CONSIDERATIONS FOR A DEDICATED GEONEUTRINO DETECTOR FOR GEOSCIENCES T21C-1978, Poster presentation in AGU Special Session: Terrestrial Heat Flux and Geoneutrinos: Estimating Earth's Heat Flux AGU Fall Meeting, December 15-19, 2008, San Francisco, CA, USA, EOS Trans. AGU, 2008, 89(53), Fall Meet. Suppl., Abstract T21C-1978. P. ILA1, W. GOSNOLD2, P. JAGAM3, G. I. LYKKEN4 1. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 The NORM Group Organization, Cambridge, MA, USA 02139 [email protected]; [email protected] 2. Department of Geology and Geological Engineering, University of North Dakota, Grand Forks, ND, USA 58202 [email protected] 3. Department of Physics, University of Guelph, Guelph, ON, Canada N1G1L9 SNO Collaboration, Canada; The NORM Group Organization, Guelph, ON, Canada N1G1L9 [email protected]; [email protected] 4. Department of Physics, University of North Dakota, Grand Forks, ND, USA 58202 [email protected] For the GRAFG collaboration [GRAFG – Geoneutrino Radiometric Analysis For Geosciences] Contact info: [email protected] [email protected] 1 ABSTRACT A combination of several sources including: radiogenic heating, processes of mantle and core formation and differentiation, delayed radiogenic heating, earthquakes, and tidal friction account for the surface heat flux in the Earth. Radiogenic heating is of much interest in various fields of geosciences. Inferences from recent experiments with reactor antineutrinos and solar neutrinos showed that the age of geoneutrinos is at hand for constraining radiogenic heat. Because of the deep penetrating properties of the neutrinos this type of radiation in the decay of the heat producing elements (HPE) is ideally suited for the investigation of the deep interiors of the Earth compared to conventional radiometric methods for HPE employing alpha-, beta- and gamma rays. This presentation will address the considerations for a dedicated geoneutrino detector to be set up for investigating the interior regions all the way to the center of the Earth. 2 INTRODUCTION Knowledge of energy sources in the Earth is of increasing interest from many different points of view in the geosciences. Using geothermal conductivity information Kelvin estimated the age of the Earth. This age estimate was not acceptable when compared to other evidence. This disagreement led to the searches for the identification of other sources of heat production within the Earth. Natural radioactivity as a heat source in the Earth and Heat Producing Elements (HPE): Natural radioactivity in the Earth was quickly recognized as a heat source soon after its discovery. Radiogenic heat was investigated as the source of heat flux over and above the primordial heat in the Earth. Abundances of the heat producing elements in the Earth, namely K, U, Th, in the Earth’s crust are investigated extensively. Dependence of investigations for assaying the HPE: These investigations were dependent on geological sampling and geochemical assay techniques. Radiometric and X-ray techniques for assaying HPE evolved rapidly with developments in instrumental analysis. Techniques based on radioactive radiations exploited the signals generated by characteristic and x-rays from the HPE with high resolution and high sensitivity radiation detectors. Limitations of geochemical assay techniques and sampling: These instruments and techniques were limited by the penetrating power of the radiation used in the assay techniques employed in the laboratory, for in-situ assaying in the field, or in the context of assaying the whole Earth. Over a period of time, the limitations of the geochemical assay techniques developed for HPE determinations based on sampling techniques were identified. In-situ sampling was needed to reduce the cost of field sampling. Techniques for sampling at ever increasing depths from the surface were needed to investigate the interior regions of the Earth. 3 Particle Physics instrumentation capability: Interest in the investigation of the particle physics properties of a weakly interacting highly penetrating radioactive radiation led to the development of sophisticated instruments and sensitive techniques for detecting this radiation. It is these instruments and techniques, which are of interest to the geoscientists for the investigation of the deep interior of the Earth. This presentation tries to discuss and understand the basic considerations in the selection and deployment of these particle physics instruments and techniques for whole Earth assay of HPE, and for tomographic assay to assign the HPE to specific regions of the Earth. Focus of this presentation: The focus of this presentation is to 1) identify the considerations for the selection and deployment of dedicated