First Excited State ofB11: Possibility of Spin-Flip Stripping

The first excited state of ${\mathrm{B}}^{11}$ at 2.14 Mev emits gamma rays which have an isotropic distribution when excited over a wide range of energy in the reaction ${\mathrm{B}}^{11}(p, p^{\prime }){\mathrm{B}}^{11*}$ and an isotropic correlation in the reaction ${\mathrm{B}}^{10}(d, p){\mathrm{B}}^{11*}$. The natural assumption is therefore that the state is of spin ½ as is suggested by the shell model. This assignment is supported by certain gamma-ray studies in the reaction ${\mathrm{Li}}^7(\alpha , \gamma ){\mathrm{B}}^{11}$. However, in both the reaction ${\mathrm{B}}^{10}(d, p){\mathrm{B}}^{11}$ and the mirror reaction ${\mathrm{B}}^{10}(d, n){\mathrm{C}}^{11}$ leading to this level or its mirror level in ${\mathrm{C}}^{11}$, an $l=1\mathrm{}$ stripping pattern is found, suggesting $\frac{3}{2}<~J<~\frac{9}{2}$. The observed isotropies could be given by emission from a state of $\frac{3}{2}-$ to the $\frac{3}{2}-$ ground state if the gamma ray were purely $E2\mathrm{}$. In the present investigation it is shown by a Doppler shift method using ${\mathrm{B}}^{11}(p, p^{\prime }){\mathrm{B}}^{11}$ that the lifetime of the first excited state is less than 4×${10}^{-14\mathrm{}}$ sec. It is shown that this limit is inconsistent with $E2\mathrm{}$ sum rules and therefore that the transition is chiefly $M1\mathrm{}$ and that the spin of the level must be ½-. This is consistent with the stripping result only if it is possible for the departing nucleon in such a reaction to flip its intrinsic spin and so transmit a further unit of angular momentum to the nucleus.