A LECTURE DEMONSTRATION OF NUCLEAR ENERGY W. H. SLABAUGH The State College of Washington, Pullman, Washington
INTHE GENERAL chemistry course the subject of nuclear energy has won for itself a n important position along with radioactivity, isotopes, and transmutation. For the student in general chemistry to grasp these fundamentals without the help of visual aids is difficult. On the other hand, there are many technical difficulties encountered in setting up mechanical models of these subjects. Certain physicists1? have suggested models for the illustration of nuclear fission, and the complexity of these models is an indication of the problem of offering a satisfactory mechanical device t o demonstrate the subject. Most of the models eniphasize the energy concepts; it was our desire not only to simplify them for lecture demonstration purposes, but also to illustrate the subject from the chemical standpoint as well. A model of the Uranium-235 atom'was constructed as shown in Figures 1and 2, which centers the attention on the nucleus where fission, radioactive, and transmutation changes occur. The model was built of approximately 235 glass marbles, cemented together with a cellulose acetate adhesive, using a mortar of suitable size and form to obtain the spherical shape. A wood block with a hole to fit the rod on a small ringstand was incorporated in the mass of marbles, and after assem- represent neutrons and protons. A hoop of '/4-inch bling, the marbles were painted orange and black to iron rod was welded to the base of the stand in order to emphasize the idea that the mass of marbles represents the nucleus. By making the nucleus of two'hemispheres I BALINKIN, I., Science Digest,22, 89 (1947). this model is readily adapted to the discussion of nu2 SUTTON, R. M., Am. 3. Physics, 15, 427-428 (1947)E clear fission. Extra orange marbles provide the neutrons which may cause the h i o n . Several orange marbles inside the lower hemisphere were not cemented, so that upon fission these neutrons become available for initiating fission of other nuclei. The same model is
Figure 3
Figu.e 1
679
680
JOURNAL OF CHEMICAL EDUCATION
Figure 4.
A Chain Reastion
adapted to the illustration of the formation and certain properties of the post-uranium elements. To construct a model showing a chain reaction which would operate slowly and simply presented difficulties. Unless some sort of neutron Geflector is used the model becomes excessively complex. Hence, a chain model was constructed using thirteen modsetraps in one plane. When its trigger is touched a trap ejects an orange hall (neutron), which is restricted in its path by a wire, the other end of which is anchored midway between the trap and the next trap, as shown in Figure 3. The hall is thus thrown through a semicircle where it lands upon the trigger of the next trap, setting it off,and thus starting a simple chain reaction. Circular areas including the traps were painted a bright yellow, and the background and wires painted a flat black, colors in keeping with the materials commonly used in the atomic pile (assuming, of course, that uranium atoms are yellow!). The orange neutrons show up well against the background during the chain reaction, which requires about two seconds for the thirteen traps t o become activated. For the sake of simplicity, other
products 6f fission such as the atom fragments were avoided in this model. A model to illustrate nuclear fission as employed in the atomic bomb is based upon a chain reaction which exactly doubles its rate as .it progresses. The first trap activates two traps, each of these activates two more, etc., until a total of thirty-one traps have been sprung. The chain carrier is the "ttil" of a trap which strikes a cross bar attached to the triggers of the next two traps. Small lead balls may be attached to the tail of the traps to represent the neutrons which are ejected and initiate the fission of the next atoms. Two No. I0 corks, one red and one green, are placed on top of the trap after it is cocked to represent the fission fragments. These corks are commonly thrown to th ceiling and rear of the lecture room holding 150 students. Figure 4 shows this model in the cocked position. These models are obviously over-simplificationsof th actual nuclear changes, but they provide a starting point as an aid for the student to formdat. these ideas in his own mind.
4
