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

2. 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!

3. 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

4. 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

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

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

7. 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

8. 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:

9. 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

10. 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

11. 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

12. 2/25/201112 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)

13. 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

14. 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 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

15. 2/25/201115 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 ? ?

16. 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

17. 2/25/2011Ray Bunker-UCSB HEP17 Electronics Rack Source Tubes A Fast-neutron Detector—Installation

18. 2/25/2011Ray Bunker-UCSB HEP18 Water Tanks Cheap Labor A Fast-neutron Detector—Installation

19. 2/25/2011Ray Bunker-UCSB HEP19 20” KamLAND Phototubes A Fast-neutron Detector—Installation

20. 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

21. 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.

22. 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