Relative Yields of Neutron Groups from the Li7(p, n)Be7, Be7* Reactions

Abstract

The relative yields of the two groups of neutrons from the ${\mathrm{Li}}^7(p, n){\mathrm{Be}}^7, {\mathrm{Be}}^{7*}$ reaction, leading to the ground state and the 430-kev state of ${\mathrm{Be}}^7$, have been measured with a time-of-flight system, using pre-acceleration pulsing of the accelerator beam. Data were taken at 30° intervals between 0° and 150° for proton energies high enough to produce ($p, n^{\prime }$) neutrons above the detection threshold (300 kev). Since the yield of the ($p, n$) reaction is more highly peaked in the forward direction than that of the ($p, n^{\prime }$) reaction, the ratio of the ($p, n^{\prime }$) to ($p, n$) intensities grows with increasing angle, severely limiting the usefulness of the ($p, n$) reaction as a neutron source above the ($p, n^{\prime }$) threshold at back angles. Absolute differential and total cross sections for both groups have been calculated from the data. A comparison with theory for total cross sections and angular distributions suggests the existence of three previously unidentified levels in ${\mathrm{Be}}^8$. One level, with $J^{\pi }=1^{-}$, near the threshold for the ($p, n^{\prime }$) reaction, is responsible for the fast rise in the ($p, n^{\prime }$) total cross section near threshold. A second level, corresponding to an incident proton energy of about 3.0 Mev, does not contribute significantly to the yield of the ($p, n^{\prime }$) reaction; the data are consistent with an assignment of $J^{\pi }=1^{+}$ and a total width of 1 Mev. The bulk of the total cross-section curve for the ($p, n^{\prime }$) reaction has been fitted by assuming a $1^{+}$ level corresponding to an incident proton energy of 3.5 Mev, with ${{\gamma }_n}^2\simeq {{\gamma }_p}^2$, ${{\gamma }_{n^{\prime }}}^2\simeq 5{{\gamma }_p}^2$, and ${{\gamma }_{p^{\prime }}}^2\ll {{\gamma }_p}^2$.