Speaker
Description
Portable 4π gamma-ray imagers are promising tools for nuclear security, decommissioning, and contamination monitoring. However, existing gamma-ray imagers are generally expensive, which limits their use in applications involving multiple imagers. One major reason for the high cost is that conventional systems require from hundreds to tens of thousands of readout channels, as well as complex processing of list-mode data including timing and energy information. Therefore, if a 4π gamma-ray imager based on a small number of detectors and simple signal processing can be realized, its cost can be significantly reduced.
In this study, we developed a compact 4π gamma-ray imager, the Coded Cube Camera for Gamma-ray (C3G), based on a small number of detectors and simple signal processing. C3G estimates the source direction using detector response patterns that vary with the incident gamma-ray direction, with detectors and lead shielding cubes arranged in a three-dimensional array. We previously reported a proof-of-concept prototype consisting of 8 detectors and 18 lead cubes in a 3×3×3 configuration [1]. In the present study, we extended this concept and designed and fabricated a practical system consisting of 24 detectors and 40 lead cubes arranged in a 4×4×4 configuration. HR-GAGG scintillators (10 mm×10 mm×10 mm) and silicon photomultipliers were used as detectors. The developed 24-detector C3G is shown on Fig. 1. The overall dimensions are 130 mm × 130 mm × 155 mm, and the total weight is 2.3 kg. A USB-C cable connected to a PC is used for both power supply and data transfer. C3G does not require coincidence detection and uses only the total counts recorded by each detector during the measurement time. It also uses far fewer detectors and readout channels than conventional gamma-ray imagers. C3G uses far fewer detectors and readout channels than conventional systems, and its cost is expected to be less than one-tenth that of conventional gamma-ray imagers.
Here, H denotes the measured detection intensity vector of the 24 detectors, R the response matrix of C3G for each gamma-ray incident direction, and W the unknown source distribution. Their relationship is expressed as H=RW. In this study, W was estimated by gradient descent to minimize σ=|H-RW|^2. The response matrix R was calculated by Monte Carlo simulation using GEANT4. 511 keV gamma rays were irradiated from a total of 614 directions defined by a 10° mesh in azimuth angle φ and polar angle θ, and the number of events with an energy deposit of 511 ± 34 keV was recorded as the detection intensity for each direction. These responses were then interpolated to a 5° mesh using spline interpolation, yielding a response matrix for a total of 2522 directions.
To experimentally evaluate the performance of the developed C3G, 11C (511 keV) produced with the AVF cyclotron at TIARA, QST, was used as the imaging target. First, in a single-source experiment, a 27 MBq 11C source was placed at φ = 180° and θ = 90°, approximately 120 cm from the C3G, and 60 consecutive measurements of 60 s each were performed, giving a total measurement time of 1 h. The reconstructed image obtained from the first 60 s measurement is shown in Fig. 2. The image was correctly reconstructed at φ = 180° and θ = 90°, and the full widths at half maximum of the line profiles in the φ and θ directions were both 5.9°. In addition, the summed intensity of the 3 × 3 pixels centered on the true source position decreased exponentially with time, and the half-life estimated from the fitted curve was 20.95 ± 0.51 min. This agrees well with the known half-life of 11C, 20.36 min, indicating that the C3G can track not only the source position but also the temporal change in radioactivity.
Next, a two-source separation experiment was performed using 28 MBq and 34 MBq 11C sources. The distance from the center of the two sources to the C3G was fixed at 100 cm, while the source-to-source distance varied to 60 cm, 40 cm, and 20 cm. Each condition was measured 300 s. When the source separation was 20 cm, the two sources could not be clearly resolved, and the minimum resolvable angular separation under this condition was found to be 23° (40 cm).
These results show that the developed 24-detector C3G realizes a low-cost compact 4π gamma-ray imager based on a small number of detectors and simple signal processing, and that its fundamental performance was confirmed through 11C imaging experiments. In future work, the target radionuclides will be expanded from below 200 keV to above 1 MeV to verify the effectiveness of this system as a low-cost 4π gamma-ray imager with a wide energy range and a wide field of view.
[1] Yoshiharu Kitayama et al., Jpn. J. Appl. Phys., 63, 076502, DOI:10.35848/1347-4065/ad5ba0, (2024)