A core scanner with a 10cm x 10cm Nal(Tl) crystal and a 256 channel analyzer has been built and calibrated for spectral gamma-ray measurement of Th, U and K in cores from boreholes. The calibration has been performed for cores of different diameters and densities, and the influence of liners, incomplete cores, and a varying core to detector distances have been investigated. A single formula for determination of the sensitivities for the window method has been developed. The stability against changes in temperature and radon background is evaluated. Proposals for an improved and faster measuring technique with two or more detectors is put forward. (au).