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  4. On the Discrepancy of Experimental and Monte-Carlo Simulated Dose Response of the MicroDiamond Detector: The Role of Charge Imbalance in Structural Components

On the Discrepancy of Experimental and Monte-Carlo Simulated Dose Response of the MicroDiamond Detector: The Role of Charge Imbalance in Structural Components

HK Looe1*, D Poppinga2 , R Kranzer2 , I Buesing1 , T Tekin1 , AB Ulrichs1 , B Delfs1 , D Vogt2 , J Wuerfel2 , B Poppe1 , (1) University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, Germany (2) PTW Freiburg, Germany

Presentations

(Sunday, 7/14/2019) 4:00 PM - 5:00 PM

Room: Stars at Night Ballroom 1

Purpose: To investigate the role of radiation induced charge imbalance in the structural components of the microDiamond (mD) detector (type 60019, PTW Freiburg, Germany) as the possible cause of the discrepancy between its experimental and Monte-Carlo simulated dose response at small field sizes.

Methods: Three modified versions of the detector were constructed: the type of charge carriers collected is reversed (mD_reversed); the detector’s contacts have been shortened (mD_shortened); and the diamond chip with the sensitive volume has been removed (mD_noChip). Output ratio (OR) were measured using a standard and the three modified detectors at nominal field sizes from 6x6 mm2 to 40x40 mm2. Output correction factors were derived from measured OR and Monte-Carlo simulations. An adapted Monte-Carlo user-code introduced previously was used to quantified the field size dependent charge imbalance in the structural components.

Results: The output correction factors differ by up to 3% between the standard and the modified mD_reversed detector. Field size dependent corrections for the effect of change imbalance have been derived from the measured OR. After correction, the mD detector shows an over-response of less than 2% in the range of investigated field sizes, which agrees to the Monte-Carlo simulated output correction factors. By shortening its contacts, the mD_shortened detector exhibits output correction factors comparable to those computed from the corrected OR.

Conclusion: The role of radiation induced charge imbalance on the dose response of mD detector in small fields dosimetry has been studied and quantified for the first time. The discrepancy between experimental and Monte-Carlo simulated output correction factors diminishes after the effect has been accounted for. A modified mD detector was constructed based on the component analysis using the adapted Monte-Carlo code with favourable behaviour requiring only corrections of less than 2%.

Keywords

Dosimetry, Small Fields, Volume Effects

Taxonomy

TH- External beam- photons: Small/nonstandard field experimental dosimetry

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