KamLAND: geo-neutrino measurement in Japan ANNALS OF GEOPHYSICS, 60, 1, 2017, S0113; doi:10.4401/ag-7388 S0113 KamLAND: geo-neutrino measurement in Japan Itaru Shimizu Research Center for Neutrino Science, Tohoku University, Sendai, Japan ABSTRACT Geoneutrinos are antineutrinos produced in radioactive isotope decays inside the Earth. Those antineutrinos are neutral particle and can be de- tected by weak interaction, however, due to extremely small cross section, there were no feasible experiments for a long time. The observation of geoneutrinos has been finally made using large-size liquid scintillator de- tectors, KamLAND and Borexino. In this article, the latest result of Kam- LAND and future prospects of geo o−e measurements are reviewed. 1. Introduction Electron antineutrinos (o−e) emitted by radioactivi- ties within Earth’s interior is called geoneutrinos. They are mainly produced by beta-decays of primordial ra- dioactivities, 238U, 232Th, and 40K, and have energies below 3.4MeV. Such series of decays also produce ra- diogenic heat, which primarily contributes to Earth’s energetics. Therefore, we can estimate Earth’s heat pro- duction through the measurement of geoneutrino flux, and make a comparison with the surface heat flow, re- sulting in an important input for geophysics. Geoneu- trino detection provides a unique tool to investigate Earth’s interior, however, it used to be regarded as al- most infeasible for a long time, due to extremely small cross section of the neutrino interaction. In the late 1990s, plans of feasible experiments to detect geo o−e´s, KamLAND in Japan and Borexino in Italy, are sug- gested for the first time. In this article, the latest result of KamLAND and future prospects of geo o−e meas- urements are reviewed. 2. KamLAND experiment KamLAND (Kamioka liquid scintillator anti-neu- trino detector) is located in Kamioka mine, Japan, at a depth of 1000 m to suppress the cosmic-ray muon back- grounds. The detector consists of 1000 ton of ultra-pure liquid scintillator (LS) contained in a transparent nylon- based balloon suspended in nonscintillating oil. The containment balloon is a 13-m-diameter spherical bal- loon made of 135-µm-thick transparent nylon/EVOH composite film, and is suspended with Kevlar ropes. The neutrino interaction produces energetic charged particle in the LS, and a LS excitation by its energy loss results in a fluorescence emission (scintillation light). This scintillation light is viewed by photomultiplier tubes (PMTs), an array of 1879 PMTs, mounted on the inner surface of the containment vessel. To recored the signals from PMTs, KamLAND has an electronics sys- tem called Front-End Electronics, based on an analog transient waveform digitizer (ATWD). When a PMT channel registers a “hit”, the charge as a function of time is recorded by an ATWD chip. Each channel has two sets of three ATWDs, which record waveforms at three available gain levels, allow us to cover signals from wide dynamic range. 3. Geo-neutrino results In the KamLAND geoneutrino measurement, the largest background comes from o−e´s produced in sur- rounding commercial nuclear reactors in Japan. The ex- pected reactor o−e flux is well calculated by neutrino oscillation parameters which are determined from Kam- LAND spectral data, as well as the nuclear fission rate based on the reactor operation records. It indicates the geoneutrino contribution can be extracted based on a statistical subtraction of reactor o−e backgrounds. In 2005, KamLAND showed the first experimental study of geo o−e´s [Araki et al. 2005], and the observed flux was consistent with Earth model expectations. It was fol- lowed by an improved geo o−e measurement owing to statistical increase and radioactive background decrease by a factor of ~20 after the LS purification [Gando et al. 2011]. After the Fukushima nuclear accident in March 2011, the reactor o−e rate significantly decreased because of the long-term shutdown of Japanese reactors in a re- view of nuclear safety standard. This reactor-off situation provided a good data for the geoneutrino observation. Article history Received July 5, 2016; accepted October 5, 2016. Subject classification: Geo-neutrino measurement, Anti-neutrino detector, KamLAND, Japan. The latest KamLAND result [Gando et al. 2013] is based on a total livetime of 8.19 years. The data set is di- vided into three periods: Period 1 (March 2002 - May 2007); before the LS purification, Period 2 (May 2007 - August 2011); during and after the LS purification, and Period 3 (October 2011 - November 2012); after the start of the KamLAND-Zen neutrino-less double beta- decay experiment. Figure 1 shows the time variation of the observed rate of o−e´s with energies above 3.4MeV in agreement with the expectations of reactor o−e´s. The o−e rate in Period 3 is remarkably smaller than other pe- riods. To exploit the time variation of the known back- ground rate, o−e candidates are analyzed with an unbinned maximum-likelihood method incorporating the event rate and the prompt energy, including the event time. The KamLAND o−e analysis constrains both the geo o−e rates and the neutrino oscillation parameters si- multaneously. As discussed in Gando et al. [2013], the precise measurement of neutrino oscillation parame- ters was achieved in KamLAND; tan2i12 = 0.436 +0. −0. 