1. 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

2. DayaBay Neutrino Experiment
DayaBay Neutrino Experiment is a precise measurement to with a near-far detector configuration.
Small-amplitude
oscillation due to 13
Determine with a sensitivity to 0.01 (90% C.L)
Large-amplitude
oscillation due to 12

4. Detector layout (Far site)
Inner Water Shield
Outer Water Shield
(separated by tyvek)
Water Pool
3-zone AD module
Target: Gd-doped scintillator
g-catcher: normal scintillator
Buffer: Oil

5. 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

6. 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.

7. 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 n-capture final state with multiple gammas. It is hard to give a general solution for all n-capture 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

8. 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 :

9. 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.

10. Other Optical Properties in AD
Detector simulation needs inputs of the optical properties of the detector components from measurements.
Light yield
Emission spectrum
Absorption length for GdLS/LS/Oil
Refractive index of acrylic vessel
Reflectivity of top/bottom reflectors
PMT Quantum Efficiency spectrum

11. Requirements on uncertainties necessary to achieve the 0
Requirements on uncertainties necessary to achieve the 0.01 sensitivity goal.

12. 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

13. 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

14. Reflector Simulation & Event Reconstruction
no reflector
simulation
with reflector
sE/E = 12%/E
sr = 13 cm
reconstruction

15. Prototype simulation v.s data
60Co
137Cs

16. Modified Gaisser Formula + MUSIC
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

17. IWS:
threshold 11PMT, efficiency 98.1%
Muon detection efficiency
OWS:
threshold 13 PMT, efficiency 97.7%

18. Energy spectrum of fast neutron backgrounds
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, (2001)).
Energy spectrum of fast neutron backgrounds

19. Muon capture Muons ( ) stop in water pool/antineutrino detector can
Decay -> Michel electron (lifetime )
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.

20. 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
neutron from Pu-C
252Cf prompt signal
252Cf delayed signal

21. Natural radioactivity simulation, give material specification for detector construction
From Stainless Steel tank
From GdLS (radioactivity accompany with Gd)
From PMT
From rocks or water (Radon) …

22. Software Framework
NuWa : our Gaudi-based offline software

23. 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