Answer to Question #8216 Submitted to "Ask the Experts"

Category: Radiation Basics — Neutrons

The following question was answered by an expert in the appropriate field:


I am a computer software researcher working in a university laboratory building that is shared with the physics department; the physics department uses the ground floor, while we use the floor above. I have recently found out that the physics department operates a Pu-Be (plutonium-beryllium) neutron source in the room right below the room I am working in. It is kept immersed in a pool of water, with a base of approximately one-by-one meter and perhaps a meter and a half high. The pool is surrounded by bricks. In the short time that I have spent in the room I haven't noticed any protection on the ceiling. The people in the physics department claim that this arrangement is completely safe, and that we should not worry. As a layman in the nuclear physics field, at first I pretty much took their word for granted, but as I found out more I have started to have some doubts.

While I expect that the pool is enough to slow the neutrons down and to eventually bind them to the hydrogen atoms, I am worried about the gamma radiation that is released in the process. How strong is it and is the concrete floor between our rooms enough to stop it? Is it safe to work in our room for four to eight hours a day and roughly how much radiation exposure are we receiving in a year? I have no idea how strong their neutron source is, but I expect it not to be very strong.


Because I do not know the actual source strength and the details of the building construction (dimensions, materials, thicknesses, etc.), I cannot provide a complete answer to your question, but I will try to provide some useful information.

As you are likely aware, although Pu-Be sources are no longer being manufactured, they have been around for several decades and have been especially popular as teaching tools in universities. Many of the sources used as such have plutonium activities of about 7.4 × 1010 Bq with associated neutron yields on the order of 3 to 4 x 106 neutrons per second. These yields are such that the neutron dose rates at the surface of a water volume of the size you mention will be quite low and should be negligible at your location on the floor above where the source is being used. The 2.2 MeV hydrogen capture gamma emission rate cannot exceed the neutron emission rate and, even if no gamma attenuation occurred, for a 7.4 × 1010 Bq Pu-Be source we would expect a soft tissue dose rate of no more than 3.1 × 10-5 µGy s-1 at a point 10 feet away, which distance is probably conservative with respect to your position relative to the source. Such an exposure rate is about equivalent to normal external radiation background levels. The real gamma dose rate would be much lower because of attenuation in the water and intervening material between the source and you.

There are some Pu-Be sources around, some designed for research and some for other applications, that have plutonium activities greater than the 7.4 × 1010 Bq that you mentioned. If you have doubts about the source activity or if questions about your radiation exposure persist, I would suggest that you contact the radiation safety officer (RSO) at your university. Discuss your concerns with him/her and if he/she does not already have results of survey measurements made at representative locations around the source in question, request that the RSO arrange to perform a survey in your work area to demonstrate that radiation levels are acceptable. I would expect that the RSO would be happy to accommodate your request. Good luck.

George Chabot, PhD, CHP

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