft (all variants) were in commercial service as of July 2018. Operators include Aeronaves TSM (8), USA Jet Airlines (6), Everts Air Cargo (4), Ameristar Charters (4) and other operators with fewer aircraft.  Delta Air Lines, since acquiring Northwest Airlines, has operated a fleet of DC-9 aircraft, most over 30 years old. With severe increases in fuel prices in the summer of 2008, Northwest Airlines began retiring its DC-9s, switching to Airbus A319s that are 27% more fuel efficient. As the Northwest/Delta merger progressed, Delta returned several stored DC-9s to service. Delta Air Lines made its last DC-9 commercial flight from Minneapolis/St. Paul to Atlanta on January 6, 2014 with the flight number DL2014.  With the existing DC-9 fleet shrinking, modifications do not appear to be likely to occur, especially since the wing design makes retrofitting difficult. DC-9s are therefore likely to be further replaced in service by newer airliners such as Boeing 737, Airbus A320, Embraer E-Jets, and the Bombardier CSeries.  One ex-SAS DC-9-21 is operated as a skydiving jump platform at Perris Valley Airport in Perris, California. With the steps on the ventral stairs removed, it is the only airline transport class jet certified to date by the FAA for skydiving operations as of 2016.

Accidents and incidents.

As of June 2018, the DC-9 has been involved in 276 aviation occurrences, including 145 hull-loss accidents, with 3,697 fatalities combined.

The McDonnell Douglas DC-9 (initially the Douglas DC-9) is a single-aisle airliner designed by the Douglas Aircraft Company.

道格拉斯DC-9是道格拉斯公司在1960年代研发出来的双涡轮引擎民航机,是为了与波音727抗衡而制造的,短航程、单层、单走道。其最大的特点就是后挂式双涡轮风扇发动机以及T型尾翼(俗称“T尾”)。


History.

This concept was first introduced in 1961 by the Council of Europe and later, the Joint FAO/WHO Expert Committee on Food Additives (JECFA), a committee maintained by two United Nations bodies: the Food and Agriculture Organization (FAO) and the World Health Organization (WHO).

Concept.

An ADI value is based on current research, with long-term studies on animals and observations of humans. First, a no-observed-adverse-effect level (NOAEL), the amount of a substance that shows no toxic effects, is determined. Usually the studies are performed with several doses including high doses. In the case of several studies on different effects, the lowest NOAEL is usually taken. Then, the NOAEL (or another point of departure such as a benchmark dose level (BMDL)) is divided by a safety factor, conventionally 100, to account for the differences between test animals and humans (factor of 10) and possible differences in sensitivity between humans (another factor of 10). safety factors with values other than 100 may be used if information on uncertainty about the value of the point of departure (NOAEL or BMDL) justify it. For instance, if the ADI is based on data from humans the safety factor is usually 10 instead of 100. The ADI is usually given in mg per kg body weight.  The ADI is considered a safe intake level for a healthy adult of normal weight who consumes an average daily amount of the substance in question. Increased safety factors for infants have been discussed, but are not needed, because elimination of chemicals is in fact often more rapid in children and as children generally suffer higher illness rates than adults, adverse effects caused by food additives can easily be disguised as any number of things children usually suffer with. It would be far more difficult to argue the case with a healthy adult. The ADI does not take into account allergic reactions that are individual responses rather than dose-dependent phenomena.  The higher the ADI, the larger amounts of a compound are safe for regular ingestion. The concept of tolerable daily intake is often used for unwanted contaminants or other chemicals.  The ADI concept can be understood as a measure to indicate the toxicity from long-term exposure to repeated ingestion of chemical compounds in foods (present and/or added), as opposed to acute toxicity.  The threshold limit value (TLV) of a chemical substance is a level to which it is believed a worker can be exposed day after day for a working lifetime without adverse effects.

Acceptable daily intake or ADI is a measure of the amount of a specific substance (originally applied for a food additive, later also for a residue of a veterinary drug or pesticide) in food or drinking water that can be ingested (orally) on a daily basis over a lifetime without an appreciable health risk. ADIs are expressed usually in milligrams (of the substance) per kilograms of body weight per day.

一日可接受摄取量 或 ADI 是指食物或饮用水中特定物质每天不影响健康下口服可摄入量。最初适用于食品添加剂,后来也适用于兽药或农药残留物。ADI通常以每公斤体重每天可摄取毫克表示。


Historical examples.

Workers' councils originated in Russia in 1905, with the workers' councils (soviets) acting as labor committees which coordinated strike activities throughout the cities due to repression of trade unions. During the Revolutions of 1917–1923, many socialists, such as Anton Pannekoek and Rosa Luxemburg, advocated for the control of the economy by the workers' councils. Several times in modern history, the idea of workers' councils has been attributed to similar forms of organization. Examples include:  Despite Lenin's declarations that "the workers must demand the immediate establishment of genuine control, to be exercised by the workers themselves", on May 30, the Menshevik minister of labor, Matvey Skobelev, pledged to not give the control of industry to the workers but instead to the state: "The transfer of enterprises into the hands of the people will not at the present time assist the revolution [...] The regulation and control of industry is not a matter for a particular class. It is a task for the state. Upon the individual class, especially the working class, lies the responsibility for helping the state in its organizational work."

Organization details.

In the workers' councils organised as part of the 1918 German revolution, factory organisations, such as the General Workers' Union of Germany (AAUD), formed the basis for organising region-wide councils. The council communists in the Communist Workers' Party of Germany advocated organising "on the basis of places of work, not trades, and to establish a National Federation of Works Committees."  Councils operate on the principle of recallable delegates. This means that elected delegates may be recalled at any time through a vote in a form of impeachment. Recall of management committee members, specialist professionals such as engineers, and delegates to higher councils was observed in the Central Workers Council of Greater Budapest during 1956, where delegates were removed for industrial, organisational, and political reasons.  Workers' councils combine to elect higher bodies for coordinating between one another. This means that the upper councils are not superior to the lower councils, but are instead built from and operated by them. The national council would therefore have delegates from every city in the country. Their nature means that workers' councils do away with traditional centralized governments and instead give power indirectly to the people. This type of democratic order is called council democracy. The Central Workers Council of Greater Budapest occupied this role in the Hungarian Revolution of 1956, between late October and early January 1957, where it grew out of local factory committees.

Councils against unions and Stalinists.

A workers' council is a deliberative assembly, composed of working class members, intended to institute workers' self-management or workers' control. Unlike a trade union, in a workers' council the workers are assumed to be in actual control of the workplace, rather than merely negotiating with employers through collective bargaining. They are a form of workplace democracy, where different workplaces coordinate production through their elected delegates.  Amongst both Marxists and anarchists there is a widespread belief that workers' councils embody the fundamental principles of socialism, such as workers' control over production and distribution. Whereas socialism from above is carried out by a centralized state run by an elite bureaucratic apparatus, here socialism from below is seen as the self-administration and self-rule of the working class.  Some left communists (particularly council communists) and anarchists support a council-based society; believing that only the workers themselves can spark a revolution and so workers' councils will be the foundation of the revolution. There are also Leninists (for example the Trotskyist International Socialist Tendency and its offshoots) who advocate a council-based society, but maintain that workers' councils cannot carry out a revolution without the leadership of a vanguard party.  During the May 1968 events in France, "[t]he largest general strike that ever stopped the economy of an advanced industrial country, and the first "wildcat general strike" in history", the Situationists, against the unions and the Communist Party that were starting to side with the de Gaulle government to contain the revolt, called for the formation of workers' councils to take control of the cities, expelling union leaders and left-wing bureaucrats, in order to keep the power in the hands of the workers with direct democracy.

A workers' council is a form of political and economic organization in which a single local administrative division, such as a municipality or a county, is governed by a council made up of temporary and instantly revocable delegates elected in the region's workplaces.<br>

工人委员会是一种经济合作组织形式,是指在一个单独的企业或工作环境(如工厂、学校或农场)中,当地工人集体管控这一实体。形式的核心即是通过临时且可随时撤换的代表管理。在这拥有临时代表的体制内,工人们自行决定工作日程以及工作需求。他们也授权临时代表发布日程和督促追赶进度。临时代表由工人群体选举而出,当被认为有悖于授权人时可被随时替换。代表通常被视为应随时更换的。体制内不存在经理,所有决定权和组织都基于这一授权系统。


Overview.

The original design of the Domain Name System (DNS) did not include any security details; instead, it was designed to be a scalable distributed system. The Domain Name System Security Extensions (DNSSEC) attempts to add security, while maintaining backward compatibility. RFC 3833 documents some of the known threats to the DNS and how DNSSEC responds to those threats.  DNSSEC was designed to protect applications (and caching resolvers serving those applications) from using forged or manipulated DNS data, such as that created by DNS cache poisoning. All answers from DNSSEC protected zones are digitally signed. By checking the digital signature, a DNS resolver is able to check if the information is identical (i.e. unmodified and complete) to the information published by the zone owner and served on an authoritative DNS server. While protecting IP addresses is the immediate concern for many users, DNSSEC can protect any data published in the DNS, including text records (TXT) and mail exchange records (MX), and can be used to bootstrap other security systems that publish references to cryptographic certificates stored in the DNS such as Certificate Records (CERT records, RFC 4398), SSH fingerprints (SSHFP, RFC 4255), IPSec

Operation.

