The calibration results are presented for a scintillation hodoscopic spectrometer of charged particles, which has been designed for an experiment on the search for a heavy electron on the bremsstrahlung photon beam at the Pakhra accelerator of the LPI. The relative energy and coordinate resolutions of the spectrometer at electron energy E = 40 MeV are δ = 22% and σ x = 9.5 mm, respectively. It has been determined that the width of electromagnetic showers in the transverse direction can be described by the energy dependences Δ ~ lnE at E < 100 MeV and Δ ~ 1/expE at E > 100 MeV.

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A brief review of searches for η-mesic nuclei is presented with emphasis on photoreactions. Results of a new experiment done at the LPI electron synchrotron are reported. They are as follows.New data on photoproduction of η-mesic nuclei off 12 C have been collected at the bremsstrahlung photon beam of E γ max = 850 MeV. An experimental setup with two plastic time-of flight spectrometers detected correlated π + n and pn pairs from annihilation of η-mesons stopped in the nuclear matter and measured their velocity distributions. Data analysis was performed using an Intra Nuclear Cascade Model in the GEANT-3 framework in order to take into account properties of the setup and physical background. A separation between charged pions and protons was achieved using information on velocities and ranges of the particles in plastic detectors. The obtained data show, apart from previously observed π + n pairs from one-nucleon annihilation of etas (via ηN → πN), a presence of emitted correlated pn pairs with velocities corresponding to the kinematics of the near-threshold reaction of two-nucleon absorption of the η-meson in the nucleus (ηNN → NN). Assuming that such π + n and pn pairs are mostly produced through formation and decay of quasi-bound states of the η-meson and a nucleus (i.e. η-mesic nuclei η A), the cross section of η-mesic nuclei formation was estimated as σ (γ + 12 C → η A + X) 10 µb.

Presented are the characteristics of systems for monitoring the intensity of the bremsstrahlung photon beam of the "Pakhra" accelerator based on Cherenkov counters.

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