93891215
Gamma-ray spectra of methane in the positron–electron annihilationprocess
Abstract Bound electron contribution to the Doppler-shift of gamma-ray spectra in the positron–electron annihilation process of molecular methane has been studied in gas phase. Two accurate ab initio quantum mechanical schemes, i.e. the delocalised molecular orbital (MO) and the localised natural bond orbital (NBO) schemes, are applied to study the multi-centred methane molecule. The present ab initio calculations of methane indicate that the C–H bonds are polarised with the partial negative charge of −0.36 a.u on the carbon atom and the partial positive charge of +0.09 a.u. on each of the hydrogen atoms. The positively charged hydrogen atoms produce repulsive Coulomb potentials to a positron. Both the MO and NBO schemes further reveal that the 2a 1 electrons of methane, that is, the 2a 1 electron component of the CH bonds rather than the whole CH bonds of methane, predominates the positron–electron annihilation gamma-ray spectra of the molecule. Electrons of a molecule which are dominant the positron–electron annihilation processes are called “positrophilic” electrons in the present study. It is further shown that the negative electrostatic potential (ESP) of methane facilitates with the density of the “positrophilic” 2a 1 electrons of methane. Other valence electrons (e.g. 1t 2 ) in the CH bonds play a minor “spectator” role in the annihilation process of methane.
119293060
Gamma-ray spectra of hexane in gas phase and liquid phase
Theoretical gamma-ray spectra of molecule hexane have been calculated and compared with the experimental results in both gas (Surko et al, 1997) and liquid (Kerr et al, 1965) phases. The present study reveals that in gas phase not all valence electrons of hexane exhibit the same probability to annihilate a positron. Only the positrophilic electrons in the valence space dominate the gamma-ray spectra, which are in good agreement with the gas phase measurement. When hexane is confined in liquid phase, however, the intermolecular interactions ultimately eliminate the free molecular orientation and selectivity for the positrophilic electrons in the gas phase. As a result, the gamma-ray spectra of hexane become an averaged contribution from all valence electrons, which is again in agreement with liquid phase measurement. The roles of the positrophilic electrons in annihilation process for gas and liquid phases of hexane have been recognized for the first time in the present study.
111221941
ASTROD and ASTROD I: Progress Report
Over the next decade the gravitational physics community will benefit from dramatic improvements in many technologies critical to the tests of gravity and gravitational-wave detection. The highly accurate deep space navigation, interplanetary laser ranging and communication, interferometry and metrology, high precision frequency standards, precise pointing and attitude control, together with the drag-free technologies will revolutionize the field of the experimental gravitational physics. Deep-space laser ranging will be ideal for gravitational-wave detection, and testing relativity and measuring solar-system parameter to an unprecedented accuracy. ASTROD I is such a mission with single spacecraft; it is the first step of ASTROD (Astrodynamical Space Test of Relativity using Optical Devices) with 3 spacecraft. In this paper, we will present the progress of ASTROD and ASTROD I with emphases on the acceleration noises, mission requirement, charging simulation, drag-free control and low-frequency gravitational-wave sensitivity.
206352395
The attachment of amino fragment to purine: inner-shell structures and spectra.
The impact of the amino fragment (-NH(2)) attachment on the inner-shell structures and spectra of unsubstituted purine and the purine ring of adenine are studied. Density functional theory calculations, using the LB94/TZ2P//B3LYP/TZVP model, reveal significant site-dependent electronic structural changes in the inner shell of the species. A condensed Fukui function indicates that all of the N and C sites, except for N((1)) and C((5)), demonstrate significant electrophilic reactivity (f(-) > 0.5 in |e|) in the unsubstituted purine. Once the amino fragment binds to the C((6)) position of purine to form adenine, the electrophilic reactivity of these N and C sites is greatly reduced. As expected, the C((6)) position experiences substantial changes in energy and charge transfer, owing to the formation of the C-NH(2) bond in adenine. The present study reveals that the N1s spectra of adenine inherit the N1s spectra of the unsubstituted purine, whereas the C1s spectra experience significant changes although purine and adenine have geometrically similar carbon frames. The findings also indicate that the attachment of the NH(2) fragment to purine exhibits deeply rooted influences to the inner-shell structures of DNA/RNA bases. The present study suggests that some fragment-based methods may not be applicable to spectral analyses in the inner shell.
109204818
Performance Limiting Effects in X‐Band Accelerators
Acceleration gradient is a critical parameter for the design of future TeV‐scale linear colliders. The major obstacle to higher gradient in room‐temperature accelerators is rf breakdown, which is still a very mysterious phenomenon that depends on the geometry and material of the accelerator as well as the input power and operating frequency. Pulsed heating has been associated with breakdown for many years however there have been no experiments that clearly separate field and heating effects on the breakdown rate. Recently, such experiments have been performed at SLAC with both standing‐wave and travelling‐wave structures. These experiments have demonstrated that pulsed heating is limiting the gradient. Also, a dual‐moded cavity has been designed to better distinguish the electric field, magnetic field and pulsed heating effects on breakdown.