DNSSEC works by digitally signing records for DNS lookup using public-key cryptography. The correct DNSKEY record is authenticated via a chain of trust, starting with a set of verified public keys for the DNS root zone which is the trusted third party. Domain owners generate their own keys, and upload them using their DNS control panel at their domain-name registrar, which in turn pushes the keys via secDNS to the zone operator (e.g., Verisign for.com) who signs and publishes them in DNS.

Resource records.

DNS is implemented by the use of several resource records. To implement DNSSEC, several new DNS record types were created or adapted to use with DNSSEC:  When DNSSEC is used, each answer to a DNS lookup contains an RRSIG DNS record, in addition to the record type that was requested. The RRSIG record is a digital signature of the answer DNS resource record set. The digital signature is verified by locating the correct public key found in a DNSKEY record. The NSEC and NSEC3 records are used to provide cryptographic evidence of the non-existence of any RR. The DS record is used in the authentication of DNSKEYs in the lookup procedure using the chain of trust. NSEC and NSEC3 records are used for robust resistance against spoofing.

Algorithms.

DNSSEC was designed to be extensible so that as attacks are discovered against existing algorithms, new ones can be introduced in a backward-compatible fashion. The following table defines, as of April 2013, the security algorithms that are most often used:

The lookup procedure.

From the results of a DNS lookup, a security-aware DNS resolver can determine whether the authoritative name server for the domain being queried supports DNSSEC, whether the answer it receives is secure, and whether there is some sort of error. The lookup procedure is different for recursive name servers such as those of many ISPs, and for stub resolvers such as those included by default in mainstream operating systems. Microsoft Windows uses a stub resolver, and Windows Server 2008 R2 and Windows 7 in particular use a non-validating but DNSSEC-aware stub resolver.

Recursive name servers.

Using the chain of trust model, a Delegation Signer (DS) record in a parent domain (DNS zone) can be used to verify a DNSKEY record in a subdomain, which can then contain other DS records to verify further subdomains. Say that a recursive resolver such as an ISP name server wants to get the IP addresses (A record and/or AAAA records) of the domain "www.example.com".  There are several exceptions to the above example.  First, if "example.com" does not support DNSSEC, there will be no RRSIG record in the answer and there will not be a DS record for "example.com" in the "com" zone. If there is a DS record for "example.com", but no RRSIG record in the reply, something is wrong and maybe

Stub resolvers.

Stub resolvers are "minimal DNS resolvers that use recursive query mode to offload most of the work of DNS resolution to a recursive name server." A stub resolver will simply forward a request to a recursive name server, and use the Authenticated Data (AD) bit in the response as a "hint to find out whether the recursive name server was able to validate signatures for all of the data in the Answer and Authority sections of the response." Microsoft Windows uses a stub resolver, and Windows Server 2008 R2 and Windows 7

Trust anchors and authentication chains.

To be able to prove that a DNS answer is correct, one needs to know at least one key or DS record that is correct from sources other than the DNS. These starting points are known as trust anchors and are typically obtained with the operating system or via some other

Signatures and zone signing.

To limit replay attacks, there are not only the normal DNS TTL values for caching purposes, but additional timestamps in RRSIG records to limit the validity of a signature. Unlike TTL values which are relative to when

Key management.

DNSSEC involves many different keys, stored both in DNSKEY records, and from other sources to form trust anchors.  In order to allow for replacement keys, a key rollover scheme is required. Typically, this involves first rolling out new keys in new DNSKEY records, in addition to the existing old keys. Then, when it is safe to assume that the time to live values have caused the caching of old keys to have passed, these new keys can be used. Finally, when it is safe to assume that the caching of records using the old keys have expired, the old DNSKEY records can be deleted. This process is more complicated for things such as the keys to trust anchors, such as at the root, which may require

DANE Working Group.

DNS-based Authentication of Named Entities (DANE) is an IETF working group with the goal of developing protocols and techniques that allow Internet applications to establish cryptographically secured communications with TLS, DTLS, SMTP, and S/MIME based on DNSSEC.  The new protocols will enable additional

History.

DNS is a critical and fundamental Internet service, yet in 1990 Steve Bellovin discovered serious security flaws in it. Research into securing it began, and progressed dramatically when his paper was made public in 1995. The initial RFC 2065 was published by the IETF in 1997, and initial attempts to implement that specification led to a revised (and believed fully workable) specification in 1999 as IETF RFC 2535. Plans were made to deploy DNSSEC based on RFC 2535.  Unfortunately, the IETF RFC 2535 specification had very significant problems scaling up to the full Internet; by 2001 it became clear that this specification was unusable for large networks. In normal operation DNS servers often get out of sync with their parents. This isn't usually a problem, but when DNSSEC is enabled, this out-of-sync data could have the effect of a serious self-created denial of service. The original DNSSEC required a complex six-message protocol and a lot of data transfers to perform key changes for a child (DNS child zones had to send all of their data up to the parent, have

Authenticating NXDOMAIN responses and NSEC.

Cryptographically proving the absence of a domain requires signing the response to every query for a non-existent domain. This is not a problem for online signing servers, which keep their keys available online. However, DNSSEC was designed around using offline computers to sign records so that zone-signing-keys could be kept in cold storage. This represents a problem when trying to authenticate responses to queries for non-existent domains since it is impossible to pre-generate a response to every possible hostname query.  The initial solution was to create NSEC records for every pair of domains in a zone. Thus if a client queried for a record at the non-existent codice_1, the server would respond with an NSEC record stating that nothing exists between codice_2 and codice_3. However, this leaks more information about the zone than

Deployment.

The Internet is critical infrastructure, yet its operation depends on the fundamentally insecure DNS. Thus, there is strong incentive to secure DNS, and deploying DNSSEC is generally considered to be a critical part of that effort. For example, the U.S. "National Strategy to Secure Cyberspace" specifically identified the need to secure DNS. Wide-scale deployment of DNSSEC could resolve many other security problems as well, such as secure key distribution for e-mail addresses.  DNSSEC deployment in large-scale networks is also challenging. Ozment and Schechter observe that DNSSEC (and other technologies) has a "bootstrap problem": users typically only deploy a technology if they receive an immediate benefit, but if a minimal level of deployment is required before "any" users receive a benefit greater than their costs (as is true for DNSSEC), it is difficult to deploy. DNSSEC can be deployed at any level of a DNS hierarchy, but it must be widely available in a zone before many others will want to adopt it. DNS servers must be updated with software that supports DNSSEC, and DNSSEC data must be created and added to the DNS zone data. A TCP/IP-using client must have their DNS resolver (client) updated before it can use DNSSEC's capabilities. What is more, any resolver must have, or have a way to acquire, at least one public key that it can trust before it can start using DNSSEC.  DNSSEC implementation can add significant load to some DNS servers. Common DNSSEC-signed responses are far larger than the default UDP size of 512 bytes. In theory, this can be handled through multiple IP fragments, but many "middleboxes" in the field do not handle these correctly. This leads to the use of TCP instead. Yet many current TCP implementations store a great deal of data for each TCP connection; heavily loaded servers can run out of resources simply trying to respond to a larger number of (possibly bogus) DNSSEC requests. Some protocol extensions, such as TCP Cookie Transactions, have been developed to reduce this loading. To address these challenges, significant effort is ongoing to deploy DNSSEC, because the Internet is so vital to so many organizations.

Early deployments.

Early adopters include Brazil (.br), Bulgaria (.bg), Czech Republic (.cz), Namibia (.na) Puerto Rico (.pr) and Sweden (.se), who use DNSSEC for their country code top-level domains; RIPE NCC, who have signed all the reverse lookup records (in-addr.arpa) that are delegated to it from the Internet Assigned Numbers Authority (IANA). ARIN is also signing their reverse zones. In February 2007 TDC became the first Swedish ISP to start offering this feature to its customers.  IANA publicly tested a sample signed root since June 2007. During this period prior to the production signing of the root, there were also several alternative trust anchors. The IKS Jena introduced one on January 19, 2006, the Internet Systems Consortium introduced another on March 27 of

Deployment at the DNS root.

DNSSEC was first deployed at the root level on July 15, 2010. This is expected to greatly simplify the deployment of DNSSEC resolvers, since the root trust anchor can be used to validate any DNSSEC zone that has a complete chain of trust from the root. Since the chain of trust must be traced back to a trusted root without interruption in order to validate, trust anchors must still be configured for secure zones if any of the zones above them are not secure. For example, if the zone "signed.example.org" was secured but the "example.org" zone was not, then, even though the ".org" zone and the root are signed, a trust anchor has to be deployed in order to validate the zone.  Political issues surrounding signing the root have been a continuous concern, primarily about some central issues:

Planning.

