2026-03-05 東京都立大学
＜関連情報＞
- https://www.tmu.ac.jp/news/topics/38416.html
- https://aapm.onlinelibrary.wiley.com/doi/10.1002/mp.70363
- https://link.springer.com/article/10.1007/s10854-026-16690-6
診断用X線線量測定のための低電圧で動作するヘテロエピタキシャルダイヤモンド電離箱の最初の評価 First evaluation of a heteroepitaxial diamond ionization chamber operating at low voltage for diagnostic X-ray dosimetry
Kiyomitsu Shinsho, Koji Koyama, Keitaro Hitomi, Mitsuhiro Nogami, Osamu Maida, Toshiyuki Onodera, Kanata Kikkawa, Shimma Hashimoto, Yuta Hirai, Yusuke Koba, Ako Haga, Daiki Maruyama, Seong-Woo Kim
Medical Physics Published: 27 February 2026
DOI:https://doi.org/10.1002/mp.70363
Abstract
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 mm3) 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 (R2 > 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.
窒素濃度の異なるヘテロエピタキシャルダイヤモンドの光刺激発光特性：前照射と発光中心活性化の影響 Optically stimulated luminescence characteristics of heteroepitaxial diamond with different nitrogen concentrations: effects of pre-irradiation and luminescence center activation
Kiyomitsu Shinsho,Go Okada,Koji Koyama,Keitaro Hitomi & Seong Woo Kim
Journal of Materials Science: Materials in Electronics Published:28 January 2026
DOI:https://doi.org/10.1007/s10854-026-16690-6
Abstract
This study systematically investigated the optically stimulated luminescence (OSL) characteristics of heteroepitaxial diamond (HED) with two different nitrogen concentrations (3 ppb and 1 ppm). The nitrogen-rich sample exhibited substantially higher OSL intensity than the low-nitrogen sample, reflecting an increased density of trapping sites and luminescent centers associated with nitrogen-related defects. Both samples showed weak dose dependence below 10 Gy and saturation at 100–200 Gy, consistent with the classical center-depletion model. A pronounced enhancement in OSL sensitivity was observed only in the nitrogen-rich sample following pre-irradiation up to 400 Gy. This enhancement is attributed to increased trapped-electron population, activation of previously inactive luminescent centers, and improved radiative recombination efficiency. The enhancement was completely removed by annealing at 500 °C for 60 s, indicating that the effect originates from metastable changes in defect occupancy rather than permanent defect formation. The OSL emission spectra exhibited a single broad band between 500 and 800 nm, peaking at 620–650 nm, corresponding to the phonon sideband of NV⁻-related luminescence. No zero-phonon line at 637 nm was resolved, consistent with the broad-band, phonon-assisted nature of the measured OSL emission. The spectral shape remained unchanged after pre-irradiation. Fading measurements further revealed that the nitrogen-rich diamond possesses more thermally stable trap levels than the low-nitrogen sample. These findings demonstrate that nitrogen incorporation is an effective strategy for tuning the OSL performance of HED and the potential of nitrogen-rich HED for high-dose and reusable dosimetry applications.
