> i Me Oe i Sink n2 be ae Fie. 1—A comparison between (A) cellulose nitrate and (B) Kodak AR-10 stripping film autoradiographs of longitudinal sections from the tibia of a mouse injected with 0.1 wCi ionic **Pu. Both sections show alpha tracks over the endosteal surfaces of the bone. The tracks registered in the cellulose nitrate detector film after two 2-hr periods are well defined at low (100 X) magnification. Higher magnifications (250 *) were reauired to demunstrate the alpha tracks in the AR-10 emulsion. aged sites was greater than the rate at which they attacked the bulk material (1 »/hr), tracks were de- veloped along the ionization paths of the alpha par- ticles. The etching process has been described") as a solid-liquid interface reaction which proceeds in two steps—the diffusion of ions from the oxidizing agent to the interface and the interface reaction. Strict control of detector film preparation is required since the etching behavior of cellulose nitrate is affected by a number of variables, such as the degree of polymerization and nitration, the plasticizer, temperature, etch‘ing time, hydroxyl concentration, and oxygen.‘®) Daylight has not been reported to be an influencing parameter, but UV light apparently increases the etchability of detector films by decomposing the bulk ceding etch period, the process will enlarge previously etched tracks as well and reduce the resolution of the autoradiograph somewhat. Figure 1A shows a typical cellulose nitrate auto- radiograph of bone from a mouse injected with ionic “39Py, The etched tracks, which have a conical shape and a 1 yw diameter, lie over the surface of the endosteum. Figure 2 shows a similar autoradiograph of trabecular bone. The apex of each track marks the initial damage site, and the broad end indicates where the particle emerged from or came to rest in the film. Cellulose nitrate films always show some background material.) Since the detector films are mechanically very stable, they can withstand repeated etching. Should, for instance, an initial exposure period prove etch pits due to melusion of debris during preparation and etching. But the background is never severe even after repeated etching and may be almost totally abolished by simply agitating or changing the etching solution at frequent intervals. The resolution of the cellulose nitrate films is at tracks registered in the detector film after each pre- vided by the nuclear emulsion Kodak AR-10. Figure 1B shows an AR-10 stripping film autoradiograph of an adjacent bone section exposed for an equivalent too short to develop a sufficient number of tracks, an autoradiograph may be stored and re-etched at a later time. However, while serial etching will develop new least as good as, and perhaps better than, that pro- ro