Spectroscopy of Dense Positronium

电子偶素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带负电荷的成分）与一个带正电荷的电子（电子的反物质孪生子，称为正电子）结合而成。电子偶素存在时间很短，最后湮灭成两到三个伽马射线光子（非常高能量的光粒子）。该项目的主要目标是产生高密度的正电子素气体，并将其冷却以产生第一个正电子素“超流体”，称为玻色-爱因斯坦凝聚体。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会引起方向和能量完全相同的其他光子的发射，这是所有类型的激光作用所需的基本活动。该项目的第二个目标是获得这种受激湮灭的证据，为第一个高度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之外，可能的最终好处包括放射线照相术和放射治疗的医学应用，高功率伽马射线束的国防和其他用途，以及伽马射线激光点燃清洁，低成本发电厂的聚变的可能性。该项目的主要目标是：（1）产生和观察电子偶素的玻色-爱因斯坦凝聚体;（2）获得其双光子湮灭辐射受激发射的证据。该项目将使用现有的慢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，并聚焦在一个直径为200微米的薄金属膜上，有效地发射慢正电子。这些将被加速并聚焦到一个5微米的点上，在一个包含空腔的靶上，在空腔内将形成高密度的正电子素并热化到低温。正电子偶素的温度将被测量为时间的函数的湮灭光子对使用现有的探测器的角度相关。冷凝物的存在将从其亚热表观温度推断出来。将使用各种腔的几何形状来实现所需的高正电子素密度和大约10 K的足够冷的正电子素温度。在制造了第一个正电子素玻色-爱因斯坦凝聚体之后，将对其湮灭伽马射线的受激发射进行研究。除了能够生产第一个伽马射线激光器外，这项工作还将为涉及高密度正电子的其他主题开辟道路，包括第一个生产和研究正电子素正离子（两个正电子和一个电子的束缚态），形成正电子素原子激光束，以及超流氦中玻色-爱因斯坦凝聚正电子偶素泡的产生-4.该奖项反映了NSF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.