The unit of radiation exposure is the millirem, abbreviated mrem. To put this unit into perspective, you should be aware that the average person in the United States receives about 200 mrem of radiation per year from cosmic radiation (40 mrem), from trace amounts of radioactive substances in the ground, brick building materials, food, and air (85 mrem), and medical procedures (61 mrem). The radiation received from a chest x-ray is about 30 mrem. A typical sealed source that is used in many lab experiments is the 1 microcurie Cobalt-60 source. A Geiger counter placed one foot from such a source reads 0.05 mrem/hr.
In most classes, sealed radioactive sources are used, in which the source activity is several microcuries or less. These sealed sources should NOT have any contamination on the outside of them. Even so, take no chances. Common sense dictates that you should not eat anything in lab, even candy or gum, around radioactive sources, and that you should wash your hands after class. Obviously, the farther you are away from a radioactive source, the fewer rays you will intercept.
Specifically, the intensity is inversely proportional to the distance from the source squared. This means that the intensity at 2 feet from the source will be 1/4 that at one foot; at 3 feet, it will be 1/9 that at 1 foot; and so on. Therefore, even when holding the source for a few seconds, it makes good sense to hold the source at arms' length rather than close to the body, and, when performing an experiment, to stand several feet from the source.
The film badge can be used to obtain a measure of the amount of radiation to which it was exposed by observing the density, or the "blackness", which appears on the film after it has been developed. The film badge cannot detect a radiation exposure less than 50 mrem.
In most labs, film badges are not worn because the radiation level is so low. However, the Department has three Geiger counters that can detect radiation as low as 0.025 mrem/hr. The instructor should use the Geiger counter to monitor the radiation and to show the level of radiation in the area of your experiment. Two Geiger counters are located in Room 203.
In a few physics labs, the students receive film badges because they use radioactive sources for a number of experiments. If so, wear your film badge at all times while in the laboratory.
Radiation causes damage because rays can break apart a molecule in tissue
causing the cells to either die, or to reproduce a different type of cell
(mutant). Maximum doses are established on a statistical basis and
you can keep the odds on your side by being exceptionally careful at all
times. A cumulative dose of 100 mr per week is regarded as an absolute
maximum. Only twice in over 25 years has a film badge shown an exposure
of more than 50 mr at the Physics Department of DePaul and these were advanced
students involved in longer experiments.