Question

Suppose one food irradiation plant uses a 137Cs source while another uses an equal activity of 60Co. Assuming equal fractions of the γ rays from the sources are absorbed, why is more time needed to get the same dose using the 137Cs source?

a) 137Cs has lower energy γ rays.

b) 60Co has lower energy γ rays.

c) 137Cs has higher energy γ rays.

d) 60Co has higher energy γ rays.

Answer 1

More time is required using a 137Cs source than a 60Co source for food irradiation because 137Cs emits lower energy gamma rays, which are less penetrative and induce less ionization, thus requiring a longer time to deliver the same dose.

Step-by-step explanation:

More time is needed to achieve the same dose using a 137Cs source compared to a 60Co source because 137Cs has lower energy gamma rays than 60Co. Specifically, the gamma rays from 137Cs average 0.67 MeV, whereas those from 60Co average 1.25 MeV.

The higher energy gamma rays from 60Co are more penetrative and produce more ionization, which means they can deliver the desired dose to the food in a shorter amount of time. Given that both 137Cs and 60Co sources have equal activity, the plant using 137Cs needs more time to irradiate the food to the same level because of the reduced energy and penetration capacity of its gamma rays.