[en] In connection with the development of a theoretical model for a gamma density tool it was investigated whether multigroup diffusion theory could be used for such a model. The geometry considered was that of a gamma point source in an infinite, homogeneous medium and a point detector in a distance R from the source. The number of neutron groups was 15. The investigation revealed that if an extra transport correction - in addition to that used for neutrons - was introduced in the highest energy group, good agreement could be obtained with moments calculations for the same geometry. This is true elements up to calcium for muR values between 4 and 10 and for gamma energies between 0.5 and 2 MeV. From this theoretical approach a model for the gamma density tool was developed. It agrees reasonable well with calibration curves for such tools. In addition it can take into account the effect of the formation composition. This paper was presented at the meeting on ''Reactor Physics Calculations in the Nordic Countries'', Oslo, Mar. 9 to 10, 1989. (author)