Laser Driven Polarized H/D Sources and Targets
PST 2003
Novosibirsk, RussiaBen 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
Introduction: Laser Driven Polarized H/D Sources and Targets
1) A circularly polarized laser is absorbed by alkali vapor,
which polarizes the vapor (optical pumping)
2) The vapor is mixed with H/D and spin is transferred to
the H/D electrons through spin-exchange collisions
3) The H/D nuclei are polarized through the hyperfine
interaction during frequent H-H or D-D collisions
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.
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)
A. Kastler, J. Phys Radium 11, 225 (1950)
A. Kastler (1950) first proposed using light to produce
atoms with nuclear polarization.
R. J. Holt et al., AIP Conf. No. 187, 499 (1989)
LDS history
Optical pumping of potassium in a ~kG magnetic field.
Electron energy levels with Zeeman splitting are shown.
+3
1
3
2
2
1jm
2
1jm
2/12S4
2/12P4
RadiativeDecays (unpolarized)Pumping
Depolarization
Optical pumping
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
3Li
11Na
19K
37Rb
55Cs
87Fr
Larger target dilution from unpolarized nucleons in the alkali nuclei
Lower spin-exchange cross section andhigher operating temperature
} Candidates for an LDS
K used at Argonne, Illinois, Erlangen, MIT
Rb used at Erlangen
Intermediate alkali metal atoms
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.
Radiation trapping
Spin Temperature Equilibrium (STE)
In the limit many HH spin exchange collisions:
1) The nucleus becomes polarized2) The population of the hyperfine states is given by
( ) /FmFm e N Where is the spin temperature
In Spin Temperature Equilibrium (STE):
Spin exchange rate to H nuclei = spin exchange rate back to H electron
Spin temperature equilibrium has been verified by:
•Breit-Rabi polarimeter (Erlangen, 1997) - Hydrogen,Deuterium•pzz polarimeter (Argonne, 1998) - Deuterium•Proton scattering (IUCF, 1998) - Hydrogen
Hydrogen atoms in STE: pz = Pe
Deuterium atoms in STE:
More details later
21STE CB B Bc= 507 G, Hydrogen
117 G, Deuterium{
-1
-0.5
0
0.5
1
-5 -2.5 0 2.5 5
Spin temp ()
Po
lari
zati
on
Pepzpzz
Nuclear polarization in Spin Temperature Equilibrium
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) configuration does not have a target cell
The target cell is used to increase the target thickness
3 2 1flow Lthickness f fd T
Molecules move more slowly than atoms
Results from sources and targets
Originally tested in a source configuration (LDS)
More wall collisions from a target cell will reduce the polarization and degree of dissociation
M. Poelker et al., Phys. Rev. A. 50 2450 (1994)M. Poelker et al., Nucl. Instr. and Meth. A 364 58 (1995)
Argonne National Laboratory
H and D typicalf = 75% under operating conditions
STEConditions
Insensitive to flow and B field
Non-STEconditions
Argonne results
1.5 W of laser power is sufficient for optical pumping
The Erlangen group obtained similar results
Extremely good results were obtained in the source configuration
H flow = 1.7 1018 atoms/s, f = 0.75, Pe = 0.51D flow = 0.86 1018 atoms/s, f = 0.75, Pe = 0.47
Argonne results
In the reaction:
D + 3H n + 4He
Neutron angular distribution is anisotropic if D is tensor polarized
J. A. Fedchak et al., Nucl. Instr. and Meth. A 417 182 (1998)
Results from the pzz polarimeter (Argonne, 1998)
pzz polarimeter based on work by Price and Haeberli
D+ ions accelerated from the target region
B = 3600 GUsed to test theory
At large B, no STE. Theory curves are calculated from non-equilibrium theory
B = 600 GTypical LDS operation
Solid and dashed lines are calculated from Pe assuming STE
A correction for wall depolarization was included
The measured Pzz is in good agreement with STE
Verification of STE using the pzz polarimeter
The Illinois target was moved to IUCF in 1996
Doct. Thesis R. V. Cadman, University of Illinois at Urbana-ChampaignR. V. Cadman et al., Phys. Rev. Lett. 86, 967 (2001)C. E. Jones et al., PST99, p 204M. A. Miller et al., PST97, p148R. V. Cadman et al., PST97, p 437H. Gao et al, PST95, p67
Modifications:
• No transport tube
• Non-uniform magnetic field in the spin-exchange cell
20mT at the top to 110-120mT at the bottom
IUCF Laser Driven Target
Target cell (storage tube):
40cm 3.2cm 1.3cm rectangular
Nuclear polarization measured using the proton beamHydrogen:
Deuterium:Average pz = 14.5%
Average pz= 10.2%
From f and Pe , we can calculate pz …
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
First physics experiment to use a laser H/D polarized target!
