A patient is treated using high-energy gamma radiation aimed at a tumour. The gamma source is outside the patient's body during the treatment and is returned to its container afterwards. The source has an activity of 240 TBq240\ \text{TBq}240 TBq and a half-life of 5 years.
State the property of gamma radiation that allows it to reach a tumour from outside the body.
Explain why healthy cells in the path of the gamma radiation may be damaged.
Calculate the activity of the source after 10 years.
Explain why the treatment does not make the patient radioactive.