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EXO-200 and nEXO
Vladimir Belov, ITEP & MEPhIfor EXO-200 and nEXO
collaborations
17th Lomonosov Conference, Moscow, 21 August 2013
21.08.2015 V.Belov EXO-200 and nEXO 2
2ν mode: a conventional 2nd order process in Standard Model
0ν mode: a hypothetical process can happen only if: <mν> ≠ 0, ν = ν |ΔL|=2, |Δ(B-L)|=2
Simulated double beta decay spectrumP.Vogel. arXiv:hep-ph/0611243
To reach high measurement sensitivity for 0ν mode one requires, • High energy resolution• Large Isotope mass• Low background
Double beta decay
21.08.2015 V.Belov EXO-200 and nEXO 3
Energy resolution is poorer than the crystalline devices (~ factor 10), but...Monolithic detector. Xenon can form detection medium, allow self shielding,
surface contamination minimized. Very good for large scale detectors.Has high Q value. Located in a region relatively free from natural
radioactivity.Isotopic enrichment is easier. Xe is already a gas & 136Xe is the heaviest
isotope.Xenon is “reusable”. Can be purified & recycled into new detector (no crystal
growth). Minimal cosmogenic activation. No long lived radioactive isotopes of Xe.Energy resolution in LXe can be improved. Scintillation light/ionization
correlation.Particle identification. Slightly limited, but can be used to tag alphas from Rn
chain.
… admits a novel coincidence technique. Background reduction by Ba daughter tagging.
Why xenon
21.08.2015 V.Belov EXO-200 and nEXO 4
80.6% 136Xe T = 167 KP = 147 kPaρ = 3.0 g/cm3
High purity HFE7000 > 50 cm
25 cm
EXO-200 detector
21.08.2015 V.Belov EXO-200 and nEXO 5
The EXO-200 TPC
• Field shaping rings: copper• Supports: acrylic• Light reflectors/diffusers: Teflon• APD support plane: copper; Au (Al) coated
for contact (light reflection)• Central cathode, U+V wires: photo-etched
phosphor bronze• Flex cables for bias/readout: copper on
kapton, no glue
Comprehensive material screening program
• 38 U triplet wire channels (charge)
• 38 V triplet wire channels, crossed at 60o (induction)
• 234 large area avalanche photodiodes (APDs, light in groups of 7)
• Wire pitch 3 mm (9 mm per channel)
• Wire planes 6 mm apart and 6 mm from APD plane
• All signals digitized at 1 MS/s, ±1024S around trigger
• Drift field 376 V/cm
40 cm40 cm
Two almost identical halves reading ionization and 178 nm scintillation, each with:
xz
y
21.08.2015 V.Belov EXO-200 and nEXO 6
21.08.2015 V.Belov EXO-200 and nEXO 7
● Signal finding. Digital filters are used on waveforms from U,V wires and APDs● Parameters of pulses (t, E) are estimated for both charge and light● Pulses are combined into clusters producing position, multiplicity (SS or MS) and energy.● Position is used in form of Standof Distance (SD) that is distance from any cluster to the nearest wall
Event reconstruction
V-wire signalsV-wire signals
U-wire signalsU-wire signals
APD signalsAPD signals
Charge clustersCharge clusters
Scintillation clustersScintillation clusters
Linked
tu
β
γ
SingleSiteEvents(SS)
tu
γ
MultipleSiteEvents(MS)
Efficiency to get into SS: 2β0ν ~90%γ 2.5MeV ~30%
But we don’t throw MS events away! We use them in the fit to help predict background
21.08.2015 V.Belov EXO-200 and nEXO 8
APD denoising
1.53 % in SS at 0νββ Q-value
21.08.2015 V.Belov EXO-200 and nEXO 9
Rotation angle chosen to optimize energy resolution at 2615 keV
Properties of xenon cause increased scintillation to be associated with decreased ionization (and vice-versa)
E. Conti et al. Phys. Rev. B 68 (2003) 054201
Use projection onto a rotatedaxis to determine event energy
Combining ionization and scintillation
21.08.2015 V.Belov EXO-200 and nEXO 10
We used gamma sources Cs, Co, Th, Ra
Energy resolution model
Resolution is dominated by by constant term (noise)
At Qββ (2458 keV):σ/Ε = 1.53 % (SS)σ/Ε = 1.65 % (MS)
Energy calibration
Data with sources is fitted by model PDFs SS/MS ratio agrees with 10% accuracy Measured source activity is 4% accurate
21.08.2015 V.Belov EXO-200 and nEXO 11
Data takingC
um
ula
tive
live
tim
e [d
ays]
Previous results
Accumulation of data in science «Run 2» from Oct'2011 till Sep'2013.«Golden» data is 447.60 ± 0.01 days livetimeThat corresponds to 100 kg y in⋅ 136Xe exposure.
