Introduction
Observational Astronomy is probably as ancient as is the very presence of man on earth. The earliest recorded observations, due to Egyptian and Central American Civilisations, go back to about 4000 years b.C., while the first written material on Astronomy is found about 1000 years later. All experimental observations, up to the 17th century had relied on the human eye as an ”optical instrument”. The situation changed drastically in 1609, when Galileo first made use of the telescope, invented one year earlier by Lippershay, a Dutch spectacles maker.
In the 19thcentury, the discovery of the Doppler effect and the development of optical spectroscopy allowed again a big step forward, yielding, at the end of the century, the discovery of optical binaries and, around 1915, that of the relationship between the star spectral type and absolute magnitude by Hetzsprung and Russell.
The field of Astronomy had gradually developed into the more general one of Astrophysics.
All observations up to the 20th century were done in the visible region of the spectrum. Astrophysics, aiming at an understanding of the Physical Processes taking place in stars, galaxies, galaxy clusters etc. could not have developed to the advanced stage in which we find it today, without an extension of the range of wavelengths used. Thus, it would not be possible to study processes occurring in the region of the center of our galaxy, without the use of detectors working in the infrared and in the radiowaves. Analogously, it is only thanks to instruments working in the radio, X and γ ray regions that pulsars have been discovered and studied in detail. A giant step forward has come with the advent of satellite-born X and γ ray detectors.
Detectors sensitive to higher and higher energy γ rays are now being built.
They fall either in the category of satellite-born tracking-calorimetric detectors, characterised by an acceptance in energy limited to ≈ 300 GeV (for the forth- coming experiment GLAST) or in that of large area surface detectors, using the Cerenkov technique, that can reach the energy range above 1010 ÷ 1011 GeV ,
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6 INTRODUCTION
where the flux has fallen to very small values. All these detectors can, however, cover a region in space around the Earth limited to about 100 M pc at 10 T eV , because of γ ray absorption by the 3oK cosmic microwave background.
Ground-based hadron detectors and underground muon detectors have been used since the beginning of the 20thcentury, for the study of cosmic rays. These may also bring useful information on the Physics taking place in stars, galaxies, pulsars, etc. However, only the highest energy protons (E > 1010 GeV ) could possibly point to their sources and, as for the photons, the region which can be explored is limited by interaction with the extragalactic backgrounds.
The neutrino, on the other hand, thanks to its tiny cross section and lack of electric charge, is a new excellent candidate for probe able to bring information from objects located at large distances and, what is even more important, from the core of such objects. In chapter 1, I will discuss advantages of using neutrinos as probes of the Universe. Detection of M eV neutrinos from the nuclear reactions in the core of the Sun or from supernova explosions in our galactic neighbourhood is by now largely used. However the main interest of neutrino astronomy at the moment is focused on neutrinos with energy above GeV ÷ T eV for several rea- sons: the neutrino cross section as well as the muon range and the detector angular resolution increase with energy, large natural target media (like water, ice or the atmosphere) can be used and much less is known on the underlying astrophysics.
In the same chapter 1, I will briefly report the astrophysics and particle-physics problems that high energy neutrino astronomy may help to solve, like e.g. the understanding of the acceleration mechanisms for high energy cosmic rays or of the composition of the “dark matter”.
The advantage that νs have compared to other particles, the tiny cross section, turns into a disadvantage when one tries to detect them. Enormous detectors are needed, and they have moreover to be well screened from the large cosmic ray muon background, through the use of massive layers of inert material.
The only means of detection that appears feasible for the low fluxes of high en- ergy neutrinos is through the use of deep lake or sea water as target, combined with large photomultiplier arrays to capture the Cerenkov light emitted by charged secondaries produced in the ν interaction.
A first attempt to build such a detector was made more than twenty years ago by the DUMAND collaboration, who chose a site close to the Hawaii islands. The experiment demonstrated that muons could indeed be detected, but it was after- wards prematurely terminated. The first real detection of neutrino-induced muons in a similar setup was achieved only much later by a Russian group working at great depths in Lake Baikal. In chapter 2, I will discuss the general features of
INTRODUCTION 7
the detection technique used in a ˇCerenkov neutrino telescope. I will briefly de- scribe the main results obtained up to now by the experiments which are already taking data, and summarize the status of new experiments which are still under development.
The work described in this document has been carried out in the framework of the ANTARES experiment which aims at building a neutrino telescope in the Mediterranean. The ANTARES architecture, electronics and data acquisition are briefly illustrated in chapter 3.
Since bioluminescence and40K radioactive decay are a non-negligible source of background in any experiment performed in the sea, I have analysed data col- lected by a prototype string of ANTARES in 2003, specifically aiming at a quan- tification and charaterisation of these noise sources. The analysis methods and the results obtained are described in chapter 4.
In addition I have tried to understand what improvement could be brought in by the use of a photodetector of a new type, the hybrid photodiode (HPD).
In this context I have designed, making use of commercial software, an HPD with a convenient geometry and verified the possibility of its use in a neutrino telescope by means of a modified version of the ANTARES Monte Carlo. Details of the HPD and of the experimental layout, together with the performance of such detector, are given in chapter 5.
A discussion of the main results obtained and conclusions are given in the closing chapter.
8 INTRODUCTION
