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Demonstration of a Fast-neutron Detector
Ray Bunker—UCSB HEP
DUSEL AARM Collaboration Meeting
The Neutron Detector Collaboration
Dan Akerib
Mike Dragowsky
Chang Lee
Mani Tripathi
Melinda Sweany
Harry Nelson
Susanne Kyre & Dean White
Ray Bunker
Carsten Quinlan
Raul Hennings-Yeomans
Prisca Cushman
Jim Beaty
Joel Sander
with support from the NSF DUSEL R&D program
&
thanks to the Department of Natural Resources & the staff of the Soudan Underground Laboratory!
A Fast-neutron Detector—The Signal
Design based on Hennings & Akerib, NIM A 574 (2007) 89-97
Light-tight Enclosure
High-energy Neutron
20” Hamamatsu PMT
2” Top Lead Shield
2” Side Lead Shield
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Capture on Gadolinium
8 MeV Gamma Cascades
Over 10’s of s
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~2.2 Metric Ton
Water Tank
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20 Ton Lead Target
Liberated
HadronicNeutrons
Shower
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A Fast-neutron Detector—The Signal
100 MeV Neutron Beam
Detector Outline
Sitting atop Pb Target
Expected Number of sub-10 MeV
Detectable Secondary Neutrons
FLUKA-simulated Hadronic Shower & Neutron Production by Raul Hennings-Yeomans
2/25/2011
Ray Bunker-UCSB HEP
4
A Fast-neutron Detector—Signal Event
Clustered Pulse Train
Relatively Large
Coincident
Pulse Heights
2/25/2011
Ray Bunker-UCSB HEP
5
A Fast-neutron Detector—Principle Background
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2/25/2011
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Accidentally Coincident
U/Th Gammas
2.6 MeV Endpoint
6
A Fast-neutron Detector—Background Event
Relatively Small
Coincident
Pulse Heights
South Tank
PMT Signals
Usually Spread
Between Tanks
Truly Random Timing
North Tank
PMT Signals
2/25/2011
7
A Fast-neutron Detector—Signal vs. Background
Primary Discriminator Based on Pulse Height
• U/Th gammas < ~50 mV
Measured U/Th
Response
• Gd capture gammas > ~50 mV
Gd Capture Response
Calibrated with
252Cf Fission Neutrons
Additional Discrimination Based on Pulse Timing
• ~½ kHz U/Th gammas
 characteristic time ~2 ms
• Gd capture time depends on
concentration
 characteristic time ~10 s
South Tank
0.2% Gd
• Gd captures cluster toward
beginning of event:

P(t , N ) ~ e
 effective ~
2/25/2011
t ( N 1)
 capture
 capture
North Tank
0.4% Gd
N 1
Ray Bunker-UCSB HEP
8
A Fast-neutron Detector—Signal vs. Background
More Neutron Like
More Gamma Like
Pulse timing Likelihood
Background U/Th
Gamma Rays
252Cf
Fission Neutrons
Pulse Height Likelihood
2/25/2011
Ray Bunker-UCSB HEP
9
A Fast-neutron Detector—GEANT4 Optical Properties
• Muons are an excellent source of Cherenkov photons—illuminate entire detector
• Use to tune MC optical properties for:
Combination of Muon Spectral Shape
Backup slides—ask me later&ifWest-East
interested Pulse Height Asymmetry
• Amino-g wavelength shifter
Used to Break Degeneracy of
Reflector’s
~150
MeV Optical Properties
• Scintered halon reflective panels
• Water
Muon Peak
95% Diffuse + 5% Specular Spike
for Best Agreement with Data
Event rate (arbitrary units)
94% Total Reflectivity for
Best Agreement with Data
2/25/2011
Stopping Muon
Decay e
50 MeV Endpoint
Ray Bunker-UCSB HEP
Pulse height (V)
10
A Fast-neutron Detector—Simulated Neutron Response
Estimated 252Cf Fission Neutrons:
• Monoenergetic 5 MeV neutrons
• Multiplicity pulled from Gaussian centered at 3.87 ( of 1.6)
Single Neutron Capture Response
Event rate (normalized)
Monte Carlo—Solid Black
Data—Shaded Red
2.5 mV/photoelectron
Scaling Required to
Match MC to Data
Implies ~½ MeV
Detection Threshold
Pulse height (mV)
2/25/2011
Ray Bunker-UCSB HEP
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A Fast-neutron Detector—Simulated Gamma-ray Response
• 1.17 & 1.33 MeV gammas from 60Co (often observe both simultaneously)
• 2.5 mV/PE 252Cf scaling applied
Event rate (normalized)
