CLAS, the Hall B detector

  CLAS is an acronym for CEBAF Large Acceptance Spectrometer, one of the major detectors installed at Jefferson Lab, it is housed in the Hall B end-station. (Figure 2.5).

Compared to the two other Halls, CLAS has two main distinctive features. First of all it is has a great solid angle coverage, the actual value in $\theta$ is $8<\theta<145$ and the toroidal magnet create 20% of dead zone in $\phi$. So the total coverage is about 3$\pi$. The other Halls must restrict their measurements to a narrow angle at a time (like the mobile arm of the Hall A spectrometer). The second and very interesting feature of CLAS is the ability to run using a secondary tagged photon beam created by Bremsstrahlung effect of the primary electron beam. The tagged photon flux is 10⁸/s with an energy resolution of 10^-3.

Because of it's geometry that provides a very large acceptance, CLAS is well adapted for experiments that require the detection of a large number of particles in the final state (like (e,e'Np$\pi$or (e,e'NN). This large acceptance also allows several experiment to run at the same time, with the help of a configurable trigger system.

In Figures 2.6 and 2.7, we can see the main layers of detectors inside CLAS. All these different detectors combined allow the tracking of particles generated by the collision.

  

Figure 2.6: Layers of detector in CLAS (3D view)

  

Figure 2.7: Layers of detector in CLAS (2D view)

The CLAS detector looks globally like a big orange of about 10 m in diameter divided in 6 sectors. So each layer of detectors will be split accordingly to this pattern.

If we go from the inside to the outside of the detector, we first see in Figure 2.7, that the first device is the toroidal shield (minitorus). It's not a detector, but a powerful magnet that prevents the great amount of low energy electrons created by electro-magnetic showers in the target (when in electron run), from reaching the drift chambers and saturating them.

The first detector layer is the drift chambers. They are arranged in three regions: Region 1 is located closest to the target, within the almost field free region inside the Torus bore, and is used to determine the initial direction of charged particle tracks. Region 2 is located between the Torus coils, in the region of strong toroidal magnetic field, and is used to obtain a second measurement of the particle track at a point where the curvature is maximal, to achieve good energy resolution. Region 3 is located outside the coils, again in a region with low magnetic field, and measures the final direction of charged particles headed towards the outer Time-of-Flight counters, Cerenkov counters and the Electro-magnetic Calorimeters.

All three regions consist of six separate sectors, one for each of the six sectors of the CLAS. Each region within a given sector contains one axial superlayer with up to 1200 sense wires in six layers (4 layers in the case of Region 1) and one stereo superlayer with sense wires in six layers at an angle of 6 degrees with respect to the axial wires. The wires are arranged into a hexagonal pattern, with up to 192 sense wire in each layer.

The next detectors are Cerenkov counters[6], that use the Cerenkov effect to discriminate between light charged particles. The Cerenkov detector is positioned between the Drift Chamber and the Time Of Flight Scintillator system (see Figure 2.7). It covers the region of polar angles in the forward direction. Each of the 6 sectors of CLAS consists of 18 segments (Figure 2.8 shows the working principle of one cell). The optical refraction index of the gas has been chosen so that most electrons will generate Cerenkov light in the detector, but most pions won't. This is due to the fact that charged pions are far heavier (140 MeV [9]) than electrons (0.511 MeV). This detector is very useful to make the pion/electron discrimination.

  

Figure 2.8: A Cerenkov cell

The third detector is the Time of Flight Detector[13] consisting of 342 fast scintillators counters. The scintillators are located at approximately 5 m from the target and cover a large portion of the 4$\pi$ solid angle. One of the purposes of this detector is to dicriminate kaons (494 MeV [9]) from pions (144 MeV) by using the speed/momentum ratio. This goal requires a time resolution better than 180 ps. We have here to mention the Start Counter, a scintillator counter placed just around the target, that gives the 'zero' for the time of flight measurement.

The latest layer is the Electro-Magnetic Calorimeter. Divided in two main parts (forward angles and large angles), it gets a direct measurement of the position, the timing and total energy of 1.0 to 6.0 GeV electrons and pions. In addition it can also detect neutral particles such as neutron and photon. The calorimeter is made of 39 sandwiches of 10 mm scintillators followed by 2 mm lead sheets, and divided into the usual 6 regions.

Using all the informations provided by these particular detectors, it is possible to reconstruct the complete event that occurred at the target.