Experimental efforts in search of 76Ge Neutrinoless Double Beta Decay
Experimental  efforts  in  search  for
 - 1 -
Experimental efforts in search of 76Ge Neutrinoless Double Beta Decay  
Somnath Choudhury †
Department of Physics & Meteorology 
Indian Institute of Technology, Kharagpur – 721302, India 
Abstract 
Neutrinoless   double  beta  decay  is  one  of  the  most  sensitive  approaches  in  non-accelerator  particle  physics  to  take  
us  into  a  regime  of  physics  beyond  the  standard  model. This  article  is  a  brief  review  of  the  experiments  in  search  
of  neutrinoless   double  beta  decay  from 76Ge.  Following a brief introduction of  the  process  of  double  beta  decay  from  
76Ge, the results of the very first experiments IGEX and  Heidelberg-Moscow which give indications of the existence of 
possible neutrinoless double beta decay mode has been reviewed. Then ongoing efforts to substantiate the early findings are 
presented and the Majorana experiment as a future experimental approach which will allow a very detailed study of the 0ν 
decay mode is discussed. 
Keywords: neutrinoless, Majorana particle, pulse shape discrimination.  
1.  Introduction 
Neutrinoless double beta decay is one of the most sensitive approaches with great perspectives to test particle 
physics beyond the Standard Model. There is immense scope to use 0νββ decay for constraining neutrino masses, 
left–right–symmetric models, interactions involving R-parity breaking in the supersymmetric model and 
leptoquark scenarios, as well as effective lepton number violating couplings. Experimental limits on 0νββ decay 
are not only complementary to accelerator experiments but at least in some cases competitive or superior to the 
best existing direct search limits. The steadily improving experimental limits on the half-life of 0νββ can be 
translated into more stringent limits on the parameters of these new physics scenarios.  
In the process of beta decay an unstable nucleus decays by converting a neutron in the nucleus to a proton and 
emitting an electron and an anti-neutrino. In order for beta decay to be possible the final nucleus must have a 
larger binding energy than the original nucleus. For some nuclei, such as Germanium-76 the nuclei with atomic 
number one higher have a smaller binding energy, preventing beta decay from occurring. In the case of 
Germanium-76 the nuclei with atomic number two higher, Selenium-76 has a larger binding energy, so the 
"double beta decay" process is allowed. In double beta decay two neutrons in the nuclei are converted to protons, 
and two electrons and two anti-neutrinos are emitted. It is the rarest known kind of radioactive decay; it was 
observed for only ten isotopes. For some nuclei, the process occurs as conversion of two protons to neutrons, with 
emission of two neutrinos and absorption of two orbital electrons (double electron capture). If mass difference 
between the parent and daughter atoms is more than 1022 keV (two electron masses), another branch of the 
process becomes possible, with capture of one orbital electron and emission of one positron. And, at last, when 
the mass difference is more then 2044 keV (four electron masses), the third branch of the decay arises, with 
emission of two positrons (β+β+ decay).  
The processes described above are also known as two neutrino double beta decay, as two neutrinos (or anti-
neutrinos) are emitted. If the neutrino is a Majorana particle, meaning that the anti-neutrino and the neutrino are 
actually the same particle then it is possible for neutrinoless double beta decay to occur. In 0νββ decay the emitted 
neutrino is immediately absorbed (as its anti-particle) by another nucleon of the nucleus, so the total kinetic 
energy of the two electrons would be exactly the difference in binding energy between the initial and final state 
nuclei.  
† Now at Indiana University – Bloomington, USA 
Experiments have been carried out and proposed to search for 0νββ decay mode, as its discovery would indicate 
that neutrinos are indeed Majorana particles and allow a calculation of neutrino mass. While the two-neutrino 
mode (1.1) is allowed by the Standard Model of particle physics, the neutrinoless mode (0νββ) (1.2) requires 
violation of lepton number (∆L=2). This mode is possible only, if the neutrino is a Majorana particle, i.e. the 
neutrino is its own antiparticle. Double beta decay, the rarest known nuclear decay process, can occur in different 
modes:  
                                                                            _   
2νββ -decay : A(Z,N) → A(Z+2, N-2)+2e ⎯ +2 ν                                                                            (1.1) 
0 ν ββ -decay : A(Z,N) → A(Z+2, N-2) + 2e⎯                                                                                           (1.2) 
0 ν(2) χ ββ -decay : A(Z,N) → A(Z+2, N-2)+2e ⎯  + (2) χ                                                                                   (1.3) 
                                                                                          
2.  Double Beta Decay:  A Rare Process 
The process arises in certain cases of even-A nuclei, where A is the mass number and is the sum of the number of 
protons and neutrons (A = Z + N). For even-A nuclei, the strong pairing force between like nucleons (neutrons 
like to be paired with other neutrons in a given nucleus, with the same true for protons), the binding energy of 
even-even nuclei (even number of protons and even number neutrons) is larger than that of odd-odd nuclei (odd 
numbers of protons and neutrons). This fact results in two separate parabolas on a plot of binding energy, one 
parabola for even-even nuclei and one for odd-odd. Consequently, one occasionally finds a situation where two 
even-even nuclei for a given mass number A are stable against ordinary beta decay. However, the heavier nucleus 
is not fully stable and can decay to the lighter nucleus via normal double beta decay, a second-order process 
whereby the nuclear charge changes by two units. The ground state of the even-even nuclei is 0+ (positive parity) 
and the nuclear transition is 0+ +→0 . 
                 
