Hybrid gaseous photomultipliers

Abstract

We have developed a hybrid gaseous photomultiplier. It consists of two planar parts: a sealed chamber with a semitransparent bi-alkali or SbCs photocathode (the “scintillation chamber”) and a position-sensitive gaseous detector with a CsI photocathode flushed with a gas at 1 atm (the “readout detector”). This device was developed according to two different designs. In the first design the scintillation chamber operates with pure Xe at 1 atm pressure. Electrons created from the photocathode by visible light move through the capillary plate (which acts as a light attenuator) and produce a weak very ultra-violet (VUV) scintillation light in the gap between the capillary plate and the collecting mesh. This VUV light is detected by the readout gaseous detector. In the second design, the sealed scintillation chamber was operated in vacuum. The photoelectrons extracted from the photocathode hit the scintillator emitting VUV light. The VUV light was detected with the readout detector. A practical quantum efficiency of a few percent was achieved at a wavelength of 400 nm, and a position resolution better than 1 mm. The main advantage of this approach is that the photocathode is kept in a sealed chamber, which ensures a high degree of cleanliness. As a result, the photocathodes have high quantum efficiency, are very stable with time and have not shown any signs of aging. The first, gas filled, version of the hybrid detector has the potential to have a large sensitive area (there are no mechanical constraints on the window size) and is also practically insensitive to magnetic fields.