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First Excited State of B 11 : Possibility of Spin-Flip Stripping

1957; American Institute of Physics; Volume: 105; Issue: 2 Linguagem: Inglês

10.1103/physrev.105.666

1536-6065

D. H. Wilkinson,

Boron Compounds in Chemistry

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 ${\mathrm{B}}^{11}(p, {p}^{\ensuremath{'}}){\mathrm{B}}^{11*}$ 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 \textonehalf{} as is suggested by the shell model. This assignment is supported by certain gamma-ray studies in the reaction ${\mathrm{Li}}^{7}(\ensuremath{\alpha}, \ensuremath{\gamma}){\mathrm{B}}^{11}$. 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 $l=1$ stripping pattern is found, suggesting $\frac{3}{2}<~J<~\frac{9}{2}$. The observed isotropies could be given by emission from a state of $\frac{3}{2}\ensuremath{-}$ to the $\frac{3}{2}\ensuremath{-}$ ground state if the gamma ray were purely $E2$. In the present investigation it is shown by a Doppler shift method using ${\mathrm{B}}^{11}(p, {p}^{\ensuremath{'}}){\mathrm{B}}^{11}$ that the lifetime of the first excited state is less than 4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$ sec. It is shown that this limit is inconsistent with $E2$ sum rules and therefore that the transition is chiefly $M1$ and that the spin of the level must be \textonehalf{}-. 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.