A method for calibrating an apparatus includes, in the case of a known process value, measuring a detector value of a first type only based on captured gamma rays that are not scattered or are scattered little; calculating a calibration assignment based on a process model; in the case of at least one unknown process value, measuring a detector value of the first type and measuring a detector value of a second type at least based on captured gamma rays that are scattered; determining the unknown process value by using the calculated calibration assignment based on the measured detector value of the first type; and modifying the calibration assignment by assigning the measured detector value of the second type to the determined process value.