Who
The programme was initiated by the Nuclear Geophysics Division of the Kernfysisch Versneller Instituut, Rijksuniversiteit Groningen around the middle of 2003. In an early stage the KVI, iThemba LABS in collaboration with the University of Cape Town, South Africa started to further develop the idea and initiated the first test experiments. Shortly afterwards contacts with the Dutch firm FOCUS OIL and GAS were established. This contact modified the original ideas and led to the present concept of the antenna. A little later the government of the Dutch Antilles was approached on their interest in the programme.

In the beginning of 2004 the first series of funding were granted, which will allow the programme to kick-off. In 2005 further funding has been applied for to demonstrate the proof of principle/concept for the detector and for antenna design.
 

When
To achieve the Earth antineutrino tomography requires a number of crucial steps each followed by a go/no-go decision. This has the advantages that the programme has well defined stages, each stage being precisely defined at the end of its predecessor. It starts out relatively small with an associated budget and will grow in time both in size and in budget.

Stage 1 (~three years)
At present we are working on the initial steps: the development of low-energy antineutrino detectors and their associated electronics, the design of a low-power consuming chip and the feasibility of drilling an antenna at our pilot site Curacao. For the detectors the directional sensitivity is a critical issue and for the chip its power consumption. Having of the order of ten million detector units per antenna, each equipped with one chip, and all arms of the antenna sending up their information can only be achieved by low-power consuming chips. This illustrates one of the technical challenges of the programme.

The drilling aspects include a literature study on the geology of Curacao and preliminary site selection, the technology of drilling, mapping the underground of the selected site by seismic. In addition we intend to have drilled two test holes on 1:1 scale to measure the characteristic of the formation with special attention to radionuclide concentrations and temperature. The latter also to investigate the mining of geothermal energy. In addition permit aspects have to be investigated.

Stage 2 (~three years)
After stage 1 it has become clear that the programme is technically feasible by testing prototype detectors at a set-up near a nuclear power plant. In stage 2 the definite drilling site and drilling configuration should become clear after which drilling should start. A laboratory has to be built to house the control and measuring units. At the same time a test arm will be setup in an underground mine to test the detectors under more realistic conditions and to install and test the data transfer and storage. Subsequently the detectors will lowered in the test holes. Moreover the data-analysis software should be developed. Also this stage is estimated to take about three years.

Stage 3 (~three years)
Stage 3 involves the lowering of the detectors and their chips, testing them and start the first series of measurements. Again in about three years the success of the method should be proven and new antennas should be built. The first results of Curacao should already indicate if the heat sources are homogeneously distributed over the Earth's interior and what nuclear processes are involved. These results are likely to influence the choice of the next set of antennas and possibly even their configuration. (Whereas for Curacao the arms of the antenna more or less evenly cover the Earth Interior, the results may indicate that further antennas should be looking in more detail to certain parts of the Earth's interior).

Stage 4
In this stage the other antennas are being built and the globalization of the project takes place.

 

Where
The detectors will in principle register antineutrinos from all sources. In addition to antineutrinos produced in the Earth's interior, antineutrinos are also produced in radioactive decay of rocks in the crust, nuclear power plants. Energetically these antineutrinos have the same energy characteristics as their counterparts in the interior and therefore form a background. We are convinced that with the anticipated directional sensitivity we will be able to distinguish between the sources at the surface and deeper in the Earth. For the first antenna we like to reduce complexity and have therefore opted for a location far away from nuclear power plants as well as from large granites rock formations, which are traditionally rich in the natural radionuclide 40K, 232Th and 238U. Such a location can be found at a number of places: e.g. Hawaii, Iceland, parts of Southern Africa and Australia, and Curacao.
 

Why
Our choice for Curacao is primarily motivated by the facts that the island is easily accessible from various parts of the world and that idea of the EARTH programme originated in the Netherlands and Curacao is part of the Kingdom of the Netherlands.

