Tamar Thesis EperimentalSetupChapt
Polarized targets and polarized beams are recently acquired research tools being used to investigate the spin structure of the nucleon. Inclusive scattering experiments using polarized targets and beams facilitate measurements of observables exhibiting spin degrees of freedom, like the spin structure of the nucleon, the electromagnetic structure of the nucleon in its ground state, etc. <ref name="Averett1999"> T.D. Averett et al. (1999). "A solid polarized target for high-luminosity experiments". Nucl. Instr. Meth A 427/3, 440-454</ref> The technology producing targets containing polarized nucleons have been developed over the past 50 years. For the experiments using electrons as probes, due to the small cross section of the electromagnetic interactions, one of the requirements for polarized targets are a large thickness and resistance to the electron beam intensity without significant radiation damage. The solid targets for the EG1B experiment were polarized via the Dynamic Nuclear Polarization (DNP) method<ref name="CrabbMeyer1997"> Crabb, D.G., Meyer, W.(1997). "Solid Polarized Targets for Nuclear and Particle Physics Experiments". Annu. Rev. Nucl. Part. Sci 47, 67-109</ref>.
EG1b polarized target system consists the following main components: the superconducting Helmholtz coils to reach 5 T magnetic field, the evaporation refrigerator for target cooling, the microwaves to induce spin flip in the target, NMR system measuring the target polarization and the housing for the solid target <ref name="Averett1999"> T.D. Averett et al. (1999). "A solid polarized target for high-luminosity experiments". Nucl. Instr. Meth A 427/3, 440-454</ref>.
A 5 T magnetic field is established using a pair of superconducting Helmholtz coils made out of Niobium-Titanium mixture with a critical temperature of 9 K. The magnet becomes superconducting below its critical temperature. In order to establish a current in the coils, a section of the magnetic is warmed above its critical temperature. This process is called "magnet energization". After the coils have been energized, the "magnet switch" section of the coils is cooled down and the current in the leads is ramped down. The full current of the magnet is carried by the superconducting Helmholtz coils.
The superconducting coils are oriented such that the magnetic field is parallel to the incident beam direction. The field induces the hyperfine splittings needed to polarize the target material using 140 Ghz RF wave. The uniformity of the field varies less thanover a cylindrical volume of 20 mm in diameter and length. This configuration is necessary for DNP. The particles with scattering angles between 0-50 degrees are detected in the CLAS as well as 75-105 degrees. The Helmholtz coils block particles scattering between 50 and 75 degrees. The target magnetic field does not interact with the electron beam, however, it is effective in shielding the drift chambers from low energy Moller electrons. The target field bends scattered particles in the azimuthal direction and falls rapidly with distance as ( ). The effect of the magnetic field on the drift chambers is negligible <ref name="Chen2006">Chen, S. (2006). First Measurement of Deeply Virtual Compton Scattering with a Polarized Proton Target. Doctoral dissertation. Florida State University, Tallahasee, FL. </ref> .
The Evaporation Refrigerator
The target material is located at the center of the magnet in a separate chamber, called the banjo. Cooling the target to approximately 1 K was achieved using aevaporation refrigerator inserted through a 20 cm diameter pumping tube. The pumping tube connects the banjo and the pumping system. The components of the refrigerator are the sintered copper separator pot, two series of heat exchangers, and two lines that supply helium to the banjo. The phase separator allows the liquid to pass, but blocks the helium vapor. The helium vapor is pumped away to cool the radiation baffles. The liquid in the phase separator is directed to the banjo one of the two lines. The heat exchangers remotely controlled by needle valves. The presence of the liquid helium in the target chamber cools target material effectively <ref name="Averett1999"> T.D. Averett et al. (1999). "A solid polarized target for high-luminosity experiments". Nucl. Instr. Meth A 427/3, 440-454</ref> .
