Li 7 ( p , n ) , ( p , p ′ γ ), and ( p , γ ) Reactions near Neutron Threshold

The ${\mathrm{Li}}^{7}(p, n)$ total cross section has been measured in detail from threshold to 2.5-Mev proton energy. Between 1.88 and 1.92 Mev the cross section follows the equation for a very broad ${2}^{\ensuremath{-}}$ resonance and for $s$-wave particles: ${\ensuremath{\sigma}}_{\mathrm{pn}}=\frac{(\frac{5}{2})\ensuremath{\pi}{\ensuremath{\lambda}}^{2}x}{{(1+x)}^{2}}$, where $x=\frac{{\ensuremath{\Gamma}}_{n}}{{\ensuremath{\Gamma}}_{p}}=1$ at 50 kev above threshold. The remainder of the cross section up to 2.35 Mev is fitted by this level and the ${3}^{+}$ level at 2.25 Mev. The neutron reduced width exceeds that of the proton by a factor of about five for both the ${2}^{\ensuremath{-}}$ and ${3}^{+}$ levels; this is surprising for the self-mirror nucleus ${\mathrm{Be}}^{8}$. A small dip is observed just above neutron threshold in the yield of 478-kev gamma rays from ${\mathrm{Li}}^{7}(p, {p}^{\ensuremath{'}}\ensuremath{\gamma})$. The shape of these data support the assumption of the broad ${2}^{\ensuremath{-}}$ neutron producing resonance with ${\ensuremath{\Gamma}}_{p}g0.5$ Mev. No anomaly is observed in the yield of Li capture gamma rays at neutron threshold; the 2.10-Mev resonance in this reaction (which probably is due to a ${3}^{\ifmmode\pm\else\textpm\fi{}}$ level) yields mainly 16-Mev gamma rays leading to the 2.9-Mev ${2}^{\ifmmode\pm\else\textpm\fi{}}$ level in ${\mathrm{Be}}^{8}$. The ratio of 16 to 19 Mev gamma rays is $\frac{3}{2}$ at both 1.7- and 2.5-Mev proton energy. The behavior of the ${\mathrm{Li}}^{7}(p, n)$ extrapolated threshold was calculated as a function of target thickness and proton energy spread; shifts in the extrapolated thresholds and the distances between them and true threshold were less than one kev. The extrapolated threshold of a \textonehalf{}-kev target taken with a proton energy spread of 1 kev was observed to be \textonehalf{} kev lower in proton energy than a thick-target threshold.