Development of a Underground High-Energy Neutron Detector Joel Sander - UCSB

Outline WIMPs High-energy neutrons underground Just measure it – The original idea Was approved – Water works – A prototype Design Calibration Data – The real McCoy Calibration Installation

Concordance Model Contributions expressed in yardage Dark Energy 70 yards Non-Baryonic Dark Matter 25 yards Baryonic Matter 5 yards “Metals” ~1cm Tytler astro-ph ENERGY MATTER ENERGY MATTER

A WIMP Coincidence? Weakly Interacting Massive Particles are a popular dark matter candidate because they solve multiple problems. Decoupling of massive particles in the big bang naturally produces the observed dark matter abundance if they have weak scale interactions. Many extensions to the standard model give rise to a WIMP candidate. WIMPs provide a nice answer to the problem of structure formation. log(z) log  Observation WIMPs Baryons Radiation Legend

Wimps Here? (Milky Way)

Direct Detection – Recoil Energy WIMP Target nuclei: A ~ 100. Typical targets: Ge, Si, I, Xe, W v/c =   0.7  Sun moves with v ~ 220km/sec through the Dark Matter halo E R   2 v 2 /m T  few 10s of keV  x-ray energy! Easy! Radioactive background gives many events of this energy! Assuming  = cm 2, R ~.5 events/kg/day

Gamma Background Rate about 20 / (kg-day) ! Strategies: shield and distinguish electron recoils from nuclear recoils Shield it! 40 K: 7x10 4  /day 1m

Neutron Background Neutron-induced nuclear recoils are a fundamental background Due to: - Radioactivity (<10MeV) - Cosmic-ray induced Solutions: - Neutron moderator - Depth - Muon veto Muon flux 50,000 times less than flux at the surface Depth (meters water equivalent) Soudan Log 10 (Muon Flux) (m -2 s -1 )

Neutron Background Detectors Low energy neutronHigh energy neutron Potentially multiple lower-energy (~10MeV) neutrons produced Neutron-interactions can fake WIMP- interactions Low-energy neutrons are moderated

Underground Neutron Flux (Mei & Hime)  100/m 2 /y factor of 3 uncertainty Results in a 0.5 events / kg / year neutron background

The Original Idea Diffuse Reflector PMT         Gadolinium Loaded Scintillator Lead Veto Hadronic Shower Liberated Neutrons   Capture on Gd, Gammas (spread over 30 μs)             Muon Veto, Large Energy Deposit Bare High Energy Neutron No Veto, Small Prompt Energy Deposit 200 cm (4 sq m) 40 cm 75 cm Original Idea: Raul Hennings-Yeomans, Postdoc at Los Alamos

Time40 μs PMT Signal Neutron Event Muon Event dE/dx Shower Initiation, backsplash, proton recoil Neutron captures Signal Differences

Detectable Multiplicities Simulated high-energy neutrons vertically incident on a horizontal 100cm thick Pb target 3 different neutron energies simulated A resulting low-energy neutron is considered detectable if it escapes the top surface of the Pb The neutron detector is sensitive to the energy of the incident neutron Long tail of high-energy neutrons 100MeV

Detectable Multiplicities 100MeV

Accidentals From Gamma Background Gamma-induced multiplicities assume a 40  s time window High-energy neutron-induced multiplicities assume 100% efficiency for observing the resulting, lower-energy neutrons Accidentals rate: Efficiency for detecting the resulting, lower-energy neutrons depends on ― Energy threshold chosen ― Size of the time window

340 cm x 170 cm (8’ x 4’) White Diffuse Reflector Water + GdCl 3 Two Kamland 20” PMTs 75 cm (30”) The Final Design – H 2 O! 2 larger tanks (12m 2 ) of H 2 O on Pb target

Reflectance - mean number of bounces = 10 - photocathode area / interior surface area = 2.7% - reflectance of glass PMT at 400nm = 2.6% - halon reflectance = 96% - probability that a photon is not absorbed by the H 2 O ~97% Measured Reflectance

340 cm x 170 cm (8’ x 4’) White Diffuse Reflector Water + GdCl 3 Two Super-K 20” PMTs 75 cm (30”) The Final Design dN  d / 2 Wavelength shifter doubles light yield 

Tuning Neutron Capture Percent of Gd by Mass in Water – Mark Vagins (UCI) Neutron Captures on Gd vs. Concentration Fraction of Captures on Gd 1 Hydrogen and Gd compete to capture neutrons Capture on hydrogen gives 2.2MeV in gammas Capture on Gd gives 8MeV in gammas Different hydrogen and Gd have different characteristic times Can tune the capture time by varying the Gd/hydrogen ratio Time (  s) % of Neutron Captures Observed % 50% 75% 100%

A Prototype Lead 30cm

Calibrating The Prototype Figure of merit: # of photo-electrons / MeV of deposited energy About 1300 PEs / muon or about 15 PEs per MeV

The Prototype – Cf-252 Run  Gd capture time /- 0.5  s

The Prototype – Surface Run Tue. Dec. 2 nd 22:31 -> Wed. Dec 3 rd 9:24 Mon. Dec. 1 st 20:07 -> Tue. Dec 2 nd 13:34 Multiplicity within a 100  s time window

The Prototype – Underground Run Operated in the Soudan Underground Laboratory (2000mwe) Exposed to Pb-shielded Cf-252 source

340 cm (8’) White Diffusive Reflector Water + GdCl 3 Two Super-K 20” PMTs 75 cm (30”) The Final Design

Leak Testing

Fabrication

Calibration 75cm 8” PMTs

Calibration 1.2 photo-electrons / MeV Expect ~13 photo-electrons / MeV with 20” PMTs and wavelength shifter

Installation – Lead Target 80” 84”16”84”

Installation - Tanks

Installation – Adding Gd

20” PMTs

A Detector

UCSB Harry Nelson (PI) Joel Sander Raymond Bunker Susanne Kyre Dean White Dave Hassan Jimmy Hynes Bobby Lanza Manuel Olmedo Carsten Quinlan Christine Nielsen Lee Tomlinson Neutron Detector Group Case University Dan Akerib (PI) Mike Dragowsky Chang Lee Matthew Harrison Los Alamos Raul Hennings-Yeomans

PMT Calibration