A procedure for determining the effective thickness of non-uniform irregular-shaped samples via radiative capture is described. In this technique, partial γ-ray production cross sections of a compound nucleus produced in a neutron-capture reaction are measured using Prompt Gamma Activation Analysis and compared to their corresponding standardized absolute values. For the low-energy transitions, the measured cross sections are lower than their standard values due to significant photoelectric absorption of the γ rays within the bulk-sample volume itself. Using standard theoretical techniques, the amount of γ-ray self absorption and neutron self shielding can then be calculated by iteratively varying the sample thickness until the observed cross sections converge with the known standards. The overall attenuation, thus, provides a measure of the effective sample thickness illuminated by the neutron beam. This procedure is illustrated through radiative neutron capture using powdered oxide samples comprising enriched ¹⁸⁶W and ¹⁸²W from which their tungsten-equivalent effective thicknesses are deduced to be 0.077(3) mm and 0.042(8) mm, respectively.

New values for thermal neutron capture cross sections of the tellurium isotopes 122Te, 124Te, 125Te, 126Te, 128Te, and 130Te are reported. These values are based on a combination of newly determined partial g-ray cross sections obtained from experiments on targets contained natural Te and gamma intensities per capture of individual Te isotopes. Isomeric ratios for the thermal neutron capture on the even tellurium isotopes are also given.

A set of prompt partial γ-ray production cross sections from thermal neutron capture were measured for the La139(n,γ) reaction using a guided beam of subthermal (thermal and cold) neutrons incident on a La2natO3 target at the Prompt Gamma Activation Analysis facility of the Budapest Research Reactor. Absolute La140 cross sections were determined relative to the well-known comparator Cl35(n,γ) cross sections from the irradiation of a stoichiometric LaCl3nat sample. The total cross section for radiative thermal neutron capture on La139 from the sum of experimentally measured cross sections observed to directly feed the La140 ground state was determined to be σ0expt=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 quasicontinuum 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 La140 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.

Prompt Gamma Activation Analysis (PGAA) at nuclear reactors has undergone a renaissance with the advent of modern spectroscopy equipment, development of a precise database for PGAA analysis, and the development of guided neutron beams and remote target facilities far from the reactor core. PGAA at these facilities is complemented by short-lived Neutron Activation Analysis (NAA) because decay gamma rays are observed either simultaneously with prompt gamma rays or separately if the neutron beam is chopped. Activities with half-lives as short as 1 ms can now be analyzed with NAA. Sensitivity of less than 0.1 mg/g of any element except Helium has been achieved with a 106 ncm-2s-1 thermal neutron beam. This kind of analysis has so far been limited to a handful of reactor facilities around the world. At Lawrence Berkeley National Laboratory (LBNL) we are developing a PGAA/NAA analysis system based on a compact, low power, >4'109 n/s D+D (En>2.5 MeV) neutron generator. The generator creates minimal gamma-ray background so detectors can be placed close to the target where the neutron flux is comparable to the guided neutron beam at a reactor. A D+D generator requires less thickness of moderator to thermalize neutrons than a conventional D+T (En>14 MeV) generator and eliminates the environmental and political concerns raised by using tritium. This paper discusses our initial PGAA/NAA experimental results for the LBNL neutron generator and future plans to develop reactor-quality PGAA/NAA analysis in the laboratory.

Partial γ-ray production cross sections and the total radiative thermal-neutron capture cross section for the Re185(n,γ)Re186 reaction were measured using the Prompt Gamma Activation Analysis facility at the Budapest Research Reactor with an enriched Re185 target. The Re186 cross sections were standardized using well-known Cl35(n,γ)Cl36 cross sections from irradiation of a stoichiometric natReCl3 target. The resulting cross sections for transitions feeding the Re186 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 Re185. 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 Re186. Additionally, 102 primary γ 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 γ-ray energies to the known Re186 decay scheme. The total capture cross section and separation energy results are comparable to earlier measurements of these values.

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.