Behavior of Pionic Hydrogen Atoms in Gas Phase

负π介子的分子和原子俘获过程至今尚未完全了解，尤其是在含氢分子中。被氢捕获的π介子转移到其他原子在整个过程中起着重要作用，尽管它使过程复杂化，并且使机制难以理解。π介子转移将受到Z-H键的化学状态和π介子氢原子从键中分离之间的关系的显著影响，因为转移被认为仅发生在激发的π介子氢原子上。最近，我们选择性地测量了分子中特定氢原子上的π介子捕获概率，使用醇和羧酸的氘代化合物[Phys.Rev.Lett.，76，2460（'96）]，并提出了一个包含大分子模型和外转移过程的组合模型[Phys.Rev.A，53，130（' 96）].本文采用气相实验研究了不考虑分子间相互作用的情况下，π-氢原子的行为（俘获、形成、分离和衰变或外转移过程）.用一种新设计的铍制高压气室测量了气体样品中的π X射线和π ^0衰变γ射线。样品为CH_4/C_2H_4/C_2H_6/H_2和稀有气体/SF_6的混合气体。目前正在高能物理国家实验室12 GeV质子同步加速器的pimu通道进行测量。

Molecular and atomic capture process of negative pions has not yet been completely understood, especially in hydrogen-containing molecules. The transfer of pions captured by hydrogen to other atoms plays an important role in the overall process, although it complicates the process and makes the mechanism hard to understand. The pion transfer will be considerably affected by the relation between the chemical state of the Z-H bond and the isolation of a pionic hydrogen atom from the bond, because the transfer is considered to occur only from an excited pionic hydrogen atom.Recently we selectively measured the pion capture probability on a particular hydrogen atom in the molecule using deuterated compounds for alcohols and carboxylic acids [Phys.Rev.Lett., 76,2460 ('96)], and proposed a combined model which involves a large mesomolecular (LMM) model and a external transfer process [Phys.Rev.A,53,130 ('96)].Here we carried out the gas phase experiments to investigate the behavior (the capture, formation, isolation, and decay or external transfer processes) of pionic hydrogen atoms without the effects of the inter-molecular interactions. The pionic X rays and pi^0 decay gamma rays from gas samples were measured with a newly designed high pressure gas chamber made of beryllium. The samples were mixtures of CH_4/C_2H_4/C_2H_6/H_2 and rare gas/SF_6. The measurements are now in progress at the pimu-channel of the 12-GeV proton synchrotron in the National Laboratory for High Energy Physics.