Low temperature test of photomultiplier tubes

Photomultiplier tubes (PMTs) are still the most widely used photosensitive devices due to their high quantum efficiency, fast response, good energy resolution, large sensitive area and high gain. The excelent scintillation propreties of the liquid rare gases and the increasing good prospects for their use in radiation detectors having different applications. To know in particular whether the connection of the quartz window with the glass body of the PMT parameters (quantum efficiency, gain, dark current and time characteristics ) depend on the temperature, we used a very simple but reliable set - up, A long cylinder of about 1 meter whose central part can be cooled down to - 170 C by a flux of liquid nitrogen vapour through a copper tube wounded around the cylinder, while its ends are kept at room temperature. The PMT is placed in the central part but the voltage divider is mounted in one of the warm ends, connected to the PMT by flexible wires. A light source was placed on the top of the cylinder, also kept at room temperature to avoid any possible changes of the emission intensity. The distance between the light source and PMT was about 0.5 m, which ensures a uniform illumination of the photocathode. The response of PMTs with photocathodes of different types and sizes in the visible and UV spectral regions was studied. As an example we refer data obtained with a 2 - inch PMT EMI 9750 Q, with bialkali photocathode, from room temperature down to -161 degree celsius. The light source was a LED excited by voltage pulses of 1 microsecond duration and 1 Khz frequency. The PMT stood a cooling rate of about 5 degree celsius / min without breaking. Concerning the anode signal amplitude, a slow decrease of 20% was observed when the temperature went down to -90 degree celsius followed by an abrupt drop. At - 161 degree celsius the signal amplitude was about 5% of its initial value. The output noise decreased approximately by a factor of 2 at -130 degree celsius, remaining constant at lower temperature. 2 figs., 5 refs. (author)