Spectroscopy of Dense Positronium

正电子是一种奇异的类氢原子，由一个普通电子（所有普通物质的轻质量负电荷成分）与一个带正电荷的电子（电子的反物质孪生体，称为正电子）结合而成。正电子的存在时间很短，最终湮灭成两个或三个伽马射线光子（非常高能的光粒子）的爆发。该项目的主要目标是产生高密度的正电子气体，并将其冷却以产生第一个正电子“超流体”，即玻色-爱因斯坦凝聚物。这种物质被预测会表现出受激湮灭，其中一个湮灭伽马射线光子诱导出完全相同方向和能量的其他光子，这是所有类型的激光作用所必需的基本活动。该项目的第二个目标是获得这种受激湮灭的证据，为第一种高穿透湮灭伽马射线激光器开辟道路。除了对这种现象的科学兴趣外，可能的最终好处包括放射照相和放射治疗的医疗应用，高功率伽马射线束的防御和其他用途，以及伽马射线激光点燃核聚变的可能性，用于清洁，低成本的发电厂。该项目的主要目标是：(1)产生和观察正电子的玻色-爱因斯坦凝聚体(2)获得其双光子湮灭辐射的受激发射的证据。该项目将使用现有的慢速正电子束和磁阱系统，产生约1亿个30%自旋极化正电子的纳秒脉冲。正电子将从陷阱的限制磁场中提取出来，并集中在薄金属膜上直径200微米的点上，该点有效地发射缓慢的正电子。这些将被加速并聚焦到一个5微米的点上，目标上有一个空腔，在这个空腔内高密度正电子将形成并热化到低温。利用现有的探测器，通过湮灭光子对的角相关来测量正电子温度作为时间的函数。凝析油的存在将从其亚热表观温度推断出来。将使用不同的腔体几何形状来实现所需的高正电子密度和足够冷的正电子温度，约为10 K。在制造了第一个正电子玻色-爱因斯坦凝聚体之后，将对其湮灭伽马射线的受激辐射进行研究。除了能够产生第一个伽马射线激光器，这项工作将为其他涉及高密度正电子的课题开辟道路，包括首次产生和研究正电子正离子（两个正电子和一个电子的结合状态），正电子原子激光束的形成，以及在超流体氦-4中产生玻色-爱因斯坦凝聚的正电子气泡。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Positronium is an exotic hydrogen-like atom composed of one ordinary electron (the light mass negatively charged components of all ordinary matter) bound to one positively charged electron, the antimatter twin of the electron, called a positron. Positronium has a brief existence which terminates in its annihilation into a burst of two or three gamma ray photons (very high energy particles of light). The primary goal of the project is to produce a high density positronium gas and cool it to produce the first positronium "superfluid", known as a Bose-Einstein condensate. This substance is predicted to exhibit stimulated annihilation, in which one annihilation gamma ray photon induces the emission of others of exactly the same direction and energy, the essential activity required for all types of laser action. The second goal of the project is to obtain evidence for this stimulated annihilation to open the way for the first highly penetrating annihilation gamma ray lasers. Besides the scientific interest in the phenomena, the possible eventual benefits include medical applications to radiography and radiation therapy, defense and other uses of high-power gamma ray beams, and the possibility of gamma ray laser ignition of fusion for clean, low-cost electric power generating plants. The principle goals of the project are to (1) produce and observe a Bose-Einstein condensate of positronium and (2) to obtain evidence for the stimulated emission of its two-photon annihilation radiation. The project will use an existing slow positron beam and magnetic trap system that produces nanosecond pulses of about 100 million 30% spin-polarized positrons. The positrons will be extracted from the confining magnetic field of the trap and focused in a 200 micron diameter spot on a thin metal film that efficiently emits slow positrons. These will be accelerated and focused to a 5 micron spot onto a target containing cavities within which high density positronium will be formed and thermalized to low temperatures. The positronium temperature will be measured as a function of time by the angular correlation of the annihilation photon pairs using an existing detector. The presence of a condensate will be inferred from its sub-thermal apparent temperature. Various cavity geometries will be used to achieve the required high positronium densities and sufficiently cold positronium temperatures around 10 K. Having made the first positronium Bose-Einstein condensate, a search will be made for the stimulated emission of its annihilation gamma rays. Besides enabling the production of the first gamma ray lasers, this work will open the way for other topics involving high density positrons, including the first production and studies of the positronium positive ion (the bound state of two positrons and one electron), formation of a positronium atom laser beam, and production of Bose-Einstein condensed positronium bubbles in superfluid helium-4.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.