Mean Lives of States inSi29below 5 MeV

Abstract

The Doppler-shift attenuation method was used to obtain the following information concerning the mean lives of the low-lying states in ${\mathrm{Si}}^{29}$: $\tau (1.27-\mathrm{MeV})=\left({{3.7}_{-0.5\mathrm{}}}^{+0.6\mathrm{}}\right)\times {10}^{-13\mathrm{}}$ sec, $\tau (2.03-\mathrm{MeV})=\left({{3.7}_{-0.6\mathrm{}}}^{+0.7\mathrm{}}\right)\times {10}^{-13\mathrm{}}$ sec, $\tau (2.43-\mathrm{MeV})\le 4.6\times {10}^{-14\mathrm{}}$ sec, $\tau (3.07-\mathrm{MeV})\le 7.4\times {10}^{-14\mathrm{}}$ sec, $\tau (3.62-\mathrm{MeV})=\left({{4.8}_{-1.5\mathrm{}}}^{+3.0\mathrm{}}\right)\times {10}^{-12\mathrm{}}$ sec, $\tau (4.08-\mathrm{MeV})=(7\mathrm{}\pm 2)\times {10}^{-14\mathrm{}}$ sec, $\tau (4.84-\mathrm{MeV})\le 1.3\times {10}^{-14\mathrm{}}$ sec. The results of a series of measurements to test the calculation of the unattenuated Doppler shift are reported. Some information concerning the branching ratios of the states at 4.84, 4.89, and 4.93 MeV in ${\mathrm{Si}}^{29}$ is also presented. A value of ${\frac{7}{2}}^{+}$ is suggested for the spin and parity of the 4.08-MeV state. Some of the results are compared to a simple pure-rotational-model interpretation of ${\mathrm{Si}}^{29}$.