Study of Low-Lying Levels ofAr37

The mean lifetimes of the levels of ${}_{}{}^{37}{}_{}{}^{}\mathrm{Ar}$ below 3.0-MeV excitation energy have been measured by applying the Doppler-shift-attenuation method to the ${}_{}{}^{34}{}_{}{}^{}\mathrm{S}(\alpha ,n){}_{}{}^{37}{}_{}{}^{}\mathrm{Ar}$ reaction ($Q_0=-4.629\mathrm{}$ MeV). A 450-μg/c${\mathrm{m}}^2$ CdS target (enriched to 85.61% in ${}_{}{}^{34}{}_{}{}^{}\mathrm{S}$) evaporated onto a 1-μ Ni foil was bombarded with $\alpha$ particles ranging in energy from 7.6 to 8.8 MeV. $\gamma$ rays were detected at 0, 37, 45, 55, 90, and 143° to the beam axis with a 30-cc Ge(Li) detector. The following mean lifetimes were found: $\tau (1.41\mathrm{}-\mathrm{M}\mathrm{e}\mathrm{V}\mathrm{l}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{l})={1.08}_{-0.13\mathrm{}}^{+0.17\mathrm{}}$ psec, $\tau \mathrm{}\left(2.22\right)=380\mathrm{}\pm 50$ fsec, $\tau \mathrm{}\left(2.49\right)={775}_{-89\mathrm{}}^{+105\mathrm{}}$ fsec, and $\tau \mathrm{}\left(2.80\right)=14\mathrm{}\pm 11$ fsec. Excitation energies for the first five excited states were determined from the ground-state transitions. All five levels were found to undergo $\gamma$ decay 100% to the ground state; upper limits were set on other possible transitions. Analysis of the $\gamma$-ray angular distributions for the levels at 1.61 and 2.49 MeV gave spins in agreement with previous assignments. The angular-distribution and lifetime measurements yield a most probable spin-parity assignment of (${\frac{7}{2}}^{+}$) for the 2.22-MeV level. The experimental transition strengths for the positive-parity states are in good agreement with predictions of many-particle shell-model calculations.