To devise an activation technique for characterizing mixed radiation fields, secondary particles from a copper target irradiated by 24 GeV/c protons were measured at the CERN High-energy AcceleRator Mixed field facility (CHARM). Activation detector sets consisting of aluminum, niobium, indium, and bismuth, were placed at 30 cm from the target at angles of 15 degrees to 160 degrees with respect to the beam axis. The nuclides generated in these detectors due to irradiation by secondary particles were analyzed by ?-ray spectrometry, and the angular distributions of the production rates were obtained. The results of Monte Carlo calculations using FLUKA code was compared with the experimental results. The calculated results well agreed with the measured data at all angles. The influence of competitive reactions on the measured data were also evaluated by FLUKA. The following nuclear reactions, with low affectivity by competitive reactions, were identified as promising tools for characterizing mixed radiation fields: the 115In(n, n ')115mIn reaction for detecting neutrons emitted by the evaporation process, the Nb-93(gamma, n)Nb-92m reaction for verifying the photon distribution generated by neutral-pion decay (Pi(0) -> 2 gamma), and the Bi-209(p, 4n)Po-206 reaction, which detects secondary protons.