Thermal Capture Cross Section Research Articles

Positive or negative gain: Role of thermal capture cross sections in impurity photovoltaic effect

In this work, we carried out a numerical study on the role of thermal capture cross sections in impurity photovoltaic (IPV) effect for silicon solar cells doped with indium. The short-circuit current density, the open-circuit voltage and the conversion efficiency of the IPV cell were calculated in dependence of variable electron and hole thermal capture cross sections of indium in silicon. We found that the thermal capture cross section of electron is crucial to the device performance and that of hole has few influence on the cell property for this IPV cell since acceptor-type indium impurity level is near the valence band edge. If the electron thermal capture cross section is less than 10−20 cm2, a positive gain of conversion efficiency for the IPV cell would be presented. We concluded that those impurities with small electron (or hole) thermal capture cross sections may be suitable for use in the IPV cell with acceptor-type (or donor-type) impurity level near the valence (or conduction) band edge. These results may help to evaluate the potential of the IPV effect for improving cell efficiency according to the thermal capture cross sections of the impurity in host semiconductor.

Capture Gamma-ray Libraries for Nuclear Applications

Author(s): Sleaford, B.W. | Abstract: The neutron capture reaction is useful in identifying and analyzing the gamma-ray spectrum from an unknown assembly as it gives unambiguous information on its composition. This can be done passively or actively where an external neutron source is used to probe an unknown assembly. There are known capture gamma-ray data gaps in the ENDF libraries used by transport codes for various nuclear applications. The Evaluated Gamma-ray Activation file (EGAF) is a new thermal neutron capture database of discrete line spectra and cross sections for over 260 isotopes that was developed as part of an IAEA Coordinated Research Project. EGAF has been used to improve the capture gamma production in ENDF libraries. For medium to heavy nuclei the quasi continuum contribution to the gamma cascades is not experimentally resolved. The continuum contains up to 90percent of all the decay energy an is modeled here with the statistical nuclear structure code DICEBOX. This code also provides a consistency check of the level scheme nuclear structure evaluation. The calculated continuum is of sufficient accuracy to include in the ENDF libraries. This analysis also determines new total thermal capture cross sections and provides an improved RIPL database. For higher energy neutron capture there is less experimental data available making benchmarking of the modeling codes more difficult. We use CASINO, a version of DICEBOX that is modified for this purpose. This can be used to simulate the neutron capture at incident neutron energies up to 20 MeV to improve the gamma-ray spectrum in neutron data libraries used for transport modelling of unknown assemblies.

Publisher’s Note: Heavy-baryon chiral perturbation theory approach to thermal neutron capture onHe3[Phys. Rev. C83, 034006 (2011)

The cross section for radiative thermal neutron capture on ${}^{3}He$ ($\He3 +n \to \He4 +\gamma$; known as the $hen$ reaction) is calculated based on heavy-baryon chiral perturbation theory. The relevant M1 operators are derived up to next-to-next-to-next-to-leading order (N${}^3$LO). The initial and final nuclear wave functions are obtained from the rigorous Faddeev-Yakubovski equations for five sets of realistic nuclear interactions. Up to N${}^3$LO, the M1 operators contain two low-energy constants, which appear as the coefficients of non-derivative two-nucleon contact terms. After determining these two constants using the experimental values of the magnetic moments of the triton and ${}^3 He$, we carry out a parameter-free calculation of the $hen$ cross section. The results are in good agreement with the data.

Heavy-baryon chiral perturbation theory approach to thermal neutron capture onHe3

The cross section for radiative thermal neutron capture on ${}^{3}\text{He}$ (${}^{3}\text{He}+n\ensuremath{\rightarrow}{}^{4}\text{He}+\ensuremath{\gamma}$; known as the $\mathit{hen}$ reaction) is calculated based on heavy-baryon chiral perturbation theory. The relevant M1 operators are derived up to next-to-next-to-next-to-leading order (N$^{3}\mathrm{L}$O). The initial and final nuclear wave functions are obtained from the rigorous Faddeev-Yakubovski equations for five sets of realistic nuclear interactions. Up to $\mathrm{N}{}^{3}$LO, the M1 operators contain two low-energy constants, which appear as the coefficients of nonderivative two-nucleon contact terms. After determining these two constants using the experimental values of the magnetic moments of the triton and ${}^{3}\text{He}$, we carry out a parameter-free calculation of the $\mathit{hen}$ cross section. The results are in good agreement with the data.

