Activation cross section measurements of O-16(n,He-3)C-14 and O-16(n,t)N-14 above neutron thresholds up to 18.1 MeV

Abstract

We have measured the cross sections of the O-16(n,He-3) C-14 and the O-16(n,t) N-14 reactions above neutron thresholds up to 18.1 MeV in an activation experiment. H2O (water) as an 160 target was irradiated with semi-monoenergetic neutrons generated from the Be-9(p,n)B-9 reaction with 20-MeV protons. The neutron flux was obtained with the aid of previous study by Uwamino et al. The (14)c and the tritium activities were measured by using a liquid scintillation counting (LSC) method. The cross sections of O-16(n,He-3) C-14 and O-16(n, t) N-14 measured by using the LSC method for water were 52 16 mb and 0.25 0.11 mb, respectively. While the present value for the cross section of O-16(n,t)N-14 agrees within a factor of four with previous values measured by using the same LSC method at similar neutron energies, the present value for the cross section of 16 0(n '3 He) C-14 turns out to be about 200 times as large as the previous accelerator mass spectrometry (AMS) value obtained by Imamura et al. This large discrepancy may be attributed to the loss Of C-14 atoms during the sample preparation in the AMS method. To further investigate such disagreement, we conducted both LSC and AMS measurements by using C3D6O (d-acetone) liquid targets which are easily combustible for extracting Co-2 for the AMS measurements. The C-14 yield of the LSC method also turned out to be 80 times as large as that of the AMS method. The reliability of the AMS method could be challenged by that of the neutron-activation measurements in view of the fact that the AMS method involves complex sample preparation, such as combustion and cryogenic trapping of CO2.