Neutrino Geophysics Conference Introduction

Long distance detection of electron anti-neutrinos from reactors at distances of order 200 km has been achieved with the 1000 ton liquid scintillator-based KamLAND instrument in Japan. In summer 2005 the KamLAND group reported the first detection of anti-neutrinos from the natural radioactivity of the earth. These measurements are due to uranium and thorium decays dominantly from the nearby crust in Japan, and are expected to have only a small contribution from the earth’s mantle (and core). Several new detectors are under consideration around the world for measurements which when taken together can reveal the location of these heavy elements, which are expected to contribute a major share of the internal earth’s heating via their radioactivity. This heating is of course associated with providing the power to drive the geomagnetic field and plate tectonics. Geologists have only indirect evidence about the deep earth, mostly from seismic wave velocity and inferences from a few meteorites. Anti-neutrino detection, on the other hand, yields direct information about earth’s interior. The location and magnitude of the earth’s uranium and thorium are crucial to understanding the origin and evolution of the earth and present day activity. Lead Article in Proceedings of Neutrino Sciences 2005     

Experimental Study of Geoneutrinos with KamLAND

The Kamioka liquid scintillator antineutrino detector (KamLAND), which consists of 1000 tones of ultra-pure liquid scintillator surrounded by 1879 photo-multiplier tubes (PMT), is the first detector sensitive enough to detect geoneutrinos. Earth models suggest that KamLAND observes geoneutrinos at a rate of 30 events/10³²-protons/year from the ²³⁸U decay chain, and 8 events/10³²-protons/year from the ²³²Th decay chain. With 7.09×10³¹ proton-years of detector exposure and detection efficiency of 0.687 ± 0.007, the ‘rate-only’ analysis gives geoneutrino candidates. Assuming a Th/U mass concentration ratio of 3.9, the ‘rate + shape’ analysis gives the 90% confidence interval for the total number of geoneutrinos detected to be from 4.5 to 54.2. This result is consistent with predictions from the Earth models. The 99% C.L. upper limit is set at 1.45×10⁻³¹ events per target proton per year, which is 3.8 times higher than the central value of the model prediction that gives 16 TW of radiogenic heat production from ²³⁸U and ²³²Th. Although the present data have limited statistical power, they provide by direct means an upper limit for the Earth’s radiogenic heat of U and Th. Sanshiro Enomoto (on behalf of the KamLAND Collaboration)     

Earth Radioactivity Measurements with a Deep Ocean Anti-neutrino Observatory

We consider the detector size, location, depth, background, and radio-purity required of a mid-Pacific deep-ocean instrument to accomplish the twin goals of making a definitive measurement of the electron anti-neutrino flux due to uranium and thorium decays from Earth’s mantle and core, and of testing the hypothesis for a natural nuclear reactor at the core of Earth. We take the experience with the KamLAND detector in Japan as our baseline for sensitivity and background estimates. We conclude that an instrument adequate to accomplish these tasks should have an exposure of at least 10 kilotonne-years (kT-y), should be placed at least at 4 km depth, may be located close to the Hawaiian Islands (no significant background from them), and should aim for KamLAND radio-purity levels, except for radon where it should be improved by a factor of at least 100. With an exposure of 10 kT-y we should achieve a 25% measurement of the flux of U/Th neutrinos from the mantle plus core. Exposure at multiple ocean locations for testing lateral heterogeneity is possible.     

Is there a Nuclear Reactor at the Center of the Earth?

In this paper we discuss the Herndon hypothesis that a nuclear reactor is operating at the center of the Earth. Recent experimental evidence shows that some uranium can have partitioned into the core. There is no viable mechanism for the small amount of uranium that is dissolved in the molten metal to crystallize as a separate uranium phase (uranium metal or uranium sulfide) and migrate to the center of the core. There is no need for an extra heat source, as the total heat leaving the core can be easily provided by “classical” heat sources, which are also more than adequate to maintain the Earth’s magnetic field. It is unlikely that nuclear georeactors (fast breeder reactors) are operating at the Earth’s center.     

