Study of positronium-gas molecule interaction using silica aerogel

SiO_2气凝胶是一种由平均粒径约为50的SiO_2微粒组成的无规三维网络结构的多孔材料。当高能正电子注入这种材料时，颗粒起到正电子慢化和正电子素（Ps）形成介质的作用，颗粒外部的大自由空间为Ps原子的自由运动提供了微腔室。因此，通过将气体引入这个空间，人们可以首次以系统的方式研究Ps气体分子。这种可能性最近由本课题组的Hyodo等人提出，并由他们的Ps-O_2散射等开创性实验所证实，本文在改进了我们的装置后，用电磁铁控制Ps原子的邻位-对位转换过程，将同样的方法应用于各种气体分子的Ps散射研究。主要结果如下：1.发现氙中对位Ps的形成截面与Ore模型的预测一致，从而解决了关于稀有气体中Ps形成的长期难题.通过仔细观察Ps在气体中的减速，我们可以得出结论，Ps的动能通过弹性碰撞只传递给气体分子的平移运动，分子的振动和旋转的激发小到可以忽略不计。如果我们认为Ps原子是电中性的，那么Ps与气体分子之间的相互作用就不可能是长程相互作用，那么这个结果是可以理解的。

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.