View source for Enriched Xenon Observatory

{{short description|Particle physics experiment}}
{{coord|32|22|18|N|103|47|37|W|display=title}}

The '''Enriched Xenon Observatory''' ('''EXO''') is a particle physics experiment searching for neutrinoless [[double beta decay]] of [[xenon]]-136 at [[Waste Isolation Pilot Plant|WIPP]] near Carlsbad, New Mexico, U.S.

Neutrinoless [[double beta decay]] (0νββ) detection would prove the [[Majorana fermion|Majorana]] nature of neutrinos and impact the [[neutrino mass]] values and ordering.  These are important open topics in [[particle physics]].

EXO currently has a 200-kilogram xenon liquid [[time projection chamber]] ('''EXO-200''') with R&D efforts on a ton-scale experiment ('''nEXO''').  [[Xenon]] double beta decay was detected and limits have been set for 0νββ.

== Overview ==
EXO measures the rate of neutrinoless decay events above the expected [[Background radiation|background]] of similar signals, to find or limit the double beta decay half-life, which relates to the effective neutrino mass using nuclear matrix elements. A limit on effective neutrino mass below 0.01&nbsp;eV would determine the neutrino mass order.  The effective neutrino mass is dependent on the lightest neutrino mass in such a way that that bound indicates the normal mass hierarchy.<ref>See P. Vogel, A. Piepke (2007). "[http://pdg.lbl.gov/2007/reviews/betabeta_s076.pdf Neutrinoless Double-beta decay]", in {{cite journal
 |author=W.-M. Yao ''et al.'' ([[Particle Data Group]])
 |year=2006
 |title=Review of Particle Physics
 |journal=[[Journal of Physics G]]
 |volume=33 |issue= 1|pages=1–1232
 |arxiv=astro-ph/0601168
 |bibcode=2006JPhG...33....1Y
 |doi=10.1088/0954-3899/33/1/001
}}</ref>

The expected rate of 0νββ events is very low, so background radiation is a significant problem.  WIPP has {{convert|650|m|ft}} of rock overburden—equivalent to {{convert|1600|m|ft}} of water—to screen incoming cosmic rays.  Lead shielding and a cryostat also protect the setup.  The neutrinoless decays would appear as narrow spike in the energy spectrum around the xenon [[Q value (nuclear science)|Q-value]] (Q<sub>ββ </sub>= 2457.8 keV), which is fairly high and above most gamma decays.

== EXO-200 ==

=== History ===
EXO-200 was designed with a goal of less than 40 events per year within two standard deviations of expected decay energy. This background was achieved by selecting and screening all materials for radiopurity.  Originally the vessel was to be made of Teflon, but the final design of the vessel uses thin, ultra-pure copper.<ref>{{cite journal
 |author=D. Leonard
 |year=2008
 |title=Systematic study of trace radioactive impurities in candidate construction materials for EXO-200
 |journal=[[Nuclear Instruments and Methods in Physics Research Section A]]
 |volume=591 |issue=3
 |pages=490–509
 |arxiv=0709.4524
 |bibcode=2008NIMPA.591..490L
 |doi=10.1016/j.nima.2008.03.001
|s2cid=118334959
 }}</ref>  EXO-200 was relocated from Stanford to [[Waste Isolation Pilot Plant|WIPP]] in the summer of 2007.<ref>{{cite press release
 |date=24 July 2007
 |title=EXO project equipment successfully placed underground at WIPP
 |url=http://www.wipp.energy.gov/pr/2007/EXO.pdf
 |publisher=[[United States Department of Energy|DOENews]]
}}</ref>  Assembly and commissioning continued until the end of 2009 with data taking beginning in May 2011. Calibration was done using <sup>228</sup>Th, <sup>137</sup>Cs, and <sup>60</sup>Co gamma sources.

=== Design ===
The prototype EXO-200 uses a copper cylindrical [[time projection chamber]] filled with {{convert|150|kg|0}} of pure liquid xenon.  Xenon is a [[scintillator]], so decay particles produce prompt light which is detected by [[avalanche photodiode]]s, providing the event time.  A large electric field drives ionization electrons to wires for collection. The time between the light and first collection determines the z coordinate of the event, while a grid of wires determines the radial and angular coordinates.

