Detection of Geoneutrinos: Can We Make the Gnus Work for Us?

arXiv:0810.3736v1 [physics.geo-ph] 21 Oct 2008 Detection of Geoneutrinos: Can We Make the Gnus Work for Us? John G. Learned Department of Physics and Astronomy, University of Hawaii, 2505 Correa Road, Honolulu, HI 96822 USA The detection of electron anti-neutrinos from natural radioactivity in the earth has been a goal of neutrino researchers for about half a century[21, 22, 23, 24, 25, 26]. It was accomplished by the KamLAND Collaboration in 2005[27], and opens the way towards studies of the Earth’s radioactive content, with very important implications for geology. New detectors are operating (KamLAND[3] and Borexino[2]), building (SNO+[4]) and being proposed (Hanohano, LENA, Earth and others) that will go beyond the initial observation and allow interesting geophysical and geochemical re- search, in a means not otherwise possible. Herein we describe the approaches being taken (large liquid scintillation instruments), the experimental and technical challenges (optical detectors, direc- tionality), and prospects for growth of this field. There is related spinoffin particle physics (neutrino oscillations and hierarchy determination), astrophysics (solar neutrinos, supernovae, exotica), and in the practical matter of remote monitoring of nuclear reactors. PACS numbers: I. INTRODUCTION: GEONEUTRINO STUDIES STARTED The preceding paper, “Why Geoneutrinos are Interesting”[1], really sets the stage for this contribu- tion to NU2008. McDonough explains that the flux of geoneutrinos coming mainly from the natural radioactive decay chains of Uranium and Thorium from throughout the earth, serves as a tag for the abundance and loca- tion of these rare isotopes. While much of this radioac- tive material is surely in the relatively thin continental crusts, roughly one half resides in the mantle. Quite surprisingly to many physicists, these most detectable of the natural decay neutrinos are thought to originate in the decay chains that constitute the major source of the Earth’s internal heating. That heating in (or under) the mantle of course is responsible for all of the man- tle circulation which produces continental drift, seafloor spreading, mid-ocean volcanoes and traveling hot spots, and of course earthquakes and tsunamis. Moreover, the geomagnetic field is thought to be produced in the outer core of the earth, which is liquid (as we know from the lack of seismic shear waves), largely composed of Iron and Nickel, and which convects much faster than the viscous mantle material (mostly silicates). There is not much certain about the depth and configuration of the mantle convection, nor of the lateral homogeneity of the U/Th abundances. Moreover, there is no consensus about the exact magnetohydrodynamical processes in operation to produce the geomagnetic fields, which do indeed change significantly on a human timescale though having been present for billions of years. In principle one can perform a kind of tomography with neutrinos to map out the distribution of the sources. The job is however a tough one, being that thousand ton scale detectors (e.g. KamLAND) are required to even begin first detection measurements. Moreover the high sensi- tivity required demands employment of expensive liquid scintillators in order to produce significant light (yielding 30-50 times that from a Cherenkov detector) and further, almost all neutrino directionality is lost. Further, delicate care must be taken for radiopurity, now well understood but not easy. The process employed for detection of the anti- neutrinos is the inverse beta decay, used by researchers since the initial observations of these neutrinos by Cowan and Reines in the 1950’s. The signature consists of two flashes of light, near in time and space, and of similar am- plitude. The first flash is due to the annihilation of the positron which results from a (free) proton being struck by an electron-antineutrino (one can think of it as the neutrino stealing a charge from the proton). The neu- tron is then free to wander about until it captures on another proton to form Deuterium, with the liberation of the 2.2 MeV binding energy. The primary interaction has a threshold of the proton-neutron mass difference, and is 1.3 MeV. Hence the key geonu signature is the detection of a primary flash equivalent to a neutrino en- ergy between roughly 1.3 and 3.6 MeV (and consisting of a thousand or so photons), a second flash equivalent to 2.2 MeV and delayed by about 200 microseconds, and ev- erything originating in a region on the order of one meter in size in the detector. This forms a beautiful discrimi- nant against non-anti-neutrino backgrounds of order 109 (depending on size, depth and other factors, including rejection of solar neutrinos which make only one flash). II. VARIOUS EXPERIMENTS The following table indicates the operating, soon to be operating, and proposed experiments of relevance around the world. KamLAND has been operating for 6 years now, and is described in the talk of Decowski[3

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