A Portable Three-Layer Compton Camera for Wide-Energy-Range Gamma-ray Imaging: Design, Simulation and Preliminary Testing

Abstract

(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.

Keywords: Compton camera; Monte Carlo simulation; image reconstruction; scintillation detector; wide energy range.

Figure 1

The principle diagram of far-field energy-domain imaging method for wide-energy-range gamma-ray imaging.

Figure 2

The detector simulation model constructed in Geant4. (a) The schematic of materials and placement. (b) The schematic of optical photon transportation in a single detector module. (c) The schematic of the three-layer detectors.

Figure 3

Percentage of effective 2-events of full absorption under different incident energies, given by using mono-energetic gamma-ray sources in Geant4.

Figure 4

The curves of detection efficiency and angular resolution with the distance between the first and second layers (D1) at different incident energies when D2 = 22 mm. (a) The detection efficiency is indicated by the percentage of effective 2-events of full absorption. (b) The angular resolution is indicated by the full width half maximum (FWHM) of the reconstructed images.

Figure 5

Photograph and signal processing flow of the three-layer Compton camera prototype.

Figure 6

The spectral performance of pixel No.29 and pixel No.112 in the three detectors at eight energies (59.5 keV, 81 keV, 122 keV, 511 keV, 661.7 keV, 834.8 keV, 1274.5 keV, 1332.5 keV). (a) The photopeak energies (keV) as functions of digital pulse amplitude. (b) The measured FWHMs of the photopeak energies and fitting curves.

Figure 7

Comparison between the experimental measurements and simulated data for the coincidence summed energy spectra of the 2-events: (a) ¹³³Ba source; (b) ¹³⁷Cs source; (c) ⁶⁰Co source.

Figure 8

The reconstructed images of single point-like source (¹³³Ba, ¹³⁷Cs, ⁶⁰Co) at the center of FOV with the conditions of simulations and experiments in Table 1. The iteration numbers of 1 and 10 are used to reconstruct the image. The profile plots taken through the maximum value of the reconstructed image on polar angle direction and azimuthal angle direction are shown.

Figure 9

The simulation and experimental results of the reconstructed images after 10 iterations for a source (¹³³Ba, ¹³⁷Cs, ⁶⁰Co) positioned at the boundary of field of view.

Figure 10

The reconstructed images with 1 iteration of MLEM for a 0.05 μSv/h ¹³⁷Cs source at (θ, φ) = (0°, 0°). The hotspot can be preliminarily identified within 10 s.

Figure 11

The reconstructed image of two ¹³⁷Cs point-like sources separated by 15° for the measurement of angular resolution. (a) The simulation results. (b) The experimental results.

Figure 12

The coincidence summed energy spectra of the 2-events for the simultaneous measurement of 3 point-like sources (¹³³Ba, ¹³⁷Cs, and ⁶⁰Co). Both the simulation and experimental results are shown.

Figure 13

Multiple source localization with the prototype under the following test conditions: a ¹³³Ba source positioned at (θ, φ) = (0°, −45°), a ¹³⁷Cs source positioned at (θ, φ) = (0°, −10°), and a ⁶⁰Co source positioned at (θ, φ) = (0°, 15°). The reconstructed images at the corresponding energy slices and the reconstructed initial energy spectra after 3 iterations are shown. The vertical axis of the reconstructed energy spectra is represented by normalized intensity. (a) The simulation results. (b) The experimental results.

Figure 14

The measured single interaction (1-events) spectra and the coincidence summed energy spectra of the 2-events, in which the data are generated from a water target under 14.1 MeV neutron bombardment in the Geant4 simulation. The relative intensity of initial gamma-ray energies is indicated by the blue line.

Figure 15

The reconstructed initial energy spectra and the reconstructed images with 1 iteration of MLEM for the water target under 14.1 MeV neutron bombardment in the Geant4 simulation. The energy slices of E_0bin(310) and E_0bin(341) represent the initial energy around 3100 keV and 6130 keV, respectively. (a) The gamma-ray emitter of water target is positioned at (θ, φ) = (0°, 0°). (b) The gamma-ray emitter of water target is positioned at (θ, φ) = (15°, 0°).