One particular type of experimental approach that hopes to determine if the neutrino is a massive Majorana 
particle is the search for neutrinoless double beta decay. This type of experiment is perhaps the only feasible 
method for determining if the neutrino is a Majorana or Dirac particle. While neutrinoless double beta decay has 
not yet been experimentally discovered, searches have been conducted for many years, with many continuing 
today. In fact, the next generation of double beta decay experiments is currently being designed and developed 
and involves a tremendous increase in the amount of source material to be studied (on the order of a half-ton or 
more). In neutrinoless double beta decay an antineutrino emitted at the first vertex is absorbed at the second as 
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seen in the figure below or that a virtual neutrino emitted by a neutron is absorbed by the second neutron 
participating in the double beta decay.   
          
                                              
The two neutrino mode is allowed in standard model. The neutrinoless mode can occur only if neutrinos have 
masses of the Majorana type. The decay rate is proportional to the squared mass. In other words, the half life is 
inversely proportional to the squared mass. Experimentally one can distinguish the two modes. In the two 
neutrino mode the electrons take away only a fraction of the energy Q released in the decay. The sum energy 
spectrum is continuous, extending from 0 to Q. In the neutrinoless mode the total energy Q is carried away by the 
electrons, and the sum energy spectrum is a peak centered at Q, with a width given by the instrumental resolution. 
            
                      
allowed in the Standard Model of physics is given by  
                                                                                   (2.1) 
the exchange of the Majorana neutrino in the absence of right-handed 
                                                               (2.2) 
tively. The nuclear 
y in the 
The decay rate for 2ν ββ decay which is 
The decay rate for the process involving 
currents can be expressed as follows: 
2/1 ),()00(
MZEGT −=→
[ ] 2
2/1 ),()00( ><−=→
νννν mM
MZEGT F
The M  and MGT F are the nuclear matrix elements of Gamow-Teller and Fermi transitions respec
atrix elements of the 0+→0+ Gamow-Teller and Fermi transition for the two neutrino mode in weak theorm
second order perturbation is given by 
 - 3 -
                                   (2
 ∑ Δ+−= n in
∑∑ ++++++ iknnjf 0110 ττ
 - 4 -
nd is given by                           and the Gamow-Teller transition operator 
                                 respectively. 
 complete orthonormal set of intermediate excited states have been introduce  denot
eutrino double beta decay mode has been expressed in terms of single beta transitions through the introduction of 
termediate excited states via which the transition from the initial 0+ to the final 0+ state occurs.  
or the neutrinoless mode, the nuclear matrix elements resulting from Fermi and Gamow-Teller transitions are 
        (2.6) 
      
          (2.7) 
here, R  ro=1.2 fm. 
he par e lepton phase space, and gV and gA are the weak vector and 
xial-vector coupling constants respectively. The <mν> is the effective electron neutrino ma . If the light 
j« few MeV) exchange is the dominant mechanism at both 
ton number 
mj) and the mixing 
arameters (
                                                                                                                                       
               
est possible construction cost. The next sections review experiments 
3.   International Germa iment  (IGEX)  
 is a unique ground to investigate the nature and properties of the neutrino. The 
tern. To 
           (2.4) 
where Δ denotes the average energy a
∑ Δ+−= n in
∑∑ ++++++ ikknnjjf 0110 τστσ
and the Fermi transition operator is given by.
A d ed by    . Thus the two 
given by 
           (2.5) 
The function H depends on the distance between the nucleons and approximately has the form 
w = ro A
⅓,  A being the mass number and
T t G2ν and G0ν results from integrating over th
neutrino (m  for the 0νββ-decay process and th
the neutrino currents are left-handed, then the 0νββ-decay amplitude is proportional to the lep
iolating parameters. This effective mass is related to the light neutrino mass eigenvalues (v
p Uej) and is give by the relation 
              
                (2.8) 
The effective light neutrino mass <mν> may be suppressed by a destructive interference between the different 
contributions in the sum of equation (2.8) if CP is conserved. In this case the mixing matrix satisfies the condition 
Uej= Uej*.ζj, where ζj = ±i is the CP parity of the Majorana neutrino νj. The absolute value has thus been inserted 
for convenience, since the quantity inside it is squared in equation (2.8) and is complex if CP is violated. 
The ideal 0νββ-decay experiment has the following dream features: the lowest possible background, the best 
possible energy resolution, the greatest possible mass of the parent isotope, detection efficiency near 100% for 
valid events, a unique signature and the low
in such an effort from the isotope 76Ge. 
nium EXper
The nuclear Double Beta Decay
neutrinoless decay mode, if it exists, would provide an unambiguous evidence of the Majorana nature of the 
neutrino, its non-zero mass, and the non-conservation of lepton number. After implication from solar and 
atmospheric neutrino oscillation results that neutrinos have non-zero mass, the process of neutrinoless Double 
Beta Decay has become the most relevant place to test the neutrino mass scale and its hierarchy pat
achieve high sensitivity limits of the effective Majorana electron neutrino mass derived from the neutrinoless half-
life lower bound required for such new objectives, it will require a large number of double beta emitter nuclei, a 
ejj Umm
++++ ∑= ik
jjkfF ErHM 0),(0
0 ττν
++++ ∑= ikjjkfGT ErHM 0.),(00 ττν kj σσ
2)( fi EEΔ = −
∑ ∑ ++
jj ττσ  and
∫ +−+=
}2/)({
fi EEE
sin2 qrqR
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 of this type of search was the IGEX. The International Germanium 
Xperiment (IGEX) was a search for the neutrinoless double beta decay of 76Ge employing large amounts of 
In the first phase of the experiment three detectors of 0.7 
ents (the most 
 sites. It provided a rejection of ~ 60 % of the events in the region of 
                                            
