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Register a new userMeasurement of the neutron capture cross-section on argon
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V. Fischer
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The use of argon as a detection and shielding medium for neutrino and dark matter experiments has made the precise knowledge of the cross section for neutron capture on argon an important design and operational parameter. Since previous measurements were averaged over thermal spectra and have significant disagreements, a differential measurement has been performed using a Time-Of-Flight neutron beam and a $sim$4$pi$ gamma spectrometer. A fit to the differential cross section from $0.015-0.15$,eV, assuming a $1/v$ energy dependence, yields $sigma^{2200} = 673 pm 26 text{ (stat.)} pm 59 text{ (sys.)}$,mb.
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We report the first measurement of the neutron cross section on argon in the energy range of 100-800 MeV. The measurement was obtained with a 4.3-hour exposure of the Mini-CAPTAIN detector to the WNR/LANSCE beam at LANL. The total cross section is measured from the attenuation coefficient of the neutron flux as it traverses the liquid argon volume. A set of 2,631 candidate interactions is divided in bins of the neutron kinetic energy calculated from time-of-flight measurements. These interactions are reconstructed with custom-made algorithms specifically designed for the data in a time projection chamber the size of the Mini-CAPTAIN detector. The energy averaged cross section is $0.91 pm{} 0.10~mathrm{(stat.)} pm{} 0.09~mathrm{(sys.)}~mathrm{barns}$. A comparison of the measured cross section is made to the GEANT4 and FLUKA event generator packages.
In a neutron lifetime measurement at the Japan Proton Accelerator Complex, the neutron lifetime is calculated by the neutron decay rate and the incident neutron flux. The flux is obtained due to counting the protons emitted from the neutron absorption reaction of ${}^{3}{rm He}$ gas, which is diluted in a mixture of working gas in a detector. Hence, it is crucial to determine the amount of ${}^{3}{rm He}$ in the mixture. In order to improve the accuracy of the number density of the ${}^{3}{rm He}$ nuclei, we suggested to use the ${}^{14}{rm N}({rm n},{rm p}){}^{14}{rm C}$ reaction as a reference because this reaction involves similar kinetic energy as the ${}^{3}{rm He}({rm n},{rm p}){}^{3}{rm H}$ reaction and a smaller reaction cross section to introduce reasonable large partial pressure. The uncertainty of the recommended value of the cross section, however, is not satisfied with our requirement. In this paper, we report the most accurate experimental value of the cross section of the ${}^{14}{rm N}({rm n},{rm p}){}^{14}{rm C}$ reaction at a neutron velocity of 2200 m/s, measured relative to the ${}^{3}{rm He}({rm n},{rm p}){}^{3}{rm H}$ reaction. The result was 1.868 $pm$ 0.003 (stat.) $pm$ 0.006 (sys.) b. Additionally, the cross section of the ${}^{17}{rm O}({rm n},{rm alpha}){}^{14}{rm C}$ reaction at the neutron velocity is also redetermined as 249 $pm$ 6 mb.
We have measured the $gamma$-ray energy spectrum from the thermal neutron capture, ${}^{157}$Gd$(n,gamma){}^{158}$Gd, on an enriched $^{157}$Gd target (Gd$_{2}$O$_{3}$) in the energy range from 0.11 MeV up to about 8 MeV. The target was placed inside the germanium spectrometer of the ANNRI detector at J-PARC and exposed to a neutron beam from the Japan Spallation Neutron Source (JSNS). Radioactive sources ($^{60}$Co, $^{137}$Cs, and $^{152}$Eu) and the reaction $^{35}$Cl($n$,$gamma$) were used to determine the spectrometers detection efficiency for $gamma$ rays at energies from 0.3 to 8.5 MeV. Using a Geant4-based Monte Carlo simulation of the detector and based on our data, we have developed a model to describe the $gamma$-ray spectrum from the thermal ${}^{157}$Gd($n$,$gamma$) reaction. While we include the strength information of 15 prominent peaks above 5 MeV and associated peaks below 1.6 MeV from our data directly into the model, we rely on the theoretical inputs of nuclear level density and the photon strength function of ${}^{158}$Gd to describe the continuum $gamma$-ray spectrum from the ${}^{157}$Gd($n$,$gamma$) reaction. Our model combines these two components. The results of the comparison between the observed $gamma$-ray spectra from the reaction and the model are reported in detail.
Natural gadolinium is widely used for its excellent thermal neutron capture cross section, because of its two major isotopes: $^{rm 155}$Gd and $^{rm 157}$Gd. We measured the $gamma$-ray spectra produced from the thermal neutron capture on targets comprising a natural gadolinium film and enriched $^{rm 155}$Gd (in Gd$_{2}$O$_{3}$ powder) in the energy range from 0.11 MeV to 8.0 MeV, using the ANNRI germanium spectrometer at MLF, J-PARC. The freshly analysed data of the $^{rm 155}$Gd(n, $gamma$) reaction are used to improve our previously developed model (ANNRI-Gd model) for the $^{rm 157}$Gd(n, $gamma$) reaction, and its performance confirmed with the independent data from the $^{rm nat}$Gd(n, $gamma$) reaction. This article completes the development of an efficient Monte Carlo model required to simulate and analyse particle interactions involving the thermal neutron captures on gadolinium in any relevant future experiments.
Background:The design of new nuclear reactors and transmutation devices requires to reduce the present neutron cross section uncertainties of minor actinides. Purpose: Reduce the $^{243}$Am(n,$gamma$) cross section uncertainty. Method: The $^{243}$Am(n,$gamma$) cross section has been measured at the n_TOF facility at CERN with a BaF$_{2}$ Total Absorption Calorimeter, in the energy range between 0.7 eV and 2.5 keV. Results: The $^{243}$Am(n,$gamma$) cross section has been successfully measured in the mentioned energy range. The resolved resonance region has been extended from 250 eV up to 400 eV. In the unresolved resonance region our results are compatible with one of the two incompatible capture data sets available below 2.5 keV. The data available in EXFOR and in the literature has been used to perform a simple analysis above 2.5 keV. Conclusions: The results of this measurement contribute to reduce the $^{243}$Am(n,$gamma$) cross section uncertainty and suggest that this cross section is underestimated up to 25% in the neutron energy range between 50 eV and a few keV in the present evaluated data libraries.
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