In September 2008, ICANN and VeriSign each published implementation proposals and in October, the National Telecommunications and Information Administration (NTIA) asked the public for comments. It is unclear if the comments received affected the design of the final deployment plan.  On June 3, 2009, the National Institute of Standards and Technology (NIST) announced plans to sign the root by the end of 2009, in conjunction with ICANN, VeriSign and the NTIA.  On October 6, 2009, at the 59th RIPE Conference meeting, ICANN and VeriSign announced the planned deployment timeline for deploying DNSSEC within the root zone. At the meeting it was announced that it

Implementation.

On January 25, 2010, the L (ell) root server began serving a "Deliberately Unvalidatable Root Zone" (DURZ). The zone uses signatures of a SHA-2 (SHA-256) hash created using the RSA algorithm, as defined in RFC 5702. As of May 2010, all thirteen root servers have begun serving the DURZ. On July 15, 2010, the first root full production DNSSEC root zone was signed, with the SOA serial 2010071501. Root trust anchors are available from IANA.

Deployment at the TLD level.

Underneath the root there is a large set of top level domains that must be signed in order to

DNSSEC Lookaside Validation - historical.

In March 2006, the Internet Systems Consortium introduced the DNSSEC Lookaside Validation registry. DLV was intended to make DNSSEC easier to deploy in the absence of a root trust anchor. At the time it was imagined that a validator might have to maintain large numbers of trust anchors corresponding to signed subtrees of the DNS. The purpose of DLV was to allow validators to offload the effort of managing a trust anchor repository to a trusted third party. The DLV registry maintained a central list of trust anchors, instead of each validator repeating the work of maintaining its own list.  To use DLV, a validator that supports it was needed, such as BIND or Unbound, configured with a trust anchor for a DLV zone. This zone contained DLV records; these had exactly the same format

DNSSEC deployment initiative by the U.S. federal government.

The Science and Technology Directorate of the U.S. Department of Homeland Security (DHS) sponsors the "DNSSEC Deployment Initiative". This initiative encourages "all sectors to voluntarily adopt security measures that will improve security of the Internet's naming infrastructure, as part of a global, cooperative effort that involves many nations and organizations in the public and private sectors." DHS also funds efforts to mature DNSSEC and get it deployed inside the U.S. federal government.  It was reported that on March 30, 2007, the U.S. Department of Homeland Security proposed "to have the key to sign the DNS root zone

DNSSEC deployment in the U.S. federal government.

The National Institute of Standards and Technology (NIST) published NIST Special Publication 800-81 Secure Domain Name System (DNS) Deployment Guide on May 16, 2006, with guidance on how to deploy DNSSEC. NIST intended to release new DNSSEC Federal Information Security Management Act (FISMA) requirements in NIST SP800-53-R1, referencing this deployment guide. U.S. agencies would then have had one year after final publication of NIST SP800-53-R1 to meet these new FISMA requirements. However, at the time NSEC3 had not been completed. NIST had suggested using

Deployment in resolvers.

Several ISPs have started to deploy DNSSEC-validating DNS recursive resolvers. Comcast became the first major ISP to do so in the United States, announcing their intentions on October 18, 2010 and completing deployment on January 11, 2012.  According to a study at APNIC, the proportion of clients who exclusively use DNS resolvers that perform DNSSEC validation rose to 8.3% in May 2013. About half of these clients were using Google's public DNS resolver.  In September 2015, Verisign announced their free public DNS resolver service, and although unmentioned in their press releases, it also performs DNSSEC validation.  By the beginning of 2016, APNIC's monitoring showed the proportion of clients who exclusively use DNS resolvers that perform DNSSEC validation had increased to about 15%.

DNSSEC support.

Google Public DNS is a freely provided, public DNS service, fully supporting DNSSEC.  On May 6, 2013, Google Public DNS enabled the DNSSEC validation by default; meaning all queries will

Issues.

Microsoft Exchange 2013 and older can break proper MX record lookup due to DNSSEC usage, causing #550 4.4.7 QUEUE.Expired errors.

Tools.

DNSSEC deployment requires software on the server and client side. Some of the tools that support DNSSEC include:

The Domain Name System Security Extensions (DNSSEC) is a suite of Internet Engineering Task Force (IETF) specifications for securing certain kinds of information provided by the Domain Name System (DNS) as used on Internet Protocol (IP) networks. It is a set of extensions to DNS which provide to DNS clients (resolvers) cryptographic authentication of DNS data, authenticated denial of existence, and data integrity, but not availability or confidentiality.

域名系统安全扩展(英语:Domain Name System Security Extensions,缩写为DNSSEC)是Internet工程任务组 (IETF)的对确保由域名系统 (DNS)中提供的关于互联网协议(IP)网络使用特定类型的信息规格套件。它是对DNS提供给DNS客户端(解析器)的DNS数据来源进行认证,并验证不存在性和校验数据完整性验证,但不提供机密性和可用性。


Beginnings.

DELAG was founded at the suggestion of Alfred Colsman, the business manager of Zeppelin Luftschiffbau. The company was having difficulty in obtaining orders from the German army, so Colsman suggested exploiting the German public's enthusiastic interest by establishing a commercial passenger-carrying company. Count Zeppelin himself distanced himself from this commercialisation of his idea: as an aristocrat and ex-officer, he saw it as a vulgar tradesman's enterprise. Colsman, who became the airline's first general director, managed to secure the cooperation of Albert Ballin, the head of the Hamburg America Line who offered 100,000 Marks a year to promote the enterprise on the condition that his offices had exclusive rights to sell tickets, and Colsman had little difficulty in raising the necessary three million marks capital. Much of the initial capital came from the cities of Frankfurt am Main and Düsseldorf, and a number of cities built airship sheds at their own expense.  The first Zeppelin to be constructed for the company was LZ 7, which was named "Deutschland". First flown on 19 June 1910, it had a useful lift of and had accommodation for 24 passengers. Cruising speed was. Given this performance it was realised that scheduled inter-city services would not be feasible, and that the company would be limited to offering pleasure cruises in the vicinity of their bases. However, the "Deutschland" was destroyed on 28 June 1910 while taking a consignment of journalists on a trip intended to generate publicity. The airship was first prevented from returning to its base in Düsseldorf and then, caught by a thunderstorm, was first carried up to a height of and then, heavy from loss of hydrogen caused by the rapid ascent and from rainwater on the envelope, forced down into the Teutoburger forest. There was only one injury. This left DELAG with only a single airship, LZ 6, which had been built the previous year with the hope of its being bought by the army and subsequently enlarged and modified for passenger-carrying purpose. Operating from Baden-Baden, successful flights were made almost daily between late August and mid-September, but on 14 September it was destroyed in a fire while in its hangar. It was insured, and DELAG could complete its next ship, LZ 8 "Deutschland II".  For the new season flights lasting between 90 minutes and two hours were offered for a price of 200 Dm. "Deutschland II" was completed on 30 March 1911, and arrived at Düsseldorf on 11 April, but after little more than a month of service was caught by a gust while being walked out of its hangar on 16 May: it was driven onto a 15 m (50 ft) high windbreak and broke its back. The passengers had to be rescued using fire ladders. Hugo Eckener, the captain, blamed the accident on his "weak-kneed" decision to let the eagerness of the passengers to fly overcome his reluctance to take the ship out in the existing conditions. The company's luck changed with the next ship, the LZ 10 "Schwaben". Completed on 26 June and delivered to DELAG on 15 July, it carried 1553 paying passengers during its career. Initially based at Baden Baden, in addition to the pleasure cruises a number of long-distance flights were made, carrying passengers to Frankfurt, Düsseldorf and eventually to Berlin. LZ 11 "Viktora Luise", named after the Kaiser's daughter, entered service on 4 March 1912. On 28 June 1912 "Schwaben" was destroyed in a hangar fire attributed to static electricity produced by its rubberised cotton gasbags, but was soon replaced by LZ 13 "Hansa", which was completed on 30 July. These airships were also used by the Imperial German Navy for crew training, with the Navy crews operating passenger flights. In 1913 LZ 17 "Sachsen" was added to the fleet.  By July 1914, one month before the start of World War I, DELAG's Zeppelins had transported 34,028 passengers on 1,588 commercial flights; the fleet had flown 172,535 kilometres in 3,176 hours.

Impact of World War I.

LZ 11, LZ 13, and LZ 17 were pressed into service for the German Army. After the war, however, DELAG's LZ 120 "Bodensee" and LZ 121 "Nordstern" helped reconnect the cities of Europe. LZ 120 flew between Friedrichshafen and Berlin-Staaken with a stopover in Munich, but both ships were surrendered as post-war reparations in 1921: LZ 120 went to Italy and was re-christened "Esperia", while LZ 121 became France's "Méditerranée" before it ever entered service for DELAG.

