Radiative Double Electron Capture (RDEC) of Ions with Quasi-free Electrons

一张流行的原子图像显示，电子围绕原子核旋转，就像行星绕太阳运行一样。为了改进这个类比，我们必须了解原子中的电子如何由于负电荷而相互排斥，以及电子如何遵守量子力学定律。这个项目研究电子之间的相互作用如何使它们的运动变得相互关联。这一点很重要，因为这种相关性对原子、分子和材料的物理和化学性质有很大影响。为了研究电子关联，这个项目将观察两个电子从一个原子转移到一个离子，同时发射一个X射线光子的情况。当这种情况发生时，整个过程可以被认为是单光子双电离(移除两个电子)的时间逆过程。这与1905年阿尔伯特·爱因斯坦最初解释的光电效应有关。然而，由于电子相互作用的方式，双光电离的速度很难预测。当两个电子转移到一个运动的弹丸上时，类似的物理学也很重要。科学家们有兴趣了解这一过程，因为它发生在投射离子与目标原子之间的碰撞中，以及单个光子对原子进行双光电离的时间反转过程中。这两个过程都可能发生在天体物理等离子体中。对两电子过程的概率进行了计算，结果大相径庭，为解决这些差异留下了实验空间。由于观察事件所需的测量时间较长，测量过程的尝试一直非常困难。博士生参与这一研究项目，为他们提供了最先进的物理实验方面的宝贵培训，以及准备合作手稿和摘要的经验，并允许他们参加会议报告结果，并在他们成长为多产的年轻科学家时帮助他们。伴随着单光子发射的两个电子到一个离子的转移被称为辐射双电子捕获，简称RDEC。这一提议的学术价值在于它在离子与准自由电子碰撞领域的内在重要性，以及它与光子与高电荷态离子相互作用的密切关系。这种关系的产生是因为RDEC是双光电离的时间逆。当入射完全剥离离子和双电子体系发生RDEC时，这些过程基本上是相同的，在这种情况下，两电子之间的相互作用，即关联，起着根本的作用。RDEC类似于通常观察到的辐射电子俘获(REC)的单电子过程，这与光电效应相反，但不需要电子关联。RDEC这个项目中关于氦、氮和霓气靶的新研究将解决先前发表的数据的复杂性，这些数据涉及氧和氟离子入射到薄的C-箔靶上的离子-固体相互作用。气体目标不存在多重碰撞效应，而固体目标则不存在多重碰撞效应。RDEC可以与几个已发表的理论计算进行比较，然而，这些计算有几个数量级的差异。测量工作正在进行中(已经获得了一些氮靶和霓虹靶的初步数据)。使用西密歇根大学的6-MV串列Van de Graaff加速器获得完全剥离的氟离子。记录了发射的X射线和单电荷和双电荷的弹丸之间的重合，后者代表了RDEC的签名。由于RDEC截面相对较小(不到~1个谷仓)，测量非常耗时，每个靶标需要一个月或更长时间的连续计数。这些研究是由国际和平研究所、联合国际和两名研究生进行的，还包括来自圣母大学和位于波兰克拉科夫的雅盖隆大学的合作者。

A popular image of atoms shows electrons circling the nucleus like planets orbiting the sun. To improve upon this analogy, one must understand how electrons in atoms repel each other due to their negative charges, and how electrons obey the laws of quantum mechanics. This project studies how interactions between electrons causes their motion to become correlated. This is important because such correlations have a big impact on the physical and chemical properties of atoms, molecules, and materials. To investigate electron correlations, this project will observe situations in which two electrons are transferred from one atom to an ion and an x-ray photon is simultaneously emitted. When this happens, the entire process can be thought of as the time inverse of double ionization (removal of two electrons) by a single photon. This is related to the photoelectric effect, which was originally explained by Albert Einstein in 1905. However, the rate for double photoionization is difficult to predict because of the way electrons interact with one another. Similar physics is important when two electrons are transferred to a moving projectile. Scientists are interested in understanding this process as it occurs in collisions between projectile ions and target atoms and from the time-reversed process of double photoionization of an atom by a single photon. Both processes are likely to occur in astrophysical plasmas. Probabilities for the two-electron process have been calculated with widely differing results, leaving the field open for experiments to resolve the differences. Attempts to measure the process have been very difficult due to the long measuring times required to observe the events. The involvement of Ph.D. students in this research project gives them valuable training in a state-of-the-art physics experiment and experience in preparing collaborative manuscripts and abstracts and allows them to attend conferences to report the results and assists them as they develop into productive young scientists.The transfer of two electrons to an ion accompanied by the emission of a single photon is called radiative-double-electron capture, or RDEC for short. The intellectual merit of this proposal lies in its intrinsic importance in the field of ion collisions with quasi-free electrons, and its close relationship to photon interactions with highly-charged ions. This relationship comes about because RDEC is the time inverse of double photoionization. These processes are essentially identical when RDEC occurs for incident fully-stripped ions and double photoionization for two-electron systems, in which case the interaction between the two electrons, i.e., correlation, plays a fundamental role. RDEC is similar to the commonly observed one-electron process of radiative-electron capture (REC), the inverse of the photoelectric effect, which, however, does not require electron correlation. New studies in this project of RDEC for gas targets of helium, nitrogen and neon will address complications of previously published data involving ion-solid interactions for oxygen and fluorine ions incident on a thin C-foil target. Multiple collision effects are not present for gas targets as they are for solid targets. RDEC can be compared with several published theoretical calculations, which, however, differ by several orders of magnitude. The measurements are being done (Some preliminary data have already been obtained for nitrogen and neon targets.) for fully-stripped fluorine ions using the 6-MV tandem Van de Graaff accelerator at Western Michigan University. Coincidences between x rays emitted and singly- and doubly-charged projectiles are recorded, with the latter representing a signature for RDEC. The measurements are very time-consuming due to the relatively small RDEC cross sections (less than ~1 barn) and require continuous counting times of a month or more for each target. The studies are carried out by the PI, the co-PI, and two graduate students and also include collaborators from the University of Notre Dame and Jagiellonian University in Krakow (Poland).