A 3D diamond dosimeter with graphitic surface connections

A new prototype 3D diamond dosimeter featuring laser-written graphitic surface connections and bonding pads has been tested. Diamond substrates are of interest to medical dosimetry as they are closer to bodily tissue equivalence than other solid-state materials. The device in this work was made with a polycrystalline chemical vapour deposition diamond substrate (pCVD) was laser processed to have internal 3D electrode columns and surface connections including the wire-bonding pads to make an all‑carbon detector, with no metal-diamond interfaces. Polycrystalline diamond can be produced with a larger area and cheaper cost than single crystal diamond, but has a relatively slow timing response due to charge trapping defects inside the substrate. To mitigate material defects, 3D sensor technology has been proposed. The 3D design has charge collection electrodes as columns spaced inside the material bulk perpendicular to the surface. When compared to the conventional ‘planar’ sensor design, the 3D arrangement decouples the charge collection distance from the thickness of the substrate, theoretically allowing for more efficient charge collection and the active volumes have smaller dimensions. The laser writing process used for the 3D columns was extended to make graphitic surface connections replacing conventionally used metal bonding contacts between the 3D graphitic columns and readout electronics. The removal of metal-diamond contacts in the detector volume reduces the distortion of the electric field close to the surface of the diamond. The prototype was tested using a laboratory X-ray tube and a clinical Elekta Synergy BM LINAC and was found to have dose-rate independence consistent with previous 3D diamond dosimeters but with lower operating voltages, the lowest being +6 V. Lower operating voltage is attributed to the removal of the electric field distortion at diamond-metal interfaces. This paper proposes the first implementation of an all‑carbon 3D diamond dosimeter using a polycrystalline substrate.