Effective Cadmium Cutoff Energies for Non-1/V Detectors

Effective cadmium cutoff energies were calculated for ²³⁹Pu, ²³⁵U, ¹¹⁵In, Au, ¹¹⁹Ir and ¹⁷⁶Lu as a function of the cadmium filter thickness, the mast probable energy of the Maxwellian flux and the epi-thermal index. An infinite slab geometry and isotropic flux were assumed in the calculations.

The calculated values of the cutoff energies are compared with those for 1/V detector, and are related to the resonance energies near the cutoff energy. If the resonance energy is lower than that of the 1/V detector, and the neutron spectrum and filter are same, the cutoff energy is lower than that of the 1/V detector. Such cases include ²³⁵U and ¹⁷⁶Lu with thin filter, ¹⁹¹Ir with thick filter, and ²³⁹Pu with either thin or thick filter. If the resonance energy is higher than the E _c of the 1/V detector, the cutoff energy is higher, such cases being ¹⁹¹Ir with thin filter, ¹⁷⁶Lu with thick filter and ¹¹⁵In with either thin or thick filter. And if the resonance energy is very much higher than the E _c of the 1/V detector, as in the case of Au with thin filter, the cutoff energy approaches the value of the 1/V detector.