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FLASH radiation therapy (RT) is a promising technique for cancer treatment making use of radiation beams delivered at high and ultra-high dose rate (> 40 Gy s^-1). However, in these irradiation conditions, some limitations hinder the translation of FLASH-RT into clinical practice, including technical issues related to the dosimetric characterization of FLASH beams. In particular, all commercially available active real time dosimeters (i.e., ionization chambers, semiconductor detectors, and scintillators) have shown to be unsuitable in ultra-high dose per pulse (UH-DPP) conditions, primarily due to saturation problems and nonlinearity of their response. The purpose of this study is to perform a systematic experimental verification to address the major limitations in the microDiamond (mD) device design affecting its response linearity in UH-DPP conditions, and to develop a novel diamond detector specifically designed and optimized for FLASH-RT applications.

A commercial mD detector and several diamond Schottky diode detector prototypes with different layouts were produced at Rome Tor Vergata University in cooperation with PTW-Freiburg. The rationale behind the design of the fabricated prototypes was to explore two key parameters affecting device linearity under UHDR beams, namely the device sensitivity and the series resistance of the Schottky diode.

The produced diamond prototypes were characterized at two Electron UHDR beam facilities equipped with ElectronFLASH Linac (SIT S.p.A., Italy). The typical experimental setup is shown in Figure 1. The linearity of the detector response was tested by varying the DPP up to a maximum value of about 26 Gy/pulse. This was performed by changing the pulse duration, the applicator size and the SSD. Gafchromic films were used as reference dosimeters.

The response of the commercial mD was found to completely saturate in UH-DPP regimes. However, by properly changing the fabrication parameters of the diamond Schottky junction it was possible to extend the linearity range of its response. In particular, a reduction of the series resistance of the detector and/or a reduction of its sensitivity allowed to operate at much higher DPPs. This was achieved by increasing the doping level of the p-type layer of the device, leading to a lower series resistance and by reducing the sensitive volume of the detector. Finally, two optimized prototypes were produced, showing a linear response from conventional dose-rate up to 26 Gy/pulse within a 5% uncertainty (Figure 2). 

The obtained experimental results demonstrated that properly designed diamond detectors can be suitable for UH-DPP FLASH radiation therapy dosimetry. 

The present work is part of the 18HLT04 UHDpulse project (http://uhdpulse-empir.eu/) which has received funding from the European Metrology Programme for Innovation and Research (EMPIR) programme, co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme.