arXiv:0810.3736v1 [physics.geo-ph] 21 Oct 2008 rtdtcinmaueet.Mroe h ihsensi- high begin the even Moreover to required measurements. are detection scale first ton KamLAND) thousand that (e.g. being The one, detectors tough sources. a the however of is distribution job the out map to neutrinos been years. having of though billions timescale for change human present indeed a do on which significantly fields, geomagnetic to the the operation about produce in consensus U/Th processes no magnetohydrodynamical the is exact of there homogeneity Moreover, mantle lateral much the abundances. the of not of configuration is nor and convection, There depth the about silicates). viscous certain the (mostly the than material from faster much know mantle and convects we Iron which of (as and composed liquid Nickel, largely waves), is shear outer seismic which the of earth, lack in the produced be of the to Moreover, core thought tsunamis. is field and geomagnetic man- earthquakes spots, course the hot of traveling of and and seafloor all volcanoes drift, for mid-ocean continental spreading, responsible produces which is under) circulation course of (or tle in of source heating mantle major That the the heating. originate constitute internal to that Earth’s thought chains the Quite are decay detectable neutrinos the most mantle. decay in natural these the the physicists, in of many continental resides thin to half relatively radioac- surprisingly one loca- the this roughly in and of much surely abundance crusts, While is the material isotopes. for tive rare tag these a of as throughout tion serves from Thorium earth, of and the flux Uranium of the radioactive chains natural contribu- that decay the explains from this mainly McDonough for coming geoneutrinos stage NU2008. the to tion sets really Interesting”[1], yee yREVT by Typeset .ITOUTO:GOETIOSTUDIES GEONEUTRINO INTRODUCTION: I. npicpeoecnpromakn ftmgah with tomography of kind a perform can one principle In are Geoneutrinos “Why paper, preceding The ntepatclmte frmt oioigo ula reac nuclear numbers: of PACS monitoring remote of matter practical r astrophysics the is in There determination), hierarchy field. describ this and of we oscillations growth Herein for t prospects and intere and possible. experimental tionality), allow otherwise the and instruments), not scintillation observation means liquid initial a (H the in proposed beyond search, being go and will (SNO+[4]) New that building geology. for Borexino[2]), implications toward and way important 23, the very 22, opens with century[21, and content, a 2005[27], in half Collaboration about KamLAND for researchers neutrino of h eeto feeto ninurnsfo aua radi natural from anti-neutrinos electron of detection The E X STARTED eateto hsc n srnm,Uiest fHawaii, of University Astronomy, and Physics of Department a eMk h nsWr o Us? for Work Gnus the Make We Can 55Cre od oouu I982USA 96822 HI Honolulu, Road, Correa 2505 eeto fGeoneutrinos: of Detection onG Learned G. John eeto fslrnurnswihmk nyoeflash). one including only factors, make which other neutrinos and solar of depth discrimi- rejection size, beautiful 10 a on order forms of (depending This backgrounds non-anti-neutrino meter detector. against one nant the of in order the size on in ev- to region and a equivalent microseconds, in flash 200 originating about second erything by a delayed and photons), MeV of so 2.2 consisting the (and or en- is MeV thousand neutrino 3.6 signature a a and geonu 1.3 to roughly equivalent key between flash ergy the primary difference, a Hence mass of MeV. proton-neutron detection 1.3 the is interaction of primary and on threshold The liberation captures energy. a the it binding has with MeV until 2.2 Deuterium, about the neu- form of wander The to to proton the proton). free another as the then it from is of charge tron think a struck can the stealing being (one of neutrino proton annihilation electron-antineutrino (free) the an a to by from due results is am- which similar flash two positron of first of and space, The consists and signature time plitude. in The near light, 1950’s. of flashes the Cowan in researchers by neutrinos Reines by these and used of decay, observations initial beta the inverse since the is neutrinos understood well now easy. radiopurity, not for but taken be delicate Further, must care lost. is further, directionality and neutrino detector) all Cherenkov almost a from (yielding that light times significant 30-50 produce liquid to order expensive in of scintillators employment demands required tivity o,adi ecie ntetl fDcwk[] twas It Decowski[3]. years of 6 talk for the operating in described been is has and KamLAND now, world. around relevance the of experiments proposed and operating, h rcs mlydfrdtcino h anti- the of detection for employed process The h olwn al niae h prtn,so obe to soon operating, the indicates table following The cnclcalne otcldtcos direc- detectors, (optical challenges echnical slrnurns uenve xtc) and exotica), supernovae, neutrinos, (solar ltdsio npril hsc (neutrino physics particle in spinoff elated 4 5 6.I a copihdb the by accomplished was It 26]. 