Measurement of Gamma (phi-> eta-prime gamma) / Gamma (phi -> eta gamma) and the pseudoscalar mixing angle

We have measured the radiative decays phi --> etagamma, phi --> eta'gamma selecting pi(+)pi(-)gammagammagamma final state in a sample of similar to 5 x 10(7) phi-mesons produced at the Frascati phi-factory DAPhiNE. We obtain Gamma(phi --> eta'gamma)/Gamma(phi --> etagamma) = (4.70 +/- 0.47 +/- 0.31) x 10(-3). From this result we derive new accurate values for the branching ratio BR(phi --> eta'gamma) = (6.10 +/- 0.61 +/- 0.43) x 10(-5) and the mixing angle of pseudoscalar mesons in the flavour basis phi(P) = (41.8(-1.6)(+1.9))degrees.