laser-driven h/d target at mit-bates
DESCRIPTION
RpEX. Laser-Driven H/D Target at MIT-Bates. Ben Clasie Massachusetts Institute of Technology Ben Clasie, Chris Crawford, Dipangkar Dutta, Haiyan Gao, Jason Seely Massachusetts Institute of Technology. Workshop on “Testing QCD through Spin Observables in Nuclear Targets” - PowerPoint PPT PresentationTRANSCRIPT
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