The EG1 experiment used the Continuous Electron Beam Accelerator Facility (CEBAF) for a source of polarized electrons, a polarized ammonia target, and the CEBAF Large Acceptance Spectrometer (CLAS).
The EG1 experiment at Jefferson Lab used five different targets during their polarized structure function measurement program. The polarized
The target materials used during the EG1b experiment were frozen ammonia,
The ammonia gas is frozen at 77 K, and then crushed into the spherical beads, about 1 - 3 mm in diameter, in order to provide good and uniform cooling with the liquid helium during the experiment.(?????????more needed)
The 5 T magnetic field parallel to the beam direction is produced by a superconducting pair of Helmholtz coils, which are cooled by the liquid helium reservoir located outside the CLAS detector. The uniformity of the field is varying less than
The Evaporation Refrigerator
The target is cooled to ~ 1 K using the helium, which is supplied from the 1 K
The Target Inserts
The target insert was designed and built by the INFN of Genova. The target cells with a thickness of 0.2 mm were made out of polychlorotrifluoroethylene (PCTFE), it is a hydrogen-free material and would not produce an NMR background, also it is not effected by radiation as much as other materials. Each target cell is 15 mm in diameter and 10 mm in length sealed by a 0.025 mm thick aluminum foil at entrance window and 0.05 mm thick kapton foil at exit window. LHe in the target cells are supplied from the tiny holes which are located at the exit window and the ammonia beads.[ref 4]
The target insert has the four target cells. Two of them were filled with ammonia beads: the top one with and the other one with . was followed by carbon ( ) disk with a thickness of 2.3 mm and the fourth cell was left empty. The carbon and the empty cells were used to measure background.[ref 4]
The Nuclear Magnetic Resonance(NMR) coils were wrapped around the outside surface of the cells in order to reduce the background, maximize the amount of ammonia in the target cells and monitor the polarization of the target. The geometry of the NMR coils were chosen so that they would be maximally sensitive to the target polarization. They were made out of CuNi material with a thickness of 0.15 mm and bent into rectangular loops. The temperature sensors(thermocouples, RuO resistors) were attached to the target insert at different locations to monitor the temperature of the target materials.[ref 4]
A second target insert was designed and built by the Jefferson Lab Polarized Target Group to measure the background using solid
The target insert during the experiment is moved up or down using the stepping motor, which means that the target can be changed automatically.[ref 4]
The Microwave System
The ammonia beads were irradiated by the microwave field to polarize the target material, without reversing the magnetic filed.
Dynamic Nuclear Polarization
The basic idea of Dynamic Nuclear Polarization method is to achieve a high polarization of the nucleons by transfering the high polarization of the electrons to these nucleons. The problem of the Dynamic Nuclear Polarization is the material itself and a paramagnetic dopant, material with unpaired or quasi-free electron. The suitable material is for which the relaxation time for electron spins is short(ms) and for nucleons long(min). Over the past decades target materials have been developed in which the paramagnetic centers have been produced by chemical or radiation method.[ref 5]
Thermal Equilibrium polarization
A polarized target can be imagined as a number of particles with magnetic moment in a high magnetic field and cooled to low temperature. The Zeeman interaction between the atoms with magnetic moment
The thermal equilibrium polarization is described by[ref 5]:
The nucleon polarization in this way is very small because of the small magnetic moment of the proton and deuteron. For example, in a 2.5 T magnetic field at 1 K temperature polarization for proton is 0.25 % and for deuteron 0.05 %. On the other hand, the magnetic moment of the electron is much higher, which gives a high polarization(92 % for the same conditions). The polarization from electrons can be transfered to the nuclear spins using Dynamic Nuclear Polarization(DNP) method.[ref 5]
The Solid-State Effect
In the Solid State Effect of the Dynamic Nuclear Polarization a solid target material with a high concentration of polarizable nucleons is doped with paramagnetic radicals which provide unpaired electron spins[wyaro_1]. As it was said above, the polarization of electron is high, due to its high magnetic moment value. After the electrons are polarized, the dipole-dipole interaction between the electron and nucleon spins provides the transformation of the polarization from electrons to the nucleons by irradiating the system with a frequency close to the frequency of the electron spin resonance.[ref 5]
There is only one parameter that can influence the Dynamic Nuclear Polarization, applied frequency. By changing the frequency one can change the direction of the nucleon polarization , it can be parallel or antiparallel to respect of the magnetic field.
Equal Spin Temperature Theory
The Dynamic Nuclear Polarization process in the target materials like ammonia is described by the Equal Spin Temperature Theory(EST). In this scheme The Dynamic Nuclear Polarization can be obtained in two steps[ref 5]:
- 1)The electron spin-spin energy reservoir is cooled by the applied rf field with the energy
- 2)In the second step which is called Thermal mixing, the electron Zeeman reservoir and the nucleon Zeeman reservoir are thermally mixed, so that they come in thermal equilibrium. For this the forbidden relaxation process which consists of a flip-flop of two electron spins together with a flip of nucleon spin. The energy of the nucleon Zeeman reservoir is changed by
The Nuclear Magnetic Resonance System
During the EG1b experiment the target polarization was monitored by a Nuclear Magnetic Resonance system. The princple of the NMR is to induce and detect nuclear magnetic transitions, where the transition rate is proportional to the population difference between the energy levels, showing the original polarization value.
