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 1

2. 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

3. 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

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

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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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.

14. 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

15. Argonne LDS Originally tested in a source configuration (LDS)
M. Poelker et al., Phys. Rev. A (1994)
M. Poelker et al., Nucl. Instr. and Meth. A (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

16. Results from the pzz polarimeter (Argonne, 1998)
J. A. Fedchak et al., Nucl. Instr. and Meth. A (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

17. Verification of STE at Argonne
J. A. Fedchak et al., Nucl. Instr. and Meth. A (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

18. 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%

19. 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

20. Erlangen Laser-Driven Source
Doct. Thesis J. Wilbert, Uni. Erlangen.
J. Stenger et al., Nucl. Instr. and Meth. A (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

21. 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

22. MIT Laser-Driven Target

23. 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

24. 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

25. 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/ ground states result from optical pumping
where, V and α are Verdet Coefficients,
J. Stenger et al., Nucl. Instr. and Meth. A (1997)

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

27. 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 (1997)

28. 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

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

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

31. 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

32. 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

33. 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

34. FOM results FOM FOM Hermes (ABS) `96 - `01 BLAST (ABS) (units)
Gas H D H
Flow (F) (1016 atoms/s)
thicknesss (t) (1013 atoms/cm2)
pz
F pz (1017 atoms/s)
t pz (1013 atoms/cm2)
FOM
E.C. Aschenauer ,International Workshop on QCD: Theory and Experiment, Martina Franca, Italy, Jun , 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) (1018 atoms/s)
thicknesss (t) (1015 atoms/cm2) f
Pe,atomic
pz
F pz (1017 atoms/s)
t pz2 (f ) (2.3) (1.5) 4.7 (2.7) (1013 atoms/cm2)
FOM
IUCF target cell had a rectangular cross section 32 x 13 mm

35. 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