Searches for d' resonance in double charge exchange reactions of pi sup + -mesons on nuclei in nuclear photoemulsion

797 events have been analyzed in double charge exchange (DCX) reactions of pi sup + -mesons registered in photoemulsion chambers exposed to positive pion beams of the phasotron of the Dzhelepov Laboratory of Nuclear Problems (JINR) at energies of 80-140 MeV. The effective invariant mass of the secondary pi sup - -meson and the two protons was determined for each three-prong event identified within the energy range of the incident pi sup + -meson between 72 and 140 MeV. The spectrum of effective masses reveals a peak in the 2060-2072 MeV interval with a mean value equal to M sub e sub f sub f =(2065.5+-3.6)MeV. The obtained value of M sub e sub f sub f is in a good agreement with the mass of the d' resonance hypothesized in 'QCD-string' model calculations and confirmed by measurements of the dependence on the incident energy of the DCX cross sections of pi sup + -mesons on nuclei and in pp->pp pi sup -pi sup + process