Towards Earth AntineutRino TomograpHy (EARTH)

The programme Earth AntineutRino TomograpHy (EARTH) proposes to build ten underground facilities each hosting a telescope. Each telescope consists of many detector modules, to map the radiogenic heat sources deep in the interior of the Earth by utilising direction sensitive geoneutrino detection. Recent hypotheses target the core-mantle boundary (CMB) as a major source of natural radionuclides and therefore of radiogenic heat. A typical scale of the processes that take place at the CMB is about 200 km. To observe these processes from the surface requires an angular resolution of about 3°. EARTH aims at creating a high-resolution 3D-map of the radiogenic heat sources in the Earth’s interior. It will thereby contribute to a better understanding of a number of geophysical phenomena observed at the Earth’s surface. This condition requires a completely different approach from the monolithic detector systems as e.g. KamLAND. This paper presents, for such telescopes, the boundary conditions set by physics, the estimated count rates, and the first initial results from Monte-Carlo simulations and laboratory experiments. The Monte-Carlo simulations indicate that the large volume telescope should consist of detector modules each comprising a very large number of detector units, with a cross section of roughly a few square centimetres. The signature of an antineutrino event will be a double pulse event. One pulse arises from the slowing down of the emitted positron, the other from the neutron capture. In laboratory experiments small sized, ¹⁰B-loaded liquid scintillation detectors were investigated as candidates for direction sensitive, low-energy antineutrino detection.     

Geo-Neutrinos: A Systematic Approach to Uncertainties and Correlations

Geo-neutrinos emitted by heat-producing elements (U, Th and K) represent a unique probe of the Earth interior. The characterization of their fluxes is subject, however, to rather large and highly correlated uncertainties. The geochemical covariance of the U, Th and K abundances in various Earth reservoirs induces positive correlations among the associated geo-neutrino fluxes, and between these and the radiogenic heat. Mass-balance constraints in the Bulk Silicate Earth (BSE) tend instead to anti-correlate the radiogenic element abundances in complementary reservoirs. Experimental geo-neutrino observables may be further (anti)correlated by instrumental effects. In this context, we propose a systematic approach to covariance matrices, based on the fact that all the relevant geo-neutrino observables and constraints can be expressed as linear functions of the U, Th and K abundances in the Earth’s reservoirs (with relatively well-known coefficients). We briefly discuss here the construction of a tentative “geo-neutrino source model” (GNSM) for the U, Th, and K abundances in the main Earth reservoirs, based on selected geophysical and geochemical data and models (when available), on plausible hypotheses (when possible), and admittedly on arbitrary assumptions (when unavoidable). We use then the GNSM to make predictions about several experiments (“forward approach”), and to show how future data can constrain a posteriori the error matrix of the model itself (“backward approach”). The method may provide a useful statistical framework for evaluating the impact and the global consistency of prospective geo-neutrino measurements and Earth models.     

Imaging the Earth’s Interior: the Angular Distribution of Terrestrial Neutrinos

Decays of radionuclides throughout the earth’s interior produce geothermal heat, but also are a source of antineutrinos; these geoneutrinos are now becoming observable in experiments such as KamLAND. The (angle-integrated) geoneutrino flux has been shown to provide a unique probe of geothermal heating due to decays, and an integral constraint on the distribution of radionuclides in the earth. In this paper, we calculate the angular distribution of geoneutrinos, which opens a window on the differential radial distribution of terrestrial radionuclides. We develop the general formalism for the neutrino angular distribution. We also present the inverse transformation which recovers the terrestrial radioisotope distribution given a measurement of the neutrino angular distribution. Thus, geoneutrinos not only allow a means to image the earth’s interior, but offer a direct measure of the radioactive earth, both revealing the earth’s inner structure as probed by radionuclides, and allowing a complete determination of the radioactive heat generation as a function of radius. Turning to specific models, we emphasize the very useful approximation in which the earth is modeled as a series of shells of uniform density. Using this multishell approximation, we present the geoneutrino angular distribution for the favored earth model which has been used to calculate the geoneutrino flux. In this model the neutrino generation is dominated by decays of potassium, uranium, and thorium in the earth’s mantle and crust; this leads to a very “peripheral” angular distribution, in which 2/3 of the neutrinos come from angles θ ≳ 60° away from the nadir. We note that a measurement of the neutrino intensity in peripheral directions leads to a strong lower limit to the central intensity. We briefly discuss the challenges facing experiments to measure the geoneutrino angular distribution. Currently available techniques using inverse beta decay of protons require a (for now) unfeasibly large number of events to recover with confidence the forward scattering signal from the background of subsequent elastic scatterings. Nevertheless, it is our hope that future large experiments, and/or more sensitive techniques, can resolve an image of the earth’s radioactive interior.     

