A measurement of number of scintillation photons in rare gases

One of the purposes in our studies is to determine the scintillation efficiency for argon, krypton and xenon through the measurements of absolute numbers of scintillation photons, where the efficiency is measured in terms of Ws-value defined as an average energy expended in a scintillator to produce one scintillation photon. An apparent has been designed and constructed recently, where methods for reduction of photons reflected at the surrounding walls were taken into consideration. In this paper the preliminary results obtained with this apparatus were described. A schematic diagram of the experimental setup is shown. The chamber is made of stainless steel with inner diameter of 101 mm and a height of 400 mm. An alpha-particle source of 241Am was electrochemically deposited on a tip of M2 screw made of stainless steel, and it was fixed on source holder trapped on a center. The chamber is connected to a vacuum system and a gas filling system. The shapes of the pulse height distribution are in good agreement with the calculated ones if it is taken into consideration that there are fluctuations in the measured pulse heights caused by noises. There is a good linear relationship between the measured pulse heights and the corresponding Fce (the collection efficiency of photons at the photocathode), except that there exists a small amount of non-linearity (a pedestal in the axis). The non-linearity is considered to be caused by a small number of photons reflected at the black-surfaced wall (photon absorber) and injected to the photocathode. In order to measure absolute numbers of scintillation photons, another PMT (Hamamatsu Photonics R208-1) was installed to the present apparatus. The measurements were made in 760 Torr and 1520 Torr xenon by applying a negative voltage of -100 V to the cathode, which is the bias to collect fully photoelectrons. As a result, the number of scintillation photons produced by 5.5 MeV alpha particles was measured to be 2.1 x 105 photons in 760 Torr xenon and 2.2 x 105 photons in 1550 Torr xenon. (S.Y.)