  The circularly polarized gamma-quanta with energy > 10 MeV are formed during
Compton backscattering of circularly polarized laser photons on an electron
beam with energy 1.26 GeV. The contribution of nonlinear processes for planned
parameters of a laser flash[T. Okugi, Y. Kurihara and M. Chiba, Jpn. J. Appl.
Phys. 35, p. 3677 (1996).] can be rather significant, that leads to
deterioration of polarization of a resulting gamma-beam. The direct method of
measurement of gamma-beam circular polarization will allow not only to estimate
a correctness of the theoretical calculations, but also to use the obtained
information for simulation of polarization parameters of positrons.
