Preliminary Simulations Study of a Pair Spectrometer for LEPS

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Preliminary Simulations Study of a Pair
Spectrometer for LEPS

• K. Hicks, Ohio University
• LEPS2 Workshop
• July 13, 2006

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What is a pair spectrometer?

• A thin foil (the radiator) is put in the beam to
  produce ee- pairs. A magnet deflects the pair
  into detectors, which record the position used to
  reconstruct the photon that produced the pair.
• It is similar to the sweep magnet in the LEPS
  laser hutch, which deflects pairs produced in the
  lead x-ray absorber.
• The resolution of the pair spectrometer depends
  on the radiator thickness and the magnet
  geometry.

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Why build a pair spectrometer?

• It would provide an independent calibration of
  the tagger energy.
• If it has sufficient resolution, it would provide
  diagnostics for the ring (e-) beam.
• CLAS has recently commissioned a pair
  spectrometer and found it to be useful.
• The pair spectrometer decouples the calibration
  of the spectrometer and the tagger.

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Goals of this study

• The idea was suggested by B. Mecking.
• Tech Note on tagger resolution, appendix B.
• Need 10-3 resolution for beam diagnostics.
• Simulations supplement analytic calculation
• Use the existing sweep magnet geometry.
• Place SSD or sci. fibers detectors at the end.
• Measure the resolution for a given geometry.

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The CLAS tagger and pair spect.
The tagger at Hall-B has a large energy range.
The pair spectrometer is placed after the
tagger and before CLAS. Microstrip (SSD)
detectors provide the position of the pair.
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Results CLAS tagger calibration
Simulations show a linear dependence of Epair/E0
on Dx (distance between pair).
Average 0.3 correction!
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The LEPS laser hutch
Ref Sumihamas thesis (not to scale!)
Sweep magnet
Place radiator here
Place detectors here
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End view the sweep magnet
B0.6 Tesla
Notes 1) The gap could be made smaller for
higher B-field. 2) The magnet could be
made wider (more bending). 3) The
whole system could be replaced with and
and electromagnet (if the fringe field is
small).
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GEANT trial geometry
sweep magnet B0.6 T, z1.0 m
e
e-
Radiator 0.1 mm Al.
Detectors sci. fibers 1 mm square
All simulations done with vacuum and zero size
beam spot, angle0!!
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Correlation position energy
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Converting position to energy
s0.3 mm resolution
Photon beam fixed at 2.000 GeV. Radiator 0.1 mm
Al.
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The dispersion Dx/DE
Dx0.5 mm per 10 MeV s 0.5 mm
Photon beam fixed at 2.010 GeV. Radiator 0.1 mm
Al.
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Preliminary results

• In the perfect world of Monte Carlo, it appears
  that we can get about 0.5 resolution by using
  the existing sweep magnet as a pair spectrometer,
  but
• Need a real magnetic field map (TOSCA?)
• Need beam size and angular divergence.
• This assumes no background, etc.
• However, improvements can also be made
• strengthen the B-field by closing the gap
• move the detectors back by about 0.5 m.
• angle the detectors to be perpendicular to the e-.

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Suggestions

• First, continue the preliminary study
• Get analytic calculations to check results.
• If the results hold up (lt1 res.) then
• add sci. fibers at the end of the sweep magnet
• fill the volume with He gas (lower mult.scatt.)
• add a radiator and a veto counter upstream
• build a trigger into the data acquisition.
• Could think about making the sweep magnet wider
  and/or with a smaller gap.

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Future Directions

• To get a 10-3 resolution pair spectrometer
• need to use a dipole electromagnet
• probably need a vacuum box and extra space
• might need SSDs for good position resolution
• need to do some more serious simulations
• The advantage of good resolution is that it
  allows for better beam diagnostics.
• This could be part of the LEPS2 project.
• Today, tagged photon beams should include a 10-3
  res. pair spectrometer to be competative.