Investigation of nuclear material using a compact modified uniformly redundant array gamma camera

We developed a compact gamma camera based on a modified uniformly redundant array coded aperture to investigate the position of a UO2 pellet emitting characteristic X-rays (98.4 keV) and γ-rays (185.7 keV). Experiments using an only-mask method and an antimask subtractive method were conducted, and the maximum-likelihood expectation maximization algorithm was used for image reconstruction. The images obtained via the antimask subtractive method were compared with those obtained using the only-mask method with regard to the signal-to-noise ratio. The reconstructed images of the antimask subtractive method were superior. The reconstructed images of the characteristic X-rays and the γ-rays were combined with the obtained image using the optical camera. The combined images showed the precise position of the UO2 pellet. According to the self-absorption ratios of the nuclear material and the minimum number of effective events for image reconstruction, we estimated the minimum detection time depending on the amount of nuclear material.