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Laser Driven Polarized H/D Sources and Targets

Laser Driven Polarized H/D Sources and Targets. PST 2003 Novosibirsk, Russia. Ben Clasie Laboratory for Nuclear Science Massachusetts Institute of Technology. Introduction Optical pumping Spin-temperature equilibrium Sources and targets Results from sources and targets

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Laser Driven Polarized H/D Sources and Targets

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  1. Laser Driven Polarized H/D Sources and Targets PST 2003 Novosibirsk, Russia Ben Clasie Laboratory for Nuclear Science Massachusetts Institute of Technology • Introduction • Optical pumping • Spin-temperature equilibrium • Sources and targets • Results from sources and targets • Comparison of ABS and LDS • The future of the MIT laser driven source • Summary C. Crawford, D. Dutta, H. Gao J. Seely, W. Xu

  2. Introduction: Laser Driven Polarized H/D Sources and Targets • A circularly polarized laser is absorbed by alkali vapor, which polarizes the vapor (optical pumping) • The vapor is mixed with H/D and spin is transferred to the H/D electrons through spin-exchange collisions • The H/D nuclei are polarized through the hyperfine interaction during frequent H-H or D-D collisions

  3. LDS history A. Kastler (1950) first proposed using light to produce atoms with nuclear polarization. A. Kastler, J. Phys Radium11, 225 (1950) After the development of lasers with high power and narrow linewidths, a LDS was developed at Argonne (1988). This early type of source operated at a low magnetic field of 10G and operated at low H/D flow rates. R. J. Holt et al., AIP Conf. No. 187, 499 (1989) T. Walker and L. W. Anderson (1993) used rate equations to show that a high magnetic field in the kG range will be suitable in a LDS. Much higher alkali densities could be used without the limiting effects of radiation trapping, and the H/D flow rate could be increased by an order of magnitude. T. Walker and L. W. Anderson, Nucl. Instr. And Meth. A334, 313 (1993)

  4. Optical pumping Optical pumping of potassium in a ~kG magnetic field. Electron energy levels with Zeeman splitting are shown. Radiative Decays (unpolarized) Pumping + Depolarization

  5. 3Li 11Na 19K 37Rb 55Cs 87Fr Lower spin-exchange cross section and higher operating temperature } Candidates for an LDS Larger target dilution from unpolarized nucleons in the alkali nuclei Intermediate alkali metal atoms • Direct optical pumping of the H/D atoms is not possible with current technology, as this will require UV light of sufficient power and narrow linewidth • Solution… • Intermediate alkali vapor atoms are polarized by absorbing photons in the near IR range • Collisions transfer polarization to the H/D atoms K used at Argonne, Illinois, Erlangen, MIT Rb used at Erlangen

  6. Radiation trapping Fluorescent photons from optical pumping are of the correct wavelength to depolarize the alkali vapor. • A high magnetic field in the kG range shifts the wavelength for + and - absorption • depolarizing fluorescent photons are not absorbed • no N2 quench gas is required like 3He targets • HOWEVER… The transfer of spin to the H/D nuclei via the hyperfine interaction is reduced at large magnetic fields • Compromise: B ~1.0 kG for hydrogen and less for deuterium.

  7. In the limit many HH spin exchange collisions: • The nucleus becomes polarized • The population of the hyperfine states is given by Where b is the spin temperature Spin Temperature Equilibrium (STE) In Spin Temperature Equilibrium (STE): Spin exchange rate to H nuclei = spin exchange rate back to H electron

  8. Bc= 507 G, Hydrogen 117 G, Deuterium Nuclear polarization in Spin Temperature Equilibrium Hydrogen atoms in STE: pz =Pe Deuterium atoms in STE: Spin temperature equilibrium has been verified by: • Breit-Rabi polarimeter (Erlangen, 1997) - Hydrogen,Deuterium • pzz polarimeter (Argonne, 1998) - Deuterium • Proton scattering (IUCF, 1998) - Hydrogen More details later

  9. A Laser Driven Target (LDT) consists of the source of polarized gas, and a target (or storage) cell, which has additional wall collisions A Laser Driven Source (LDS) configuration does not have a target cell The target cell is used to increase the target thickness Sources and targets Molecules move more slowly than atoms

