(1) Background: The imaging energy range of a typical Compton camera is limited due to the fact that scattered gamma photons are seldom fully absorbed when the incident energies are above 3 MeV. Further improving the upper energy limit of gamma-ray imaging has important application significance in the active interrogation of special nuclear materials and chemical warfare agents, as well as range verification of proton therapy. (2) Methods: To realize gamma-ray imaging in a wide energy range of 0.3~7 MeV, a principle prototype, named a portable three-layer Compton camera, is developed using the scintillation detector that consists of an silicon photomultiplier array coupled with a Gd₃Al₂Ga₃O₁₂:Ce pixelated scintillator array. Implemented in a list-mode maximum likelihood expectation maximization algorithm, a far-field energy-domain imaging method based on the two interaction events is applied to estimate the initial energy and spatial distribution of gamma-ray sources. The simulation model of the detectors is established based on the Monte Carlo simulation toolkit Geant4. The reconstructed images of a ¹³³Ba, a ¹³⁷Cs and a ⁶⁰Co point-like sources have been successfully obtained with our prototype in laboratory tests and compared with simulation studies. (3) Results: The proportion of effective imaging events accounts for about 2%, which allows our prototype to realize the reconstruction of the distribution of a 0.05 μSv/h ¹³⁷Cs source in 10 s. The angular resolution for resolving two ¹³⁷Cs point-like sources is 15°. Additional simulated imaging of the 6.13 MeV gamma-rays from 14.1 MeV neutron scattering with water preliminarily demonstrates the imaging capability for high incident energy. (4) Conclusions: We conclude that the prototype has a good imaging performance in a wide energy range (0.3~7 MeV), which shows potential in several MeV gamma-ray imaging applications.

This work presents a novel stereo coded aperture gamma camera based on four detection modules placed within four quadrants. Each module includes 48 × 48 strips of Bi₄Ge₃O₁₂ scintillators, 24 × 24 pieces of silicon photomultipliers (SiPMs), and a set of data acquisition circuits. All signals from the SiPMs are individually extracted and finally simplified into 144 outputs to calculate the positions and energies of gamma-rays. Two masks of modified uniformly redundant arrays are fixed in the front of the top-right and bottom-left modules and two anti-masks are fixed in the top-left and bottom-right modules to achieve simultaneous imaging without dual measure. Once the projections of radioactive sources are determined, their source-to-detector distances can be estimated by our new multocular ranging algorithm. A series of lab and field experiments, including flood histogram and energy spectrum testing, field-of-view and angular resolution testing, real-time imaging, long-distance imaging, and ranging ability testing, with different radioactive sources, such as ¹³⁷Cs, ²²Na, ²⁴¹Am, ⁶⁰Co, ¹³³Ba, and ⁷⁵Se, have been completed to evaluate the comprehensive ability of the stereo gamma camera. The results show that the stereo gamma camera has a strong ability for the three-dimensional localization of radioactive sources over long distances with a high image sensitivity. So far, the stereo gamma camera has been successfully used in practical nuclear security and safety tasks although the angular resolution and the ranging algorithm robustness should be more optimized in the future.

Y2SiO5: Ce (YSO) scintillation with characteristics of high light yield, fast decay time and high Compton scattering fraction shows good application potential for Compton imaging. In this study, we propose a three-dimensional (3-D) scintillator detector, which is segmented by the YSO monolithic rods and pixelated by sub-surface laser engraving (SSLE) in the depth direction. Two arrays of silicon photomultipliers are optically coupled to the YSO array at each end, in which the pixels are designed for a one-to-one match. By identifying the location number of array pixels and measuring depth-of-interaction (DOI) of pixelated scintillator rods, the interaction coordinates of gamma photon can be obtained accurately. After energy consistent calibration, the energy resolutions of the pixels in 3-D detector were evaluated. The imaging test results indicate that the detector has a capability to locate a 0.1 µSv/h Cs-137 gamma-ray source within 90 seconds in 4π field-of-view (FOV). The efficiency of the imaging events is 2.2 %. By using the maximum likelihood expectation maximization (MLEM) algorithm with 10 iterations, the full width at half maximum (FWHM) of the reconstructed hotspot decreases to less than 9.0 °. This configuration mainly simplifies the calculation of interaction coordinates compared to the center of gravity method. Moreover, the higher segment precision for DOI estimation are realized thanks to the SSLE technique. The 3-D scintillator detector achieves 4π Compton imaging with high detection efficiency and high localization accuracy.