1. Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy
KOPIO Beam Catcher
Tadashi Nomura
(Kyoto U.)
Contents
What is Beam Catcher?
Concept and Design
Expected Performance
Proof of Principle
Summary
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

2. 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
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

3. Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy
Challenge
“In-beam” environment
High intense beam (necessary to observe >100 Kpnn; Br~10-11)
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
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

4. Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy
Solution
Utilize Cherenkov radiation Aerogel tile (n=1.05) 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)
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

5. Aerogel Catcher in Beam
flat mirror
Cerenkov light
lead sheet
5inch PMT
Module
funnel
Converter (Lead) + Radiator (Aerogel tile)
Sensitive area of 30cm x 30cm
Like “Sandwich” detector by modules’ array g
red: e+/e-, blue: photon
Example of g event (MC)
Plan view
Coincidence
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

6. Guard Counter in Halo Region
Converter (Lead) + Radiator (Acrylic slab)
Sensitive area of 15cm x 15cm
“Sandwich” detector
Total reflection condition in light transportation can reduce neutron’s 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
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

7. KOPIO Catcher System In-beam Aerogel Catcher Halo Guard counter
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
Top view
12m downstream of main detector
Beam envelope
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

8. Location of Catcher System
Beam Catcher
Decay Region
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

9. Expected Performance by MC (1)
Aerogel Catcher Efficiency
Y position dependence
300MeV
Beam core
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

10. Expected Performance by MC (2)
Hit probability for Neutrons
Hit probability for KLs
Dominated by decays in the Catcher
800MeV
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

11. Signal Loss due to Catcher False Hit
Accidental hit due to neutrons may kill Kpnn signal
Neutron rate
False hit (rate x hit prob.)
False hit rate due to neutron
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…
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

12. 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)
Hit pattern of prompt beam g
Overlay on g event
Expected event map
This cluster survives
This cluster be killed
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

13. Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy
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
response to proton (as substitute for neutron)
Check single layer eff. / two-layers’ coincidence
Good agreement with MC (with gas scintillation)
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

14. Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy
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 300MeV)
Low fake hit probability due to neutrons Ekin=800MeV)
Signal loss due to false hit will be ~5%
Catcher efficiency loss due to blindness ~1%
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

15. 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
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy