Investigations of reactions of the type
are of interest for their own sake, and also for the fact that, if is a radioactive nucleus, as it usually is, being a stable isobar, activation of a few different A nuclei in the same fast neutron flux will yield some information about the neutron spectrum. This follows from the fact that these (np) reactions (and the similar (nα) reactions) are nearly all endothermic, showing a threshold of neutron energy below which no activation takes place. Thus knowledge of the excitation functions of a few such reactions would enable the above-mentioned crude neutron spectroscopy to be realized. The detectors (A nuclei) would have a standard shape and size, and the induced β-activity would be measured with a standard counter in fixed geometrical conditions. Though exact spectroscopy would not be possible, any change in the relative activations of the detectors would be a quite sensitive indicator of a change in the incident neutron spectrum.