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Register a new userThe 14C(n,g) cross section between 10 keV and 1 MeV
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The neutron capture cross section of 14C is of relevance for several nucleosynthesis scenarios such as inhomogeneous Big Bang models, neutron induced CNO cycles, and neutrino driven wind models for the r process. The 14C(n,g) reaction is also important for the validation of the Coulomb dissociation method, where the (n,g) cross section can be indirectly obtained via the time-reversed process. So far, the example of 14C is the only case with neutrons where both, direct measurement and indirect Coulomb dissociation, have been applied. Unfortunately, the interpretation is obscured by discrepancies between several experiments and theory. Therefore, we report on new direct measurements of the 14C(n,g) reaction with neutron energies ranging from 20 to 800 keV.
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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.
We have carried out photodisintegration cross-section measurements on 86Kr using monoenergetic photon beams ranging from the neutron separation energy, S_n = 9.86 MeV, to 13 MeV. We combine our experimental 86Kr(g,n)85Kr cross section with results from our recent 86Kr(g,g) measurement below the neutron separation energy to obtain the complete nuclear dipole response of 86Kr. The new experimental information is used to predict the neutron capture cross section of 85Kr, an important branching point nucleus on the abundance flow path during s-process nucleosynthesis. Our new and more precise 85Kr(n,g)86Kr cross section allows to produce more precise predictions of the 86Kr abundance from s-process models. In particular, we find that the models of the s-process in asymptotic giant branch stars of mass < 1.5 Msun, where the 13C neutron source burns convectively rather than radiatively, represent a possible solution for the highest 86Kr/82Kr ratios observed in meteoritic stardust SiC grains.
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.
As suggested in a Comment by Peters, Phys. Rev. C {bf 96}, 029801 (2017), a correction is applied to the $^{13}$C($alpha$,n)$^{16}$O data of Harissopulos {it et al.}, Phys. Rev. C {bf 72}, 062801(R) (2005). The correction refers to the energy-dependent efficiency of the neutron detector and appears only above the ($alpha$,n$_1$) threshold of the $^{13}$C($alpha$,n)$^{16}$O reaction at about $E_alpha approx 5$ MeV. The corrected data are lower than the original data by almost a factor of two. The correction method is verified using recent neutron spectroscopy data and data from the reverse $^{16}$O(n,$alpha$)$^{13}$C reaction.
The Coulomb Dissociation (CD) cross sections of the stable isotopes 92,94,100Mo and of the unstable isotope 93Mo were measured at the LAND/R3B setup at GSI Helmholtzzentrum fur Schwerionenforschung in Darmstadt, Germany. Experimental data on these isotopes may help to explain the problem of the underproduction of 92,94Mo and 96,98Ru in the models of p-process nucleosynthesis. The CD cross sections obtained for the stable Mo isotopes are in good agreement with experiments performed with real photons, thus validating the method of Coulomb Dissociation. The result for the reaction 93Mo(g,n) is especially important since the corresponding cross section has not been measured before. A preliminary integral Coulomb Dissociation cross section of the 94Mo(g,n) reaction is presented. Further analysis will complete the experimental database for the (g,n) production chain of the p-isotopes of molybdenum.
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