N.C. Summers

Investigation of the tungsten isotopes via thermal neutron capture

�5 ( 184 W m ,8.33 µs) = 0.0247(55) b; �0( 184 W) = 1.43(10) b and �11/2+( 185 W m ,1.67 min) = 0.0062(16) b; and, �0( 186 W) = 33.33(62) b and �9/2+( 187 W m ,1.38 µs) = 0.400(16) b. These results are consistent with earlier measurements in the literature. The 186 W cross section was also independently confirmed from an activation measurement, following the decay of 187 W, yielding values for �0( 186 W) that are consistent with our prompt -ray measurement. The cross-section measurements were found to be insensitive to choice of level density or photon strength model, and only weakly dependent on Ecrit. Total radiative-capture widths calculated with DICEBOX showed much greater model dependence, however, the recommended values could be reproduced with selected model choices. The decay schemes for all tungsten isotopes were improved in these analyses. We were also able to determine new neutron separation energies from our primary -ray measurements for the respective (n,) compounds: 183 W (Sn = 6190.88(6) keV); 184 W (Sn = 7411.11(13) keV); 185 W (S n = 5753.74(5) keV); and, 187 W (S n = 5466.62(7) keV).

Radiative Capture Cross Sections of 155,157 Gd for Thermal Neutrons

Abstract Thermal neutron radiative capture cross sections σ0γ of 155,157Gd are determined by summing the transition cross sections feeding the ground states of the respective product nuclei. The transition cross sections feeding the ground states from the discrete states in the low-excitation region, where the decay schemes are known completely, were measured using a guided cold neutron beam at the Budapest Research Reactor. Transitions from the states at the higher excitation, the so-called quasi-continuum levels, are determined from simulations with the extreme statistical model normalized to the intensity balance through the low-lying discrete levels. A significant non-1/v correction was applied to 155,157Gd, leading to σ0γ(155Gd) = 56 700(2100) b and σ0γ(157Gd) = 239 000(6000) b.

Improved capture γ-ray libraries for nuclear applications

The neutron-capture reaction is of fundamental use in identifying and analyzing the γ-ray spectrum from an unknown object as it gives unambiguous information on exactly what isotopes are absorbing the neutrons. There are many applications where this can be used passively (nonproliferation), or actively where an external neutron source is used to probe an unknown assembly (planetary studies). There are known capture-γ data gaps in the ENDF libraries used by transport codes for various nuclear applications. A new database, EGAF, containing thermal neutron-capture γ-ray data is used to improve the capture-γ information in the ENDF libraries. For many nuclei the unresolved quasi-continuum part of the γ cascade is not available experimentally. In this work, we have modeled this contribution using the Monte Carlo statisticaldecay code DICEBOX, in addition to improving level-scheme evaluations. For capture of higher-energy neutrons there is little experimental data available, making evaluation of modeling codes problematic. We plan to continue the DICEBOX approach through the resolved resonance region where spin and parity information is partially known. In the unresolved resonance region, and up to 20-MeV incident neutron energy, we are applying Hauser-Feshbach models to predict the capture-γ spectrum.

Neutron Capture Gamma‐Ray Libraries for Nuclear Applications

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 is being 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 90% of all the decay energy and 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. They are investigating the capture spectra from higher energy neutrons experimentally using surrogate reactions and modeling this with Hauser-Feshbach codes. This can then be used to benchmark CASINO, a version of DICEBOX modified for neutron capture at higher energy. This can be used to simulate spectra from neutron capture at incident neutron energies up to 20 MeV to improve the gamma-ray spectrum in neutron data libraries used for transport modeling of unknown assemblies.

Data evaluation methods and improvements to the neutron‐capture γ‐ray spectrum

Improved neutron‐capture γ‐ray spectra, not only of interest to the nuclear structure and reactions communities, are needed in a variety of applied and non‐proliferation programs. This requires an evaluation of the existing experimental capture‐γ data. Elemental neutron‐capture data taken from direct measurements at the Budapest Reactor have been used to collate the Evaluated Gamma‐ray Activation File, a database of capture γ‐ray cross sections. These cross sections are then compared to Monte Carlo simulations of γ‐ray emission following the thermal neutron‐capture process using the statistical‐decay code DICEBOX. The aim of this procedure is to obtain the total radiative neutron‐capture cross section and confidently increase the number of levels and γ rays that can be assigned to a given isotope in the neutron data libraries. To achieve these goals and provide as complete information as possible in the neutron data libraries, it is also necessary to remain current with recent advances in nuclear structur...