Cancer cells are more vulnerable to x and gamma radiation than are healthy cells. In the past, the standard source for radiation therapy was radioactive , which decays into an excited nuclear state of , which immediately drops into the ground state, emitting two gamma-ray photons, each with an approximate energy of 1.2 MeV. The controlling beta-decay half-life is 5.27 y. We have to find how many radioactive nuclei were present in a 6000 Ci source of the type used in hospitals.

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The beta-decay half-life of is 5.27 y. Its decay constant will be

Unit Ci is defined as

.

.

The decay rate

Therefore, the number of nuclides that will have activity of 6000 Ci will be