\(I\ \left(trans\right)\) is the transmitted intensity which is equivalent to integration over the photo-peak with scatterer. Similarly, \(I\ \left(incident\right)\) is the intensity of the incident ray or integration without scatterer. \(\mu\) is the mass attenuation coefficient which at the present energy level, 0.662 MeV, is \(\mu = 0.078\) for aluminium . \(\rho\) is the density of the aluminium, 2.7 g/cm^2. The variable d stands for the square thickness of the scatterer. The integrations are calculated using Riemann integral around the peak with \(\Delta x\) of 1 channel. In essence, the integral is summation of data points around the Gaussian peak. Rearranging the equation 16, d can be expressed as