The many uses of electron antineutrinosWilliam F. McDonough, John G. Learned, and Stephen T. Dye Citation: Phys. Today 65(3), 46 (2012); doi: 10.1063/PT.3.1477 View online: http://dx.doi.org/10.1063/PT.3.1477 View Table of Contents: http://www.physicstoday.org/resource/1/PHTOAD/v65/i3 Published by the American Institute of Physics. Additional resources for Physics TodayHomepage: http://www.physicstoday.org/ Information: http://www.physicstoday.org/about_us Daily Edition: http://www.physicstoday.org/daily_edition

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46 March 2012 Physics Today www.physicstoday.org

The kind of neutrinos emitted in nuclear betadecay—namely electron antineutrinos (‾νe)—are helping scientists implement a diverserange of intriguing applications beyond fun-damental particle-physics research. Like all

neutrinos, they’re very difficult to detect becausethey interact so feebly with matter. Nevertheless,

they have in recent years begun providing valuableclues about the origin and thermal history of Earth(see PHYSICS TODAY, September 2011, page 14). Theyare also providing critical information about thefuel cycle in nuclear reactors and, hopefully soon,new insight on heavy-element production in super-novae.

The many uses ofelectron antineutrinosWilliam F. McDonough, John G. Learned, and Stephen T. Dye

They have become tools for understanding Earth’s internal heat engine andfor surveillance of nuclear reactors.

William McDonough is a professor of geology at the University of Maryland, College Park. John Learned is a professor ofphysics at the University of Hawaii, in Honolulu. Stephen Dye is a professor of physics at Hawaii Pacific University, Kaneohe.

Figure 1. Interior of the SNO+ antineutrino detector nearing comple-tion at the Sudbury Neutrino Observatory in Ontario, Canada. The fore-ground acrylic vessel that will hold almost a kiloton of liquid scintillator is surrounded by thousands of phototubes that will look for light flashessignaling antineutrino interactions in the liquid. The detector will beused for particle physics and geology.

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Two kiloton-sized ‾νe detectors are now moni-toring Earth’s interior to help geologists determinethe abundance and distribution of uranium and thorium—the planet’s principal heat- producing radioactive elements. A third big detector will soonjoin them (see figure 1). They and much smaller detectors can also monitor nuclear reactors far andnear. Under consideration is a next generation ofeven bigger multitask detectors that could con-tribute broadly to astroparticle physics, geology, reactor studies, and fundamental particle physics.

All neutrino varieties are impervious to theelectromagnetic and strong-nuclear forces, and areat least a million times lighter than the electron (seethe box on page 48). But the minuscule interactioncross sections that make them so hard to see allowus to peer deep into the bowels of exploding starsas well as our own planet.

Of course, the various neutrino mass and flavoreigenstates, and their metamorphoses, are them-selves subjects of intense study by particle physicists.But in this article, we focus primarily on the useful-ness of neutrinos for geology and nuclear security.

Seeing neutrinosNeutrinos were first hypothesized in 1930 by Wolf-gang Pauli to conserve momentum in beta decay. He presumed that they were massless and un -detectable—except as missing momentum. In 1956,however, Frederick Reines, Clyde Cowan, andcoworkers achieved the first sighting of neutrinos,specifically electron antineutrinos, with a cubic-meter-size detector placed about 10 meters from anuclear reactor at the Savannah River power plantin Georgia (see figure 2). Thus began the science ofdetecting neutrinos and their use for monitoringwhat’s going on inside nuclear reactors.

A decade later, Raymond Davis’s experimentdeep inside the Homestake mine in Lead, SouthDakota, first detected neutrinos from the Sun—inthat case νes. But he saw only about a third as manyas solar models predicted. That shortfall was thefirst indication of a phenomenon now well attestedby many detector experiments: neutrino oscillation,the oscillatory metamorphosis of neutrino flavors.1

In 2003 the KamLAND (Kamioka Liquid Anti-Neutrino Detector) collaboration, whose detectorinside the Kamioka zinc mine in Japan monitorsdozens of reactors near and far, reported that, likesolar neutrinos, the ‾νe oscillates between flavorstates.2 The conclusion was now inescapable thatthere are three different neutrino mass eigenstates,only one of which might be massless. It may be thatthe interplay of neutrino mass and flavor eigen-states provides the symmetry-breaking mechanismthat accounts for the cosmic preponderance of mat-ter over antimatter (see the article by Helen Quinnin PHYSICS TODAY, February 2003, page 30).