Results from the experiment provided further evidence for the three nucleon force.
IUCF 1998 H and D run (CE 66 and CE 68)
Elastic p-p or p-d
target polarization
Developed many diagnostic tools for the LDS
Dissociator optical monitorFaraday rotation monitorBreit-Rabi polarimeter
All important operating parameters can be monitored and/or optimized
http://eomer.physik.uni-erlangen.de/publikationen/dateien/pdf/stenger_koeln_procs.pdf
Laser
University of Erlangen: source configuration
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)
University of Erlangen: Optical and Faraday monitors
Requires a probe laserTwo 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)
University of Erlangen: Faraday monitor
J. Stenger et al., Phys. Rev. Lett. 78, 4177 (1997)
Hydrogen flow 41017 atoms/sB = 1500 GPe = 0.51 0.02
A Breit-Rabi polarimeter is an inverted ABS
Transitions between the hyperfine states are possible
All results are consistent with STE
Verification of STE by Breit-Rabi polarimeter (Erlangen, 1997)
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!
MIT-Laser Driven Target
Preliminary results
0
20
40
60
0.75 1 1.25 1.5 1.75 2
H flow rate (1018 atoms/s)
(%)
Pe
f
Unlike the Argonne LDS, there is no direct path from the spin-exchange cell to the polarimeter
WithoutEOM !!!
LDS
Target cell
Electronpolarimeter
Drifilm coating
MIT-Laser Driven Target
D = 1.25 cmL = 40 cm
Faraday vapor monitor
Recent progress on the MIT-LDT
Electro-Optic Modulator (EOM)
41.2
41.6
42
42.4
0 50 100
Time (ms)
Ro
tatio
n a
ng
le (
de
g)
PumplasershutterOPEN CLOSED
= 4.22 +- 0.1 ms
Comparison of ABS and LDS
ABS is the traditional target for polarized H/D experiments. Why?
Advantages of the LDSHigher FOMHigher target thicknessCompact design
Disadvantages of the LDSDeterioration of the coating over time due to alkali vapor after operating ~100 hrsLow D tensor polarizationAdditional dilution from the pumping alkali
Technology well established
High deuterium tensor polarization
High nuclear vector polarization
Pure atomic specieshttp://blast.lns.mit.edu/targets/abs_web/
Doct. Thesis J. Wilbert, Uni. Erlangen.
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 0.48 0.32 (1017 atoms/s)
t(f pz,at)2 5.5 10.0 (1013 cm-2)
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)
E.C. Aschenauer ,International Workshop on QCD: Theory and Experiment, Martina Franca, Italy, Jun 16 - 20, 2001
Summary of results
The future of the MIT LDT
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 second is being addressed by exploring the use of a diamond coating.
The first is being addressed in the MIT lab by using a double-dissociator design
(Diamond coated target cells may also be more resistant to radiation damage in an accelerator)
Bates Large Acceptance Spectrometer Toroid (BLAST)
Large symmetric acceptance
Covers: 20 < < 90, -15 < < 15
Solid angle ~ 1 sr
The Proton Charge Radius Experiment (RpEX) will will provide the most precise determination of the proton charge radius
BLAST and RpEX
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)
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