CLAS apparatus - Revision history

Oborn: /* Target Material */

2009-08-10T19:00:51Z

Target Material

Oborn: /* Target */

2009-08-10T18:38:11Z

Target

Oborn: /* Notes */

2008-07-29T20:54:51Z

Notes

Oborn: /* Dynamic Nuclear Polarization */

2008-07-29T20:39:32Z

Dynamic Nuclear Polarization

Oborn: /* The Target Inserts */

2008-07-29T20:37:42Z

The Target Inserts

Oborn: /* The Evaporation Refrigerator */

2008-07-29T20:37:06Z

The Evaporation Refrigerator

Oborn: /* The Magnet */

2008-07-29T20:36:45Z

The Magnet

Oborn: /* Target Material */

2008-07-29T20:36:23Z

Target Material

Oborn: /* Target Material */

2008-07-29T20:36:08Z

Target Material

Oborn: /* Dynamic Nuclear Polarization */

2008-07-29T16:45:53Z

Dynamic Nuclear Polarization

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 ===Target Material===
-The target materials used during the EG1b experiment were frozen ammonia, <math>15NH_3</math> for the polarized protons and <math>15ND_3</math> for the polarized deuterons. Ammonia targets were selected because of their ability to produce high polarization and they are less effected by beam radiation. On the other hand, the damage caused by this radiation can be repair by annealing(heating process). In addition, ammonia target has a high ratio of free nucleons  (~3/18) approximately 16.5 % for <math>15NH_3</math> and 26.6 % for <math>15ND_3</math>. One disadvantage of choosing ammonia is the polarization background caused by 15N(spin - 1/2), or 14N(spin - 1). However, for 15N the polarization can be measured easily then for 14N. Also, it does not have any free nucleons, while 14N has one free neutron. That concludes, in case of using 15N ammonia rather than 14N the spin asymmetry measurement should have a smaller correction and error.['''reference 1'''] <br>
+The target materials used during the EG1b experiment were frozen ammonia, <math>15NH_3</math> for the polarized protons and <math>15ND_3</math> for the polarized deuterons. Ammonia targets were selected because of their ability to produce high polarization and they are less effected by beam radiation. On the other hand, the damage caused by this radiation can be repair by annealing(heating process). In addition, ammonia target has a high ratio of free nucleons  (~3/18) approximately 16.5 % for <math>15NH_3</math> and 26.6 % for <math>15ND_3</math>. One disadvantage of choosing ammonia is the polarization background caused by 15N(spin - 1/2), or 14N(spin - 1). However, for 15N the polarization can be measured easily then for 14N. Also, it does not have any free nucleons, while 14N has one free neutron. That concludes, in case of using 15N ammonia rather than 14N the spin asymmetry measurement should have a smaller correction and error.['''SChen_FSU_thesis'''] <br>
 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)<br>

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 =Target=
-The EG1 experiment at Jefferson Lab used five different targets during their polarized structure function measurement program.  The polarized <math> NH_3</math> and <math>ND_3</math> targets were designed and built by collaboration of the Italian Istituto di Fisica Nucleare, TJNAF, Oxford instruments and the University of Virginia. The solid targets(<math> NH_3</math> and <math>ND_3</math>) were polarized using the method of Dynamic Nuclear Polarization[ insert reference].  A 5 Tesla magnet establish the yperfine splittings needed to polarize the target material using X Ghz RF waves.  The polarization was monitored by pickup coils around the target using standard NMR techniques.  Three other targets, <math>C^{12}</math>, liquid <math>He^{4}</math> and frozen <math>N^{15}</math> were used to employ several methods for removing the Nitrogen contributions to the measured structure functions made using <math> NH_3</math> and <math>ND_3</math>.  <br>
+The EG1 experiment at Jefferson Lab used five different targets during their polarized structure function measurement program.  The polarized <math> NH_3</math> and <math>ND_3</math> targets were designed and built by collaboration of the Italian Istituto di Fisica Nucleare, TJNAF, Oxford instruments and the University of Virginia. The solid targets(<math> NH_3</math> and <math>ND_3</math>) were polarized using the method of Dynamic Nuclear Polarization[ insert reference].  A 5 Tesla magnet establish the hyperfine splittings needed to polarize the target material using X Ghz RF waves.  The polarization was monitored by pickup coils around the target using standard NMR techniques.  Three other targets, <math>C^{12}</math>, liquid <math>He^{4}</math> and frozen <math>N^{15}</math> were used to employ several methods for removing the Nitrogen contributions to the measured structure functions made using <math> NH_3</math> and <math>ND_3</math>.  <br>
 ===Target Material===

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 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.
-==Notes==
+==References==
-<references/>
+.<br>