21.08.2015 V.Belov EXO-200 and nEXO 12
2β0ν measurement
Single site
Multi site
Efficiency to 2β events85 %
Full livetime477.6 d100.0 kg y
2β events~37000
The lowest background index among comparable detectorsBI = 1.7 ± 0.2 × 10-3 keV-1 kg-1 y-1
[Nature 510, 229 (2014)]
13
2β0ν measurement
T1/20νββ > 1.1 x 1025 yr
〈 mββ 〉 < 190 – 450 meV (90% C.L.)
[Nature 510, 229 (2014)]
Pro
file
likel
ihoo
d
Backgrounds in ± 2σ ROI
Th chain 16.0
U chain 8.1
Xe-137 7.0
Total 31.1 ± 3.8
21.08.2015 V.Belov EXO-200 and nEXO 14
Comparison of 2β0ν
EXO-200:Nature 510, 229 (2014)
GERDA Phase 1: PRL 111 (2013) 122503
KamLAND-Zen:PRL 110 (2013) 062502 KK&K Claim:Mod. Phys. Lett., A21 (2006) 1547
21.08.2015 V.Belov EXO-200 and nEXO 15
Searching for majoron
Phys.Rev D90 9, 092004 (2014)
21.08.2015 V.Belov EXO-200 and nEXO 16
EXO-200 results● Operated a 100 kg scale LXe TPC for 2 years● Discovered a double beta decay of 136Xe● Made the most precise measurement of its halflife● Measured residual backgrounds are very low● Reached design (anticorrelated) energy resolution, σ/E(Q)=1.5%● Achieved stable electron lifetime of ~3 ms or better● Utilized self-shielding in monolithic detector● Demonstrated power of β/γ discrimination (SS/MS)
● Implemented novel detector solutions and analysis techniques● 500 LAAPDs for VUV (175 nm) scintillation detection● Photo- etched, charge collection wires, cathode, and fasteners● Epoxy- poned, kapton flat cable feedthroughs● HFE- 7000 thermal bath and radiation shield● Ultra- light design, no solder joints
21.08.2015 V.Belov EXO-200 and nEXO 17
nEXO concept
● 5 tonnes of enriched LXe● Monolithic design dramatically improves
performance with size● Using enhanced self- shielding● x100 better T
1/2 sensitivity
● < 1% energy resolution● no central cathode● *no* Ba tagging (initially)
Conceptual design for SNOLab150 kg
5000 kg
2.5MeV γ attenuation length (8.5cm)
21.08.2015 V.Belov EXO-200 and nEXO 18
nEXO R&D: TPC
Charge readout tiles
Silicon Photo Multipliers
TCP: ~ 1.3 m x 1.3 m
Key features:● single drift volume● thin Copper or Carbon vessel● minimal use of plastics● charge collected by pads on top● 10 ms electron lifetime● scintillation collected by SIPMs on side barrel behind the field cage● VUV reflection● Optimization of fiducial volume
nEXO R&D in full swing
21.08.2015 V.Belov EXO-200 and nEXO 19
nEXO R&D: Charge• Charge readout structures on low background substrates• SS/MS discrimination (readout pitch)● No tension and microphonics (not wires)
Prototype Charge Readout Quartz Tile LXe Test Apparatus
21.08.2015 V.Belov EXO-200 and nEXO 20
nEXO R&D: Light• High light collection efficiency (>15% at 175 nm)• Ultra‐low background (~ppt levels for Th & U)• VUV sensitive SiPMs from FBK, Hamatsu, Ketek or Hamamatsu MEG devices or Wave-Length-Shifters• Already measuring samples
FBK SiPMs, 1st gen Hamamatsu MEG
21.08.2015 V.Belov EXO-200 and nEXO 21
