The Neutron Multiplicity Meter at Soudan Ray Bunker—Syracuse University AARM Collaboration Meeting

1. The Neutron Multiplicity Meter at Soudan Ray Bunker—Syracuse University AARM Collaboration Meeting June 22–23, 2012

2. The Neutron Multiplicity Meter (NMM) Collaboration Dan Akerib Mike Dragowsky Chang Lee Mani Tripathi Raul Hennings-Yeomans Harry Nelson Susanne Kyre Carsten Quinlan Dean White Melinda Sweany SYRACUSE UNIVERSITY Richard Schnee Ray Bunker Yu Chen Joel Sander Prisca Cushman Jim Beaty Anthony Villano With support from the NSF DUSEL R&D program & AARM, and thanks to the Minnesota Department of Natural Resources & the staff of the Soudan Underground Laboratory!

3. The Neutron Multiplicity Meter High-energy Neutron Light-tight Enclosure 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 Ray Bunker-Syracuse University

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 neutron production taken from R. Hennings-Yeomans and D.S. Akerib, NIM A574 (2007) 89 Ray Bunker-Syracuse University

5. NMM Candidate Signal Event Clustered Pulse Train Relatively Large Coincident Pulse Heights Ray Bunker-Syracuse University

6. Principle Neutron-detection Background     Accidentally Coincident U/Th Gammas 2.6 MeV Endpoint  Ray Bunker-Syracuse University

7. NMM Background Event Relatively Small Coincident Pulse Heights South Tank PMT Signals Usually Spread Between Tanks Truly Random Timing North Tank PMT Signals Ray Bunker-Syracuse University

8. 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-Syracuse University

9. Pulse-height Discrimination More Gamma Like More Neutron Like

10. Combined Timing & Pulse-height Discrimination 100 More Neutron Like More Gamma Like 50 U/ThBackground Gamma Rays 252Cf Fission Neutrons Pulse-timing Likelihood 0 -50 -10 -20 -15 10 -25 0 5 -5 Pulse-height Likelihood (-log of likelihood ratio)

11. Geant4 NMM Detector Model Background Gammas from U/Th Calibration Neutrons from 252Cf Monte Carlo—Solid Black Data—Shaded Red Monte Carlo—Solid Black Data—Shaded Red Event rate (normalized) Event rate (normalized) Pulse height (mV) Pulse height (mV) Muons and Michele Electrons Calibration Gammas from 60Co Monte Carlo—Solid Black Data—Shaded Red Monte Carlo—Solid Black Data—Shaded Red Event rate (arb. units) Event rate (normalized) Pulse height (V) Pulse height (mV)

12. 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: Ray Bunker-Syracuse University

13. 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

14. Additional Studies via Correlations with the Soudan LBCF Muon Shield • Recently instrumented acquisition • of veto-shield trigger signals • Further reject backgrounds • Umbrella-veto effectiveness • High-energy neutron event topology Veto Shield Proportional Tubes

15. The NMM Installation Source Tubes Ray Bunker-Syracuse University

16. The NMM Installation Ray Bunker-Syracuse University

17. The NMM Installation Ray Bunker-Syracuse University

18. The NMM Installation Ray Bunker-Syracuse University

19. The Neutron Multiplicity Meter—Concluding Remarks • The underground flux of cosmogenicallyinduced 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 MeVhas 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 Ray Bunker-Syracuse University

20. Backup Slides Ray Bunker-Syracuse University

21. NMM 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-Syracuse University

22. 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. Ray Bunker-Syracuse University

23. NMM Geant4 Detector Model—Optical Properties Absorption & Emission Spectra for Amino G Wavelength Shifter 20” PMT Quantum Efficiency Probability (%) Wavelength (nm) Wavelength (nm) Ray Bunker-Syracuse University

24. NMM Geant4 Detector Model—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-Syracuse University