Even if only a crystal located directly above the source were to be hit corresponding to area A in Fig. II.XXX the location of the developed silver could be found anywhere within a larger area A in this figure. The size of the silver halide crystals and the possible dislocation of the developed silver in relation to the crystal are factors limiting the resolution of autoradiograms. For example, a very even distribution of silver halide crystals can be achieved by centrifuging the crystals in a very dilute emulsion onto the specimen grid placed at the bottom of a centrifuge tube. The specimen grids used for autoradiography should be made of molybdenum rather than copper which dissolves during developing of the autoradiograph.
Thin sections produce a problem with respect to the intensity of the radiation.
Only a very minute amount of isotope is contained in the thin sections, so it is important for the emulsion to be highly sensitive and also to have high specific activity. Exposure times are still long - up to several months. The low energy of b -particles emitted from tritium favors sensitivity since the probability for ionization per unit path length increases with decreasing velocity of the particles. The location of the silver halide grain which is reduced by the b -radiation will therefore be close enough to the source to allow high precision in determining the location of the source. Exposure and development of the autoradiogram Exposure times can be estimated by preparing specimens for light microscopy autoradiography at the same time as those for TEM autoradiography and developing the former after 1-2 weeks.