Gas gain and space charge effects in aging tests of gaseous detectors

We present the results of investigations of the gas gain in straw drift-tubes under different irradiation intensities from a 90Sr β-source. Tests were performed with a gas mixture of 70% Xe+10% CO2+20% CF4. The goal of this investigation was to show how to optimize the choice of high voltage for quick aging studies of modern gaseous detectors. In order to perform aging tests within a reasonable time period, the rate of accumulating charge must be at least 10 times higher than what is expected in a real experiment. This means a reasonably quick charge accumulation of about 0.1–0.2 C/cm per day (1–2.3 μA/cm). In order to be as close as possible to the real experimental conditions, the high voltage should be chosen so as to compensate for the space charge effect, which changes the gas gain in such quick aging tests. It is shown that the limited streamer mode becomes quite noticeable starting at a gas gain of 4.5×104.