Experimental determination of the PTW 60019 microDiamond dosimeter active area and volume

Small field output correction factors have been studied by several research groups for the PTW 60019 microDiamond (MD) dosimeter, by comparing the response of such a device with both reference dosimeters and Monte Carlo simulations. A general good agreement is observed for field sizes down to about 1 cm. However, evident inconsistencies can be noticed when comparing some experimental results and Monte Carlo simulations obtained for smaller irradiation fields. This issue was tentatively attributed by some authors to unintentional large variations of the MD active surface area. The aim of the present study is a nondestructive experimental determination of the MD active surface area and active volume.

Ten MD dosimeters, one MD prototype, and three synthetic diamond samples were investigated in the present work. 2D maps of the MD response were recorded under scanned soft x-ray microbeam irradiation, leading to an experimental determination of the device active surface area. Profiles of the device responses were measured as well. In order to evaluate the MD active volume, the thickness of the diamond sensing layer was independently evaluated by capacitance measurements and alpha particle detection experiments. The MD sensitivity, measured at the PTW calibration laboratory, was also used to calculate the device active volume thickness.

An average active surface area diameter of (2.19 ± 0.02) mm was evaluated by 2D maps and response profiles of all the MDs. Average active volume thicknesses of (1.01 ± 0.13) μm and (0.97 ± 0.14) μm were derived by capacitance and sensitivity measurements, respectively. The obtained results are well in agreement with the nominal values reported in the manufacturer dosimeter specifications. A homogeneous response was observed over the whole device active area. Besides the one from the device active volume, no contributions from other components of the housing nor from encapsulation materials were observed in the 2D response maps.

The obtained results demonstrate the high reproducibility of the MD fabrication process. The observed discrepancies among the output correction factors reported by several authors for MD response in very small fields are very unlikely to be ascribed to unintentional variations of the device active surface area and volume. It is the opinion of the authors that the role of the volume averaging as well as of other perturbation effects should be separately investigated instead, both experimentally and by Monte Carlo simulations, in order to better clarify the behaviour of the MD response in very small fields.