Experimental Status of Geo-reactor Search with KamLAND Detector

A natural nuclear fission reactor operating in the center of the Earth has been proposed by Herndon (Hollenbach and Herndon, 2001) as the energy source that powers the geo-magnetic field. The upper limit on the expected geo-reactor power is set by the estimated 12 TW (Buffett, 2003) heat flow from the Earth’s core. If it exists, a nuclear reactor of that size emits a strong anti-neutrino flux. Emitted electron anti-neutrinos can be detected by the Kamioka liquid scintillator anti-neutrino detector (KamLAND) (Raghavan, 2002), and the geo-reactor power level is proporional to the anti-neutrino emission rate. KamLAND measures the geo-reactor power as a constant positive offset in detected anti-neutrino rate on top of the varying anti-neutrino rate coming from man-made reactors. Here we present the first attempt to measure the geo-reactor power. Based on a 776 ton-year exposure of KamLAND to electron anti-neutrinos, the detected flux corresponds to (6 ± 6) TW. The upper limit on the geo-reactor power at 90% confidence level is 18 TW, which is below the lower limit of the total Earth’s radiogenic heat, estimated to be between 19 and 31TW (Anderson, 2003).     

The content of uranium,thorium, and potassium in the rocks of Venus as measured by Venera 8

A gamma ray spectrometer recording on the surface of Venus from Venera 8 reveals a content of radioactive potassium, uranium, and thorium very similar to acid magmatic rocks on the Earth. Venus is evidently a differentiated planet.     

Sample return of interstellar matter (SARIM)

The scientific community has expressed strong interest to re-fly Stardust-like missions with improved instrumentation. We propose a new mission concept, SARIM, that collects interstellar and interplanetary dust particles and returns them to Earth. SARIM is optimised for the collection and discrimination of interstellar dust grains. Improved active dust collectors on-board allow us to perform in-situ determination of individual dust impacts and their impact location. This will provide important constraints for subsequent laboratory analysis. The SARIM spacecraft will be placed at the L2 libration point of the Sun–Earth system, outside the Earth’s debris belts and inside the solar-wind charging environment. SARIM is three-axes stabilised and collects interstellar grains between July and October when the relative encounter speeds with interstellar dust grains are lowest (4 to 20 km/s). During a 3-year dust collection period several hundred interstellar and several thousand interplanetary grains will be collected by a total sensitive area of 1 m². At the end of the collection phase seven collector modules are stored and sealed in a MIRKA-type sample return capsule. SARIM will return the capsule containing the stardust to Earth to allow for an extraction and investigation of interstellar samples by latest laboratory technologies.     

Probing the Earth’s Interior with the LENA Detector

A future large-volume liquid scintillator detector such as the proposed 50 kton LENA (Low Energy Neutrino Astronomy) detector would provide a high-statistics measurement of terrestrial antineutrinos originating from β-decays of the uranium and thorium chains. Additionally, the neutron is scattered in the forward direction in the detection reaction . Henceforth, we investigate to what extent LENA can distinguish between certain geophysical models on the basis of the angular dependence of the geoneutrino flux. Our analysis is based on a Monte-Carlo simulation with different levels of light yield, considering an unloaded PXE scintillator. We find that LENA is able to detect deviations from isotropy of the geoneutrino flux with high significance. However, if only the directional information is used, the time required to distinguish between different geophysical models is of the order of severals decades. Nonetheless, a high-statistics measurement of the total geoneutrino flux and its spectrum still provides an extremely useful glance at the Earth’s interior.     

Helium in the Martian atmosphere: Thermal loss considerations

Helium concentrations in the Martian atmosphere are estimated assuming that the helium production on Mars, comparable to its production on Earth, via the radioactive decay of uranium and thorium, is in steady state equilibrium with its thermal escape. Although non-thermal losses would tend to reduce the estimated concentrations, these concentrations are not necessarily an upper limit since higher production rates and/or a possibly lower effective exospheric temperature over the solar activity cycle could increase them to even higher values. The computed helium concentration at the Martian exobase (200 km) is 8 × 10⁶ atoms cm^?3. Through the lower exosphere, the computed helium concentrations are 30–200 times greater than the Mariner-measured atomic hydrogen concentrations. It follows that helium may be the predominant constituent in the Martian lower exosphere and may well control the orbital lifetime of Mars-orbiting spacecraft. The estimated helium mixing ratio is greater at the Martian turbopause than at the terrestrial turbopause, and the helium column density in the lower Martian atmosphere may be comparable to that on Earth.     

