Laser driven sources of H/D for internal gas targets Ben Clasie MIT Laboratory for Nuclear Science C. Crawford, D. Dutta, H. Gao, J. Seely, W. Xu 1

Outline Introduction and motivation Physics motivation for polarized gas targets Storage rings and internal gas targets Atomic Beam Sources (ABS) Polarized H/D Laser-Driven Sources/Targets (LDS/LDT) Optical pumping Spin-temperature equilibrium Previous efforts on LDS/LDT MIT laser-driven target Experimental setup (some details on Faraday rotation diagnostics) Results and simulations Summary

Introduction and Motivation Polarized beams and polarized targets are relatively new technologies By flipping either the beam or target polarization, small (~10%) changes in the scattering rates are observed This is an extremely powerful technique as: detector efficiencies cancel, and, such double-polarization asymmetries are more sensitive to quantities otherwise difficult to access, for example the nucleon electromagnetic form factors Nucleon electromagnetic form factors describe the electromagnetic structure of the nucleon

Form factors Kinematics k k' q p p' Form factor

The Proton Electromagnetic Form Factors Unpolarized scattering Polarization transfer Super-ratio

Storage rings and internal gas targets Many passes through the target gas Large stored current, typically 0.1 to 1A , COSY, IUCF, RHIC , HERA, Bates, NIKEF, VEPP Internal gas target Nuclear polarized H/D for the target can only be produced in small quantities Windowless storage cell Storage cell increases target thickness vs. jet targets Stacking Storage

Storage cells 1966 – Idea to use a storage cell to increase the target density (Willy Haeberli) 1980 – First test of a storage cell at Wisconsin scattering 1000 wall collisions No observable depolarization The polarized target gas is produced by breaking H/D molecules into atoms, which depolarize quickly on most surfaces Recombination produces molecules where little (if any) nuclear polarization is retained The storage cell walls are usually coated with teflon or drifilm Erhard Steffens and Willy Haeberli, Rep. Prog. Phys. 66 (2003) 1887

Atomic Beam Sources (ABS) Standard technology |1 |2 |3 |4 Unpol. H MFT 2-3 Polarized H Single state Zeeman splitting of the hydrogen hyperfine energy levels

Atomic Beam Source (ABS) Conventional polarized H/D source Pure atomic species High Deuterium tensor polarization Laser Driven Source (LDS) Potentially higher Figure Of Merit Larger target thickness Compact design However … Dilution from alkali vapor (potassium or rubidium) Drifilm coating deteriorates (~100 hrs) due to the presence of the alkali

Polarized H/D Laser-Driven Sources and Targets (LDS/LDT) A circularly polarized laser is absorbed by potassium vapor, which polarizes the potassium (optical pumping) The vapor is mixed with hydrogen (H) and spin is transferred to the H electrons through spin-exchange collisions The H nuclei are polarized through the hyperfine interaction during frequent H-H collisions

Optical pumping The potassium D1 line is split in a magnetic field of ~1kG Photon angular momentum is transferred to the potassium vapor  polarized potassium No N2 quench gas is required like 3He targets Spin-exchange collisions

Spin-Temperature equilibrium (STE) The H/D nuclear polarization is given by the spin temperature, β The H or D nucleus becomes polarized through H-H or D-D collisions STE is reached when: H/D hyperfine state population: Hydrogen polarization: pz = Pe Deuterium polarization: Spin exchange rate to H nuclei = spin exchange rate back to H electron

Radiation trapping Fluorescent photons can depolarize the alkali vapor T. Walker and L. W. Anderson (1993) suggested using a larger magnetic field in an LDS A magnetic field in the kG range shifts the wavelength for + and - absorption depolarizing fluorescent photons are not absorbed T. Walker and L. W. Anderson, Nucl. Instr. And Meth. A334, 313 (1993) 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.

Previous efforts on LDS/LDT A. Kastler (1950) first proposed using light to produce atoms with nuclear polarization A. Kastler, J. Phys Radium 11, 225 (1950) After the development of lasers with high power and narrow linewidths, development of an early LDS began at Argonne National Laboratory in the late 1980’s In 1998, an LDT was used for the first time in a physics experiment at IUCF In the mid to late 1990’s, LDS and LDT projects were begun at the University of Erlangen and at MIT

Argonne LDS Originally tested in a source configuration (LDS) 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 storage cell will reduce the polarization and degree of dissociation Extremely good results were obtained in this 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

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

Verification of STE at Argonne J. A. Fedchak et al., Nucl. Instr. and Meth. A 417 182 (1998) B = 600G STE conditions B = 3600G Non-STE Transfer of polarization to the nucleus is suppressed at large magnetic fields Solid and dashed line in the first graph are from theory that assumes STE Non-STE theory was used in the second graph Correction for wall depol. pzz under operating conditions agree with STE

IUCF Laser-Driven Target 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 The Illinois target was moved to IUCF in 1996 Modifications: No transport tube Low B field region Storage cell was 40cm 3.2cm 1.3cm with rectangular cross section Nuclear polarization from proton scattering Hydrogen: Deuterium: Average pz = 14.5% Average pz= 10.2%

