Answer to Question #12968 Submitted to "Ask the Experts"

If an object is exposed to radiation does it in turn become radioactive in a similar way that you can magnetize a piece of metal by having it close to a magnet? If someone has been exposed to radiation, do they carry the radiation in them or is it the contamination—dust, etc.—that is radioactive? I know the person exposed to radiation will be affected by the radiation, but does it intrinsically make them radioactive just by being exposed to the rays?

For most radiations that people, especially laypeople, encounter, which would include x rays, gamma radiation, beta particles, and alpha particles, the exposure of the people or objects to the radiation does not produce any radioactivity within them. When such radiation impinges on a person or an object it may interact and deposit some or all of its energy in the person or object, but this energy is dissipated in the form of very small amounts of heat or excitation of atoms and molecules with no induced radioactivity. The radiation impinging on a person or object is different from a case in which someone gets exposed to radioactive materials in the environment, such as airborne radioactivity that may be attached to small particles, as you have inferred, which might be inhaled into the respiratory system or attach to the surfaces of clothing and/or skin.

There is one radiation, well-known but less common than those mentioned above, that is capable of inducing radioactivity in an irradiated person or object; that is neutron radiation. The reason that neutrons are effective in that regard and other radiations are not is because radioactivity is a property of the nucleus of an atom, and the common x, gamma, beta, and alpha radiations interact with the electrons of atoms, but not within the nuclei, whereas neutrons are able to penetrate the electron cloud around an atom and be absorbed by the nucleus, changing the nuclear configuration and, in some cases, making it unstable against radioactive decay. For example, if a person is exposed to significant neutron radiation, one of the most notable radioactive products that we would expect to be produced would be ²⁴Na (sodium-24), produced when stable ²³Na captures a neutron. Sodium is a relatively abundant element in the body and is readily activated to the ²⁴Na product. Similarly, many other materials, especially metals, are subject to neutron activation.

A piece of steel containing small amounts of stable cobalt when irradiated with low energy neutrons will produce radioactive products from both the iron and the cobalt in the metal (as well as other possible species). In fact, as a consequence of the atomic bombing of Japan at the end of World War II, stable ⁵⁹Co in steel contained in many buildings was activated to radioactive ⁶⁰Co. The measurement and detection of the gamma radiation from this ⁶⁰Co provided a means years after the bombing for estimating the effective bomb-produced fluence of neutrons, a quantity useful in helping to establish the doses to some of the irradiated Japanese survivors.

I hope this adequately addresses your questions.

George Chabot, PhD