Neutron Capture gamma ENDF libraries for modeling and identification of neutron sources

There are a number of inaccuracies and data omissions with respect to gammas from neutron capture in the ENDF libraries used as field reference information and by modeling codes used in JTOT. As the use of Active Neutron interrogation methods is expanded, these shortfalls become more acute. A new, more accurate and complete evaluated experimental database of gamma rays (over 35,000 lines for 262 isotopes up to U so far) from thermal neutron capture has recently become available from the IAEA. To my knowledge, none of this new data has been installed in ENDF libraries and disseminated. I propose to upgrade libraries of {sup 184,186}W, {sup 56}Fe, {sup 204,206,207}Pb, {sup 104}Pd, and {sup 19}F the 1st year. This will involve collaboration with Richard Firestone at LBL in evaluating the data and installing it in the libraries. I will test them with the transport code MCNP5

On the Use of Rossi Alpha Critical Assembly Measurements for Validating and Constraining Nuclear Data

Critical assemblies are exquisitely sensitive to details of the microscopic nuclear reactions that govern neutron multiplication. For this reason experimental studies of critical assemblies represent a cornerstone in the process of validating nuclear data. Several different characteristics of a critical system can be measured. The most commonly considered is the so-called effective k eigenvalue, k{sub eff}. Another well-measured property of these systems is {alpha}{sub 0}, the inverse e-folding time of the neutron population in the absence of {beta}-delayed neutrons. Through Monte Carlo calculations and appear to perturbation theory they show that for fast critical systems {alpha}{sub 0} and k{sub eff} can be viewed as lying on a single straight line for any reasonable assumptions about the underlying nuclear data. This means that the two quantities provide the same constraint on nuclear data. In principle, {alpha}{sub 0} could be associated with a very small uncertainty, and this would make the measurements for neutron multiplication rates more useful than k{sub eff} for constraining nuclear data. In practice, though, uncertainties in k{sub eff} and {alpha}{sub 0} are dominated by uncertainties in the representation of the critical system, and not by pure measurement errors for these quantities. This, together with the linear relation between {alpha}{sub 0} and k{sub eff}, implies that the two measured quantities provide exactly the same constraint on nuclear data. They do not consider other measured or inferred characteristics of critical assemblies, such as neutron generation times or spectral indices, that may be valuable in the validation process

A structural evaluation of the tungsten isotopes via thermal neutron capture

Total radiative thermal neutron-capture $\gamma$-ray cross sections for the $^{182,183,184,186}$W isotopes were measured using guided neutron beams from the Budapest Research Reactor to induce prompt and delayed $\gamma$ rays from elemental and isotopically-enriched tungsten targets. These cross sections were determined from the sum of measured $\gamma$-ray cross sections feeding the ground state from low-lying levels below a cutoff energy, E$_{\rm crit}$, where the level scheme is completely known, and continuum $\gamma$ rays from levels above E$_{\rm crit}$, calculated using the Monte Carlo statistical-decay code DICEBOX. The new cross sections determined in this work for the tungsten nuclides are: $\sigma_{0}(^{182}{\rm W}) = 20.5(14)$ b and $\sigma_{11/2^{+}}(^{183}{\rm W}^{m}, 5.2 {\rm s}) = 0.177(18)$ b; $\sigma_{0}(^{183}{\rm W}) = 9.37(38)$ b and $\sigma_{5^{-}}(^{184}{\rm W}^{m}, 8.33 \mu{\rm s}) = 0.0247(55)$ b; $\sigma_{0}(^{184}{\rm W}) = 1.43(10)$ b and $\sigma_{11/2^{+}}(^{185}{\rm W}^{m}, 1.67 {\rm min}) = 0.0062(16)$ b; and, $\sigma_{0}(^{186}{\rm W}) = 33.33(62)$ b and $\sigma_{9/2^{+}}(^{187}{\rm W}^{m}, 1.38 \mu{\rm s}) = 0.400(16)$ 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 $\sigma_{0}(^{186}{\rm W})$ that are consistent with our prompt $\gamma$-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 E$_{\rm crit}$. 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.Comment: 25 pages, 15 figures, 15 table

Investigation of \u3csup\u3e186\u3c/sup\u3eRe via radiative thermal-neutron capture on \u3csup\u3e185\u3c/sup\u3eRe

