Spectral perturbations from silicon diode detector encapsulation and shielding in photon fields
2010 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 37, no 11, 6055-6060 p.Article in journal (Refereed) Published
Silicon diodes are widely used as detectors for relative dose measurements in radiotherapy. Generally two types of diode mountings are used. Plastic encapsulation is used for electron fields while the encapsulation for diodes intended for photon fields include a shield of high density material (typically tungsten). The purpose of the shield is to absorb low energy scattered photons to which a silicon diode over-responses. However, new models based on spectra calculations have been proposed for direct correction of the readout from unshielded (e.g.”electron”) diodes in photon fields. This raises the question whether it is correct to assume that the spectrum calculated for water is not disturbed by the detector encapsulation. This work aims at investigating the spectral effects of the encapsulation materials typical for typical silicon diodes used in radiotherapy clinics, including the effects of the shielding traditionally used for photon field diodes.
The effects of detector encapsulation of an unshielded and a shielded commercial diode on the spectra at the silicon chip location are studied through Monte Carlo simulations with PENELOPE-2005. Variance reduction based on importance sampling and correlated sampling is applied to reduce the CPU-time needed for the simulations.
The use of variance reduction is proved to be efficient and to not introduce any significant bias of the results. Compared to reference spectra calculated in water, the encapsulation for an unshielded diode is demonstrated to not perturb the spectrum while tungsten shielded diode caused not only the desired decrease in low energy scattered photons but also a large increase of primary electrons of all energies. Measurements with a shielded diode in a 6MV photon beam proved that the shielding does not completely remove the field-size dependence of the detector response caused by the over response from low energy photons.
Spectra calculated for water can be directly used for modeling the response of silicon diodes with plastic only encapsulations. For photon dose measurements, an unshielded diode used together with appropriate corrections gives more accurate results than the traditionally used shielded diodes. Variance reduction for diode simulations can effectively be applied, however with great considerations considering choice of application.
Place, publisher, year, edition, pages
2010. Vol. 37, no 11, 6055-6060 p.
silicone diode; photon spectra; detector response
Medical and Health Sciences
IdentifiersURN: urn:nbn:se:uu:diva-120602DOI: 10.1118/1.3501316ISI: 000283747600051OAI: oai:DiVA.org:uu-120602DiVA: diva2:303653