25, 24, tors. eetr r prtn (KamLAND[3] operating are detectors tn epyia n eceia re- geochemical and geophysical sting atvt nteerhhsbe goal a been has earth the in oactivity h prahsbigtkn(large taken being approaches the e nhn,LN,Erhadothers) and Earth LENA, anohano, I AIU EXPERIMENTS VARIOUS II. tde fteErhsradioactive Earth’s the of studies s 9 2 the first to report detection of neutrinos from the earth, as was done in a cover article in Nature in 2005[27]. This TABLE I: List of various geoneutrino experiments. detection, now improved since publication, presented the Detector Region Location Size Status (Start) KamLAND Japan Mine 1000 T Operating (2002) first demonstration of the expected signal from terrestrial Borexino Italy Tunnel 100 T Operating (2007) radioactivity and, though feeble in statistical power, is SNO+ Canada Mine 1000 T Construction (2010) roughly in agreement with expectations (to order 20%). Hanohano Pacific Ocean 10,000 T Proposed (2013?) Unfortunately measurements on the continental crust LENA Finland? Mine 50,000 T Proposed (?) will mostly measure only those neutrinos originating in EARTH Curacao Drill Holes ?? Discussed (??) the crust in the detector’s neighborhood (500 km or so), with the mantle originating neutrinos only contributing on the order of 25% to the total. Since even the predic- elsewhere[10]. The observatory will record interactions tions of the local crustal neutrino flux are uncertain at of electron antineutrinos of Eν > 1.8MeV by inverse the 20% level, one cannot discern the mantle contribution β-decay in a monolithic cylindrical detector of 10 kt of from a detector location on or near the continental plates. ultra-pure scintillating liquid. An outer surface array of Nonetheless, measurements of the geoneutrino flux from inward-looking 10-inch photomultiplier tubes in 13-inch continental locations are interesting, since neither the to- glass pressure housings will collect scintillation photons. tal amount of the U & Th in the Earth is certain, nor is The planned energy resolution is 3.5%/√Evis, with the distribution between crust and mantle. Evis = Eν 0.8MeV the visible energy. Sufficient The Borexino detector in a tunnel of the Gran Sasso overburden (3− km or more for most sensitive geonu stud- Laboratory in the Apennines in Italy has started in 2007 ies), adequate shielding (from ocean radioactivity), and and has solar neutrino data already[2], but at 100 tons radio-pure detector components will limit background to mass is too small to make much contribution to the negligible levels providing very high detection efficiency. geoneutrino business. Considerable science potential derives from the abil- The SNO+ detector[4], as a 1000 ton liquid scintilla- ity to deploy the observatory at various deep ocean lo- tor conversion of the older SNO (heavy water) detector cations. An initial deployment offshore a nuclear reactor in Subury Canada, will make interesting crustal measure- complex for measuring neutrino mixing parameters could ments in a location above ancient continental plate. be followed, for example, by a deployment near Hawaii The LENA detector[8] has been talked about for a few for measuring terrestrial antineutrinos. This flexibility years in Europe as a very large, mine based, liquid scin- presents a significant advantage over similar observato- tillator detector in the 50-100 kiloton class. It is now ries at a fixed underground locations. part of the trio of detectors being studied as a European The preliminary design, resulting from a two year engi- Megaproject (MEMPHYS, along with a megaton water neering study by Makai Ocean Engineering, specifies the Cherenkov instrument, Laguna, and a 100 kiloton liquid detector to be a right cylindrical shape, transported in a argon device, Glacier). Various locations have been sug- special barge from which it may be deployed, and recov- gested but the favorite for LENA appears to be in a mine ered. The inner volume of liquid scintillator is separated in Pyhalsalmi, Finland. The LENA team is centered in from the photodetectors by a segmented acrylic layer. Munich, and they have done many excellent studies of The individual optical units are in clusters, in plain oil. the physics and technology for this proposed project. An Outside this stainless steel vessel will be a further layer option to place LENA underwater co-located with the of 2 meters of pure water, and veto photomultipliers (as NESTOR Project near Pylos, Greece has been discussed, well this layer provides access to the inner tank for instal- but appears not to be the main plan at this time. lation and repairs). Very importantly in this design, all The EARTH project has been presented by a fiber-optic and electrical connections will be made pier- Dutch/South African team, with the goal of putting side, tested and calibrated prior to deployment (avoiding a many-armed detector underground on the Island of previous bad experience with unreliable connectors in the Curacao[9]. The notion is to employ long, relatively thin, ocean, and difficult remote connections via robot). The layered neutrino detectors, getting some directionality detector design is aimed at multiple deployments on a from the relative rates in each arm. A detailed detec- roughly annual cycle, allowing changes of venue to follow tor design has not been presented yet, and performance the science. is as yet not well determined.

