the neutron multiplicity meter at soudan ray bunker—syracuse university aarm collaboration meeting...

The Neutron Multiplicity Meter at Soudan

Ray Bunker—Syracuse University

AARM Collaboration MeetingJune 22–23, 2012

With support from the NSF DUSEL R&D program & AARM, and thanks to theMinnesota Department of Natural Resources & the staff of the Soudan Underground Laboratory!

Harry NelsonSusanne Kyre

Carsten QuinlanDean White

Prisca CushmanJim Beaty

Anthony Villano

Mani Tripathi

The Neutron Multiplicity Meter (NMM) Collaboration

Raul Hennings-Yeomans

Joel Sander

Dan AkeribMike Dragowsky

Chang Lee

Melinda Sweany

SYRACUSE UNIVERSITY

Richard SchneeRay Bunker

Yu Chen

Ray Bunker-Syracuse University 3

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

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The Neutron Multiplicity Meter


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 neutron production taken fromR. Hennings-Yeomans and D.S. Akerib, NIM A574 (2007) 89

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Clustered Pulse Train

NMM Candidate Signal Event

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Relatively LargeCoincident

Pulse Heights


Principle Neutron-detection Background

Accidentally Coincident

U/Th Gammas2.6 MeV Endpoint

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Ray Bunker-Syracuse University 76/22/2012

South TankPMT Signals

North TankPMT Signals

Relatively SmallCoincident

Pulse Heights

Truly Random Timing

Usually SpreadBetween Tanks

NMM Background Event


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 Discrimination

More Neutron LikeMore Gamma Like

106/22/2012Pulse-height Likelihood (-log of likelihood ratio)

Puls

e-tim

ing

Like

lihoo

d

252Cf Fission Neutrons U/Th BackgroundGamma Rays

More Neutron Like More Gamma Like

Combined Timing & Pulse-height Discrimination

-25 -20 -15 -10 -5 0 5 10

100

50

0

-50

Geant4 NMM Detector Model

Pulse height (mV)

Even

t rat

e (n

orm

aliz

ed)

Monte Carlo—Solid BlackData—Shaded Red

Background Gammas from U/Th

Pulse height (mV)

Even

t rat

e (n

orm

aliz

ed)


Calibration Gammas from 60Co

Pulse height (mV)

Even

t rat

e (n

orm

aliz

ed)


Calibration Neutrons from 252Cf

Pulse height (V)

Even

t rat

e (a

rb. u

nits

)


Muons and Michele Electrons


Constraining the Underground Flux of High-energy Neutrons

Throw Mei & Hime parameterized distribution of neutron energies:(see, e.g., D.-M. Mei and A. Hime. Phys. Rev. D73 (2006) 053004)

Compare secondary-neutron multiplicity distributions for events accepted by Geant4 detector model to actual events from ~6 months of data:

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Constraining the Flux via a Top-Down Simulation

• Propagate muons using MUSIC/MUSUN in 2-meter shell of rock surrounding Soudan experimental hall

• Use Geant4.9.5.r00 with updated μ-nuclear interactions (shielding physics list) to produce high-energy neutrons entering Soudan cavern

• Measure multiplicity-meter response with well-developed & calibrated detector model

• Compare to ~1 year’s worth of data (now in hand) recorded by multiplicity meter, searching for candidate events with more advanced likelihood-based analysis

Ray Bunker-Syracuse University

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Additional Studies via Correlationswith the Soudan LBCF Muon Shield

Recently instrumented acquisitionof veto-shield trigger signals• Further reject backgrounds• Umbrella-veto effectiveness• High-energy neutron event topology

Veto ShieldProportional Tubes


Source Tubes

The NMM Installation

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The Neutron Multiplicity Meter—Concluding Remarks

• The underground flux of cosmogenically induced neutrons is an important background for a variety of next-generation rare-event searches, but it is not yet accurately characterized by current simulations

• A high-energy neutron detector with sensitivity to neutron energies ≳40 MeV has been successfully installed underground at the Soudan Mine (late 2009)

• Preliminary analysis of ~6 months worth of data indicates larger than expected neutron flux relative to Mei & Hime parameterization.

• A full, top-down simulation of neutron production and subsequent NMM detection is under way with updated Geant4 physics

• A more sophisticated likelihood-based event selection is being developed for analysis of full year’s worth of data

• Correlated operations with the LBCF muon shield are under way, allowing for a more detailed investigation of muons and showers associated with high-energy neutron production


Backup Slides


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

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

NMM Muon Response


NMM Geant4 Detector Model—Optical Properties

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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Absorption & Emission Spectra forAmino G Wavelength Shifter

Wavelength (nm)Wavelength (nm)

Prob

abili

ty (%

)

20” PMT Quantum Efficiency



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

the neutron multiplicity meter at soudan ray bunker—syracuse university aarm collaboration meeting...

Documents

pulse timing

pulse height uth gammas

pulseheight likelihood

mv gd capture gammas

ms gd capture time

lead shield2

gd primary discriminator

geant4 detector model