Putting them to workThe KamLAND experiment was designed to studyfundamental neutrino physics by monitoring ‾νesemitted, with energies up to 8 MeV, by reactors atvarious distances during their normal fuel cycles.The detector’s kiloton of liquid scintillator, watched

over by thousands of photomultiplier tubes, revealsthe positrons and neutrons created in the inverse-beta-decay reaction

‾νe + p → e+ + n

between an incident ‾νe and a hydrogen nucleus inthe organic scintillator. The kinematic threshold ‾νeenergy for that reaction is 1.8 MeV.

But KamLAND can also monitor the actions ofthose who operate reactors. Early in a power reac-tor’s fuel cycle, its rods produce weapons-grade plu-tonium along with abundant other fissile and fissionisotopes. Legitimate power reactors burn the rodsuntil they are largely (but not entirely) depleted offissile material—typically in about 18 months. Atelltale signature of surreptitious use of reactors tomanufacture weapons-grade material is frequentshutdowns to allow shuffling of the rods.

Many isotopes contribute to a reactor’s ‾νe flux,whose mean detectable energy in KamLAND isnear 4 MeV. By contrast, the ‾νe geoneutrino spec-trum from beta decays of abundant, long-lived iso-topes naturally present in Earth’s crust and mantlepeters out at 3.3 MeV (see figure 3.)

KamLAND’s ability to detect geoneutrinos is abyproduct of its primary particle-physics purpose:recording the flavor-oscillation disappearance of ‾νe s that emanate from reactors at different distances.In fact, KamLAND’s purposeful placement in themidst of several dozen reactors creates a trouble-some background for geoneutrino searches. The in-ternational collaboration first reported detectinggeoneutrinos in 2005, albeit with a significant back-ground reactor signal.3 More recently the team hasobserved more than 100 geoneutrino events withgreatly reduced background noise, thanks to im-provements to the detector and unanticipated shut-downs of nearby reactors.4

www.physicstoday.org March 2012 Physics Today 47

Figure 2. Frederick Reines in 1953 with a component of the 250-kg liquid-scintillator detector with which he and Clyde Cowan woulddemonstrate the existence of neutrinos three years later.

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Borexino, the only other detector so far in thegeoneutrino business, is smaller than KamLAND.But in a tunnel under the Gran Sasso d’Italia, thehighest peak in the Apennines, it’s much lessplagued by reactor backgrounds; the nearest reactoris 1000 km away. The Borexino team reported itsfirst geoneutrino measurements5 in 2010. So geol-ogy now has the benefit of two large detectors sitedin markedly different geological environments, con-tending with different background issues.

Unlocking Earth’s secretsOne can think of Earth as an “antineutrino star”emitting about 6 million ‾νes per square centimeterper second. Such a high surface flux emerges freelyfrom the interior because Earth, like all material, isessentially transparent to neutrinos of all varieties.An MeV neutrino can pass through a light-year’slength of lead without interacting.

Geoneutrino observations offer an opportunityto establish independently the absolute amount ofU and Th in Earth’s crust and mantle, thus settlinga 150-year-long discussion about the Earth’s internalheat sources initiated by Lord Kelvin. Increasinglyprecise measurement of the surface flux of geo -neutrinos will make it possible to discriminate be-tween competing models of the planet’s bulk com-position. The data will test models that considervarious accretion materials that went into Earth’sformation—for example, the primitive, undifferen-tiated material that formed in the Sun’s proto -planetary nebula 4.5 billion years ago and nowshows up in chondritic meteorites.