<gallery>
Image:EXO cryostat.jpg|The EXO-200 cryostat installed underground at WIPP.
Image:EXO underground laboratory.JPG|The EXO-200 cleanrooms installed underground at WIPP.
</gallery>

=== Results ===
The background from earth radioactivity(Th/U) and <sup>137</sup>Xe contamination led to ≈2&times;10<sup>−3</sup> counts/(keV·kg·yr) in the detector.  Energy resolution near Q<sub>ββ</sub> of 1.53% was achieved.<ref name=":0">{{Cite journal|title = Search for Majorana neutrinos with the first two years of EXO-200 data|date = 12 June 2014|journal = Nature |volume=510 |issue = 7504|pages=229–234|doi = 10.1038/nature13432|pmid = 24896189|arxiv = 1402.6956 |bibcode = 2014Natur.510..229T |last1 = Albert|first1 = J. B.|last2 = Auty|first2 = D. J.|last3 = Barbeau|first3 = P. S.|last4 = Beauchamp|first4 = E.|last5 = Beck|first5 = D.|last6 = Belov|first6 = V.|last7 = Benitez-Medina|first7 = C.|last8 = Bonatt|first8 = J.|last9 = Breidenbach|first9 = M.|last10 = Brunner|first10 = T.|last11 = Burenkov|first11 = A.|last12 = Cao|first12 = G. F.|last13 = Chambers|first13 = C.|last14 = Chaves|first14 = J.|last15 = Cleveland|first15 = B.|last16 = Coon|first16 = M.|last17 = Craycraft|first17 = A.|last18 = Daniels|first18 = T.|last19 = Danilov|first19 = M.|last20 = Daugherty|first20 = S. J.|last21 = Davis|first21 = C. G.|last22 = Davis|first22 = J.|last23 = Devoe|first23 = R.|last24 = Delaquis|first24 = S.|last25 = Didberidze|first25 = T.|last26 = Dolgolenko|first26 = A.|last27 = Dolinski|first27 = M. J.|last28 = Dunford|first28 = M.|last29 = Fairbank Jr|first29 = W.|last30 = Farine|first30 = J.|s2cid = 2740003|display-authors = 29}}</ref>

In August 2011, EXO-200 was the first experiment to observe double beta decay of <sup>136</sup>Xe, with a [[half life]] of 2.11×10<sup>21</sup> years.<ref name="Ackerman">{{cite journal
 |author=N. Ackerman
 |display-authors=etal
 |year=2011
 |title=Observation of Two-Neutrino Double-Beta Decay in <sup>136</sup>Xe with EXO-200
 |journal=[[Physical Review Letters]]
 |volume=107 |issue=21 |page=212501
 |arxiv=1108.4193
 |bibcode=2011PhRvL.107u2501A
 |doi=10.1103/PhysRevLett.107.212501
 |pmid=22181874
|s2cid=40334443
 }}</ref> This is the slowest directly observed process. An improved half life of 2.165 ±0.016(stat) ±0.059(sys) &times; 10<sup>21</sup> years was published in 2014.<ref>{{Cite journal|title = An improved measurement of the 2νββ half-life of Xe-136 with EXO-200|date = 2014|journal = Phys. Rev. C |volume=89 |issue = 1|pages=015502 |doi = 10.1103/PhysRevC.89.015502|arxiv = 1306.6106 |bibcode = 2014PhRvC..89a5502A |last1 = Albert|first1 = J. B.|last2 = Auger|first2 = M.|last3 = Auty|first3 = D. J.|last4 = Barbeau|first4 = P. S.|last5 = Beauchamp|first5 = E.|last6 = Beck|first6 = D.|last7 = Belov|first7 = V.|last8 = Benitez-Medina|first8 = C.|last9 = Bonatt|first9 = J.|last10 = Breidenbach|first10 = M.|last11 = Brunner|first11 = T.|last12 = Burenkov|first12 = A.|last13 = Cao|first13 = G. F.|last14 = Chambers|first14 = C.|last15 = Chaves|first15 = J.|last16 = Cleveland|first16 = B.|last17 = Cook|first17 = S.|last18 = Craycraft|first18 = A.|last19 = Daniels|first19 = T.|last20 = Danilov|first20 = M.|last21 = Daugherty|first21 = S. J.|last22 = Davis|first22 = C. G.|last23 = Davis|first23 = J.|last24 = Devoe|first24 = R.|last25 = Delaquis|first25 = S.|last26 = Dobi|first26 = A.|last27 = Dolgolenko|first27 = A.|last28 = Dolinski|first28 = M. J.|last29 = Dunford|first29 = M.|last30 = Fairbank|first30 = W.|display-authors = 29}}</ref>  EXO set a limit on neutrinoless beta decay of 1.6×10<sup>25</sup> years in 2012.<ref>{{cite journal
 |author=M. Auger
 |display-authors=etal
 |year=2012
 |title=Search for Neutrinoless Double-Beta Decay in <sup>136</sup>Xe with EXO-200
 |journal=[[Physical Review Letters]]
 |volume=109 |issue=3 |page=032505
 |arxiv=1205.5608
 |bibcode=2012PhRvL.109c2505A
 |doi=10.1103/PhysRevLett.109.032505
|pmid=22861843
 |s2cid=29698686
 }}</ref>  A revised analysis of run 2 data with 100&nbsp;kg·yr exposure, reported in the June issue of [[Nature (journal)|''Nature'']] reduced the limits on half-life to 1.1×10<sup>25</sup> yr, and mass to 450 meV.<ref name=":0" />  This was used to confirm the power of the design and validate the proposed expansion.

