Hybrid PhotoDiodes (HPDs) are very simple photo detectors which cover the applicability field of Photo Multipliers but show some better features. In particular HPDs show a very good photon counting capability, due to their peculiar one-stage gain process. It was found that HPD's photoelectron spectrum has a low-energy continuum which could be attributed to a backscattering process from the diode surface. In order to clearly distinguish the exact role of backscattering, the single photoelectron response (SER) of a 61 pixel proximity focused HPD was measured in standard conditions and in a 5 kG axial magneticfield. The results clearly indicate that the backscattering process gives the greatest contribution to the low-energy continuum spectrum. Monte Carlo simulations, using the EGS4 code, supported the analysis and allowed some interpretation of the effect, which greatly depends on diode surface characteristics, such as contact layer thickness.
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