A method of screening one or more items of freight or baggage, the method comprising the steps of: irradiating the one or more items using a source of incident radiation; detecting packets of radiation emanating from within or passing through the one or more items of freight or baggage as a result of the irradiation by the incident radiation, using a plurality of detectors, each detector being configured to produce an electrical pulse caused by the detected packets having a characteristic size or shape dependent on an energy of the packets; processing each electrical pulse using one or more digital processors to determine the characteristic size or shape; generating a screening detector energy spectrum for each detector of the energies of the packets detected; and measuring an effective atomic number Z of an unknown material associated with the one or more items of freight or baggage based on the screening detector energy spectra in comparison with calibration measurements made during a calibration step; characterised in that the calibration step comprises taking measurements obtained from irradiation of varying thickness samples of different calibration materials of known effective atomic number Z, so as to generate calibration detector energy spectra I(Z,x,B) as a function of Z, calibration material thickness x, and histogram bins B; summing the spectra received at each thickness to minimize the noise in the spectra; taking a reference spectrum I 0 for zero material thickness; computing calibration transmission characteristics Tx(Z,B,x) as a function of Z, material thickness x and energy histogram bins B by dividing the calibration detector energy spectra I(Z,x,B) by a reference spectrum I 0; and computing calibration total transmission ratios R(Z,x) using a sum over the energy histogram bins B of the calibration detector energy spectra I(Z,x,B) divided by a sum over energy histogram bins B of the reference spectrum I 0. irradiating the one or more items using a source of incident radiation; detecting packets of radiation emanating from within or passing through the one or more items of freight or baggage as a result of the irradiation by the incident radiation, using a plurality of detectors, each detector being configured to produce an electrical pulse caused by the detected packets having a characteristic size or shape dependent on an energy of the packets; processing each electrical pulse using one or more digital processors to determine the characteristic size or shape; generating a screening detector energy spectrum for each detector of the energies of the packets detected; and measuring an effective atomic number Z of an unknown material associated with the one or more items of freight or baggage based on the screening detector energy spectra in comparison with calibration measurements made during a calibration step; characterised in that the calibration step comprises taking measurements obtained from irradiation of varying thickness samples of different calibration materials of known effective atomic number Z, so as to generate calibration detector energy spectra I(Z,x,B) as a function of Z, calibration material thickness x, and histogram bins B; summing the spectra received at each thickness to minimize the noise in the spectra; taking a reference spectrum I 0 for zero material thickness; computing calibration transmission characteristics Tx(Z,B,x) as a function of Z, material thickness x and energy histogram bins B by dividing the calibration detector energy spectra I(Z,x,B) by a reference spectrum I 0; and computing calibration total transmission ratios R(Z,x) using a sum over the energy histogram bins B of the calibration detector energy spectra I(Z,x,B) divided by a sum over energy histogram bins B of the reference spectrum I 0.