Transatlantic service.

In September 1928, DELAG began operating the successful rigid airship "Graf Zeppelin", which made regular, nonstop, transatlantic flights possible before airplanes had flight ranges sufficient to cross the ocean in either direction without stopping. For DELAG's first transatlantic trip, Dr. Eckener commanded the "Graf Zeppelin" leaving Friedrichshafen, Germany, at 07:54 on 11 October 1928, arriving at Lakehurst Field, New Jersey, on 15 October. In 1931 the "Graf Zeppelin" began offering regular scheduled passenger service between Germany and South America which continued until 1937. During its career Graf Zeppelin crossed the South Atlantic 136 times.

Evolution.

The "Graf Zeppelin" was the final airship flown by DELAG. In 1935, the successor to DELAG, the state-sponsored "Deutsche Zeppelin-Reederei" (DZR) was founded. Its fleet included the LZ 127 "Graf Zeppelin", LZ 129 "Hindenburg" and LZ 130 "Graf Zeppelin".  In 2001, a modern firm also named Deutsche Zeppelin Reederei was established as a subsidiary of Zeppelin Luftschifftechnik GmbH (ZLT). It operates the Zeppelin NT airships from Friedrichshafen at Lake Constance, mainly for sightseeing flights around Germany.

Airships.

Prior to World War I:  Following World War I:  In March 1935 the LZ 127 was transferred to the newly founded Deutsche Zeppelin-Reederei company, which also received as start capital the "Hindenburg", which was at the time under construction.

DELAG, acronym for "Deutsche Luftschifffahrts-Aktiengesellschaft" (German for "German Airship Travel Corporation"), was the world's first airline to use an aircraft in revenue service. It was founded on 16 November 1909 and operated Zeppelin rigid airships manufactured by the Luftschiffbau Zeppelin Corporation. Its headquarters were located in Frankfurt, Germany.

德国飞艇旅行公司,缩写为DELAG,是世界上按成立日期排列的航空公司列表将飞行器用于商业运营的航空公司。它于1909年11月16日成立,主要运营齐柏林系列硬式飞船,该飞船系列由齐柏林公司(Luftschiffbau Zeppelin)制造。总部位于德国法兰克福。


History.

The lyrics of "Denes nad Makedonija" were penned by Vlado Maleski, a writer who was active in the Partisan movement during World War II. In particular, he took charge of Radio Skoplje in December 1944, penned the manuscript for the first Macedonian-language movie, sat on the editorial board of the first Macedonian publishing house, and was part of the Commission for Language and Orthography that submitted recommendations to the government on standardizing the Macedonian alphabet, which were subsequently accepted. For his extensive contributions to the country's literature, Maleski is regarded as part of "the first generation of Macedonian prose writers".  The musical portion was composed by Todor Skalovski, one of Macedonia's most distinguished composers who also served as the conductor of its opera. He is also regarded as one of the trailblazers in composing music inspired by and incorporating Macedonian culture and mythology. The song was first played in 1942, among groups affiliated with the communist and Partisan resistance in Struga (which also happens to be Maleski's birthplace). According to oral recounts, the hymn was played by Maleski himself on New Year's Eve in the presence of 24 youths, whose identities were subsequently documented in writing only in 1981. Even though the resistance consisted of fighters from the different ethnic groups across Yugoslavia, the aforementioned youths are believed to have all identified as Macedonians. From the end of the war, it was utilized – albeit unofficially – as the regional anthem of the Socialist Republic of Macedonia (which was part of Yugoslavia at the time) until 1989.  Meanwhile, during the Informbiro period, a small change was made in the lyrics. In the new version the names of Nikola Karev and Dimitar Vlahov were scrapped and that of Dame Gruev was added. Karev's name was removed, as he and his brothers were suspected of being Bulgarophile elements. Vlahov also was dismissed, because he came from the Bulgarian Communist Party, communicated much better in Bulgarian than in the newly codified Macedonian language, and had no political supporters in Macedonia. The initial idea of then communist elite to remove also the name of Gotse Delchev and not to include that of Dame Gruev, because of their Bulgarophile sentiments, was finally abandoned. On 14 April 1989 the socialist republic's Assembly amended the Constitution of 1974 to expressly affirm "Denes nad Makedonija" as its official regional anthem. This change was short-lived, however, as the Assembly declared Macedonia's sovereignty two years later in January. This was approved on 8 September 1991 in a referendum that saw Macedonians vote overwhelmingly in favour of independence.  Soon after independence, the Macedonian national legislature held a contest to determine a national anthem for the newly-sovereign state. "Denes nad Makedonija" was one of several candidates in contention – the others in the running were "Himma" (, ) by Taki Hrisik and "Dani bideš večna" () by Aleksandar Džambazov – and ended up finishing runner-up in the final results. In spite of this, most of the Assembly Commission voted to make it the national anthem of the nascent state, and it was duly adopted on 11 August 1992.

Context of lyrics.

The lyrics of "Denes nad Makedonija" are reflective of a military marching song, which is fitting given its connection to the Yugoslav Macedonian theatre of the Second World War. They are not a call to arms for Macedonians; rather, the hymn uses imagery to take the person singing it back to the time the anthem was written, and purports that they themselves were engaging in combat at the time. Furthermore, the lyrics previously alluded to the concepts of mothers and motherhood (specifically in the second stanza). This is in line with the national anthems – both past and present – of other Southeast European countries like Bosnia and Herzegovina ("Jedna si jedina"), Croatia ("Horvatska domovina"), and Montenegro ("Oj, svijetla majska zoro"). It discussed how mothers mourn for their fallen sons, who died fighting for the rights and liberty of their country. They are comforted for their loss by being reminded of the bravery of their sons and the nobleness of the cause for which they died. Although this theme has been described as a "standard anthemic device", it was subsequently expunged from the official words of the state anthem.

Legal protection.

Macedonia's Sobranie (legislative chamber) passed legislation titled "The Law on the Anthem of the Republic of Macedonia" on 11 August 1992. This officially adopted "Denes nad Makedonija" as the national anthem of the then-nascent country, with Article 2 specifically confirming the song's status as such. However, it did not stipulate which stanzas were to be recognized as official. Curiously, the statute did not technically pass with the requisite two-thirds majority as stipulated by Article 5 of the country's constitution for proposals concerning national symbols (the anthem, the flag, and the national emblem). Although 88 out of the 120 members of the assembly voted for it, representatives elected from the Albanian community were not involved.

Lyrics.

"Denes nad Makedonija" as originally written consists of four stanzas. North Macedonia's law does not exactly specify which stanzas are officially constitute the national anthem, though the second stanza is often omitted from the lyrics of the national anthem that are posted on North Macedonia's government websites.

"" (, ; ; ) is the national anthem of North Macedonia; both the music and lyrics date from the early 1940s. Todor Skalovski composed the music, while the lyrics were written by Vlado Maleski. It was adopted as the national anthem in 1992, a year after the state's independence was declared from Yugoslavia. Before its adoption as a national anthem, it was used as the regional anthem of the Socialist Republic of Macedonia, a constituent state of Yugoslavia.

《今天在马其顿之上》()是北马其顿共和国的国歌,写作于1943年,由托多尔·斯卡洛夫斯基作曲,弗拉多·马莱斯基作词,第二次世界大战后,马其顿加入南斯拉夫民主联盟,这首歌就被定为国歌。1991年当马其顿脱离南斯拉夫独立后,国歌并没有改变,这首歌仍然是独立后的国歌。


Signs and symptoms.

Hypochondriasis is categorized as a somatic amplification disorder—a disorder of "perception and cognition"—that involves a hyper-vigilance of situation of the body or mind and a tendency to react to the initial perceptions in a negative manner that is further debilitating. Hypochondriasis manifests in many ways. Some people have numerous intrusive thoughts and physical sensations that push them to check with family, friends, and physicians. For example, a person who has a minor cough may think that they have tuberculosis. Or sounds produced by organs in the body, such as those made by the intestines, might be seen as a sign of a very serious illness to patients dealing with hypochondriasis.  Other people are so afraid of any reminder of illness that they will avoid medical professionals for a seemingly minor problem, sometimes to the point of becoming neglectful of their health when a serious condition may exist and go undiagnosed. Yet others live in despair and depression, certain that they have a life-threatening disease and no physician can help them. Some consider the disease as a punishment for past misdeeds.  Hypochondriasis is often accompanied by other psychological disorders. Bipolar disorder, clinical depression, obsessive-compulsive disorder (OCD), phobias, and somatization disorder are the most common accompanying conditions in people with hypochondriasis, as well as a generalized anxiety disorder diagnosis at some point in their life.  Many people with hypochondriasis experience a cycle of intrusive thoughts followed by compulsive checking, which is very similar to the symptoms of obsessive-compulsive disorder. However, while people with hypochondriasis are afraid of having an illness, patients with OCD worry about getting an illness or of transmitting an illness to others. Although some people might have both, these are distinct conditions.  Patients with hypochondriasis often are not aware that depression and anxiety produce their own physical symptoms, and mistake these symptoms for manifestations of another mental or physical disorder or disease. For example, people with depression often experience changes in appetite and weight fluctuation, fatigue, decreased interest in sex and motivation in life overall. Intense anxiety is associated with rapid heartbeat, palpitations, sweating, muscle tension, stomach discomfort, dizziness, shortness of breath, and numbness or tingling in certain parts of the body (hands, forehead, etc.).  If a person is ill with a medical disease such as diabetes or arthritis, there will often be psychological consequences, such as depression. Some even report being suicidal. In the same way, someone with psychological issues such as depression or anxiety will sometimes experience physical manifestations of these affective fluctuations, often in the form of medically unexplained symptoms. Common symptoms include headaches; abdominal, back, joint, rectal, or urinary pain; nausea; fever and/or night sweats; itching; diarrhea; dizziness; or balance problems. Many people with hypochondriasis accompanied by medically unexplained symptoms feel they are not understood by their physicians, and are frustrated by their doctors’ repeated failure to provide symptom relief.

