Silicon semiconductor detectors used in radiation dosimetry have different properties, just as e.g. ionisation chambers, affecting the interaction of radiation with matter in the vicinity of the sensitive volume of the detector, e.g. wall materials, and also the collection of the charges liberated in the detector by the radiation. The charge collection depends on impurities, lattice imperfections and other properties of the semiconductor crystal. In this paper the relevant parameters of a silicon semiconductor detector intended for dosimetry are reviewed. The influence of doping material, doping level, various effects of radiation damage, mechanical construction, detector size, statistical noise and connection to the electrometer is discussed.

A p-type silicon radiation detector has been constructed and the effect of radiation damage on sensitivity and dose rate dependence has been studied. The dose rate dependence showed, in contrast to an n-type silicon detector, a linear dose rate response for clinically relevant radiation qualities, dose rates and pre-irradiation doses.

Semicondunor detectors based on ptype silicon but with different doping levels have been investigated. The linearity response with dose rate (dose per pulse in a pulsed beam). the sensitivity drop and the sensitivity variation with temperature have been investigated prehdiation, radiation damage, in different radiation qualities. It was shown that a p-type detector with a low doping lev& high resistivity, showed a non-linear dose rate response if radiation damaged in a high energy photon beam, which contains neuImns. By increasing the doping level it was shown that a detector with a resistivity of 03 Oan stay& linear after preirradiation in radiation fields from high energy elech.ons, photons and protons. Other parameters did not show any changes of clinical importance ai the dilYerent doping levels.

A theoretical investigation concerning sensitivity variations in semiconductor detectors used in the short-circuit mode at different temperatures and pre-irradiation dose levels has been compared with experimental results. Initially, a sensitivity drop of more than 15% kGy-1 and a sensitivity increase with temperature of less than 1% per 10 degrees C was found for p-type silicon detectors. After a pre-irradiation dose of 5 kGy with 20 MeV electrons these values were changed to about 6.5% kGy-1 and 3% per 10 degrees C, respectively. As a temperature rise of about 10 was obtained in the detector when applied to a patient, the change in sensitivity has to be taken into consideration when the detector is used in patient dosimetry.

This paper describes a dual-scattering-foil technique for flattening of radiotherapeutic charged particle beams. A theory for optimization of shapes and thicknesses of the scattering foils is presented. The result is a universal optimal secondary-scatterer profile, which can be adapted to any charged particle beam by a simple scaling procedure. The calculation of the mean square scattering angle of the beam after passing through the scattering foils is done using the generalized Fermi-Eyges model for charged particle transport. It is shown that the fluence profile in the plane of interest can be made flat to better than 1% inside a predefined beam radius provided the shaped secondary scatterer has the universal radial thickness profile. The thicknesses of the two foils are optimized to minimize the total energy loss. The theory has been tested experimentally in an 180 MeV clinical proton beam. The measured distributions agree well with the calculations.

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