Ben Clasie - Massachusetts Institute of Technology 11
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:1) detector efficiencies cancel, and,2) 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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. 3
2 2
( ) ( )
11 ...
6
iq xF q x e d x
q r
Form factors
Kinematics
Form factor
k k'
q
p p'
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The Proton Electromagnetic Form Factors
Unpolarized scattering
2 2 2 22 2
2 4
cos 2 2 tan 214 sin 2
p ppE M
M
d E G GG
d EE
Polarization transfer
tan2 2
pt eE
pM l p
PG E E
G P M
Super-ratioPE
PL LL M
PER
PR RM
Ga bA G
GA a bG
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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
p
d
e
e
Storage rings and internal gas targets
Stacking
Storage
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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 scattering1000 wall collisionsNo 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
( , )H p
Erhard Steffens and Willy Haeberli, Rep. Prog. Phys. 66 (2003) 1887
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Atomic Beam Sources (ABS)
Standard technology
Zeeman splitting of the hydrogen hyperfine energy levels
|1|2
|3|4
{Unpol.H
Polarized HSingle state
MFT2-3
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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
Atomic Beam Source (ABS)
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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
hydrogen
potassium
hydrogen
potassium
hydrogen
potassium
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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
RadiativeDecays(unpolarized)
+3
1
3
2
2
1jm2
1jm
2/12S4
2/12P4
PumpingRadiativeDecays(unpolarized)
+3
1
3
2
2
1jm2
1jm
2/12S4
2/12P4
Pumping +3
1
3
2
2
1jm2
1jm
2/12S4
2/12P4
Pumping
3Li
11Na
19K
37Rb
55Cs
87Fr
Larger target dilution
Lower spin-exchange cross sectionHigher operating temperature
} Candidates for an LDS
3Li
11Na
19K
37Rb
55Cs
87Fr
Larger target dilution
Lower spin-exchange cross sectionHigher operating temperature
} Candidates for an LDS
HH KK
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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:
Deuterium polarization:
Spin exchange rate to H nuclei = spin exchange rate back to H electron
( ) /FmFm e N
H/D hyperfine state population:
Hydrogen polarization: pz = Pe
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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
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.
T. Walker and L. W. Anderson, Nucl. Instr. And Meth. A334, 313 (1993)
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A. Kastler (1950) first proposed using light to produce
atoms with nuclear polarization
Previous efforts on LDS/LDT
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
A. Kastler, J. Phys Radium 11, 225 (1950)
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
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Argonne LDS
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
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
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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
B = 600GSTE conditions
B = 3600GNon-STETransfer 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
J. A. Fedchak et al., Nucl. Instr. and Meth. A 417 182 (1998)
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IUCF Laser-Driven Target
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
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 scatteringHydrogen:
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://eomer.physik.uni-erlangen.de/forschung/forschung.htmlJ. 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 monitorFaraday rotation monitorBreit-Rabi polarimeter
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Verification of STE at Erlangen
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
Measurements from a Breit-Rabi polarimeter
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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
ˆ //k B
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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
Ln n
c
n+, n- refractive index for σ+, σ-
where, V and α are Verdet Coefficients,
J. Stenger et al., Nucl. Instr. and Meth. A 384 333 (1997)
Population differences in the Ms = +1/2 and -1/2 ground states result from optical pumping
( , ) 1 ( , )
n n
V B NL P B
,P n n
Adapted from D. Budker, et. al., Rev. Mod. Phys. 74, 1153 (2002)
1n n
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Probe laser system
Ti:Sapph laser tunable from 700 to 850nm
0.001nm linewidth
Low power required <1mW
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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
,
,
arctanv v backgroundh
v h h background
P P
P P
Analyzing power is greatest when the initial 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
B=0
B=0
B = 155 mT
J. Stenger et al., Nucl. Instr. and Meth. A 384 333 (1997)
pumpblocked
Pump open
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Faraday rotation results
Theory curves
Pump beam choppedProbe beam chopped
Characteristic time for the potassium polarization to decay
Make best fit using Verdet Coeffs nK = 1.6 1011 atoms/cm3
PK = -41% (EOM off), -56% (EOM on)
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Monte-Carlo simulation
H/D atoms move in straight lines between wall collisions (molecular flow)
Depolarization and recombination coefficients, depol = 0.00146, recomb = 0.0006
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Monte-Carlo results
Spin-exchange Transport TargetCell Tube Cell Total
(1) 1180 200 135 1515(2) 1145 195 155 1495(3) 1140 195 295 1630
(1) Atoms that leave the center sampling hole(2) Atoms that leave the off-center hole(3) Atoms that leave the ends of the target cell
Spin-exchange Transport TargetCell Tube Cell Total
(1) 1180 200 135 1515(2) 1145 195 155 1495(3) 1140 195 295 1630
(1) Atoms that leave the center sampling hole(2) Atoms that leave the off-center hole(3) Atoms that leave the ends of the target cell
Average number of wall collisions
Wall collision results Polarization results
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MIT LDT preliminary results
fα = degree of dissociation
Pe = H electron polariz.
H nuclear polariz. (pz)
FOM = Figure Of Merit
= flowpz2, or, thickness pz2
Results for hydrogen only (first priority)
Measurements were made without an Electro-Optic Modulator (EOM)
Future tests with a diamond coating0
0.1
0.2
0.3
0.4
0.75 1 1.25 1.5 1.75 2
H2 Flow rate (1018 atoms/s)
FO
M (
1017 a
tom
s/s
)
0
1
2
3
4
5
6
FO
M (
1013 a
tom
s/c
m2 )
Preliminary results
0
20
40
60(%)
f
Pe
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)
•
, dLThickness t
2FOM t p
p = average polarization as seen by the beam
3flow Lt
d T
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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
•
2t p
2
expbeam erimentt p f f
2 2
collisions geometry temperature recombination dilusourc ne tioFlt p fow f f fp f
1temperaturef
T
sourc ae tomsf pp 1
. 2 1recombf f f
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FOM resultsHermes (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 pz2 0.50 0.33 0.051 (1017 atoms/s)
t pz2 5.8 10.1 0.61 (1013 atoms/cm2)
IUCF (LDT) 1998 MIT (LDT) Prelim. (units)Gas H D HFlow (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 pz2 0.21 0.10 0.34 (1017 atoms/s)
t pz2 (f ) 0.63 (2.3) 0.42 (1.5) 4.7 (2.7) (1013 atoms/cm2)
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 target cell had a rectangular cross section 32 x 13 mm
FOM
FOM
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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