instruments for detecting the HPE concentrations in the deep interiors of the Earth, which are otherwise inaccessible by conventional sampling techniques, 2) discuss and understand the basic considerations in the selection and deployment of these particle physics instruments and techniques for whole Earth assay of HPE and for tomographic assay to assign the HPE to specific regions of the Earth. NEUTRINO TERMINOLOGY AND PROPERTIES Terminology: Neutrinos and antineutrinos are emitted in nuclear positive and negative decays. Together they are generally called neutrinos except when referring to the specific type. An example of neutrino emission from a HPE is shown below in the decay of Potassium-40. Positive decay Negative decay 4 The subscript on the symbol for neutrino indicates that the neutrinos are emitted in nuclear beta- decay compared to other types of neutrinos emitted in other types of radioactive decays, which emit other types of neutrinos. Positive beta decay accompanied by neutrinos: A proton in an unstable nucleus becomes a neutron emitting a positron and a neutrino. Negative beta decay accompanied by antineutrinos: A neutron in an unstable nucleus becomes a proton emitting an electron and an antineutrino. Inverse beta decay capturing antineutrinos by protons in a detector: An incoming antineutrino interacts with a proton in the detection medium releasing a neutron and a positron. Penetrating power and directionality: Compared to the and x-rays emitted in the radioactive decay of unstable elements, neutrinos are weakly interacting particles. Therefore, they are not stopped or scattered from their initial direction of travel or, their intensity attenuated by absorbers or shielding materials commonly used with the other radiations as shown in figure 1. Figure 1. Relative advantage of using antineutrinos as a radiation probe in radiometric techniques for the assay of the heat producing elements (HPE). Neutrinos travel in straight lines from the point of origin to the point of detection. This characteristic is advantageous for bulk in-situ assay in the field or, for whole Earth assay and for tomography of the radioactivity in the Earth. 5 GEONEUTRINOS AND HEAT PRODUCING ELEMENTS Geoneutrinos are the antineutrinos produced during the negative beta-decay of the long-lived isotopes of the unstable elements in the Earth. The predominant production is from Potassium, Uranium, and Thorium, which are usually referred to as radiogenic Heat Producing Elements (HPE). The antineutrino production rates for the HPE are given in Table 1. Table 1. Radioactive half-lives and antineutrino production rates of the predominant heat producing elements in the Earth [Ref. Eder]. The production rates quoted above are estimates from isotopic abundances of elemental uranium, thorium and potassium by weight. The antineutrino production rates given in Table 2 are calculated from known concentrations of HPE in the specified regions of the Earth. For the lower mantle, observational data are not available; the production rates are based on the Bulk Silicate Earth model that describes the present crust-plus- mantle system based on geochemical arguments. According to geochemical arguments, negligible amounts of U, Th, and K should be present in the core [Ref. Mantovani et al]. 6 Table 2. Estimated antineutrino production rates of the predominant heat producing elements in the Earth. Note that the estimate from the core regions of the Earth is zero [Ref. Mantovani et al]. 7 Antineutrinos of K, U and Th: From the above table it can be seen that potassium in the Earth is the dominant producer of antineutrinos. Thorium and uranium contribute about the same to the antineutrino production rate. However, the antineutrinos emitted by potassium have lower distribution of energies compared to those emitted from thorium and uranium. This difference in the antineutrino spectra has to be taken into account when considering the detection of HPE by instrumental techniques. In addition, to identify thorium and uranium separately with antineutrino detectors, consideration should be given to the instrumentation capabilities for resolving the data by energy dispersive spectrometry. These considerations will be elaborated below. Earth as a source of antineutrinos: The different Earth regions are shown schematically in figure 2. It should be recognized that the Earth is a distributed volume source of antineutrinos emitted by the HPE. In addition, the average concentrations in the different regions of the individual HPE are widely different over and above the local variations due to mineralization. Figure 2. Cross-sectional view of the interior regions of the Earth based on Anderson (2007). The radial thicknesses of