General characteristics of the use of silicon diode detectors for clinicaldosimetry in proton beams

The properties of silicon diode detectors, used for dosimetry in clinical p roton beams, were investigated with special regard to the measurement of re lative dose distributions in water. Different types of silicon diode detect or were studied, and the resulting distributions of detector signal versus depth in the water phantom were compared with the corresponding distributio ns obtained with a plane-parallel NACP ionization chamber. The measurements were performed in a proton beam with an initial energy of 173 MeV. It is shown that the Hi-p silicon detector gives a signal which is proporti onal to the ionization density in the silicon crystal in all parts of the B ragg curve, and for all levels of accumulated dose to the detector. This is in contrast to detectors based on n-type silicon, or on low resistivity p- type silicon. After pre-irradiation, these latter detectors show a stopping -power dependent recombination, yielding an increase in the detector signal per unit dose with increasing LET. This effect leads to an over-response i n the Bragg peak, which increases gradually with the accumulated detector d ose. Using the Hi-p silicon diode detector, the depth ionization distribution wa s found to be equal to the distribution obtained with the plane-parallel NA CP ionization chamber at all pre-irradiation levels, within the experimenta l accuracy. This implies that the quotient between the ionization in the de tector and the absorbed dose to the surrounding water is equal for these de tectors.