Evaluation of Neutron Nuclear Data on Bromine and Krypton Isotopes for JENDL-4.0

Neutron nuclear data on 79,81Br and 78,80,82,83,84,85,86Kr have been evaluated for the evaluated nuclear data library JENDL-4.0 in the energy region from 10−5 eV to 20 MeV. The resolved resonance parameters were adjusted so as to reproduce recommended or measured thermal capture cross sections for some isotopes. The statistical model was applied to calculate the cross sections above the resolved resonance region. In the calculations, coupled-channel optical model parameters were used for neutrons. Preequilibrium and direct-reaction processes were taken into account in addition to the compound process. The present evaluation is consistent with available experimental data. The evaluated results were compiled into JENDL-4.0.

Determination of the 209Bi( [formula omitted]) 210Bi and 209Bi( [formula omitted]) 210m,gBi reaction cross sections in a cold neutron beam

Gamma-ray transitions following neutron capture in 209Bi have been studied at the cold neutron beam facility of the Budapest Neutron Centre. Measurements have been performed using a coaxial HPGe detector with Compton suppression. Partial capture cross sections at a velocity of 2200 m/s (or a neutron energy of 25.3 meV) have been deduced relatively to the one for the 4055 keV transition after thermal neutron capture in 209Bi. This partial cross section of (8.07 ± 0.14) mb has in turn been obtained from measurements with a bismuth nitrate target relatively to the cross section for the 1884 keV transition after thermal neutron capture in 14N. Shell model calculations have been performed to deduce the half-life of the 3rd excited state at 320 keV and to assign the M1 multipolarity of the 320 keV transition to the ground state. Lower limits of (21.4 ± 0.8) mb and (18.2 ± 0.7) mb for the cross sections to the ground state and to the isomeric state, respectively, have been deduced from the data for the gamma rays feeding these states. The results obtained in this work are in good agreement with experimental data reported in the literature. The thermal total capture cross section has been compared with the contribution due to observed s-wave resonances.

Interpretation of pile-oscillation measurements by the integral data assimilation technique

The Integral Data Assimilation (IDA) was designed to deduce values of infinite dilute neutron cross-sections from specific integral measurements. Performances of the IDA procedure are demonstrated with pile-oscillation measurements carried out on 155Gd in the pool type reactor MINERVE (CEA Cadarache, France). At low neutron energies, the Integral Data Assimilation is based on the Neutron Resonance Shape Analysis technique routinely used in neutron spectroscopy measurements. As a result of the IDA analysis, a value of (61 900±1500) b has been obtained for the 155Gd thermal capture cross-section at ( E th =25.3 meV).

Identification of traps in an epitaxied AlGaAs/GaAs/AlGaAs quantum well structure

An epitaxied AlGaAs/GaAs/AlGaAs heterostructure is studied by using the low frequency noise technique over a wide range of temperature from 300 K down to 4 K as a function of bias. Emphasis is placed on the generation—recombination noise because it is present on a wide range of temperature and frequency. Thermal activation energies and capture cross sections of traps responsible for this kind of noise are extracted from an arrhenius plot. Six kinds of traps have been identified and their electrical nature has been suggested.

Morphological, structural and electrical investigations on non-polar a-plane ZnO epilayers

We report on the growth of non-polar a-plane ZnO by CVD on r-plane-sapphire-wafers, a-plane GaN-templates and a-plane ZnO single-crystal substrates. Only the homoepitaxial growth approach leads to a Frank–van-der–Merwe growth mode, as shown by atomic force microscopy. The X-ray-diffraction spectra of the homoepitaxial thin films mirror the excellent crystalline quality of the ZnO substrate. The morphological and the structural quality of the homoepitaxial films is comparable to the best results for the growth on c-plane ZnO-substrates. The impurity incorporation, especially of group III elements, seems to be reduced when growing on the non-polar a-plane surface compared to the c-plane films as demonstrated by secondary ion mass spectrometry (SIMS). Optical properties have been investigated using low temperature photoluminescence measurements. We employed capacitance–voltage measurements ( C– V) to measure the background carrier density and its profile from substrate/film interface throughout the film to the surface. In thermal admittance spectroscopy (TAS) specific traps could be distinguished, and their thermal activation energies and capture cross sections could be determined.

