Scissors resonance in the quasicontinuum of Th, Pa, and U isotopes

Abstract

The $\ensuremath{\gamma}$-ray strength function in the quasicontinuum has been measured for ${}^{231--233}$Th, ${}^{232,233}$Pa, and ${}^{237--239}$U using the Oslo method. All eight nuclei show a pronounced increase in $\ensuremath{\gamma}$ strength at ${\ensuremath{\omega}}_{\mathrm{SR}}\ensuremath{\approx}2.4$ MeV, which is interpreted as the low-energy $M1$ scissors resonance (SR). The total strength is found to be ${B}_{\mathrm{SR}}=9--11\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{N}^{2}$ when integrated over the 1--4 MeV $\ensuremath{\gamma}$-energy region. The SR displays a double-hump structure that is theoretically not understood. Our results are compared with data from ($\ensuremath{\gamma}$, ${\ensuremath{\gamma}}^{\ensuremath{'}}$) experiments and theoretical sum-rule estimates for a nuclear rigid-body moment of inertia.