The Microwave System
The 140 GHz microwave used for DNP are supplied by the Extended Interaction Oscillator (EIO) tube providing
Microwaves are delivered to the target material using a rectangular gold-plated wave guide located outside the cryostat and a tube made out of inside the wave guide system. The tube and the wave guide are connected by a rectangular-circular adapter. This allows microwaves to travel through the tube and into the rectangular segment of the wave guides suppling microwaves to the desired target cell during the experiment <ref name="Prok2004"> Prok, Y. A. (2004). Measurement of The Spin Structure Function of The Proton in The Resonance Region. Doctoral dissertation. University of Virginia, Richmond, VA.</ref>.
The Nuclear Magnetic Resonance System
The polarization of the target material during the experiment was monitored using Nuclear Magnetic Resonance (NMR) techniques. The technique relies on detecting nuclear magnetic spin transitions. The rate of transitions is related to the population difference of the energy levels giving information on the original target polarization. The magnetization of the sample, of volume
When the external magnetic field is placed perpendicularly under a rotating resonance frequency field with amplitude and frequency of
"A measurement of the ratio of the strengths of the NMR signal with and without RF applied gives the polarization of the target relative to the calculable thermal equilibrium (TE) polarization."<ref name="Averett1999"> T.D. Averett et al. (1999). "A solid polarized target for high-luminosity experiments". Nucl. Instr. Meth A 427/3, 440-454</ref> The polarization of protons (the spin ) and deuterons at thermal equilibrium are given as
The Target Chamber
The target chamber is placed at the top of the cryostat. The chamber is filled with
During the experiment, two of the target cells were filled withand , a third cell with a thick graphite disk, and the last cell was left empty. The and targets were used for physics measurements, while the carbon and the empty cells for background measurements. The desired target cell can be placed along the beam axis using a stepping motor. The NMR coils were wrapped around the outside surface of the cells. The coils, a in diameter tubing, were shaped rectangularly. Only one loop of NMR coils was used for the target, while the target required four loops to measure the polarization. Temperature sensors were located at several places of the target chamber and heater coils were attached below each target cell for annealing.
Polarized Target Materials
A polarized solid target's limited resistance to radiation damage is one of the remaining challenges for using polarized targets in scattering experiments. At present, solid ammonia and lithium deuteride are the target materials with the highest resistance to radiation damage<ref name="Baum1996"> Baum G, et al. A proposal for a Common Muon and Proton Apparatus for Structure and spectroscopy, CERN/SPSLC/96-14(1996).</ref>. . For EG1b experiment, ammonia targets were selected because of their ability to produce high polarization and be resistant to high radiation dose caused by the incident electron beam. Another advantage of ammonia target is a high ratio of free nucleons (~3/18), approximately 16.7 % for
The target materials for the EG1b experiment were prepared by slowly freezing ammonia gas at
Dynamic Nuclear Polarization (DNP) is a process in which the polarization of free electrons is transferred to a nucleus<ref name="Chen2006">Chen, S. (2006). First Measurement of Deeply Virtual Compton Scattering with a Polarized Proton Target. Doctoral dissertation. Florida State University, Tallahasee, FL. </ref> . In DNP the target is doped with paramagnetic impurities by chemical doping or by irradiating the target in an electron beam. For low temperatures, on the order of
The CEBAF Large Acceptance spectrometer
The CEBAF Large Acceptance Spectrometer (CLAS), located in Jefferson Lab's Hall B, was used to measure the final state particles resulting from the scattering of a polarized electron by a polarized nucleon. The CLAS uses six superconducting coils to establish a toroidal magnetic field encircling the incident electron's momentum direction. A set of three drift chambers are positioned to determine the trajectories of particles which pass through the 6 gaps between the magnet coils. The first drift chamber, Region 1 (R1), is placed at the entrance to the magnetic coil. A second chamber, Region 2 (R2), is placed in the center of the coils. The final chamber, Region 3 (R3), measures charged particles leaving the toroidal field. A total of 18 drift chambers are available to reconstruct the trajectory of charged particles passing through the magnetic field. After the drift chamber system, the CLAS detector is equipped with a Cherenkov counter for separating electrons from pions and with scintillators, to determine the time of flight of a charged particle. An electromagnetic calorimeter is placed at the exit of the detector to detect neutral particles and improve the detector's ability to distinguish between electrons and hadrons.