                                   IGEX spectrum with and without the PSD background rejection. 
The IGEX detectors had the initial objective of the detection of the double beta decay of 76Ge. At the end of 1999 
certain modifications were made to adapt the detectors to the detection at low energy where the signal of WIMPs 
(Weak Interacting Massive Particles) is relevant. The shielding, shared by three IGEX detectors (2 kg germanium 
detectors isotopically enriched to 86% in 76Ge) and the COSME detector, included from inside to outside 40 cm 
of lead, a PVC box (silicone sealed and flushed with nitrogen), 2 mm of cadmium, plastic scintillators working in 
anticoincidence with the Ge detectors and 20 cm of polyethylene. The shielding was modified on July 2001 as it 
included only one 2 kg germanium detector inside a more efficient neutron shielding. These techniques of passive 
very low background and a sharp energy resolution in the Q-value region, and effective methods to disentangle 
signal from noise. A typical example
HPGe detectors, isotopically enriched to 86% in 76Ge.  
kg active volume each were operated: one in the Homestake gold mine (4000 m.w.e.), other in the Baksan 
Neutrino Observatory (660 m.w.e.) and the other in the Canfranc underground laboratory (Laboratory 2 at 1380 
m.w.e.). A conservative lower bound on the neutrinoless half-life of about 1024 years was derived.  
The International Germanium EXperiment (IGEX) took data at the Canfranc Underground Laboratory in Spain at 
a depth of 2450 m.w.e. in a search of neutrinoless double beta decay.  Three Germanium detectors (RG1, RG2 
and RG3), of ~2 kg each, enriched to 86% in 76Ge were used. Efforts were made to reduce part of the radioactive 
background by discriminating it from the expected signal by comparison of the shape of the pulses (PSD) of both 
types of events. The method was applied to the data recorded by two Ge detectors of the IGEX, which has 
produced one of the two best current sensitivity limits for the Majorana neutrino mass parameter. In the second 
phase, three large detectors (2 kg each) were fabricated (with improvements derived from the analysis of data of 
Phase 1). They are installed in the Canfranc underground laboratory (Laboratory 3 at 2450 m.w.e.) inside a low 
background shielding consisting of 40 cm of lead, a PVC box (silicone sealed and flushed with nitrogen), 2mm of 
cadmium, 20 cm of polyethylene and an active veto (plastic scintillators). A pulse shape discrimination (PSD) 
technique capable to distinguish single site events (ββ decay events for example) from multisite ev
dominant background events) is implemented. New limits on the neutrinoless half-life and the neutrino mass 
parameter were thus obtained from here. 
In large intrinsic Ge detectors, the charge carriers take 300 - 500 ns to reach their respective electrodes. These 
drift times are long enough for the current pulses to be recorded at a sufficient sampling rate. The current pulse 
contributions from electrons and holes are displacement currents, and therefore dependent on their instantaneous 
velocities and locations. Accordingly, events occurring at a single site (ββ-decay events for example) have 
associated current pulse characteristics which reflect the position in the crystal where the event occurred. More 
importantly, these single-site events (SSE) frequently have pulse shapes that differ significantly from those due to 
the background events that produce electron-hole pairs at several sites by multi-Compton-scattering process, for 
example (the so-called Multi-Site Events (MSE)). Consequently, pulse-shape analysis was used to distinguish 
between these two types of energy depositions since DBD events belong to the SSE class of events and will 
deposit energy at a single site in the detector while most of the background events belong to the MSE class of 
events and will deposit energy at several
interest, accepting the criterion that those events having more than two lobes cannot be due to DBD event. 
 - 6 -
and active shielding, along with the extreme radiopurity of the detectors and their components, allowed a low 
energy background as well as a low enough threshold which are unique in this type of detectors. So, very stringent 
contour limits for cross sections and masses of dark matter particles interacting with Ge nuclei through spin-
independent interactions were derived from here. The need to understand and reject backgrounds in Ge-diode 
detector double-beta decay experiments thus gave rise to the development of the pulse shape analysis technique in 
such detectors to distinguish DBD single-site energy deposits from the multiple-site deposits. Henceforth the 
analysis was extended by DBD people to segmented Ge detectors to study the effectiveness of combining 
segmentation with pulse shape analysis to identify the multiplicity of the energy deposits. 
The IGEX calculations for a lower bound to the half-life for the neutrinoless mode where there were fewer than 
3.1 candidate events (90% Confidence Level) under a peak having FWHM = 4 keV and centered at 2038.56 keV 
corresponded to:  
he requirements for a next generation experiment can easily be deduced by reference to                                 (3.1) 
             
where N is the number of parent nuclei, t is the counting time, and c is the upper limit on the number of 0νββ-
decay counts consistent with the observed background. To improve the sensitivity of ‹mν› by a factor of 100, the 
quantity Nt/c must be increased by a factor of 104. The quantity N can feasibly be increased by a factor of ~102 
over present experiments, so that t/c must also be improved by that amount. Since practical counting times can 
only be increased by a factor of 2 to 4, the background should be reduced by a factor of 25 to 50 below present 
levels.  These are approximately the target parameters of the next generation neutrinoless double-beta decay 
experiments.  
                                               
