Prompt thermal neutron capture $\gamma$-ray cross sections ${\sigma }_{\gamma }$ were measured for the ${}^{23}$Na($n$,$\gamma$) reaction with guided cold neutron beams at the Budapest Reactor. The ${}^{24}$Na $\gamma$-ray cross sections were internally standardized with a stoichiometric NaCl target by using standard ${}^{35}$Cl($n$,$\gamma$)${}^{36}$Cl $\gamma$-ray cross sections. Transitions were assigned to levels in ${}^{24}$Na based primarily upon the known nuclear structure information from the literature, producing a nearly complete neutron capture decay scheme. The total radiative thermal neutron cross section ${\sigma }_0$ was determined from the sum of prompt $\gamma$-ray cross section populating the ground state as 0.540 (3) b, and from the activation $\gamma$-ray cross sections for the decay of ${}^{24}$Na as 0.542 (3) b. The isomer cross section ${\sigma }_0$ (${}^{23}$Na${}^m$, $t_{1/2}=20.20\text{ms})=0.501\left(3\right)$ b and the ${}^{24}$Na neutron separation energy $S_n=6959.352\left(18\right)$ keV were also determined in these experiments. New level spins and parities were proposed on the basis of new transition assignments and the systematics of reduced transition probabilities for the primary $\gamma$ rays.

• Received 16 October 2013

DOI:https://doi.org/10.1103/PhysRevC.89.014617