Laser Driven Polarized HD Sources and Targets

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Laser Driven Polarized H/D Sources and Targets
PST 2003 Novosibirsk, Russia
Ben Clasie Laboratory for Nuclear Science Massac
husetts 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
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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

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LDS history
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, 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)
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Optical pumping
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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
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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.

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Spin Temperature Equilibrium (STE)
In Spin Temperature Equilibrium (STE)
Spin exchange rate to H nuclei spin exchange
rate back to H electron
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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
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Sources and targets
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) configur
ation does not have a target cell
The target cell is used to increase the target
thickness
Molecules move more slowly than atoms
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Results from sources and targets
Argonne National Laboratory
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 w
ill reduce the polarization and degree of
dissociation
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Argonne results
H and D typical f? 75 under operating conditio
ns
STE Conditions Insensitive to flow and B field
Non-STE conditions
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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 ato
ms/s, f? 0.75, Pe 0.47
1.5 W of laser power is sufficient for optical
pumping
The Erlangen group obtained similar results
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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 angula
r distribution is anisotropic if D is tensor
polarized
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Verification of STE using the pzz polarimeter
B 3600 G Used to test theory At large B, no
STE. Theory curves are calculated from
non-equilibrium theory
B 600 G Typical LDS operation Solid and dash
ed lines are calculated from Pe assuming STE
A correction for wall depolarization was
included The measured Pzz is in good agreement wi
th STE
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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. Re
v. Lett. 86, 967 (2001) C. E. Jones et al., PST99
, p 204 M. A. Miller et al., PST97, p148 R. V. C
adman et al., PST97, p 437 H. Gao et al, PST95, p
67
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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
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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 slo
wly 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 p
rovided further evidence for the three nucleon
force.
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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
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University of Erlangen Optical and Faraday
monitors
Doct. Thesis J. Wilbert, Uni. Erlangen.
http//eomer.physik.uni-erlangen.de/forschung/fors
chung.html
Light output from the dissociator
Monitored for a change in intensity
Calibrated to give the degree of dissociation
Faraday polarimeter Rotation of linearly polari
zed light by the alkali vapor
J. Stenger et al., Nucl. Instr.
and Meth. A 384 333 (1997)
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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 (19
98)
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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 pos
sible
All results are consistent with STE
Hydrogen flow 4?1017 atoms/s B 1500 G Pe 0.5
1 ? 0.02
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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!
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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 !!!
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Recent progress on the MIT-LDT
Electro-Optic Modulator (EOM)
Faraday vapor monitor
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Comparison of ABS and LDS
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ABS is the traditional target for polarized H/D
experiments. Why?
Technology well established High deuterium tensor
polarization High nuclear vector polarization P
ure 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 co
ating over time due to alkali vapor after
operating 100 hrs Low D tensor polarization A
dditional dilution from the pumping alkali
Doct. Thesis J. Wilbert, Uni. Erlangen.
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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 0.4
8 0.32 (1017 atoms/s) t(f? pz,at)2 5.5 10.0 (
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,a
tomic 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)
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The future of the MIT LDT
Two most pressing items for laser driven
sources 1) Consistent results with high performa
nce at high flow rates needs to be established
2) Maintenance and reliability associated with
coating/recoating (Drifilm deteriorates after
100 hours)
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)
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BLAST and RpEX
Bates Large Acceptance Spectrometer Toroid (BLAST)
Large symmetric acceptance Covers 20? -15? Solid angle 1 sr
The Proton Charge Radius Experiment (RpEX) will
will provide the most precise determination of
the proton charge radius
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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 at
oms/s, f? 0.75, Pe0.51 ? D 0.86 ? 1018 ato
ms/s, f? 0.75, Pe0.47 High FOM results need t
o 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 o
ptimization of B field and dwell times ? requires
BRP Limitations of the coating reduce the overa
ll 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)
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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 Juergen Wilbert 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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