Histogram of the IGEX data in the energy region of interest for the 0ν -ββ decay. The limits on the half-life and 
neutrino mass parameter are also shown. 
The Effective ν Mass: The section of KKDK on effective neutrino mass (“Critical View to the IGEX neutrinoless 
ouble-beta decay experiment...” published in Phys. Rev. D, Volume 65 (2002) 092007, by H. V. Klapdor-
Kleingrothaus, A. Dietz, and I. V. Krivosheina) begins with: “Starting from their incorrectly determined half-life 
limit the authors claim a range of effective neutrino mass of (0.33-1.35) eV.” In response the IGEX collaboration, 
came out stating that KKDK selected only the 52.51 mole·years of the IGEX data that had been subjected to PSD 
and obtained T½0ν > 7.1×1024 y using the maximum number of counts, 3.1, from the entire 117 mole·years of data 
which was erroneous and unjustified. In another case, KKDK also decided to arbitrarily use the entire IGEX data 
set prior to PSD selection from which they obtained 0ν a bound of T½0ν > 1.1 × 1025 y for which there was no 
scientific justification for selecting only PSD corrected data on one hand and totally ignoring the PSD corrected 
data on the other hand. In the conclusion of KKDK it states: “the IGEX paper - apart from the too high half-life 
limits presented, as a consequence of an arithmetic error - is rather incomplete in its presentation”. In response to 
this paper the IGEX collaboration published the article “The IGEX experiment revisited: a response to the critique 
of Klapdor-Kleingrothaus, Dietz, and Krivosheina” where they stated that there was absolutely no arithmetic error 
yryrGe 25
76 1057.1
1087.4
>T 0 (ν2/1 1.3
).2(ln0
2/1 =
 - 7 -
and that the analysis of the published IGEX data presented in KKDK stands illegitimate. To obtain a much shorter 
bound on the half-life, they arbitrarily analyzed two ~ halves of the data separately. Instead of having 4.88×1025 y 
in the numerator (ln2 N.t) they used 2.2×1025 y. Yet they used the 90% CL upper limit on the number of counts 
under the peak, obtained by IGEX from all of the data. In another analysis, they ignored the fact that 52.51 
mole·years were corrected with PSD and treated the complete uncorrected data set. Naturally, the lower limits on 
T1/2oν (76Ge) obtained by these completely unjustified procedures are shorter than that obtained from properly 
analyzing the complete data set. This paper henceforth states “the lower limit quoted by IGEX, T1/20ν ≥ 1.57 × 1025 
years, is correct and that there was no arithmetical error as claimed in the Critical View article.” 
4.  The  HEIDELBERG - MOSCOW  Experiment  
The Heidelberg-Moscow experiment at the Gran Sasso underground laboratory is now claimed to be the most 
sensitive neutrinoless double beta decay experiment worldwide. It has contributed in an extraordinary way to the 
research in neutrino physics and particularly to beyond standard model physics, and limits for the latter are 
competing with those from the largest high-energy accelerators. The emphasis on the first indication for 
y is found in the Heidelberg-Moscow experiment ving first evidence of the lepton neutrinoless double beta deca gi
number violation and a Majorana nature of the neutrinos. The neutrinoless double beta decay could answer 
questions to the absolute scale of the neutrino mass and the fundamental character of the neutrino whether it is a 
Dirac or a Majorana particle. 
                                      
                                        Entrance of the highway tunnel under Gran Sasso mountain. 
With the support of the LNGS the experimental building of the experiment was built between Halls A and B in 
Gran Sasso, into which the first enriched 76 76Ge detector (the first high-purity enriched Ge detector worldwide) 
was installed in July 1990 . First preparation work had been done since 1989 in a provisional tent in Hall C. The 
ll amount of five enriched 76Ge detectors of in total 11 kg was finally installed in 1995 and were operated since 
n method  
lead (detectors ## 1,2,3,5). Each setup is coated with stainless steal casing. Non-
dioactive pure nitrogen was blown through casings to reduce radon emanation contribution. To reduce neutron 
background the casing with detectors ##1,2,3,5 was coated with borated polyethylene and two anticoincidence 
plates of plastic scintillator were located over the casing in order to reduce muon component. The setup was 
located in Gran Sasso underground laboratory, Italy at a depth of 3500 metres of water equivalent of the lab 
1996 with a newly developed pulse shape discriminatio
High purity germanium crystals, enriched by Germanium-76 isotope up to 86% are used as the main detecting 
elements. Five coaxial detectors with the total weight of 11.5 kg (125 moles in the active volume of detectors) are 
used. Each detector is located in a separate cryostat made of electrolytic copper with low content of radioactive 
impurities. The quantity of other designed materials (iron, bronze, light material insulators) is minimized in order 
to reduce the feasible radioactive impurities contribution to the total background of the detectors. The detectors 
were located in two separate shielded boxes. One of them, 270 mm thick is made of electrolytic copper (detector 
#4), the other consists of two layers of lead – inner -100mm of high purity LCD2-grade lead and outer – 200 mm 
of low  background Boliden 
reduces influence of cosmic rays on background conditions of the experiment. The electronics and the system of 
collecting data allow to record each event – the number (or numbers) of acted detector, amplitude and pulse shape, 
and anticoincidence veto. The Heidelberg-Moscow experiment, with five enriched 86%-88% high-purity p-type 
Germanium detectors, of in total 10.96 kg of active volume, used the largest source strength of all double beta 
experiments at present, and reached a record low level of background. The detectors were the first high-purity Ge 
detectors ever produced. The degree of enrichment was checked by investigation of tiny pieces of Ge after crystal 
production using the Heidelberg MP-Tandem accelerator as a mass spectrometer.  
The detectors, except detector # 4, were operated in a common Pb shielding of 30 cm, which consisted of an inner 
shielding of 10 cm radiopure LC2-grade Pb followed by 20 cm of Boliden lead. The whole setup was placed in an 
air-tight steel box and flushed with radiopure nitrogen in order to suppress the 222Rn contamination of the air. The 
shielding was improved in the course of the measurement. The steel box operated since 1994 centered inside a 10-
cm boron-loaded polyethylene shielding to decrease the neutron flux from outside. An active anticoincidence 
shielding was placed on top of the setup since 1995 to reduce the effect of muons. Detector # 4 was installed in a 
separate setup, which had an inner shielding of 27.5 cm electrolytical Cu, 20 cm lead, and boron-loaded 
below the steel box, but no muon shielding. The setup was kept air-tight closed since 
stallation of detector #5 in February’95. Since then no radioactive contaminations of the inner of the 
he sensitivity for the 0ν ββ half-life is given by                                                                   (4.1) 
ent are: energy resolution, background and 
e strength ever operated in a double beta decay 
xperiment. The background reached to the experiment, was 0.113 ± 0.007 events/kg y keV (in the period 1995-
was the lowest lim
polyethylene shielding 
experimental setup by air and dust from the tunnel could occur. 
- 8 -
With     denoting the degree of enrichment, ε the efficiency of the detector for detection of a double beta event, M 
the detector (source) mass, ∆E the energy resolution, B the background and t the measuring time, the sensitivity of 
our 11 kg of enriched 76Ge experiment corresponds to that of an at least 1.2 ton natural Ge experiment. After 
enrichment - the other most important parameters of a ββ experim
source strength. The high energy resolution of the Ge detectors of 0.2% or better, assures no background for a 
0νββ line from the two-neutrino double beta decay in this experiment (5.5 × 10-9 events expected in the energy 
range 2035-2039.1keV), in contrast to most other present experimental approaches, where limited energy 
resolution is a severe drawback. 
                                 
