The role of radiation‐induced charge imbalance on the dose‐response of a commercial synthetic diamond detector in small field dosimetry

Abstract

Purpose Discrepancy between experimental and Monte Carlo simulated dose–response of the microDiamond (mD) detector (type 60019, PTW Freiburg, Germany) at small field sizes has been reported. In this work, the radiation‐induced charge imbalance in the structural components of the detector has been investigated as the possible cause of this discrepancy. Materials and methods Output ratio (OR) measurements have been performed using standard and modified versions of the mD detector at nominal field sizes from 6 mm × 6 mm to 40 mm × 40 mm. In the first modified mD detector (mD_reversed), the type of charge carriers collected is reversed by connecting the opposite contact to the electrometer. In the second modified mD detector (mD_shortened), the detector's contacts have been shortened. The third modified mD detector (mD_noChip) is the same as the standard version but the diamond chip with the sensitive volume has been removed. Output correction factors were calculated from the measured OR and simulated using the EGSnrc package. An adapted Monte Carlo user‐code has been used to study the underlying mechanisms of the field size‐dependent charge imbalance and to identify the detector's structural components contributing to this effect. Results At the smallest field size investigated, the OR measured using the standard mD detector is >3% higher than the OR obtained using the modified mD detector with reversed contact (mD_reversed). Combining the results obtained with the different versions of the detector, the OR have been corrected for the effect of radiation imbalance. The OR obtained using the modified mD detector with shortened contacts (mD_shortened) agree with the corrected OR, all showing an over‐response of less than 2% at the field sizes investigated. The discrepancy between the experimental and simulated output correction factors has been eliminated after accounting for the effect of charge imbalance. Discussions and conclusions The role of radiation‐induced charge imbalance on the dose–response of mD detector in small field dosimetry has been studied and quantified. It has been demonstrated that the effect is significant at small field sizes. Multiple methods were used to quantify the effect of charge imbalance with good agreement between Monte Carlo simulations and experimental results obtained with modified detectors. When this correction is applied to the Monte Carlo data, the discrepancy from experimental data is eliminated. Based on the detailed component analysis using an adapted Monte Carlo user‐code, it has been demonstrated that the effect of charge imbalance can be minimized by modifying the design of the detector's contacts.

Technical Note: Characterization of the new microSilicon diode detector

“…As shown in previous studies, the factors k / k vol of the silicon diode detectors are dominated by the density effect with values less than unity, while the microSilicon exhibits less density perturbation mainly owed to the reduced density of the epoxy layers. For the microDiamond detector, it has been recently shown that the observed over‐response is partly caused by radiation‐induced charge imbalance in the detector’s components . Nevertheless, this over‐response is largely compensated in small‐fields by the volume‐averaging effect, resulting in overall correction factors, k, similar to that of microSilicon.…”

Three‐dimensional characterization of the active volumes of PTW microDiamond, microSilicon, and Diode E dosimetry detectors using a proton microbeam

“…Furthermore, a recent study has demonstrated the significant role of radiation‐induced charge imbalance in the contacts and cable of the microDiamond detector on its dose response in small photon fields. Although the thickness of the active volume plays directly a minor role in dose perturbation effects, this type of field size‐dependent radiation‐induced charge will be collected in addition to the charge released in the detector’s sensitive volume.…”

Evaluation of the PTW microDiamond in edge‐on orientation for dosimetry in small fields

“…Recent work has also quantified an additional effect of radiation-induced charge imbalance, in the PTW microDiamond. 23 At field sizes of length <2 cm radiation induced charge in the electrical contact of the PTW microDiamond is reported to result in an overresponse of the device. Despite these limitations for dosimetry applications the microDiamond remains an area of interest for QA with its response having recently been characterized for use in an MRI linac 24 and for the MRI-associated surface dose from electron contamination.…”