Design of a composite collimator for the hard X-ray camera in HL-2M tokamak

A new hard X-ray (HXR) camera is being designed for the HuanLiu-2M (HL-2M) tokamak to measure the spatial-temporal evolution of the fast electrons in the energy range of 20-200 keV during radio frequency (RF) drive or heating experiments. The new camera integrates 63 channels of independent detectors consisting of 63 lutetium yttrium oxyorthosilicate scintillators (LYSO) coupled to silicon photomultipliers (SiPMs). A composite collimator (pinhole + collimating tube) is designed for the HXR camera to arrange the 63 channels in a compact space and reduce background rays. This paper introduces the design of composite collimator and the calculation of geometric parameters of the pinhole and collimating tube. The geometric factor uniformity (GFU) and effective count rate (ECR) of the composite collimator were evaluated by Geant4 simulation for different pinhole inner wall thicknesses T-1. The results show that when the energy is 200 keV, GFU increases and ECR increases and then decreases when reducing T-1. The optimal T-1 is 2 mm, and the resulting GFU is 0.83 and ECR is 78.2% for 200 keV.