  10. Argonne National Laboratory Results from sources and targets M. Poelker et al., Phys. Rev. A.50 2450 (1994) M. Poelker et al., Nucl. Instr. and Meth. A 364 58 (1995) Originally tested in a source configuration (LDS) More wall collisions from a target cell will reduce the polarization and degree of dissociation

  11. Argonne results H and D typical f = 75% under operating conditions STE Conditions Insensitive to flow and B field Non-STE conditions

  12. Argonne results Extremely good results were obtained in the source configuration H flow = 1.7  1018 atoms/s, f = 0.75, Pe = 0.51 D flow = 0.86  1018 atoms/s, f = 0.75, Pe = 0.47 1.5 W of laser power is sufficient for optical pumping The Erlangen group obtained similar results

  13. Results from the pzz polarimeter (Argonne, 1998) J. A. Fedchak et al., Nucl. Instr. and Meth. A 417 182 (1998) pzz polarimeter based on work by Price and Haeberli D+ ions accelerated from the target region In the reaction: D + 3H  n + 4He Neutron angular distribution is anisotropic if D is tensor polarized

  14. Verification of STE using the pzz polarimeter B = 600 G Typical LDS operation Solid and dashed lines are calculated from Pe assuming STE B = 3600 G Used to test theory At large B, no STE. Theory curves are calculated from non-equilibrium theory A correction for wall depolarization was included The measured Pzzis in good agreement with STE

  15. IUCF Laser Driven Target The Illinois target was moved to IUCF in 1996 Target cell (storage tube): 40cm  3.2cm  1.3cm rectangular • Modifications: • No transport tube • Non-uniform magnetic field in the spin-exchange cell • 20mT at the top to 110-120mT at the bottom Doct. Thesis R. V. Cadman, University of Illinois at Urbana-Champaign R. V. Cadman et al., Phys. Rev. Lett.86, 967 (2001) C. E. Jones et al., PST99, p 204 M. A. Miller et al., PST97, p148 R. V. Cadman et al., PST97, p 437 H. Gao et al, PST95, p67

  16. IUCF 1998 H and D run (CE 66 and CE 68) Nuclear polarization measured using the proton beam Hydrogen: Deuterium: Average pz = 14.5% Average pz= 10.2% From fand Pe , we can calculate pz …

  17. IUCF 1998 H and D run (CE 66 and CE 68) From graphs, for both H and D, f 0.45, Pe  0.41 From STE, and that molecules move more slowly than atoms, the expected nuclear polarizations are: Hydrogen: 13.7% Deuterium: 17.4% Conclusion… H is in STE, D is not in STE Elastic p-p or p-d  target polarization First physics experiment to use a laser H/D polarized target! Results from the experiment provided further evidence for the three nucleon force.

  18. Laser http://eomer.physik.uni-erlangen.de/publikationen/dateien/pdf/stenger_koeln_procs.pdf University of Erlangen: source configuration Developed many diagnostic tools for the LDS Dissociator optical monitor Faraday rotation monitor Breit-Rabi polarimeter All important operating parameters can be monitored and/or optimized

  19. University of Erlangen: Optical and Faraday monitors Doct. Thesis J. Wilbert, Uni. Erlangen. http://eomer.physik.uni-erlangen.de/forschung/forschung.html Light output from the dissociator: Monitored for a change in intensity Calibrated to give the degree of dissociation Faraday polarimeter: Rotation of linearly polarized light by the alkali vapor J. Stenger et al., Nucl. Instr. and Meth. A 384 333 (1997)

  20. University of Erlangen: Faraday monitor Requires a probe laser Two modes of operation The first can be used to measure the alkali density and polarization The second can be used to measure the alkali “pump up” and decay time W. Nagengast et al., J. Appl. Phys.83, 5626 (1998)

  21. Verification of STE by Breit-Rabi polarimeter (Erlangen, 1997) J. Stenger et al., Phys. Rev. Lett. 78, 4177 (1997) A Breit-Rabi polarimeter is an inverted ABS Transitions between the hyperfine states are possible All results are consistent with STE Hydrogen flow 41017 atoms/s B = 1500 G Pe = 0.51  0.02

  22. MIT-Laser Driven Target This target is being developed for a polarized e-p scattering experiment at 275 MeV beam energy (MIT-Bates Proposal 00-02) Polarized hydrogen is the first priority This may be the first use of an LDT in an electron scattering experiment!

