Nuclear polarization in the B11(d,p)B12 reaction. II

Measurements have been made of the polarization of the B12 recoil nuclei resulting from the B11(d,p)B12 reaction for Ed in the range 0.90 to 3.20 MeV. The polarization is observed by measuring the β-decay asymmetries of the B12 recoil nuclei with respect to the reaction plane. The B12 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 kd×kp. The magnitude of the polarization is strongly dependent on Ed, the recoil emission angle and the material used to stop the recoils. It is found that the polarization of B12 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 B12 recoil polarization plotted as a function of Ed 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 C13 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.