A Prototype Gamma Imaging System for Measuring High-Intensity Proton Beam Spots Based on Pixelated CdZnTe Detectors

Measuring the high-power proton beam distribution on the target is critical for the stable operation of a spallation neutron source or an accelerator-driven subcritical system (ADS). This work preliminarily implemented a step of a proposed methodology for indirectly measuring the proton beam spot using pinhole imaging by detecting back-streaming secondary gammas. A prototype gamma imaging system based on pixelated cadmium zinc telluride (CdZnTe) detectors was developed to serve as a foundation for implementing the proposed method. Each detector consists of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$11\times 11$ </tex-math></inline-formula> pixel array, with a pixel pitch of 1.72 mm. Readout electronic modules for two detectors were designed and realized. The energy and timing information of anode channels is obtained by analog application-specific integrated circuits (ASICs) called JCF032EB based on charge-sensitive amplifiers (CSAs), while the cathode channel signals are read out using CSAs and analog-to-digital converters (ADCs). The anode channels have a maximum input charge of up to 49 fC, while the equivalent noise charge (ENC) of most channels is less than 0.1 fC. Energy correction methods based on depth sensing using the cathode-to-anode (C/A) ratio and election drift time were applied. According to test results using radioactive sources, more than half of the anode channels achieve an energy resolution [full-width at half-maximum (FWHM)] better than 1.8% at 662 keV after depth correction, with the best channel reaching better than 1.1%. The imaging results of a 137Cs point source verified the functioning of the gamma imaging system. To verify the practical feasibility of the imaging method, a nickel target was activated on the proton beam line of the Associated Proton beam Experiment Platform (APEP) at the China Spallation Neutron Source (CSNS) and then imaged by the prototype system, which produced the expected results.