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

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

2. 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 Joel Sander Prisca Cushman Jim Beaty 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 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 ~2.2 Metric Ton Water Tank       Capture on Gadolinium 8 MeV Gamma Cascades Over 10’s of s     20 Ton Lead Target   Liberated Neutrons Hadronic Shower

4. A Fast-neutron Detector—The Signal 100 MeV Neutron Beam Expected Number of sub-10 MeV Detectable Secondary Neutrons Detector Outline Sitting atop Pb Target FLUKA-simulated Hadronic Shower & Neutron Production by Raul Hennings-Yeomans Ray Bunker-UCSB HEP

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

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

7. 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

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

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

10. A Fast-neutron Detector—GEANT4 Optical Properties • Muons are an excellent source of Cherenkov photons—illuminate entire detector • Use to tune MC optical properties for: • Water • Amino-g wavelength shifter • Scinteredhalon reflective panels Combination of Muon Spectral Shape & West-East Pulse Height Asymmetry Used to Break Degeneracy of Reflector’s Optical Properties Backup slides—ask me later if interested ~150 MeV Muon Peak 95% Diffuse + 5% Specular Spike for Best Agreement with Data 94% Total Reflectivity for Best Agreement with Data Event rate (arbitrary units) Pulse height (V) Stopping Muon Decay e 50 MeV Endpoint Ray Bunker-UCSB HEP

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

12. A Fast-neutron Detector—Simulated Gamma-ray Response • 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) Monte Carlo—Solid Black Data—Shaded Red Event rate (normalized) Pulse height (mV)

13. 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 Event rate (normalized) Keith Ruddick 1996-NuMI-L-210 Monte Carlo—Solid Black Data—Shaded Red Pulse height (mV) Gammas/second/sq.m GEANT Event rate (normalized) Monte Carlo—Black Data—Red Pulse height (mV) Gamma energy (MeV) Ray Bunker-UCSB HEP

14. 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 Ray Bunker-UCSB HEP

15. 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. D73 (2006) ? ?

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

17. A Fast-neutron Detector—Installation Electronics Rack Source Tubes Ray Bunker-UCSB HEP

18. A Fast-neutron Detector—Installation Cheap Labor Water Tanks Ray Bunker-UCSB HEP

19. A Fast-neutron Detector—Installation 20” KamLAND Phototubes Ray Bunker-UCSB HEP

20. A Fast-neutron Detector—Muon Response • Large dE/dx events (>80% of all recorded events) • Large initial pulse with prominent after pulsing • Large individual channel multiplicities, but few coincidences Ray Bunker-UCSB HEP

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