CIATION ACHERS
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DEMONSTRATING RADIOACTIVITY BY AUTORADIOGRAPHY-' LAURENCE S. FOSTER Watertown Arsenal Laboratory, Watertown, Massachusetts
HmRI BECQUEREL in 1895 discovered that uranium compounds are capable of fogging photographic plates. The spontaneous emission of radiat,ions by uranium, that was shown to be the cause of the fogging, he termed radioactivity. Thus, the process now known as autoradiography goes hack to the very beginning of the science of nucleonics. Experiments on autoradiography, therefore, have a historical significance and for that reason are worth a place in introductory conrses. Furthermore, they are easy and inexpensive to do, requiring little special equipment or supplies. The photographic test for radioactivity in rocks, described by the U. S. Atomic Energy Commission in its valuable little h a n d b ~ o kis , ~a good illustration of the simple autoradiographic technique. The test is made by wrapping a strip of unexposed camera film in opaque photographic paper, placing a key or other small metallic object on the wrapped film and the rock sample on top of both. If activity is present in the rock, an autoradiograph of the metal ohject is obtained when the film is developed. A specimen of pitchblende will produce a clear image in one day. Special emulsion^,^ like the Eastman Kodak Company's Type NTA, have been developed which are not
sensitive to light, betaparticles, or gamma radiation, but which are highly sensitive to such densely ionizing particles as the mesons, protons, titons, alpha particles, and fission fragments. Other types of special emulsions, designated as NTB-1, KTR-2, and NTB-3, have varying sensitivities to beta rays. Type NTC is used for recording fission fragments6 Fortunately, these special emulsions are no more needed for demonstrating autoradiography than they were in Becquerel's day, since ordinary photographic film, lantern slides, and X-ray emulsions can be used for this purpose. These optical emulsions are sensitive in varying degree to light, gamma radiation, beta particles, and alpha particles. If uranium compounds are used to expose photographic material, still wrapped in its black cover paper, for the most part only the beta particles and gamma radiations of the daught,er elements present penetrate into the emulsion. U23sitself is a pure alpha emitter and its alpha particles would be stopped by the wrapping paper. SamInformation concerning these special e:nulsions may be ohtained from the Industrisl Photographic Sales Division, Enstman Kodak Company, Rochester, h\ierv York.
Bmed on a paper presented a t the 254th meeting of the New England Associntion of Chemistry Teachers, Rhode Island School of Design, Providrno::, IIhode Islend, February 11, 1950. Tho statements and opinions expressed are those oi the author and do not necesmrily repi.esent the views of the Ordnance Department. '"Prospecting far Uranium," U. S. Atomic Energy Commission and U. S. Geological Survey, Washington, 1949, p. 23. 1 Obtainable from U.S.A.E.C., Document Sales Agency, P. 0. Box 62, Oak Ridge, Tennessee, or from Superintendent of Dacuments, Government Printing Office, Washington 25, D. C. (Price 306). ' YAGODA, HERMAN, "Radio&ctive Meesurements with Nuclear Emulsions," John Wiley and Sons, New York, 1949; cf. also, by the same author, "Tracks of densely ionizing particles in nuclear ,,;F emulsions," Nucleonics, 2, 2 (1948).
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csyamic plate end cereal ~ i s h colored - ~ of uranium i n GI==-
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So far as health hazards are concerned, these dishes are not dangerously radioactive. The half l i e of uranium ( U Z 3is very long, being 4.51 X lo8 years. Such dishes are not so radioactive as a luminous wrist watch dial containing radium. To obtain a visible darkening on the developed photogra.phic plate, one needs about loa alpha or beta c~unts/cm.~, and for a reasonably dense negative about lo7c o ~ n t s / c m . This ~ fact can he used to emphasize the extreme sensitivity of the autoradiographic process, by having the student calculate the number of uranium atoms needed to produce this number of counts from the half life. The alpha activity of uranium is 740 disintegrations per mill&ram per minute. The literature on the autoradiographic technique is extensive. A survey on biological applicationsais available from the U. S. Atomic Energy Commission. A laboratory experiment can be worked out to determine (1) the optimum exposure time, the best type of film, and the processing variables; (2) the penetrating Dower of the effective radiations. usine different thickhesses of aluminum and lead sheet; and (3) the dependence of shielding efficiency of thin metallic sheets on the "density" (in grams per square centimeter) and not directly on the atomic number. For such studies standard size sources and use of optical densitometers in making the comparisons would he desirable. Recently a precise autoradiographic procedure was reported that served to determine the half life and to identify a radioactive species responsible for fogging of
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I Figure 2.
Autoredi.maph of Plate
Positive print from negative made by setting Dlate right side up on x-ray film. Dark oircle showa where &re bas been worn off.
ples of purified uranium compounds that have been allowed to age are, therefore, more effective than when freshly made. The demonstration of radioactivity can be carried out in the classroom. A dense metallic object is dated on the wrapped photographic plate (a iantern slide plate is and covered with a radioactive source. The assembly can be tied together with "Scotch Tape" and put in the lecture table dra~veruntil thene.ut class meetina. When the class has reassembled, the lecture hall k darkened except for a red photographic safe light, and the exposed plate developed and fixed in front of the glass. The fixed negative thus produced autoradiographically can be projected while still wet for inspection by the class. In place of samples of radioactive ores, an even more interesting source of radiation is the orange-colored pottery, made extensively up until 1942. One trade name was "Fiesta-Ware." The pigment for the orange or tangerine-colored glaze contains uranium, added probably as the yellow trioxide, UOa, or as sodium diuranate, NadJaO,. 6Ha0. The orange color developed is closer to that of the peroxide, UO,. It will be recalled that up until 1942, uranium was a by-product material of little value, used primarily in ceramic colors. Dishes colored with uranium are sufficiently radioactive to give a good autoradiograph in 24 or 48 hours exposure. The photographs that have been reproduced in Figures 2 and 3 were made with such dishes, shown in Figure 1, using Eastman Type A Industrial X-ray film, with a 48-hour exposure. If the orange dishes are brought near a GeigerMiiller counter tube, attached to a radiation survey type instrument with a loudspeaker, a satisfying, rapidfire barrage is heard, reflecting the individual nuclear explosions.
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' AXELROD,DOROTAY J., "The Radioautographic Technique," Isotopes Div&ion Circular No. A-4, January, 1948, is obtainable fro, the ~~~t~~~~Division, Oak Ridge Operations, U. 9. Atomic ~ n e r g yCommission, Osk Ridge, Tennessee.
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photographic plates in their original shipping containers. I t was possible t o show that the cause was a cerium isotope, Ce141, trapped in the strawboard, made in Vincennes, Indiana, on August 6, 1945,in which the film was packed, that probably resulted from the test explosion of the atom bomb on July 16, 1945,over 1000 miles away in New Mexico.l 7 WEBB,J. il., "The fogging of photographic film by radioactive contaminants in cardboard packaging materials," Phys. Reu., 76, 375-80 (Aug. 1, 1949).
Wl In calling attention to the radioactivity of orangecolored Fiesta-Ware, efforts should be made to avoid causing undue alarm. On the other hand, it is apparent that under the Atomic Energy Act, uranium can no longer be used for ceramic colors. Dishes colored with uranium will become increasingly rare l'antiques." It behooves those who wish to procure examples of such dishes to start their collections now while they rnay still occasionally be found for sale, or still remain whole on the pantry shelf.
