Answer to Question #13014 Submitted to "Ask the Experts"
Category: Radiation Basics — Neutrons
The following question was answered by an expert in the appropriate field:
Due to a previously fractured ankle, I have a large metal plate and around 20 metal screws implanted inside my ankle. I believe the plate and screws may be a metal composite containing cobalt, and the implants will remain in my ankle forever.
I am traveling for about 20 hours. I have read that inside an airplane during flight, there is exposure to a high level of neutron radiation resulting from cosmic rays (which has been increasing over the years), and there is an even higher level during solar flares.
Is there a possibility that this exposure to neutron radiation during my air flights would cause any induced radioactivity in the metal implants in my ankle, especially considering that the implants contain cobalt? I have read that stable cobalt has a high susceptibility to induced radioactivity from neutron radiation, and it converts to radioactive cobalt-60 (60Co) upon exposure to neutron radiation. I don't know though, if this only would occur at very high levels of neutron radiation exposure, much higher than the levels that would be encountered in an airplane or anywhere else in normal everyday life.
Can you advise if the level of neutron radiation and amount of exposure time during my air flights, would cause the metal implants inside my ankle to become radioactive at all?
A very interesting question. I will state first that small amounts of radioactive 60Co may be produced, but I do not believe you need to be concerned about the potential radioactivity that might be induced in your implant components. I'll attempt to explain my reasons for this conclusion. You are correct in many of your observations:
- Many metallic alloy implants, especially those used as reinforcements necessary in severe fracture cases, do contain significant amounts of cobalt.
- Stable cobalt is 100% 59Co. This isotope has a relatively large thermal neutron absorption cross section, meaning that in a significant flux of thermal (slower and lower energy) neutrons, appreciable numbers of such nuclei may capture a neutron and become radioactive 60Co, which is characterized by a 5.26 year half-life.
- The flux of neutrons at high altitudes is much higher than that near the surface of the earth.
I do not know the specific alloy used in your case, but one of the more common ones used in medical implants as you describe contains roughly 65% cobalt, 30% chromium, and 5% molybdenum. For the likely mass of the device you describe, this would represent plenty of cobalt to produce large amounts of radioactive 60Co if it were exposed to a high fluence of thermal neutrons as might be available in a nuclear reactor. The key words here are italicized. The flux of effective neutrons (cm-2 s-1, number of neutrons per square centimeter per second incident on the target material that you might encounter would be on the order of perhaps 1011 to 1012 lower than what would be available in a typical research reactor. The fluence is the product of the exposure time and the flux, and the exposure time, in your instance is limited to about 20 hours.
To give you a better idea of the significance of the neutron exposure, I should note that the earth is continuously bathed in neutrons produced from cosmic ray interactions and decay of certain induced products. The neutrons produced in the upper atmosphere are virtually all high energy neutrons. Those near the surface of the earth tend to be much lower in energy because of losses in energy associated with both scatter in the atmosphere and scatter with and reflection from atoms within the earth and the oceans. The flux of such "earthly" neutrons is in the typical range from about 1 to 3 neutrons per square centimeter per second. You, I, and your implant are continuously subjected to this flux. If all of these neutrons were thermal (which they are not) and if we assume a flux of 2 cm-2 s-1, incident on your implant, and further assume that your implant (whose mass I do not know) contained 100 grams of cobalt, a likely upper bound, we could show that the expected eventual maximum amount of 60Co that would be expected would be about 75 bequerel (Bq) or a little more than one atom decaying per second. In reality, the number would be lower because of attenuation of the neutrons in the body tissues overlying the implant. This activity is quite small. For example, 75 Bq is about 500 times lower than the amount of 60Co that the US Nuclear Regulatory Commission defines as "exempt," meaning it may be obtained without a license and used without necessity for adhering to typical regulations required for radiation protection purposes. It certainly would present no health concern.
At higher elevations the neutron flux will be considerably higher. Here is a link to a convenient website that allows one to estimate the neutron flux for specified elevations above the surface of the earth at a particular location; the results are given as relative to the ground neutron flux for New York City. For example, I entered the latitude and longitude for Toronto, Canada (45.6532 N and 79.3832 W, respectively) and an elevation of 9,144 m (30,000 ft) and got a relative flux of 235. For an assumed New York ground flux of 2 cm-2 s-1, the expected flux at 9,144 m above Toronto would then be 470 cm-2 s-1. These would be primarily high energy neutrons that would not be readily absorbed by stable cobalt. Some of them would be attenuated as they passed through the aircraft and into a passenger's body and some would get thermalized (slowed down) by scattering in the body and aircraft components and, in your situation, some of these would find their way to your alloy implant and be absorbed by cobalt to yield some 60Co. If we assumed, I believe conservatively, that 10% of the original flux outside the aircraft might be thermalized and be available to activate the cobalt, the effective flux would then be 47 cm-2 s-1. This flux is about 24 times that assumed when we made the above calculation for the activation of 100 grams of cobalt at the earth's surface. The exposure time for your planned flight, however, was only 20 hours. For a 20-hour exposure, I calculate an expected 60Co activity of 0.53 Bq.
The 0.53 Bq is less than 1% of the value obtained for the earth surface estimation. I recognize that I do not have sufficient details about your implant or about the exact fate of neutrons that enter the aircraft and are available for activation to provide an exact determination of the expected 60Co activity. I believe, however, that the relatively simple estimations we have made here are sufficient to provide significant confidence that the 60Co that might be produced in your implant will not be sufficient to produce any effects of concern. I do expect, based on what we have done, that very small amounts of activity may be produced whether you're flying at over 9,000 m or living out your life on the surface of this planet. In fact, unless you are an extremely frequent flyer, the activity produced in your earthly exposure may well outweigh that produced when you're flying high. Additionally, the radioactive cobalt produced on earth where you spend almost all of your time will remain at a constant level as long as the neutron flux remains constant because 60Co is being produced on a continuous basis and is decaying at the same time so that it reaches an equilibrium value after sufficient time has passed. Any 60Co produced during flight will continue to decay after return to earth and will disappear with a half-life of 5.26 years.
The extent of activity production depends on both the neutron flux intensity and the duration of exposure. As long as any thermal neutrons are available to irradiate the cobalt, and they always are, some 60Co will be produced, although the amounts will be expectedly trivial and probably immeasurable by common techniques from outside your body.
Please also keep in mind that all of the above estimations are based on the supposition that your implant does contain significant cobalt. Naturally, other devices available that do not contain significant cobalt would not be of concern here. I hope this is helpful to you.
George Chabot, PhD