Question

the bacteria in some foods are sterilized by placing the food near a source of ionizing radiation. Does that mean that the food becomes radioactive? Explain your answer.

Answer 1

Irradiation or ionizing radiation doesn't make food radioactive.

Gamma radiation and sometimes X-rays are used for this sterilization by irradiation technique purpose. These kind of radiation are form of electromagnetic radiations with high energy photons. The bacteris and other organisms can't withstand this highly energetic photons. Thus, the irradiations interact with food by killing bacteria and other organisms present in the food.

A substance must have unstable nucleus to become radioactive. Gamma rays cannot add or remove neutron or proton to or from the nucleus. In photoneutron production, about 5-8 MeV gamma rays are required. A free neutron then could be absorbed by another atom and possibly make it radioactive. The two isotopes most commonly used for food irradiation are Co-60 and Cs-137. All the gammas produced from them being less than the 2 Mev.

Therefore, the level of gamma radiation that certain foods are exposed to are simply far too low to leave any residual radioactivity. The radiation is in the form of high energy particles which pass through the substance -- not in the form of particulate matter that is itself radioactive and so the food doesn't become radioactive if it is placed near a source of ionizing radiation. Only the food is sterilized by killing the bacteria with this technique.