• Additional resolution required for agreement
 Gaussian smear with energy-dependent width,  ~ 0.9*sqrt(pulse height)
2/25/2011
Monte Carlo—Solid Black
Data—Shaded Red
Pulse height (mV)
12
A Fast-neutron Detector—Simulated U/Th Background Response
• Throw Ruddick spectrum from cavern walls
• Apply scaling and energy-dependent smearing indicated by 252Cf and 60Co
 Ruddick spectrum is softer than observed data
• Enhancing 2.6 MeV endpoint resolves discrepancy
 Implies that cavern/materials near detector have 40% more Thorium in U/Th ratio
GEANT
Event
(normalized)
rate (normalized)
Event rate
Gammas/second/sq.m
Keith Ruddick 1996-NuMI-L-210
Pulse height
(mV) (mV)
Pulse height
Gamma energy (MeV)
2/25/2011
Monte Carlo—Black
Monte Carlo—Solid
Black
Data—Red
Data—Shaded
Red
Ray Bunker-UCSB HEP
13
A Fast-neutron Detector—Concluding Remarks
• Constructed a water Cherenkov, Gd-loaded high-energy neutron detector
• Response to U/Th & 60Co gammas, muons, and 252Cf fission neutrons understood via GEANT4
• Demonstrated ability to separate signal from background
• Have operated in Soudan Mine since November 2009... calibration + neutron-search data
• Rough analysis of search data shows a clear excess of high-multiplicity events!
• Goals:
• Absolute flux measurement & Monte Carlo Benchmarking: MCNP, FLUKA, GEANT4, …
• Unfold energy spectrum from multiplicity distribution
Background
Signal
2/25/2011
Ray Bunker-UCSB HEP
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A Fast-neutron Detector—Multiplicity = Energy?
FLUKA Demonstration of
Secondary Neutron Multiplicity
Dependence on Energy of Primary
Raul Hennings-Yeomans
Underground Neutron Flux
Mei & Hime Phys. Rev. D73 (2006)
?
?
2/25/2011
15
A Fast-neutron Detector—Multiplicity 27 Candidate
Event # 2314 from 2nd Fast-neutron Run: South Tank PMT Traces
Pulse height (volts)
— Channel 1—South East PMT
2/25/2011
— Channel 2—South West PMT
Triggering
Pulses
Ray Bunker-UCSB HEP
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A Fast-neutron Detector—Installation
Electronics Rack
Source Tubes
2/25/2011
Ray Bunker-UCSB HEP
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A Fast-neutron Detector—Installation
Cheap Labor
Water Tanks
2/25/2011
Ray Bunker-UCSB HEP
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A Fast-neutron Detector—Installation
20” KamLAND
Phototubes
2/25/2011
Ray Bunker-UCSB HEP
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A Fast-neutron Detector—Muon Response
• Large dE/dx events (>80% of all recorded events)
• Large initial pulse with prominent after pulsing
• Large individual channel multiplicities, but few coincidences
2/25/2011
Ray Bunker-UCSB HEP
20
A Fast-neutron Detector—GEANT4 Optical Properties of Water
Water absorption and refractive index taken from LUXSim package:
Refraction  The equation for the refractive index is evaluated by D. T. Huibers, 'Models for the wavelength dependence of the index
of refraction of water', Applied Optics 36 (1997) p.3785. The original equation comes from X. Qua and E. S. Fry, 'Empirical
equation for the index of refraction of seawater", Applied Optics 34 (1995) p.3477.
Absorption:
• 200-320 nm: T.I. Quickenden & J.A. Irvin, 'The ultraviolet absorption spectrum of liquid water', J. Chem. Phys. 72(8) (1980) p4416.
• 330 nm: A rough average between 320 and 340 nm. Very subjective.
• 340-370 nm: F.M. Sogandares and E.S. Fry, 'Absorption spectrum (340-640 nm) of pure water. Photothermal measurements',
Applied Optics 36 (1997) p.8699.
• 380-720 nm: R.M. Pope and E.S. Fry, 'Absorption spectrum (380-700 nm) of pure water. II. Integrating cavity measurements',
Applied Optics 36 (1997) p.8710.
A Fast-neutron Detector—GEANT4 Optical Properties
20” KamLAND Phototubes
(~17” photocathode)
~20% Peak Quantum Efficiency
Amino-g Wavelength Shifter
Absorbs UV, Emits Blue
(most Cherenkov photons are UV)
>2 Increase in Light Yield
2/25/2011
Ray Bunker-UCSB HEP
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