029 025 , Dm221 = 7.53 +0. −0. 18 18 × 10 −5 eV2, and sin2i13 = 0.023 +0. −0. 002 002 , incorporating constraints from solar neutrino, acceler- ator, and short-baseline reactor neutrino experiments. The energy spectrum of the observed events together with the best-fit contributions in the energy region of geo o−e´s is shown in Figure 2. The main background contributors are the reactor o−e, 13C(a, n)16O reactions, and accidental coincidence events in descending order. In Period 2 and later, the background from 13C(a, n)16O decreased due to the LS purification, and in Period 3, the reactor o−e background decreased. It results in the significant low background condition in Period 3. We find the clear event excess in the backgroundsubtracted spectrum as shown Figure 2 (top panel), and the back- ground-subtracted rate and spectral shape agree with Earth model expectations for each data set. The best-fit geo o−e´s are 116 and 8 events from U and Th decays, re- spectively, with confidence level (C.L.) contours as shown in Figure 3. Assuming the Th/U mass ratio of 3.9 based on the geochemical model [McDonough and Sun 1995], the total number of geo o−e events is 116 +28 −27, which corre- sponds to an oscillated o−e flux of 3.4 +0.8 −0.8 × 10 6 cm−2s−1 at KamLAND. 4. Earth model comparison The result of geo o−e flux measurements in Kam- LAND will provide a new constraint on both geophysics SHIMIZU 2 Figure 1. Time variation of expected and observed rates of o−e´s with energies above 3.4MeV in KamLAND data over 10 years [Gando et al. 2013]. The vertical grey bands indicate deadtime during the liquid scintillator purification and the detector modification. The significant re- duction of o−e flux from reactors after March 2011 earthquake (red dashed line) is remarkable, and provides a new opportunity of the “Re- actor on-off ” study for neutrino oscillations and geo o−e´s below 3.4MeV. Figure 2. Energy spectrum of the KamLAND observed events [Gando et al. 2013]. The best-fit contributions are indicated by col- ored histograms (bottom panel), and the spectrum after subtracting the non-geo-neutrino contributions is compared with the Earth model prediction (top panel). 3 and geochemistry. Firstly, we calculate the geo o−e flux at Kamioka for different bulk silicate Earth (BSE) compo- sitional estimates based on simple but appropriate as- sumption, to compare with the KamLAND geo o−e data. The crustal contribution for U and Th can be estimated by compositional data through rock sampling. On the other hand, the chemical composition of the mantle, most massive layer of Earth’s interior, is uncertain. So we estimate the total amount of U and Th, predicted by the BSE models, and assume that the excess above the crustal contribution uniformly distribute in the mantle [Enomoto et al. 2007]. The KamLAND data tested three BSE compositional estimates of geo o−e flux and radi- ogenic heat from 238U and 232Th [Šrámek et al. 2013], as shown in Figure 4. The uncertainties from the abun- dances and the distributions of U and Th in the mantle are indicated by the vertical bands. The upper solid line with a steeper slope corresponds to the estimate in the homogeneous mantle assumption, and the lower solid line the estimate in the sunken-layer which assumes that all of the U and Th below the crust are concen- trated at the mantle-core boundary. While the statistical treatment of geological uncertainties is not straight- forward, assuming Gaussian errors for the crustal con- tribution and for the BSE abundances, the KamLAND data disfavor the geodynamical models with the ho- mogeneous hypothesis at 89% C.L. However, due to the limited statistical power of the data, all BSE com- position models are still consistent within ~2v C.L. The measured geo o−e flux can be converted into a total ra- diogenic heat of 11.2+7.9−5.1 TW from U and Th (additional ~3.0 TW is expected from other radioactive nuclei). After the publication of the KamLAND result in Gando et al. [2013], new reference models with refined crustal compositions become available in the literature. If the reference model [Enomoto et al. 2007] used in the analysis in Gando et al. [2013] is replaced with a modern model [Huang et al. 2013], the radiogenic heat is esti- mated to be 14.9+9.8−8.3 TW from U and Th. The increase of the radiogenic heat estimation is mainly due to the change in the crustal geo o−e flux contribution, which is the offset of the flux-heat conversion function as shown in Figure 4. It indicates, in addition to the determination of the geo o−e flux, a precise estimate of the crustal contribution is of critical importance in proving the radiogenic heat in the mantle. In the future, the estimation needs to be further improved by the revision of the regional crustal compo- sitions around Japan, compiling the latest knowledge in geophysics, geochemistry, and local geology. The calcu- lations based on the different Earth models support that the total radiogenic heat is smaller than the heat flow of 47 ± 2 TW from Earth’s surface [Davies and Davies 2010], indicating secular cooling of the Earth. 