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

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! Harry Nelson Susanne Kyre Carsten Quinlan Dean White Prisca Cushman Jim Beaty Anthony Villano Mani Tripathi The Neutron Multiplicity Meter (NMM) Collaboration Raul Hennings-Yeomans Joel Sander Dan Akerib Mike Dragowsky Chang Lee Melinda Sweany SYRACUSE UNIVERSITY Richard Schnee Ray Bunker Yu Chen

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 6/22/2012Ray Bunker-Syracuse University3 The Neutron Multiplicity Meter

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 neutron production taken from R. Hennings-Yeomans and D.S. Akerib, NIM A574 (2007) 89 6/22/2012Ray Bunker-Syracuse University4

Clustered Pulse Train NMM Candidate Signal Event 6/22/2012Ray Bunker-Syracuse University Relatively Large Coincident Pulse Heights 5

Principle Neutron-detection Background      Accidentally Coincident U/Th Gammas 2.6 MeV Endpoint 6/22/2012 Ray Bunker-Syracuse University6

6/22/2012 South Tank PMT Signals North Tank PMT Signals Relatively Small Coincident Pulse Heights Truly Random Timing Usually Spread Between Tanks NMM Background Event Ray Bunker-Syracuse University7

6/22/2012Ray Bunker-Syracuse University 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: 8

6/22/20129 Pulse-height Discrimination More Neutron LikeMore Gamma Like

6/22/2012 Pulse-height Likelihood (-log of likelihood ratio) Pulse-timing Likelihood 252 Cf Fission Neutrons U/Th Background Gamma Rays More Neutron Like More Gamma Like Combined Timing & Pulse-height Discrimination

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

6/22/2012Ray Bunker-Syracuse University12 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) ) Compare secondary-neutron multiplicity distributions for events accepted by Geant4 detector model to actual events from ~6 months of data:

6/22/ 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

6/22/ 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

6/22/2012Ray Bunker-Syracuse University Source Tubes The NMM Installation 15

6/22/2012Ray Bunker-Syracuse University The NMM Installation 16

6/22/2012Ray Bunker-Syracuse University17 The NMM Installation

6/22/2012Ray Bunker-Syracuse University18 The NMM Installation

6/22/2012Ray Bunker-Syracuse University19 The Neutron Multiplicity Meter—Concluding Remarks The underground flux of cosmogenically induced 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 MeV has 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

6/22/2012Ray Bunker-Syracuse University20 Backup Slides

6/22/2012Ray Bunker-Syracuse University Large dE/dx events (>80% of all recorded events) Large initial pulse with prominent after pulsing Large individual channel multiplicities, but few coincidences NMM Muon Response 21

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 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 /22/2012Ray Bunker-Syracuse University22

6/22/2012Ray Bunker-Syracuse University NMM Geant4 Detector Model—Optical Properties Absorption & Emission Spectra for Amino G Wavelength Shifter Wavelength (nm) Probability (%) 20” PMT Quantum Efficiency 23

6/22/2012Ray Bunker-Syracuse University NMM Geant4 Detector Model—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 24