Additional running for two years was taken.

EXO-200 has performed two scientific operations, Phase I (2011-2014) and after upgrades, Phase II (2016 - 2018) for a total exposure of 234.1 kg·yr. No evidence of neutrinoless double beta decay has been found in the combined Phase I and II data, giving the lower bound of <math>3.5 \cdot 10^{25}</math> years for the half-life and upper mass of 239 meV.<ref>{{cite journal |last1=Anton |first1=G. |last2=Badhrees |first2=I. |last3=Barbeau |first3=P. S. |last4=Beck |first4=D. |last5=Belov |first5=V. |last6=Bhatta |first6=T. |last7=Breidenbach |first7=M. |last8=Brunner |first8=T. |last9=Cao |first9=G. F. |last10=Cen |first10=W. R. |last11=Chambers |first11=C. |last12=Cleveland |first12=B. |last13=Coon |first13=M. |last14=Craycraft |first14=A. |last15=Daniels |first15=T. |last16=Danilov |first16=M. |last17=Darroch |first17=L. |last18=Daugherty |first18=S. J. |last19=Davis |first19=J. |last20=Delaquis |first20=S. |last21=Der Mesrobian-Kabakian |first21=A. |last22=DeVoe |first22=R. |last23=Dilling |first23=J. |last24=Dolgolenko |first24=A. |last25=Dolinski |first25=M. J. |last26=Echevers |first26=J. |last27=Fairbank |first27=W. |last28=Fairbank |first28=D. |last29=Farine |first29=J. |last30=Feyzbakhsh |first30=S. |last31=Fierlinger |first31=P. |last32=Fudenberg |first32=D. |last33=Gautam |first33=P. |last34=Gornea |first34=R. |last35=Gratta |first35=G. |last36=Hall |first36=C. |last37=Hansen |first37=E. V. |last38=Hoessl |first38=J. |last39=Hufschmidt |first39=P. |last40=Hughes |first40=M. |last41=Iverson |first41=A. |last42=Jamil |first42=A. |last43=Jessiman |first43=C. |last44=Jewell |first44=M. J. |last45=Johnson |first45=A. |last46=Karelin |first46=A. |last47=Kaufman |first47=L. J. |last48=Koffas |first48=T. |last49=Krücken |first49=R. |last50=Kuchenkov |first50=A. |last51=Kumar |first51=K. S. |last52=Lan |first52=Y. |last53=Larson |first53=A. |last54=Lenardo |first54=B. G. |last55=Leonard |first55=D. S. |last56=Li |first56=G. S. |last57=Li |first57=S. |last58=Li |first58=Z. |last59=Licciardi |first59=C. |last60=Lin |first60=Y. H. |last61=MacLellan |first61=R. |last62=McElroy |first62=T. |last63=Michel |first63=T. |last64=Mong |first64=B. |last65=Moore |first65=D. C. |last66=Murray |first66=K. |last67=Njoya |first67=O. |last68=Nusair |first68=O. |last69=Odian |first69=A. |last70=Ostrovskiy |first70=I. |last71=Piepke |first71=A. |last72=Pocar |first72=A. |last73=Retière |first73=F. |last74=Robinson |first74=A. L. |last75=Rowson |first75=P. C. |last76=Ruddell |first76=D. |last77=Runge |first77=J. |last78=Schmidt |first78=S. |last79=Sinclair |first79=D. |last80=Soma |first80=A. K. |last81=Stekhanov |first81=V. |last82=Tarka |first82=M. |last83=Todd |first83=J. |last84=Tolba |first84=T. |last85=Totev |first85=T. I. |last86=Veenstra |first86=B. |last87=Veeraraghavan |first87=V. |last88=Vogel |first88=P. |last89=Vuilleumier |first89=J.-L. |last90=Wagenpfeil |first90=M. |last91=Watkins |first91=J. |last92=Weber |first92=M. |last93=Wen |first93=L. J. |last94=Wichoski |first94=U. |last95=Wrede |first95=G. |last96=Wu |first96=S. X. |last97=Xia |first97=Q. |last98=Yahne |first98=D. R. |last99=Yang |first99=L. |display-authors=1 |last100=Yen |first100=Y.-R. |last101=Zeldovich |first101=O. Ya. |last102=Ziegler |first102=T. |title=Search for Neutrinoless Double-$\ensuremath{\beta}$ Decay with the Complete EXO-200 Dataset |journal=Physical Review Letters |date=18 October 2019 |volume=123 |issue=16 |pages=161802 |doi=10.1103/PhysRevLett.123.161802| arxiv=1906.02723|pmid=31702371 |s2cid=174803277 }}</ref> Phase II was the final operation of EXO-200.

