I've been working in the undergraduate labs lately, checking the setups for the undergraduate laboratory project course. The course has any number of interesting projects, like spatial filtering of images using Fourier optics, measurements of the Hall effect, demonstrations of Rutherford scattering (after all, he had his laboratory in the building that is, now that it has been decontaminated, our library), and so on. The setup for the experiment on Compton scattering, pictured to the right, has all sorts of terrifying warning signs all over it, because it necessarily uses a powerful gamma emitter. Just how dangerous is it?

This is not purely a theoretical question, since the head lab technician caught a student looking down the beam at the unshielded source, prompting the numerous warning signs. So just how much harm did that student do themselves?

The source is about 100 millicuries of cesium 137, and emits 661.6 keV gamma radiation. In SI units, that's four billion becquerels (decays per second). But what does this translate to in terms of exposure (measured in sieverts)? This is a more complicated calculation, since the kind of radiation matters, as well as the geometry and time of the exposure.

For the purposes here, though, I'll just point to a list of gamma ray dose constants, which give the exposure rate in rem/hour for a one curie source at a distance of one meter, and note that in these units cesium-137 has a dose constant of about 0.4. So a hundred millicurie source produces roughly 0.04 rem/hour, or 0.4 millisievert per hour.

At this rate, if you stood there and stared into the source for two and a half hours, you'd get Health Canada's recommended yearly radiation limit for the general public. It'd take a hundred and fifty hours to get the limit recommended for people who work with radiation. At two hundred and fifty hours (if that still counts as "acute") you might raise your cancer risk by 0.8%.

All this is to say that while I don't think it's a good idea to look into the source, and certainly not to touch it, and I wouldn't want to work with it every day, the warning signs are perhaps a bit over-emphatic.