Partial -ray production cross sections and the total radiative thermal-neutron capture cross section for the 185Re(n,)186Re reaction were measured using the Prompt Gamma Activation Analysis facility at the Budapest Research Reactor with an enriched 185Re target. The 186Re cross sections were standardized using well-known 35Cl(n,)36Cl cross sections from irradiation of a stoichiometric natReCl3 target. The resulting cross sections for transitions feeding the 186Re ground state from low-lying levels below a cutoff energy of Ec=746keV were combined with a modeled probability of ground-state feeding from levels above Ec to arrive at a total cross section of σ0=111(6)b for radiative thermal-neutron capture on 185Re. A comparison of modeled discrete-level populations with measured transition intensities led to proposed revisions for seven tentative spin-parity assignments in the adopted level scheme for 186Re. Additionally, 102 primary rays were measured, including 50 previously unknown. A neutron-separation energy of Sn=6179.59(5)keV was determined from a global least-squares fit of the measured -ray energies to the known 186Re decay scheme. The total capture cross section and separation energy results are comparable to earlier measurements of these values

Developments in Capture- γ Libraries for Nonproliferation Applications

The neutron-capture reaction is fundamental for identifying and analyzing the γ-ray spectrum from an unknown assembly because it provides unambiguous information on the neutron-absorbing isotopes. Nondestructive-assay applications may exploit this phenomenon passively, for example, in the presence of spontaneous-fission neutrons, or actively where an external neutron source is used as a probe. There are known gaps in the Evaluated Nuclear Data File libraries corresponding to neutron-capture γ-ray data that otherwise limit transport-modeling applications. In this work, we describe how new thermal neutron-capture data are being used to improve information in the neutron-data libraries for isotopes relevant to nonproliferation applications. We address this problem by providing new experimentally-deduced partial and total neutron-capture reaction cross sections and then evaluate these data by comparison with statistical-model calculations

Radiative-capture cross sections for the La139(n,γ) reaction using thermal neutrons and structural properties of La140

A set of prompt partial γ-ray production cross sections from thermal neutron capture were measured for the 139La (n,γ) reaction using a guided beam of subthermal (thermal and cold) neutrons incident on a nat La2O3 target at the Prompt Gamma Activation Analysis facility of the Budapest Research Reactor. Absolute 140La cross sections were determined relative to the well-known comparator 35Cl(n,γ) cross sections from the irradiation of a stoichiometric nat LaCl3 sample. The total cross section for radiative thermal neutron capture on 139La from the sum of experimentally measured cross sections observed to directly feed the 140 La ground state was determined to be σ0 = 8.58(50) b. To assess completeness of the decay scheme and as a consistency check, the measured cross sections for transitions feeding the ground state from levels below a critical energy of Ec = 285 keV were combined with a modeled contribution accounting for ground-state feeding from the quasi continuum to arrive at a total cross section of σ0 = 9.36(74) b. In addition, a neutron-separation energy of Sn = 5161.001(21) keV was determined from a least-squares fit of the measured primary γ-ray energies to the low-lying levels of the 140La decay scheme. Furthermore, several nuclear structure improvements are proposed for the decay scheme. The measured cross-section and separation-energy results are comparable to earlier measurements of these quantities

Gamma-Ray Detection with Pbo Glass Converters In Mwpc: Electron Conversion Efficiency And Time Resolution

The development of glass tubing converters for efficient gamma-ray detection in multiwire proportional chambers (MWPC) has led to an investigation on the improvement of conductivity on glass surfaces and to an investigation of gas mixtures which will improve on the electron conversion efficiency and electron transit time within the tubes. Efforts to establish uniform electric field lines within small diameter tubes has resulted in an improved H{sub 2} reducing treatment. For a 0.91 mm I.D., 1.10 mm O.D., 2 cm thick converter the electron conversion efficiency {epsilon} was measured to be 9.0% and 10.4% at 511 keV, using Ar mixtures containing 10% CF{sub 4} and 30% isobutane, respectively. The effects of gas mixtures on {epsilon} and on {tau}, the mean transit time on conversion electrons within the converter, and the projection of these results on the performance of a modified MWPC positron camera will be presented