The efficiency of Ge detectors for detection of 0ν ββ decay events is close to 100%. The source strength in the 
Heidelberg-Moscow experiment of 11kg was the largest sourc
~0 〉〈×ν ε mand
. 02/1 Δ ννTBE
2003) in the 0ν ββ decay region (around Qββ). This it ever obtained in such type of experiment. 
 - 9 -
he statistics collected in this experiment during 13 years of stable running is the largest ever collected in a 
 presented a paper concerning “Measurement of the Bi spectrum in the energy 
gion around the Q-value of Ge neutrinoless double-beta decay”. In this work they presented the measurements 
f the 214Bi spectrum from a 226Ra source with a high purity germanium detector. Their attention was mostly 
focused on the energy region around the Q-value of 76Ge neutrinoless double-beta decay (2039.006 keV). The 
results of the measurement strongly relates to the first indication for neutrinoless double beta decay of 76Ge. An 
analysis of the data collected during ten years of measurements by the Heidelberg-Moscow experiment, at Gran-
Sasso Underground Laboratory, yields a first indication for the neutrinoless double beta decay of 76Ge. An 
important point of this analysis is the interpretation of the background, in the region around the Q-value of the 
double beta decay (2039.006 keV), as containing several weak photopeaks. It was suggested and has been shown 
that four of these peaks are produced by a contamination from the isotope 214Bi, whose lines are present 
throughout the Heidelberg-Moscow background spectrum.  
In this work they performed a measurement of a 226Ra source with a high-purity germanium detector. The aim of 
this work was to study the spectral shape of the lines in the energy region from 2000 to 2100keV and, most 
important, to show the difference in this spectral shape when changing the position of the source with respect to 
the detector, and to verify the effect of TCS (True Coincidence Summing) for the weak 214Bi lines seen in the 
Heidelberg-Moscow experiment. The activity of the 226Ra source is 95.2kBq. The isotope 226Ra appears in the 
238U natural decay chain and from its decays also 214Bi is produced. The γ-spectrum of 214Bi is clearly visible in 
the 226Ra measured spectrum. 214Bi is a naturally occurring isotope: it is produced in the 238U natural decay chain 
through the β- decay of 214Pb and the alpha decay of 218At. With a subsequent β- reaction, 214Bi decays then into 
214Po (the branching ratio with respect to the α decay into 210Tl is 99.979%). The decay, however, does not lead 
d state of 214Po, but to its excited states. From the decays of those excited states to the ground 
tate the well known γ-spectrum of 214Bi is obtained, which contains more than hundred lines. 
 the table given below, one can see in the energy region around the Q-value of the 0νββ decay (2000-2100keV), 
ur γ-lines and one E0 transition with energy 2016.7keV are expected. The E0 transition can produce a 
double beta decay experiment. The experiment took data during ~ 80% of its installation time. The Q value for 
neutrinoless double beta decay was recently determined with high precision. 
The background of the experiment: (1) primordial activities of the natural decay chains from 238 232U, Th, and 40K; 
(2) anthropogenic radio nuclides, like 137 134 125 207Cs, Cs, Sb, Bi; (3) cosmogenic isotopes, produced by activation 
due to cosmic rays during production and transport; (4) the bremsstrahlungs spectrum of 210Bi (daughter of 210Pb); 
(5) elastic and inelastic neutron scattering; and (6) direct muon-induced events. 
H.V. Klapdor-Kleingrothaus, O. Chkvorez, I.V. Krivosheina and C. Tomei at Max-Planck-Institut fur Kernphysik 
in the Heidelberg-Moscow group 214
directly to the groun
conversion electron or an electron-positron pair but it could not contribute directly to the γ-spectrum in the 
considered energy region if the source is located outside the detector active volume.  
               