nEXO R&D: Electronics• Cold electronics close to the detector• improve energy resolution to 1%• low background, low noise• keep required power low enough• deliver all the signals without much loss• multiplexing to reduce cabling/feedthroughs
Cold charge readout ASIC Prototype circuit for SiPMs
21.08.2015 V.Belov EXO-200 and nEXO 22
EXO-200 and nEXO projected sensitivities
Combining all ourexperience and thebest technologies
we plan
T1/2
= 6 · 1027 yr
in 5 years of
counting
Majorana
neutrino mass
<mββ
> sensitivity
of 7- 18 meV
nEXO is an active international R&D program for a x100 the sensitivity of EXO- 200!
University of Alabama, Tuscaloosa AL, USA - T. Didberidze, M. Hughes, A. Piepke, R. TsangUniversity of Bern, Switzerland - S. Delaquis, R. Gornea, T. Tolba, J-L. Vuilleumier California Institute of Technology, Pasadena CA, USA - P. Vogel Carleton University, Ottawa ON, Canada - V. Basque, M. Dunford, K. Graham, C. Hargrove, R. Killick, T. Koffas, C. Licciardi, D. SinclairColorado State University, Fort Collins CO, USA - C. Chambers, A. Craycraft, W. Fairbank, Jr., T. WaltonDrexel University, Philadelphia PA, USA - M.J. Dolinski, Y.H. Lin, E. Smith, Y.-R YenDuke University, Durham NC, USA - P.S. BarbeauIHEP Beijing, China - G. Cao, W. Cen, X. Jiang, L. Wen, J. ZhaoUniversity of Illinois, Urbana-Champaign IL, USA - D. Beck, M. Coon, S. Homiller, J. Ling, J. Walton, L. Yang Indiana University, Bloomington IN, USA - J. Albert, S. Daugherty, T. Johnson, L.J. Kaufman, T. O’ConnerUniversity of California, Irvine, Irvine CA, USA - M. MoeITEP Moscow, Russia - V. Belov, A. Burenkov, M. Danilov, A. Dolgolenko, A. Karelin, A. Kuchenkov, V. Stekhanov, O. ZeldovichLaurentian University, Sudbury ON, Canada - B. Cleveland, A. Der Mesrobian-Kabakian, J. Farine, B. Mong, U. WichoskiUniversity of Maryland, College Park MD, USA - C. Davis, C. Hall University of Massachusetts, Amherst MA, USA - J. Abdollahi, S. Feyzbakhsh, S. Johnston, A. Pocar, D. ShyIBS Center for Underground Physics, Daejeon, South Korea - D.S. LeonardSLAC National Accelerator Laboratory, Menlo Park CA, USA - M. Breidenbach, R. Conley, T. Daniels, J. Davis, A. Dragone, K. Fouts, R. Herbst, A. Johnson, K. Nishimura, A. Odian, C.Y. Prescott, A. Rivas, P.C. Rowson, J.J. Russell, K. Skarpaas, M. Swift, A. Waite, M. WittgenUniversity of South Dakota, Vermillion SD, USA, – R. MacLellanStanford University, Stanford CA, USA - T. Brunner, J. Chaves, R. DeVoe, D. Fudenberg, G. Gratta, M. Jewell, S.Kravitz, D. Moore, I. Ostrovskiy, A. Schubert, K. Twelker, M. WeberStony Brook University, SUNY, Stony Brook, NY, USA – K. Kumar, O. Njoya, M. TarkaTechnical University of Munich, Garching, Germany - W. Feldmeier, P. Fierlinger, M. MarinoTRIUMF, Vancouver BC, Canada – J. Dilling, R. Krücken, F. Retière, V. Strickland
21.08.2015 V.Belov EXO-200 and nEXO 24
21.08.2015 V.Belov EXO-200 and nEXO 25
What else from EXO-200?
