Noble liquid and gas detectors for nuclear security Adam Bernstein Advanced Detector Group Leader Lawrence Livermore National Laboratory LBL-LLNL xenon workshop Nov 17 2009 Noble liquid and gas detectors for nuclear security Dennis Carr, Darrell Carter, Mike Heffner, Kareem Kazkaz, Peter Sorensen - LLNL Tenzing Joshi, Rick Norman UCB Nuclear Engineering Michael Foxe, Igor Jovanovic, Purdue University Nucl Eng.

Talk outline The Nuclear Materials Problem And its Connection With Dark Matter and Neutrino Science Current Detectors and Detection Needs High Pressure Xenon for Spectroscopy and Imaging in the Field Applied Antineutrino Physics and Coherent Scatter Detection Improvements in sub-MeV neutron Detection with Liquid Argon Detectors Conclusions

The world is awash in civil and military plutonium and highly enriched uranium Category Plutonium (tonnes) HEU (tonnes) Civil 1675 175 Military 155 1725 Total 1830 1900 Separated plutonium: 340 tons civil stocks (includes military surplus) 150 tons military stocks 490 tons total separated plutonium Estimate from http://www.isis-online.org In units of Hiroshima style fission weapons … From HEU ~75,000 From separated Plutonium ~ 60,000 From all plutonium ~ 230,000

What is being done to monitor and reduce global stockpiles of nuclear materials and weapons ? Civil nuclear fuel cycle monitoring:IAEA safeguards regime, Euratom, ABACC.. Weapons dismantlement verification: START I and II, SORT.. Military nuclear materials control and monitoring – Nunn-Lugar, Fissile Material Cutoff Treaty, HEU Purchase Domestic nuclear security in individual states – DHS etc. ‘National Technical Means’

Detection and monitoring of plutonium and HEU is central to all of these efforts Quiescent nuclear material: Plutonium and HEU emit penetrating gamma rays and neutrons that can be passively detected out to many tens of meters Critical systems: Reactors emit huge fluxes of antineutrinos, which can be detected at stand-off distances of tens of meters to hundreds of kilometers Lawrence Livermore National Laboratory

Rare neutral particle detection underlies nuclear security and fundamental nuclear science Fissile Material Search and Monitoring are top priorities for global nuclear security Rare Event Detection Dark Matter and Neutrino Physics are top priorities in 21rst century physics Reactor antineutrino signature Neutrino Physics: oscillations and neutrino mass ~1-10 MeV antineutrinos SNM gamma/neutron signatures Dark Matter signatures: Axions and WIMPS ~1 keV to 10 MeV Neutrons and Gamma-rays Both areas require improved keV to MeV-scale neutral particle rare event detectors

Talk Outline The Nuclear Materials Problem And its Connection With Dark Matter and Neutrino Science Current Detectors and Detection Needs High Pressure Xenon for Spectroscopy and Imaging in the Field Applied Antineutrino Physics and Coherent Scatter Detection Improvements in HEU/PU Characterization with Liquid Argon Detectors Conclusions

Nuclear security needs impose unique constraints on detectors Fissile Material Search /Monitoring Dark Matter and Neutrino Physics High efficiency for the signal of interest Excellent background rejection through: Energy resolution Particle tracking Particle identification Active/passive shielding Robust, easy to operate and to interpret non-cryogenic usually preferred.. but not always Little or no overburden Simplicity a secondary consideration Cryogenic detectors often used 100-5000 m.w.e. overburden Unique to applications Common Needs Unique to fundamental science

Current detectors and possible improvements from noble liquid/gas detectors Particle Current detectors Example Detector Noble Liquid candidate Benefit of noble liquid detector Gamma-ray HPGe NaI(Tl) plastic scintillator Mechanically cooled handheld HPGe HPXe Non-cryogenic high resolution Imaging and spectroscopy Antineutrino Liquid scintillator Dual phase Argon Higher rate/smaller footprint Neutron 3He (thermal) liquid scintillator (fast) Liquid Argon with PSD 50 keV  1 MeV neutron identification

