Answer to Question #12454 Submitted to "Ask the Experts"

I have some questions regarding neutron activation. Can you tell me some basic facts like what materials can be activated? In the medical neutron therapy, do patients (possibly with metal implants), medical personnel, and parts of the machine become radioactive due to neutron activation? If electronic products such as cellphones and Geiger counters are exposed to neutron radiations, will they become activated and radioactive and do they still work properly?

Thank you for the question. When significant neutrons are present in the vicinity of various materials, including many metals, human tissue and body fluids, or other materials, including air, there is the likelihood that some neutrons will be captured within some atomic nuclei, and some of the products will be radioactive. Some attempts have been made at using fast (high energy) neutrons to irradiate tumors, but these techniques have generally not been very successful, and I will not directly address possible activation products associated with high-energy neutron reactions.

The more successful neutron therapy techniques have used lower-energy neutrons. In the past, nuclear reactors have served as sources of neutrons, but in more recent years accelerators have often been used to generate neutrons. In either case a beam-modifying system is required to slow neutrons down into the epithermal energy region; these neutrons are slowed further by scattering in tissue on their way to the targeted tumor. Prior to irradiation, in a technique called boron neutron capture therapy (BNCT) the target tissue(s) of the patient are ideally subjected to uptake of borated compounds with the intention that the slower thermal or near thermal energy neutrons will be captured by the ¹⁰B in the cancerous tissue; the ¹⁰B has a large thermal neutron absorption cross section and the induced neutron, alpha (n.α) reaction releases appreciable charged particle energy in the tumor, capable of causing tumor cell death. The propensity of a particular nucleus to absorb a neutron depends on the size of its absorption cross section. Naturally, the magnitude of the neutron fluence and the abundance of particular atomic species exposed to neutrons will also affect the extent of activation that will be seen.

The human body contains a number of elements that are subject to activation by thermal neutrons. The most significant of these is probably ²³Na, present throughout the body in body fluids and most body tissues. It produces radioactive ²⁴Na, which has a 15-hour half-life, upon capture of a neutron. Other radioactive species that one might expect in the body include ⁴²K (12.4 hours half-life), ³⁸Cl (37 min), and ⁵⁹Fe (45 days). The patient being treated is the one who would be subject to such activation. Medical staff would not be in the treatment room during irradiation and would not be a concern from the point of view of neutron activation. Parts of the machine being used for neutron production may be subject to significant neutron fluences and possible neutron activation. Metallic components are common. Various steels contain iron and other activatable species such as manganese and cobalt; resultant radionuclides may include ⁵⁹Fe, ⁵⁶Mn (2.6 hours), and ⁶⁰Co (5.3 years). The beam-shaping/modifying materials used to produce the desired spectral distribution may be variable and may include various metals that may contribute other species to the radioactive inventory—e.g., ²⁸Al (2.3 min.), ¹²²Sb (2.7days), ¹²⁴Sb (60 days), ⁵¹Cr (28 days).

I would not expect cellphones to be present in a medical treatment room, but if they were, they would be subject to possible activation of some metallic species. Reactions such as ⁶³Cu(n,γ)⁶⁴Cu (12.7 hours) and ¹⁹⁷Au(n,γ)¹⁹⁸Au (2.7 days) are possible. A Geiger counter left in a neutron environment would also be subject to some activation of iron, copper, and other constituents. If such devices were inadvertently left in a treatment room, unless they were in the direct treatment beam, I would not expect them to lose functionality. Even if placed in the direct beam for, say, the duration of a patient treatment, I would expect such devices to maintain functionality. The most damaging neutron-induced nuclear events are displacement reactions in which high-energy neutrons knock atoms from their metallic matrices. At the energies involved in typical neutron therapy, such events are not very significant. Naturally, the activation could be significant and one would want to monitor the affected instrument(s) to ensure that the radiation dose rates are not a concern.

I hope this addresses most of your concerns.

George Chabot, PhD