The Torus Magnet
The CLAS's torus magnet consists of six superconducting coils located around the beam line in a toroidal geometry, producing a magnetic field in the
The drift chamber system in the CLAS detector is divided into three regions, each consisting of six separate chambers(sectors). The drift chambers contain three types of wires stretched between the endplates: sense, guard and field. The end plates are attached to the drift chamber so that the angle they form is equal to 60 degrees. Each drift chamber is subdivided into two separate superlayers. Each superlayer has six layers of drift cells. Each drift cell has one sense wire and is surrounded by six field wires forming a hexagonal shape. Each superlayer is surrounded by guard wires at a positive potential to complete the cell symmetry establishing a radial electric field within 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 <ref name="Metayer2000"> Mestayer, M. D., et. al. (2000). The CLAS drift chamber system. NIM, A(449), 81-111.</ref> . The CLAS drift chamber gas is a 90 - 10% mixture of the argon(Ar) and
The drift chamber system is used to track charged particles. A drift chamber is a particle tracking detector that measures the drift time of liberated electrons in a gas to calculate the spacial position of the ionizing particle.
An electric field in a drift chamber is produced by the anode(sense) and cathode(field) wires. A charged particle traveling through the drift chamber ionizes the gas, freeing electrons that drift to the anodes. After a drift time $(\delta t)$, electrons are collected at the anode(sense wire) generating a pulse for the time measurement. The distance of the traversing particle from the known position of the sense wire can be calculated using the drift time and drift velocity.
The threshold CLAS Cherenkov detector is used to distinguish electrons from pions. The gas mixture used to fill the Cherenkov counter is perfluorobutane
Light is collected using a system of mirrors to focus the light onto cones, which are connected to the Phillips XP4500B type photomultiplier tubes(PMTs). In the extreme regions of the spectrometer's angular acceptance, the number of detected photoelectrons is too low. Additional photomultiplier tubes were placed in these regions to compensate for the low photoelectron detection efficiency.
The CLAS is equipped with 288 scintillator counters. The scintillators are used to determine the time of flight for a charged particle and to determine coincidences between particles. The time of flight system has a time resolution at small polar angles of
The CLAS detector contains 8 electromagnetic calorimeter modules. A calorimeter measures the total energy deposited by a particle that enters the detectors acceptance. Calorimeters are sensitive neutral particles with sufficient energy to generate showers and can be used to distinguish between the energy deposited by electrons and hadrons. The CLAS calorimeter has three main functions:
- Detection of electrons at energies above 0.5 GeV;
- Detection of photons with energies higher than 0.2 GeV;
- Detection of neutrons, with discrimination between photon and neutrons using time-of-flight techniques.
Calorimeter detectors are placed in the forward region of each sector that span polar angles of 10-45 degrees. Two additional calorimeters are located in sectors 1 and 2 that span polar angles of 50 to 70 degrees. The forward calorimeter has a lead/scintillator thickness ratio of 0.2, with 40 cm of scintillators and 8 cm of lead per module. The lead-scintillator sandwich is shaped to form an equilateral triangle in order to match the hexagonal geometry of the CLAS detector. Each scintillator layer contains 36 paddles parallel to one side of the triangle, with this configuration each orientation is rotated by 120 degrees from each other. This gives three views, each containing 13 layers providing stereo information locating the energy deposition. There is a longitudinal sampling of the shower to improve hadron identification. Each set of 13 layers are subdivided into 5 inner layers and 8 outer layers.