2. Detectors (CLAS) Yarulin Rafael Department of Physics, Graduate School September 2003 Kyungpook National University

3. Detector Any device used to sense the passage of a particle; also a collection of such devices designed so that each serves a particular purpose in allowing physicists to reconstruct particle events. Currently in HEP are used a multi-layer detectors to identify particles. Each layer gives different information about the events. One important function of the detector is to measure a particle's charge and momentum. For this reason, the inner parts of the detector, especially the tracking device, are in a strong magnetic field. The signs of the charged particles can easily be read from their paths, since positive and negative particles curve in opposite directions in the same magnetic field.

4. What must a detector be capable of doing? 1) Measure the directions, momentum, and signs of charged particles. The momentum of particles can be calculated since the paths of particles with greater momentum bend less than those of lesser momentum. This is because a particle with greater momentum will spend less time in the magnetic field or have greater inertia than the particle with lesser momentum, and thus bends less in a magnetic field. 2) Measure the energy carried by electrons and photons in each direction from the collision. 3) Measure the energy carried by hadrons (protons, pions, neutrons, etc.) in each direction. 4) Identify which charged particles are generated from the collision, if any, are electrons. 5 Identify which charged particles from the collision, if any, are muons.

5. 6) Identify whether some of the charged particles originate at points a few millimeters from the collision point rather than at the collision point itself (signaling a particle's decay a few millimeters from the collision point). 7) Infer (through momentum conservation) the presence of undetectable neutral particles such as neutrinos. 8) Have the capability of processing the above information fast enough to permit flagging about 10-100 potentially interesting events per second out of the billion collisions per second that occur, and recording the measured information. 9) The detector must also be capable of long and reliable operation in a very hostile radiation environment.

6. The Many Layers of the Detector Surround the Collision Point  The innermost layer, the vertex detector gives most accurate location of any outgoing charged particles as they pass through it.  The next layer, the drift chambers detects the positions of charged particles at several points along the track. The curvature of the track in the magnetic field reveals the particle's momentum.  The middle layer, the Cherenkov detector measures particle velocity. It occurs when a particle travels through a medium (here, Freon gas) at a speed that is faster than the speed of light in the medium (but slower than the speed of light in a vacuum, of course), just as a sonic boom occurs when an object travels in a medium (air) faster than the speed of sound in the medium.  The next layer, calorimeter stops most of the particles and measures their energy. this is the first layer that records neutral particles.  The Electromagnetic Calorimeter absorbs the energies of all electrons and photons traversing it (this constitutes the "electromagnetic energy"), and produces signals proportional to those energies. It is finely subdivided so that it can measure the directional dependence of the electromagnetic energy.

7. 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. The Laboratory hosts CEBAF (Continuous Electron Beams Accelerator Facility) a 4 GeV electron beam accelerator, with a maximum current of 200 A. This accelerator delivers it's beam into 3 experimental Halls, as shown in Figures 1.All these different detectors combined allow the tracking of particles generated by the collision. (Schematics 3D view of CEBAF) Figure 1.

8. In this figure one can see the main layers of detectors inside CLAS. Figure 2.

9. The CLAS detector 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, there is 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 super layer with up to 1200 sense wires in six layers (4 layers in the case of Region 1) and one stereo super layer 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.

10. The CLAS detector The next detectors are Cerenkov counters, 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. It covers the region of polar angles in the forward direction. Each of the 6 sectors of CLAS consists of 18 segments. 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) than electrons (0.511 MeV). This detector is very useful to make the pion/electron discrimination. The another detector is the Time of Flight Detector 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 solid angle. One of the purposes of this detector is to dicriminate kaons (494 MeV) 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.

11. The CLAS detector 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 information provided by these particular detectors, it is possible to reconstruct the complete event that occurred at the target.

