Method for calibrating an apparatus for measuring a process value of at least one substance, - wherein the apparatus has a detector device, wherein the detector device is embodied for measuring a detector value by means of capturing gamma rays that have at least partially penetrated at least the substance, and for producing the detector value based on the captured gamma rays, - wherein the method includes the steps of: a) 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, b) calculating a calibration assignment based on a process model, wherein the process model is based on a description of the penetration only taking into account unscattered gamma rays, and based on the measured detector value of the first type, wherein the calibration assignment assigns in each case a detector value of the first type to different process values, c) 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 or scattered more, d) determining the unknown process value by means of the calculated calibration assignment based on the measured detector value of the first type, and e) modifying the calibration assignment by means of assigning the measured detector value of the second type to the determined process value, - wherein the method includes: - measuring the detector values of the first type in each case only based on captured gamma rays having gamma energies that are the same as or greater than a first energy threshold value, and - measuring the detector value of the second type at least based on captured gamma rays having gamma energies that are smaller than the first energy threshold value. - wherein the apparatus has a detector device, wherein the detector device is embodied for measuring a detector value by means of capturing gamma rays that have at least partially penetrated at least the substance, and for producing the detector value based on the captured gamma rays, - wherein the method includes the steps of: a) 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, b) calculating a calibration assignment based on a process model, wherein the process model is based on a description of the penetration only taking into account unscattered gamma rays, and based on the measured detector value of the first type, wherein the calibration assignment assigns in each case a detector value of the first type to different process values, c) 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 or scattered more, d) determining the unknown process value by means of the calculated calibration assignment based on the measured detector value of the first type, and e) modifying the calibration assignment by means of assigning the measured detector value of the second type to the determined process value, a) 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, b) calculating a calibration assignment based on a process model, wherein the process model is based on a description of the penetration only taking into account unscattered gamma rays, and based on the measured detector value of the first type, wherein the calibration assignment assigns in each case a detector value of the first type to different process values, c) 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 or scattered more, d) determining the unknown process value by means of the calculated calibration assignment based on the measured detector value of the first type, and e) modifying the calibration assignment by means of assigning the measured detector value of the second type to the determined process value, - wherein the method includes: - measuring the detector values of the first type in each case only based on captured gamma rays having gamma energies that are the same as or greater than a first energy threshold value, and - measuring the detector value of the second type at least based on captured gamma rays having gamma energies that are smaller than the first energy threshold value.