Talk outline The Nuclear Materials Problem And its Connection With Dark Matter and Neutrino Science Current Detectors and Detection Needs High Pressure Xenon for Spectroscopy and Imaging in the Field Applied Antineutrino Physics and Coherent Scatter Detection Improvements in HEU/PU Characterization with Liquid Argon Detectors Conclusions

Location and monitoring of nuclear material with gamma-rays Current spectroscopic systems Cryogenic detectors (e.g. Ge) have the best resolution but are hard to field - though this is getting easier 3-6% resolution (662 keV FWHM) is far more common in fieldable devices Current imaging devices Few gamma-ray imaging devices used in nuclear security applications –mostly demonstrations or lab devices low resolution and/or restricted field of view A handheld Ge detector Imaging a MIRved warhead with a CsI coded aperture device

Xenon for spectroscopy Possible advantages of xenon gamma-ray spectrometers and imagers for nuclear security applications Xenon for spectroscopy High Z (good photo-absorption capability) 0.56% FWHM resolution @ 662 keV (within 3-4x of HPGe) Non-cryogenic/room temperature operation Stable against temperature variations Highly linear, no nonproportional response as in for example NaI(Tl) Xenon for imaging Spectroscopy advantages, plus.. nearly 4p field of view Potential for 10-20x improvement in imaging efficiency using Compton camera approach (relative to segmented Ge) Performance range of current xenon gas detectors – 2-4% FWHM for 662 keV Theoretical limit in resolution 0.6% FWHM A. Bolotnikov, B. Ramsey /NIM. A 396 (1997) 360-370

A recent industrial effort at HPXe spectroscopy "Field-Deployable, High-Resolution, High Pressure Xenon Gamma Ray Detector” www.proportionaltech.com DTRA funded project ca 2001-2005: No fragile Frisch grid as in prior high resolution designs Correct event energy based on event radius derived from primary and secondary scintillation on wire Result – ~2% resolution Fundamental limitations – electronics noise, statistical fluctuations, loss of electrons to impurities

Can we build a better gas spectrometer ? These numbers can’t be improved N = number of liberated electrons F = fano factor in HPXe ~0.15 But these might be.. L= 1-ε = loss factor/inefficiency for electrons G= fluctuations in gain on wires or other readout mechanism n = rms electronics noise (m = gain factor) 0.56% FWHM resolution @ 662 keV may be possible in a fieldable spectrometer (G=L=δE(electronics) << Fano factor

Negative Ion Drift to achieve the theoretical limit in gamma-ray energy resolution Principle: electronegative dopants capture ionization electrons, slowly drift them to a readout plane, and release them one at a time Benefits – ideal resolution-. No e- losses, no gain fluctuations, lower purity requirements Xenon gas with electronegative dopant E 2) Electronegative ions capture electrons and drift (slowly) 1) Xenon ion recoils, inducing ionization e- 0) Incoming gamma 3) Electron released to Large Electron Multiplier or other gain device 4) LEM amplifies individual electron by 500-10000 well above electronics noise floor (200 e-) Catch (for nonproliferation) – slow drift implies low rate ~1-10 kHz (modest sizedetectors/drift lengths, not for imaging) But low rate not an issue for zero-rate experiments – see Mike Heffner talk on DOE-OS funded DUSEL R&D project for neutrinoless double beta decay

Compton imaging in HPXe using electron drift cosq = E = E1 + Eabsorption Segmented Compton camera HPXe Compton camera Scatter and absorption plane thicknesses must be optimized Imaging efficiency is ~2% Scatter and absorption ‘planes’ throughout the detector Imaging efficiencies >10%

GEANT simulation of efficiencies for Compton scatter + absorption in 1 cubic meter of HPXe Imaging efficiency (Compton+p.e.) Photon energy (MeV) Pressure (atm) 8-12% efficiency from 0.4-0.9 MeV at 10 atm (simulation by Steve Dazeley)

