Study of positronium-gas molecule interaction using silica aerogel

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Silica aerogel is a porous material composed of a random three-dimensional network of SiO_2 fine grains whose mean diameter is about 50A. When energetic positrons are injected into this material, the grains function as positron-moderation and positronium (Ps)-formation medium, and large free space outside the grains provides a micro chamber for free motions of the Ps atoms. By introducing a gas into this space, therefore, one can investigate Ps-gas molecule for the first time in a systematic way. This possibility was recently pointed out by Hyodo et al. of our group, and verified by their pioneering experiments on Ps-O_2 scattering and so on.In the present wori, we have applied the same method to the study of Ps scattering by various gas molecules after improving our apparatus by equipping it with an electromagnet to control ortho-para conversion process of the Ps atoms. Our main results are as follows:1. Formation cross section of para-Ps in Xenon was found to be consistent with the prediction of the Ore model, thus solving a long puzzle concerning the Ps formation in rare gases.2. It has been concluded from the careful observation of Ps slowing down in gases that the Ps kinetic energy is transferred only to the translational motion of the gas molecules via elastic collisions, and that the excitations of vibrations and rotations of the molecules are negligibly small. This result is understandable if one noticesthat Ps atoms are electrically neutral, and hence the interaction between Ps and gas molecules cannot be a long-ranged one.