Study of Low-Lying Levels ofAr37

Abstract

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 $^{34}\mathrm{S}(\ensuremath{\alpha},n)^{37}\mathrm{Ar}$ reaction (${Q}_{0}=\ensuremath{-}4.629$ MeV). A 450-\ensuremath{\mu}g/c${\mathrm{m}}^{2}$ CdS target (enriched to 85.61% in $^{34}\mathrm{S}$) evaporated onto a 1-\ensuremath{\mu} Ni foil was bombarded with $\ensuremath{\alpha}$ particles ranging in energy from 7.6 to 8.8 MeV. $\ensuremath{\gamma}$ rays were detected at 0, 37, 45, 55, 90, and 143\ifmmode^\circ\else\textdegree\fi{} to the beam axis with a 30-cc Ge(Li) detector. The following mean lifetimes were found: $\ensuremath{\tau}(1.41\ensuremath{-}\mathrm{M}\mathrm{e}\mathrm{V}\phantom{\rule{0ex}{0ex}}\mathrm{l}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{l})={1.08}_{\ensuremath{-}0.13}^{+0.17}$ psec, $\ensuremath{\tau}(2.22)=380\ifmmode\pm\else\textpm\fi{}50$ fsec, $\ensuremath{\tau}(2.49)={775}_{\ensuremath{-}89}^{+105}$ fsec, and $\ensuremath{\tau}(2.80)=14\ifmmode\pm\else\textpm\fi{}11$ fsec. Excitation energies for the first five excited states were determined from the ground-state transitions. All five levels were found to undergo $\ensuremath{\gamma}$ decay 100% to the ground state; upper limits were set on other possible transitions. Analysis of the $\ensuremath{\gamma}$-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.