Question

Determine how many times the intensity of a narrow beam of thermal neutrons will decrease after passing through the heavy water layer of thickness d = 5.0 cm. The effective cross-sections of interaction of deuterium and oxygen nuclei with thermal neutrons are equal to σ₁ = 7.0 b and σ₂ = 4.2 b respectively.

Answer 1

Here

where 1 refers to O and 2 to D nuclei 

Using n₂ = 2n, n₁ = n = concentration of O nuclei in heavy water we get

Now using the data for heavy water 

Thus substituting the values