The data will also test models that predict theexistence of local areas of enriched radioactivity in the mantle, some of which may have been se-questered to the core–mantle boundary early inEarth’s history. Geoneutrino results will also ad-

dress models that predict the fractional contributionof radioactive heating to the planet’s total heat flow. There’s also the open question of the level ofradioactivity—presumably small—in Earth’s core.(See the article by Bernard Wood in PHYSICS TODAY,December 2011, page 40.)

Many other debates in geodynamics can be nar-rowed and reshaped by geoneutrino data collectedon land and in the oceans. One wants to know howmuch, if any, of Earth’s radiogenic power is drivingplate tectonics or running the geodynamo in the liquid-iron core. Knowing how much radioactivepower Earth generates would, in turn, tell us aboutthe building blocks that made the planet and the ini-tial conditions of its formation. A subject of livelydebate is what proportion of the surface heat flowcomes from present radioactive decay and howmuch is from primordial heat sources such as the kinetic energy of accretion bombardment, ancientradioactivity, and the gravitational energy of coreformation by percolation of molten metal from theouter layers.

Earth radiates away its internal heat at 46 ± 3 ter-awatts.6 From estimates of Th, U, and potassium inthe continental crust, one can conclude that about8 TW of that heat is from 40K decay.7 The remainingradiogenic heating is presumed to come almost en-tirely from the decay chains of 238U and 232Th inroughly equal measure. The Th/U atomic abundanceratio (about 4:1) is taken from the impressively uni-form compositions of chondritic meteorites. The K/Uratio is 104:1, but most of that is stable 39K. Unfortu-nately, the entire ‾νe spectrum from 40K decay is belowthe 1.8-MeV inverse-beta threshold.

Subtracting the 8-TW contribution from 40K, weneed to explain the source of the remaining 38 TW.What fraction of that deep power is radiogenic? Theanswer relates to compositional models of Earth’sformation.

48 March 2012 Physics Today www.physicstoday.org

Electron antineutrinos

The standard model of particle theory admits three flavors ofneutrinos: νe, νμ, and ντ, associated, respectively, with the threecharged leptons: the electron, the muon, and the tau.The model, regarding neutrinos as Dirac fermions,assigns each flavor f a distinct antineutrino (‾νf). Conser-vation of lepton number L then decrees that the cre-ation of an electron (L = +1) in beta decay requires thecreation of an antineutrino ‾νe, for which L = −1.

Beta decay, in which a neutron becomes a proton, iscommon for many long-lived, neutron-rich isotopes of,for example, uranium, thorium, and potassium. The figure shows this metamorphosis at a more funda -mental level: a down quark d in the neutron becomesan up quark u by emitting a virtual W− boson thatinstantly decays into an electron and a ‾νe. In stellarfusion reactions, by contrast, protons become neu-trons as proton-rich light nuclei fuse. So in the solarneutrino flux, neutrinos (as distinguished from anti-neutrinos) predominate.

It’s widely conjectured that the neutrino is not, infact, a Dirac fermion but rather a Majorana fermion

that’s its own antiparticle. (See the article by Alfred Goldhaberand Maurice Goldhaber in PHYSICS TODAY, May 2011, page 40.) In that case, helicity (spin handedness) replaces lepton numberas the conserved quantity, with no significant effect on the neutrino properties discussed in this article.

Neutrinos, antineutrinos, and beta decay

u

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νe‾

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w−

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www.physicstoday.org March 2012 Physics Today 49

How much K, Th, and U is there below thecrust? Earth-formation models are constrained byseismic and geodetic data that describe the planet asit is today. Determining its earliest stages is a chal-lenge. Three different approaches are used for esti-mating Earth’s composition: cosmochemical, geo-chemical, and geophysical. Each has its strengthsand weaknesses. Together they narrow the totalmass of U in the crust and mantle—a proxy for theplanet’s total radiogenic heat production—to some-where between 5 × 1016 and 1.3 × 1017 kg. The remainder of heat production, from Th and K, isprojected from the Th/U and K/U abundance ratios.(Uranium is, by the way, the scarcest naturally occurring element in the solar system.)

