A sealed source has a dose rate of 6 mSv/h at 0.5 m. What is the predicted dose rate at 1.5 m using the inverse square law?

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Multiple Choice

A sealed source has a dose rate of 6 mSv/h at 0.5 m. What is the predicted dose rate at 1.5 m using the inverse square law?

Explanation:
Radiation dose rate from a point source decreases with the square of the distance. If you know the dose rate at one distance, you can scale it by the square of the distance ratio: D2 = D1 × (r1/r2)². Here, the distance goes from 0.5 m to 1.5 m, a factor of 3. So D2 = 6 mSv/h × (0.5/1.5)² = 6 × (1/3)² = 6 × 1/9 ≈ 0.67 mSv/h. So the predicted dose rate at 1.5 m is about 0.67 mSv/h. A common mistake is to assume a linear 1/r relationship, which would yield 2 mSv/h, but the inverse square law requires squaring the distance ratio.

Radiation dose rate from a point source decreases with the square of the distance. If you know the dose rate at one distance, you can scale it by the square of the distance ratio: D2 = D1 × (r1/r2)².

Here, the distance goes from 0.5 m to 1.5 m, a factor of 3. So D2 = 6 mSv/h × (0.5/1.5)² = 6 × (1/3)² = 6 × 1/9 ≈ 0.67 mSv/h.

So the predicted dose rate at 1.5 m is about 0.67 mSv/h. A common mistake is to assume a linear 1/r relationship, which would yield 2 mSv/h, but the inverse square law requires squaring the distance ratio.

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