Deep Ocean Neutrino Sciences

Large Scintillating Liquid Detector

A large scintillating liquid detector with modest energy threshold and resolution is capable of significant multidisciplinary science. The specific suite of scientific measurements depends on detector location. Many of the potential measurements are possible with the proposed 50 kiloton LENA project at the Pyhasalmi mine in Finland. Deployment of a similar detector in the deep ocean accesses unique geosciences measurements without sacrificing any on the LENA program. The following describes some of the scientific measurements possible with a 50 kiloton scintillating liquid detector deployed in the deep ocean.

     Neutrino Geosciences

The deep ocean detector would efficiently detect antineutrinos from the natural decay series of uranium-238 and thorium-232. These antineutrinos, which are called geo-neutrinos, have energies of less than 3.3 MeV. The interaction rate of geo-neutrinos in a 50 kiloton detector located in the deep ocean far from continents and nuclear reactors is predicted to be high enough to uniquely sample radioactivity in the mantle. Such sampling is not possible with continental detectors due to systematic uncertainties associated with the predicted distribution of radioactivity in the lithosphere. Of significant importance are measurements that constrain radiogenic heating in the mantle, resolve the predicted sub-chondritic thorium to uranium elemental ratio in the mantle, and probe radioactivity levels in seismically-resolved structures (LLSVPs) in the deep mantle.

     Astrophysics

A significant source of antineutrinos with energy above 10 MeV and below 30 MeV is all past core-collapse supernovae. Measuring these antineutrinos, which are known as the diffuse supernova neutrino background and relic supernova neutrinos, and their energy spectrum probes the rate of star formation in the universe at different epochs. One challenge of this as yet unrealized measurement is identification of neutral current quasi-elastic atmospheric neutrino interactions. The energy range of 10 - 30 MeV is set by background from nuclear reactors at the low end and from charged current atmospheric neutrino interactions at the high end. An advantage of the deep oceean detector is the opportunity for deployment at locations minimizing the fluxes of antineutrinos from reactors and the atmosphere, thereby enhancing sensitivity to relic supernova neutrinos.

     Particle Physics

The unification of the strong and electroweak forces in the very early universe is an important prediction of supersymmetric grand unified theory. A clear signature of this prediction is proton decay to neutrino plus kaon. Large scintillating liquid detetors, such as LENA, have unique sensitivity to the kaon mode of proton decay. Experimental evidence for supersymmetric grand unification indicates an explanation for the dominance of matter over antimatter in the unverse by the process of leptogenesis.

The discovery of flavor conversion of solar neutrinos by the LMA-MSW-predicted survival probability clearly establishes the mixing of neutrino flavor states and mass states. Deviations of the predicted survival probability in the energy spectrum of boron-8 solar neutrinos are sensitive probes of new physics, such as non-standard neutrino interactions and light sterile neutrinos. A large, self-shielding scintillating liquid detector, like LENA, has the ability to measure the boron-8 solar neutrino energy spectrum between 2 and 4 MeV, which is the region where deviations from the LMA MSW prediction indicate new physics.

     Cosmic Rays

The flux of secondary cosmic rays, including atmospheric neutrinos, at the surface of Earth is strongly influenced by the geomagnetic field. In addition to being a major source of background for experiments searching for nuclean decay and relic supernova neutrinos, atmospheric neutrinos encode Earth matter effects with sensitivity to neutrino mass ordering. A large scintillting liquid detetor in the deep ocean offers an opportunity to measure atmospheric neutrinos at various geomagnetic latitudes for improving experimental sensitivities by validating calculated predictions.

A significant source of background for experiments operating with an overburden of less than 5 km of water equivalent is cosmic ray muons. Cosmic ray muons produce fast neutrons and nuclear spallation, which interfere with measurements of many rare processes. The production rates of fast neutron and nuclear spallation, which depend on the flux and average energy of cosmic ray muons at the detector, are typically calculated from emprical data based on estimates of overburden and material properties of surrounding material. A large scintillating liquid detector, which is readily deployed at varying depth at specific geomagnetic latitudes, is capable of measuring the production rates of fast neutrons and nuclear spallation of carbon. Such measurements improve sensitivity to rare processes by validating and adding precision to the calculated estimates at other sites.