Evaluation of Neutron Nuclear Data on Arsenic-75 for JENDL-4

Neutron nuclear data on 75As have been evaluated for the evaluated nuclear data library the energy region from 10−5 eV to 20 MeV. The thermal capture cross section was updated by considering recent measurements. The statistical model was applied to calculate the cross sections above the resolved resonance region. In the calculation, coupled-channel optical model parameters were used for neutrons. Pre-equilibrium and direct-reaction processes were taken into account in addition to the compound process. The present calculations are almost consistent with available experimental data. The measured leakage neutron spectrum is well reproduced by the presentlyevaluated data at 14 MeV.

Temperature dependent carrier lifetime studies on Ti-doped multicrystalline silicon

Carrier lifetime measurements were performed on deliberately Ti-doped multicrystalline silicon wafers using a temperature controlled photoconductance device. The dominant recombination center was found to be the double-donor level associated with interstitial titanium. The interstitial Ti concentrations in multicrystalline silicon wafers were determined by measuring the Shockley–Read–Hall time constant for holes and using the known values of the thermal velocity and capture cross section for holes of the double-donor level at different temperatures. The measured values of the Ti concentration were then used to determine the electron capture cross section of the double-donor level over the temperature range of 140–270 °C via the measured values of the Shockley–Read–Hall time constant for electrons and the known thermal velocity. Multiphonon emission was found to be the most likely capture mechanism for this temperature range for electron capture into the double-donor level of Ti in silicon. The effective segregation coefficient for Ti was estimated by fitting Scheil’s equation to the measured values of the Ti concentrations and their respective vertical positions in the ingot. If all Ti were present as the interstitial double-donor, a lower limit of 1.8×10−6 can be ascribed to the segregation coefficient, which is very close to the equilibrium value.

Validation of the IRDF-2002 dosimetry library

A Data Development Project was instigated by the International Atomic Energy Agency (IAEA) to update the library of dosimetry cross-sections “International Reactor Dosimetry File: IRDF-2002” and this was completed recently. Thermal capture cross-sections and resonance integrals were compared against the database of integral constants for neutron activation analysis in a validation exercise; this new IRDF-2002 file was also used in the analysis of several benchmark experiments performed at the Frascati Neutron Generator (FNG), ENEA, Frascati, and in the ASPIS facility, AEA Technology, Winfrith. The results exhibited good consistency and improved the “calculation/experiment” ratios. Reduction of some uncertainties in the new IRDF-2002 file is supported by the analysis. Inclusion of pointwise cross-section data is an important enhancement of the library for the Monte Carlo methods, as well as for codes using multigroup data for cases where self-shielding of the resonances is important.

Neutron Capture and Total Cross-Section Measurements and Resonance Parameters of Gadolinium

Neutron capture and transmission measurements were performed by the time-of-flight technique at the Rensselaer Polytechnic Institute linac facility using metallic and liquid Gd samples. The liquid samples were isotopically enriched in either 155Gd or 157Gd. The capture measurements were made at the 25-m flight station with a multiplicity-type capture detector, and the transmission measurements were performed at 15- and 25-m flight stations with 6Li glass scintillation detectors. The multilevel R-matrix Bayesian code SAMMY was used to extract resonance parameters.Among the significant findings are the following. The neutron width of the largest resonance in Gd, at 0.032 eV in 157Gd, has been measured to be (9 ± 1)% smaller than that given in ENDF/B-VI updated through release 8. The thermal (2200 m/s) capture cross section of 157Gd has been measured to be 11% smaller than that calculated from ENDF. The other major thermal resonance, at 0.025 eV in 155Gd, did not display a significant deviation from the thermal capture cross section given by ENDF.In the epithermal region, the analysis provided here represents the most extensive to date. Twenty-eight new resonances are proposed, and other resonances previously identified in the literature have been revisited. The assignment of resonances within regions of complicated structure incorporated the observations of other researchers, particularly on the six occasions where ENDF resonances are recommended to be removed. The poor match of the ENDF parameters to the current data is significant, and substantial improvement to the understanding of gadolinium cross sections is presented, particularly above 180 eV where the ENDF resolved region for 155Gd ends.

Determination of thermal neutron radiative capture cross section of 6Li

The thermal neutron radiative capture cross section of 6Li was determined relative to the 1H(n,γ) reaction emitting the 2223.25 keV γ-ray. The sample used was a mixture of enriched 6Li 2CO 3 and melamine. Background correction was performed by considering the scattering effect of the sample. The determined thermal neutron radiative capture cross section of 6Li was 37.7 ± 3.0 mb.