  23. LDS Target cell Electron polarimeter Drifilm coating f Pe MIT-Laser Driven Target D = 1.25 cm L = 40 cm Unlike the Argonne LDS, there is no direct path from the spin-exchange cell to the polarimeter Without EOM !!!

  24. Recent progress on the MIT-LDT Electro-Optic Modulator (EOM) Faraday vapor monitor

  25. Comparison of ABS and LDS

  26. ABS is the traditional target for polarized H/D experiments. Why? Technology well established High deuterium tensor polarization High nuclear vector polarization Pure atomic species http://blast.lns.mit.edu/targets/abs_web/ Advantages of the LDS Higher FOM Higher target thickness Compact design Disadvantages of the LDS Deterioration of the coating over time due to alkali vapor after operating ~100 hrs Low D tensor polarization Additional dilution from the pumping alkali Doct. Thesis J. Wilbert, Uni. Erlangen.

  27. Summary of results Hermes (ABS) (units) Gas H D F 6.5 4.6 (1016 atoms/s) T 7.5 14 (1013 cm-2) f0.93 0.95 pz,atomic 0.92 0.89 F(fpz,at)2 0.480.32(1017 atoms/s) t(fpz,at)2 5.510.0(1013 cm-2) E.C. Aschenauer ,International Workshop on QCD: Theory and Experiment, Martina Franca, Italy, Jun 16 - 20, 2001 Argonne (LDS) IUCF (LDT) MIT (LDT) 1995 1998 Preliminary (units) Gas H D H D H F 1.7 0.86 1.0 1.0 1.1 (1018 atoms/s) t 0.3 0.4 1.5 (1015 cm-2) f 0.75 0.75 0.48 0.48 0.56 pz,atomic 0.51 0.42 0.37 pz,total 0.145 0.102 F(fpz,at)2 2.5 1.1 0.32 0.15 0.47 (1017 atoms/s) t(fpz,at)2 0.93 0.61 6.4 (1013 cm-2)

  28. Two most pressing items for laser driven sources… 1) Consistent results with high performance at high flow rates needs to be established 2) Maintenance and reliability associated with coating/recoating (Drifilm deteriorates after ~ 100 hours) The future of the MIT LDT The first is being addressed in the MIT lab by using a double-dissociator design The second is being addressed by exploring the use of a diamond coating. (Diamond coated target cells may also be more resistant to radiation damage in an accelerator)

  29. BLAST and RpEX Bates Large Acceptance Spectrometer Toroid (BLAST) Large symmetric acceptance Covers: 20 < q < 90, -15 < j < 15 Solid angle ~ 1 sr The Proton Charge Radius Experiment (RpEX) will will provide the most precise determination of the proton charge radius

  30. Summary: Laser Driven Polarized H/D Sources and Targets Very high FOM compared to ABS for source was established at Argonne  H: 1.7  1018 atoms/s, f =0.75, Pe=0.51  D: 0.86  1018 atoms/s, f =0.75, Pe=0.47 High FOM results need to be produced in a target configuration (current work) Nuclear polarization has been seen (IUCF) and STE verified (Argonne, Erlangen). Deuterium LDS (e.g. IUCF) needs a very careful optimization of B field and dwell times  requires BRP Limitations of the coating reduce the overall performance of laser driven targets A diamond coating may offer an alkali-resistant surface, and its feasibility for use in the spin-exchange cell, transport tube and target cell needs to be determined (current work)

  31. Acknowledgment We thank Tom Wise and Willy Haeberli for the construction of the MIT-LDT storage cells We thank Michael Grossman and George Sechen for their technical support, and Tom Hession for the fabrication of the spin-exchange cells We also thank Bob Cadman,Hauke Kolster, Matt Poelker, Erhard Steffens and JuergenWilbert for their help in preparing this talk This work is supported in part by the U.S. Department of Energy under contract number DE-FC02-94ER40818 H. Gao acknowledges the support of an Outstanding Junior Faculty Investigator Award from the U.S. Department of Energy

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