A. Hanohano Geoneutrino Studies

The Hanohano team aims for measuring the flux of III. HANOHANO U/Th geo-neutrinos from earth’s mantle with 25% un- certainty in one year of operation near Hawaii[28]. In- A deep ocean antineutrino observatory called cluded in this statistic-dominated result is 9% system- Hanohano (Hawaii Anti-Neutrino Observatory) is atic error due to uncertainty in the U/Th content of being developed at Hawaii and with collaborators the crusts. This same uncertainty limits the precision of 3 measurements of the mantle flux at continental locations reactor[16, 17, 18]. The experiment utilizes a near detec- to > 50%. Not included in the analysis is uncertainty tor at the reactor complex for normalizing flux and cross of the neutrino mixing angles θ12[3] and θ13[30]. The section with a far detector at the first minimum of sur- survival probability for fully mixed geo-neutrinos is 59% vival probability. There is agreement that an exposure (+6%, - 15%), as measured at present. The upper (lower) of 60 GW-kt-y of a far detector at a distance of 60 km 2 value obtains with minimum (maximum) present values yields an uncertainty of 2% in the value of sin (θ12) at of the mixing angles. Imprecise knowledge of mixing an- the 68% confidence level, assuming a detector systematic gles and U/Th content of earth’s crusts introduce com- uncertainty of 4% or less. This experiment is analogous parable uncertainties to the measurement by Hanohano to methods proposed for precision measurement of the of geo-neutrinos from the mantle. Nonetheless, deploy- sub-dominant mixing angle θ13[19], namely the reactor ments at several widely-spaced mid-ocean locations test antineutrino flux sampling defines one-half cycle of the lateral heterogeneity of uranium and thorium in the man- oscillating survival probability. The difference is the dis- tle. tance of the first minimum of survival probability, which Geo-neutrinos with energy between 1.8 MeV and 2.3 is 2 km for the θ13 measurement. 238 232 MeV come from both U and Th decay products, The far detector for the θ12 experiment records mul- 2 while those between 2.3 MeV and the maximum energy tiple cycles of δm 13 oscillation given that θ13 > 0, the of 3.3 MeV are only from the 238U decay product 214Bi. detector has adequate energy resolution, and sufficient This spectral feature allows a measurement of the Th/U exposure. There is a plan to measure these cycles in L/E ratio. Although geology traditionally ascribes the chon- space by sampling the Fourier power at different values dritic Th/U ratio of 3.9 0.1 to the bulk earth, samples of δm2[20]. This self-normalizing, robust method offers ± 2 2 from the upper mantle reveal a substantially lower value a precision measurement of δm 13 for sin (2θ13) > 0.05, of 2.6 suggesting layered mantle convection[31]. Geo- determines neutrino mass hierarchy by evaluating asym- neutrino flux measurements sample large volumes of the metry of the Fourier power spectrum, and measures θ13; deep earth providing an important test of mantle convec- all without the need for a near detector. tion models. Hanohano is capable of performing the experiments An earth-centered natural fission reactor[32, 33] is a described above with a one-year deployment offshore a speculative, untested hypothesis. Predicted to be in the suitable nuclear reactor complex. At least two candidate power range of 1-10 TW, it has the potential to explain sites with considerable overburden exist. One is in 1100 the variability of the geo-magnetic field and the anoma- m of water West of the 7 GW San Onofre reactors in lously high helium-3 concentrations in hot-spot lavas. California and the other is in 2800 m of water east of the Fission products from such a geo-reactor would undergo 6 GW Maanshan reactors in Taiwan. Other locations are β-decay, producing antineutrinos with the characteris- possible. tic nuclear reactor spectrum. A one-year deployment of Hanohano at a mid-ocean location well distant from nu- clear power plants tests the existence of the geo-reactor. C. Hanohano Other Physics Opportunities This deployment would set a 99% CL upper limit to the geo-reactor power at 0.3 TW or, were a 1 TW geo-reactor The existence of a detector with the capabilities of to exist, produce nearly a 5σ measurement[29]. Hanohano and the location deep inside the ocean offers an opportunity for several exciting discoveries. Here we indicate some of them briefly. One is a search for an B. Hanohano Neutrino Studies anti-neutrino signal from relic supernovae from the dis- tant past. The signal can be as large as 4 events per Neutrino mixing and oscillation[11] are responsible for year[34]. Of course, the signal from a galactic super- the deficit of solar neutrinos[12], the spectral distortion of nova is much more robust: about 2000 conventional anti- reactor antineutrinos[13], and the deficit of atmospheric electron neutrino capture events, with additional charged muon neutrinos[14], which has been confirmed using an current events from 12N, 12B, and neutral current events accelerator-produced muon neutrino beam[15]. These from scattering on electrons and 12C. There are also initial observations reduce the allowed regions of neu- about 2000 events of elastic scattering on protons. This trino mixing parameter space, guiding future precision would be a signal in addition and complimentary to those measurements of mixing angles and mass-squared differ- from all other detectors around the world. ences, including resolution of the spectrum of neutrino If the purity levels in the scintillating liquid can be masses. Positioning Hanohano 60 km distant from a made as low as in Borexino, then one may search for the nuclear reactor complex enables precision measurement neutrino signal from solar neutrinos especially the pep 2 of θ12 and, for non-zero θ13, δm 31. This latter mea- line and the CNO neutrinos. The signal from pep+CNO surement can lead to a determination of neutrino mass in the window of 0.8 to 1.3 MeV is expected to be about hierarchy. 