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"I have done a terrible thing, I have posulated aparticle that cannot be detected" W. Pauli, a great physicist in the 20th century sighed over a kind of new particle he proposed in early 1930s. This kind of new particle is the neutrino, and scientists believe they exist in large number in the universe. Take the solar neutrinos, for example, every second about one million billions (1014) neutrinos reach the Earth and stream through our bodies. But why are such a huge flux of particles so difficult to detect? The reason is neutrinos only have very weak interaction with matters. They hardly interact with other particles such as electrons, neutrons and protons strongly enough to be detected. This weakness of the force gives the neutrinos the property that matter is almost transparent to them. In other words, they have great penetration power and can speed through the interior of the Sun or in the vast realm of the universe unhindered. Neutrinos have been playing an important role in the rapidly developing field of physical science, but their significance to astronomy was recognized only since the 1960s. That was the famous Solar Neutrino Problem. Since 1950s, Raymond Davis Jr, whose career as a chemist at the US Department of Energy's Brookhaven National Laboratory, has been thinking of capturing the solar neutrinos. After series of setbacks, he succeeded in developing the first experiment to detect the neutrinos in 1967 at an old mine 1.5 km underground in the Homestake Gold Mine in South Dakota. The experiments continued for over some 20 years since then. Let us take a look at Davis' results. Based on the Standard Solar Model well known to astronomers, we can envisaged that the 380,000 litres of perchloroethyylene (C2Cl4) in the detector should be able to capture 7.7 Solar Neutrino Unit (SNU) per chlorine atom per second. However, only 2.6 SNU were detected for the some 20 years. Kamioka, situated 300 km west of Tokyo of Japan, was originally planned to construct a laboratory for the purpose of studying proton decay. The detector was called Kamiokande (Kamioka nucleaon decay experiement). It was ultimately turned into the second solar neutrino detector in history with the assistance of USA in 1984 and was renamed Kamiokande II. The detector, located 1 km underground, used 680 tonnes of water and should theoretically be able to capture 5.2 units of neutrino. But over 10 years they had found only 2.8 units. So, what's wrong? Is it the experiment itself? Or is it we do not understand the interior of the Sun correctly? Or is it because there are something about neutrinos that scientists do not know well enough? There are actually three types of neutrino, the electron-neutrino the muon-neutrino and the tau-neutrino. The neutrinos produced by the Sun are the electron-neutrinos. Scientists now generally believe that the electron-neutrinos, when on their way to the Earth, undergo a process called neutrino oscillation during which they are transformed into the muon and the tau neutrinos. Since the experiments of both Davis and Kamioka could only detect electron-neutrinos, the neutrino number was therefore much lower than expected. Many detectors sprang up in 1980s and 1990s for detection of solar neutrinos. The SAGE, GNO and GALLEX all used Gallium for the detection. Another one was the Super-Kamiokande which was 30 times larger than Kamiokande II and used highly purified water. The Sudbury Neutrino Observatory (SNO), built at a depth of 2 km in Ontario in Canada, used heavy water (D2O) instead. By detecting the very faint interaction of neutrinos with water and determining their different types, the latter two detectors had recently found evidence for neutrino oscillations. If the theory of neutrino oscillation could finally be established, it will prove that neutrinos do possess mass, and this might alter our understanding of the future evolution of the entire universe. ˇ@ ˇ@
ˇ@ Photo credits ˇG University of Maryland, Queen's University ˇ@ |
