49 U.S.C. § 5114

1. (a)Material that emits ionizing radiation spontaneously may be transported on a passenger-carrying aircraft in air commerce (as defined in section 40102(a) of this title) only if the material is intended for a use in, or incident to, research or medical diagnosis or treatment and does not present an unreasonable hazard to health and safety when being prepared for, and during, transportation.
2. (b)The Secretary shall prescribe procedures for monitoring and enforcing regulations prescribed under this section.
3. (c)This section does not apply to material the Secretary decides does not pose a significant hazard to health or safety when transported because of its low order of radioactivity.