A FILTER TECHNIQUE FOR OPTIMIZING THE PHOTON ENERGY RESPONSE OF A SILICON PIN DIODE DOSIMETER

Silicon PIN diodes are used routinely as radiation detectors in electr onic pocket dosemeters. Unless they are energy compensated, a signific ant over-response occurs below 200 keV. At least one manufacturer (Sie mens) is using three diodes in parallel with individual filters to pro duce excellent energy and angular response. It is also possible to use an algorithm based on the photon spectrum of a single diode to flatte n the energy response. However, the more cost effective commercial pra ctice is to use a single diode with a simple filter to flatten the ene rgy response-with the major disadvantage of sacrificing low energy pho ton response. The technique of using a filter with an opening has been used previously, typically for energy compensating GM detectors and p roportional counters. A new variation of this technique has been inves tigated, which not only compensates the energy response of a silicon P IN diode but also maintains an extended low energy response. The techn ique consists of using a composite filter of two or more materials tog ether with several openings whose individual area is in the range of 1 5% to 25% of the diode's active area. One of the openings is centred o ver the diode's active area, while the others are located al the perip hery of the active area to preserve a good polar response to +/-45 deg rees. Monte Carlo radiation transport methods were used to simulate th e coupled electron-photon transport through a Hamamatsu S2506-01 diode and to determine the energy response of the diode for a variety of fi lters. Composite filters were designed, and optimised for both current and pulse mode operation of the diode. In current mode, the resultant dosemeter energy response relative to air dose was within -15% and +3 0% for 0 degrees incidence over the energy range from 15 keV to 1 MeV. For +/-45 degrees incidence, the energy response was within -30% and +20% relative to air dose from 30 keV to 1 MeV. In pulse mode, the res ultant dosemeter energy response was within -25% and +50% for 0 degree s incidence over the energy range from 30 keV to 10 MeV. For +/-45 deg rees incidence, the energy response was within -25% and +40% from 40 k eV to 10 MeV. This filter technique is applicable to a variety of sili con diodes. The work and results described in this paper are intended to show the theoretical viability of the filter technique.