Eos, Vol. 87, No. 26, 27 June 2006

E-mail: [email protected]; Wilfrid Schroeder, Program in Ecology and Natural Resource Man- Federal University of Acre, Brazil; and Jose Marengo, Department of Geography, University of Maryland, agement, Federal University of Acre, Rio Branco, Center for Weather Forecasting and Climate Stud- College Park; Alberto Setzer, National Institute of Acre, Brazil; Nara Pantoja, Departments of Forestry ies, National Institute of Space Research, Cachoeira Space Research, São Jose dos Campos, Sao Paulo, and Geography, Federal University of Acre, Brazil; Paulista, Sao Paulo, Brazil. Brazil; Monica de Los Rios Maldonado, Master’s Alejandro Duarte, Department of Natural Sciences, Exploring Earth’s Composition and Energetics With Geoneutrinos

PAGES 253, 260 are one of the most important controls on The production of heat by radioactive the planet’s subsequent thermal, chemical, decay plays a major role in the evolution of and mechanical evolution. As a result, they the Earth. Present constraints of the amount are crucial to understanding processes such of heat production come from conflicting as mantle convection, plate tectonics, and assessments of global surface heat flow, the core’s dynamics and magnetic field. which can be directly measured. A high value of 44 ± 1 terawatts accounts for signifi- Neutrino Detection cant heat flow through the ocean floor by Fig. 1. The beta-decay radioactive process in both conduction and hydrothermal convec- Detecting geoneutrinos employs essentially which a neutron decays into a proton, emitting tion [Stein, 1995]. A low value of 31 ± 1 tera- the same technology used five decades ago an electron (beta-minus particle) and an anti- watts assumes only a minor hydrothermal to discover neutrinos from a nuclear reactor: neutrino, and releasing heat. effect [Hofmeister and Criss, 2005]. This heat a large vat of clear liquid viewed by inward- flux is thought to result from heat produc- looking photomultiplier tubes. Mineral oil, a tion and cooling of the planet. relatively inexpensive, clear liquid containing The relative size of these contributions and a significant fraction of hydrogen and whose their origin allow a range of models. Models free proton nucleus is a good neutrino target, are often compared via the ratio of radioactive is usually used within the vat. Scintillating heat generation to heat flow, called the Urey material, which emits visible light propor- ratio. Estimates of the Urey ratio, based on tional to the energy of ionizing particles cre- chemical models of the early Earth, range from ated in a neutrino interaction, is dissolved in 0.4 to 0.8 [Kellogg et al., 1999; Korenaga, 2003], the oil. The scintillation light is collected by reflecting the cooling of the planet and indi- photomultipliers and used to identify neu- cating the time for heat conduction to the sur- trino interactions by reconstructing their face. Different models have significantly differ- energy and position. ent implications for Earth’s composition, The challenge of detecting geoneutrinos formation, and evolution. Thus, resolving the comes from their tiny probability of interact- question of how much heat is produced by ing. Unless neutrino flux is extremely high, as Fig. 2. Antineutrinos (ν bar) are detected by radioactive decay within the Earth will refine e it is close to a nuclear reactor, only an enor- inverse beta decay, which causes a characteris- the models that help describe the Earth. mous detector can observe a reasonable tic sequence of events. A new technology has emerged that pro- interaction rate. The KamLAND detector, vides the first direct constraints on poorly located in a kilometer-deep zinc mine on of neutrinos. The energy available to the known concentrations of radioactive ele- the Japanese island of Honshu and which antineutrino, derived from the daughter ments in the Earth and the heat they generate. was built for measuring reactor neutrinos, nucleus being more tightly bound than the This involves measuring the flux of geoneu- weighs 1000 tons and records about one unstable parent, is shared in varying amounts trinos, which are produced within the Earth geoneutrino per month. with the electron. Sometimes the electron along with heat by the decay of radioactive This small detection rate makes reduction gets no energy, defining a cutoff to the anti- isotopes of uranium, thorium, and potassium. of background noise, which is due primarily neutrino spectrum characteristic of the par- Neutrinos come from nuclear beta decay in to cosmic rays and radioactivity in and ticular transformation. If the daughter nucleus the Earth, but they also can be produced by around the detector itself, a primary experi- itself is unstable, transformation continues similar processes in the Sun and nuclear mental concern. The