Noble liquid and gas detectors for nuclear security

1. 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.

2. 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

3. The world is awash in civil and military plutonium and highly enriched uranium • Separatedplutonium: • 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

4. 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’

5. 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

6. Rare neutral particle detection underlies nuclear securityand fundamental nuclear science Dark Matter and Neutrino Physics are top priorities in 21rst century physics Rare Event Detection Fissile Material Search and Monitoring are top priorities for global nuclear security 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

7. 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

8. 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 detectorsoften used • 100-5000 m.w.e.overburden Unique to applications Common Needs Unique to fundamental science

9. Current detectors and possible improvements from noble liquid/gas detectors

10. 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

11. 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

12. Possible advantages of xenon gamma-ray spectrometers and imagers for nuclear security applications Performance range of current xenon gas detectors – 2-4% FWHM for 662 keV • 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) Theoretical limit in resolution 0.6% FWHM A. Bolotnikov, B. Ramsey /NIM. A 396 (1997) 360-370

13. 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 fromprimary and secondary scintillation on wire Result – ~2% resolution Fundamental limitations – electronics noise, statistical fluctuations, loss of electrons to impurities

14. 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

15. 0) Incoming gamma 2) Electronegative ions capture electrons and drift (slowly) e- e- e- e- 1) Xenon ion recoils, inducing ionization Xenon gas with electronegative dopant E Negative Ion Drift to achieve the theoretical limit in gamma-ray energy resolution Principle: electronegative dopantscapture 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 3) Electron released to Large Electron Multiplier or other gain device 4) LEM amplifies individual electronby 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

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

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

18. 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

19. 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 The history of Applied Antineutrino Physics 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

20. 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 mx 1.5 m • Discover and exploit coherent neutrino nucleus scattering • Shrink footprint to 1 mx 1 m ? • Slight problem – no one has ever measured this process after 3 decades of trying

21. Energies  Eυ(MeV)‏ <Erecoil> (keV)‏ Reactor υ 1 8 0.018 1.15 Solar υ 2 15 0.07 4.0 Supernova υ 10 50 1.8 44.8 Q(Germanium) 0.2 Q(Argon) ?=0.2 Detection of few hundreds of eV The basic principles of coherent scattering in argon – signature is very similar to the higher energy WIMP recoil 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 AtomicNumber • Quenching  detectable ionization energy only a fraction of the recoil energy Cross-section Neutron Number

22. A limiting background: solar neutrinos also scatter coherently Estimated counts/day kg • 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

23. Monte Carlo Simulation of signal and backgrounds in 10 kg Ar, per day Shield: Inner: 2cm Lead Outer: 10cm borated polyethylene Rates in plot and table simulated@ 20 mwe 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-

24. 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

25. 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” 7Li (p,n) 7Be, 10-100 keV neutrons 2-MeV LINAC Li-target neutron generator 100 Hz rep. rate, ~105 neutrons / spill Argon detector Lawrence Livermore National Laboratory

26. Predicted nuclear recoil spectrum – very low energy recoils generated by10-100 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)‏

27. 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…

28. Improvements on the gas detector test bed replace single PMT with 4 PMT array Understand optical collection v. position with movable55Fe source Purifier off demonstrate purification with getterplot show 55Fe peak stability versus time in days Next step: directly measure low energyquench factors in gas, then liquid

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

30. 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

31. 21rst century multiplicity counters:Exploiting the theory of the fission chain in quiescent material • Problem: Neutrons and gammas from HEU and Pu, downscatteredby 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 eitherat > 0.5 MeV with fast timingor at 0.025 eV (thermal energy) with slow (hundred microsecond interevent times)

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

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

34. 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

35. 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

36. Backups Lawrence Livermore National Laboratory

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