Behavior of Pionic Hydrogen Atoms in Gas Phase

负子的分子和原子俘获过程尚未完全了解，尤其是在含氢分子中。被氢俘获的子子转移到其他原子的过程在整个过程中起着重要作用，尽管它使这个过程变得复杂，并使机制难以理解。Z-H键的化学态与质子氢原子与键的分离程度之间的关系对π转移有很大的影响，因为这种转移被认为只发生在激发态的质子氢原子上。最近，我们使用醇和羧酸的重氢化合物选择性地测量了分子中特定氢原子上的π俘获几率[Phys.Rev.Lett，76,2460(‘96)]，并提出了一个包含大分子介观(LMM)模型和外部转移过程的组合模型[Phys.Rev.A，53,130(‘96)]。在这里，我们进行了气相实验，以研究在不受分子间相互作用影响的情况下，质子氢原子的行为(捕获、形成、隔离、衰变或外部转移过程)。用新设计的铍高压气室测量了来自气体样品的离子X射线和pi^0衰变伽马射线。这些样品是CH_4/C_2H_4/C_2H_6/H_2和稀有气体/SF_6的混合物。目前正在高能物理国家实验室12 GeV质子同步加速器的PIMU通道上进行测量。

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.