Role of multiparticle-multihole states ofO18,19inO18(n,γ)O19reactions at keV energy

Abstract

We have determined for the first time the partial-capture cross sections for discrete $\ensuremath{\gamma}$ rays from neutron capture by $^{18}\mathrm{O}$ to the $5/{2}^{+}$ground, $1/{2}^{+}$second, and $3/{2}^{+}$ first excited states in $^{19}\mathrm{O}$ at $10\ensuremath{\leqslant}{E}_{n}\ensuremath{\leqslant}80$ keV. The first two cross sections agree with the calculated ones based on a nonresonant direct $p$-wave capture by $^{18}\mathrm{O}$ within a factor of two. On the other hand, the cross section to the $3/{2}^{+}$ state with a three-particle $({d}_{5/2}){}^{3}$configuration was found to be about 180 times larger than the calculated one. We discussed a possible origin of the large discrepancy by referring to recent results of $^{18}\mathrm{O}$($n,\ensuremath{\gamma}$)$^{19}\mathrm{O}$ at thermal energy.