Details of Grant 

EPSRC Reference:	EP/D033055/1
Title:	Photomultiplier Optimisation Device: with a unique method of sequential improvements across the operating spectrum with extended red performance
Principal Investigator:	Townsend, Professor P
Other Investigators:	Hole, Dr D E
Researcher Co-Investigators:
Project Partners:
Department:	Sch of Engineering and Design
Organisation:	University of Sussex
Scheme:	Standard Research (Pre-FEC)
Starts:	01 January 2006	Ends:	30 November 2006	Value (£):	47,579
EPSRC Research Topic Classifications:	Materials Processing Optoelect. Devices & Circuits
EPSRC Industrial Sector Classifications:	Electronics
Related Grants:
Panel History:
Summary on Grant Application Form
Photomultiplier Optimisation Device, POD. Photon detection is central to modern science. The most sensitive detectors for the UV and visible regions are photomultiplier tubes (PM) as they have sufficient amplification to detect individual photons and so are used in all branches of science, from communications to mineralogy, basic research, oil well prospecting, pollution detection, DNA coding to cancer diagnosis. Nevertheless their detection efficiency falls from ~35% in the UV/blue region to much less than 1% for long wavelengths (red and near infrared). This severely limits their range of applications for low light levels. The photocathode converts the photon energy into emitted electrons but at longer wavelengths a major reason for their poor response is that the light is not absorbed(e.g. typically multi-alkali cathodes are >95% transparent by ~800 nm). The number of electrons produced per incident photon, the quantum efficiency, is the key measure of the sensitivity of the detector and this is dominated by the absorption of the light. The proposed prototype apparatus will almost totally overcome the absorption problem and offer nearly100% absorption at all wavelengths. Therefore the effect on quantum efficiency is to dramatically increase it across the spectrum, with values raised by x2 in the blue to >50 times in the red region. Thicker cathodes can increase absorption of the light but for the photoemissive process there is often no advantage, or the situation is worse, as electrons generated near the input surface fail to travel across the cathode to escape into the vacuum region of the PM tube. This electron transport is degraded by the multi-layer deposition technique. However, absorption is strongly influenced by the angle at which the light strikes the cathode. Moving from normal incidence to angles near 60 or 70 degrees can increase absorption to almost 100%. In normal use there is a cut-off in angle near 45 degrees from total internal reflection for light incident from air into the glass window above the cathode. Overcoming this limit allows exploitation of the higher absorption. To achieve this we will use a surface dome so the input light is always normal to the dome. By illuminating at different angles very high internal incidence angles can be achieved at the cathode. The optimum angles vary with wavelength, polarisation of the light and cathode thickness. We will use a graded thickness and rotate and/or move sideways across the dome to track the optimum position as the wavelength is varied. Since cathode deposition varies for every photomultiplier tube it will be necessary to record the angular, thickness and polarisation dependence to give a look-up table of optimal values at each wavelength. This table will determine the scanning and positioning of the tube in the device in normal operation. The overall effect will be a sensitive detector with unprecedented performance. The benefits in the long wavelength region are particularly valuable as not only is this the region where tubes are currently poor in efficiency, but it is precisely the region where there are numerous new opportunities for use in the biological and medical fields. A parallel new experiment of pulsed laser annealing will be tried to improve the uniformity of the cathodes by pulse heating the multi-alkali deposited layers. This should improve the electron transport within the cathode. It may be needed prior to the final caesium monolayer deposition but would significantly benefit all types of cathode.New uses include luminescence and light probes to detect breast cancer which require wavelengths where the tissue is transparent in the spectral range from ~750 to 900 nanometres (shorter wavelengths <750nm are absorbed by haemoglobin in the blood and water absorbs light beyond 900 nm). The new detector would have a very valuable role in a wide range of medical applications, as well as in all other current PM usage.
Key Findings
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Potential use in non-academic contexts
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Organisation Website:	http://www.sussex.ac.uk