430 THEAMERICAN BIOLOGY TEACHER the National Bureau of Standards available from No. 51, Radiological Monitoring Methods and the Superintendent of Documents, Government Instruments, $.20. Printing Office, Washington 25, D. C., at the prices No. 69, Maximum Permissible Body Burdens and indicated. Maximum Permissible concentrations of No. 42, Safe Handling of Radioactive Isotopes, Radionucleides in Air and in Water for $.20 Occupational Exposure, $.35. No. 48, Control and Removal of Radioactive No. 80, A Manual of Radioactivity Procedures, Contamination in Laboratories, $.15. $.50. Types of Radiation-the PatternsThey Produce in a CloudChamber Downloaded from http://online.ucpress.edu/abt/article-pdf/27/6/430/21573/4441004.pdf by guest on 28 September 2021 * C. T. Lange, ClaytonHigh School, Clayton,Missouri, and Glen Farrell,Holt, Rinehartand Winston,San Francisco,California A description of the practical uses of a cloud chamber is followed by the instructions for the building and use of an inexpensive cloud chamber in the classroom. Introduction a biologist, yet we can use it to help build a Every minute of the day, whether we basic understanding of the nature and prop- realize it or not, we are being pelted by an erties of ionizing radiations. The cloud invisible stream of very high energy particles chamber was the tool which Rutherford used known as cosmic rays. The number of these to study transmutation of one element to striking our body is about one thousand per another. He observed that nitrogen atoms minute. We also are being subjected to the under alpha particle bombardment were alpha, beta, and gamma radiations from changed into oxygen and hydrogen. The natural radioactive materials in our external cloud chamber also figured in the 1932 dis- environment, as well as radiations of internal covery by Chadwick of the neutral particle origin from the body burden of radioactive of matter known as the neutron. atoms such as potassium-40, carbon-14, The diffusion type cloud chamber is most radium-226, strontium-90 and cesium-133. satisfactory for classroom use. This type of Some of the cosmic rays with energies as cloud chamber is patterned after one devel- high as several billion electron volts (Bev) oped by A. Langsdorf of the United States in can go right through us. Some of the radia- 1939. This continuous type cloud chamber tions from natural sources are equally as utilizes principles first employed by C. T. R. penetrating, and we cannot avoid the radia- Wilson of Cambridge, England. It is based tions from atoms that are decaying within us. on the principle that whenever an electrically It seems remarkable that we are not aware charged fast-moving particle passes through of this incessant bombardment. The explana- a gas it produces ionizations along its track. tion is that our senses are not nearly sensitive If the radiations, in passage, can be observed enough to enable us to perceive the minute in a confining space where the gaseous con- effects we experience when radioactive par- tents are in a state of super-saturation when our as ticles or rays penetrate body. Yet, the particle passes, the ions formed by the scientists, as biologists, we are concerned radiation become the foci for the conden- with the ionizing radiations, and desire to sation of liquid from the vapor phase. Thus learn their secrets by whatever means we like are made can use to demonstrate their presence and/or particle tracks, streaks of fog, interaction with matter. to appear in the chamber. Pictures of radia- It is the function of the cloud chamber, tion tracks in a cloud chamber show clusters which will be described in this exercise, to of droplets along the path of the track. These implement our senses so that the paths of paired drops are the result of the ion pair ionizing radiations become visible. The cloud which forms during radiation passage; one chamber is more the tool of a physicist than ion by the ejection of an electron from an TYPESOF RADIATION 431 atom to give a positive ion, and the other by radioactive source in the cloud chamber. the capture of this electron to give a negative These alpha tracks come from the radioac- ion. What is seen in the cloud chamber is not tive decay of radon-222. The alpha tracks the particle itself but the path that it takes from the decay of this atmospheric com- through the cloud chamber. ponent are about four centimeterslong. Ra- don is a Types of Tracks Visible in the Cloud Chamber gas decay productof radium.Higher 1. Cosmic ray traces. When no source of than normal radon gas concentrationsin the radiation is placed in or near the cloud atmosphere of uranium mines constitutes chamber, almost all the tracks seen will be a health hazard for miners. the result of cosmic rays. Cosmic