Laser-Driven H/D Target at MIT-Bates

Ben ClasieMassachusetts Institute of Technology

Ben Clasie, Chris Crawford, Dipangkar Dutta, Haiyan Gao, Jason Seely

Massachusetts Institute of Technology

RpEX

Workshop on “Testing QCD through Spin Observables in Nuclear Targets”

University of Virginia 2002

Introduction• The Laser-Driven Target (LDT) is a source of nuclear spin polarized hydrogen

or deuterium atoms

• The H or D nuclei are polarized through collisions with polarized, intermediate alkali-metal atoms

• The LDT is similar to the atomic beam source as both targets are a source of nuclear spin polarized H or D atoms

Atomic Beam Source (H)Well established technology ~1014 atoms/cm2 thickness~84% degree of dissociation ~80% polarization

Laser-Driven Target (goal, H)Compact design ~ 1015 atoms/cm2 thickness~ 60% degree of dissociation ~ 50% polarization

• The LDT flow rate (greater than 1018 atoms/s) is approximately 25 times larger than the atomic beam source and has the potential of a higher figure-of-merit

• The LDT is being developed for the South Hall Ring at the MIT-Bates Linear Accelerator Center; an ABS target is currently being installed

• The LDT is planned to be used in the conditionally approved “Precision measurement of the Proton charge Radius” EXperiment (RpEX)

• RpEX will measure the proton charge radius with sub 1% precision – a factor of three smaller than any single existing measurement

• Several groups have demonstrated the feasibility of the laser-driven technique:

1. The Argonne group reported results of high atomic H/D polarization for flow rates in excess of 1018 atoms/s (Poelker 1995)

2. Nuclear polarization in a LDT was established by the Erlangen group (Stenger 1997) and the Argonne group (Fedchak 1998)

3. A laser-driven target that operated with flow rates above 1018 atoms/s was used in two proton scattering experiments at the IUCF Cooler Ring, 1998

4. The Erlangen group is developing a laser driven source to be installed at COSY

• Optical pumping is the process by which the angular momentum of the photon is transferred to the alkali-metal atom

• Optical pumping of potassium vapor in a high magnetic field (1kG) produces high potassium electron polarization at high vapor densities (nK ~ 7x1011 atoms/cm3)

+3

1

3

2

2

1jm2

1jm

2/12S4

2/12P4

RadiativedecaysPumping

Potassium fine structure (dashed

lines) and Zeeman splitting (solid lines)

of the electron energy levels

Optical pumping

Spin temperature equilibrium (STE)

• Atomic potassium polarization is transferred to the H/D nuclei through spin-exchange collisions, without RF transitions

• KH spin exchange, spin is transferred to the hydrogen electron

• HH spin exchange, spin is transferred to the hydrogen nucleus through the hyperfine interaction

• In STE, the polarization of the hydrogen nucleus equals the electron polarization

• The nuclear polarization of deuterium is slightly larger than that of the electron in STE

LDT description

• The potassium ampoule is slowly heated- introducing vapor into the spin cell, which is polarized through optical pumping

• Atomic H/D from the dissociator is polarized through spin-exchange collisions with the potassium, and flows into the storage cell

• The dwell time for the hydrogen in the spin cell must be sufficient for spin temperature equilibrium

• The Pyrex spin cell and aluminum storage cell are typically heated to 200oC and are coated with drifilm to reduce recombination/depolarization

• There are two holes in the storage cell at 90o to the transport tube for measuring the polarization along the target length, with no direct path to the spin cell

• To reduce the number of wall collisions, the spin cell is spherical

Pump laser and probe laser• The Ti-Sapphire laser, typically 2.3W with 0.8

GHz linewidth, is tuned to 770.1nm• An electro-optic modulator is planned• The pump beam is formed by expanding and

circularly polarizing the laser

• A small fraction of the Ti-Sapphire laser is used to make the probe beam

• The Faraday rotation of the probe beam gives the potassium density and polarization rise time (Stenger 1997)

Gas flow• The gas supply is

research purity (1ppm) bottled H(D)

• H2 or D2 pass through an MKS mass flow control, a pneumatic valve, and into the dissociator

H/D electron polarimeter• In the first stage a 1 Tesla permanent sextupole magnet

focuses one spin state and defocuses the other• The focused spin state is chopped at 20Hz and detected

with a cross beam Quadrupole Mass Analyzer (QMA)• A bellows and gimble mount connects the polarimeter

to the target chamber for alignment• The H/D electron polarization has been calculated to be

in spin temperature equilibrium

Experimental results

• Large background due to the target chamber pressure

• Uncorrected degree of dissociation = offRF2onRF2 MM1

The two holes in the target cell can be

studied individually by changing the

angle of the polarimeter

Molecular hydrogen signal

0

10

20

30

QM

A (

mV

)

Dissociator off

Dissociator on

0

20

40

60

80

100

0 5 10 15 20 25 30

Distance at target cell (mm)

De

gre

e o

f dis

soci

atio

n (

%)

Backgroundsubtracted

Uncorrected

50

60

70

80

90

100

0 1 2 3 4

H 2 flow rate (sccm)

Be

st

de

gre

e o

f d

iss

oc

iati

on

( %

)

100 MHz160 MHz100 MHz, dissociator only160 MHz, dissociator only

Approximate dissociator frequency

• A dissociator was made, without a spincell, and the degree of dissociation then measured directly at the dissociator (Blue)

• The complete glassware using similar dissociator dimensions was then made and the degree of dissociation measured at the target cell (Red)

• Dissociator aperture diameter = 1 mm

80

90

100D

eg

ree

of d

isso

cia

tion

(%

)

0

100

200

0 60 120 180 240 300

Time (min)

Sp

in c

ell

tem

pe

ratu

re (

OC

)

• The spin cell is heated to prevent potassium from condensing on the walls with a small increase in recombination

• The Ti-Sapphire laser is tuned to the two potassium resonances

• Hydrogen flow rate = 1.5 sccm

• Laser shutter closed unpolarized Laser shutter open 36% polarization

• Atomic polarization=

Figure Of Merit (FOM) =

flow rate (degree dissociation polarization)2

0.00

0.20

0.40

0.60

0.5 1 1.5 2 2.5

H2 flow (sccm)

FO

M (1

017

atom

s/s) Positive helicity

Negative helicity

111 closedopen MM0

1

2

3

4

5

6

7

8

Laser wavelength (nm)

QM

A a

tom

ic s

ign

al (

mV

).Laser shutter open

Laser shutter closed

770.105 770.112

+ -

Conclusions

• The LDT has produced high atomic polarized H and D atoms at flow rates exceeding 1018 atom/s, which have been calculated to be in spin temperature equilibrium

• The design goal for LDT is a flow rate of 2 x 1018 atoms/s with 60% degree of dissociation and 50% polarization

• Further improvements are expected with the use of an electro-optic modulator and optimization of the operating parameters of the new spincell

• M. Poelker et al., Nucl. Instr. And Meth. A 364, 58 (1995).

• J. A. Fedchak et al., Nucl. Instr. And Meth. A 417, 182 (1998).

• J. Stenger et al., Phys. Rev. Lett. 78, 4177 (1997).

• J. Stenger et al., Nucl. Instr. And Meth. A 384, 333 (1997).

References