Low temperature performance of photomultiplier tubes illuminated in pulsed mode by visible and vacuum ultraviolet light

Abstract

Detecting the ultraviolet scintillation light of the liquefied rare gases, in particular liquid xenon, requires the use of photomultiplier tubes (PMTs) in a temperature range where their operation is not ensured by manufacturers. Simultaneously, monitoring of the PMTs should be done in the visible for technical reasons. Bialkali photocathodes, that present high quantum efficiency and low dark noise, exhibit a very strong increase of electrical resistivity upon cooling that, apart from other reasons, can lead to operation failure at low temperature. Photomultiplier tubes with bialkali photocathodes and quartz windows (two EMI 9750Q and one Philips XP 2020Q) and another with S11-type photocathode and glass window (FEU 85A) were tested down to −160 °C with a pulsed light source, both in the visible region and at the xenon scintillation wavelength in the vacuum ultraviolet. For the visible range, the results obtained with bialkali photomultipliers display a sharp drop of response at about −100 °C, depending on the count rate, regardless of the wavelength, and a shift of the spectral sensitivity curve towards shorter wavelengths. Concerning the vacuum ultraviolet region, a significant increase of the response was observed between room temperature and that of the efficiency drop. For some photomultiplier tubes, the response as a function of temperature was observed to depend on their past history, probably due to mechanical effects arising in the multiplier system upon cooling. It is concluded that photomultipliers with bialkali photocathode can, in principle, be used for the detection of liquid xenon scintillation, although they have to be carefully monitored while in operation.