]{} \_[ ]{}\^[11]{}[C]{}) + N p + n + X + N \[reac:NN\] have recently been done at the LPI synchrotron and we report below on the obtained results.

Experimental setup at LPI {#experimental-setup-at-lpi .unnumbered}
=========================

[r]{}[0.3]{} ![image](fig-setup.eps){width="30.00000%"}

Our experiment was performed at the bremsstrahlung photon beam of the 1.2-GeV electron synchrotron of the Lebedev Physical Institute. Photons were produced with an electron beam of intensity $I_e \simeq 10^{12}
~\rm s^{-1}$ and the duty factor $\simeq 10\%$. The energy of the beam was usually $E_e = E_{\gamma\;\rm max} = 850~\rm MeV$ (i.e. above the $\eta$ photoproduction threshold off free nucleons, $E_{\eta\;\rm
thr}=708~\rm MeV$); additional measurements of subthreshold backgrounds have been done at $E_e = E_{\gamma\;\rm max} = 650~\rm
MeV$.

The experimental setup included two time-of-flight arms (two scintillation telescopes — C and N arms) for detecting in coincidence charged and neutral particles (back-to-back pairs), see Fig. \[exp-setup\]. These arms were both positioned at $90^\circ{-}90^\circ$ with respect to the beam axis in order to minimize background.

The C-arm used for detection of charged particles is a plastic TOF spectrometer for charged pions and protons. It consists of a start detector T1 ($20\times 20\times 2~\rm cm^3$), a stop detector T2 ($50\times 50\times 