AUTORADIOGRAPHY WITH THE ELECTRON MICROSCOPE A Procedure for Improving Resolution, Sensitivity, and Contrast

Autoradiographic techniques for use with the electron microscope have been developed and considerably improved in recent years. A review of this historical development is given by Meek and Moses (14), and a theoretical analysis of the problems involved by Pelc (17) and Caro (4). In this paper, we describe a method for preparing autoradiographic specimens, using a new, fine-grained emulsion and a special, high-sensitivity developing technique. This procedure gives higher resolution, sensitivity, and specimen contrast than previously available. Fine-grained emulsion: The size of the undeveloped silver halide crystal and the thickness of the emulsion are among the limiting factors in autoradiographic resolution. Ideally, one wants a fine-grained emulsion coated in a closely packed monolayer. The Ilford L4 nuclear track emulsion with a silver halide grain size of 1200 to 1600 A is most frequently used for electron microscopic autoradiography. The Gevaert (Scientia Nuc. 307) emulsion has finer grains (700 A), but is reported to be difficult to handle (7). We obtained an experimental emulsion from Eastman Kodak ~ with an undeveloped grain size of 300 to 500 A. (Fig. 4 compares the grain size of this emulsion with that