Talk outline The Nuclear Materials Problem And its Connection With Dark Matter and Neutrino Science Current Detectors and Detection Needs High Pressure Xenon for Spectroscopy and Imaging in the Field Applied Antineutrino Physics and Coherent Scatter Detection Improvements in HEU/PU Characterization with Liquid Argon Detectors Conclusions

The history of Applied Antineutrino Physics W. Pauli, 1930: “I have done a terrible thing, I have postulated a particle that cannot be detected.” Reines and Cowan, 1960: Detect antineutrinos using a reactor source Mikelyan Group, 1975-1984: First to suggest/demonstrate reactor monitoring with an antineutrino detector Our group, 2007: Demonstrated practical, self- calibrating, low channel count, non-intrusive, automated antineutrino detectors IAEA Spokesperson, …. “The American group has done the first practical demonstration, and its detector is promising, because it is not much bigger than other systems the IAEA currently deploys at reactors.” IEEE Spectrum, April 2008

Reduction of the detector footprint is an important consideration for the end user, the IAEA Current useful prototypes are ~ 3 meter on a side Smaller detectors would be more attractive Increase efficiency of inverse beta detectors Shrink footprint to 1.5 m x 1.5 m Discover and exploit coherent neutrino nucleus scattering Shrink footprint to 1 m x 1 m ? Slight problem – no one has ever measured this process after 3 decades of trying

<Erecoil> (keV)‏ The basic principles of coherent scattering in argon – signature is very similar to the higher energy WIMP recoil 44.8 1.8 50 10 Supernova υ 4.0 0.07 15 2 Solar υ 1.15 0.018 8 1 Reactor υ <Erecoil> (keV)‏ Eυ(MeV)‏ Energies  q << 1/(nucleus radius) ~ tens of MeV (condition of coherence) among the nobles Argon (Z=18) gives the greatest number of detectable ionizations per unit mass Recoil energies Atomic Number Quenching  detectable ionization energy only a fraction of the recoil energy Cross-section Neutron Number Q(Germanium) 0.2 Q(Argon) ?=0.2 Detection of few hundreds of eV 21

A limiting background: solar neutrinos also scatter coherently Estimated counts/day kg Detector type from reactor , R_core=20m from solar  Distance where solar and reactor counts are equal (Km) Argon 52.3 6.3e-4 5.8 The solar neutrino background is comparable with the reactor neutrino signal at distances >1.5 km from the reactor core. The solar background prevents using coherent scatter detectors to monitor reactors beyond a few kilometers Detector

Estimates of antineutrino signal & backgrounds @ 10 mwe overburden 10 kg Ar, 25 m standoff, 3.4 GWt Signal: estimated after quenching: 1-10 free e- Shield: Inner: 2cm Lead Outer: 10cm borated polyethylene Monte Carlo Simulation of signal and backgrounds in 10 kg Ar, per day Rates in plot and table simulated@ 20 mwe Signal Rate ~200 per day (1 or more liquid e-) Background Rates  counts/ dy/10 kg Dominant: 39Ar (sim.; depleted Ar reduces 20x) 1000 External U/Th/K : (sim., after 2 cm Pb shield) ~ 100 External neutrons: (sim. after 10 cm borated poly shield,) ~ 20 Internal gammas: (as measured in XENON10): ~ 50 per day @ 3 keVee; but ~1 Hz of single liquid electrons

Detection concept for coherent scatter – dual phase, S2 only current test-bed: gas-phase ~1 liter drift volume Only look for liquid electrons via secondary scintilation Primary signal is too small

Attempted neutron-nuclear recoil measurement with gas phase detector Nuclear collisions produce fewer ionizations than electronic collisions. We want to measure this quenching factor. lead or borated poly shielding Neutron beam Gamma Background 478 keV from 7Li(p,p’)‏ 12” Only keep lead animation Argon detector 7Li (p,n) 7Be, 10-100 keV neutrons 2-MeV LINAC Li-target neutron generator 100 Hz rep. rate, ~105 neutrons / spill Lawrence Livermore National Laboratory 25

