Lifetime of the 1.52-MeV Level inCa42

Nuclear resonance fluorescence from the first excited state of ${\mathrm{Ca}}^{42}$ has been observed. The ${\mathrm{K}}^{42}$ beta decay provided the recoil needed to restore the resonance condition. To fully utilize this recoil, a gaseous source of ${\mathrm{K}}^{42}$ iodide was used. The measured cross section for resonance scattering corresponds to a width $\Gamma =(4.8\mathrm{}\pm 1.1)\times {10}^{-4\mathrm{}}$ eV for the 1.52-MeV $2^{+}$ level in ${\mathrm{Ca}}^{42}$, or a mean life of 1.4×${10}^{-12\mathrm{}}$ sec. In arriving at this result, the coefficient $\lambda$ in the beta-neutrino angular correlation function $W\left(\theta \right)=1+\lambda \left(\frac{v}{c}\right)cos\theta$ was assumed to have the value $\frac{1}{3}$. The error quoted above for the width $\Gamma$ takes into account the possibility that $\lambda$ may range from $-\frac{1}{3}$ to +1. The reduced $E2\mathrm{}$ transition probability corresponding to the measured width, $B(E2, 2\to 0)=74e^2{F}^4$, is eight times larger than the Weisskopf estimate, and more than 20 times larger than the prediction of a recent calculation by Bertsch.