Low cost enhancements of photomultiplier sensitivity

Abstract

Photomultiplier tubes are widely used detectors of low level light signals; however their performance is often limited, especially at long wavelengths. Input signals are reduced both by surface reflection and by transmission through the photocathode layer. Earlier methods of overcoming these weaknesses are summarized. New predictive modelling of the reflectivity and absorption reveals dependencies that are a function of angle of incidence, cathode thickness and polarization. Improvements on normal usage using extremely simple and low cost techniques are effective. These are demonstrated using retrofits that can improve the overall sensitivity of many types of photomultiplier. Examples include a simple external conical torch reflector, which has raised the efficiency of an S20 multialkali photocathode by between 20 to 10% across the blue to red spectral range. A second example, of a semi-cylindrical glass coupler, improved the absorption efficiency by exploiting 60 degree, rather than normal incidence of the light. Enhancements are up to 500% at longer wavelengths. Such gains are particularly valuable as this is the region of lowest quantum efficiency for the standard operation of the tubes.

Unexploited Information from Luminescence Spectra

“…For less than 2 Euros, we [6,8] used a reflector cone from a torch (flashlight) and, without any optimisation, this raised sensitivity from between 20 to 10% from the blue to the red regions. The simplicity of adding a reflector, plus the improvements it offered, is shown in Fig.…”