Earth attenuates cos- until a stable daughter is produced. Because reactors. mic rays, so geoneutrino detectors operate reactor neutrinos and geoneutrinos come The Earth is glowing with neutrinos. Mil- best in tunnels, mines, or the deep ocean. from different radioactive isotopes, they have lions of these uncharged elementary parti- Deeper is better, with an overburden equiva- different energy spectra. cles flow out of every fingernail-sized area of lent to several kilometers of water usually Antineutrino detection relies on inverse Earth’s surface every second. However, considered a minimum. Size, radio-purity of beta decay, the reverse of the reaction that because they interact with other particles materials, and shielding make detecting geo- produced the antineutrinos. Antineutrinos only via the weak force, almost all neutrinos neutrinos a major project. with enough energy initiate a distinct signa- pass unaffected through the Earth, making Although geoneutrinos and reactor neutri- ture upon colliding with a proton. In the colli- them difficult to detect. nos both are produced in the decay of neu- sion, the proton converts to a neutron while Nevertheless, detecting neutrinos is cru- tron-rich nuclei, they can be distinguished the antineutrino becomes a positron (Figure cial for understanding the planet’s geologi- by their energy spectra. In this form of beta 2). Both products interact quickly in ordinary cal history. The concentrations of radioac- decay, a radioactive isotope emits an elec- matter, producing ionizing particles. The posi- tive elements, which reflect the process by tron and an antineutrino when one of its tron annihilates with an atomic electron, and which Earth accreted 4.6 billion years ago, neutrons spontaneously changes to a proton later the slowed neutron is captured by a (Figure 1). Geoneutrinos and reactor neutri- nearby hydrogen nucleus, forming deuterium. BY S. DYE AND S. STEIN nos are thus antineutrinos, the antiparticles The positron’s ionization signal is proportional Eos, Vol. 87, No. 26, 27 June 2006 to the antineutrino energy, providing informa- tion about the radioactive source, and the Table 1. Comparison of Characteristics of and Annual Event Rates neutron’s signal corresponds to the binding for Geoneutrino Projectsa energy of deuterium. These two signals, KamLAND Borexino SNO+ Hanohano closely separated in time and space, form a Location distinctive coincidence that distinguishes Japan Italy Canada Hawaii antineutrino detections from background. Crust continental continental continental oceanic Geoneutrino measurements reflect the concentrations in the Earth of radioactive Current status or start date operating 2007 2008 planning isotopes with decay times comparable to Depth (meters water equivalent) 2700 3700 6000 4500 the Earth’s age. Because shorter-lived iso- topes are no longer present, only uranium, Target (1032 free protons) 0.35 0.18 0.57 8.7 thorium, and potassium continue to pro- Geoneutrinos per year, total 13 8 30 110 vide significant internal heating. Radioac- tive isotopes of uranium and thorium Geoneutrinos per year, mantle 4 2 5 81 undergo a series of alpha and beta decays Reactor neutrinos per year 89 6 32 12 leading to stable isotopes of lead. Radioac- aReactor neutrino rates assume all reactors running at full power. tive potassium decays directly to calcium. The geoneutrino energy spectrum is punc- projects, summarized in Table 1, which are Canada (Table 1) should begin measuring tuated by cutoffs determined by the excess drawing considerable interest among an continental radioactivity within the next binding energy of the daughter nuclei. emerging community of physicists and geo- few years. Additional projects, including an Only the highest-energy geoneutrinos from physicists. The initial KamLAND measurement oceanic project near Hawaii, are under the decay series of uranium and thorium initi- announced last year [Araki et al., 2005; design. This marine project faces the chal- ate inverse beta decay, whereas geoneutrinos McDonough, 2005] demonstrates the potential lenge of adapting instrumentation for the from potassium do not have enough energy. of geoneutrinos for investigations of Earth’s ocean but, by minimizing the effects of con- Because potassium has a different chemical internal heat sources. A subsequent workshop tinental crust, the project promises an behavior from uranium and thorium, it may in December 2005 in Hawaii, which reported important measurement of mantle and core have a much different distribution within the present and planned instrumentation and radioactivity. Earth. In particular, it may be the light element explored the future use of geoneutrino data, Because geoneutrino studies yield impor- in the core [Gessman and Wood, 2002; Rama reflected interest in ensuring that the program