A DEMONSTRATION FOG CHAMBER W. H. SLABAUGH Oregon State College, Cornallis, Oregon
WITH an increasing interest in subatomic phenomena there has been a corresponding interest in demonstrating the characteristics of radioactivity for the general student. With this in mind the author has devised an instmment, based upon the fog-chamber technique, which serves as a classroom tool for instruction. More than a score of students may observe the fog tracks simultaneously. In view of the economical construction of the instrument, as well as its dependability in operation, it is presented here as a guide for others who may wish to construct one for use with their own classes. The fog chamber is essentially a glass-enclosed box in which there is a region of supersaturated vapor of a suitable liquid. The region of supersaturation, sometimes called the sensitive layer, may be two or three centimeters thick, although usually it is somewhat thinner. In this region any charged particle emanatingfrom a radioactive substance causes molecules of the air to become ionized, which in turn serve as nuclei for the precipitation of vapor, thus forming a visible fog. The resulting fog track marks the trail of the radioactive particle. Wide fog tracks indicate alpha particles and narrow tracks are formed by beta particles. If a track suddenly appears to start a t some distance from the radioactive substance, this may be caused by a gamma ray which strikes a molecule, ejecting a charged particle which then forms a fog track. Cosmic rays also produce fog tracks in the sensitive layer. The instrument is made primarily of two components. First, a glass-walled, cubical box of approximately one foot on each edge (Figure 1). The box is constmcted of grooved l-in. X l-in. stock and ordinary window glass. The top of the box is removable; a pad is attached to the underside of the top panel to which a liberal quantity of isopropyl or ethyl alcohol is applied when the instrument is operated. A wire screen is attached to the underside of the top panel; it is attached to one side of a higb-voltage supply. The other side of the high voltage is attached to the bottom of the box which is a piece of sheet metal painted black on its upper surface. The box is supported on a frame in which a slab of dry ice is placed. A one-inch slab of dry ice provides a sensitive layer in the instrument for a period of about two hours. An alternative to constructing a glass-walled box is to use the case from an ancient or otherwise useless analytical balance, a supply of which frequently adorns the back comer shelves of many academic storerooms. In this instance, a sheetmetal bottom should be substituted for the stone base of the balance case.
The second component is a high-voltage supply. The circuit diagram of the power supply constructed for this instmment is shown in Figure 2. As a precaution it is well to remember that the voltages produced in such a power supply are potentially hazardous; they should be used carefully. The purpose of the electrical field superimposed upon the vapor system is to sweep out excess ions that otherwise serve as fog nuclei. Ideally, only those ions created by the radioactive emanations should give fog nuclei. In operation, several samples of radioactive salts such as uranium and thorium nitrates are placed on the floor of the fog chamber. Each sample is individually mounted on a thin slice of a rubber stopper, and coated with a lacquer or cement which resists the alcohol that condenses upon the floor of the chamber. The fog chamber is adequately illuminated with a single desk lamp placed next to one side of the box. The fog tracks
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JOURNAL OF CHEMICAL EDUCATION
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are visible through all four sides of the box, but appear most intense when viewed from the side opposite the lamp. The fog chamber will operate contiuuously as long
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Acknowledgment is given to the k i d assistance of Mr. D. N. Moutan in coustructing the high-voltage supply.
