The quantity of a radioactive material is often measured by its activity (measur

Question

The quantity of a radioactive material is often measured by its activity (measured in curies or mill curies) rather than by its mass. In a brain scan procedure, a 64.0-kg patient is injected with 16.0 mCi of^99mTc, which decays by emitting y-ray photons with a half-life of 6.01 h. Given that the RBE of these photons is 0.98 and exactly two-thirds of the photons are absolved by the body, calculate the rem dose received by the patient. Assume all the^99m Tc nuclei decay while in the body. The energy of a gamma-ray photon is 2.29 times 10^-14 J.

Explanation / Answer

Disintegrations per second of 16mCi of 99Tc = 16 * 10^-3 * 3.7 * 10^10 = 5.92 * 10^8 disintegrations/sec

Total energy of gamma photons: 5.92 * 10 ^8 * 2.29*10^-14 = 1.36 * 10^-5 J

Energy absorbed by the body (E) = 2/3 * 1.36 * 10^-5 = 9.1 * 10^-6 J

Absorbed dose (rad) = Energy/mass = 9.1 * 10^-6/64 = 1.4 * 10^-7

rem dose = rad * RBE = 1.4 * 10^-7 * 0.98 = 1.37 * 10^-7

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