IUCF 1998 H and D run (CE66 and CE68) Measurements with the electron polarimeter should agree with the nuclear polarization However: from the graphs and for both H and D, f  0.45, Pe  0.41 From STE, we should get H vector pol: 13.7% D vector pol: 17.4% Conclusion: H is in STE, D is not in STE

Erlangen Laser-Driven Source Doct. Thesis J. Wilbert, Uni. Erlangen. http://eomer.physik.uni-erlangen.de/forschung/forschung.html J. Stenger et al., Nucl. Instr. and Meth. A 384 333 (1997) Developed many diagnostic tools for the LDS All important operating parameters can be monitored and/or optimized Dissociator optical monitor Faraday rotation monitor Breit-Rabi polarimeter

Measurements from a Breit-Rabi polarimeter Verification of STE at Erlangen J. Stenger et al., Phys. Rev. Lett. 78, 4177 (1997) Measurements from a Breit-Rabi polarimeter 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

MIT Laser-Driven Target

MIT Laser-Driven Target Gas panel Magnetic field Pump laser system Probe laser system Glassware/coating Dissociator Storage cell Heaters Polarimeter Vacuum pumps Control software Polarimeter

Faraday rotation diagnostics The Faraday effect is the rotation of linear polarized light by a medium in a magnetic field ( ) Provides information on the alkali vapor: density, polarization, and, polarization time constants

Faraday rotation diagnostics Linearly polarized light can be decomposed into two circular counter-rotating components σ+ and σ- Faraday effect occurs when a B-field is applied Adapted from D. Budker, et. al., Rev. Mod. Phys. 74, 1153 (2002) n+, n- refractive index for σ+, σ- Population differences in the Ms = +1/2 and -1/2 ground states result from optical pumping where, V and α are Verdet Coefficients, J. Stenger et al., Nucl. Instr. and Meth. A 384 333 (1997)

Probe laser system Ti:Sapph laser tunable from 700 to 850nm 0.001nm linewidth Low power required <1mW

Measurement of Faraday rotation Pp = incident probe laser power Ph = (horizontally polarized transmitted power)/Pp Pv = (vertically polarized transmitted power)/Pp Analyzing power is greatest when the initial q is 45º  rotate the Faraday polarimeter (or a half waveplate) Faraday rotation from the glassware must be subtracted Technique is very useful when the incident power, Pp , is not constant Pump open B = 155 mT B=0 pump blocked B=0 J. Stenger et al., Nucl. Instr. and Meth. A 384 333 (1997)

Faraday rotation results Probe beam chopped Pump beam chopped Make best fit using Verdet Coeffs nK = 1.6  1011 atoms/cm3 PK = -41% (EOM off), -56% (EOM on) Characteristic time for the potassium polarization to decay Theory curves

Monte-Carlo simulation H/D atoms move in straight lines between wall collisions (molecular flow) Depolarization and recombination coefficients, gdepol = 0.00146, grecomb = 0.0006

Monte-Carlo results Wall collision results Polarization results Average number of wall collisions

MIT LDT preliminary results Results for hydrogen only (first priority) fα = degree of dissociation Pe = H electron polariz.  H nuclear polariz. (pz) FOM = Figure Of Merit = flowpz2, or, thickness pz2 Measurements were made without an Electro-Optic Modulator (EOM) Future tests with a diamond coating  Improves Pe

Figure Of Merit (FOM) (atoms/cm2) FOM is a measure of the target performance, it is inversely proportional to the running time of an experiment = average polarization as seen by the beam

Figure Of Merit (FOM) (cont.) is the usual definition of FOM, however, there are other considerations How do we compare the performance of two different types of polarized targets? - smallest error bars may be more useful How do we compare the performance of polarized sources at different facilities? - storage cell geometry is usually restricted by beam halo This comparison is difficult as there are spin-exchange collisions and wall collisions in the storage cell

FOM results FOM FOM Hermes (ABS) `96 - `01 BLAST (ABS) (units) Gas H D H Flow (F) 6.5 4.6 2.5 (1016 atoms/s) thicknesss (t) 7.5 14 3.0 (1013 atoms/cm2) pz 0.88 0.85 0.45 F pz2 0.50 0.33 0.051 (1017 atoms/s) t pz2 5.8 10.1 0.61 (1013 atoms/cm2) FOM E.C. Aschenauer ,International Workshop on QCD: Theory and Experiment, Martina Franca, Italy, Jun 16 - 20, 2001 HERMES target cell has elliptical cross section 29 x 9.8 mm IUCF (LDT) 1998 MIT (LDT) Prelim. (units) Gas H D H Flow (F) 1.0 1.0 1.1 (1018 atoms/s) thicknesss (t) 0.3 0.4 1.5 (1015 atoms/cm2) f 0.48 0.48 0.56 Pe,atomic 0.45 0.45 0.37 pz 0.145 0.102 F pz2 0.21 0.10 0.34 (1017 atoms/s) t pz2 (f ) 0.63 (2.3) 0.42 (1.5) 4.7 (2.7) (1013 atoms/cm2) FOM IUCF target cell had a rectangular cross section 32 x 13 mm

Summary Laser driven sources and targets can provide H/D with high polarization at flow rates in excess of 1018 atoms/s These offer a more compact design than conventional atomic beam sources and may provide a higher overall FOM Faraday rotation diagnostics provide important information on the alkali number density, polarization and time constants