Positron spin-relaxation (e(+)SR) study of carbon phases, SiC, and fused quartz

The positron spin-relaxation (e(+)SR) method permits the detection and inve stigation of positronium (Ps) in condensed matter with very small formation probabilities. In a set-up that makes use of spin-polarized positrons emit ted from a Ge-68/Ga-68 source, the effect of reversals of an external magne tic field applied longitudinally to the positron spin polarization on the D oppler broadening of the annihilation photon line is measured. For the firs t time, the fractions r of Ps-forming positrons and normalized electron den sities K at the positron in Ps have been determined for natural type-IIa di amond (r = 0.07, K = 1) and crystalline SiC (r = 0.04, K = 0.15). The two m aterials show a hitherto not observed anomalous dependence of the Doppler b roadening on the magnetic field strength that indicates that a large Ps fra ction annihilates before becoming thermalized. Fused quartz (r = 0.67, K = 1.4), C-60 fullerite (r = 0.03, K = 0.3), and highly orientated pyrolytic g raphite (r = 0.01, K = 0.7) showed the 'normal' field dependence.