Ionisation of Atomic Hydrogen and Helium by Low Energy Antiprotons

碰撞过程的实验研究，其中基本带电粒子电离简单的原子和分子，提供了必要的数据，我们的理解多体相互作用和发展理论描述这一基本的碰撞动力学尚未解决的问题。电离过程在天体物理和技术等离子体以及地球大气中分子的解离中起着重要作用。通过比较等速度质子、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目标的单次和多次电离，可以确定弹丸质量和电荷的影响。检验理论的最简单的碰撞系统是氢原子被反质子单次电离。该系统只有一个活性电子，一个精确已知的波函数，并且与电子碰撞情况不同，没有交换效应。此外，反质子不能捕获氢原子电子，可以采用直线等速抛射路径。在欧洲核子研究中心（CERN）的低能反质子环（LEAR）上，由Knudsen和来自奥胡斯大学的同事（包括M Charlton教授）领导的PS194协作小组在30 keV / 1 MeV的能量范围内测量了原子氘的单电离截面（没有预期的同位素效应）。结果发现，对于反质子速度大于目标电子速度（2.2e8cm s-1或反质子能量为25 keV）的反质子，实验数据和所有理论都是一致的。然而，在较低的能量，有显著的差异。大多数理论表现出几乎与能量无关的趋势，正如费米-泰勒机制所预测的那样，碰撞具有准分子性质，目标电子结合能在一定的临界距离上消失。迫切需要在30 keV以下的能量范围内进行实验测量，以确定这种最基本的碰撞系统的横截面的大小和能量依赖性

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