14538301
Measuring dark energy with gamma-ray bursts and other cosmological probes
It has been widely shown that cosmological parameters and dark energy can be constrained by using data from Type Ia supernovae (SNe Ia), the cosmic microwave background (CMB) anisotropy, the baryon acoustic oscillation (BAO) peak from Sloan Digital Sky Survey (SDSS), the X-ray gas mass fraction in clusters, and the linear growth rate of perturbations at z = 0.15 as obtained from the 2dF Galaxy Redshift Survey. Recently, gamma-ray bursts (GRBs) have also been argued to be promising standard candles for cosmography. In this paper, we present constraints on the cosmological parameters and dark energy by combining a recent GRB sample including 69 events with the other cosmological probes. First, we find that for the ΛCDM cosmology this combination makes the constraints stringent, and the best fit is close to the flat universe. Second, we fit the flat Cardassian expansion model and find that this model is consistent with the ΛCDM cosmology. Third, we present constraints on several two-parameter dark energy models and find that these models are also consistent with the ΛCDM cosmology. Finally, we reconstruct the dark energy equation of state parameter w(z) and the deceleration parameter q(z). We see that the acceleration could have started at a redshift from zt = 0.40 to 0.65. This difference in the transition redshift is due to different dark energy models that we adopt. The most stringent constraint on w(z) lies in the redshift range z ~ 0.3-0.6.
124800347
U(1/1)q q-Coherent States and Path Integrals for the q-Deformed Jaynes-Cummings Model*
The q-coherent states for the quantum superalgebra U(1/1)q, are introduced. Path integrals in the q-coherent state representations of the U(1/1)q, are constructed and applied to the q-deformed Jaynes-Cummings model whose dynamical superalgebra is the quantum super-algebra U(1/1)q.
18159661
Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae
The property of dark energy and the physical reason for the acceleration of the present universe are two of the most difficult problems in modern cosmology. The dark energy contributes about two-thirds of the critical density of the present universe from the observations of type-Ia supernovae (SNe Ia) and anisotropy of cosmic microwave background (CMB). The SN Ia observations also suggest that the universe expanded from a deceleration to an acceleration phase at some redshift, implying the existence of a nearly uniform component of dark energy with negative pressure. We use the ``Gold'' sample containing 157 SNe Ia and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the properties of dark energy and the transition redshift. For a flat universe with the cosmological constant, we measure Ω M = 0.28 −0.05 +0.04 , which is consistent with Riess et al. The transition redshift is z T = 0.60 −0.08 +0.06 . We also discuss several dark energy models that define w ( z ) of the parameterized equation of state of dark energy including one parameter and two parameters ( w ( z ) being the ratio of the pressure to energy density). Our calculations show that the accurately calculated transition redshift varies from z T = 0.29 −0.06 +0.07 to z T = 0.60 −0.08 +0.06 across these models. We also calculate the minimum redshift z c at which the current observations need the universe to accelerate.
185692670
Study on the High Order Modes of the 3.5cell Cavity at Peking University
As part of the updated DC-SC injector, a 3.5cell cavity has been fabricated at Peking University, which includes two Coaxial High Order Mode (HOM) couplers. The effect of the HOM couplers has been studied by numerical simulation and measurement. The results are highly uniform and show that the two couplers do effectively damp the HOMs. INTRODUCTION The 3.5cell cavity is part of the DC-SC injector, the feasibility of which is proved in 2004 [1] with beam done test. The updated 3.5cell cavity was designed and fabricated from large grain niobium, and the vertical test result (Eacc=23.4MV/m@Q0>10) at JLAB is exciting. The main RF parameters are listed in Table1. Table 1: RF Parameters of 3.5 Cell Cavity Type of accelerating structure Standing wave Accelerating mode TM010, pi mode fundamental frequency 1300MHz Quality factor ≥5x10 Active length 41.7cm Cell-to-cell coupling 1.8% Effective shunt impedance 418Ω Transit-time factor 0.74 Total longitudinal loss factor* 3.95(V/pC) transverse loss factor* 2.15(V/pC/cm) *the standard deviation length of the bunch is 2mm Among those RF parameters, the total longitudinal loss factor and transverse loss factor are from ABCI simulation; all the others are from Superfish. NUMERICAL CALCULATIONS OF HOMS The electron bunches passing through the cavity excite the eigenmodes of higher frequency which