Thermal-neutron capture by N-14

The energies and intensities of 58 gamma rays emitted in thermal-neutron capture by nitrogen (99.63% N-14) have been measured accurately. A major reason was to establish this reaction as a standard for similar measurements on other nuclides. These gamma rays have been placed between 19 known levels (including the ground state and the capturing state) in N-15. The primary gamma rays of both electric dipole (E1) and magnetic dipole (M1) types have been analyzed with existing theories of slow-neutron capture. Unlike many other light nuclides, the cross sections for E1 transitions in N-15 differ drastically from the calculations of pure direct-capture theory. The role of the resonance-capture contribution from the proton-unbound, neutron-bound level at 29+/-2 keV below the neutron separation energy was considered. Some of the properties of this level are quite well known from the C-14(p, gamma) reaction, and others can be derived from an R-matrix analysis of the total cross section as a function of neutron energy. The thermal-neutron capture gamma-ray spectrum is different from the proton-capture gamma-ray spectrum, but if proper account is taken of the interference among the compound-nuclear processes, the valence-neutron mechanism, and potential capture, the data can be satisfactorily explained. In the thermal-neutron reaction, compound-nuclear E1 and direct-capture E1 contributions are of comparable magnitude. Valence-neutron capture forms a significant component of capture by the neutron-bound level at -29 keV. Largely destructive interference between compound-nuclear and valence processes in a few transitions in thermal-neutron capture gives rise to a much smaller total cross section than would be obtained from the compound-nuclear process alone. The M1 transitions also show some evidence of a direct process but not a dominant one. The magnitudes of the compound-nuclear transitions, both E1 and M1, are largely consistent with the values implied by giant resonance theories. The resonance parameters deduced for the -29-keV level are: total radiation width=565+/-24 meV, reduced neutron width=51.6+/-0.3 keV (for a channel radius of 3.5 fm), and proton width=160+/-30 meV.