First evaluation of a heteroepitaxial diamond ionization chamber operating at low voltage for diagnostic X-ray dosimetry.

BACKGROUND: Heteroepitaxial diamond has recently gained attention as a radiation detector material due to its wide bandgap, radiation hardness, and near-tissue equivalence. Despite these advantages, its use as a solid-state ionization chamber for diagnostic X-ray dosimetry has not yet been established. Demonstrating stable, high-efficiency operation at low voltage would enable compact dosimeters with a very small sensitive volume, which is difficult to achieve with conventional air ionization chambers. PURPOSE: To perform the first characterization of a heteroepitaxial diamond ionization chamber (HED-IC) operated at low bias voltage under diagnostic X-ray conditions and to evaluate its feasibility as a compact, high-efficiency dosimeter. METHODS: A heteroepitaxial diamond detector (4 × 4 × 0.5 mm) with Ti/Au electrodes was fabricated and evaluated using diagnostic X-ray beams at tube voltages from 50 to 120 kV. Charge-collection characteristics, dose linearity, energy dependence, and temporal response were assessed at negative bias voltages with magnitudes between -1 and -100 V. Monte Carlo simulations were performed using PHITS to compute the expected diamond-to-air sensitivity ratio under the same beam qualities for comparison with the experimental measurements. RESULTS: The HED-IC exhibited excellent dose linearity (R > 0.997) and weak energy dependence (< 10%) across effective energies from 28.4 to 40.1 keV. The detector enables dose measurements within a very small sensitive volume, only 1/1250 of that of a typical air ionization chamber. The volume-normalized sensitivity exceeded theoretical expectations, suggesting enhanced effective ionization efficiency. An increased response with higher bias voltage further indicated potential for high-sensitivity operation. CONCLUSIONS: The results demonstrate that the HED-IC can operate as a low-voltage, high-efficiency solid-state ionization chamber under diagnostic X-ray conditions. Owing to the scalability of heteroepitaxial diamond growth, this detector concept provides a promising basis for compact, tissue-equivalent dosimeters capable of real-time dose monitoring across a wide range of radiological applications.