By Richard A. Herrmann | The Journal of Chemical Education | 1974, v 51, pp 420-421


For nearly two decades pupils have been hearing about nuclear power plants, atomic submarines, and nuclear missiles. They always desire to “see” the strange powers of radiation that are associated with those enormous man-made nuclear devices, especially since they have been taught to fear the phenomenon of radiation by their preceding generations.

By concentrating radioactivity contained on luminous dial, you can make your own high reading source for impressive classroom experiments at practically no cost. Your source will be at least four times more powerful than standard purchased gamma courses selling for about 30 dollars. It will also be permanent as the half-life of your alpha and gamma emitter is 1622 years.


While preparing the source, and for later experiments, you will need an audible detecting instrument such as a rate meter or Geiger-Mueller counter. Readings can reach 50 mr/hr at cm from the source. If you do not have a meter, check with your local Defense Civil Preparedness Agency (formerly Civil Defense) for the possibility of borrowing one.

Obtaining Radioactive Material

Radium dial alarm clocks that were manufactured in abundance 30-50 years ago, or surplus World War Two U.S. Army compasses, serve as source material. A request for these in the classroom usually results in obtaining at least one clock per class. The zinc sulfide fluorescent paint containing radium from four or five dials and hands will be sufficient.


Dismantle the front of the clock, pry off the hands, and remove the face. Prepare the work area as noted from proper procedure when working with radioactive materials.

Grasp the face with tongs and hold it vertically over a 3 or 4-inch watch glass. Squirt a few numerals at a time with acetone from a squeezable wash bottle. The radium paint will start to dissolve within 20 seconds and the excess acetone will drip into the watch glass. Assist the dissolving by skillfully dabbing the numerals with an ordinary medium size water-color brush, dipping repeatedly into the acetone reservoir contained in the watch glass. This procedure is about the opposite of how the numerals were painted on decades ago (Chemistry, April 1969, page 18).

Within a minute the group of numerals will be dissolved. Flush them off completely with more quick rinses from the wash bottle. Use the same procedure for the hands.

By carefully rocking the watch glass, the heavier radium will migrate to the center area. Transfer the acetone residue, by using an eyedropper, into another nearby watch glass where it can evaporate. The cleaned radium will stick to the first watch glass as the solvent completely evaporates. Repeat the procedure for successive dials.

If you wish to build up a supply, wash each settlement of cleaned radium into a lead-wrapped pill bottle. Allow the solvent to evaporate or remove it with an eye dropper, before capping. Label the supply and store behind sufficient shielding and away from inquisitive fingers.


Treat the radioactive materials during procedures with thoughtful respect and your maximum exposure dose per clock face or compass dial will not exceed 1 milli rem. The hazards of radiation of this amount are far less dangerous than the dozens of toxic, caustic, and corrosive chemicals that students handle in a school year. Use caution to prevent unwarranted contamination upon yourself or the surroundings. The following points should be kept in mind.

Work in a private area that is free from public access.
Cover the surface of the counter with aluminum foil. Then cover again with a smaller piece, edges folded upward, where the dissolving procedure is carried out. This will contain any spills and they be salvaged with the brush.

Wear protective clothing such as a lab coat and waterproof gloves that will not be harmed by acetone.

Have an audible radiation detector nearby to constantly monitor background or to check possible contamination spots.

Do not try to scrape the numerals off. This would introduce airborne alpha contamination that will be inhaled.

Do not use a pipet in place of an eyedropper.

Decontaminate if necessary with toweling and acetone. (The clock face cannot be decontaminated. It will still give off readings because it has become irradiated over the years from the paint.)

Discard solid contaminated wastes such as the clock faces, hands, compass needles, paint brush, aluminum foil, etc. in a sturdy plastic bag or container and personally dispose of it in a land-fill dump.


It is an audible radiation detector and substantial gamma readings that produce for the experimentor an unforgettable introduction to “seeing” the mysteries of radiation. By moving the source stepwise toward the probe (or vice versa) from 128, 64, 32, and 16 cm, you can test the inverse square law as it applies to distance when you subtract the background readings. With a Geiger-Mueller counter you can move in to 8 cm from the source.

Shieldings of various types of thickness, including lab counter tops, bricks, water, and volunteer hands, are most impressive with this gamma source. An avalanche of eager questions will erupt from demonstrations so be ready.

Please also see:
Index of Radioactive Materials
Atomic Glossary