Photocathodes—past performance and future potential

Abstract

Light detection and imaging at very low intensities, even down to individual photon detection, has been made possible by the development of photomultiplier and imaging tubes. They have two main components: a photocathode detector and an electron multiplication section, but throughout their long history their performance has been limited by the response of the photocathodes. Reappraisal of the underlying science of cathode preparation emphasizes that they are still performing well below their full potential. The reasons for this are discussed and some indications of how improvements could be made are suggested. Since many of the potential advances are within the scope of current technology it is certainly feasible to achieve enhancements in performance by factors of two to ten across the blue to near-infrared spectral range. Higher improvement factors are implied but will be dicult to realize in the normal spectral range for wavelengths below 1 mm. Signi®cant gains in sensitivity, and perhaps in response speed, will open many new opportunities for low light level sensors and the prediction is that they will be an enabling technology for developments in biological and medical applications, among others.

Enhancement of photomultiplier sensitivity with anti-reflective layers

“…This is important as some impedance matching layers may be slightly lossy and therefore the total absorptance cannot be used to predict performance gains. Note also that the overall quantum efficiency is influenced by the relative separation of the absorption site and the vacuum surface as photoexcited electrons need to travel through the layer and have sufficient energy to escape through the cathode/vacuum interface [1,7]. By applying equations (11), (12), (13) and (14) we can find the absorptance of the j th layer, noting that the Poynting vector obtained from applying equation (13) to equations (11) and (12) will be a function of the distance z through the j th layer.…”

“…despite the continued importance of this optoelectronic device there has been only a slow improvement in the performance of the commercially available PMTs in the last 50 years [1]. Reasons for reduced efficiency arise from signal losses of reflection from the photocathode material and transmission through it, as well failures to convert absorbed photons into electron emission into the vacuum chamber.…”

“…BeO) reducing the reflectance from the PMT window and reducing the transmittance through the photocathode layer. These techniques rely on controlling the angle that the incident light impinges upon the PMT window using additional optical elements, resulting in higher absorptance values than are achievable with normal or near normally incident light [1,2]. Other methods propose the use of multiply reflecting structures, such as pyramids or cones, fabricated on the inside of the PMT window to reduce reflectance and enhance absorptance, or waveguiding within the tube window to enhance the overall absorption efficiency [3][4][5].…”

Unexploited Information from Luminescence Spectra

“…However, the performance across the range falls steeply at long wavelengths. In order to consider improvements it is helpful to consider the inherent stages [3]. The photo-emissive material is usually a mixture of alkali metals but there is considerable empirical "black magic" in the conditions for deposition, rather than a full understanding of the science.…”

Common mistakes in luminescence analysis

“…The response of such tubes can be quite variable at the long wavelength end of the spectrum so they need individual calibration [9,10]. They may also vary with age and, more critically, with exposure to light.…”