Development of a high-rate MUSIC for particle identifications of unstable nuclei

一种用于重离子的高速、稳定的粒子识别探测器是为像即将到来的RI-BEAM工厂那样的不稳定核束实验中的束流监测器而研制的。该探测器采用无栅格的多层平行板气体电离室。多层结构能够使腔室的每个单元非常薄，从而能够以高速运行。然而，由于无栅极结构，该电离室的阳极脉冲不仅由漂移的电子产生的快速电流形成，而且还由极显着的正离子电流形成。在高速运行时，后一种电流的存在会造成很大的问题。通过对阳极脉冲应用特殊的脉冲整形可以避免这一问题。可以利用正离子电流的慢度来有效地抑制来自阳极脉冲的该电流的贡献。利用含有不同重离子的次级束流对多层电离室(高速MUSIC)进行了测试。结果表明，该探测器的粒子识别能力几乎等于入射粒子能量损失的统计涨落所确定的极限值。这就是说，电噪声或探测器的性质根本不影响它。通过增加能量损失可以无限期地提高分辨率。至于高倍率性能，在测试实验的最大计数速率150kcps之前，没有观察到分辨率下降。因此，可以开发出一种高度实用的探测器。

A high-rate and stable particle identification detector for heavy ions was developed for using as a beam monitor in the unstable nuclear beam experiment like at the forthcoming RI-beam Factory. A multi-layered parallel plate gas ionization chamber having no grid was tried for this detector. The multi-layered configuration enables to make each unit of the chamber very thin and thus to operate it at high speed. However, because of the grid less construction, anode pulses of this ionization chamber are formed not only by the rapid current due to drifting electrons but also by the extremely show positive ion current. The existence of the latter current causes a great problem when operating at high rate. This problem was found to be avoided by applying the special pulse shaping to the anode pulses. The slowness of the positive ion current can be utilized to effectively reject the contribution of this current from the anode pulses. The multi-layered ionization chamber (high-rate MUSIC) thus constructed was tested using secondary beams containing various different heavy ions. It was found that the particle identification capability of this detector was almost equal to the limiting value determined by the statistical fluctuations of energy loss of the incident particles. That is to say, electric noises or detector properties did not at all affect it. The resolution can be improved indefinitely by increasing energy loss. As for the high-rate performance, no deterioration of the resolution was observed up to the maximum counting rate, 150kcps, of the test experiment. As a result of this, a highly practical detector could be developed.