The cosmochemists seek to reconcile the chem-ical and isotopic compositions of chondritic mete-orites with those of Earth. Noting that so-called en-statite chondrites—a rare subclass characterized byits paucity of oxides—share with Earth the same isotopic compositions for many elements, one con-cludes that they are the essential building blocks ofEarth’s formation. That argument leads cosmo-chemists to opt for the low end of the range of pos-sible U abundances.8

From the meteoritic materials, one wouldtherefore conclude that the lower two-thirds of themantle is markedly different, chemically and min-eralogically, from the upper mantle. But many geol-ogists reject the idea of such large-scale heterogene-ity in the mantle as inconsistent with seismologicalimages of subducting oceanic plates that plungeinto the deep mantle, continually stirring the entireconvecting mantle.

An alternative modeling approach is based onusing actual geochemical samples of the mantle andcrust to estimate the concentration of elements inthe primitive mantle before the crust was formed.9

Those models predict about 8 × 1016 kg U in the crustand mantle, and they conclude that the mantle hasa relatively homogeneous composition throughout.They are consistent with elasticity models of themantle and with broader chondritic models thatdeemphasize the enstatites. The shortcoming of thegeochemical approaches, however, is that rocks thatemerge from the mantle only sample depths of a few

hundred kilometers and do not reveal whether theyhad resided at greater mantle depths in the past.

Finally there are the geophysical models. Theyuse present-day boundary conditions given by themeasured surface heat flux to find solutions for theplanet’s thermal evolution that specify the relativecontributions of primordial heat and ongoing radio -genic heat production.10 Those models parameterizemantle convection in terms of thermal and momen-tum diffusivities, buoyancy, and viscosity. Typically,the geophysics models predict the highest U abun-dances for the mantle, and they conclude that morethan half the present heat flow is radiogenic. Thusthey are at odds with the cosmochemical and geo-chemical models with regard to the structure, abun-dance, and distribution of Earth’s heat-producingelements.

ProgressSince the first geoneutrino sighting at KamLAND in2005, there have been considerable advances. Asshown in figure 4, KamLAND’s 2011 results havemuch improved statistics.4 Its new value for thecombined U and Th contribution to Earth’s heat fluxremains consistent with all three broad classes ofEarth models. But it now excludes, with 97% confi-dence, a fully radiogenic model that presumes thatall the planet’s primordial heat is long gone and attributes Earth’s entire heat flux to current radio -activity. The 2010 Borexino measurement, with itscapacious error bar, had been compatible with thatfully radiogenic model, but it’s also compatible withthe new KamLAND measurement.

Soon a third geoneutrino detector will come online, the SNO+ detector deep inside a nickel mine inSudbury, Ontario (see figure 1). SNO+ is a thor-oughly revamped version of the Sudbury NeutrinoObservatory’s heavy-water solar-neutrino detector(see PHYSICS TODAY, July 2002, page 13). The originalSNO detector’s kiloton of heavy water will havebeen replaced by liquid scintillator by the time itstarts counting ‾νes early next year. Geoneutrinos willbe a sideline for SNO+, as it is for KamLAND andBorexino. An important role of the revamped detec-tor will be to look for neutrinoless double betadecay, which could only happen if neutrinos are

NEUTRINO ENERGY (MeV)

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Figure 3. Expected detection rates from varioussources of electron antineutrinos (‾νe) per kiloton ofscintillator liquid for a scintillation detector at a typicalcontinental site, plotted against the ‾νe’s incident energy. The 1.8-MeV cutoff is the threshold for the inverse-beta-decay reaction that reveals the ‾νe. Below3.5 MeV, the expected spectrum is dominated bybeta-decay ‾νes from the decay chains of uranium-238and thorium-232 in Earth’s crust and mantle. Thespectrum of ‾νes from nuclear reactors peaks near 4 MeV, where the geoneutrino spectrum has alreadypetered out, making it possible to distinguish between natural and reactor sources.

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50 March 2012 Physics Today www.physicstoday.org

Electron antineutrinos

indeed Majorana particles11 (see the box). Because ofits location, SNO+ should have more than twiceKamLAND’s geoneutrino count rate and a muchlower reactor background.

