DOI responses of all crystals in an array (a) at five individual depths of 2, 6, 10, 14, 18 mm and (b) for simultaneous irradiation of all depths in the array. The two vertical lines in (b) mark the DOI ratio at half of the maximum counts on the left and right hand side of the DOI responses for the entire array, which are treated as corresponding to irradiation depths of 0 and 20 mm (two ends of the arrays). This information is used to derive the linear calibration curve. (c) DOI responses for all 49 crystals in one detector array.