STUDY OF OPERATION CHARACTERISTICS OF LIQUID XENON PHOTOIONIZATION DETECTORS

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Rare gas liquids have good properties both for ionization and scintillation as radiation detectors. It has been shown that these can be excellent detectors when ionization and scintillation are simultaneously utilized. Photoionization is one of the methods. In this method, photons from rare gas liquid molecules are absorbed by doped organic molecule and ionized the molecule. Thus scintillation is converted to ionization. One of important characteristics of the photoionization detectors is time stability, which we measured in this study.We measured the stability of allene doped liquid argon for more than 60 hours. Amount of collected charge varied with time. Although the absolute charge decreases, it gradually collected charge varied with time. Although the absolute charge decreases, it gradually approachs to the charge amount of pure liquid argon and does not reduce to less than the level. This means that the concentration of allene decreases with time. Measurement of attenuation length of drifting electrons showed concentration of electronegative impurity also decreases. As a results, effective abount of mixed molecules, whether it is desired or undesired, decreases with time.It was found that, in trimethylamine (TMA) doped liquid, xenon, undesired electronegative impurity was introduced together with TMA to the liquid. This can be avoided by purify TMA at low temperature as -110ﾟC and operating detectors also at the temperature. In this way, the collected charge of TMA doped liquid xenon does not change for more than 50 hours.In conclusion, it has been shown that rare gas liquid ionization detectors can be opetated stably.