Detectors are getting progressively better.They’re being made with purer materials that min-imize backgrounds from radioactive contaminants.Borexino holds less than half a kiloton of scintillator,but the exquisite purity of the liquid and its vesselallow it to detect neutrinos by elastic scattering aswell as inverse beta decay. As clean as Borexino, theSNO+ detector, 2 km underground, will be the deep-est of the geoneutrino detectors. Depth is crucial forminimizing backgrounds due to cosmic-ray muons.

Planned large-scale experiments propose to in-clude developments in detector technology thathave applications in particle physics, geology, na-tional security, and astroparticle physics. A group at the Technical University of Munich is planning agargantuan 50- kiloton scintillation detector calledLENA (Low Energy Neutrino Astronomy).12 A pro-posed US experiment called Hanohano (Hawaiian Anti- Neutrino Observatory, stated twice in Hawai-ian linguistic tradition) envisages a mobile detector,with between 10 and 50 kilotons of liquid scintillator,that would operate on the sea floor. It could be de-ployed out in the middle of the ocean, far removedfrom continental crustal radioactivity—or at differ-ent distances from specific nuclear reactors.13 Theformer deployment would address radioactivity inthe mantle as distinguished from the crust, and thelatter would facilitate the study of neutrino oscilla-tion as a function of distance from the source.

A particularly interesting deployment of such amobile detector would be in the middle of the SouthPacific, roughly equidistant from South America,Australia, and the core–mantle boundary. Out thereabove the thin oceanic crust, which is relatively freeof radioactivity, it would have better access thanland-based facilities to the mantle’s radioactivity.

Supernovae generate ‾νes. Thus far only the 1987supernova in the nearby Large Magellanic Cloud

has yielded a detectable neutrino pulse. But the newgeneration of detectors will be searching for moreof them, and for the cosmic neutrino background expected from the sum of all past supernovae.

Achieving directionalityThe positron created in the inverse-beta-decay reac-tion produces a prompt light flash as it moves justa few millimeters through the scintillator and thenannihilates with an electron. The flash’s intensityprovides a rough measure of the instigating neu-trino’s energy. About 0.2 ms later, the much heavierneutron, having wandered about a meter with verylittle of the incident ‾νe’s kinetic energy, provides aconfirming flash when it fuses with a proton to forma deuteron and emit a telltale 2.2-MeV gamma. Suchdouble-flash events within a narrowly specifiedtime, space, and energy window are very hard forspurious background processes to mimic. But noneof the detectors that rely on inverse beta decay candetermine incident ‾νe directions event by event.

Identifying the source direction of a ‾νe recordedby a scintillation detector would be of great impor-tance for geology, astrophysics, and nuclear securitymonitoring. But thus far KamLAND and Borexinohave been blind to source direction. The muchsmaller CHOOZ and Palo Verde ‾νe detectors, respec-tively in France and Arizona, have demonstratedthe determination of a source’s direction to within20° for statistical samples of a few thousand eventsfrom a reactor a few kilometers away. That’s possi-ble because the random meter-long thermalizingneutron excursions sum vectorially, for a bigenough sample from a single source, to pointroughly away from that source. The present kilotondetectors can’t do that statistical trick because theydon’t localize the beginning and end of each neu-tron’s excursion to better than 10 cm.

We hope that the CHOOZ and Palo Verdedemonstrations are only the beginning. The Univer-sity of Hawaii contingent of the KamLAND collab-oration is developing a tiny neutrino detector thatcan localize the ‾νe interaction to a few millimetersand a doped scintillator liquid that can similarly localize the neutron’s endpoint.

Directionality greatly enhances the ability tomonitor reactors from afar by identifying the neu-trinos one wants and rejecting those coming fromelsewhere. It would let geologists make tomo-graphic neutrino images of the mantle that point outareas of high Th and U accumulation. And particlephysicists want anything that enhances the signal-to-noise ratio of neutrino-oscillation data. Direction-ality might even allow actual imaging of a reactor’sfusion furnace.

