Spectroscopy of Dense Positronium

正电子是一种类氢原子，其中带正电的粒子是正电子，而不是质子，正电子是电子的反粒子。与普通氢原子不同，正电子原子会自发衰变--电子和正电子湮没，能量最初以质量的形式转化为伽马射线。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生产一种高密度的正电子气体，并对其进行冷却，以产生第一种正电子超流体，即众所周知的玻色-爱因斯坦凝聚体(BEC)。这种凝聚体已经被研究用于其他原子，并表现出丰富的行为范围，但需要新的实验技术来观察正电子中的玻色凝聚。科学上很有趣的是，如果能产生正电子素的凝聚体，正电子素原子衰变成伽马射线的事实就开启了创造伽马射线激光的可能性。据预测，在足够数量和低温下的正电子将表现出受激发湮灭，在这种情况下，一个湮灭的伽马射线光子会诱导出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发湮灭的证据，为第一台高穿透性湮没伽马射线激光开辟道路。学生将参与这项研究的所有方面，并将学习广泛应用于物理学的重要实验室技术。除了对这一现象的科学兴趣外，伽马激光的长期可能好处包括医疗应用于放射照相和放射治疗、高功率伽马射线束的国防和其他用途，以及为清洁、低成本发电厂的聚变提供伽马激光点火的可能性。该项目的主要目标是(1)产生和观测正电子的玻色-爱因斯坦凝聚体，以及(2)获得其双光子湮没辐射的受激发射的证据。该项目将使用现有的慢正电子束和磁陷阱系统，该系统可产生约1亿个自旋极化的30%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，并聚焦在直径200微米的金属薄膜上，该薄膜有效地发射缓慢的正电子。它们将被加速并聚焦到一个含有空腔的5微米的靶上，在空腔中将形成高密度的正电子，并将其加热到较低的温度。正电子温度将通过使用现有探测器的湮没光子对的角关联作为时间的函数来测量。凝析油的存在将从其亚热表观温度中推断出来。不同的腔型将被用来获得所需的高正电子密度和足够低的正电子温度，在10K左右。在制造了第一个正电子玻色-爱因斯坦凝聚体后，将搜索其湮没伽马射线的受激发射。除了能够生产第一台伽马射线激光器，这项工作还将为其他涉及高密度正电子的课题开辟道路，包括第一次生产和研究正电子正离子(两个正电子和一个电子的束缚态)，形成正电子原子激光束，以及在超流氦-4中产生玻色-爱因斯坦凝聚的正电子气泡。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.