are HOMs. Here Microwave Studio is used to simulation the HOMs. To check whether there are trapped modes, two different boundary conditions at the end of beam pipes are used in two separate runs [2]. Large frequency shift of one mode indicates the mode depends on the boundary condition sensitivity; otherwise the mode could be trapped. The simulation shows TM110(2) and TM020(1)could be trapped. But they will not be excited by the beam as the beam bunch repetition frequency is 81.25MHz. All of the interested modes with ideal electric boundary condition below 3GHz are listed in Table 2. Table 2: HOMs of 3.5 Cell Cavity by Simulation Mode Mode f(MHz) *R/Q Mode1 TM010(1) 1278.69 9.75 Mode2 TM010(2) 1286.80 13.09 Mode3 TM010(3) 1296.83 16.63 Mode4 TM010(4) 1301.07 417.26 Mode5 TE111(1) 1634.35 0.07 Mode6 TE111(2) 1681.53 2.49 Mode7 TE111(3) 1747.03 10.07 Mode8 TM110(1) 1798.05 3.43 Mode9 TM110(2) 1849.61 3.47 Mode10 TM110(3) 1872.20 1.42 Mode11 TM110(4) 1883.84 0.77 Mode12 TM011(1) 2391.49 0.62 Mode13 TE111 (4) 2391.79 0.18 Mode14 TM011(2) 2421.67 12.65 Mode15 TM011(3) 2449.83 84.33 Mode16 TM020(1) 2658.30 23.97 Mode17 TM020(2) 2690.74 8.93 Mode18 TM020(3) 2728.82 4.26 Mode19 TM020(4) 2762.00 2.08 Mode20 TM111(1) 2819.80 0.71 Mode21 TM111(2) 2880.96 0.85 *unit of R/Q isΩ/cm; for monopole mode, n=0; dipole mode,n=2. The number next to the mode indicates the passband of the mode. For some modes, the electric field along the beam centre axis is almost zero, so the voltage is zero. Here we integrate the electric field along the axis 10 mm away from the beam centre and get the accelerating THPD009 Proceedings of IPAC’10, Kyoto, Japan 4296 03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques A08 Linear Accelerators voltage Vc , and the effective shunt impedance is ( ) ( ) U V Q R C ω / * / 2 * 0 = , where ω is the angular frequency of the mode, U is the stored energy. The effects of HOM couplers are simulated on a 2cell cavity also by CST Microwave Studio, which deduces the simulation time obviously [3]. The simulation shows Qe of HOM coupler on 2-cell TESLA cavity is 10 for fundamental mode, and 10~10 for HOMs. MEASUREMENTS OF THE RESONATE MODES In order to confirm the type of the modes, the electric field profile of the modes is measured by bead pulling. Two beads are used, one is a teflon cylinder with diameter of 4 mm and length of 4mm, the other is a copper cylinder with diameter of 5mm and length of 4mm. By comparing the measured electric fields with the simulated ones, 16 of all the 21 interested modes are obtained. The block diagram of the test bench is shown is Figure 1. Figure 1: Block diagram of the test bench. RF MEASUREMENTS OF THE HOM DAMPING ON THE CAVITY Before the HOM couplers are welded to the cavity, they are tested on a self-designed coaxial transmission line [4], to ensure the tuning range of the resonate frequency. The antenna of the HOM coupler is match to the load and with a round tip to increase the coupling as shown in Figure 2. Figure 2: The shape of the antenna. The antenna is made of copper in a series of length. With these antennas amounted, the gap from the antenna to the HOM coupler is 0.5mm, 1mm, 1.5mm, 2mm, 2.3mm. During this change, the Qext does not change much. Finally 1mm is chosen and the result is shown in Table 3. Table 3: Qe of the HOMs by Measurements mode f (MHz) Qe TE111(1) 1652.406 9.55x10 TE111(2) 1693.484 3.82x10 TE111(3) 1757.300 peak overlaps TM110(1) 1797.265 1.66x10 6 TM110(2) 1864.493 1.89x10 7 TM110(3) 1876.990 1.06x10 7 TM110(4) 1882.496 3.45x10 6 TE111(4) 2374.582 4.69x10 6 TM011(1) 2412.774 7.46x10 5 TM011(2) 2429.367 2.27x10 5 TM011(3) 2462.537 7.26x10 5 TM020(1) 2678.580 1.57x10 5 TM020(2) 2703.662 1.60x10 4 TM020(3) 2732.066 1.50x10 4 TM020(4) 2769.828 8.93x10 4 During the measurements, one HOM coupler is used because the available vector network analyzer has only two ports. For TE111(3) pass band, the two polarization modes are too close to measure directly by the vector network analyzer. Table 3 only lays out one polarization mode of each passband, the Qe of the other one is much bigger. Obviously one coupler is not enough for all the modes. CONCLUSION The test results of the coaxial HOM coupler on the 3.5cell large grain niobium cavity at room temperature indicate that the coupler works efficiently after a careful adjustment. The effects at 2K with beam loading are looking forward. REFERENCES [1] J. Hao et al., NIM A 557 (2006) 138-141. [2] F. Marhauser, P. Huelsmann, H. Klein, Proceedings of the 1999 Particle Accelerator Conference, New York, 1999. 3405-3407. [3] X. Luo, CPC (HEP & NP), to be published. [4] E. Wang, CPC (HEP & NP), 2009, 33(5): 401-405 Proceedings of IPAC’10, Kyoto, Japan THPD009 03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques A08 Linear Accelerators 4297