12. Drift chambers Drift chambers are used to measure where a charged particle has crossed a virtual plane. For this purpose thin wires are fixed in a volume filled with a special gas (Argon/Ethan) in a way, that the wires form cells: Inside these cells a traversing charged particle ionizes the gas. Due to the electrical potentials applied to the wires the electrons drift to the sense wire, the connected electronics measures the charge of the signal and when it appears. The difference between this time and the time when the particle traversed the cell (measured by other detectors) is used to reconstruct the impact point of the particle in the chambers midplane. In order to reconstruct the particles track several chamber planes are needed: Figure 3 Figure 4

13. Cerenkov Counters The Cerenkov counters also differ in size and form. These attributes depend on the practical necessities. Invariably all of them use the Cerenkov effect to detect and to identify particles. They work when the particles have velocities close to the velocity of light in the vacuum. The Cerenkov effect is the following: When particles move in a material medium with a velocity greater than the velocity of the light in the medium, part of the emitted light by the excited atoms appears in form of coherent wave front at a fixed angle with respect to the particle trajectory. The angle at which the coherent front of waves appears, is directly related with the velocity of the particle. This detector gives us some information, but not enough: a Cherenkov detector only detects charged particles moving faster than a critical speed. Thus very little information is gained here. The non-charged particles are impossible to see with this detector. Cherenkov counters for electron identification.

14. Cherenkov Counter The detector works by detecting the Cerenkov radiation emitted by particles when they move through a medium at velocities greater than c/n, where c is the speed of light in vacuum, and n is the index of refraction of the material. Charged particles moving above the speed of light in the medium will emit light in a forward pointing cone with an opening angle, defined by: where is the velocity of the particle relative to the speed of light ( ). By choosing the index of refraction of the material properly, the threshold velocity (=c/n) can be made such that electrons at the spectrometer momentum will emit Cerenkov radiation, and pions will not. Mirrors are used to focus the light onto photomultiplier tubes, which measure the Cerenkov light. The medium must be a material that will allow the Cerenkov light to propagate without significant loss, and which does not generate significant light from scintillation.

15. Time of Flight Detector The requirements for the TOF system include excellent resolution for particle identification, and good segmentation for flexible triggering and prescaling. The design parameters are chosen to allow separation of pions and kaons up to 2GeV/c.The most energetic particles are produced at small angles. The system specifications called for a time resolution approx.120ps at angles above 90 degree. Particle identification is achieved by off-line analysis that combines leading- edge time measurements with pulse-height information for time-walk corrections. The TOF system is also used for energy-loss measurements and velocity determination in specific instances. Pulse-height information, being proportional to the energy loss in the counter, provides an independent means for the identification for slow particles. Also, the flight time can provide a more accurate measurement of particle energy then magnetic analyses for slow particles when the tracking resolution is dominated by multiple scattering.

16. Time-Of-Flight System The time of flight (TOF) system measures the flight time of particles with high resolution. Combined with the momentum of the particle (derived from the curvature of its tracked in the magnetic field, measured with (central detector tracking chamber), this gives a constraint on the particle mass that aids in identifying the particle type. Usually the detector are 5 or 10 cm thick bars of scintillator--a material that emits a flash of light when a charged particle passes through it--that run the length of the detector barrel. At both ends of each scintillator is a lucite light pipe that channels light to a phototube light detector that sits outside the magnetic field of the superconducting coil. This system allows measuring the flight time of particles produced at the interaction point to 150 picoseconds. Figure 5

17. Calorimeters  A calorimeter measures the energy lost by a particle that goes through it. It is usually designed to fully stop or "absorb" most of the particles coming from a collision event, forcing them to deposit within the detector all of their energy.  Calorimeters typically consist of layers of "passive" or "absorbing" high density material (lead for instance) interleaved with layers of "active" medium such as solid lead-glass or liquid argon.  Interactions between the particles and the high-density material of the calorimeter induce the production of "showers" of low energy particles.  The active medium samples this lost energy, from which physicists can determine the total energy of the incoming particle.