Studies of interface dosimetry for photon energies below 1.25 MeV by thermally stimulated exoelectron emission
Following radiation excitation, exoelectrons which are emitted from the surface of a phosphor material on heating are produced from a depth of the order of ten nanometres thick. This permits the preparation of thin film TSEE dosimeters. The energies of these electrons are so small that only a multiplying device can be used to detect them. The study therefore involves building and developing a dosimetry system based on this principle. An energy band model has been used to describe the mechanism of exoelectron emission. The application of this model to experimental data shows that it can predict with accuracy the exoelectron energies and their frequencies of escape from traps which are eventually emptied by thermal stimulation. Deviations from theoretically predicted values are described and partially interpreted. A TSEE reader system has been developed which employs an electronic quenched gas-flow GM counter, ohmic regulated heating system and thermocouple for temperature measurements. The counting circuit is optimised for maximum pulse rate (25KHz for a resolution time of 40μsec), speed and minimum background electronic noise. The heating system can be controlled within 0.1°C/s - 10°C/s and the dosimeter actual temperature, accurately located to within ±2oC. The whole set of TSEE data is accumulated on a BBC microcomputer. The most intense emitter, BeO was selected from among the commonly used exoelectron phosphors. The physical and chemical treatments of Na2O and Li2O doped thin film BeO which enhance TSEE dosimetry properties are considered. The description of preparation of the thin films is given. Characteristics of most importance to TSEE dosimetry obtained are as follows; a dose reproducibility within ±2.92&37, good linearity of response even from background level 10^-5Gy to 1Gy and an energy independence of response. The field in which this dosimetry system has proved very useful is in the study of interface dosimetry. It provides the necessary spatial resolution required for measurements in the steep dose gradient encountered between two dissimilar media of lead-; tin-; copper-; aluminium-; perspex; and carbon. From the results of measurements information can be gleaned about the finite dimensions of the interfaces for photon energies below 1.25MeV for which reliable information is scanty.