Demonstration of a Fast-neutron Detector

Ray Bunker—UCSB HEP

DUSEL AARM Collaboration Meeting

The Neutron Detector Collaboration

Raul Hennings-Yeomans

Joel Sander

Mani TripathiMelinda Sweany

Prisca CushmanJim Beaty

Harry NelsonSusanne Kyre & Dean White

Ray BunkerCarsten Quinlan

Dan AkeribMike Dragowsky

Chang Lee

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

High-energy Neutron

Hadronic ShowerLiberated Neutrons

Capture on Gadolinium8 MeV Gamma Cascades

Over 10’s of s

Light-tight Enclosure

20” Hamamatsu PMT

2” Top Lead Shield

2” Side Lead Shield

~2.2 Metric TonWater Tank

20 Ton Lead Target

3

Design based on Hennings & Akerib, NIM A 574 (2007) 89-97

A Fast-neutron Detector—The Signal

100 MeV Neutron Beam

Detector OutlineSitting atop Pb Target

Expected Number of sub-10 MeVDetectable Secondary Neutrons

FLUKA-simulated Hadronic Shower & Neutron Production by Raul Hennings-Yeomans

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Clustered Pulse Train

A Fast-neutron Detector—Signal Event

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Relatively LargeCoincident

Pulse Heights

A Fast-neutron Detector—Principle Background

Accidentally Coincident

U/Th Gammas2.6 MeV Endpoint

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South TankPMT Signals

North TankPMT Signals

Relatively SmallCoincident

Pulse Heights

Truly Random Timing

Usually SpreadBetween Tanks

A Fast-neutron Detector—Background Event

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A Fast-neutron Detector—Signal vs. Background

Gd Capture ResponseCalibrated with

252Cf Fission Neutrons

Measured U/ThResponse

North Tank0.4% Gd

South Tank0.2% Gd

capture

Nt

eNtP

)1(

~),(

1~ N

captureeffective

Primary Discriminator Based on Pulse Height

• U/Th gammas < ~50 mV • Gd capture gammas > ~50 mV

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 • Gd captures cluster toward beginning of event:

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Pulse Height Likelihood

Puls

e tim

ing

Like

lihoo

d

252Cf Fission Neutrons

Background U/ThGamma Rays

More Neutron Like More Gamma Like

A Fast-neutron Detector—Signal vs. Background

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A Fast-neutron Detector—GEANT4 Optical Properties

Pulse height (V)

Even

t rat

e (a

rbitr

ary

units

)

~150 MeVMuon Peak

Stopping MuonDecay e

50 MeV Endpoint

• Muons are an excellent source of Cherenkov photons—illuminate entire detector

• Use to tune MC optical properties for:

• Water

• Amino-g wavelength shifter

• Scintered halon reflective panels

Backup slides—ask me later if interested

Combination of Muon Spectral Shape& West-East Pulse Height Asymmetry

Used to Break Degeneracy of Reflector’s Optical Properties

95% Diffuse + 5% Specular Spikefor Best Agreement with Data

94% Total Reflectivity forBest Agreement with Data

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A Fast-neutron Detector—Simulated Neutron Response

Pulse height (mV)

Even

t rat

e (n

orm

aliz

ed)

Monte Carlo—Solid BlackData—Shaded Red

Estimated 252Cf Fission Neutrons:

• Monoenergetic 5 MeV neutrons• Multiplicity pulled from Gaussian centered at 3.87 ( of 1.6)

2.5 mV/photoelectronScaling Required toMatch MC to Data

Implies ~½ MeVDetection Threshold

Single Neutron Capture Response

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A Fast-neutron Detector—Simulated Gamma-ray Response

Pulse height (mV)

Even

t rat

e (n

orm

aliz

ed)

Monte Carlo—Solid BlackData—Shaded Red

• 1.17 & 1.33 MeV gammas from 60Co (often observe both simultaneously)

• 2.5 mV/PE 252Cf scaling applied

• Additional resolution required for agreement Gaussian smear with energy-dependent width, ~ 0.9*sqrt(pulse height)

Gam

mas

/sec

ond/

sq.m

Gamma energy (MeV)

Keith Ruddick 1996-NuMI-L-210

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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

Pulse height (mV)

Even

t rat

e (n

orm

aliz

ed)

Monte Carlo—Solid BlackData—Shaded Red

GEANT

Pulse height (mV)

Even

t rat

e (n

orm

aliz

ed)

Monte Carlo—BlackData—Red

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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

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Underground Neutron FluxMei & Hime Phys. Rev. D73 (2006)

A Fast-neutron Detector—Multiplicity = Energy?

FLUKA Demonstration ofSecondary Neutron Multiplicity

Dependence on Energy of PrimaryRaul Hennings-Yeomans

??

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A Fast-neutron Detector—Multiplicity 27 Candidate

Event # 2314 from 2nd Fast-neutron Run: South Tank PMT Traces

TriggeringPulses

Puls

e he

ight

(vol

ts)

— Channel 1—South East PMT

— Channel 2—South West PMT

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Electronics Rack

Source Tubes

A Fast-neutron Detector—Installation

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Water Tanks

Cheap Labor

A Fast-neutron Detector—Installation

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20” KamLANDPhototubes

A Fast-neutron Detector—Installation

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• Large dE/dx events (>80% of all recorded events)

• Large initial pulse with prominent after pulsing• Large individual channel multiplicities, but few coincidences

A Fast-neutron Detector—Muon Response

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.

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A Fast-neutron Detector—GEANT4 Optical Properties

Amino-g Wavelength ShifterAbsorbs UV, Emits Blue

(most Cherenkov photons are UV)>2 Increase in Light Yield

20” KamLAND Phototubes(~17” photocathode)

~20% Peak Quantum Efficiency