0.0502010.71
Intensity(%)Energy (keV)
                                    
0.0782052.94
0.05020889.7
0.0202021.8
0.00582016.7
0.0502010.71
Intensity(%)Energy (keV)
0.0202021.8
0.00582016.7
0.0782052.94
0.05020889.7
The intensity of each line is defined as the number of emitted photons, with the corresponding energy, per 100 
decays of the parent nuclide. The considerations for the measurement were the efficiency of the detector (which 
depends on the size of the detector and on the distance source-detector) and the effect called True Coincidence 
Summing (TCS). The lifetimes of the atomic excited levels are much shorter than the resolving time of the 
detector. If two gamma-rays are emitted in cascade, there is a certain probability that they will be detected 
 - 10 -
lled in 
. The measurement of Bi 
pectrum, with a high purity germanium detector, in the energy region around the Q-value of 76Ge neutrinoless 
ta decay (2039.006keV) was done with the 226Ra source used for the measurements positioned, in a first 
step the source was positioned on the top of the detector, directly in contact with the copper cap (close geometry) 
and in a second step the source was moved 15cm away from the detector cap (far geometry). The results of the 
measurements show that, if the source is close to the detector, the intensities of the weak Bi lines in the energy 
region 2000- 2100keV are not in the same ratio as reported by Table of Isotopes.  The results of the analysis of the 
data collected by the Heidelberg-Moscow experiment with all the five detectors, yielding a first indication for the 
neutrinoless double beta decay of 76Ge, shows that four 214Bi lines are present in the energy region from 2000 to 
2080keV (many other strong lines from the same isotope are present in the spectrum), due to the presence of 
bismuth in the experimental setup, especially in the copper in the vicinity of the Ge crystals. 
                        
together. If this happens, then a pulse will be recorded which represents the sum of the energies of the two 
individual photons, instead of two separated pulses with different energies. The TCS effect can result both in 
lower peak-intensity for full-energy peaks and in bigger peak-intensity for those transitions whose energy can be 
given by the sum of two lower-energy gamma-rays. In this case, the lines at 2010.7 keV and 2016.7 keV can be 
given by the coincidence of the 609.312 keV photon (strongest line, intensity = 46.1%) with the 1401.50keV 
photon (intensity = 1.27%) or with the 1407.98keV photon (intensity = 2.15%). The degree of TCS depends on 
the probability that two gamma-rays emitted simultaneously will be detected simultaneously which is a function 
of the detector geometry and of the solid angle subtended at the detector by the source and for this the intensities 
of the two lines mentioned above (2010.71keV and 2016.7keV) are expected to depend on the position of the 
source with respect to the detector. 
The 226Ra γ-ray spectra were measured using a γ-ray spectroscopy system based on an HPGe detector insta
the operation room of the HEIDELBERG-MOSCOW experiment in Gran Sasso Underground Laboratory, Italy. 
The coaxial germanium detector had an external diameter of 5.2cm and 4.9cm height. The distance between the 
top of the detector and the copper cap was kept at 3.5cm. The relative detection efficiency of the detector was 
23% and the energy resolution being 3.6keV for the energy range 2000-2100keV 214
double-be
                 
