Laser driven sources of H/D for internal gas targets

1
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
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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

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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

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Form factors

• Kinematics

k
k'
q
p
p'

• Form factor

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The Proton Electromagnetic Form Factors

• Unpolarized scattering

• Polarization transfer

• Super-ratio

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Storage rings and internal gas targets

• Storage ring
• 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

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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
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Atomic Beam Sources (ABS)

• Standard technology

Unpol. H
Polarized H Single state
MFT 2-3
Zeeman splitting of the hydrogen hyperfine energy
levels
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Atomic Beam Source (ABS)

• 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

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Polarized H/D Laser-Driven Sources and Targets
(LDS/LDT)

1. A circularly polarized laser is absorbed by
   potassium vapor, which polarizes the potassium
   (optical pumping)
2. The vapor is mixed with hydrogen (H) and spin is
   transferred to the H electrons through
   spin-exchange collisions
3. The H nuclei are polarized through the hyperfine
   interaction during frequent H-H collisions

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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
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Spin-Temperature equilibrium (STE)
The H/D nuclear polarization is given by the spin
temperature, ß 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
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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.
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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 1980s

• In 1998, an LDT was used for the first time in a
  physics experiment at IUCF

• In the mid to late 1990s, LDS and LDT projects
  were begun at the University of Erlangen and at
  MIT

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Argonne 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
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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

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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

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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
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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

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Erlangen Laser-Driven Source
Doct. Thesis J. Wilbert, Uni. Erlangen. http//eom
er.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
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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
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MIT Laser-Driven Target
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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
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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

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Faraday rotation diagnostics
Linearly polarized light can be decomposed into
two circular counter-rotating components s and
s- 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 s, s-
Population differences in the Ms 1/2 and -1/2
ground states result from optical pumping
where, V and a are Verdet Coefficients,
J. Stenger et al., Nucl. Instr. and Meth. A 384
333 (1997)
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Probe laser system

• TiSapph laser tunable from 700 to 850nm
• 0.001nm linewidth
• Low power required lt1mW

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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
B0
pump blocked
B0
J. Stenger et al., Nucl. Instr. and Meth. A 384
333 (1997)
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Faraday rotation results

• Pump beam chopped

• Probe 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
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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

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Monte-Carlo results

• Wall collision results

• Polarization results

Average number of wall collisions
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MIT LDT preliminary results

• Results for hydrogen only (first priority)

fa 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
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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
• (atoms/cm2)

average polarization as seen by the beam
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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

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FOM results
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
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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