Simulation for DayaBay Detectors
Liangjian Wen
Institute of High Energy Physics
Sep 19, 2008
Topical Seminar on Frontier of Particle Physics 2008:
Neutrino Physics and Astrophysics
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DayaBay Neutrino Experiment
• DayaBay Neutrino Experiment is a precise measurement to
with a near-far detector configuration.
1.1
1
Nosc /Nno_osc
0.9
0.8
0.7
0.6
Small-amplitude
oscillation due to 13
0.5
Determine
with a
sensitivity to 0.01 (90% C.L)
0.4
0.3
0.1
Large-amplitude
oscillation due to 12
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10
Baseline (km)
100
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3
• Detector layout (Far site)
Water Pool
Inner Water Shield
Outer Water Shield
(separated by tyvek)
3-zone AD module
Target: Gd-doped scintillator
g-catcher: normal scintillator
Buffer: Oil
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DayaBay simulation
• Our simulation is based on Geant4 (G4dyb), but with
modifications and extensions to accommodate the
specific requirements of the DayaBay Experiment.
• We did many validations for our simulation by
 Comparison with other simulations (FLUKA, MCNPX, Geant3)
 Comparison with previous measurement data
 Comparison with our prototype experiment data
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Neutrino Detection at DayaBay
Inverse
reaction in Gadolinium-doped Liquid Scintillator (GdLS)
Two critical things in simulation
Neutron capture processes
Optical model of the detector.
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Neutron Capture Process
• We validate the neutron capture
cross section for Gd/H/C
targets in Geant4 with Geant3
simulation and experimental
data.
• Geant4 is incorrect for the ncapture final state with multiple
gammas. It is hard to give a
general solution for all ncapture targets in Geant4.
• We modified the G4 neutron
capture processes for Gd/C/H
targets, based on experimental
spectrum.
Simulated and measured
gamma energy spectrum for
n-Gd capture
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Scintillation Process: Quenching
• No quenching effect in G4Scintillation
• We write our own scintillation process with the
quenching effect be considered (J.B.Birks’ law):
• We measured the quenching factor for the
DayaBay GdLS and LS:
GdLS (ppo, bis-MSB, LAB, 0.1% Gd)
6.49(±1.06)
LS (ppo, bis-MSB, LAB)
8.21(±1.23)
unit :
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Scintillation Process: Re-emission
• Use the measured GdLS/LS emission
spectrum in scintillation processes
simulation
• Re-emission of Cerenkov light and
scintillation light are very important.
• Currently an assumed re-emission
possibility spectrum is used and we are
doing the measurement for our
GdLS/LS.
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Other Optical Properties in AD
• Detector simulation needs inputs of the optical properties of the
detector components from measurements.
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Light yield
Emission spectrum
Absorption length for GdLS/LS/Oil
Refractive index of acrylic vessel
Reflectivity of top/bottom reflectors
PMT Quantum Efficiency spectrum
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• Requirements on uncertainties necessary to achieve the 0.01
sensitivity goal.
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Positron & Neutron Efficiency
• Positron event：Evis>1MeV
 Efficiency = 99.8%
 Error ~ 0.05% assuming 2% energy scale
error
• Neutron event：Evis>6MeV
 Efficiency ~ 90.7% (overall)
 Error ~ 0.2% assuming 1% energy scale
error
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3 Zone detector
Neutron efficiency v.s LS thickness
42.5cm, 91%
Sensitivity v.s target mass
4x20 ton
15cm
Detector response for different
e- positions in detector
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Reflector Simulation & Event Reconstruction
no reflector
simulation
with reflector
sE/E = 12%/E
sr = 13 cm
reconstruction
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• Prototype simulation v.s data
60Co
137Cs
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Muon Simulation
Modified Gaisser Formula + MUSIC
Modified Gaisser Formula:
More reasonable muon flux parameterization at sea level
MUSIC (MUon SImulation Code) :
A three-dimensional code transports muons through the
rock to underground lab
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IWS:
threshold 11PMT,
efficiency 98.1%
Muon detection efficiency
OWS:
threshold 13 PMT,
efficiency 97.7%
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Neutron Simulation
• Muon-induced fast neutron background is an
important background. Its rates calculated
using Geant4 are consistent with earlier Geant3
calculations that used empirical
parametrization of measurements, accurate to
~20% (Y.Wang et al., PRD64, 013012 (2001)).
n
n
Energy spectrum of fast neutron backgrounds
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Muon capture
• Muons (
) stop in water pool/antineutrino detector can
 Decay -> Michel electron (lifetime 2.2 )
 Capture on C, O, Fe and emit a neutron (fast neutron background)
 Capture on C and form a 12B (delayed signal)
• Muon stopping ratio from Geant4 simulation is consistent with
FLUKA simulation.
• Its capture rate on C, O, Fe from Geant4 simulation is
consistent with experimental data.
• Yet the neutron energy spectra given by Geant4 is not in good
agreement with measurements. So we implemented new
neutron spectrum according to experimental data.
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Radioactivity Generator
• Full decay chains of U, Th, K
simulated to provide HEPevt input
to G4dyb ( U,Th using programs of
A.Piepke)
• 60Co simulation for calibration
source and background
• 68Ge, 252Cf and Pu-C simulation for
calibration sources
252Cf
neutron from Pu-C
prompt signal
252Cf
delayed signal
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• Natural radioactivity simulation, give material specification for
detector construction
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From Stainless Steel tank
From GdLS (radioactivity accompany with Gd)
From PMT
From rocks or water (Radon)
…
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Software Framework
NuWa : our Gaudi-based offline software
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Summary
• DayaBay simulation is based on Geant4, and we
made extensions and modifications to accommodate
the specific requirements of the DayaBay Experiment.
• Many validations have been done for current
simulation.
• Important simulations are based on G4dyb
 Positron and neutron efficiency and its error estimation
 Detector design related issues
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