5. Future prospect Currently, the geoneutrino measurements by two large LS detectors are available; KamLAND in Japan and Borexino in Italy [Bellini et al. 2013]. In the future, there are several plans of geoneutrino detectors at dif- ferent locations; the SNO+ detector close to complet- ing the construction in Canada [Chen 2006], the JUNO detector in China [Han et al. 2016], the LENA detector in Europe [Wurm et al. 2012], and the movable Hanohano detector in a deep ocean [Learned et al. 2008]. Those detectors will have LS mass larger than KamLAND, so we expect the reduction of the statistical uncertainties on the measured o−e flux. At the KamLAND location, KAMLAND: GEO-NEUTRINO MEASUREMENT IN JAPAN Figure 3. Confidence level (C.L.) contours for the observed number of geo o−e events in U and Th. The small shaded region represents the prediction from the reference model of Enomoto et al. [2007]. Figure 4. Three BSE compositional estimates of geo o−e flux and ra- diogenic heat from 238U and 232Th [Gando et al. 2013]. The colored shaded regions indicate the different mantle model predictions from cosmochemical, geochemical, and geodynamical estimates, and are compared with the KamLAND measurement (gray band). The vari- ations due to the radiochemical distributions are represented by the two solid lines with different slopes; the homogeneous and sunken- layer hypotheses. The variations after adding the uncertainty in the crustal contribution are indicated by the two dashed lines. the geo o−e contribution from the mantle is only about one fourth of the total geo o−e flux, and may be esti- mated based on the subtraction of the crustal contri- bution depending on the crustal model. On the other hand, a detector in an oceanic location (e.g. Hanohano) has less exposure to geo o−e´s from thick continental crusts, and will be more appropriate to measure the mantle flux. Such multi-site data at different geological locations will be useful to construct a detail map of neutrino sources inside the Earth. In addition, detec- tors distant from commercial reactors will reduce the uncertainties on the measured geo o−e flux. Alternative approach to mapping neutrino sources is the directional measurement of o−e´s using a detector with a sensitivity of coming o−e direction [Shimizu 2007]. Such measurements will provide the information on separate contributions from the crust and mantle even at locations near continental crusts, based on the nadir angle distribution. A 6Li-loaded detector with rel- atively good angular resolution can be a candidate, and the possibility of applications for exploring hypotheti- cal magmatic reservoirs was studied in Tanaka and Watanabe [2014]. They are new experimental techniques imaging Earth’s interior using neutrinos. Further de- velopment of the detector performance and application researches are anticipated in the future. 6. Conclusion The KamLAND data collected between March 2002 and November 2012 provides a most precise meas- urement of the geoneutrino flux. The observed energy spectra are consistent with the expectation of 238U and 232Th decays in the Earth. Based on an appropriate geo- physical assumption, the observed geoneutrino flux is in agreement with the predictions from three BSE models within ~2v C.L. Currently, due to the limited statistical power, the clear discrimination between models has not been achieved yet. In addition, the crustal contribution needs to be estimated more pre- cisely based on the latest knowledge in geophysics, geo- chemistry, and local geology. In the future, if multi-site measurements with larger statistics at different geolog- ical locations become available, a detail map of neu- trino sources inside the Earth can be constructed. The directional measurement of o−e can be an alternative ex- perimental technique imaging Earth’s interior. References Araki, T., et al. 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Watanabe (2014). 6Li-loaded directionally sensitive anti-neutrino detector for pos- sible geo-neutrinographic imaging applications, Sci. Rep.-UK, 4, 4708 EP. Wurm, M., et al. (2012). The next generation liquid- scintillator neutrino observatory LENA, Astropart. Phys., 35, 685-732. Corresponding author: Itaru Shimizu, Research Center for Neutrino Science, Tohoku University, Sendai, Japan; email: shimizu@awa.tohoku.ac.jp. © 2017 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved. 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