Interaction between paramagnetic centers at surface and positroniums

Angular correlation of positron-annihilation radiation from silica aerogel was measured in the temperature range between 30 and 295 K. The angular correlation of annihilation radiation (ACAR) curve can be resolved into two components. The broad component was mainly due to the positrons annihilating with electrons bound in SiO2. Pick-off annihilation of a positronium atom also contributed to this component. The narrow component at the center of the ACAR curve was dur to the self-annihilation of parapositronium atoms. On cooling the sample at 30 K, the intensity of the narrow component increased, but the broad component was almost unchanged. This result indicated that at 30 K orthopositronium atoms converted into parapositroniums which annihilated into 2γ and contributed to the ACAR curve, while at 295 K they annihilated into 3γ and did not contribute to the ACAR curve. The origin of the ortho-para conversion should be spin-exchange collision between positroniums and paramagnetic centers induced by positron irradiation. The disappearance of this effect at 295 K was due to thermal annealing of these centers. The model of the paramagnetic centers was determined from the measurements by ESR. (Y.K.)