We have developed a method for studying the as-grown defect density spectra of Czochralski silicon wafers by infrared light scattering tomography These spectra show the defect density vs. size or stability temperature. Different annealing procedures were used to reveal both parts of the spectra, one stable at high (1000 to 1280 degrees C) and the other at low temperature (500 to 1000 degrees C). The studies were performed on wafers grown in the vacancy-rich regime and on wafers with a stacking fault ring 150 and 200 mm in diam. In the as-grown state, only a very small part of the defect spectrum lies above the detection limit. In general, three main peaks appear in the spectrum. Their total height and exact positions are determined by the growth and cooling rates of the crystal, and by the initial concentration of interstitial oxygen. Due to the different locations of the maxima in the low temperature spectra, standard precipitation tests can be misleading for the estimation of the oxygen precipitation capability. A high temperature preanneal at 1000 or 1100 degrees C shifts the low temperature part of the spectrum to smaller sizes. However, a dissolution of the grown-in defects was never detected. We conclude that during the nucleation anneals for internal gettering, only existing as-grown nuclei grow except under conditions when a time lag in nucleation is observed. The defect spectra provide a unique basis for the simulation of defect generation during wafer processing, and facilitate the selection of wafer material for a given technology. Furthermore, they provide the basis for effective generation of a bulk defect zone for internal gettering.