CLAS apparatus

Apparatus

Target

The target materials used in the EG1b experiments were [math] ^{15}NH_3[/math] and [math](15)ND_3[/math]. The first one was used for the polarized protons and the second for the polarized deuterons. These materials satisfy several conditions which are suitable for scattering experiments. Ammonia targets produce high polarization and are resistant to radiation damage(it will be changed) which can be easily repaired by an annealing process. Also,it has high ratio of free nucleons(3/18).
The target material was prepared in the following way. The ammonia gas was frozen at 77 K and then crushed into little pieces, about 1-3 mm in diameter, because it heps to have them efficiently cooled by the liquid helium during the experiment and prevent from potential damage by the beam heating.

Cherenkov detector

The CLAS Cherenkov detector is a threshold gas counter. It was filled with perfluorobutane [math]C_4 F_{10}[/math] gas at atmospheric pressure. Perfluorobutane [math]C_4 F_{10}[/math] was chosen for its high index of refraction n=1.00153, which results in a high photon yield and the following energy thresholds, for electrons 9 MeV and for pions it is 2.5 GeV. The Cherenkov detector is used to distinguish electrons from pions. The six superconducting coils placed by angles of 60 degrees in the azimuthal angle [math]\Phi[/math] around the electron beam line produces the 5 T magnetic field.It is important to minimize the amount of material in all of the detectors to minimize hadron and electron absorption and secondary particle production upstream of the time-of-flight scintillators and calorimeters. The detector was divided in the six sectors with each sector independently instrumented to be an effective spectrometer. The Cherenkov detector was designed to maximize the coverage in each of the sectors up to an angle [math]\theta=45[/math] degrees.

As a light collector were used the system of mirrors , the light collecting cones and photomultiplier tubes(PMTs). In the extreme regions of the angular acceptance of the spectrometer the number of detected photoelectrons is too low. To get acceptable efficiency of the detector in these regions were placed photomultiplier tubes.

The charged particle trajectories are in planes of almost constant azimuthal angle, because of the toroidal configuration of the magnetic field. Under this conditions, the light collection can be designed to focus the light in the azimuthal angle direction. However, the polar angle is constant. Each of the six sectors was divided into 18 regions of the polar angle [math]\theta[/math] and each [math]\theta[/math] segment was bisected into two modules by the symmetry plane. This results in a total of 12 identical subsectors around the azimuthal direction for each the polar angle [math]\theta[/math] interval and a total of 216 light collection modules.

The optical elements of each [math]\theta[/math] module were two focusing mirrors one elliptical and the other one hyperbolic, a "Winston" light collection cone and a cylindrical mirror at the base of the cone to improve the focusing. The light was detected by means of 5 in. Phillips XP4500B PMT placed at the base of the Winston cone.

The photomultiplier tubes were surrounded with high permeability magnetic fields,because they were located in the fringe field region of the spectrometer(??????).