The  above  figure  shows  the  sum  spectrum  of  the  76Ge  detectors 1,2,3,4  and  5  over  the  period  August  
1990  to  May  2003 as recorded by the Heidelberg-Moscow experiment. 
 - 11 -
 There is no null hypothesis analysis demonstrating that the data require a peak. Furthermore, no simulation has 
to demonstrate that the analysis correctly finds true peaks or that it would find no peaks if none 
existed. Monte Carlo simulations of spectra containing different numbers of peaks are needed to confirm the 
significance of any found peaks. 
2. There are three unidentified peaks in the region of analysis that have greater significance than the 2039-keV 
peak. There is no discussion of the origin of these peaks. 
3. There is no discussion of how sensitive the conclusions are to different mathematical models. There is a 
previous Heidelberg-Moscow publication that gives a lower limit of 1.9 × 1025 y (90% confidence level). This is 
in conflict with the “best value” of a newer KDHK paper of 1.5 × 1025 y. This indicates a dependence of the 
results on the analysis model and the background evaluation. 
In this paper they state that a number of other cross checks of the result should also be performed. For example, 
there is no discussion of how a variation of the size of the chosen analysis window affects the significance of the 
hypothetical peak. There is no relative peak strength analysis of all the 214Bi peaks. Quantitative evaluations 
should be made on the four 214Bi peaks in the region of interest. There is no statement of the net count rate of the 
peaks other than the 2039-keV peak. There being no presentation of the entire spectrum, is difficult to compare 
relative strengths of peaks. There is no discussion of the relative peak strengths before and after the single-site-
event cut.  
On the other hand the Heidelberg-Moscow group claims that the signal found at Qββ is consisting of single site 
events and is not a γ line. The signal does not occur in the Ge experiments not enriched in the double beta emitter 
76Ge, while neighbouring background lines appear consistently in these experiments. On this basis they translated 
the observed numbers of events into half-lives for neutrinoless double beta decay.  
The Heidelberg-Moscow experiment continued regularly from 1990 till 2003. The analysis of the full data taken 
with the Heidelberg-Moscow experiment in the period 2 August 1990 until 20 May 2003 is presented. The 
completed  Heidelberg-Moscow  76Ge  Experiment -71.7 kg y  after 13 years of operation presents their mass 
calculation limit status as  mν (eV) = 0.24 - 0.58  ( 99.997%  C.L.) with the best value of 0.4 eV (95% C.L.). 
hile an unambiguous interpretation of all of the neutrino oscillation experiments is not yet possible, it is 
bundantly clear that neutrinos exhibit properties not included in the standard model, namely mass and flavor 
arch which will employ 500 kg of Ge, 
a. The Majorana experiment is proposed for a US deep underground laboratory, 
eriments. Furthermore, new segmented Ge detector 
yogenic performance and background reduction and 
Moscow and IGEX experiments both utilized Germanium enriched to 86% in Ge and operated deep 
In a paper by Klapdor-Kleingrothaus, Dietz, Harney, and Krivosheina (hereafter referred to as KDHK) evidence is 
claimed for zero-neutrino double-beta decay in 76 Ge. The high quality data, upon which this claim is based, was 
compiled by the 2 careful efforts of the Heidelberg-Moscow collaboration, and is well documented. However, the 
analysis in KDHK makes an extraordinary claim, and therefore requires very solid substantiation according to 
another paper “Comment on Evidence for Neutrinoless Double Beta Decay” C.E.Aalseth et al. They state that a 
large number of issues were not addressed in KDHK some of which are: 
been presented 
5.  The proposed MAJORANA experiment 
mixing. Accordingly, sensitive searches for neutrinoless double-beta decay (0νββ-decay) are more important than 
ever. Experiments with large quantities of Ge, isotopically enriched in 76Ge, have thus far proven to be the most 
sensitive, specifically the Heidelberg-Moscow and IGEX experiments with lower limits in half-life sensitivities 
1.9×1025 y and 1.6×1025 y respectively. A new generation of experiments will be required to make significant 
improvements in sensitivity one of which is the proposed Majorana Experiment.  
The Majorana Experiment is a next-generation Ge double-beta decay se
isotopically enriched to 86% in Ge, in the form of ~200 detectors in a close-packed array for high granularity. 
Each crystal will be electronically segmented, with each region fitted with pulse-shape analysis electronics. A 
half-life sensitivity is predicted of 4.2 × 1027 years or < mν> ~ 0.02 - 0.07 eV, depending on the nuclear matrix 
elements used to interpret the dat
and requires very little R&D as it stands on the technical shoulders of the IGEX experiment and other previous 
successful double-beta decay and low-background exp
technology has recently become commercially available, while Pacific Northwest National Laboratory 
(PNNL)/University of South Carolina (USC) researchers have developed new pulse-shape discrimination 
techniques. 
Several configurations have been evaluated with respect to cr
rejection. It will concentrate on a conventional modular design using ultra-low background cryostat technology 
developed by IGEX. It will also utilize new pulse-shape discrimination hardware and software techniques 
developed by the Majorana collaboration and detector segmentation to reduce background. The Heidelberg-
underground. The projection for the Majorana is that the background will be reduced by a factor of 65 over the 
early IGEX results prior to pulse shape analysis (from 0.2 to ~0.003 keV-1
 - 12 -
germanium by limiting the time above ground after crystal growth, careful material selection 
marily comprised of multiple 
o or 
ore of the independent segments. When coincidences are found, the output from all detector segments is 
nly of the full-energy peak lying above a featureless 
 kg-1 y-1). This will occur mainly by the 
decay of the internal background due to cosmogenic neutron spallation reactions that produce 56 58 60Co, Co, Co, 
65Zn and 68Ge in the 
and electroforming copper cryostats. One component of the background reduction will arise from the 
segmentation and granularity of the detector array.  
Most of the Compton continuum consists of single Compton scatterings followed by escape of the scattered 
gamma ray, whereas full-energy events at typical gamma-ray energies are pri
scattering sequences followed by a photoelectric absorption. The peak-to-Compton ratio can therefore be 
enhanced by requiring a recorded event to correspond to more than one interaction within the detector before its 
acceptance. In germanium detectors, this selection is usually accomplished by subdividing the detector into 
several segments (or providing several adjacent independent detectors) and seeking coincident pulses from tw
summed and recorded. The resulting spectrum is made up o
continuum that is greatly suppressed and has no abrupt Compton edges. New Ge experiments must not simply be 
a volume expansion of IGEX or Heidelberg–Moscow. They must have superior background rejection and better 
electronic stability. The summing of 200 individual energy spectra can result in serious loss of energy resolution 
for the overall experiment which can be avoided by segmenting n-type intrinsic Ge detectors, advanced PSD 
techniques and electronic stability in measurement.  
                                         