150 events/day/10kt. This is detectable against an ex- Several authors discuss a precision measurement of the pected background of about 130 events/day/10kt, mostly solar mixing angle θ12 using antineutrinos from a nuclear from 11C. This is very useful in confirming the conven- 4 tionally accepted neutrino parameters and to rule out is given in Bowden[7], where the focus is upon close-in re- some non-standard neutrino properties[35]. actor monitoring. Remote monitoring of reactor activity Finally, Hanohano has the capability to detect the pro- will be possible out to hundreds of kilometers with next ton decay mode p ν + K+. This is expected to occur generation instruments, and some can envisage a world- in super-symmetric→ models. Liquid scintillation detectors wide network of neutrino monitors contributing to anti- an advantage over conventional water-Cherenkov detec- nuclear weapons proliferation efforts in the future[36]. tors by being able to detect the kaon directly. Hanohano can reach a sensitivity of almost 1034 y.

IV. THE SAD STORY OF K40 DETECTION VII. SUMMARY

Geologists would very much like to detect the neutrinos from Potassium-40 decay[1]. This common isotope has The business of geoneutrino detection is already under- different chemical properties than the heavy elements, way at existing large instruments and those being built. and is thus to be found in different regions of the Earth, The first results are in hand, and they will continue to im- from the oceans and crust to the core. The unfortunate prove. The most desirable information about U and Th case is that the end point energy (1.3 MeV) is below content of the mantle (and core) will not be obtained un- the inverse β threshold, and hence the signature of these til a 10 kiloton scale deep ocean instrument is deployed. neutrinos’ interactions is only one flash of light, to be Hanohano is proposed for this task, but is at least several extracted from the solar neutrino and other backgrounds. years from operations. Studies are underway for further We will not say more here, other than to report it as a improvements in detector sensitivity and directional ca- major challenge for which good ideas are needed[24]. pability. Operation of the class of multi-kiloton neutrino detec- tors with MeV level sensitivities will not only open a new V. TOWARDS DIRECTIONALITY area in geology, but will make contributions to neutrino physics and astrophysics, and will pave the path towards A great challenge for a next generation of low energy the creation of networks of detectors for remote nuclear anti-neutrino detectors is achieving some directional res- reactor monitoring. It would appear that there is a bright olution. People have discussed tracking detectors, but future in this emerging field. the sizes required are formidable and not yet practical. The nearest opportunity seems to be with the inverse-β decay itself. The neutron acquires a tiny (few keV/c) amount of momentum from the striking neutrino. If one can record the positron appearance location and the neu- A. Acknowledgments tron absorption location, one can make a poor (O(20o)) angular determination. Improvements in present technol- ogy can come through better vertex resolution, track res- I want to acknowledge help from many others in olution, shorter scintillator emission times, heavier mate- preparing this survey. Particular thanks to Steve Dye, rials (shorter gamma travel distances), and greater neu- Sanshiro Enomoto, Gene Guillian, Eligio Lisi, Bill Mc- tron cross sections. Employment of an alpha emitter can Donough, Sandip Pakvasa, Bob Svoboda, Nikolai Tolich, help in locating the neutron absorption, but the alpha’s and members of the DUSEL, Hanohano, KamLAND, suffer from greatly decreased light yield due to saturation LENA, and SNO groups for graphics and physics help. of the scintillator by heavily ionizing particles. Imaging This work was partially funded by U.S. Department of can help by permitting one to recognize the annihilation Energy grant DE-FG02-04ER41291 and the University of gamma’s topology and getting a better initial neutrino Hawaii. We would like to acknowledge our collaborators interaction vertex. Ditto for the neutron absorption loca- on the Hanohano Collaboration Council: A. Bernstein, tion. Advances in CCDs and optics may make some sig- M. Cribier, F. von Feilitzsch, G. Horton-Smith, K. Inoue, nificant progress here, but not immediately. Studies are T. Lasserre, J. Mahoney, J. Maricic, W. McDonough, S. underway at several institutions (particularly at RCNS, Parke, A. Piepke, R. Raghavan, N. Solomey, R. Svoboda Tohoku) to push ahead in this area. and J. Wilkes.

