TOXICITY OF RADIOELEMENTS APPLICATION OF CELLULOSE NITRATE FILMS FOR ALPHA AUTORADIOGRAPHY OF BONE Alan Cole,* D. J. Simmons, Helen Cummins, F. J. Congel, and Jacob Kasiner Sr nTENOT Meee ene ER When an alpha particle strikes the surface of a dielectric material such as a cellulose nitrate foil and gives up more energy per unit distance along its trajectory than the critical value of the material, latent etchable tracks are produced.“ The ionization and subsequent electrostatic repulsion produced by the particle creates vacancies along its path by ejecting ne HOES ~ cae rateaM Re atoms from the nitrocellulose molecules into interstitial positions. The resulting molecular fragments TERARORIReo capt rey te ae Cellulose nitrate can be used to record the tracks of alpha particles from bone and may have some advantages over nuclear emulsions. The tracks are made visible by etching the cellulose nitrate film with NaOH and no darkroom is required. Beta rays and electrons do not produce tracks, so background fogging is eliminated. The etching process makes visible only those alpha particle tracks which penetrate the film, so the attainable resolution may be better than that of nuclear emulsions. Finally, the alpha tracks are enlarged and so may be more easily counted by a photoelectric scanner. tion and etching which has produced high quality autoradiographs of bone sections from mice injected intravenously with a monomeric solution of 1% sodium citrate with 90% ultrafilterable °Pu (0.1 pCi). The animals were sacrificed 6 days after injection. The long bones obtained at autopsy were fixed in 95% alcohol, embedded undecalcified in methyl methacrylate, and were cut on a high speed rotary saw longitudinally at 100 ». The tissue sections had been used previously to prepare contact auto- radiographs using Kodak Type A autoradiographic plates, and the results of that study have recently been published by Rosenthal et al. Weprepared a stock solution of nitrocellulose according to Benton’s‘*) method: 17 g_ nitrocelluloset were first dissolved in a solvent composed of 5.1 g isopropyl! alcohol, 4.0 g butyl alcohol, and 8 g cello- solve acetate. This solution was then dissolved in 61.9 g ethyl acetate, and 4 g dioctyl phthalate were are more soluble than the parent molecule.“-®) The process of recording alpha particle tracks in nitrocellulose foil by etching radiation damaged sites has been called alphagraphy. It is a simple and direct method, and because nitrocellulose films offer certain technical advantages over photographic nuclear emulsions, it has found application in health physics as an autoradiographic detector film. In practice, we diluted 25 ml of the stock solution by adding 75 films are insensitive to electrons, thermal neutrons, or the dilute cellulose nitrate solution for 10 sec. The slides were removed vertically and allowed to drain for dosimetry.) This technique can discriminate between heavyand light charged particles, and thus the gamma irradiation. In addition, they will not fog during long term exposures, they are remarkably stable, and they can be processed (etched) in daylight and at room temperatures. It is possible to prepare thin films of cellulose ni- trate which are selectively sensitive to alpha par- ticles which have an energy range up to 5 MeV. This suggested that the technique might be useful when high resolution autoradiographic studies are required to visualize the tissue localization of bone-seeking added as a plasticizer. The mix was permitted to age for 4 days to achieve chemical equilibrium before use ml ethyl acetate in order to obtain thin (3 ») detector films, Autoradiographs were prepared by simply dipping the bone sections mounted on microscope slides into for an additional 10 sec. The underside of each slide was wiped free of the solution and the preparations were air dried horizontally overnight under a glass dish to avoid contamination with dust. The coated slides. were then annealed at 70° C for 4 hr to temper the detector film as recommended by Benton,‘ and were wrapped in aluminum foil during the exposure period. The autoradiographs were developed in a stendor dish by etching the detector films with radioisotopes such as plutonium and radium. Wewill 6.5 N NaOH at 24° C for 2 hr. Since the rate at which the hydroxyl ions attacked the radiation dam- * Present address: Department of Anatomy, The Bowman Gray School of Medicine, Winston-Salem, North Carolina. lulose RS 11.8-12.2% Ne, 5+ to 6-sec viscosity, 30% isopropyl alcohol by weight. describe a method of cellulose nitrate film prepara- Tt Hercules Powder Co.. Wilmington, Delaware: nitrocel-