KOPIO Beam Catcher

1
KOPIO Beam Catcher

• Tadashi Nomura
• (Kyoto U.)
• Contents
• What is Beam Catcher?
• Concept and Design
• Expected Performance
• Proof of Principle
• Summary

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Task

• K?pnn experiment needs hermetic veto system

Signature 2g nothing
Veto detectors surrounding Decay Region
Particles can escape through the beam hole

• Charged particles
• Can be swept out from the beam and detected
  outside
• Photons
• Must be detected inside the beam
• Need in-beam photon detector
• Beam Catcher

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Challenge

• In-beam environment
• High intense beam(necessary to observe gt100
  K?pnn Br10-11)
• A vast amount of neutrons (30G/spill in KOPIO)
• Produce protons, pions, (g and e/e-) in the
  detector
• Most KLs survive after decay region (300M/spill)
• Decay into p, g ,e/e- in the detector
• ? These secondary particles fire the counter
  and disturb its primary function !!

Spill length 5 sec
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Solution

• Utilize Cherenkov radiation Aerogel tile
  (n1.05) or Acrylic slab radiator
• Avoid detection of slow particles from neutron
  interactions Slow p, p and other hadrons cannot
  emit lights.
• Use direction information Segment the detector
  into many modules and require coincidence
  along the beam direction
• Catch forward photons only Reduce fake signal
  due to g from secondary p0 (neutron interaction
  and KL decay in the detector)

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Aerogel Catcher in Beam

• Converter (Lead) Radiator (Aerogel tile)
• Sensitive area of 30cm x 30cm
• Like Sandwich detector by modules array

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Guard Counter in Halo Region

• Converter (Lead) Radiator (Acrylic slab)
• Sensitive area of 15cm x 15cm
• Sandwich detector
• Total reflection condition in light
  transportationcan reduce neutrons signal
• e/e- emit large angle Cherenkov lights ? meet
  the total reflection condition
• Slow particles cannot emit light or generate
  only small angle emission which escape to
  outside of the slab

Photon
Neutron
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KOPIO Catcher System

• In-beam Aerogel Catcher
• Module size 30cm x 30cm
• Pb converter 2mm per layer
• Number of modules 420
• 12-21 in horizontal with beam divergence
• 25 layers along beam(8.3 X0 in total)
• Z gap between layers 35cm
• Halo Guard counter
• Just before Aerogel Catcher
• ( 2mm Pb 10mm Acrylic ) x 8 layers x 3
  modules

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Location of Catcher System
Beam Catcher
Decay Region
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Expected Performance by MC (1)
Aerogel Catcher Efficiency
Y position dependence
99 _at_ 300MeV
Beam core
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Expected Performance by MC (2)
Hit probability for Neutrons
Hit probability for KLs
Dominated by decays in the Catcher
0.3 _at_ 800MeV
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Signal Loss due to Catcher False Hit

• Accidental hit due to neutrons may kill K?pnn
  signal

• Total false hit probability was found 0.4
  events / m-bunch
• Integrated over the duration consistent with
  the arrival time of g from our signal KL
• If we set the time window to be 3ns,signal loss
  due to false hit will be 4.6
• Calculation based on random effect
• Detailed studies by MC under way

Apply timing cut
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Efficiency Loss due to Blindness

• Catcher may be blinded due to other activities
• Blinded modules become less efficient for photons
• Photon efficiency loss found to be 1.1 (prompt g
  neutrons)
• Double pulse resolution is quite important
  !! (We used 3ns in the calculation above)

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Proof of Principle
PT2
PT1

• Prototype 1 (2001-2)
• 1/4 size, flat mirror
• light yield
• Prototype 2 (2002-3)
• 1/4 size, parabolic mirror
• light yield
• response to proton (as substitute for neutron)
• Check single layer eff. / two-layers
  coincidence
• Good agreement with MC(with gas scintillation)

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Summary (1)

• In-beam Photon Veto in KOPIO Beam Catcher
  System
• Aerogel counter inside beam
• Acrylic sandwich counter in halo region
• Expected performance meets our requirement
• High efficiency for photons (gt99 _at_ 300MeV)
• Low fake hit probability due to neutrons (0.3 _at_
  Ekin800MeV)
• Signal loss due to false hit will be 5
• Catcher efficiency loss due to blindness 1

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Summary (2)

• Proof-of-Principle experiments have been done
• two generations of prototype
• MC is found reliable
• We are ready for production !!

Full size prototype now under construction