Novel method to study neutron capture of U 235 and U 238 simultaneously at keV energies

The neutron capture cross sections of the main uranium isotopes, U235 and U238, were measured simultaneously for keV energies, for the first time by combining activation technique and atom counting of the reaction products using accelerator mass spectrometry. New data, with a precision of 3%-5%, were obtained from mg-sized natural uranium samples for neutron energies with an equivalent Maxwell-Boltzmann distribution of kT∼25keV and for a broad energy distribution peaking at 426 keV. The cross-section ratio of U235(n,γ)/U238(n,γ) can be deduced in accelerator mass spectrometry directly from the atom ratio of the reaction products U236/U239, independent of any fluence normalization. Our results confirm the values at the lower band of existing data. They serve as important anchor points to resolve present discrepancies in nuclear data libraries as well as for the normalization of cross-section data used in the nuclear astrophysics community for s-process studies

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

Precise measurement of the thermal and stellar 54^{54}Fe(n,γn, \gamma)55^{55}Fe cross sections via AMS

The detection of long-lived radionuclides through ultra-sensitive single atom counting via accelerator mass spectrometry (AMS) offers opportunities for precise measurements of neutron capture cross sections, e.g. for nuclear astrophysics. The technique represents a truly complementary approach, completely independent of previous experimental methods. The potential of this technique is highlighted at the example of the $^{54}$Fe($n, \gamma$)$^{55}$Fe reaction. Following a series of irradiations with neutrons from cold and thermal to keV energies, the produced long-lived $^{55}$Fe nuclei ($t_{1/2}=2.744(9)$ yr) were analyzed at the Vienna Environmental Research Accelerator (VERA). A reproducibility of about 1% could be achieved for the detection of $^{55}$Fe, yielding cross section uncertainties of less than 3%. Thus, the new data can serve as anchor points to time-of-flight experiments. We report significantly improved neutron capture cross sections at thermal energy ($\sigma_{th}=2.30\pm0.07$ b) as well as for a quasi-Maxwellian spectrum of $kT=25$ keV ($\sigma=30.3\pm1.2$ mb) and for $E_n=481\pm53$ keV ($\sigma= 6.01\pm0.23$ mb). The new experimental cross sections have been used to deduce improved Maxwellian average cross sections in the temperature regime of the common $s$-process scenarios. The astrophysical impact is discussed using stellar models for low-mass AGB stars

Compositional studies of functional orthodontic archwires using prompt-gamma activation analysis at a pulsed neutron source

Prompt-gamma activation analysis (PGAA) measurements were carried out at the ISIS Spallation Neutron Source on two sets of functional commercial stainless steel orthodontic archwires, aiming at providing insights into the elemental and isotopic composition differences of two nominally equivalent archwires. The results were compared to those obtained from parallel cold neutron PGAA measurements on the same samples at the Budapest Neutron Centre in order to test the current status of PGAA at a pulsed neutron source and eventually to inform improvement in set-up and acquisition methods. In addition, time-resolved PGAA (T-PGAA) that combines PGAA and neutron time-of-flight methods was applied to the present set of samples, allowing the measurement of the neutron energy dependence of the PGAA spectra. The advantages of this technique were demonstrated to be that through incident neutron energy selection, spanning 0.07–67.94 eV, enhancement or decrease of specific gamma lines associated with isotopes of interest could be achieved. These were shown to reduce peak interference and to increase the signal-to-background ratio for certain species in order to facilitate accurate elemental identification. Suggestions for potential performance improvement for this evolving technique are proposed

Structure and reactivity of ceria-zirconia catalysts for bromine and chlorine production via the oxidation of hydrogen halides

The impact of zirconia on the activity and stability of ceria has been investigated in the gas-phase oxidation of HBr and HCl to the corresponding halogens. Homogeneous and non-homogeneous ceria-zirconia catalysts with a Ce:Zr ratio of 75:25 were prepared and characterized by X-ray diffraction, temperature-programmed reduction in hydrogen, transmission electron microscopy, and X-ray photoelectron spectroscopy. Catalytic tests demonstrated that ZrO2 promotes the activity of CeO2 independently of the metal homogeneity in the mixed oxide. Upon cycling of the temperature and feed composition, no differences with respect to chlorine formation were observed. On the other hand, a hysteresis of the reaction rate was measured in HBr oxidation, which was more pronounced over the non-homogeneous mixed oxide due to the higher extent of bromination. A moderate degree of bromination was attained in the homogeneous mixed oxide, leading to an improved long-term stability. The impact of phase homogeneity on the halogenation properties of the catalysts, and thus on the lifetime, was further rationalized by the determination of the halogen uptake by operando prompt-gamma activation analysis. While the chlorine uptake under different reaction conditions was comparable over both materials, the bromine uptake on the non-homogeneous sample was up to 50% higher compared to the homogeneous counterpart. This indicates not only that the catalysts are more prone to bromination than chlorination, but also that their robustness depends on the intermixing of the Ce and Zr phases, suggesting that the mixed oxides are more stable than supported ceria catalysts. The degree of halogenation of the homogeneous ceria-zirconia catalysts can be controlled by tuning their Ce:Zr ratio, minimizing the halogen uptake at a Zr content in the range of 70-90 mol.%

New Experimental 235U(n,f) Prompt Fission Neutron Spectrum and Old Disagreement between Microscopic and Macroscopic Data

A measurement of the 235U Prompt Fission Neutron Spectrum (PFNS) was performed at the Budapest Nuclear Research Reactor at 100 oK incident neutron energy. The motivation of this experiment is based on the discrepancy between literature data and the fact that the measured PFNS at thermal incident neutron energy cannot be reproduced by theory. The measured spectra using three neutron detectors are in excellent agreement with each other. The average spectrum confirms literature data within the error bars in the neutron energy range 0.7 - 10 MeV. However, the present PFNS shape cannot predict integral experimental data. It seems to be clear now, that the disagreement between microscopic and macroscopic data is not connected with a systematic experimental error in the PFNS at low incident neutron energy.JRC.DG.D.5-Nuclear physic