Predicted nuclear recoil spectrum – very low energy recoils generated by 10-100 keV source overlap with the antineutrino (and WIMP) recoil region Incident neutron spectrum Predicted nuclear recoil spectrum (With an assumed quenching factor)‏ Nuclear recoil spectrum (keVr)‏ Quenched spectrum (keVee)‏ (assume q = 0.25)‏ Simulated detected spectrum (keVee)‏ (geometric losses, quenching)‏ Amplitude Incident neutrons within this 80keV resonance will contribute to the bulk of measured n-Ar recoils Energy (keVr) or (keVee)‏ 26

First attempt in 2008 - nuclear recoil data analysis using neutrons Lead data: neutrons & residual gammas Poly data: Mostly residual gammas preliminary Result 8 keVr, 1.8 keVee recoil Momenta comparable to what is needed for coherent scatter But detector calibration was an issue… 27

Improvements on the gas detector test bed replace single PMT with 4 PMT array Understand optical collection v. position with movable 55Fe source Purifier off demonstrate purification with getter plot show 55Fe peak stability versus time in days Next step: directly measure low energy quench factors in gas, then liquid

Design of the dual-phase detector is underway now 1 kg liquid target 60 keV neutron source Fiducialization with 4 1” square PMTs May consider LEM readout 5mm x-y resolution would keep multiple scatters to < 8% of total

Talk outline The Nuclear Materials Problem And its Connection With Dark Matter and Neutrino Science Current Detectors and Detection Needs High Pressure Xenon for Spectroscopy and Imaging in the Field Applied Antineutrino Physics and Coherent Scatter Detection Improvements in HEU/PU Characterization with Liquid Argon Detectors Conclusions

21rst century multiplicity counters: Exploiting the theory of the fission chain in quiescent material Problem: Neutrons and gammas from HEU and Pu, downscattered by shielding, are hard to detect Timing at the scale of tens of nanoseconds helps select this rare signal from backgrounds Particle ID is essential Good energy spectroscopy desirable Current methods work either at > 0.5 MeV with fast timing or at 0.025 eV (thermal energy) with slow (hundred microsecond interevent times)

A recent success: detection of shielded HEU in minutes through 6" lead and 2" polyethylene using 200 kg liquid scintillator array No HEU HEU present [Analysis and plot by Ron Wurtz/Neal Snyderman] Lawrence Livermore National Laboratory

Emission spectra and sensitivity bands compared to neutron background – energy weighted flux E*Dphi/DE Cosmic background neutrons Implosion weapon with 1” steel shielding (contains some hydrogenous material) Pu sphere LAr could improve PSD and preserve timing in this important region LAr detector Liquid scintillatir Fast timing information 1/1000 PSD degrades below ~0.5 MeV MeV

Liquid argon compared to liquid scintillator Liquid scintillator woes: Part-per-thousand gamma/neutron discrimination Discrimination does not work below 500 keV 35% energy resolution Pure liquid argon Part-per-100-million gamma/neutron discrimination Works down to 50-60 keV ~5% energy resolution Can be mechanically cooled 10 kg scale detectors demonstrated An operating 7-kg detector— SNOLAB design Gamma/neutron discrimination in Ar Lawrence Livermore National Laboratory

Conclusions Significant and useful overlap between nuclear security and dark matter/neutrino applications HPXe for imaging and spectrometry  relevant for double beta detection Dual phase Ar for coherent scattering  closely analogous to DM detectors and an interesting discovery anyway Liquid Ar for multiplicity measurements closely analogous to DM detectors Lawrence Livermore National Laboratory

Backups Lawrence Livermore National Laboratory

Ways in which nuclear security is not like nuclear science Ways in which nuclear security is not like nuclear science.. S1/S2, PSD, shielding are not the main issues LLNL Physicist and IAEA inspector George Anzelon being kicked out of North Korea Following a recent nuclear inspection - April 15 2009 November 13 2009