Cause.

The genetic contribution to hypochondriasis is probably moderate, with heritability estimates around 10-37%. Non-shared environmental factors (i.e., experiences that differ between twins in the same family) explain most of the variance in key components of the condition such as the fear of illness and disease conviction. In contrast, the contribution of shared environmental factors (i.e., experiences shared by twins in the same family) to hypochondriasis is approximately zero.  Although little is known about exactly which non-shared environmental factors typically contribute to causing hypochondriasis, certain factors such as exposure to illness-related information are widely believed to lead to short-term increases in health anxiety and to have contributed to hypochondriasis in individual cases. Overly protective caregivers and an excessive focus on minor health concerns have also been implicated as potential causes of hypochondriasis.  In the media and on the Internet, articles, TV shows and advertisements regarding serious illnesses such as cancer and multiple sclerosis often portray these diseases as being random, obscure and somewhat inevitable. In the short term, inaccurate portrayal of risk and the identification of non-specific symptoms as signs of serious illness may contribute to exacerbating a fear of illness. Major disease outbreaks or predicted pandemics can have similar effects.  There is anecdotal evidence that it is common for serious illnesses or deaths of family members or friends to trigger hypochondria in certain individuals. Similarly, when approaching the age of a parent's premature death from disease, many otherwise healthy, happy individuals fall prey to hypochondria. These individuals believe they are suffering from the same disease that caused their parent's death, sometimes causing panic attacks with corresponding symptoms.

Diagnosis.

The ICD-10 defines hypochondriasis as follows:  The DSM-IV defines hypochondriasis according to the following criteria:  A. Preoccupation with fears of having, or the idea that one has, a serious disease based on the person's misinterpretation of bodily symptoms. B. The preoccupation persists despite appropriate medical evaluation and reassurance. C. The belief in Criterion A is not of delusional intensity (as in Delusional Disorder, Somatic Type) and is not restricted to a circumscribed concern about appearance (as in Body Dysmorphic Disorder). D. The preoccupation causes clinically significant distress or impairment in social, occupational, or other important areas of functioning. E. The duration of the disturbance is at least 6 months. F. The preoccupation is not better accounted for by Generalized Anxiety Disorder, Obsessive-Compulsive Disorder, Panic Disorder, a Major Depressive Episode, Separation Anxiety, or another Somatoform Disorder. In the fifth version of the DSM (DSM-5), most who met criteria for DSM-IV hypochondriasis instead meet criteria for a diagnosis of somatic symptom disorder (SSD) or illness anxiety disorder (IAD).

Classification.

The classification of hypochondriasis in relation to other psychiatric disorders has long been a topic of scholarly debate, and has differed widely between different diagnostic systems and influential publications.  In the case of the DSM, the first and second versions listed hypochondriasis as a neurosis, whereas the third and fourth versions listed hypochondriasis as a somatoform disorder. The current version of the DSM (DSM-5) lists somatic symptom disorder (SSD) under the heading of "somatic symptom and related disorders", and illness anxiety disorder (IAD) under both this heading and as an anxiety disorder.  The ICD-10, like the third and fourth versions of the DSM, lists hypochondriasis as a somatoform disorder. The ICD-11, however, lists hypochondriasis under the heading of "obsessive-compulsive or related disorders".  There are also numerous influential scientific publications which have argued for other classifications of hypochondriasis. Notably, since the early 1990s, it has become increasingly common to regard hypochondriasis as an anxiety disorder, and to refer to the condition as "health anxiety" or "severe health anxiety".

Treatment.

Approximately 20 randomized controlled trials and numerous observational studies indicate that cognitive behavioral therapy (CBT) is an effective treatment for hypochondriasis. Typically, about two-thirds of patients respond to treatment, and about 50% of patients achieve remission, i.e., no longer suffer from hypochondriasis after treatment. CBT for hypochondriasis and health anxiety may be offered in various formats, including as face-to-face individual or group therapy, via telephone, or as guided self-help with information conveyed via a self-help book or online treatment platform. Effects are typically sustained over time.  There is also evidence that antidepressant medications such as selective serotonin reuptake inhibitors can reduce symptoms. In some cases, hypochondriasis responds well to antipsychotics, particularly the newer atypical antipsychotic medications.

Etymology.

Among the regions of the abdomen, the hypochondrium is the uppermost part. The word derives from the Greek term ὑποχόνδριος "hypokhondrios", meaning "of the soft parts between the ribs and navel" from ὑπό "hypo" ("under") and χόνδρος "khondros", or cartilage (of the sternum). "Hypochondria" in Late Latin meant "the abdomen".  The term "hypochondriasis" for a state of disease without real cause reflected the ancient belief that the viscera of the hypochondria were the seat of melancholy and sources of the vapor that caused morbid feelings. Until the early 18th century, the term referred to a "physical disease caused by imbalances in the region that was below your rib cage" (i.e., of the stomach or digestive system). For example, Robert Burton's "The Anatomy of Melancholy" (1621) blamed it "for everything from 'too much spittle' to 'rumbling in the guts'".  Immanuel Kant discussed hypochondria in his 1798 book, "Anthropology from a Pragmatic Point of View", like this: The disease of the hypochondriac consists in this: that certain bodily sensations do not so much indicate a really existing disease in the body as rather merely excite apprehensions of its existence: and human nature is so constituted – a trait which the animal lacks – that it is able to strengthen or make permanent "local impressions" simply by paying attention to them, whereas an "abstraction" – whether produced on purpose or by other diverting occupations – lessens these impressions, or even effaces them altogether.

Hypochondriasis or hypochondria is a condition in which a person is excessively and unduly worried about having a serious illness. An old concept, the meaning of hypochondria has repeatedly changed. It has been claimed that this debilitating condition results from an inaccurate perception of the condition of body or mind despite the absence of an actual medical diagnosis. An individual with hypochondriasis is known as a hypochondriac. Hypochondriacs become unduly alarmed about any physical or psychological symptoms they detect, no matter how minor the symptom may be, and are convinced that they or others have, or are about to be diagnosed with, a serious illness.

疑病症(英语:Hypochondriasis),也作疑病性神经症(英语:Hypochondriacal neurosis)、虑病症,是指对自身出现的一些身体状况作出不合实际的解释,担心自己身患一种极为严重的疾病。疑病症患者会在未经诊断的情况下对自己的生理、心理状况作出不恰当的评估,并且无论症状严重与否,他们都会对自我的状况产生不必要的警觉,坚信自己已经患有或即将确诊为一种严重的病症。


Overview.

Like other solids, semiconductor materials have an electronic band structure determined by the crystal properties of the material. Energy distribution among electrons is described by the Fermi level and the temperature of the electrons. At absolute zero temperature, all of the electrons have energy below the Fermi level; but at non-zero temperatures the energy levels are filled following a Boltzmann distribution.  In undoped semiconductors the Fermi level lies in the middle of a "forbidden band" or band gap between two "allowed bands" called the "valence band" and the "conduction band". The valence band, immediately below the forbidden band, is normally very nearly completely occupied. The conduction band, above the Fermi level, is normally nearly completely empty. Because the valence band is so nearly full, its electrons are not mobile, and cannot flow as electric current.  However, if an electron in the valence band acquires enough energy to reach the conduction band (as a result of interaction with other electrons, holes, photons, or the vibrating crystal lattice itself), it can flow freely among the nearly empty conduction band energy states. Furthermore, it will also leave behind a hole that can flow as current exactly like a physical charged particle.  Carrier generation describes processes by which electrons gain energy and move from the valence band to the conduction band, producing two mobile carriers; while recombination describes processes by which a conduction band electron loses energy and re-occupies the energy state of an electron hole in the valence band.  These processes must conserve both quantized energy and crystal momentum, and the vibrating lattice plays a large role in conserving momentum as, in collisions, photons can transfer very little momentum in relation to their energy.

