Particle Identification of High-Energy Heavy Ions by a Tilted Electrode Gas Ionization Chamber

本文介绍了一种用于即将建成的RI束流工厂的高能重离子高速粒子鉴别装置。研制了一种倾斜电极气体电离室气体探测器，并在RIKEN环形回旋加速器上进行了实验。气体长度依赖性的检测器分辨率测量使用核碎片的56 Fe。结果表明，当分辨率仅由电离损失的统计涨落决定时，分辨率与气体长度的平方成反比。但是，分辨率并不趋向于零向无限气体长度和有一个小的偏移。该偏移值随着被检测粒子的原子序数Z的增加而增加。直到Z=25，分辨率保持在可接受值内。但是如果我们将增加的趋势外推到更大的Z，则分辨率超过了Z=50附近的可接受值。这种分辨率下降的原因似乎是由于我们的探测器中使用的电极箔。因此，对于Z= 50附近的重离子，需要使用更薄的探测箔。用~（40）Ar核碎片测量了探测器分辨率与计数率的关系。在该测量中，比较了Ar-CH 4和Ar-CF 4两种气体混合物。结果发现，气体混合物Ar-CH 4的分辨率是非常好的计数率为300 kcps，但迅速恶化超过这个计数率。与此相反，发现气体混合物Ar-CF 4的分辨率几乎恒定，直至测量的最大计数率1 Mcps。气体混合物Ar-CF 4的更好的速率依赖性可能是由于与气体混合物Ar-CH 4相比该气体的更大的电子漂移速度。在高速运行时，获得了满意的结果。

A high rate particle identification device for high-energy heavy ions, which is intended to be used in the forthcoming RI beam factory has been developed. A gas detector named tilted electrode gas ionization chamber was constructed and was tested at the RIKEN ring cyclotron. Gas length dependence of the detector resolution was measured using nuclear fragments of 56Fe. It was found that the resolution was nearly proportional to inverse square of gas length just as expected when the resolution is determined simply by statistical fluctuations of ionization loss. But the resolution does not tend to zero toward the infinite gas length and has a small offset. This offset value increases with the increase of the atomic number Z of the detected particles. Up to Z=25, the resolution remains well within the acceptable value. But if we extrapolate the increasing trend toward larger Z, the resolution exceeds the acceptable value around Z=50. The reason of this deterioration of the resolution seems to be due to the electrode foils used in our detector. It is thus necessary to use thinner foils for the identification of heavy ions around Z=50.Counting rate dependence of the detector resolution was measured using nuclear fragments of 40Ar. In this measurement two kinds of gas mixtures, Ar-CH4 and Ar-CF4, were compared. It was found that the resolution of the gas mixture Ar-CH4 is very good up to a counting rate of 300kcps but deteriorates rapidly above this counting rate. In contrast to this, the resolution of the gas mixture Ar-CF4 was found to be almost constant up to the measured maximum counting rate 1Mcps. The better rate dependence of the gas mixture Ar-CF4 is probably due to a larger electron drift velocity of this gas compared to the gas mixture Ar-CH4. As for high-rate operation of the detector, a satisfactory result was obtained.