Thermal neutron capture cross section for theKisomerLu177m

The thermal neutron radiative capture cross section for the K isomeric state in $^{177}\mathrm{Lu}$ has been measured for the first time. Several $^{177}\mathrm{Lu}$${}^{m}$ targets have been prepared and irradiated in various neutron fluxes at the La\"ue Langevin Institute in Grenoble and at the CEA reactors OSIRIS and ORPHEE in Saclay. The method consists of measuring the $^{178}\mathrm{Lu}$ activity by $\ensuremath{\gamma}$-ray spectroscopy. The values obtained in four different neutron spectra have been used to calculate the resonance integral of the radiative capture cross section for $^{177}\mathrm{Lu}$${}^{m}$. In addition, an indirect method leads to the determination of the $^{177}\mathrm{Lu}$${}^{g}$ neutron radiative capture cross section.

Admittance spectroscopy of a phosphorus-doped n-diamond homoepitaxial layer

Generation and recombination processes of electrons through phosphorus (P) donor are analyzed by admittance spectroscopy for a Ni/Au Schottky contact on an n-diamond epilayer. The dependence of capacitance and conductance frequency on temperature is interpreted by Shockley–Read–Hall statistics. The thermal ionization energy and capture cross-section of P donor are evaluated to be 0.54 ± 0.02 eV and (4.5 ± 2.0) × 10 − 17 cm 2, respectively. Broadening of the conductance–frequency curve is observed, which is believed to be evidence of a long Debye tail of electron distribution at the depletion layer edge.

Revisiting the 238U Thermal Capture Cross Section and Gamma-Ray Emission Probabilities from 239Np Decay

The precise value of the thermal capture cross section of 238U is uncertain, and evaluated cross sections from various sources differ by more than their assigned uncertainties. A number of the original publications have been reviewed to assess the discrepant data, corrections were made for more recent standard cross sections and other constants, and one new measurement was analyzed. Because of the strong correlations in activation measurements, the gamma-ray emission probabilities from the β– decay of 239Np were also analyzed. As a result of the analysis, a value of 2.683 ± 0.012 b was derived for the thermal capture cross section of 238U. A new evaluation of the gamma-ray emission probabilities from 239Np decay was also undertaken.

Frequency dependence of junction capacitance of GaN p–i–n UV detectors

In this paper frequency dependence of small-signal capacitance of p–i–n UV detectors, which were fabricated on GaN grown on sapphire substrate by metalorganic chemical vapor deposition, has been studied. The Schibli–Milnes model was used to analyze the capacitance–frequency characteristics. According to high frequency C– V measurements, the deep level mean concentration is about 2.98 × 10 20 cm −3. The deep level is caused by the un-ionised Mg dopant. The calculated Mg activation energy is 260 meV and the hole thermal capture cross section of the deep level is about 2.73 × 10 −22 cm 2. The applicability of the Schibli–Milnes model is also discussed when the concentration of deep levels exceeds that of the heavily doped n-side. It is concluded that the analytic expression of the Schibli–Milnes model can still be used to describe the capacitance–frequency characteristics of GaN p–i–n UV detectors in good agreement with experiment.

Admittance spectroscopy for phosphorus-doped n-type diamond epilayer

Capture and emission processes of electron at phosphorus (P) donor in a n-diamond epilayer are analyzed by admittance spectroscopy. Dependence of temperatures on capacitance and conductance-frequency curves is well explained by Shockley-Read-Hall statistics. Thermal ionization energy and capture cross section of P donor are evaluated to be 0.54±0.02eV and (4.5±2.0)×10−17cm2, respectively. Broadening of the conductance-frequency curve is observed, which is believed to be due to a large Debye tail of electron distribution at a depletion layer edge.

Neutron capture cross section ofPd102

The cross sections for radiative neutron capture by $^{102}\mathrm{Pd}$ have been deduced from a measurement of the \ensuremath{\gamma} rays emitted by 17.0-d $^{103}\mathrm{Pd}$. The thermal cross section has been determined to be $\ensuremath{\sigma}=1.82\ifmmode\pm\else\textpm\fi{}0.20$ b, and the effective resonance integral is $I=23\ifmmode\pm\else\textpm\fi{}4$ b. We also report thermal and resonance capture cross sections for $^{108}\mathrm{Pd}$ and note possible inconsistencies with the presently accepted values of the $^{110}\mathrm{Pd}$ cross sections.