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Energy Dependence of the Response of a Silicon Diode Designed for Small-Field Dosimetry

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S Aeffner

S Aeffner*, M Yarahmadi, O Sauer, Julius-Maximilians-University, Wuerzburg, Germany

SU-E-T-80 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
To investigate the energy dependence/spectral sensitivity of silicon diodes designed for small-field dosimetry and obtain correction factors for arbitrary photon spectra using Monte Carlo simulations.

Methods:
The dose to the active volume of a stereotactic diode field detector (IBA Dosimetry) was measured for several low-energy photon spectra (mean energy 40 to 200 keV) as well as for Co60-radiation (mean energy 1.25 MeV) and an Elekta Synergy beam (mean energy 2.9 MeV) under reference conditions. The corresponding dose to water was obtained by a PTW 31010 ionization chamber.
Monte Carlo calculations corresponding to the experiments were carried out by the EGS user code DOSRZnrc. Dose to the volume of the diode as well as dose to water were calculated using the photon spectra of the measurements.

Results:
At the lowest photon energies used, the diode over-responds by a factor of more than 5 due to the increasing mass energy absorption coefficient of silicon compared to that of water. This general behavior is reproduced in both our measurements and simulations. Energy correction factors for determination of dose-to-water obtained by experiments and simulations differ by less than 5% at mean energies below and <1% at energies above 1 MeV.

Conclusion:
While at high photon energies the change of response is marginal, it is extreme at low energies. Therefore it is desirable to benchmark response calculations also in the low energy domain. If calculations are accurate in this region, it is likely that they are also accurate for a realistic high energy spectrum.
Our first results, with a simple geometry, indicate that validation of the simulations by the experimental results is achievable. Further refinement of the Monte Carlo modeling, as well as extension to other diode detectors is in progress. In a second step, corrections required for small field sizes will be studied.


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