Development and application of miniature ionising radiation detectors based on pin photodiodes.

Research has been carried out at the University of Surrey to investigate the response of the relatively inexpensive photodiode-scintillator type gamma radiation detector. The results otained were compared with other published work in this field. The use of windowless photodiodes as direct semiconductor detectors for both alpha and X-ray / gamma ionising radiation was also studied. The work was carried out as a collaborative project with Vinten Analytical Systems Ltd. of Sandy in Bedfordshire. Results obtained by direct detection of Am-241 photons were particularly good using the inexpensive Hamamatsu S1223-01 photodiode (with a small active area of only 13.7mm2) providing excellent energy resolution for the gamma photons and the associated X-rays. The results obtained from the diode-scintillator format of detector compare well with published findings. Comparison of output pulses from gamma photons directly detected in the silicon photodiode (used as a stable reference) with the output from scintillation events provided an absolute method of calculating pulse size in terms of charge (in electron-hole pairs) generated in the photodiode. A reflector paint manufactured by Kodak based on barium sulphate was investigated as a substitute for the widely used yet bulky and inconvenient MgO powder. Manufacturer's literature indicate a reflectance significantly greater than for MgO powder. However, the results obtained when used in conjunction with CsI(Tl) were close to that obtained with MgO powder. Following initial work with 1cm2 diodes (Hamamatsu S1790-01, S1790-02 and S3590-01), even more compact and still less expensive detectors were developed using the small active area Hamamatsu S1223-01 type photodiodes. Unlike the majority of the diodes supplied by the manufacturers these have no protective coating over the active surface. The manufacture of these detectors involved the difficult task of coating the bare active surface of the diodes with a passive resin which would not. unduly affect the photodiodes characteristics. Although the performance of these detectors was not as good as for those constructed using the relatively large area photodiodes, they were of sufficient promise to encourage the development of compact and inexpensive readout electronics. Applications of the range of detectors and the associated readout electronics developed during the research period were also considered and are listed in broad categories below: (a) low cost educational demonstration systems (b) tomography systems where many detector arrays (often close together) and corresponding read-out electronics are required (c) gamma radiation monitors and assay applications (d) monitors for use in environments of high magnetic flux where conventional photomultiplier based detectors would be affected (e) gamma source verification for the well type ionisation chambers used in radiotherapy treatment. Information about the spectrum of the source being measured can be used to automatically select the scaling factor necessary to correct for the chamber response. Also, the purity of a source can be assessed where grow-in of gamma emitting daughters is a possibility in a personal dose meter to obtain spectroscopic information so that the effects of absorbed dose may be corrected for energy. Also, to assist in identifying the cause and location of a source of radiation in a working environment