CLAS apparatus

Apparatus

Target

The target materials used in the EG1b experiments were [math] NH_3[/math], [math]ND_3[/math], [math]C^{12}[/math], and [math]N^{15}[/math]. The first one was used as a polarized proton target and the second provided 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 a high ratio of free nucleons(3/18).
To prepare the target material, ammonia gas was frozen at 77 K and then crushed into little pieces, about 1-3 mm in diameter. In the case of [math]ND_3[/math] deuterated ammonia was used. The target are kept in solid form during the experiment by liquid helium.

Scintillators

The CEBAF Large Acceptance Spectrometer (CLAS) is equipped with 288 scintillator counters. The purpose of the scintillator is to determine the time of flight for the charged particles and to trigger it in coincidence with another detector system for the particle identification. The time of flight system is built so that time resolution at small polar angles [math]\sigma =120[/math]ps and at angles above 90 degrees [math]\sigma=250[/math]ps. This time resolution helps to discriminate pions and kaons up to 2 GeV/c.

The time of flight system is located between the Cherenkov detectors and electromagnetic calorimeters. The scintillator strips(BC_408) are located perpendicularly to the average particle trajectory with an angular polar coverage of 1.5 degrees. Each sector of The CLAS detector consists of 48 strips with a thickness of 5.08 cm. The length of the scintillators varies from 30 cm to 450 cm and the width is between 15 cm at small polar angles and 22 cm for the large angles.

Each scintillator of the CLAS detector is surrounded with a photomultiplier tube. When particle hits the scintillator strip, part of its energy can excite atoms in the scintillator which in the end produces light(visible). The produced light is transmitted to the photomultiplier tubes by light guides.

For each photomultiplier tube the time and pulse height are measured. This is important to evaluate the time-walk correction and in addition, the measure of the pulse height gives information of the energy released by the crossing particle.

Cherenkov detector

The CLAS Cherenkov detector is a threshold gas counter 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 at angles of 60 degrees in the azimuthal angle [math]\Phi[/math] around the electron beam line produce a 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(??????).
Below is shown the scheme of CLAS detector