Combined magnetic dipole and electric quadrupole interaction and radiation damage effects: The Po in Pb system

Time-differential perturbed angular distribution measurements were performed for Po isomers using the (α,xn) reaction with pulsed 25-MeV α beams. Enriched lead targets were maintained at temperatures between 80 K and 575 K. In most measurements, less than the maximal initial nuclear alignment was observed, and a damping of the alignment on a 10-6 s time scale was observed at other than the highest temperatures. A simple model was developed in which the loss of initial alignment is due to strong electric field gradients (EFG) at the Po sites from nearest neighbor lattice defects, produced by the recoiled Po nuclei. The further damping is due mainly to interference of the magnetic interaction with weak quadrupole interactions from radiation damage that is further away from the probe nucleus. Larmor oscillations are confined to a Gaussian-shaped envelope, the width of which depends on the quadrupole interaction strength and the nuclear spin. To test the validity of the calculations, the quadrupole moment of the 54Fe(10+) isomer was measured as Q = 72 +- 7 fm2. The isomer was then implanted into Zn in the presence of an external magnetic field. The perturbation calculation was found to be valid for magnetic fields such that ω/sub L/ greater than or equal to 5 Iω/sub Q/. The formalism was then used to extract for each temperature the average electric field gradientarising from radiation damage. The results indicated that significant annealing takes place in a time short compared to the observation time with the system