Eight activation detectors, 235U(n, f), Np(n, f), 238U(n,f), P(n, p), Al(n, p), 56Fe(n, p), Al(n, α), and 63Cu(n, 2n) are placed on a firm experimental foundation to prepare for the investigation of fission-neutron spectra in Part II. Gross beta-gamma counting with methane-flow counters is employed to achieve the exceptional stability of detector response required for spectrum determinations. Calibration and specification of detector excitations are based on relative cross-section measurements performed at the Los Alamos Van de Graaff. The latter have been tailored to supplement existing cross-section information and to aid in its evaluation. Deduced detector excitations are specified in fifty energy groups with uncertainties assigned in a separate seven-group excitation structure appropriate for the investigation of distributed neutron spectra.