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SPIN MUON COLLABORATION

Measurement of the Spin-Dependent Structure Functions of the Deutron and Proton

The SMC Experiment

The main aim of the experiment of the Spin Muon Collaboration (SMC) is to measure the proton and the deuteron spin dependent structure functions using polarized muon and polarized target. The SMC experiment is situated at the end of the CERN SPS muon beam line M2. Muons are produced from decay in flight of charged pions and kaons from the interaction of 450 GeV/c protons with a beryllium target. The muon beam intensity reached is 4.5E7 particles per pulse with a 14.4s repetition and with a 2.4s spill time . The muons are naturally polarized due to parity violation in Pion and Kaon decays. The momentum of the incident muons is measured in the beam momentum station (BMS) which consist of a magnet and two scintillator planes before and after the magnet.

The position and the angle if the incoming muons are measured by using two sets of beam hodoscopes; the first, BHA, after the last bending magnet of the beam line and the second, BHB, just in front of the target. The halo particles remaining within the ac ceptance of the apparatus are recorded in five scintillation counter arrays. A high precision measurement of the scattering angle and of the momentum of charged particles is provided by a large aperture dipole magnet (integral Bdl=2.3 Tm) instrumented wit h multiwire Proprotional Chambers (MWPCs) and drift chambers. The momentum measurement stage is followed by a muon identification stage which consists of a hadron absorber and a large-surface array of streamer tubes and drift tube. Drift chambers and stre amer tubes are complemented by MWPCs to improve the rate capability of the spectometer close to the beam. Three arrays of scintillator hodoscopes behind the hadron absorber provide the muon trigger of the experiment.

In order to measure the beam polarization with an accuracy of better than 5%, a muon polarimeter was built downstream of the SMC spectometer. The muon polarization is determined by two independent methods: The decay method of positron decay of the muon is based on the shape of the positron energy spectrum which is related to the polarization of the muon beam. The scattering method is based on the measurement of the cross-section asymmetry in elastic scattering of longitudinally polarized muons on a longit udinally polarized electrons (Moller scattering). The polarized electron target is realized by a magnetized iron foil. The polarizations of 10 GeV and 190 GeV muon beam are found to be, P(muon)= -0.82 +- 0.06 and P(muon)= -0.811 +- 0.028 +- 0.029 respecti vely. The SMC uses a polarized target which is subdivided in two separate sections. Each target section is 65 cm long with 5 cm diameter. There is a 20 cm gap between the two target sections. In order to measure the deuteron (proton) structure function, deutera ted butanol (butanol) beads have been used as target material. (In 1996 we are using irradiated amonia beads as the target material. Here some pictures of the amonia before and after irradiation.HERE ) The superconducting solenoid provides a longitudinal magnetic field of 2.5 T. A dilution refrigerator cools the target to 0.5 K during microwave polarization, and to 0.05 K during frozen mode. The polarization is performed by the method of Dynamic Nuclear Polarization (DNP). The target sections are operated with opposite polarizations by two separate microwave systems with different frequencies. The polarization is measured by 10 NMR coils emb eded in the target material. The direction of the polarizations are reversed frequently by rotating the magnetic field direction. This allows to reduce the time dependent acceptance differences of the detectors. The average polarization achieved in butano l target was 84% and in deuterated butanol target was 48%.

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