An underwater inspection robot travels through mud containing an alpha-emitting radioactive substance. Some of the mud remains on its wheels. A second robot is exposed to gamma radiation from a sealed source during a materials test. No radioactive substance touches the second robot. Each robot is then driven over a different clean floor.
Which statement explains why the floor used by the first robot may need checking for radioactive contamination, but the floor used by the second robot does not?
Alpha radiation changes atoms in the first robot into unstable nuclei; gamma radiation only changes molecules in the second robot.
Only alpha-emitting substances can contaminate surfaces; gamma-emitting substances can cause irradiation but cannot cause contamination.
Contamination means that ionisation has made the first robot positively charged; gamma radiation is too weakly ionising to charge the second robot.
Radioactive material in the mud can be transferred from the wheels to the floor; gamma irradiation transfers energy to the second robot but no radioactive material.