Some consequences of a steady thermal moon

The Moon is represented as an inhomogeneous spherical body in a steady thermal state. Radioactive heat sources are supposed distributed in a manner which is consistent both with the total measured heat flux near the surface and with the broad seismic evidence. Surface concentrations of uranium and thorium are those suggested by the study of Apollo 11 samples. The resultant internal temperature profile allows the details of Sonett's electrical conductivity profile to be understood if it is accepted that the Moon was not cold 4.5 × 10⁹ yr ago. It would appear further that at least one of the maria was formed by the impact of planetesimals.     

Geo-neutrinos in SNO+

There are plans to fill the Sudbury Neutrino Observatory with liquid scintillator after measurements with heavy water are completed. The new experiment, known as SNO+, would make an excellent detector for geo-neutrinos. SNO+ would be located amidst a thick and uniform region of continental crust, away from nuclear power reactors. As a result, the geo-neutrino signal to reactor background ratio in SNO+ will exceed that from previous measurements. Geo-neutrino measurements by SNO+ will shed light on the amount of uranium and thorium radioactivity in the crust, as well as deeper inside the Earth. Spectral information from SNO+ geo-neutrino detection will provide the first direct measurement of the U/Th ratio.     

Euv Blinkers: The Significance of Variations in the Extreme Ultraviolet Quiet Sun

A search for microflare activity in the extreme ultraviolet (EUV) quiet Sun using the Coronal Diagnostic Spectrometer (CDS) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft has not resulted in the identification of microflare activity, but has resulted in the identification of a hitherto unknown phenomenon: enhancements of a factor of 2–3 in the flux of transition region lines at network junctions. A total of some 6 hours of observation of 5 different target areas showed this ‘blinker’ activity at each area, with durations ranging from 1 to 30 min and averaging 13 min, and thermal energy content of order 10^-6 that of a ‘standard’ flare. Assuming that the observations are of typical quiet Sun, and projecting these data to predict a distribution of these events over the entire Sun, the total thermal energy content of these ‘blinkers’ is insignificant when compared to the energy required to heat the corona. The nature of these events and their significance are discussed in this paper.     

On the internal structures of mercury and venus

Recent radar measures of the radius and mass of Mercury imply a composition for the planet containing about 60% iron. One or other of two conclusions seems inescapable: either that Mercury is a highly exceptional object among terrestrial planets, or that all measures to date of the planet involve substantial systematic error. In either case the situation is such that independent checking of the radius and mass of Mercury by some entirely different means has become of the greatest importance to planetary physics and cosmogony.The recent radar and other determinations of the solid radius of Venus imply an internal structure similar to that of the Earth, namely a liquid core surrounded by a solid mantle and outer-shell zone. The theory also implies that the temperatures within Venus should be slightly higher than at the corresponding parts of the Earth. The proportion of mass in the core of Venus (about 25% of the whole) is entirely consistent with the phase-change hypothesis as to its nature, as of course is also the absence of any liquid or iron core in both Mars and the Moon. On the older iron-core hypothesis, Venus with considerably less iron content by mass than the Earth, and Mars and the Moon with none, would all present problems in different degrees to account for the differences of composition.If Venus began as an all-solid planet, the initial radius would have been about 6300 km, and the total amount of surface reduction to date owing to contraction of the planet would have been almost 40 million km², and as a proportion of the total area only slightly less than the contraction of the Earth. The theory thus predicts the existence of folded and thrusted mountain-systems of terrestrial type at the surface of Venus.     

Heat flow of the Earth and resonant capture of solar <Superscript>57</Superscript>Fe axions

In a very conservative approach, supposing that all heat flow of the Earth is exclusively due to resonant capture inside the Earth of axions emitted by ⁵⁷Fe nuclei on Sun, we obtain limit on the mass of hadronic axion: m _a < 1.8 keV. Taking into account release of heat from decays of ⁴⁰K, ²³²Th, ²³⁸U inside the Earth, this estimation could be improved to the value: m _a < 1.6 keV. Both the values are less restrictive than limits set in devoted experiments to search for ⁵⁷Fe axions (m _a < 216–745 eV), but are much better than limits obtained in experiments with ⁸³Kr (m _a < 5.5 keV) and ⁷Li (m _a < 13.9–32 keV). Published in Ukrainian in Kinematika i Fizika Nebesnykh Tel, 2009, Vol. 25, No. 2, pp. 143–149. The article was translated by the authors.     

