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When the salt is irradiated, gamma rays pass through the crystals and the energy deposited there excites electrons and causes them to move to a higher energy state. Due to the nature of salt crystals, the electrons become trapped in that higher energy state. After being irradiated, the salt appears as a cinnamon color rather than white; that is because the repositioned electrons affect the way that light is reflected by the crystal.
Preheat a dry frying on a hot plate set at its highest temperature. OR, put the pan above a lab burner. Continue heating the pan. In a completely darkened room, sprinkle or pour some of the irradiated salt into the hot frying pan. Carefully observe what happens! Then, observe the salt which remains in the bottom of the frying pan after your experiment.
You should see tiny flashes of light as the irradiated salt comes into contact with the frying pan surface. (You must be fairly close; the flashes are not bright.)
Heating the salt causes increased motion (vibration) in the salt crystal. This allows the electrons to return to their normal (somewhat lower) energy state. As the electrons move to lower energy states, the previously stored energy is released in the form photons of visible light. After the electrons return to normal energy states, the salt crystals reflect light as normally and appear white.
Check the irradiated salt with a radiation monitor (Geiger counter) to see if it is radioactive. (Make sure you have a reading for background radiation, too.)
The salt was irradiated, but it is not radioactive. Readings from a radiation monitor should be the same as background.
Concepts you can teach:
If the irradiated salt is exposed to sunlight or artificial light, it will gradually lose its coloration and turn back to white. The light exposure causes some changes in the lattice, the electrons gradually return to their original energy states and the salt returns to its original white color. Be sure to keep it protected in a dark or opaque container.
Purchasing Irradiated Salt:
Penn State University, Breazeale Reactor, phone 814-865-6351
Other scientific supply companies may offer irradiated salt; check with your normal supply sources.
Frequently Asked Question: Is the irradiated salt safe to eat? The dose of radiation given to the salt was higher than FDA allows for this type of food; the laboratory where it was irradiated does not meet USDA/FDA standards for food handling. However, the salt is not radioactive – either before or after heating in the demo. The salt never releases ionizing radiation, only visible light.
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