Spectroscopy of Dense Positronium

电子偶素（Positronium）是一种类氢原子，其带正电的粒子是电子的反粒子--正电子，而不是质子。与普通氢原子不同，电子偶素原子会自发衰变--电子和正电子湮灭，最初以质量形式存在的能量转化为伽马射线。这个由教师、学生和博士后组成的研究团队的主要工作将是产生一种高密度的正电子素气体，并将其冷却，以产生第一种正电子素超流体，称为玻色-爱因斯坦凝聚体（BEC）。 这种凝聚物已经被其他原子研究过，并表现出丰富的行为，但需要新的实验技术来观察电子偶素中的玻色凝聚。 科学上有趣的是，如果正电子素的凝聚体可以产生，正电子素原子衰变为伽马射线的事实打开了创造伽马射线激光器的可能性。电子偶素在足够的数量和低温下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其他方向和能量完全相同的光子的发射，这是所有类型的激光作用所必需的基本活动。因此，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。学生将参与这项研究的各个方面，并将学习广泛适用于物理学的重要实验室技术。 除了对该现象的科学兴趣之外，伽马射线激光器的长期可能益处包括放射照相术和放射治疗的医学应用、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用于清洁、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，有效地发射出慢正电子。这些将被加速并聚焦到包含空腔的靶上的5微米点，在该空腔内将形成高密度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 a hydrogen-like atom where, instead of a proton, the positively charged particle is a positron, the anti-particle of the electron. Unlike ordinary hydrogen atoms, positronium atoms spontaneously decay – the electron and positron annihilate and the energy, originally in the form of mass, is converted into gamma rays. The primary work of this research team of faculty, students and a post-doc will be to produce a high-density gas of positronium and cool it to produce the first positronium superfluid, known as a Bose-Einstein condensate (BEC). Such condensates have been studied for other atoms and exhibit a rich range of behaviors, but new experimental techniques will be required to observe Bose condensation in positronium. Scientifically interesting in its own right, if a condensate of positronium can be produced, the fact that positronium atoms decay to gamma rays opens the possibility of creating a gamma ray laser. Positronium in sufficient quantities and at low temperatures 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 therefore to obtain evidence for this stimulated annihilation to open the way for the first highly penetrating annihilation gamma ray lasers. Students will be involved in all aspects of this research and will learn important laboratory techniques widely applicable in physics. Besides the scientific interest in the phenomena, long term possible benefits from a gamma ray laser 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 principal 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 thermalize 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.