Delayed electron emission in photomultiplier tubes

Abstract

The time distributions of pulses generated by noise or external light sources in the ХР2020, ХР2232В, ХР1021, FEU-85, FEU-87, FEU-93, FEU-130, and R7600U-200 photomultiplier tubes (PMTs) were investigated using an autocorrelation delayed-coincidence time spectrometer. The observed pulse time correlations were of two types: non-exponential, related to the detection of the effects of feedback ions; and exponential, caused by the interaction of secondary-emission electrons with the PMT dynodes. Detection of multi-electron events arising from the action of external radiators leads to intensification of residual-gas ionization processes in the PMT. However, no analogy is observed in the shapes of the feedback ion spectra for different types of PMT despite the identical composition of photocathodes or dynodes. This might be related to differences in PMT manufacturing processes and to possible different restructuring of molecular electron shells in the photocathode material constituents. The detection of single-electron events (noise pulses) in some of the PMTs studied was not always characterized by nanosecond-scale emissions that might be expected to result from metastable states related to properties of their photocathodes and dynodes, and on the material processing technology used in PMT manufacture. It is difficult to describe exactly the mechanisms leading to delayed electron emission. The source of such events remains open to attempts at explanation and further research.