Efficiency determination of a 4πγ-detector by numerical simulation

4πγ-counting is a well established method for direct activity measurements, and is especially suited for radionuclides with complex gamma-ray spectra. It is shown that numerical simulation is a convenient way to obtain the counting efficiency of a large well-type NaI(TI) detector. The decay-scheme parameters of a given nuclide are introduced as input data to the program to simulate the disintegration process itself. The corresponding efficiency and calculated spectrum are generated for comparison with the experimental data. Scattering in the material surrounding the detector as well as the bremsstrahlung, conversion electrons and fluorescence X-rays are automatically taken into account. Different source geometries (e.g. thick sources and bottles) are easily treated. The accuracy of such a calibration is of the same order (in the range of some tenth of percent) as that obtained by other methods, as illustrated by experimental results.