Relation between generation and recombination.

Recombination and generation are always happening in semiconductors, both optically and thermally. As predicted by thermodynamics, a material at thermal equilibrium will have generation and recombination rates that are balanced so that the net charge carrier density remains constant. The resulting probability of occupation of energy states in each energy band is given by Fermi–Dirac statistics.  The product of the electron and hole densities (formula_1 and formula_2) is a constant formula_3 at equilibrium, maintained by recombination and generation occurring at equal rates. When there is a surplus of carriers (i.e., formula_4), the rate of recombination becomes greater than the rate of generation, driving the system back towards equilibrium. Likewise, when there is a deficit of carriers (i.e., formula_5), the generation rate becomes greater than the recombination rate, again driving the system back towards equilibrium. As the electron moves from one energy band to another, the energy and momentum that it has lost or gained must go to or come from the other particles involved in the process (e.g. photons, electron, or the system of vibrating lattice atoms).

Carrier generation.

When light interacts with a material, it can either be absorbed (generating a pair of free carriers or an exciton) or it can "stimulate" a recombination event. The generated photon has similar properties to the one responsible for the event. Absorption is the active process in photodiodes, solar cells and other semiconductor photodetectors, while stimulated emission is the principle of operation in laser diodes.  Besides light excitation, carriers in semiconductors can also be generated by an external electric field, for example in light-emitting diodes and transistors.  When light with sufficient energy hits a semiconductor, it can excite electrons across the band gap. This generates additional charge carriers, temporarily lowering the electrical resistance of materials. This higher conductivity in the presence of light is known as photoconductivity. This conversion of light into electricity is widely used in photodiodes.

Recombination mechanisms.

Carrier recombination can happen through multiple relaxation channels. The main ones are band-to-band recombination, Shockley–Read–Hall (SRH) trap-assisted recombination, Auger recombination and surface recombination. These decay channels can be separated into radiative and non-radiative. The latter occurs when the excess energy is converted into heat by phonon emission after the mean lifetime formula_6, whereas in the former at least part of the energy is released by light emission or luminescence after a radiative lifetime formula_7. The carrier lifetime formula_8is then obtained from the rate of both type of events according to: formula_9From which we can also define the internal quantum efficiency or quantum yield, formula_10 as:formula_11

Radiative Recombination.

Band-to-band radiative recombination.

"Band-to-band recombination" is the name for the process of electrons jumping down from the conduction band to the valence band in a radiative manner. During band-to-band recombination, a form of spontaneous emission, the energy absorbed by a material is released in the form of photons. Generally these photons contain the same or less energy than those initially absorbed. This effect is how LEDs create light. Because the photon carries relatively little momentum, radiative recombination is significant only in direct bandgap materials. This process is also known as "bimolecular recombination""."  This type of recombination depends on the density of electrons and holes in the excited state, denoted by formula_12 and formula_13 respectively. Let us represent the carrier generation rate as formula_14 and the radiative recombination as formula_15. Then considering the case in which only band-to-band recombination occurs we can express the change in carrier density as a function of time as:formula_16In thermal equilibrium the radiative recombination formula_17 is equal to thermal generation rate formula_18, with the mass action law formula_19, it can be written as:formula_20where formula_21 is called the radiative recombination rate, formula_22 is the intrinsic carrier density, formula_23 and formula_24 are the equilibrium carrier densities. In the presence of electron hole pairs, the charge neutrality remains formula_25 and the non-equilibrium densities are given by :formula_26  formula_27Then the net recombination rate formula_15 becomes,formula_29Solving this equation by considering charge neutrality, the radiative lifetime formula_30 is then given byformula_31

Stimulated emission.

"Stimulated emission" is a process in which an incident photon interacts with an excited electron causing it to recombine and emit a photon with the same properties of the incident one, in terms of phase, frequency, polarization, and direction of travel. Stimulated emission together with the principle of population inversion are at the heart of operation of lasers and masers. It has been shown by Einstein at the beginning of the twentieth century that if the excited and the ground level are non degenerate then the absorption rate formula_32and the stimulated emission rate formula_33are the same. Else if level 1 and level 2 are formula_34-fold and formula_35-fold degenerate respectively, the new relation is:formula_36

Non-radiative recombination.

Non-radiative recombination is a process in phosphors and semiconductors, whereby charge carriers recombine with releasing phonon instead of photons. Non-radiative recombination in optoelectronics and phosphors is an unwanted process, lowering the light generation efficiency and increasing heat losses.  Non-radiative life time is the average time before an electron in the conduction band of a semiconductor recombines with a hole. It is an important parameter in optoelectronics where radiative recombination is required to produce a photon; if the non-radiative life time is shorter than the radiative, a carrier is more likely to recombine non-radiatively. This results in low internal quantum efficiency.

Shockley–Read–Hall (SRH).

In "Shockley-Read-Hall recombination" ("SRH"), also called "trap-assisted recombination", the electron in transition between bands passes through a new energy state (localized state) created within the band gap by a dopant or a defect in the crystal lattice; such energy states are called "traps". Non-radiative recombination occurs primarily at such sites. The energy is exchanged in the form of lattice vibration, a phonon exchanging thermal energy with the material.  Since traps can absorb differences in momentum between the carriers, SRH is the dominant recombination process in silicon and other indirect bandgap materials. However, trap-assisted recombination can also dominate in direct bandgap materials under conditions of very low carrier densities (very low level injection) or in materials with high density of traps such as perovskites. The process is named after William Shockley, William Thornton Read and Robert N. Hall, who published it in 1962.

Types of traps.

Electron traps vs. hole traps.

Even though all the recombination events can be described in terms of electron movements, it is common to visualize the different processes in terms of excited electron and the electron "holes" they leave behind. In this context, if trap levels are close to the conduction band, they can temporarily immobilize excited electrons or in other words, they are "electron traps". On the other hand, if their energy lies close to the valence band they become "hole traps."

Shallow traps vs. deep traps.

The distinction between shallow and deep traps is commonly made depending on how close electron traps are to the conduction band and how close hole traps are to the valence band. If the difference between trap and band is smaller than the thermal energy kT it is often said that it is a "shallow trap". Alternatively, if the difference is larger than the thermal energy, it is called a deep trap. This difference is useful because shallow traps can be emptied more easily and thus are often not as detrimental to the performance of optoelectronic devices.

SRH Model.

In the SRH model, four things can happen involving trap levels:  When carrier recombination occurs through traps, we can replace the valence density of states by that of the intragap state. The term formula_37is replaced by the density of trapped electrons/holes formula_38. formula_39 Where formula_40 is the density of trap states and formula_41 is the probability of that occupied state. Considering a material containing both types of traps, we can define two trapping coefficients formula_42 and two de-trapping coeficients formula_43. In equilibrium, both trapping and de-trapping should be balanced (formula_44 and formula_45). Then, the four rates as a function of formula_41become:formula_47 Where formula_48 and formula_49 are the electron and hole densities when the quasi Fermi level matches the trap energy. In steady-state condition, the net trapping rate of electrons should match the net recombination rate for holes, in other words: formula_50. This eliminates the occupation probability formula_41 and leads to the Shockley-Read-Hall expression for the trap-assisted recombination:formula_52Where the average lifetime for electrons and holes are defined as:formula_53

Auger recombination.

In "Auger recombination" the energy is given to a third carrier which is excited to a higher energy level without moving to another energy band. After the interaction, the third carrier normally loses its excess energy to thermal vibrations. Since this process is a three-particle interaction, it is normally only significant in non-equilibrium conditions when the carrier density is very high. The Auger effect process is not easily produced, because the third particle would have to begin the process in the unstable high-energy state.  In thermal equilibrium the Auger recombination formula_54 and thermal generation rate formula_18 equal each otherformula_56where formula_57 are the Auger capture probabilities.  The non-equilibrium Auger recombination rate formula_58 and resulting net recombination rate formula_59 under steady-state conditions areformula_60The Auger lifetime formula_61 is given byformula_62The mechanism causing LED efficiency droop was identified in 2007 as Auger recombination, which met with a mixed reaction. In 2013, an experimental study claimed to have identified Auger recombination as the cause of efficiency droop. However, it remains disputed whether the amount of Auger loss found in this study is sufficient to explain the droop. Other frequently quoted evidence against Auger as the main droop causing mechanism is the low-temperature dependence of this mechanism which is opposite to that found for the drop.

Surface recombination.

Trap-assisted recombination at the surface of a semiconductor is referred to as surface recombination. This occurs when traps at or near the surface or interface of the semiconductor form due to dangling bonds caused by the sudden discontinuation of the semiconductor crystal. Surface recombination is characterized by surface recombination velocity which depends on the density of surface defects. In applications such as solar cells, surface recombination may be the dominant mechanism of recombination due to the collection and extraction of free carriers at the surface. In some applications of solar cells, a layer of transparent material with a large band gap, also known as a window layer, is used to minimize surface recombination. Passivation techniques are also employed to minimize surface recombination.

