1. The problem statement, all variables and given/known data In an experiment carried out with a beam of thermal neutrons it is found that on traversing a 2mm thick foil of 197Au, some 70% of the neutrons are removed. What is the total thermal neutron cross-section for this isotope of gold? (Density of gold: 19300 kg m-3) 2. Relevant equations 1. N = N0 exp(-xnσ) - x is thickness, n is number of nuclei per unit volume, σ is cross section 2. moles = mass/A 3. n0 = Na x moles - Na is avagadros number, n0 is number of nuclei 3. The attempt at a solution I have attempted this question and appear to have used the same method as in the solution given by the professor, but i get a different answer... Am i right? Firstly, to find n i substitute equation 2 into 3, giving n0 = (Na x m)/A Then i divide this by the volume: n = n0/V = (Na x m)/(A x V) = (Na/A) x ρ (ρ = density) Next i rearrange eqn 1: N/N0 = (70%)/(100%) = 0.7 = exp(-xnσ) This gives: σ = (-ln(0.7))/(xn) Now im not sure if i should be using m in grams or kilograms? My answer: σ = 3.02 x 10^(-30) m^2 = 0.0302 barns (using m in grams) σ = 3.02 x 10^(-27) m^2 = 30.2 barns (using m in kg) Neither of these answers matches the answer given, 102 barns.... Even though my method is identical to the professors method (although he gives no intermediate results, so i dont know if/where i have gone wrong...) Any help is much appreciated.