Circular Polarization Measurement of Gamma Rays Following Beta Decay ofAl24andNa24to Determine the Meson Exchange Effect

Abstract

We have measured and compared the $\beta$ circularly polarized $\gamma$ angular correlations for ${\mathrm{Al}}^{24}$ and ${\mathrm{Na}}^{24}$, as suggested by Bouchiat, in order to determine the size of the Fermi matrix elements $M_{\pi }$ due to meson exchange and $a_0\mathrm{}\left(1\right)\mathrm{}\sqrt{2}$ due to isospin impurities. The polarimeter was a cylindrical magnet that produces 19000 G in the form of a square wave oscillating at 140 cps. At this frequency the 2.1-sec half-life presents no new problem except one of source handling. A pneumatic tube was used to transport sources between the polarimeter and the UCLA cyclotron internal beam. The results (using superscripts + and - to denote ${\mathrm{Al}}^{24}$ and ${\mathrm{Na}}^{24}$, respectively) are $A^{+}$ (the circular polarization asymmetry parameter) = -0.084±0.054, $A^{-}=0.091\mathrm{}\pm 0.017$, ${M_F}^{+}=(0.06\mathrm{}\pm 5.4)\times {10}^{-3\mathrm{}}$, and ${M_F}^{-}=(0.8\mathrm{}\pm 1.7)\times {10}^{-3\mathrm{}}$. From $M_{\pi }=\frac{1}{2}({M_F}^{-}-{M_F}^{+})$ and $a_0\mathrm{}\left(1\right)\mathrm{}\sqrt{2}=\frac{1}{2}({M_F}^{-}+{M_F}^{+})$, we get $M_{\pi }=(0.4\mathrm{}\pm 3)\times {10}^{-3\mathrm{}}$ and $a_0\mathrm{}\left(1\right)\mathrm{}\sqrt{2}=(0.4\mathrm{}\pm 3)\times {10}^{-3\mathrm{}}$ in this isobaric triplet. With the CVC conserved vector-current hypothesis $M_{\pi }$ should be zero.