Langevin recombination.

For free carriers in low-mobility systems, the recombination rate is often described with the Langevin recombination rate. The model is often used for disordered systems such as organic materials (and is hence relevant for organic solar cells) and other such systems. The Langevin recombination strength is defined as formula_63.

In the solid-state physics of semiconductors, carrier generation and carrier recombination are processes by which mobile charge carriers (electrons and electron holes) are created and eliminated. Carrier generation and recombination processes are fundamental to the operation of many optoelectronic semiconductor devices, such as photodiodes, light-emitting diodes and laser diodes. They are also critical to a full analysis of p-n junction devices such as bipolar junction transistors and p-n junction diodes.

在固体物理学中,载流子的复合(英语:recombination)指半导体中的载流子(电子和空穴)成对消失的过程。这一过程在许多光电半导体材料中都会涉及。在一些具有PN结的器件,例如二极管和双极性晶体管中,为了研究其详细的工作原理,分析载流子的复合过程也是必要的。


Discovery.

The first mutagens to be identified were carcinogens, substances that were shown to be linked to cancer. Tumors were described more than 2,000 years before the discovery of chromosomes and DNA; in 500 B.C., the Greek physician Hippocrates named tumors resembling a crab "karkinos" (from which the word "cancer" is derived via Latin), meaning crab. In 1567, Swiss physician Paracelsus suggested that an unidentified substance in mined ore (identified as radon gas in modern times) caused a wasting disease in miners, and in England, in 1761, John Hill made the first direct link of cancer to chemical substances by noting that excessive use of snuff may cause nasal cancer. In 1775, Sir Percivall Pott wrote a paper on the high incidence of scrotal cancer in chimney sweeps, and suggested chimney soot as the cause of scrotal cancer. In 1915, Yamagawa and Ichikawa showed that repeated application of coal tar to rabbit's ears produced malignant cancer. Subsequently, in the 1930s the carcinogen component in coal tar was identified as a polyaromatic hydrocarbon (PAH), benzo[a]pyrene. Polyaromatic hydrocarbons are also present in soot, which was suggested to be a causative agent of cancer over 150 years earlier.  The association of exposure to radiation and cancer had been observed as early as 1902, six years after the discovery of X-ray by Wilhelm Röntgen and radioactivity by Henri Becquerel. Georgii Nadson and German Filippov were the first who created fungi mutants under ionizing radiation in 1925. The mutagenic property of mutagens was first demonstrated in 1927, when Hermann Muller discovered that x-rays can cause genetic mutations in fruit flies, producing phenotypic mutants as well as observable changes to the chromosomes, visible due to the presence of enlarged "polytene" chromosomes in fruit fly salivary glands. His collaborator Edgar Altenburg also demonstrated the mutational effect of UV radiation in 1928. Muller went on to use x-rays to create Drosophila mutants that he used in his studies of genetics. He also found that X-rays not only mutate genes in fruit flies, but also have effects on the genetic makeup of humans. Similar work by Lewis Stadler also showed the mutational effect of X-rays on barley in 1928, and ultraviolet (UV) radiation on maize in 1936. The effect of sunlight had previously been noted in the nineteenth century where rural outdoor workers and sailors were found to be more prone to skin cancer.  Chemical mutagens were not demonstrated to cause mutation until the 1940s, when Charlotte Auerbach and J. M. Robson found that mustard gas can cause mutations in fruit flies. A large number of chemical mutagens have since been identified, especially after the development of the Ames test in the 1970s by Bruce Ames that screens for mutagens and allows for preliminary identification of carcinogens. Early studies by Ames showed around 90% of known carcinogens can be identified in Ames test as mutagenic (later studies however gave lower figures), and ~80% of the mutagens identified through Ames test may also be carcinogens. Mutagens are not necessarily carcinogens, and vice versa. Sodium azide for example may be mutagenic (and highly toxic), but it has not been shown to be carcinogenic.

Effects.

Mutagens can cause changes to the DNA and are therefore genotoxic. They can affect the transcription and replication of the DNA, which in severe cases can lead to cell death. The mutagen produces mutations in the DNA, and deleterious mutation can result in aberrant, impaired or loss of function for a particular gene, and accumulation of mutations may lead to cancer. Mutagens may therefore be also carcinogens. However, some mutagens exert their mutagenic effect through their metabolites, and therefore whether such mutagens actually become carcinogenic may be dependent on the metabolic processes of an organism, and a compound shown to be mutagenic in one organism may not necessarily be carcinogenic in another.  Different mutagens act on the DNA differently. Powerful mutagens may result in chromosomal instability, causing chromosomal breakages and rearrangement of the chromosomes such as translocation, deletion, and inversion. Such mutagens are called clastogens.  Mutagens may also modify the DNA sequence; the changes in nucleic acid sequences by mutations include substitution of nucleotide base-pairs and insertions and deletions of one or more nucleotides in DNA sequences. Although some of these mutations are lethal or cause serious disease, many have minor effects as they do not result in residue changes that have significant effect on the structure and function of the proteins. Many mutations are silent mutations, causing no visible effects at all, either because they occur in non-coding or non-functional sequences, or they do not change the amino-acid sequence due to the redundancy of codons.  Some mutagens can cause aneuploidy and change the number of chromosomes in the cell. They are known as aneuploidogens.  In Ames test, where the varying concentrations of the chemical are used in the test, the dose response curve obtained is nearly always linear, suggesting that there may be no threshold for mutagenesis. Similar results are also obtained in studies with radiations, indicating that there may be no safe threshold for mutagens. However, the no-threshold model is disputed with some arguing for a dose rate dependent threshold for mutagenesis. Some have proposed that low level of some mutagens may stimulate the DNA repair processes and therefore may not necessarily be harmful. More recent approaches with sensitive analytical methods have shown that there may be non-linear or bilinear dose-responses for genotoxic effects, and that the activation of DNA repair pathways can prevent the occurrence of mutation arising from a low dose of mutagen.

Types.

Mutagens may be of physical, chemical or biological origin. They may act directly on the DNA, causing direct damage to the DNA, and most often result in replication error. Some however may act on the replication mechanism and chromosomal partition. Many mutagens are not mutagenic by themselves, but can form mutagenic metabolites through cellular processes, for example through the activity of the cytochrome P450 system and other oxygenases such as cyclooxygenase. Such mutagens are called promutagens.

DNA reactive chemicals.

A large number of chemicals may interact directly with DNA. However, many such as PAHs, aromatic amines, benzene are not necessarily mutagenic by themselves, but through metabolic processes in cells they produce mutagenic compounds.

Metals.

Many metals, such as arsenic, cadmium, chromium, nickel and their compounds may be mutagenic, but they may act, however, via a number of different mechanisms. Arsenic, chromium, iron, and nickel may be associated with the production of ROS, and some of these may also alter the fidelity of DNA replication. Nickel may also be linked to DNA hypermethylation and histone deacetylation, while some metals such as cobalt, arsenic, nickel and cadmium may also affect DNA repair processes such as DNA mismatch repair, and base and nucleotide excision repair.

Protection.

Antioxidants are an important group of anticarcinogenic compounds that may help remove ROS or potentially harmful chemicals. These may be found naturally in fruits and vegetables. Examples of antioxidants are vitamin A and its carotenoid precursors, vitamin C, vitamin E, polyphenols, and various other compounds. β-Carotene is the red-orange colored compounds found in vegetables like carrots and tomatoes. Vitamin C may prevent some cancers by inhibiting the formation of mutagenic N-nitroso compounds (nitrosamine). Flavonoids, such as EGCG in green tea, have also been shown to be effective antioxidants and may have anti-cancer properties. Epidemiological studies indicate that a diet rich in fruits and vegetables is associated with lower incidence of some cancers and longer life expectancy, however, the effectiveness of antioxidant supplements in cancer prevention in general is still the subject of some debate.  Other chemicals may reduce mutagenesis or prevent cancer via other mechanisms, although for some the precise mechanism for their protective property may not be certain. Selenium, which is present as a micronutrient in vegetables, is a component of important antioxidant enzymes such as gluthathione peroxidase. Many phytonutrients may counter the effect of mutagens; for example, sulforaphane in vegetables such as broccoli has been shown to be protective against prostate cancer. Others that may be effective against cancer include indole-3-carbinol from cruciferous vegetables and resveratrol from red wine.  An effective precautionary measure an individual can undertake to protect themselves is by limiting exposure to mutagens such as UV radiations and tobacco smoke. In Australia, where people with pale skin are often exposed to strong sunlight, melanoma is the most common cancer diagnosed in people aged 15–44 years.  In 1981, human epidemiological analysis by Richard Doll and Richard Peto indicated that smoking caused 30% of cancers in the US. Diet is also thought to cause a significant number of cancer, and it has been estimated that around 32% of cancer deaths may be avoidable by modification to the diet. Mutagens identified in food include mycotoxins from food contaminated with fungal growths, such as aflatoxins which may be present in contaminated peanuts and corn; heterocyclic amines generated in meat when cooked at high temperature; PAHs in charred meat and smoked fish, as well as in oils, fats, bread, and cereal; and nitrosamines generated from nitrites used as food preservatives in cured meat such as bacon (ascobate, which is added to cured meat, however, reduces nitrosamine formation). Overly-browned starchy food such as bread, biscuits and potatoes can generate acrylamide, a chemical shown to cause cancer in animal studies. Excessive alcohol consumption has also been linked to cancer; the possible mechanisms for its carcinogenicity include formation of the possible mutagen acetaldehyde, and the induction of the cytochrome P450 system which is known to produce mutagenic compounds from promutagens.  For certain mutagens, such as dangerous chemicals and radioactive materials, as well as infectious agents known to cause cancer, government legislations and regulatory bodies are necessary for their control.

