Ionisation of Atomic Hydrogen and Helium by Low Energy Antiprotons

碰撞过程是基本带电粒子电离简单原子和分子的过程，碰撞过程的实验研究为我们理解多体相互作用提供了必要的数据，并为碰撞动力学中这个基本但尚未解决的问题提供了理论描述。电离过程在天体物理和技术等离子体以及地球大气层中分子的解离中发挥着重要作用。通过比较等速质子、电子及其对应的反质子和正电子反粒子对简单原子和分子靶单电离和多次电离的影响，可以确定入射质量和电荷的影响。用来检验理论的最简单的碰撞系统是原子氢被反质子单次电离。该系统只有一个活跃的电子，一个确切已知的波函数，而且与电子碰撞的情况不同，没有交换效应。此外，反质子不能俘获氢电子，可以使用直线恒速弹射路径。在欧洲核子研究中心低能反质子环(LEAR)的30keV/1 MeV能量范围内，由诺森和奥胡斯大学的同事，包括M·查尔顿教授(目前的申请者之一)领导的PS194合作小组测量了氢原子的单电离截面(没有预期的同位素效应)。结果表明，当反质子速度大于靶电子速度(2.2e8 cm S~(-1)或25keV反质子能量)时，实验数据与理论符合得很好。然而，在较低的能量下，存在显著的差异。正如费米-泰勒机制所预言的那样，大多数理论都显示出一种几乎与能量无关的趋势，即碰撞是准分子性质的，靶电子结合能在一定的临界距离上消失。在低于30kev的能量范围内，迫切需要实验测量来确定这个最基本的碰撞系统的截面的大小和能量相关性。

Experimental studies of collision processes, in which fundamental charged particles ionize simple atoms and molecules, provide the data necessary for our understanding of the many-body interaction and the development a theoretical description of this fundamental yet unsolved problem in collision dynamics. The ionization process plays an important role in astrophysical and technological plasmas and in the dissociation of molecules in the Earth's atmosphere. A comparison of single and multiple ionization of simple atomic and molecular targets by equi-velocity protons, electrons and their corresponding antiproton and positron antiparticles makes it possible to determine the effects of projectile mass and charge. The simplest collision system for testing theory is the single ionization of atomic hydrogen by antiprotons. The system has only one active electron, a precisely known wave function and, unlike in the electron impact case, there are no exchange effects. In addition, the antiproton cannot capture the hydrogen electron and straight-line constant velocity projectile paths may be used. The cross section for single ionization of atomic deuterium (no isotope effects expected) was measured in the energy range 30 keV / 1 MeV at the Low Energy Antiproton Ring (LEAR) at CERN by the PS194 collaborative grouping lead by Knudsen and co-workers from the University of Aarhus and including Prof. M Charlton (one of the present applicants). It was found that for antiproton velocities larger than the target electron velocity (2.2e8cm s-1 or 25 keV antiproton energy) the experimental data and all theories are in agreement, as might be expected. However, at lower energies there are significant discrepancies. Most of the theories show an almost energy independent trend, as predicted by the Fermi-Teller mechanism, in which the collision is of a quasi-molecular nature and the target electron binding energy vanishes at a certain critical distance. Experimental measurements are required urgently in the energy regime below 30 keV to determine the magnitude and energy dependence of the cross section for this most fundamental of collision systems