The CEBAF Large Acceptance spectrometer
The CEBAF Large Acceptance spectrometer is a detector at Thomas Jefferson National Accelerator Facility which covers particle detection almost on 360 degree. The detector is divided into six sectors, each of them including a superconducting coil, which produce a toroidal magnetic field along the beam direction. As a charge particle tracking system was used a set of drift chambers divided by three regions, totally consisting of 18 drift chambers. After the drift chamber system the CLAS detector is equipted with Cherenkov counter for seperating electrons from the pions and with scintillators for determination of the Time of Flight of a charge particle. At the end of the detector the particle identification and timing(resolution) is achieved using electromagnetic calorimeters.
The Torus Magnet
The torus magnet for CLAS detector consists the six superconducting coils located around the beam line in a toroidal geometry, producing the magnetic field in the
The maximum current for the CLAS magnet is 2860 A, in the forward direction the total magnetic field is 2.5 T-m and at a polar scattering angle with 90 degrees 0.6 T. The magnet itself is around 5 m in diameter and 5 m in length. The coils of the magnet are cooled by liquid helium circulating through cooling tubes to a superconducting temperature of 4.5 K. The cooling tubes are placed at the edge of the windings The magnetic field form by the torus magnet bends charged particles toward or away from the beam direction depending of the charge of the particle and the polarization of the torus(inbending and outbending).<ref>Yelena Alexsandrovna Pork, Measurement of The spin Structure Functions of The Proton in The resonance Region.</ref> One can find out the charged particles momentum knowing the trajectory of a particle. In Eg1b experiment, the operated torus values were: 2250, -2250, 1500, -1500.
In order to track the charged particles in the EG1b experiment a drift chamber system is used. A drift chamber is a particle tracking detector that measures the drift time of ionization electrons in a gas to calculate the spacial position of an ionizing particle.
The electric field in a drift chamber is produced by the anode(sense wire) and cathode(field wire) wires. The charge particle traveling trough the drift chamber ionizes the gas, producing the electrons that drift to the anodes. After time(drift time) electrons are collected at the anode(sense wire) generating the pulse at t time. The distance from the traversing particle to the sense wire can be calculated using the drift time and velocity.
The drift chamber system in CLAS detector is divided into three regions, each consisting six separate chambers(six sectors). Region 1 (R1) chambers are placed closest to the target, where the magnetic filed is low. Region 2 (R2) chambers are in a high magnetic field region, they are located between the magnetic coils. Region 3 (R3) chambers are outside the torus coils and they are largest ones. The drift chambers contain three type of wires stretched between the endplates: sense, guard and field. The endplates are attached to the drift chamber so that the angle the form is equal to 60 degrees. Each drift chamber is subdivided into two separate superlayers. Each superlayer with six layers of drift cells, and each drift cell with one sense wire surrounded by six field wires forming a hexagonal shape(). Each superlayer is surrounded by guard wires at a positive potential to stimulate the electric field caused by the drift cells. The sense wire is operated at positive potential and the field wire at negative. In each superlayer the distance between the sense and field wire increases with the radial distance from the target. In R1 the average distance between the sense and field is 0.7 cm, in R2 1.15 cm and in R3 2.0 cm.
The gas used to fill the CLAS drift chamber is a 90 - 10 % mixture of the argon(Ar) and , where Ar has an ionization gain of . Inside the drift chamber the constant pressure is provided by outflowing the gas. The chamber endplates are equipped with the circuit board with a single channel differential pre-amplifier for each sense wire.
The threshold CLAS Cherenkov detector is used to distinguish electrons from pions. The mixture gas used to fill the Cherenkov counter is perfluorobutane
The six superconducting coils placed at angles of 60 degrees in the azimuthal angle
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
The optical elements of each
The calibration of the Cherenkov detector is in terms of the collected number of photoelectrons.
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
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 scintillator 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 on the energy released by the crossing particle.
The CLAS detectur contains 8 modules of electromagnetic calorimeter. A calorimeter is a device that measures the total energy deposited by a crossing particle. They are useful in detecting neutral particles and distinguishing between electrons and hadrons due to their different mechanism of depositing energy. The CLAS calorimeter has three main functions:
1) detection of electrons at energies above 0.5 GeV;
2) detection of photons with energies higher than 0.2 GeV;
3) detection of neutrons, with discrimination between photon and neutrons using time-of-flight technique.
6 calorimeter modules of the CLAS detector are placed in each sector in the forward region (polar angle of 10-45 degrees, forward angle calorimeter), while the other two modules are located at large angles in sectors 1 and 2(50-70 degrees, large angle calorimeter). The forward calorimeter has a lead/scintillator thickness ratio 0.2, with 40 cm of scintillators and 8 cm of lead per module. The lead-scintillator sandwich is shaped as a equlaterial triangle in order to match the hexagonal geometry of the CLAS detector. It is made of 39 layers of a 10 mm BC_412 scintillator and lead sheet of thickness of 2.2 mm. Each scintillator layer contains 36 strips parallel to one side of the triangle, with this configuration each orientation is rotated by 120 degrees from another one. This gives three views each containing 13 layers providing stereo information of the location of the energy deposition. To improve hadron identification, there was provided longitudinal sampling of the shower. Each set of 13 layers were subdivided into an inner 5 layers and outer 8 layers stack.
5. D. G. Crabb and W. Meyer, Annu. Rev. Nucl. Part. Sci. 47, 67-109 (1997).