On the model of the accumulation of the moon compatible with the data on the composition and the age of lunar rocks

It is suggested that the overall early melting of the lunar surface is not necessary for the explanation of facts and that the structure of highlands is more complicated than a solidified anorthositic ‘plot’. The early heating of the interior of the Moon up to 1000K is really needed for the subsequent thermal history with the maximum melting 3.5 × 10⁹ yr ago, to give the observed ages for mare basalts. This may be considered as an indication that the Moon during the accumulation retained a portion of its gravitational energy converted into heat, which may occur only at rapid processes. A rapid (t < 10³ yr) accretion of the Moon from the circumterrestrial swarm of small particles would give necessary temperature, but it is not compatible with the characteristic time 10⁸ yr of the replenishment of this swarm which is the same as the time-scale of the accumulation of the Earth. It is shown that there were conditions in the circumterrestial swarm for the formation at a first stage of a few large protomoons. Their number and position is evaluated from the simple formal laws of the growth of satellites in the vicinity of a planet. Such ‘systems’ of protomoons are compared with the observed multiple systems, and the conclusion is reached that there could have been not more than 2–3 large protomoons with the Earth. The tidal evolution of protomoon orbits was short not only for the present value of the tidal phase-lag but also for a considerably smaller value. The coalescence of protomoons into a single Moon had to occur before the formation of the observed relief on the Moon. If we accept the age 3.9 × 10⁹ yr for the excavation of the Imbrium basin and ascribe the latter to the impact of an Earth satellite, this collision had to be roughly at 30R, whereR is the radius of the Earth, because the Moon at that time had to be somewhere at this distance. Therefore, the protomoons had to be orbiting inside 20–25R, and their coalescence had to occur more than 4.0x10⁹ yr ago. The energy release at coalescence is equivalent to several hundred degrees and even 1000 K. The process is very rapid (of the order of one hour). Therefore, the model is valid for the initial conditions of the Moon.     

Helium loss from Martian meteorites mainly induced by shock metamorphism: Evidence from new data and a literature compilation

Abstract— Noble gas data from Martian meteorites have provided key constraints about their origin and evolution, and their parent body. These meteorites have witnessed varying shock metamorphic overprinting (at least 5 to 14 GPa for the nakhlites and up to 45–55 GPa (e.g., the lherzolitic shergottite Allan Hills [ALH] A77005), solar heating, cosmic‐ray exposure, and weathering both on Mars and Earth. Influences on the helium budgets of Martian meteorites were evaluated by using a new data set and literature data. Concentrations of ³He, ⁴He, U, and Th are measured and shock pressures for same sample aliquots of 13 Martian meteorites were determined to asses a possible relationship between shock pressure and helium concentration. Partitioning of ⁴He into cosmogenic and radiogenic components was performed using the lowest ⁴He/³He ratio we measured on mineral separates (⁴He/³He = 4.1, pyroxene of ALHA77005). Our study revealed significant losses of radiogenic ⁴He. Systematics of cosmogenic ³He and neon led to the conclusion that solar radiation heating during transfer from Mars to Earth and terrestrial weathering can be ruled out as major causes of the observed losses of radiogenic helium in bulk meteorites. For bulk rock we observed a correlation of shock pressure and radiogenic ⁴He loss, ranging between ?20% for Chassigny and other moderately shocked Martian meteorites up to total loss for meteorites shocked above 40 GPa. A steep increase of loss occurs around 30 GPa, the pressure at which plagioclase transforms to maskelynite. This correlation suggests significant ⁴He loss induced by shock metamorphism. Noble gas loss in rocks is seen as diffusion due to (1) the temperature increase during shock loading (shock temperature) and (2) the remaining waste heat after adiabatic unloading (post shock temperature). Modeling of ⁴He diffusion in the main U, Th carrier phase apatite showed that post‐shock temperatures of ?300 °C are necessary to explain observed losses. This temperature corresponds to the post‐shock temperature calculated for bulk rocks shocked at about 40 GPa. From our investigation, data survey, and modeling, we conclude that the shock event during launch of the meteorites is the principal cause for ⁴He loss.