Recently the cross sections of the neutron disintegration of 28Si have become available. The primary Si(n,q) breakup is to the ground state of the residual nucleus and secondary modes of breakup to excited states are also known. For any given energy interval one may calculate the neutron flux incident on a silicon semiconductor detector by observing the charged particle flux corresponding to the Q-value of the reaction. The primary mode of breakup is 28Si(n, p 0 + p 1) or 28Si( n, α 0) according as the neutron energy is less or greater than 8.5 MeV. Corrections for disintegration to secondary modes of breakup must be taken into account. The energetics of the interaction limits this method to energies greater than 5 MeV. Present limitations on our knowledge of the appropriate cross sections indicate that an upper energy limit of 10.2 MeV is appropriate but may be removed by further knowledge of the silicon-neutron disintegration. As an example of the method the Pu-Be spectrum is calculated with ∆En = 400 keV.