High School

A 1.2 µCi Cs-137 source is used for 1.4 hours by a 62-kg worker. Radioactive Cs-137 decays by beta decay followed by a gamma-ray, with a half-life of 30 years. The total emitted energy is 850 keV per decay. Assuming the person absorbs all emitted energy, what effective dose (in Sv) is received?

Answer :

The effective dose equivalent (EDE) received by the worker is 1.23 x 10-8 Sv given that a 1.2 uCi Cs-137 source is used for 1.4 hours by a 62-kg worker.

The absorbed dose is given by the formula; D = A x t x (0.693/λ) x (1/ M)Sv = D x Q, where Q = Radiation Weighting Factor (WRF) and for beta/gamma = 1

The dose equivalent is;H = D x Q x N, where N = Quality Factor (QF) = 1 for beta/gamma. The effective dose equivalent (EDE) is; EDE = ΣH x Wr Where Wr is the radiation weighting factor of a tissue or organ which is equal to 1 for gamma-rays.The calculation is shown below;

Activity of Cs-137, A = 1.2 µCi = 1.2 x 10-6 x 3.7 x 1010 Bq = 4.44 x 104 Bq Time, t = 1.4 hours = 1.4 x 60 x 60 = 5040 seconds

Decay constant, λ = 0.693 / t½ = 0.693 / 30 = 0.0231 year-1

The number of decayed atoms (disintegrations), N = A x t = 4.44 x 104 x 5040 = 2.24 x 108Total absorbed dose, D = A x t x (0.693/λ) x (1/M) = 1.23 x 10-8 Gy

Dose equivalent, H = D x Q x N = 1.23 x 10-8 x 1 x 1 = 1.23 x 10-8 Sv

Effective dose equivalent (EDE) = ΣH x Wr = 1.23 x 10-8 x 1 = 1.23 x 10-8 Sv

Therefore, the effective dose equivalent (EDE) received by the worker is 1.23 x 10-8 Sv.

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