LaserDriven HD Target at MITBates

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Laser-Driven H/D Target at MIT-Bates
Ben ClasieMassachusetts Institute of
TechnologyBen Clasie, Chris Crawford, Dipangkar
Dutta, Haiyan Gao, Jason Seely Massachusetts
Institute of Technology
Workshop on Testing QCD through Spin
Observables in Nuclear Targets University of
Virginia 2002
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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

• 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

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• 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
• The Argonne group reported results of high atomic
  H/D polarization for flow rates in excess of 1018
  atoms/s (Poelker 1995)
• Nuclear polarization in a LDT was established by
  the Erlangen group (Stenger 1997) and the Argonne
  group (Fedchak 1998)
• 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
• The Erlangen group is developing a laser driven
  source to be installed at COSY

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Optical pumping
Potassium fine structure (dashed lines) and
Zeeman splitting (solid lines) of the electron
energy levels

• 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)

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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

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LDT description
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• 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

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Experimental results
The two holes in the target cell can be studied
individually by changing the angle of the
polarimeter

• Large background due to the target chamber
  pressure
• Uncorrected degree of dissociation

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• 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

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

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• 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
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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

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References

• 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).