Development of a pulshed monoenergetic positron beam line

为了研制正电子减速剂，建立了单能正电子束流线。在这条波束线中，正电子从22Na源发射。单能正电子在磁场引导下被植入铝样品中。利用位于样品室的HP-Ge探测器测量了高能正电子向单能正电子的转化率。本研究开发了由钨箔、钨带和镍锥组成的减速剂。本研究设计的正电子脉冲系统由三个主要器件组成；反射型斩波器、亚谐波预聚束器和双谐波聚束器。斩波器产生波束宽度为3- 8ns的脉冲光束。将切碎的光束加速到250 eV，然后将其引导到SHPB中。SHPB将切碎光束的脉冲宽度压缩到约2ns。然后，束束器将样品处的脉冲宽度压缩到约100ps。集束器的基频f_b为150 MHz。SHPB的工作频率为基频（f_b/4）的四分之一。斩波频率为f_b/4n MHz （n=1,2…），其中后续脉冲之间的时间间隔可以改变。寿命谱是通过测量从脉冲系统电输出的定时信号与BaF_2闪烁探测器探测到的湮灭伽马射线之间的时间间隔得到的。

A monoenergetic positron beam line was constructed in order to develop a postitron moderator. In this beam line, positrons emitted from a 22Na soruce were used. The monoenergetic positrons were guided by a magnetic field and were implanted into the Al specimen. The conversion rate from high energy positrons to monoenergetic positrons was measured with a HP-Ge detector located at the sample chamber. In the present study, the moderators consist of tungsten foils, tungsten ribbons and nickel cones were developed.The positron pulsing system designed in the present study consists of three main devices ; a reflection type chopper, a sub-harmonic prebuncher (SHPB) and a double harmonic buncher. The chopper generates a pulsed beam with beam width of 3-8 ns. The chopped beam is accelerated up to 250 eV,then it is guided into the SHPB.The SHPB compresses the pulse width of the chopped beam to about 2 ns. Then, the buncher compresses the pulse width to about 100 ps at the specimen. The fundamental frequency of the buncher, f_b, is 150 MHz. The operating frequency of the SHPB is a quarter of the fundamental frequency (f_b/4). The chopper frequency is f_b/4n MHz (n=1,2 ...), where the time interval between subsequent pulses can be varied. The lifetime spectra are obtained by measuring the time interval between the timing signal derived electrically from the pulsing system and the annihilation gamma ray detected by a BaF_2 scintillation detector.