Theory of the γ+A→B+y+z reaction near the theshold of the γ+A→1+2+z channel

A theory of the γ+A→B+y+z reaction is developed near the threshold Q/sub γ//sub ,12//sub z/ of the γ+A→1+2+z channel. It is shown that there is a peak in the distribution of events of the first reaction in the relative energy ω/sub B//sub y/ of the particles B and y at the value ω/sub B//sub y/ = m1+m2-m/sub B/-m/sub y/. The triangular Feynman diagram with the successive virtual processes A→1+3, 3+γ→2+z, and 1+2→B+y is responsible for this peak. No such peak exists at the threshold of the γ+A→1+2+z reaction. The excitation function of the γ+A→B+y+z reaction, in contrast to the γ+A→1+2+z reaction, has a resonance form near the threshold Q/sub γ//sub ,12//sub z/. The cross sections and the integrated cross sections for the γ+A→B+y+z and γ+A→1+2+z reaction at a γ-ray energy of the order of a few MeV measured from the threshold Q/sub γ//sub ,12//sub z/ differ by several times. This can be explained by the reaction mechanism indicated and must not be interpreted as the charge noninvariance of nuclear forces