Laser Driven Polarized H/D Sources and Targets

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