1. Photomultipliers: Their Cause and Cure†

Publisher Summary This chapter describes the basic physics and construction of photomultipliers. Photoelectric photometry is a fundamental technique in observational astronomy. The photo emissive detector produces a current supposedly proportional to the light intensity falling on it, thus reducing photometry to a simple electrical measurement that can be made very accurately. A thick, internal cold-box window can be helpful in lowering cathode temperature, but it increases cosmic-ray noise and so it may not reduce the total dark noise. A lower string current reduces the heat dissipated in the string, and hence the heat input and thermal gradients at the tube base. Nonlinearity can vary with the position of the illuminated spot on the cathode, and with wavelength. It is suggested that the area of the photocathode illuminated during measurements, and the wavelength of the radiation, are the same as those used during calibration. The pulse-height distribution due to thermionic or field emission from the cathode is similar to that caused by photoelectrons.