tant new data on the bulk radioactive con- Murthy et al., 2003], in which case radioactive grows. A session at the 2006 AGU Joint Assem- tent of Earth’s crust and mantle, these results heat would be produced there as well as in bly in Baltimore, Md., provided a venue for fur- are likely to significantly advance the under- the mantle and crust. The consequences of ther development of the program. In addition standing of our planet’s composition and radioactivity in the core make the develop- to presenting plans for geoneutrino measure- energetics. ment of techniques for detecting potassium ments of radioactivity from throughout the geoneutrinos a high priority. Earth, the session explored how these mea- References Detecting geoneutrinos at several locations surements can help resolve crucial issues such provides insight into spatial variations of as the role of hydrothermal heat transfer and Araki, T., et al. (2005), Investigation of geologically radioactivity. The geoneutrino flux at all loca- produced antineutrinos with KamLAND, Nature, the possible presence of radioactive heat 436, 499–503. tions has a baseline component from the sources in the core. mantle, and a contribution originating mainly Fiorentini, G., M. Lissia, F. Mantovani, and R. Vannucci The KamLAND study, which first detected (2006), Geo-neutrinos, Earth Planet. Sci. Lett., in press. from crust within several hundred kilometers geoneutrinos, measured the energy but not Gessman, C., and B. Wood (2002), Potassium in of the detector. Continental crust is thicker Earth’s core, Earth Planet. Sci. Lett., 200, 63–78. the direction of neutrinos from local nuclear Hofmeister, A., and R. Criss (2005), Earth’s heat flux and has a higher concentration of radioactiv- reactors. After careful analysis, the geoneu- ity than oceanic crust. Compared with the revised and linked to chemistry, Tectonophysics, trino spectrum was resolved at the low- 395, 159–177. geoneutrino flux from the mantle, the flux energy end of the reactor spectrum. The data Kellogg, L., B. Hager, and R. van der Hilst (1999), from continental crust should be several Compositional stratification in the deep mantle, constrained the total abundance of uranium Science, 283, 1881–1884. times greater, whereas the flux from the oce- and thorium to values producing less than anic crust should be several times less. Korenaga, J. (2003), Energetics of mantle convec- 60 terawatts at 99 percent confidence, with a tion and the fate of fossil heat, Geophys. Res. Lett., Although the mantle is predicted to have central value of 16 terawatts. 30(8), 1437, doi:10.1029/2003GL016982. the lowest concentration of radioactivity, its McDonough, W. (2005), Ghosts from within, Nature, This measurement is consistent with 436, 467–468. enormous volume results in a measurable geochemical estimates, but background geoneutrino flux. Thus, a detector in the Rama Murthy, V., W. van Westrenen, and Y. Fei (2003), noise and reactor signal make the uncer- Experimental evidence that potassium is a sub- middle of the ocean should effectively mea- tainties too large to discriminate between stantial radioactive heat source in planetary cores, sure the radioactive content of the mantle Nature, 423, 163–165. models. Nonetheless, this initial result high- Stein, C. (1995), Heat flow of the Earth, in Global and core. In contrast, a detector in the mid- lights potential geological benefits and dle of a continent effectively measures the Earth Physics: Handbook of Physical Constants, experimental challenges. Further observa- AGU Ref. Shelf Ser., vol. 1, edited by T. Ahrens, radioactive content of the surrounding tions with more exposure, less noise, and pp. 144–158, AGU, Washington, D. C. crust. Because the oceanic flux is lower less signal from nuclear reactors are than the continental, a seafloor detector needed at both continental and oceanic Author Information needs to be larger than a terrestrial detec- locations. tor to observe geoneutrinos at the same The magnitude and complexity of geo- Stephen Dye, Department of Physics and Astron- rate. Continental and oceanic detectors neutrino detection is a challenge for physi- omy, University of Hawaii at Manoa, Honolulu; would provide a complete estimate of cists, Earth scientists, and engineers. With and Seth Stein, Department of Geological Sciences, Earth’s geoneutrino flux and constrain its the initial observation complete, refined Northwestern University, Evanston, Ill., E-mail: radioactive abundance. measurements and new investigations are [email protected] Observational Program planned. The KamLAND project in Japan is preparing to filter radioactive contaminants Measuring and interpreting geoneutrino from the scintillating fluid to reduce back- flux is the goal of several major experimental ground noise. Projects in Italy and eastern