Demonstration of a Fastneutron Detector Ray BunkerUCSB HEP

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 Capture on Gadolinium 8 Me. V Gamma Cascades Over 10’s of s ~2. 2 Metric Ton Water Tank 20 Ton Lead Target Liberated Hadronic. Neutrons Shower 3

A Fast-neutron Detector—The Signal 100 Me. V Neutron Beam Detector Outline Sitting atop Pb Target Expected Number of sub-10 Me. V 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 2/25/2011 Accidentally Coincident U/Th Gammas 2. 6 Me. V 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 m. V Measured U/Th Response • Gd capture gammas > ~50 m. V Gd Capture Response Calibrated with 252 Cf Fission Neutrons Additional Discrimination Based on Pulse Timing • ~½ k. Hz 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: North Tank 0. 4% Gd 2/25/2011 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 252 Cf Fission Neutrons Pulse Height Likelihood 2/25/2011 Ray Bunker-UCSB HEP 9

A Fast-neutron Detector—GEANT 4 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&if. West-East interested Pulse Height Asymmetry • Amino-g wavelength shifter Used to Break Degeneracy of Reflector’s ~150 Me. V 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 Me. V Endpoint Ray Bunker-UCSB HEP Pulse height (V) 10

A Fast-neutron Detector—Simulated Neutron Response Estimated 252 Cf Fission Neutrons: • Monoenergetic 5 Me. V 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 m. V/photoelectron Scaling Required to Match MC to Data Implies ~½ Me. V Detection Threshold Pulse height (m. V) 2/25/2011 Ray Bunker-UCSB HEP 11

A Fast-neutron Detector—Simulated Gamma-ray Response • 1. 17 & 1. 33 Me. V gammas from 60 Co (often observe both simultaneously) • 2. 5 m. V/PE 252 Cf 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 (m. V) 12

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 Me. V endpoint resolves discrepancy Implies that cavern/materials near detector have 40% more Thorium in U/Th ratio GEANT 2/25/2011 Gamma energy (Me. V) Event rate (normalized) Gammas/second/sq. m Keith Ruddick 1996 -Nu. MI-L-210 Monte Carlo—Black Monte Carlo—Solid Black Data—Red Data—Shaded Red Pulse height (m. V) Pulse height 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 & 60 Co gammas, muons, and 252 Cf fission neutrons understood via GEANT 4 • 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, GEANT 4, … • Unfold energy spectrum from multiplicity distribution Background Signal 2/25/2011 Ray Bunker-UCSB HEP 14

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. D 73 (2006) ? 2/25/2011 ? 15

A Fast-neutron Detector—Multiplicity 27 Candidate Event # 2314 from 2 nd 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 16

A Fast-neutron Detector—Installation Electronics Rack Source Tubes 2/25/2011 Ray Bunker-UCSB HEP 17

A Fast-neutron Detector—Installation Cheap Labor Water Tanks 2/25/2011 Ray Bunker-UCSB HEP 18

A Fast-neutron Detector—Installation 20” Kam. LAND Phototubes 2/25/2011 Ray Bunker-UCSB HEP 19

A Fast-neutron Detector—Muon Response • Large d. E/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—GEANT 4 Optical Properties of 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) p 4416. • 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—GEANT 4 Optical Properties 20” Kam. LAND 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 22