Li7(p, n), (p, p′γ), and (p, γ) Reactions near Neutron Threshold

Abstract

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 $2^{-}$ resonance and for $s$-wave particles: ${\sigma }_{\mathrm{pn}}=\frac{\left(\frac{5}{2}\right)\pi {\lambda }^{2}x}{{\mathrm{}(1+x)\mathrm{}}^2}$, where $x=\frac{{\Gamma }_n}{{\Gamma }_p}=1\mathrm{}$ 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 $2^{-}$ 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 ${\mathrm{Li}}^7(p, p^{\prime }\gamma )$. The shape of these data support the assumption of the broad $2^{-}$ neutron producing resonance with ${\Gamma }_p>0.5\mathrm{}$ 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 $3^{\pm }$ level) yields mainly 16-Mev gamma rays leading to the 2.9-Mev $2^{\pm }$ 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 ½-kev target taken with a proton energy spread of 1 kev was observed to be ½ kev lower in proton energy than a thick-target threshold.