The above figure depicts a standard Ge detector segmentation scheme. This is the configuration of the SEGA 
detector undergoing tests by the Majorana collaboration. A configuration with six-azimuthal-segment by two-
axial-segment geometry is shown in the above figure. 
Efforts are thus on with the Majorana experiment for the search of neutrinoless double beta decay that would give 
a new shape to the standard model of physics. Majorana cannot not simply be a volume expansion of IGEX, but 
must have superior background rejection. As it was conclusively shown that the limiting background in at least 
some previous experiments has been cosmogenic activation of the germanium itself, it is necessary to mitigate 
those background sources. Cosmogenic activity fortunately has certain factors which discriminate it from the 
signal of interest. For example, while 0νββ -decay would deposit 2 MeV between two electrons in a small, 
perhaps 1 mm3 volume, internal 60Co decay deposits about 318 keV (endpoint) in beta energy near the decaying 
atom, while simultaneous 1173 keV and 1332 keV gammas can deposit energy elsewhere in the crystal, most 
probably both in more than one location, for a total energy capable of reaching the 2039 keV region-of-interest. A 
similar situation exists for internal 68Ge decay. Thus deposition-location multiplicity distinguishes double-beta 
decay from the important long lived cosmogenics in germanium. Isotopes such as 56 57 58Co, Co, Co and 68Ge are 
produced at a rate of roughly 1 atom per day per kilogram on the earth’s surface. Only 60Co and 68Ge have both 
the energy and half-life to be of concern. To pursue the multiplicity parameter, firstly, the detector current pulse 
shape carries with it the record of energy deposition along the electric field lines in the crystal; that is, the radial 
 - 13 -
imension of cylindrical detectors. This information may be exploited through pulse-shape discrimination. 
econdly, the electrical contacts of the detector may be divided to produce independent regions of charge 
collection. The ability of new techniques to be easily calibrated for individual detectors makes them practical for 
large detector arrays. Calibration for single-site event pulses was trivially accomplished by collecting pulses from 
thorium ore; the 2614.47-keV gamma ray from 208Tl produces a largely single-site double-escape peak at 1592.47 
keV. The PSD discriminator was then calibrated to the properties of the double-escape peak A slightly improved 
double-escape peak was be made from the 26Al gamma ray of 2938.22-keV. The double-escape appears at 
1916.22 keV, only about 120 keV away from the expected region of interest for 0νββ-decay. The obvious and 
direct use of pulse- shape discrimination and segmentation is the rejection of cosmogenic pulses in the germanium 
itself. However, the approach should be also effective on gamma rays from the shielding and structural materials. 
The background effects of neutrons of both high energy (cosmic muon generated) and low energy (fission and 
(α,n) from rock) could be protected by the segmentation and granularity of the detectors. These neutrons could 
also produce other unwanted activities like the formation of 3H and 14C in nitrogen from high and low energy 
neutrons, respectively. Fortunately, Majorana detectors will not be surrounded by nitrogen at high density. 
                              
               
The GERDA (GERmanium Detector Assembly), which is another next generation 76Ge double beta decay 
experiment at the Gran Sasso Underground Laboratory, has projected a sensitivity in the half-life of the 0νββ-
decay mode which is less than the proposed Majorana experiment. In conclusion, the Majorana project has been 
designed in a compact, modular way such that it can be built and operated with high confidence in the approach 
and the technology. The initial years of construction will allow alternate cooling methods to be employed if they 
have an advantage and should they be shown to overcome long-term concerns due to surface contamination, 
muon-induced ions, and diffusion. The Majorana Collaboration has made an extensive analysis of the predicted 
backgrounds and their impact on the final sensitivity of the experiment. The Majorana experiment represents a 
great increase in Ge mass over IGEX with new segmented Ge detectors and the newest electronic systems for 
pulse-shape discrimination. Their conclusion is that with 500 kg of Ge, enriched to 86% in the isotope 76Ge, the 
Majorana array operating over 10 years including construction time, can reach a lower limit on T1/20ν of 4×1027 
years. This corresponds to an upper bound of < m  > of 0.038 ± 0.007eV.  One advantage of 76ν Ge is that it may 
well be a candidate for a future more reliable microscopic calculation of the 0ν ββ- decay nuclear matrix element.  
6   Conclusion 
eutrinoless double beta decay is thus one of the most sensitive approaches with great perspectives to test pN article 
hysics beyond the Standard Model. The possibilities to use 0νββ decay for constraining neutrino masses, left–
ght symmetric models, SUSY and leptoquark scenarios, as well as effective lepton number violating couplings, 
have been reviewed. It is a very sensitive probe to the lepton number violating terms in the Lagrangian such as the 
Majorana mass of the light neutrinos, right-handed weak couplings involving heavy Majorana neutrinos, as well 
as Higgs and other interactions involving violation of chirality conservation. 
 - 14 -
In search for neutrinoless double beta decay 76Ge as the source material has multiple advantages. It has high 
resolution (< 4 keV at Qββ) with no background from 2ν mode. A huge leap in sensitivity is possible applying 
ultra-low background techniques and 0ν- ββ signal discrimination. There can be a phased approach in the 
experiment with the increment of target mass. The source and detector are the same material thereby reducing 
background and maintaining the 4π geometry and the only way to scrutinize 0ν – DBD claim on short time scale: 
since it tests T1/2 and not mν. The consequences of Neutrinoless Double Beta Decay are- [1] Total Lepton number 
violation: The most important consequence of the observation of neutrinoless double beta decay is that lepton 
number is not conserved. This is fundamental for particle physics. [2] Majorana nature of neutrino: Another 
fundamental consequence is that the neutrino is a Majorana particle. Both of these conclusions are independent of 
any discussion of nuclear matrix elements. [3] Effective neutrino mass: The matrix element enters when we derive 
a value for the effective neutrino mass - making the most natural assumption that the 0νββ decay amplitude is 
dominated by exchange of a massive Majorana neutrino.  
Acknowledgements 
I would like to thank the IGEX collaboration, the Heidelberg-Moscow collaboration and the Majorana 
n for having used information from th imental works to write up this bri  review. 
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