Following 2 accidents at the WIPP mine, EXO- 200 personnel is
now granted regular access to the site and recovery operations
are ongoing
● EXO-200 can still contribute to the leading set of experiments
● Upgrades had bee installed before the accident:● Radon suppression system for air around the detector● Upgraded electronics (could get to 1% energy resolution)
● Approx. 2 years of data are still being worked on:● Cosmogenic backgrounds● Gamma, beta and alpha backgrounds
26
Efficiency to 2β 58 % (87 %)
Full exposure127.6 сут23.14 кг л
2β events18984
Reanalyzed Run 2a data from (PRL 109, 032505, 2012)
2β2ν measurementThe most precise measurement of halflife of any isotope to date
T1/22νββ = 2.165 ± 0.016(stat) ± 0.059(sys) × 1021 yr
[PRC 89, 015502 (2014)]total relative uncertainty 2.85%
21.08.2015 V.Belov EXO-200 and nEXO 27
Xenon purity
Непрерывная циркуляция ксенона через высокотемпературные очистители SAES с использованием специально сконструированного насоса.
Neilson et al. (2011) arXiv:1104.5041v1
Среднее время жизни электрона ~3 мс обеспечивает на максимальном времени дрейфа 110 мкс уменьшение сигнала <3%.
Восстановление после остановок занимает несколько дней
21.08.2015 V.Belov EXO-200 and nEXO 28
nEXO expected backgrounds and signalsSimulated nEXO spectrum near single-site ROI:
Full 4.8 tonnes, 5 yr
Central 1 tonneonly, 5 yr
T1/20νββ=
1.8·1027 yr
● Have developed Geant4 simulation for nEXO, using experience gained from EXO-200
● Spectra on right show expected backgrounds in 5 yr exposure, and 0νββ at discovery threshold
● Background calculation assumes measured activity for detector materials
● This procedure was verified with EXO-200 data, and assumes several improvements for nEXO
21.08.2015 V.Belov EXO-200 and nEXO 29
Ba tagging
2448 keV214Bi
2615 keV208Tl
Simulated nEXO spectrum near single-site ROI:
Full 4.8 tonnes, 5 yr
● In addition, R&D to develop techniques to identify Ba daughter nucleus of 0νββ decay (“Ba tagging”) is continuing
● Candidate 0νββ events would be identified in real time and daughter Ba ion collected by probe inserted into the TPC at the decay location
● Identity of Ba daughter can be confirmed spectroscopically
•This technology would eliminate all non-ββ backgrounds near ROI•Could extend ultimate reach of nEXO into the normal hierarchy since full 5 tonne mass would be background free
21.08.2015 V.Belov EXO-200 and nEXO 30
Ba tagging
Twelker et al., arXiv:1407.0618
Several techniques are currently being pursued:
• Tagging from solid Xe by fluorescence spectrum
• Resonance ionization spectroscopy (RIS):
• Also studying ion extraction and tagging from high pressure Xe gas
389.7 nm553.5 nm1064 nm
To Time of FlightSpectrometer
Ba+
Wavelength (nm)
Typical time of flight spectrum:
Fluorescence of Ba in SXe:
Excited by 558 nm laser
<128 atoms
<28 atoms<18 atoms
<6 atoms
Cold probe
Solid Xe
Liquid Xe Ba+
Detect single ion or atom on the probe with
laser-induced fluorescence
Co
un
ts