FEBRUARY, 1949
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A GENERAL CHEMISTRY EXPERIMENT IN RADIOACTIVITY1 GEORGE A. SCHERER Earlham College, Richmond, Indiana
A CONSIDERATIOK of radioactivity has been included in general chemistry courses for many years. The methods that have been used to detect and distinguish the kinds of radioactivity are usually described. Some times some of these methods are demonstrated. But the student does not have f i s t hand experience with this subject which is so fascinating to him. The reason is that much of the apparatus for such work is too complicated and expensive to be placed in the hands of inexperienced students. This difficulty can be overcome by the experiments which are described below which can be performed with home-made apparatus and easily obtainable materials. Construction of Chalk Box Electroscope (see Figure 1). The case for the leaf electroscope is an ordinary wood chalk box placed on end. The sliding wood lid is replaced by a piece of window glass which forms the door. The sample holder is a shallow tray with 2-mm. sides made from sheet aluminum. It is placed in the bottom of the electroscope. The conducting rod is a heavy copper wire which can be topped with a metal ball. The bottom of the copper wire is flattened, drilled, and a piece of sheet aluminum 1 em. X 4.5 cm., a leaf of aluminum foil (which can be obtained from a sign painter) and a cardboard scale are bolted to it. This assembled unit is mounted in the case by drilling a a/l-in. hole in the center of the top of the case, filling the hole with molten sulfur, and then drilling a hole through the solidified sulfur as near as possible the size of the copper wire. If the hole is too large for the copper wire it can be sealed with more molten sulfur. Tests for Alpha Pa~ticles. (1) The electroscope is, charged by touching the metal ball with a stick of sealing wax which has been charged by rubbing on woolen cloth. If the wea;her is not too humid, the leaf swings up on the scale, gradually falling back as the charge leaks off. The time required for the electroPresented before the Division of Chemical Education a t the 114th meeting of the American Chemical Society in Washington, D. C ,August3l. 194%
scope leaf to p8ss from one point to another on the scale is taken with a watch. In later determinations the same reference points are always taken. This natural rate of fall is subtracted from the observed rate in each subsequent determination. The radioactive substance to be tested (pitchblende, carnotite, radium barium chloride, uranium or thorium salts) is introduced on the sample holder and spread out flush with the sides of the tray and the rate of fall of the leaf determined again.
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It can he shown that i t is the surface area of the radioactive material rather than the mass that emits the greater amount of alpha particles. Sprinkle about 0.5 g. of black uranium oxide on a 16-cm. square of cardboard and time the leaf fall. Then fill a paper box 1 cm. square by 5 cm. deep with about 2 gr. of the same uranium oxide, and time the fall. The smaller mass hut the larger area will discharge the electroscope faster thau the larger mass, because alpha particles from near the bottom of the pile cannot penetrate the upper layers. (2) A home-made radioscope may be used to test for alpha particles. To prepare the fluorescent screen a thin pool of a weak solution of gum arabic is poured on a glass plate. Stretch cheesecloth over the neck of a zinc sulfide hottle, gently tap the powder onto the gum arabic and let dry. Half fill a wide-mouthed 4-oz. hottle with water and add some radioactive substance. Shake thoroughly, then uncork it and lay the screen over the top of the hottle with the sulfide down. Flashes can be seen with a magnifying glass if the eyes have been adjusted by remaining in the dark for 10 to 15 minutes. (3) Taylor2 describes the construction of a simple fog-track apparatus. His design can he simplified by putting a very small pinch of pitchblende on the bottom of the flask and covering it with a thin layer of household waterproof cement (Figure 2). Tests for Beta Particles. (1) If a specimen is placed in the electroscope and covered with a thin sheet of aluminum foil the alpha particles will he stopped by the thin metal but most of the heta particles will pass through. Therefore it will take considerably longer for the electroscope leaf to pass between the two points than if alpha particles were not stopped, hut the time will be shorter than if no specimen a t all is present. A. E., J. CEEM.EDUC., 11,576 (1934). 'TAYLOR,
JOURNAL OF CHEMICAL EDUCATION
(2) Becquerel's classic photographic test can easily he repeated. In a darkrooin a piece of photographic paper is sealed in a light-proof envelope. On top of this is laid a flat piece of metal (key, coin, etc.) which is covered with powdered carnotite, pitchblende or uranium nitrate crystals. When the paper is developed a t the next period (at least 3 days later) there is a perfect shadow-graph of the piece of metal. It can he shown that beta particles, and to a lesser extent gamma rays, are the ones that darken the photographic paper. Flow gum arahic over a glass plate and sprinkle it with black uranium oxide. When it is dry cover half of it with a sheet of aluminum and place a piece of photographic paper in a light-proof envelope on top. At the next period develop the paper. The paper will he but slightly darker where the alpha particles have bombarded the paper thau where they have been cut off by the aluminum. The greater penetration of the beta particles and gamma rays can be shown by placing a piece of photographic paper in a light-proof envelope over two piles of carnotite, one about 3 mm. high and the other 2.5 om. high. When the paper is developed at the next period the smaller pile will be shown to have made only a faint image while the larger pile has produced a dense black area. This is because the beta particles and gamma rays can penetrate the upper layers. Separation and Detection of Uranium and Thorium284 (Uranium XI). Since uranium emits alpha particles and thorium-234 emits beta particles, and the half-life period for uranium is 4.5 X 109 years and for thorium-234 is 24.5 days, the following interesting experiment can be performed with uranium nitrate. First test the uranium nitrate a t the start in the electroscope for alpha particles and for beta particles, recording the time of fall of the leaf in each case. Also place some of the crystals on top of a piece of photographic paper in a light-proof envelope for a day, develop the paper and put it aside. The thorium-234 in the nitrate can now he precipitated and filtered out attached to ferric hydroxide. Dissolve 20 grams of the uranium nitrate in 60 ml. of. water, add one or two drops of a ferric salt solution, and slowly pour into this solution a 0.5 N solution of ammonium carbonate until a yellow precipitate of the uranium salt forms and just dissolves. There will be left some trace of an orange-brown precipitate which contains the thorium-234 which should be filtered off. The precipitate is heated gently until it no longer gives off a smell of ammonia and then dried. The uranium nitrate is obtained by evaporation of the filtrate. Test both samples in the electroscope without delay. The thorium-234 emits no alpha particles and the uranium little or no beta. Next try the two samples on the photographic paper, exposing it for a day. The pure uranium will darken it but slightly. Finally, 24 days later, test them again for a day on another piece of paper. Both samples now show heta particles because the uranium has disintegrated to form more thorium-234.
