Some asteroid fragments are large enough to not completely burn up in the atmosphere and they end up on the surface of the Earth. It is possible for such a fragment to be radioactive. What is the chief cause of radioactivity? If you had a radiation detector that could measure the amount of radiation—but not the type of radiation—how could you determine which type of radiation was being emitted?
As an asteroid passes through the atmosphere it comes in contact with the gases that the atmosphere is made up of. The extremely high speed with which asteroids travel generates a lot of heat due to frictional forces. This results in a part of the asteroid burning up, the extent to which this happens and the portion that is left unaffected in the portion that crashes on the Earth is dependent on the chemical constitution of the asteroid.
Radioactivity is not a result of the asteroid burning up. It is an inherent property of some elements that spontaneously undergo nuclear decay with the formation of smaller sized atoms of other elements. In this process, radiation in the form of alpha, beta or gamma radiation can be given off. The nature of the decomposition reaction determines which type of radiation is given off. If a radiation detector can only determine the amount of radiation but not what type of radiation is emitted, it would not be possible to identify which type of radiation is being emitted. A study of the mechanism by which the radiation detector works could help, and it may enable the type of radiation to be identified. Generally, radiation detectors are built to be able to identify the type of radiation as well as how much of it is being released.