Thermal-neutron capture by 14N

Abstract

The energies and intensities of 58 \ensuremath{\gamma} rays emitted in thermal-neutron capture by nitrogen (99.63% ${}^{14}\mathrm{N}$) have been measured accurately. A major reason was to establish this reaction as a standard for similar measurements on other nuclides. These \ensuremath{\gamma} rays have been placed between 19 known levels (including the ground state and the capturing state) in ${}^{15}\mathrm{N}.$ The primary \ensuremath{\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 ${}^{15}\mathrm{N}$ 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 $29\ifmmode\pm\else\textpm\fi{}2\mathrm{keV}$ below the neutron separation energy was considered. Some of the properties of this level are quite well known from the ${}^{14}\mathrm{C}(p,\ensuremath{\gamma})$ 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 \ensuremath{\gamma}-ray spectrum is different from the proton-capture \ensuremath{\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 $\ensuremath{-}29\mathrm{keV}.$ 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 $\ensuremath{-}29\ensuremath{-}\mathrm{keV}$ level are: total radiation $\mathrm{w}\mathrm{i}\mathrm{d}\mathrm{t}\mathrm{h}=565\ifmmode\pm\else\textpm\fi{}24\mathrm{}\mathrm{meV},$ $\mathrm{reduced}\mathrm{}\mathrm{neutron}\mathrm{}\mathrm{w}\mathrm{i}\mathrm{d}\mathrm{t}\mathrm{h}=51.6\ifmmode\pm\else\textpm\fi{}0.3\mathrm{}\mathrm{keV}$ (for a channel radius of 3.5 fm), and proton $\mathrm{w}\mathrm{i}\mathrm{d}\mathrm{t}\mathrm{h}=160\ifmmode\pm\else\textpm\fi{}30\mathrm{}\mathrm{meV}.$