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Exploring Earth's Composition and Energetics with Geoneutrinos

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Exploring Earth's Composition and Energetics with Geoneutrinos 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
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