Identification of atomic numbers and mass numbers of high-energy heavy ions using a counter telescope

利用倾斜电极气体电离室作为Δ电子计数器，NaI(Tl)探测器作为能量计数器，塑料闪烁体作为TOF计数器，研制了由三种不同类型的探测器组成的计数望远镜。其中，A电子计数器和NaI(Tl)探测器的组合首次在美国国家放射科学研究所的HIMAC加速器上用于识别高能重离子的原子序数。使用了能量高达400 MeV/N的Kr(Z=36)和Xe(Z=54)重离子束，这接近于RI束流工厂的能区。结果表明，对于Z=54以下的重离子，原子序数的识别效果很好。但NaI(Tl)探测器的质量仅限于100个粒子/溢出的低束强区域。在较高的束流强度下，NaI(Tl)探测器的输出脉冲高度随束流强度的增加而减小，束流能量与脉冲高度之间的线性关系被扭曲。当NaI(Tl)探测器与TOF计数器一起用于识别质量数时，这种失真导致质量分辨率的恶化。对于用作TOF计数器的塑料闪烁体，通过使用快速光电倍增管读取方形闪烁体四边的光输出来测试时间分辨率。光到达四个光管的时间不是使用恒定分数鉴别器之类的电子电路来确定的，而是通过分析由4通道数字存储示波器测量的光管信号的波形来确定的。通过这种分析，可以减少噪声对波形数据的影响。在一次使用宇宙射线的测试实验中，我们获得了90ps的时间分辨率。对于产生比宇宙射线大得多的光输出的重离子来说，预计时间分辨率将远远好于90ps。

A counter telescope composed of three different types of detectors was developed using a tilted electrode gas ionization chamber as a ΔE-counter, an NaI(Tl) detector as an energy counter and a plastic scintillator as a TOF counter. Among them a combination of the A E-counter and the NaI(Tl) detector was first tested for identifying atomic numbers of high-energy heavy ions at the HIMAC accelerator of the National Institute of Radiological Sciences. Heavy ion beams of Kr(Z=36) and Xe(Z=54) with energies up to 400MeV/N, which is close to the energy region of the RI beam factory, were used. It was found that the identification of atomic numbers was quite good for heavy ions up to Z=54. But the quality of the NaI(Tl) detector was limited to the region of low beam intensity around 100 particles/spill. At the higher beam intensities, output pulse heights of the NaI(Tl) detector decreased with increasing beam intensities and the linear relation between beam energy and pulse height was distorted. Such distortion renders deterioration of mass resolution when the NaI(Tl) detector together with a TOF counter was used to identify mass numbers. Thus it remains necessary to develop a high-rate energy counter.As for a plastic scintillator used as a TOF counter, time resolution was tested by reading light outputs from four sides of the square scintillator using fast photomultiplier tubes. Arrival times of light to the four phototubes were determined not using electronic circuits, like constant fraction discriminators, but by analyzing waveforms of the phototube signals measured by a 4-channel digital storage scope. By this analysis, we could reduce the effect of noise on the waveform data. In a test experiment using cosmic rays, we obtained the time resolution of 90 ps. For heavy ions which yield much larger light outputs than cosmic rays, a timing resolution of much better than 90 ps is expected.