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Spectral perturbations from silicon diode detector encapsulation and shielding in photon fields
Author(s) -
Eklund Karin,
Ahnesjö Anders
Publication year - 2010
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3501316
Subject(s) - diode , detector , electromagnetic shielding , photon , optoelectronics , dosimetry , shielded cable , materials science , spectral line , monte carlo method , optics , silicon , particle detector , radiation , physics , nuclear medicine , computer science , telecommunications , medicine , statistics , mathematics , astronomy , composite material
Purpose: Silicon diodes are widely used as detectors for relative dose measurements in radiotherapy. The common manufacturing practice is to encapsulate the diodes in plastic for protection and to facilitate mounting in scanning devices. Diodes intended for use in photon fields commonly also have a shield of a high atomic number material (usually tungsten) integrated into the encapsulation to selectively absorb low‐energy photons to which silicon diodes would otherwise over‐response. However, new response models based on cavity theories and spectra calculations have been proposed for direct correction of the readout from unshielded (e.g., “electron”) diodes used in photon fields. This raises the question whether it is correct to assume that the spectrum in a water phantom at the location of the detector cavity is not perturbed by the detector encapsulation materials. The aim of this work is to investigate the spectral effects of typical encapsulations, including shielding, used for clinical diodes. Methods: The effects of detector encapsulation of an unshielded and a shielded commercial diode on the spectra at the detector cavity location are studied through Monte Carlo simulations with PENELOPE‐2005 . Variance reduction based on correlated sampling is applied to reduce the CPU time needed for the simulations. Results: The use of correlated sampling is found to be efficient and to not introduce any significant bias to the results. Compared to reference spectra calculated in water, the encapsulation for an unshielded diode is demonstrated to not perturb the spectrum, while a tungsten shielded diode caused not only the desired decrease in low‐energy scattered photons but also a large increase of the primary electron fluence. Measurements with a shielded diode in a 6 MV 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. Response factors of a properly corrected unshielded diode were shown to give comparable, or better, results than the traditionally used shielded diode. Conclusions: Spectra calculated for photon fields in water can be directly used for modeling the response of unshielded silicon diodes with plastic encapsulations. Unshielded diodes used together with appropriate corrections can replace shielded diodes in photon dose measurements.