The ratio R(D-D)/R(D-3He) of yields of fusion reactions was used as a diagnostic tool for studying the effective ion energy of plasma produced in the 1-MJ plasma focus device at Frascati. The device is operated in the neutron-optimized mode at energies of 250, 390, and 490 kJ. The reaction yields are determined by measuring the activity induced in the 63 Cu(p,n)63 Zn (Ep = 14.6 MeV) and 115 In(n,n')115m In (En = 2.45 MeV) reactions. A detailed discussion of the energy slowing down of neutrons is given and a new calibration of the (p,n) cross section of the monitor reaction is performed. The measurements are carried out simultaneously for the end-on and side-on positions, and no significant differences are observed. A comparison with other experimental data is given. Conversion of the ratio of fusion reaction yields to ion temperatures or effective ion energies of streams interacting with a cold gas medium gives kTt ≅ 14 keV or Ed(kTe = 0.5 keV) = 72 keV, respectively.