Workshops

Cool Websites

References

Ocean Bottom KamLAND

Ocean Bottom KamLAND (OBK) is a proposed large liquid scintillation detector designed to be transportable on the ocean surface, deployable to great depths, and recoverable for operation at different locations. At 50 kilotons of target mass it is capable of multidisciplinary science, including geology, astrophysics, particle physics, and cosmic rays. When deployed to depths of 6 kilometers or more, background due to cosmic ray muons is virtually eliminated.

Hawaii Anti-Neutrino Observatory

HANOHANO is a proposed 10 kiloton liquid scintillation detector designed to be transportable and deployable in the deep ocean. For particle physics use it can be positioned ~50km offshore from a reactor complex and make measurements of the unknown neutrino oscillations mixing angle theta_13. Moreover if this angle is not too small, then it can determine the neutrino mass hierarchy, in a means not dependent upon matter effects, and relatively insensitive to systematic effects.

The other goal is to measure for the first time the neutrino flux from uranium and thorium from the Earth's mantle. This opens a new field, after the initial geoneutrino detections from KamLAND, and will help solve the mystery of the source of heat in the Earth. The heat which drives tectonics and the geomagnetic field remains controversial in source and magnitude.

Initial engineering and design studies have been carried out, along with some critical laboratory tests, and feasibility has been established. (Link to report completed under CEROS Contract No. 53439)

     Neutrino Oscillation Studies

We describe a method for determining the hierarchy of the neutrino mass spectrum and theta_13 through remote detection of electron antineutrinos from a nuclear reactor. This method utilizing a single, 10-kiloton scintillating liquid detector at a distance of 50-64 kilometers from the reactor complex measures mass-squared differences involving nu_3 with a one (ten) year exposure provided sin^2(2theta_13)>0.05 (0.02). Our technique applies the Fourier transform to the event rate as a function of neutrino flight distance over neutrino energy. Sweeping over a relevant range of delta_m^2 resolves separate spectral peaks for delta_m^2_31 and delta_m^2_32. For normal (inverted) hierarchy |delta_m^2_31| is greater (lesser) than |delta_m^2_32|. This robust determination requires a detector energy resolution of 3.5%/sqrt(E). (Link to arXiv paper)

     Geoneutrino Studies

The Hawaii Anti-Neutrino Observatory (Hanohano) is a deep ocean project to detect natural neutrinos throughout the Earth and its core. (Link to arXiv paper) Natural neutrinos (anti-electron neutrinos) arise from the decay of radioactive isotopes of uranium, thorium, and potassium in the crust and mantle (geo-neutrinos) and possibly from a nuclear reactor in the core (geo-reactor neutrinos). The spectrum of neutrinos from the decay chains of uranium and thorium extends above the energy threshold for inverse neutron decay (1.8 MeV). Detection (background reduction) is facilitated by the delayed coincidence signal from the capture of the neutron. Potassium neutrinos are below threshold for this reaction.

The detector concept is similar to that employed by the operating KamLAND experiment in Japan. Detection of inverse neutron decay using scintillation liquid viewed by photomultiplier tubes is traditional, starting with the seminal experiments of Reines and Cowan. The KamLAND experiment, which is situated for observing neutrinos from nuclear power reactors, demonstrates the capability for detecting lower energy geo-neutrinos. The geo-neutrino signal at the KamLAND site originates primarily from radioactive elements in the continental crust.





To reduce background from nuclear power reactors and to gain sensitivity to radioactive elements in Earth's mantle Hanohano is placed on the oceanic crust near Hawaii. There are significant engineering challenges associated with operating a sophisticated detector in a remote, deep ocean environment. Physicists and geologists at the University of Hawaii, Tohoku University, the University of Maryland, and others along with engineers at Makai Ocean Engineering are investigating these challenges and developing solutions. (Link to Hanohano paper)

this site developed by steve dye, feb. 2005