Answer to Question #13561 Submitted to "Ask the Experts"

Does a Geiger-Mueller (GM) tube (pancake-style) detect neutron-induced radioactivity from the background neutrons and from some significant neutron source?

The short answer is yes, it may be possible for a GM detector-based instrument to detect radioactivity induced in the body of someone who has been irradiated with neutrons. Background neutrons would not be sufficiently intense to produce measurable activity, however, but significantly greater exposures may yield measurable activity. In fact, this is a long-known and used phenomenon in situations where individuals have received possibly large neutron doses, as in some cases of accidental nuclear criticality events that have occurred in the past.

In such situations, the Geiger probe may be placed against the midsection of the individual (having the individual curl his/her body around the detector probe will improve detection efficiency). The most likely induced radionuclide that would produce measurable gamma radiation is sodium-24 (²⁴Na). Stable sodium, ²³Na, is present relatively uniformly throughout the body at a concentration of about 1.4 g kg^-1 of body mass and in the blood at about 1.9 mg L^-1 of blood. The thermal neutron absorption cross for ²³Na is greater than 0.5 barns, large enough to allow appreciable activation to ²⁴Na in a significant neutron fluence, and the major radiations are a 1.27 MeV and a 2.75 MeV gamma ray, each with a yield of about 1 gamma ray per disintegration.

The technique of using a detector placed close to the body following a possible neutron exposure is most useful as an initial screening process to quickly identify individuals who were exposed to significant neutron fluences. The amount of radioactivity produced in the body depends not only on the fluence of neutrons but also on the energies of the neutrons, with lower energy distributions typically yielding greater ²⁴Na activities. At any rate, the fluence of neutrons necessary to yield measurable activity with the GM probe would be likely considerably higher than a radiation worker or a layperson would be expected to experience in any situation other than an accidental exposure involving doses close to, or higher than, the allowed legal limits for occupational exposure. The method suffers additionally from the fact that the clothing and/or skin of the measured individual may be contaminated with other radioactive material, depending on the nature of the incident.

More sensitive and more accurate methods have been used to establish quantitative estimates of neutron doses. One of these involves taking and measuring ²⁴Na in blood samples; this has been developed and used in accident situations. If you have further interest in this, you might like to review the report on the topic.

George Chabot, CHP, PhD