On the large COMPASS polarized deuteron target

The spin structure of the nucleons is investigated in deep inelastic scattering of a polarized muon beam and a polarized nucleon target in the COMPASS experiment at CERN since 2001. To achieve high luminosities a large solid polarized target is used. The COMPASS polarized target consists of a high cooling power $^{3}$He/$^{4}$He dilution refrigerator capable to maintain working temperature of the target material at about 50mK, a superconducting solenoid and dipole magnet system for longitudinal and transversal magnetic field on the target material, respectively, target cells containing polarizable material, microwave cavities and high power microwave radiation systems for dynamic nuclear polarization and the nuclear magnetic resonance system for nuclear spin polarization measurements. During 2001–2004 experiments superconducting magnet system with opening angle $\pm$69 mrad, polarized target holder with two target cells and corresponding microwave and NMR systems have been used. For the data taking from 2006 on an important upgrade of COMPASS polarized target has been done. In a new target system the new large acceptance superconducting magnet with opening angle $\pm$180 mrad and three cells target holder with corresponding microwave and NMR systems have been implemented. The spin reversal by magnetic field rotation to decrease possible systematic error is incorporated. New three cells target holder configuration leads to a further reduction – by an order of magnitude – of possible instrumental false asymmetries. The new large acceptance COMPASS polarized target system with $^{6}$LiD target material for deuteron polarization was successfully used in the COMPASS 2006 run. Preliminary figures of the maximum average longitudinal deuterium polarizations in three target cells obtained at the begining of the COMPASS 2006 run were $+$54%$/ − $52%$/ +$ 56%, respectively. In the experiments with polarized proton target the irradiated NH$_{3}$ material is planned to be used.