Hall D Photon Beam Simulation and Rates

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Hall D Photon BeamSimulation and Rates
Hall D Beam Line and Tagger Review Jan. 23-24,
2006, Newport News
Richard Jones, University of Connecticut
Part 1 photon beam line Part 2 tagger
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I. Photon Beam Line Simulation

• estimate background rates
• evaluate options for shielding
• has a detailed model of coherent bremsstrahlung
• describes the detailed beam line geometry, fields
• simulates electromagnetic processes accurately
• includes photonuclear interactions at some level

To accomplish these goals, we need a Monte Carlo
simulation that
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Photon Beam Line Simulation

• Detailed photon beam line simulation HDGeant
• has built-in coherent bremsstrahlung generator to
  simulate beam line with a realistic
  intensity spectrum
• beam photons tracked from exit of radiator
• assumes beam line vacuum down to a few cm from
  entry to primary collimator,
  followed by air
• beam enters vacuum again following secondary
  collimator and continues down to a few cm from
  the liquid hydrogen target
• includes all shielding and sweep magnets in
  collimator cave
• monitors background levels at several positions
  in cave and hall
• The same simulation also includes the complete
  GlueX target and spectrometer, detector systems,
  dump etc.

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Photon Beam Line Simulation

• HDGeant all of the std Geant3 physics models,
  plus
• muon pair production
• modified Geant to turn a fraction of the e- e
  pairs into µ- µ
• cross section formulas translate simply me
  mm
• rates are down by factor (me/mm)2 -- but not
  insignificant
• photonuclear reactions
• large mass of data on these cross sections, not
  all relevant
• final results will depend on hadronic
  interactions in any case.
• GELHAD package from BaBAR was adopted
• gN quasi-elastic scattering, single p/p0
  production, pp- production (vector dominance
  model), pn emission (quasi-deuteron model)

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Photon Beam Line Simulation
cut view of simulation geometry through
horizontal plane at beam height
Hall D
collimator cave
Fcal
tagger building
Cerenkov
vacuum pipe
spectrometer
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Photon Beam Line Simulation
overhead view of collimator cave cut through
horizontal plane at beam height
12 m
collimators
concrete
air
vac
vac
sweep magnets
iron blocks
lead
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Photon Beam Line Simulation
cut view of simulation geometry through
horizontal plane at beam height
Hall D
collimator cave
virtual detector plane
x
z
Fcal
tagger building
Cerenkov
vacuum pipe
spectrometer
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Photon Beam Line Simulation Development

• Progress so far
• Simulation has been used to optimize the amount
  and placement of shielding in the collimator
  cave.
• Study of background rates in the GlueX start
  counter and trackers are being used to constrain
  the design of those detectors.
• Ongoing development
• The Hall D team is working with the Jlab RadCon
  group to cross-check our rate results for the
  Hall D beam line.
• Elements from HDGeant (geometry, coherent
  bremsstrahlung generator, fields) have been
  shared, and many things checked.
• Latest results (P. Degtiarenko) are consistent
  with conclusions based on previous Hall D
  studies.
• Some errors have been corrected results follow

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Beam Energy Spectrum
counts
photon energy (GeV)
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Background Rates z -1 m
particle flux (/cm2/s)
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Bethe-Heitler muons
m/m flux vs. position

• At these energies, the primary source of muons
  is pair production in the collimator.
• With appropriate shielding (included in the
  simulation) the rates at the detector are within
  an order of magnitude of cosmic rays.

flux (muons/cm2/s)
radial position (cm)
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II. Photon Tagger Simulation

• estimate background rates in focal plane counters
• evaluate options for shielding
• detailed field map of the tagger
• design for the vacuum box downstream exit region
• focal plane hodoscope model

Elements needed for tagger simulation (in
addition to beam line)
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Photon Tagger Simulation
cut view of simulation geometry through
horizontal plane at beam height
Tagger Building
goniometer
Hall D
photon beam line
quadrupole magnet
tagger dipoles
fixed array hodoscope
microscope
vacuum chamber
to electron beam dump
detailed magnetic field map (from TOSCA) 350 x 30
x 1600 17 M points
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Photon Tagger Simulation

• events begin with a CB g,e- pair inside the
  diamond radiator
• beam emittances realistic for the 12 GeV machine
• electron tracked in the magnetic field of the
  quadrupole and dipoles
• focal plane scintillators are sensitive volumes

photon beam exits from tagger inside
vacuum, continues 70 m down to collimator
cave without leaving vacuum
Runge-Kutta method

• hodoscope exit window is 1 mm Kapton
• other walls of vacuum chamber are 1 cm Al
• exposed side of electron exit channel is 5 mm Al

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Photon Tagger Simulation
One issue highlighted in the GlueX Detector
Review report, October 2004
A potential concern is the high flux of electrons
with energies close to the endpoint interacting
with the mechanical structure of the vacuum
chamber or the dump pipe. Because of the shallow
bend angle of the spectrometer, downstream spray
could cause background in the tagging detectors.
Recommendation Perform a Monte Carlo simulation
of the tagging system with particular attention
to background in the tagging counters caused by
high-energy electrons.
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Photon Tagger Simulated Events
concrete wall
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Photon Tagger Simulation
Resolution in focal plane microscope in energy
and emission angle
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Photon Tagger Simulation
monitor all particles reaching one of the central
fibers of the FP microscope
gammas only
gammas
positrons
electrons
all hits, single fiber
all hits, combine neighboring hits
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Summary

• A complete set of tools for detailed physics
  simulation has been developed covering the Hall D
  Photon Beam and Tagger systems.
• Backgrounds at the entrance to the detector and
  also at the tagger focal plane have been
  examined.
• The shielding for the collimator region has been
  optimized based on the simulation, leading to
  acceptable background rates in the GlueX
  detector.
• Backgrounds coming from the exit region of the
  tagger vacuum will be minimized using the
  simulation by refining the mechanical design and
  optimizing the shielding in that area.