Nuclear Polarization in theB11(d,p)B12Reaction. II

Measurements have been made of the polarization of the ${\mathrm{B}}^{12}$ recoil nuclei resulting from the ${\mathrm{B}}^{11}\mathrm{}(d,p)\mathrm{}{\mathrm{B}}^{12}$ reaction for $E_d$ in the range 0.90 to 3.20 MeV. The polarization is observed by measuring the $\beta$-decay asymmetries of the ${\mathrm{B}}^{12}$ recoil nuclei with respect to the reaction plane. The ${\mathrm{B}}^{12}$ recoil nuclei formed in the ground state are separated kinematically from those in excited states and are then stopped in various solids in the presence of a small magnetic field normal to the reaction plane. The sign of the observed polarization for all measurements is in the direction of the vector ${\mathbf{k}}_d\times {\mathbf{k}}_p$. The magnitude of the polarization is strongly dependent on $E_d$, the recoil emission angle and the material used to stop the recoils. It is found that the polarization of ${\mathrm{B}}^{12}$ nuclei can be maintained in either metallic Pd or Au at room temperature, using holding fields smaller than 20 G. For recoils stopped in Pt metal, a magnetic field of about 200 G is required to maintain the nuclear polarization. The ${\mathrm{B}}^{12}$ recoil polarization plotted as a function of $E_d$ shows definite structure. Polarization resonances are observed at 1.5, 2.1, and 3.0 MeV. At these energies the measured relative polarizations are +(8.01±0.30)%, +(4.29±0.30)%, and +(3.86±0.27)%, respectively, for a Pt metal recoil stopper and a 49° (lab) recoil angle. These resonances are tentatively identified with states of the ${\mathrm{C}}^{13}$ compound nucleus near 20.0 MeV and at 20.52 and 21.28 MeV. It is believed that the polarization resonances may result from an interference between direct interaction and two-particle compound resonance processes.