Test systems.

Many different systems for detecting mutagen have been developed. Animal systems may more accurately reflect the metabolism of human, however, they are expensive and time-consuming (may take around three years to complete), they are therefore not used as a first screen for mutagenicity or carcinogenicity.

Yeast.

Systems similar to Ames test have been developed in yeast. "Saccharomyces cerevisiae" is generally used. These systems can check for forward and reverse mutations, as well as recombinant events.

"Drosophila".

Sex-Linked Recessive Lethal Test – Males from a strain with yellow bodies are used in this test. The gene for the yellow body lies on the X-chromosome. The fruit flies are fed on a diet of test chemical, and progenies are separated by sex. The surviving males are crossed with the females of the same generation, and if no males with yellow bodies are detected in the second generation, it would indicate a lethal mutation on the X-chromosome has occurred.

Plant assays.

Plants such as "Zea mays", "Arabidopsis thaliana" and "Tradescantia" have been used in various test assays for mutagenecity of chemicals.

Cell culture assay.

Mammalian cell lines such as Chinese hamster V79 cells, Chinese hamster ovary (CHO) cells or mouse lymphoma cells may be used to test for mutagenesis. Such systems include the HPRT assay for resistance to 8-azaguanine or 6-thioguanine, and ouabain-resistance (OUA) assay.  Rat primary hepatocytes may also be used to measure DNA repair following DNA damage. Mutagens may stimulate unscheduled DNA synthesis that results in more stained nuclear material in cells following exposure to mutagens.

Chromosome check systems.

These systems check for large scale changes to the chromosomes and may be used with cell culture or in animal test. The chromosomes are stained and observed for any changes. Sister chromatid exchange is a symmetrical exchange of chromosome material between sister chromatids and may be correlated to the mutagenic or carcinogenic potential of a chemical. In micronucleus Test, cells are examined for micronuclei, which are fragments or chromosomes left behind at anaphase, and is therefore a test for clastogenic agents that cause chromosome breakages. Other tests may check for various chromosomal aberrations such as chromatid and chromosomal gaps and deletions, translocations, and ploidy.

Animal test systems.

Rodents are usually used in animal test. The chemicals under test are usually administered in the food and in the drinking water, but sometimes by dermal application, by gavage, or by inhalation, and carried out over the major part of the life span for rodents. In tests that check for carcinogens, maximum tolerated dosage is first determined, then a range of doses are given to around 50 animals throughout the notional lifespan of the animal of two years. After death the animals are examined for sign of tumours. Differences in metabolism between rat and human however means that human may not respond in exactly the same way to mutagen, and dosages that produce tumours on the animal test may also be unreasonably high for a human, i.e. the equivalent amount required to produce tumours in human may far exceed what a person might encounter in real life. Mice with recessive mutations for a visible phenotype may also be used to check for mutagens. Females with recessive mutation crossed with wild-type males would yield the same phenotype as the wild-type, and any observable change to the phenotype would indicate that a mutation induced by the mutagen has occurred. Mice may also be used for dominant lethal assays where early embryonic deaths are monitored. Male mice are treated with chemicals under test, mated with females, and the females are then sacrificed before parturition and early fetal deaths are counted in the uterine horns. Transgenic mouse assay using a mouse strain infected with a viral shuttle vector is another method for testing mutagens. Animals are first treated with suspected mutagen, the mouse DNA is then isolated and the phage segment recovered and used to infect "E. coli". Using similar method as the blue-white screen, the plaque formed with DNA containing mutation are white, while those without are blue.

In anti-cancer therapy.

Many mutagens are highly toxic to proliferating cells, and they are often used to destroy cancer cells. Alkylating agents such as cyclophosphamide and cisplatin, as well as intercalating agent such as daunorubicin and doxorubicin may be used in chemotherapy. However, due to their effect on other cells which are also rapidly dividing, they may have side effects such as hair loss and nausea. Research on better targeted therapies may reduce such side-effects. Ionizing radiations are used in radiation therapy.

In fiction.

In science fiction, mutagens are often represented as substances that are capable of completely changing the form of the recipient or granting them superpowers. Powerful radiations are the agents of mutation for the superheroes in Marvel Comics's Fantastic Four, Daredevil, and Hulk, while in the Teenage Mutant Ninja Turtles franchise the mutagen is chemical agent also called "ooze", and for Inhumans the mutagen is the Terrigen Mist. Mutagens are also featured in television series, and video games, such as the "Cyberia," "The Witcher", "", "", "Resident Evil", "Infamous", "Freedom Force, Command & Conquer", "Gears of War 3", "StarCraft", "BioShock", and "Fallout". In the "nuclear monster" films of the 1950s, nuclear radiation mutates humans and common insects often to enormous size and aggression; these films include "Godzilla", "Them!", "Attack of the 50 Foot Woman", "Tarantula!", "The Amazing Colossal Man", etc.

In genetics, a mutagen is a physical or chemical agent that changes the genetic material, usually DNA, of an organism and thus increases the frequency of mutations above the natural background level. As many mutations can cause cancer, mutagens are therefore also likely to be carcinogens, although not always necessarily so. All mutagens have characteristic mutational signatures with some chemicals becoming mutagenic through cellular processes. Not all mutations are caused by mutagens: so-called "spontaneous mutations" occur due to spontaneous hydrolysis, errors in DNA replication, repair and recombination.

突变原(英语:Mutagen,又译致变原、致突变原、致突变剂或诱变剂等)是指一些能使生物体内的遗传讯息(通常是脱氧核糖核酸)发生变化的物理或化学因子。生物若处于这些因子的作用下,发生突变的机会将高于在一般自然状况中。许多突变会造成癌症的发生,因此突变原很可能也是致癌物质。不过,并非所有突变皆因突变原而产生,在DNA复制、修复或重组过程中,也可能发生错误,进而导致突变。


Characteristics.

Ataxites are composed mainly of meteoric iron, a native metal found in meteorites that consists of the mineral taenite with minor amounts of plessite, troilite, and microscopic lamellae of kamacite. Ataxites are the most nickel-rich meteorites known; they usually contain over 18% nickel. The high nickel content is the reason that they do not develop a Widmanstätten structure, because in this case kamacite can be exsolved from taenite only at such a low temperature (below about 600°C) where diffusion is already too slow.

Classification.

Most ataxites belong to the IVB group or are classified as "Iron, ungrouped" because they do not fit in any of the groups currently recognized by meteorite classification. Some ataxites belong to the IAB group and fall into the sHL (high-Au, low-Ni subgroup), sLH (low-Au, high-Ni subgroup), sHH (high-Au, high-Ni subgroup) and "ung" subgroup (does not fit any subgroup of IAB). Only a couple of ataxites have been classified into the IAB complex and the IIF, IVA, IIAB, IIIAB groups.

Abundance.

They are a rare class, with none of the about 50 observed iron meteorite falls being an ataxite, however, the largest meteorite ever, the Hoba meteorite (found 1920 in Namibia, weight 60 tons), belongs to this class. A Tibetan Buddhist statue, the "Iron Man", was likely carved from an ataxite meteorite. It might even be made from a fragment of the Chinga meteorite. Other examples of ataxites are the Dronino meteorite and pieces of the Gebel Kamil.

Ataxites (from Greek meaning "without structure") are a structural class of iron meteorites with a high nickel content and show no Widmanstätten patterns upon etching.

无纹陨铁是铁陨石的一种,它的主要成分是铁镍合金、镍纹石,也包含有合纹石、陨硫铁和用显微镜才能看见的锥纹石薄片,但没有可见的魏德曼花纹。无纹陨铁是镍含量最高的陨石,含量都在18%以上。高镍含量是不能发展出魏德曼花纹的原因,因为锥纹石只有在较低的温度(大约600°C以下)才能从镍纹石中熔出,而此时扩散的速率已经太慢