== nEXO ==

A ton-scale experiment, nEXO ("next EXO"), must overcome many backgrounds.  The EXO collaboration is exploring many possibilities to do so, including barium tagging in liquid xenon.  Any double beta decay event will leave behind a daughter barium ion, while backgrounds, such as radioactive impurities or neutrons, will not.  Requiring a barium ion at the location of an event eliminates all backgrounds.  Tagging of a single ion of barium has been demonstrated and progress has been made on a method for extracting ions out of the liquid xenon.  A freezing probe method has been demonstrated, and gaseous tagging is also being developed.<ref>{{cite journal
 |author=P. Fierlinger
 |display-authors=etal
 |year=2008
 |title=A microfabricated sensor for thin dielectric layers
 |journal=[[Review of Scientific Instruments]]
 |volume=79 |issue=4 |pages=045101–045101–7
 |arxiv=0706.0540
 |bibcode=2008RScI...79d5101F
 |doi=10.1063/1.2906402 <!--|doi-access=subscription-->
 |pmid=18447546
|s2cid=2950473
 }}</ref>

The 2014 EXO-200 paper indicated a 5000&nbsp;kg TPC can improve the background by xenon self-shielding and better electronics. Diameter would be increased to 130&nbsp;cm and a water tank would be added as shielding and muon veto.  This is much larger than the attenuation length for gamma rays. Radiopure copper for nEXO has been completed. It is planned for installation in the [[SNOLAB]] "Cryopit".<ref>{{cite conference
 |title=Status and Prospects for the EXO-200 and nEXO Experiments
 |first=Liang
 |last=Yang
 |url=http://neutrino2016.iopconfs.org/IOP/media/uploaded/EVIOP/event_948/10.05__5__Yang.pdf
 |type=presentation
 |date=8 July 2016
 |location=London
 |conference=XXVII International Conference on Neutrino Physics and Astrophysics
 |conference-url=http://neutrino2016.iopconfs.org/programme
 |access-date=16 November 2016
 |archive-date=17 November 2016
 |archive-url=https://web.archive.org/web/20161117071925/http://neutrino2016.iopconfs.org/IOP/media/uploaded/EVIOP/event_948/10.05__5__Yang.pdf
 |url-status=dead
 }} Video available at {{YouTube|id=VIm2RkDPOlQ#t=78m9s|Neutrino Conference 2016 - Friday (part 1)}}.<!--That's the "in SNOLAB" statement; talk starts at 66m--></ref>{{Rp|17}}<ref>{{cite conference
 |title=nEXO: a tonne-scale next-generation double-beta decay experiment
 |first=Ryan |last=MacLellan
 |url=https://indico.cern.ch/event/606690/contributions/2591417/
 |type=presentation
 |conference=XV International Conference on Topics in Astroparticle and Underground Physics (TAUP 2017)
 |conference-url=http://taup2017.snolab.ca/
 |date=25 September 2017 |location=[[Greater Sudbury|Sudbury]], Canada
}}</ref>{{Rp|7}}

An Oct. 2017 paper details the experiment and discusses the sensitivity and the discovery potential of nEXO for neutrinoless double beta decay.<ref>{{cite journal
 |first=J. B. |last=Albert |display-authors=etal
 |collaboration=nEXO Collaboration
 |year=2018
 |title=Sensitivity and Discovery Potential of nEXO to Neutrinoless Double Beta Decay
 |journal=Physical Review C |volume=97 |issue=6 |pages=065503 |arxiv=1710.05075 |doi=10.1103/PhysRevC.97.065503 |bibcode=2018PhRvC..97f5503A |s2cid=67854591 }} LLNL-JRNL-737682</ref>
Details on the ionization readout of the TPC have also been published.<ref>{{cite journal
 |first=M. |last=Jewell  |display-authors=etal
 |collaboration=nEXO Collaboration
 |date=14 October 2017
 |title=Characterization of an Ionization Readout Tile for nEXO
 |journal=Journal of Instrumentation  |volume=13  |pages=P01006  |arxiv=1710.05109 |doi=10.1088/1748-0221/13/01/P01006  |s2cid=56297955 }}</ref>

The pre-Conceptual Design Report (pCDR) for nEXO was published in 2018. The planned location is [[SNOLAB]], Canada.

== References ==
{{reflist}}

== External links ==
*[http://www-project.slac.stanford.edu/exo/ EXO web site]
*[https://nexo.llnl.gov/ nEXO web site]
*[https://inspirehep.net/experiments/1399409 EXO experiment] record on [[INSPIRE-HEP]]

{{neutrino detectors}}

[[Category:Particle experiments]]
[[Category:Neutrino experiments]]
[[Category:Radioactivity]]
[[Category:Xenon]]