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 ===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. <br>
+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'''] <br>
 ====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 <math>\mu</math> and the external magnetic field B gives a set of 2I+1 sublevels, where the spin I=1/2 for proton and I=1 for deuteron. At thermal equilibrium the population of two magnetic sublevels at the T temperature is given by the Boltzmann Law['''wyaro''']: <br>
+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 <math>\mu</math> and the external magnetic field B gives a set of 2I+1 sublevels, where the spin I=1/2 for proton and I=1 for deuteron. At thermal equilibrium the population of two magnetic sublevels at the T temperature is given by the Boltzmann Law['''ref 5''']: <br>
 ;(1) <math>N_1 = N_2 \times exp (\frac{-\Delta E}{k_B T})</math> <br>
-where <math>k_B</math> is the Boltzmann constant and <math>N_{1,2}</math> are the corresponding population numbers of the magnetic sublevels. The definition of the polarization for a system with spin-1/2 and spin-1 is following:['''wyaro''']:<br>
+where <math>k_B</math> is the Boltzmann constant and <math>N_{1,2}</math> are the corresponding population numbers of the magnetic sublevels. The definition of the polarization for a system with spin-1/2 and spin-1 is following:['''ref 5''']:<br>
 ;(2) <math>P(1/2) = \frac{(N_{1/2} - N_{-1/2})}{(N_{1/2} + N_{-1/2})}</math><br>
 ;(2) <math>P(1) = \frac{(N_1 - N_{-1})}{(N_1 + N_0 + N_{-1})}</math><br>
-The thermal equilibrium polarization is described by['''wyaro''']:<br>
+The thermal equilibrium polarization is described by['''ref 5''']:<br>
 ;(3) <math>P(1/2) = tanh(\frac{g_i \mu_i B}{2 k_B T})</math><br>
 ;(3) <math>P(1) = \frac{4 tanh(\frac{g_i \mu_i B}{2 k_B T})}{3 + tanh^2 (\frac{g_i \mu_i B}{2 k_B T})}</math><br>
-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.<br>
+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''']<br>
 ====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.<br>
+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''']<br>
 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.<br>
 ====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:<br>
+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''']:<br>
 :1)The electron spin-spin energy reservoir is cooled by the applied rf field with the energy <math>h (\nu_e - \Delta)</math>. The electron Zeeman energy is changed by <math>h \nu_e</math>   and the rest of it is absorbed by the electron spin-spin system. The electron spin-spin system(reservoir) is heated, if <math>\Delta>0</math> and if <math>\Delta<0</math> than the system emits this energy and it cools down.<br>
 :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 <math>h \nu_e</math>, while the electron Zeeman reservoir is unchanged. Responsible for the relaxation mechanism is the spin-spin interaction reservoir, without participation of the lattice. The energy <math>h \nu_e</math> is exchanged between the electron spin-spin and nuclear Zeeman reservoirs. Because of this energy exchange the two reservoirs are equalized thermally.<br>

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 ===The Target Inserts===
 [[Image:The_Target_Insert.jpg|300px|thumb|The Target Insert]]<br>
-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.<br>
+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''']<br>
-The target insert has the four target cells. Two of them were filled with ammonia beads: the top one with <math>ND_3</math> and the other one with <math>NH_3</math>. <math>ND_3</math> was followed by carbon (<math>C^{12}</math>) 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.<br>
+The target insert has the four target cells. Two of them were filled with ammonia beads: the top one with <math>ND_3</math> and the other one with <math>NH_3</math>. <math>ND_3</math> was followed by carbon (<math>C^{12}</math>) 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''']<br>
-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.<br>
+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''']<br>
 [[Image:The_N_15_Insert_Stick.jpg|300px|thumb|The N 15 Insert Stick]]<br>
 A second target insert was designed and built by the Jefferson Lab Polarized Target Group to measure the background using solid <math>N^{15}</math>. The first measurements of electron scattering from <math>N^{15}</math> target provided the data which was subtracted from the <math>NH_3</math> scattering data in order to eliminate the contribution of the <math>N^{15}</math>. The insert cell is made out of a Torlon of 15.7 mm in diameter and 12.7 mm  in length. Both sides of the cell are covered by kapton windows. The target cell was filled with isotopically enriched <math>N^{15}</math> gas at room temperature and the insert was located into the target chamber in order to cool it by LHe, causing the gas to condense. To avert the target from freezing a heater wire was wrapped around the filling tubes and plugging the tubes before the cell was filled with the gas.<br>
-The target insert during the experiment is moved up or down using the stepping motor, which means that the target can be changed automatically.<br>
+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''']<br>
 ===The Microwave System===

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 ===The Evaporation Refrigerator===
-The target is cooled to ~ 1 K using the helium, which is supplied from the 1 K <math>He^4</math> evaporation refrigerator. Approximately 0.8 W cooling power is provided by a system of Roots and rotary-vane vacuum pumps that reach a pumping speed for helium gas of 3300 <math>m^3/h</math>[reference !!!!!!!!].<br>
+The target is cooled to ~ 1 K using the helium, which is supplied from the 1 K <math>He^4</math> evaporation refrigerator. Approximately 0.8 W cooling power is provided by a system of Roots and rotary-vane vacuum pumps that reach a pumping speed for helium gas of 3300 <math>m^3/h</math>['''reference 3'''].<br>
 ===The Target Inserts===

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 ===The Magnet===
-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 <math>10^{-4}</math> over a cylindrical volume of 20 mm in diameter and length. This configuration is necessary for DNP. The particles with scattering angles between 0-50 are detected in the CLAS as well as 75-105 due to the 8 cm gap between the Helmholtz coils. The target magnetic field does not interact with the electron beam, however it is effective in shielding the drift chambers from the low energy Moller electrons.The target field bents the scattered particles in the azimuthal direction and it falls rapidly with distance as (<math>~ 1/r^3</math>). The effect of magnetic field at the drift chambers is negligible.<br>
+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 <math>10^{-4}</math> over a cylindrical volume of 20 mm in diameter and length. This configuration is necessary for DNP. The particles with scattering angles between 0-50 are detected in the CLAS as well as 75-105 due to the 8 cm gap between the Helmholtz coils. The target magnetic field does not interact with the electron beam, however it is effective in shielding the drift chambers from the low energy Moller electrons.The target field bents the scattered particles in the azimuthal direction and it falls rapidly with distance as (<math>~ 1/r^3</math>). The effect of magnetic field at the drift chambers is negligible.['''ref 2''']<br>
 ===The Evaporation Refrigerator===

@@ Line 33: / Line 33: @@
 ===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.<br>
+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. <br>
 '''should read'''(ar dagaviwydes wyaros mititeba tamuna !)<br>