Effect of higher harmonics in the area-ratio pulsed neutron source technique

The area-ratio pulsed neutron source technique, which can yield a subcriticality in dollar units, is given by Ap/Ad, the area ratio of the prompt neutron component to the delayed neutron component. In this paper, both components are reconstructed by the summation of the spatial harmonics that are obtained by solving the α-mode eigenvalue equations based on the neutron transport and diffusion theories. Monte Carlo simulations for the area-ratio method in a one-dimensional slab are performed. The reconstructed prompt and delayed components agree well with the simulation results. The areas Ap and Ad contain a higher harmonic effect, and the ratio Ap/Ad differs from the true subcriticality. The effect of the higher harmonics in Ap can be reduced by the extrapolated area-ratio method if the earlier period after the pulse generation is successfully excluded from the fitting period. However, there is no way to remove the effect of the higher harmonics from the delayed neutron component, but it alleviates itself because it is given by the subtraction of the prompt component from the total (delayed and prompt) component. The subcriticality in dollar units adjusted by the extrapolated area-ratio method is considerably improved compared with the unadjusted Ap/Ad. The improvement is remarkable for a near-critical system in which the denominator Ad is relatively large, and the effect of the higher harmonics in Ad is minor.