Spectral Comparisons with High Energy Activation Detectors

The responses of the activation detectors P(n, p), Al(n, p), Fe(n, p), Al(n, α), and Cu(n, 2n) to the neutron spectra at the center of the bare U²³⁵ and Pu²³⁹ critical assemblies, Godiva and Jezebel, are the basis of a precision comparison of these two spectra above 2 Me v. Results, interpreted in terms of the spectral function E^1/2_e–βE, show that the average energy of this component of the spectrum in Godiva is 4.7 ± 0.8% lower than in Jezebel. From some preliminary measurements a close resemblance to the corresponding fission neutron spectra is indicated in this energy region. Results of photoplate measurements in the leakage spectra of both assemblies are considered and briefly analyzed. A discussion and determination of effective thresholds is appended.