Michishio K, Chiari L, Tanaka F, Oshima N, Nagashima Y
Department of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan.
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.
Rev Sci Instrum. 2019 Feb;90(2):023305. doi: 10.1063/1.5060619.
We constructed a new apparatus, built upon a trap-based slow positron beam, for the production of a collimated, energy-tunable positronium beam under ultra-high vacuum conditions employing the photodetachment of positronium negative ions. A slow positron generator consisting of a Na radioisotope (20 mCi) combined with a buffer-gas positron trap is employed to generate high-quality, nano-second positron bursts with a repetition rate of 1 Hz-1 kHz. The positron bursts are focused onto an efficient positron-to-positronium negative ion converter, a Na-coated W thin film in a transmission geometry, using a magnetic lens system. The ions emitted from the opposite surface of the film are electrostatically accelerated to a given energy and photodetached by a pulsed infrared laser to form a mono-energetic positronium beam with kinetic energies of 0.2 keV-3.3 keV. The achieved detection rate of Ps atoms is 23 cps at the energy of 3.3 keV with a signal-to-background ratio as high as 300. The energy spread of the beam was evaluated by comparing the result of the time-of-flight measurements and particle-tracking simulations. With the use of a collimator of 1 mm diameter, a coherent beam with an angular divergence of less than 0.3° is obtained. The obtained Ps beam, having a much higher quality than those reported hitherto, will open up a new field of experimental investigations, such as Ps interacting with a variety of materials and fundamental studies on Ps spectroscopy.
我们构建了一种基于陷阱型慢正电子束的新装置，用于在超高真空条件下通过正电子素负离子的光剥离产生准直的、能量可调的正电子素束。使用由钠放射性同位素（20毫居里）与缓冲气体正电子陷阱组合而成的慢正电子发生器，以1赫兹至1千赫兹的重复率产生高质量的纳秒级正电子脉冲。利用磁透镜系统将正电子脉冲聚焦到一个高效的正电子到正电子素负离子转换器上，该转换器是处于透射几何结构的镀钠钨薄膜。从薄膜相对表面发射出的离子被静电加速到给定能量，并通过脉冲红外激光进行光剥离，以形成动能为0.2千电子伏至3.3千电子伏的单能正电子素束。在3.3千电子伏能量下，正电子素原子的探测率达到23计数每秒，信背比高达300。通过比较飞行时间测量结果和粒子追踪模拟来评估束流的能量展宽。使用直径为1毫米的准直器，可获得角发散小于0.3°的相干束。所获得的正电子素束质量比迄今报道的那些要高得多，这将开辟一个新的实验研究领域，例如正电子素与各种材料的相互作用以及正电子素光谱的基础研究。
