Answer to Question #1389 Submitted to "Ask the Experts"

Q

Would you please introduce me to some references concerning "the effect of photomultiplier thickness on its efficiency?"

A

You will need to contact the photomultiplier tube (PMT) manufacturer for technical data on a particular type. The factors that could impact a PMT "efficiency" include photocathode glass thickness, light attenuation in the scintillator, electron emitter coating material composition (for example, bialkali), optical coupling, electron multiplication, counting electronics, etc., all of which will impact observed electrical pulse output detection. The scintillation light pulse detection "efficiency," in pulses per detected radiation, will no doubt also depend on the incident radiation type, fraction of energy deposition, scintillator emission spectra (that is, for NaI, CsI, ZnS, plastic, glass, etc.), and matching to the spectral sensitivity of the photocathode. If your concern is thickness of the scintillator, that too will impact efficiency. In the common case of gamma-ray detection with NaI scintillators, the thinner crystals are better for the low-energy photons (for example, from ¹²⁵I or ²⁴¹Am), while the thicker crystals are used for the higher-energy emitters such as ¹³⁷Cs and ⁶⁰Co. A good introduction text is Radiation Detection and Measurement by Glenn Knoll (John Wiley & Sons, New York). David J. Allard, CHP