Low-Lying Levels inNa25

Abstract

Information about the 90-, 1068-, 2201-, 2417-, 2788-, 2914,- and 3353-keV levels in ${\mathrm{Na}}^{25}$ has been obtained by measuring $\gamma$-ray angular correlations in a collinear geometry. The ${\mathrm{Mg}}^{26}(t,\alpha ){\mathrm{Na}}^{25}$ reaction was used to populate the levels of ${\mathrm{Na}}^{25}$ at an incident triton energy of 2.6 MeV. NaI (Tl) detectors were used for $\gamma$-ray spectroscopy. $\gamma$-ray angular correlations were obtained by measuring the yield of $\gamma$ rays at angles between 0° and 90° in coincidence with reaction-produced $\alpha$ particles detected near 180° with respect to the incident-beam direction. The $\alpha -\gamma$ coincidence data were collected employing two-parameter analysis. $\gamma$-ray branching data were obtained for all the levels mentioned above, and angular correlations were extracted from these data for $\gamma$ rays from the decay of the 1068-, 2201-, 2914-, and 3353-keV levels. Analysis of these data resulted in a definite spin assignment for only one level: $J=\frac{3}{2}$ for the 2201-keV level. The spin of the 90-keV level was restricted to either $J=\frac{3}{2} or \frac{5}{2}$, while the 2914- and 3353-keV levels have spin $J\ge \frac{3}{2}$. The angular correlation of the 979-keV $\gamma$ ray following the decay of the 1068-keV level was found to be isotropic, supporting the $J=\frac{1}{2}$ assignment to this level. More information about the $\gamma$-ray branching modes of these levels was obtained using the ${\mathrm{Na}}^{23}(t,p\gamma ){\mathrm{Na}}^{25}$ reaction. A Ge (Li) $\gamma$-ray spectrometer located at 90° to the beam direction was used to measure $\gamma$-ray pulse-height distributions coincident with reaction-produced protons. Two-parameter techniques were used. Intensity ratios $\frac{{\gamma }_0}{{\gamma }_1}$ of $\gamma$-ray transitions to the ground and the first excited states were extracted from these spectra; $\frac{{\gamma }_0}{{\gamma }_1}$ was found to be 0.13±0.01, 2.6±0.2,>12, >50, ${\mathrm{Na}}^{25}$. Measured $\gamma$-ray energies are compared with transition energies computed from ${\mathrm{Na}}^{25}$ energy levels quoted by Hinds, Marchant, and Middleton; good agreement was obtained, and as a result, we adopt level energies (in keV) for ${\mathrm{Na}}^{25}$ of 90±1, 1068±2, 2201±2, 2417±3, 2788±3, 2914±4, 3353±3, 3455±7, 3685±4, 3928±7, 3950±4, and 3995±5. An upper limit ${\tau }_m\le 20\times {10}^{-9\mathrm{}}$ sec was determined for the lifetime of the levels mentioned above. Several of these low-lying states are tentatively described in terms of the shell model.