Title: EARTH: 3-D geoneutrino TOMOGRAPHY
R.J. de Meijer1,2), H.J. W÷rtche2), F.D. Smit3), R.W. Fearick4), E.R. van der Graaf2) and R.G.E. Timmermans2)
1) Foundation EARTH, 9321XS Peize, the Netherlands
2) KVI, Rijksuniversiteit Groningen, 9747 AA Groningen, the Netherlands
3) iThemba Labs, P.O. Box, Somerset West, South Africa
4) Department of Physics, University of Cape Town, Rondebosch, South Africa

Abstract: The multi-disciplinary programme EARTH (Earth AntineutRino TomograpHy) aims at mapping radiogenic heat sources in the Earth's Interior with ultimately an angular resolution of about 3 degrees. Similar to medical tomography this 3-D image is obtained by placing a number (~10) of detector arrangements (antennas) over the Earth's surface. With such an angular resolution the spatial resolution for objects at the Earth's centre or at the core/mantle boundary becomes 300, respectively 150-200km. This will provide unprecedented information on radiogenic heat sources as engines in the machinery of the Earth's Interior. The first antenna is planned to be installed at Curašao, Dutch Antilles.
Antennas are designed to contain a mass of about 4kilotonnes of solid scintillation material. Contrary to the KamLAND detector or the ones planned for Borexino, LENA, Hawaii, or Baksan, all monolithic, spherical arrangements. Angular resolution can only be obtained with small sized detectors, the EARTH antennas will be modular and will consist of many modules, each containing a large number of rod-shaped detector units. The inverse beta-decay results in a positron and a neutron. The positron is almost emitted isotropically, carries almost the full available energy, and is almost instantaneously stopped. The neutron initially carries the direction information of the incident antineutrino, but tends to lose this energy in subsequent collisions with predominantly H nuclei. Geoneutrino detection hence becomes a problem of neutron tracking and small becomes comparable to the distance neutrons travel (a few centimetres). Addition of 10B reduces this loss of direction significantly. Moreover it shortens the time between positron detection and neutron capture and produces an alpha-particle of almost constant energy. Neutron tracking simulations indicate an angular resolution ranging with incident angle from 15 degrees (axial) to 60 degrees (radial) for small-size detectors. The requirements of delayed coincidence within 2 microseconds in one or two adjacent detector units, a spatial separation of the coincident events of <5cm, the constant alpha-particle energy, differences in pulse shape and the reduced sensitivity to muons (simultaneous detection in more than two units) is expected to reduce background by a factor 10^9-10^12. These restrictions are also thought to reduce significantly our sensitivity to the 13C(alpha,n) reaction.
To achieve our ambitions the University of Groningen, the foundation for Astronomical research ASTRON and a private, Curašao foundation JADE. They jointly with physicist of South Africa, industries in the Netherlands and geoscientists in the Netherlands and Germany have started the development of the detector units, their electronics and the geological suitability of the antenna. Depending on the reduction of sensitivity to cosmic induced background the antenna will consist of several directionally drilled shafts or will be placed in a shallow underground laboratory. In a shallow underground laboratory the antenna can be "focussed" on certain hotspots for more detailed information.