Optimization of photomultiplier spectral sensitivity with a graded thickness photocathode

Abstract

The efficiency of S20 multialkali photocathodes is a function of thickness, wavelength, polarization and angle of incidence of light into the cathode. Therefore no single set of conditions can maximize performance over the entire spectral range. Consequently, two prototype photocathodes have been made with a gradation in thickness so that during monochromatic analysis the optical beam can be addressed to different thickness regions at preferred angles of incidence. This potentially enables a spectrum to be recorded under optimal conditions at a single interaction event. Relative to normal commercial S20 photomultipliers the quantum efficiency (QE) has been significantly raised across the entire spectral range. The current data were primarily obtained between 450 and 800 nm and at 450 nm it resulted in values of at least 65% QE, which is the highest value ever cited at this wavelength. Signals at longer wavelengths, for example at 750 and 800 nm, were recorded with up to 20 and 10% QE, respectively. Once again these are new record values that match performance from multiple interactions in waveguide cathodes. The data from this new design of photocathode underline the potential for improvements in efficiency for non-normal incidence in graded thickness photocathodes and indicate that current S20 technology could be significantly enhanced. Alternative enhancement methods are mentioned, particularly for spectrally dispersed signals. The enhancements are compared with data for a standard high quality S20 photocathode.

Low cost enhancements of photomultiplier sensitivity

“…In order to assess the maximum QE values trials have been made with graded thickness cathodes as a function of wavelength, polarization and incidence angle. These reveal the potential for remarkably high QE values of >60% in the blue and >10% at say 800 nm [5] . For existing systems there is interest in simple in-line retrofits which could offer similar benefits.…”

Improvements of the PLD (Pulsed Laser Deposition) Method for Fabricating Photocathodes in ICMOS (Intensified CMOS) Sensors

“…It is also relatively stable under short exposure to atmosphere, which substantially simplifies production and handling of intensifiers with CsTe photocathodes. In the visible spectra one of the most efficient photocathodes is multi-alkali S20 [3], [5].…”

Unexploited Information from Luminescence Spectra

“…Manufacturers therefore now try to overcome these difficulties with better deposition reproducibility, inbuilt anti-reflection coatings or structured surfaces which minimise reflection, and/or increase absorption, plus topographies that enhance electron extraction [4][5][6][7]. Far less appreciated is that even with standard PMT systems it is often simple to greatly improve on the manufacturer's original performance specification, merely by making simple changes in the way light is coupled into the PMT.…”