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CHO detectability indices of Gaussian spheres of various sizes for two sets of hot spheres: (a) spheres with 2:1 maximum-activity-to-mean-background ratio [Figs. 3(c) and 3(d)] and (b) spheres with 3:1 maximum-activity-to-mean-background ratio [Figs. 3(e) and 3(f)]. The solid lines connecting calculated data points are added as visual aids. The estimated errors are smaller than the symbols.
