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Construction of a spectrometer magnet equipped with multiwire proportional chambers

Dipole spectrometer magnet with symmetric tapered poles for HERMES experiment, designed by DESY and INFN-Sanita' groups to satisfy geometrical and magnetic requirements of the experiments and to minimize the power consumption, was manufactured in Russia at the Efremov Scientific Research Institute of Elecrophysical Apparata in St. Petersburg. It has been installed in the East Hall of the HERA ring as the core part of the experiment spectrometer. Field maps were measured by a joint effort of DESY, PNPI and INFN-Sanita' groups and an agreement within 2% was found with the project model calculations done with three-dimensional computer codes.

Six Multiwire Proportional Chambers, each consisting of three independent modules (U, X and V planes) with separate high voltage supply, were designed and manufactured by the PNPI group taking into account the compelling spatial requirements of the chamber positions within the magnet. Gas supply system of non-flammable mixture (Ar/CO2/CF4) as well as the on-board electronic cards cooling system, using a negative pressure water loop, was also designed and provided by the St. Petersburg group.

The electronic read-out was provided by the INFN-Sanita' group who, working in collaboration with the manufacturer, designed a new configuration of the PCOS 4 read-out system in order to match geometrical limitations connected with the installation within the poles magnet. INFN- Sanita' group designed and manufactured special backplanes boards for the data streaming taking specially into account the requirement of reliability of the read-out system that is inaccessible for the whole period of data taking and can be cured only during the annual shut-down of the accelerator.

Magnet Chamber were commissioned by PNPI and INFN-Sanita' physicists and with the whole HERMES system of detectors are currently taking data. Efficiency of each of the 18 planes is about 99%. They have been demonstrated to be a unique tool to define moment of the low energy charged particles.

Neutron and proton spin structure functions have been measured using longitudinal polarized electron/positron beam off polarized pure helium-3 and hydrogen as target then with a different technique than previous measurements and in some case increasing the precision of the measurements. Data confirm CERN and SLAC results that, interpreted in the framework of the naive quark parton model, assign only a fraction of the nucleon spin to the quark spins.

Semi-inclusive deep-inelastic reactions with detection of pions in the final state have also been measured off hydrogen and deuterium targets showing a flavor asymmetry in the nucleon sea quarks.

Study of the photoproduction of pairs of hadrons with high transverse momentum in the final state when polarized positrons scatter off a polarized hydrogen target has shown a negative asymmetry that could be originated by a positive gluon polarization in the proton.

Other results, obtained measuring deep-inelastic scattering off unpolarized light and medium A nuclei, indicate effects of the nuclear medium on electroproduction of r meson and on the ratio of longitudinal to transverse cross section.

Spin transfer in deep-inelastic L electroproduction has been studied and the results compared to various models for the L spin structure.

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Istituto Nazionale di Fisica Nucleare
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