Total thermal radiative neutron cross sections have been measured on natural and enriched isotopic targets containing Li6,7,Be9,B10,11,C12,13, and N14,15 with neutron beams from the Budapest Reactor. Complete neutron capture γ-ray decay schemes were measured for each isotope. Absolute transition probabilities have been determined by a least-squares fit of the transition intensities, corrected for internal conversion, to the (n,γ) decay schemes. The γ-ray cross sections were standardized using stoichiometric compounds containing both the isotope of interest and another element whose γ-ray cross sections are well known. Total cross sections σ0 were then determined for each isotope from the γ-ray cross sections and transition probabilities. For the B11(n,γ)B12 reaction decay transition probabilities were determined for the γ rays from B12 (t1/2=20.20 ms) β- decay.

Thermal neutron capture γ-ray spectra for O16,17,18 and H2 have been measured with guided cold neutron beams from the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) reactor and the Budapest Research Reactor (BRR) on natural and O17,18 enriched D2O targets. Complete neutron capture γ-ray decay schemes for the O16,17,18(n,γ) reactions were measured. Absolute transition probabilities were determined for each reaction by a least-squares fit of the γ-ray intensities to the decay schemes after accounting for the contribution from internal conversion. The transition probability for the 870.76-keV γ ray from O16(n,γ) was measured as Pγ(871)=96.6 ±0.5% and the thermal neutron cross section for this γ ray was determined as 0.164±0.003 mb by internal standardization with multiple targets containing oxygen and stoichiometric quantities of hydrogen, nitrogen, and carbon whose γ-ray cross sections were previously standardized. The γ-ray cross sections for the O17,18(n,γ) and H2(n,γ) reactions were then determined relative to the 870.76-keV γ-ray cross section after accounting for the isotopic abundances in the targets. We determined the following total radiative thermal neutron cross sections for each isotope from the γ-ray cross sections and transition probabilities; σ0(O16)=0.170±0.003 mb; σ0(O17)=0.67±0.07 mb; σ0(O18)=0.141±0.006 mb; and σ0(H2)=0.489±0.006 mb.

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.

The Fe56(n,γ) thermal neutron capture cross section and the Fe57 level scheme populated by this reaction have been investigated in this work. Singles γ-ray spectra were measured with an isotopically enriched Fe56 target using the guided cold neutron beam at the Budapest Reactor, and γγ-coincidence data were measured with a natural Fe target at the LWR-15 research reactor in ŘeŽ, Czech Republic. A detailed level scheme consisting of 448 γ rays populating/depopulating 97 levels and the capture state in Fe57 has been constructed, and ≈99% of the total transition intensity has been placed. The transition probability of the 352-keV γ ray was determined to be Pγ(352)=11.90±0.07 per 100 neutron captures. The Fe57 level scheme is substantially revised from earlier work and ≈33 previously assigned levels could not be confirmed while a comparable number of new levels were added. The Fe57γ-ray cross sections were internally calibrated with respect to H1 and S32γ-ray cross section standards using iron(III) acetylacetonate (C15H21FeO6) and iron pyrite (FeS2) targets. The thermal neutron cross section for production of the 352-keV γ-ray cross section was determined to be σγ(352)=0.2849±0.015 b. The total Fe56(n,γ) thermal radiative neutron cross section is derived from the 352-keV γ-ray cross section and transition probability as σ0=2.394±0.019 b. A least-squares fit of the γ rays to the level scheme gives the Fe57 neutron separation energy Sn=7646.183±0.018 keV.

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.

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.

The emission of γ rays from neutron-bound and neutron-unbound states in Mo95, populated in the Mo94(d,p) reaction, has been investigated. Charged particles and γ radiation were detected with arrays of annular silicon and Clover-type high-purity Germanium detectors, respectively. Utilizing p-γ and p-γ-γ coincidences, the Mo95 level scheme was greatly enhanced with 102 new transitions and 43 new states. It agrees well with shell model calculations for excitation energies below ≈2 MeV. From p-γ coincidence data, a new method for the determination of spins of discrete levels is proposed. The method exploits the suppression of high-angular momentum neutron emission from levels with high spins populated in the (d,p) reaction above the neutron separation energy. Spins for almost all Mo95 levels below 2 MeV (and for a few levels above) have been determined with this method.