Monitoring reactorsSmall detectors are being developed specifically formonitoring ‾νe emission from nuclear reactors. Oneprototype is essentially a modern version of theoriginal Reines– Cowan detector. Containing abouta ton of liquid scintillator, it has been successfullydeployed at gigawatt nuclear power plants. Suchdevices, without control-room information, havedemonstrated their ability to observe daily power

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Figure 4. Heating of Earth by uranium and thorium decay, as deduced from three geoneutrino experiments.3–5 Both KamLANDmeasurements are consistent with the predictions of the principalclasses of Earth models (blue lines). But the 2011 measurement excludes, with 97% confidence, the fully radiogenic model (red) that assumes no remaining primordial heat. The right-hand axis translatesthe U + Th heating power into the deduced total mass of U in themantle and crust.

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cycles and fuel-cycle evolution. Unlike deep- underground kiloton detectors that enjoy massiveshielding from cosmic rays, cubic-meter monitorsoperating at or near the surface would be floodedby cosmic-ray muons. But reactors are extremelybright ‾νe sources; a gigawatt nuclear power plant radiates about 1021 per second. So signal-to-noise ratios are favorable at close proximity. At a distanceof 25 meters, a one-ton monitor would detect some-thing like a thousand events per day.

Endorsement of modern compact, standaloneneutrino detectors by the International Atomic Energy Agency would represent a major step for-ward. More conventional monitors have to tap intoa reactor’s plumbing or other infrastructure. It’sthought that neutrino detectors near cooperating reactor facilities will become commonplace. Suchnonadversarial monitoring might offer economicbenefit by helping operators tune the reactor formaximal power output. In disasters like the March2011 earthquake in Japan, neutrino detectors mightprovide the signal for timely reactor shutdown.

There’s much that geologists don’t yet under-stand about the source of Earth’s deep heat. We doknow that radioactivity has been and remains a significant contributor. But recent results demon-strate that the planet’s primordial heat is not yet exhausted.4 The biggest unknown is the amount ofradioactive material inside Earth, and that’s where‾νe detectors can come to the rescue. In the futurethey should also map spatial variations in radio -genic mantle heating and reveal much about the

nature of the geodynamo. The ghostly neutrino has gone from being a

peripheral curiosity in particle physics to playing a prominent role in cosmology and in the quest fora comprehensive theory of the fundamental forces.Now tools originally developed for particle physicsand astrophysics are being adapted to probe theotherwise inaccessible deep interior of our planetand to provide unimpeachable monitoring of nu-clear reactors.

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Q. R. Ahmad et al., Phys. Rev. Lett. 89, 011302 (2002).2. K. Eguchi et al., Phys. Rev. Lett. 90, 021802 (2003).3. T. Araki et al., Nature 436, 499 (2005).4. A. Gando et al., Nat. Geosci. 4, 647 (2011).5. G. Bellini et al., Phys. Lett. B 687, 299 (2010).6. C. Jaupart, S. Labrosse, J.-C. Mareschal, in Treatise

on Geophysics, vol. 7, D. Bercovici, G. Shubert, eds.,Elsevier, Amsterdam (2007), p. 253.

7. R. Arevalo, W. F. McDonough, M. Luong, Earth Planet.Sci. Lett. 278, 361 (2009).

8. M. Javoy et al., Earth Planet. Sci. Lett. 293, 259 (2010).9. W. F. McDonough, S.-S. Sun, Chem. Geol. 120, 223

(1995).10. G. Schubert, D. Stevenson, P. Cassen, J. Geophys. Res.

85, 2531 (1980). 11. M. C. Chen, Earth Moon Planets 99, 221 (2006).12. M. Wurm et al., Acta Phys. Polon. B 41, 1749 (2010).13. J. G. Learned, S. T. Dye, S. Pakvasa, in XII International

Workshop on Neutrino Telescopes, Venice, 2007, H. Baldo Ceolin, ed., Papergraf, Padua, Italy (2007), p. 235. ■

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