Demonstration of a Fast-neutron Detector Ray Bunker—UCSB HEP DUSEL AARM Collaboration Meeting

The Neutron Detector Collaboration Raul Hennings-Yeomans Joel Sander Mani Tripathi Melinda Sweany Prisca Cushman Jim Beaty Harry Nelson Susanne Kyre & Dean White Ray Bunker Carsten Quinlan Dan Akerib Mike 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 Shower Liberated Neutrons Capture on Gadolinium 8 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 Ton Water 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 Outline Sitting atop Pb Target Expected Number of sub-10 MeV Detectable Secondary Neutrons FLUKA-simulated Hadronic Shower & Neutron Production by Raul Hennings-Yeomans 2/25/20114Ray Bunker-UCSB HEP

Clustered Pulse Train A Fast-neutron Detector—Signal Event 2/25/20115Ray Bunker-UCSB HEP Relatively Large Coincident Pulse Heights

A Fast-neutron Detector—Principle Background      Accidentally Coincident U/Th Gammas 2.6 MeV Endpoint 2/25/20116

7 South Tank PMT Signals North Tank PMT Signals Relatively Small Coincident Pulse Heights Truly Random Timing Usually Spread Between Tanks A Fast-neutron Detector—Background Event

2/25/2011Ray Bunker-UCSB HEP8 A Fast-neutron Detector—Signal vs. Background Gd Capture Response Calibrated with 252 Cf Fission Neutrons Measured U/Th Response North Tank 0.4% Gd South Tank 0.2% Gd 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:

2/25/2011Ray Bunker-UCSB HEP9 Pulse Height Likelihood Pulse timing Likelihood 252 Cf Fission Neutrons Background U/Th Gamma Rays More Neutron LikeMore Gamma Like A Fast-neutron Detector—Signal vs. Background

2/25/2011Ray Bunker-UCSB HEP10 A Fast-neutron Detector—GEANT4 Optical Properties Pulse height (V) Event rate (arbitrary units) ~150 MeV Muon Peak Stopping Muon Decay 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 Spike for Best Agreement with Data 94% Total Reflectivity for Best Agreement with Data

2/25/2011Ray Bunker-UCSB HEP11 A Fast-neutron Detector—Simulated Neutron Response Pulse height (mV) Event rate (normalized) Monte Carlo—Solid Black Data—Shaded Red Estimated 252 Cf Fission Neutrons: Monoenergetic 5 MeV neutrons Multiplicity pulled from Gaussian centered at 3.87 (  of 1.6) 2.5 mV/photoelectron Scaling Required to Match MC to Data Implies ~½ MeV Detection Threshold Single Neutron Capture Response

2/25/ A Fast-neutron Detector—Simulated Gamma-ray Response Pulse height (mV) Event rate (normalized) Monte Carlo—Solid Black Data—Shaded Red 1.17 & 1.33 MeV gammas from 60 Co (often observe both simultaneously) 2.5 mV/PE 252 Cf scaling applied Additional resolution required for agreement  Gaussian smear with energy-dependent width,  ~ 0.9*sqrt(pulse height)

Gammas/second/sq.m Gamma energy (MeV) Keith Ruddick 1996-NuMI-L-210 2/25/2011Ray Bunker-UCSB HEP13 A Fast-neutron Detector—Simulated U/Th Background Response Throw Ruddick spectrum from cavern walls Apply scaling and energy-dependent smearing indicated by 252 Cf and 60 Co  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) Event rate (normalized) Monte Carlo—Solid Black Data—Shaded Red GEANT Pulse height (mV) Event rate (normalized) Monte Carlo—Black Data—Red

2/25/2011Ray Bunker-UCSB HEP14 A Fast-neutron Detector—Concluding Remarks Constructed a water Cherenkov, Gd-loaded high-energy neutron detector Response to U/Th & 60 Co gammas, muons, and 252 Cf fission neutrons understood via GEANT4 Demonstrated ability to separate signal from background Have operated in Soudan Mine since November 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/ Underground Neutron Flux Mei & Hime Phys. Rev. D73 (2006) A Fast-neutron Detector—Multiplicity = Energy? FLUKA Demonstration of Secondary Neutron Multiplicity Dependence on Energy of Primary Raul Hennings-Yeomans ? ?

2/25/2011Ray Bunker-UCSB HEP16 A Fast-neutron Detector—Multiplicity 27 Candidate Event # 2314 from 2 nd Fast-neutron Run: South Tank PMT Traces Triggering Pulses Pulse height (volts) — Channel 1—South East PMT — Channel 2—South West PMT

2/25/2011Ray Bunker-UCSB HEP17 Electronics Rack Source Tubes A Fast-neutron Detector—Installation

2/25/2011Ray Bunker-UCSB HEP18 Water Tanks Cheap Labor A Fast-neutron Detector—Installation

2/25/2011Ray Bunker-UCSB HEP19 20” KamLAND Phototubes A Fast-neutron Detector—Installation

2/25/2011Ray Bunker-UCSB HEP20 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 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 Absorption: nm: T.I. Quickenden & J.A. Irvin, 'The ultraviolet absorption spectrum of liquid water', J. Chem. Phys. 72(8) (1980) p nm: A rough average between 320 and 340 nm. Very subjective nm: F.M. Sogandares and E.S. Fry, 'Absorption spectrum ( nm) of pure water. Photothermal measurements', Applied Optics 36 (1997) p nm: R.M. Pope and E.S. Fry, 'Absorption spectrum ( nm) of pure water. II. Integrating cavity measurements', Applied Optics 36 (1997) p.8710.

2/25/2011Ray Bunker-UCSB HEP22 A Fast-neutron Detector—GEANT4 Optical Properties Amino-g Wavelength Shifter Absorbs UV, Emits Blue (most Cherenkov photons are UV) >2 Increase in Light Yield 20” KamLAND Phototubes (~17” photocathode) ~20% Peak Quantum Efficiency