VI. OTHER APPLICATIONS

Due to lack of space we cannot elaborate upon the References practical applications of these detectors. An introduction 5

[1] W. McDonough, “Why Geoneutrinos are Interesting”, [20] J.G. Learned et al. (2008) Phys. Rev. D (in press); these Proceedings. hep-ex/0612022. [2] C. Galbiati, “Borexino”, these Proceedings. [21] G. Eder, Nucl. Phys. 78 (1966) 657. [3] P. Decowski, “KamLAND”, these Proceedings. [22] G. Marx, Czech. J. Phys. B 19 (1969) 1471. [4] H. Robertson, “SNO”, these Proceedings. [23] C. Avilez, G. Marx, and B. Fuentes, Phys. Rev. D 23 [5] T. Lasserre, “Θ13: Double Chooz and non-Asian Exper- (1981) 1116. iments”, these Proceedings. [24] L.M. Krauss, S.L. Glashow, and D.N. Schramm, Nature [6] C. White, “Θ13: Daya Bay and Asian Experiments”, (London) 310 (1984) 191. these Proceedings. [25] R.S. Raghavan et al., Phys. Rev. Lett. 80 (1998) 635. [7] N. Bowden, “Neutrino Monitoring of Reactors”, these [26] C.G. Rothschild, M.C. Chen, and F.P. Calaprice, Geo- Proceedings. phys. Res. Lett. 25 (1998) 103. [8] A. Rubbia, “Megaton Detectors”, these Proceedings. [27] T. Araki et al., Nature (London) 436 (2005b) 499. [9] R.J. deMeijer, E.R. van der Graaf and K.P. Jung- [28] S. Dye and S. Stein, Eos 87 (2006) 253. man, “Quest for a Nuclear Georeactor”, Nuclear [29] S.T. Dye et al. (2006) hep-ex/0609041. Physics News International, arxiv.org/pdf/0404046. [30] M. Apollonio et al., Eur. Phys. J. C27 (2003) 331-374. Also, arxiv.org/pdf/nucl-ex/0404015. See also [36]. [31] D.L. Turcotte, D. Paul, and W.M. White, J. Geophys. [10] See Hanohano web site, Res. 106 (2001) 4265. http://www.phys.hawaii.edu/∼sdye/hanohano.html. [32] J.M. Herndon, Proc. Nat. Acad. Sci. 93 (1996) 646. [11] R. N. Mohapatra et al., Rept. Prog. Phys. 70 (2007) 1757. [33] D.F. Hollenback and J.M. Herndon, Proc. Nat. Acad. Sci. [12] B. Aharmin et al., Phys. Rev. C72 (2005) 055502. 98 (2001) 110. [13] T. Araki et al., Phys. Rev. Lett. 94 (2005a) 081801. [34] L. Strigari et al., JCAP, 7 (2004) 0403. [14] Y. Ashie et al., Phys. Rev. D 64 (2005) 112005. [35] A. Friedland, C. Lunardini and C. Pena-Garay, Phys. [15] E. Aliu et al., Phys. Rev. Lett. 94 (2005) 081802; D. G. Lett. B594 (2004) 347; O.G. Miranda, M.A. Torlola and Michael et al., Phys. Rev. Lett. 97 (2006) 191801. J. W. F. Valle, JHEP, 8 (2006) 0610. [16] A. Bandyopadhyay, S. Choubey, and S. Goswami, Phys. [36] S.T. Dye, ed., “Neutrino Geophysics: Pro- Rev. D67 (2003) 113011. ceedings of Neutrino Sciences 2005”, Springer [17] A. Bandyopadhyay et al., Phys. Rev. D72 (2005) 033103. , Netherlands 2007; also see web page [18] H. Minakata et al., Phys. Rev. D71 (2005) 013005. http://www.phys.hawaii.edu/∼sdye/nsc.html [19] K. Anderson et al. (2004) hep-ex/0402041.