ray tracks Alpha sources usually contain radium. are thin and faint and are usually bent and With an alpha source in the chamber,tracks crooked, although a few are very straight which start from the source and fan out in Downloaded from http://online.ucpress.edu/abt/article-pdf/27/6/430/21573/4441004.pdf by guest on 28 September 2021 and will pass completely across the cham- a starbursteffect will be noted. The tracks ber without any deviation. Those tracks grow progressively thicker as they come traveling horizontally through the narrow to their termination.The specific ionization sensitive layer of the cloud chamber are of the alpha tracks is very intense and in- usually electrons. Vertical tracks in the cloud creases as the alpha particle is slowed by chamber, while more difficult to observe, are interaction with matter. Alpha tracks fre- caused generally by cosmic ray particles quently terminate with a short hook. The called mesons. The cloud chamber, Figs. 1 alpha tracks pictured in Fig. 3 were taken and 2, shows a beta track due to cosmic ray. in a high school science laboratoryfollowing 2. Alpha particles. These particles ema- a technique which is noted later. nate spontaneously from certain atoms which 3. Beta particle tracks. Beta particles are have an atomic mass of over 209. Alpha par- electronsof nuclearorigin. They have a mass ticle tracks are frequently noted with no about 1/1836 that of a proton, or 1/7344 that of an alpha particle. Their single nega- tive charge as opposed to the double posi- tive charge of the alpha particle, coupled with the much lighter mass, gives rise to a track with a low specific ionization per unit length as compared to the alpha. The beta tracks are described as being long, much longer than the alpha, thin, and wiggly. Probably 1 ftrom source a track, in and a from Decry of out of focal Radon in Air End of 5 track Plane a track \ rapidly leaving A - f oro focal Pane f ro, sburc of Particle from Source a from Source Notice the appearance of the end of the track Figure 1. Photograph of tracks in a classroom cloud chamber. f romSour t Courtesyof AtomicLaboratories, Inc. 3100 Crow Canyon Road San Ramon,California Figure2. Identificationof radiation tracks. 432 THEAMERICAN BIOLOGY TEACHER available instrument, or with a diffusion type cloud chamber which can be constructed by following these instructions: Materials. Fig. 4 illustrates the materials required to assemble a cloud chamber in the laboratory: coffee can or tobacco can, blotter material, short length of lamp cord or in- sulated wire, plastic sheeting (plastic wrap), battery (45-90 volts) methyl alcohol (spirit duplicating fluid will do), battery clips, masking tape, bright light source (slide pro- jector), radioactive sources. This may be Downloaded from http://online.ucpress.edu/abt/article-pdf/27/6/430/21573/4441004.pdf by guest on 28 September 2021 done by scraping powder from numbers or hands of luminous watch dial, by placing a drop of plastic cement in the eye of a sewing needle and rolling the eye end of needle in powder from dial. Great care must be exercised to avoid contaminating hands or the work area with this powder, as materials of this type figured in the poisoning of ra- dium watch dial painters many of whom developed bone tumors from accidental in- take of radioactive material. Other materials are: small corks to fit %"hole, black shoe dye, dry ice, solder, misc. tools, i.e., tin snips, soldering iron, etc. Figure3. Alpha particle cloud chambertracks. The beta particles are frequently diverted from their path by interaction with mole- cules or atoms in the air passage, which re- sults in the thin tortuous pathway. Fig. 1 shows beta particle tracks. Strontium-90is generally the active material used for dem- onstration sources. 4. Gamma rays. The passage of electro- magnetic radiation leaves no specific path in the cloud chamber as do particulate ra- diations. Gamma rays cause indirect ioniza- tions by collision with one or more electrons. These electrons,called secondary radiations, frequently cause ionization tracks to scatter out from the path of the gamma radiation. ,,By tracing the path of these secondaries back to their origin in cloud chamber pho- tographs,it is frequentlypossible to indicate the path of the gamma radiation. Constructionand Use of the DiffusionType Cloud Chamber The foregoing, regarding the interaction of radiation with the gaseous matter in the cloud chamber, should be verified by con- ducting an exercise with a commercially Figure 4. Materials for cloud chamber construction. TYPESOF RADIATION 433 Construction. 9. Wrap a layer of masking tape around 1. Cut large circular hole in top of can. top of can with 34"overhang. Press tape in 2. Solder a 6" length of wire to can.