Mankind has an irresistible need to explore unknown territories. For that purpose new technologies have been developed from better sailing ships to spacecrafts. Even today missions are on their way to learn more about Mercury and Saturn. The successful landing of the Huygens satellite on Titan is one of the most recent examples. Nevertheless Terra Incognita remains very close, right under our feet. By mining and drilling mankind has drilled to only 13 km into the interior of our own planet.

In the beginning of the 20th century the knowledge on the interior of our planet advanced by the analysis of the reflection and absorption of seismic waves produced in earthquakes. This analysis has led to the present image of an onion like structure with a solid inner core, a liquid outer core and a mantle on which a crust is present. It inspired Jules Verne for its book on a journey to the centre of the Earth.

The interior of the Earth plays an important role for life at its surface. The magnetic field of our planet protects us from a lethal dose of cosmic radiation, whilst earthquakes and volcanic eruptions often notice the drift of the continents. Nowadays some scientists even relate the uneven internal heat flow at the Earth's surface to El Niño and its related impact on weather systems.

These phenomena raise the question where the internal heat of the Earth is generated, by what mechanism and how diffuse these heat sources are. Hypotheses on the internal structure hard to verify and are often invoked to explain certain aspects. Most hypotheses consider a more or less spherically symmetric structure and ignore possible local concentrations. The global surface heat flow, however, ranges over more than an order of magnitude. Is that an indication for energy clustering? And if so, how large are such clusters, where are they located and do they influence the motion in the liquid core, which is responsible for the geomagnetic field?

There is more or less consensus that at least half of the heat is generated by radioactivity. For most scientists this means the decay of the naturally occurring radionuclide. Recently, however, it has been proposed that at the centre of the Earth there is an ~8 km diameter nuclear breeder reactor. Is it possible to prove or disprove these hypotheses?
 

How
A tomographic image is a well-known diagnostic method in nuclear medicine localizing a radioactive substance in the body of a patient by an array of detectors placed around the patient's body (SPECT: Single Photon Emission Computed Tomography). Also in geophysics tomographic images by seismic of the near surface are obtained. In EARTH the radioactive heat sources emit antineutrinos that cross the Earth's interior almost without interaction. Therefore, they are assumed to have traveled along straight lines. The flux of antineutrinos is estimated to be a million per second through an area the size of a human fingernail. This large flux combined with the very low cross section for interaction requires a large-volume detector.

The detection of high-energy (antineutrinos often proceeds via the production of Cerenkov light. The light is emitted in a cone in the direction of the incoming (anti)neutrino. This provides directional sensitivity. For low-energy antineutrinos a capture reaction on a proton is used. In this reaction, referred to as the inverse beta-decay, a positron and a neutron are produced. Roughly speaking, due to the kinematics, the light positron carries the energy information whilst the much heavier neutron is emitted in virtually the same direction as the incoming antineutrino. The neutron slows down in the medium and, depending on the medium, is captured after about 0.2ms. During that time the neutron travels a few centimeters. The positron slows down almost instantly and annihilates with an electron in the detector matter.

The detection mechanism in the proposed antenna detector differs at two important points from detector set-up of KamLAND in Kamioka, Japan. The KamLAND detector is a large-volume detector filled with a scintillation liquid. The neutron is detected via capture of the thermalised neutron by hydrogen, producing a 2.2 MeV gamma ray that produces in turn scintillation light. Our antennas consist of a manifold of small-size detectors, making up the same volume. The other difference is the doping of the scintillator. The mean free path of the gamma ray is considerably larger than that of the neutron and thereby would deteriorate the directional sensitivity. For that reason we opted for an alpha-particle emitting nucleus (7Li or 10B), despite the fact that in liquid scintillator the alpha signal is quenched.

The doping has a number of major advantages for directional sensitivity:

the number of scatters, that the neutron undergoes before it is captured, is strongly reduced the range of the gamma rays is larger than the distance the neutron travels and hence the position where the neutron is stopped is blurred. With the doping an alpha particle is produced that is instantaneously stopped, preserving the neutron capture position in a proper scintillator the pulse shape of an alpha particle clearly differs of that of a positron.