Imaging Gas Scintillation Proportioanl Counters with Very Low Background

成像气体闪烁比例计数器（GSPC）具有能量分辨率好、有效面积宽、对高能x射线灵敏度高的特点，已成功应用于x射线天文卫星ASCA。本研究的目的是通过引入无漂移型气电池来提高这些计数器在背景抑制效率和位置分辨率方面的性能。已生产出内径93mm，高度20-25mm的无漂移设计气室。首先用普通的（不位置敏感的）光电管进行测试，以研究背景性能。用波形数字化仪对GSPC输出信号进行处理，得到气池内气体闪烁引起的光发射的时间变化以及x射线吸收的深度的详细信息。通过比较总光前半部分和后半部分的发光时间，可以在高能量范围内抑制背景。在3 keV以下的低能量范围内，x射线总是在离入口窗口非常近的区域被吸收，利用x射线吸收深度的信息来消除背景。我们已经将背景比率降低到ASCA标准的10%以下。将气体电池与成像光电管相结合，研究了无漂GSPC的位置分辨率。将x射线源的x射线准直到直径为0.1mm，在6 keV下获得了0.3mm （FWHM）的位置分辨率。这几乎是ASCA GSPC的两倍。因此，本研究显著提高了GSPC成像的性能。我们希望该仪器能广泛应用于各个研究领域。

Imaging gas scintillation proportional counters (GSPC) have been successfully used in the X-ray astronomy satellite ASCA with good energy resolution, wide effective area, and high sensitivity to high-energy X-rays. The aim of this research is to improve the performance of these counters in the efficiency of background rejection and in the position resolution by introducing a driftless type of gas-cell. Gas cells of driftless design with an inner diameter of 93mm and a height of 20-25mm have been produced. This is first tested with an ordinary (not position sensitive) phototube to study the background performance. The output signals of GSPC are processed with an wave-form digitizer which gives detaled information about the time variation of the light emission due to gas scintillation in the gas cell as well as the depth of the X-ray absorption. By comparing the time of light emission between the first and the second halves of the total light, we can suppress the background in the high energy range. In the low energy range below 3 keV where X-rays are always absorbed in the very close region from the entrance window, the information of the depth of the X-ray absorption is used to eliminate the background. We have reduced the background rate down to less than 10% of the ASCA level. The position resolution of the driftless GSPC is studied by combining the gas cell with an imaging phototube. The X-rays from an X-ray generator are collimated to 0.1mm diameter, and a position resolution of 0.3mm (FWHM) at 6 keV has been obtained. This is nearly twice as good as that ot the ASCA GSPC.Therefore, a significant improvement in the performance of imaging GSPC has been resulted from the present study. We hope that this instrument will be widely used in various research areas.