Reaction. pi. /sup -/p. -->. X/sup 0/n, X/sup 0/. -->. 2. gamma. at momenta 15 and 40 GeV/c

The cross sections of the reactions ..pi../sup -/p..-->..X/sup 0/n with the decay X/sup 0/..-->..2..gamma.. were measured at momenta 15 and 40 GeV/c. About 500 events were registered. The ..gamma../sup -/ quantum pairs were detected with a hodoscopic spectrometer on line with a computer. The t-dependence of the differential cross section was found to be similar to that of the reaction ..pi../sup -/p..-->..etan. The angle of the singlet-octet mixing of the pseudoscalar mesons was determined from the ratio of the differential cross sections for the production of X/sup 0/ and eta mesons at t=0 and found to be ..beta..=-(16 +- 2) /sup 0/.