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Discovery, detection of cosmic particles and radiation
THIS YEAR'S Nobel Prize in Physics is concerned with the discoveries and detection of cosmic particles and radiation, from which two new fields of research have emerged, neutrino astronomy and X-ray astronomy. The prize is awarded with one half jointly to: Raymond Davis Jr, Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, U. S, and Masatoshi Koshiba, International Centre for Elementary Particle Physics, University of Tokyo, Japan, "for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos", and the second half to Riccardo Giacconi, Associated Universities, Inc., Washington, DC, USA, "for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources".
Riccardo Giacconi of the Associated Universities Inc., of Washington,D.C sent the first X-ray telescope to space which provided relatively sharp images of the universe. He also constructed an improved X-ray observatory
Why does the Sun shine?
In the 19th C there were discussions about the source of the Sun's energy. One theory was that this solar reaction was due to the release of gravitational energy when the Sun's material contracted.
However, in this case, the calculated life expectancy of the Sun was, in our eyes, short. It was approximately 20 million years, compared with the age of the Earth, which we know today is approximately 5 billion years.
The British astrophysicist Sir Arthur Eddington realised that nuclear reactions in which hydrogen was transformed into helium might be the basis of the Sun's energy supply. The transformation of hydrogen into helium in the Sun gives rise to two neutrinos for each helium nucleus that is formed by a series of reactions.
The dream of verifying this theory by detecting neutrinos was considered impossibile. But in the 1950s the Nobel Prize Laureate Frederick Reines and his colleagues succeeded in showing that it was possible to prove the existence of neutrinos.
The flux of neutrinos from the Sun was estimated to be large. In the late 1950s Raymond Davis Jr was the only scientist who dared to try to prove the existence of solar neutrinos. The Italian physicist Bruno Pontecorvo proposed that it ought to be possible to detect this neutrino after it had reacted with a nucleus of chlorine, forming a nucleus of argon and an electron. In the 1960s Davis placed a tank filled with 615 tonnes of the common cleaning fluid tetrachloroethylene in a gold mine in South Dakota, U. S. Altogether there were some 2·1030 chlorine atoms in the tank. He calculated that every month approximately 20 neutrinos ought to react with the chlorine, or in other words that 20 argon atoms ought to be created.
Davis's pioneering approach was the development of a method for extracting these argon atoms and measuring their number. He released helium gas through the chlorine fluid and the argon atoms attached themselves to it an achievement considerably more difficult than finding a particular grain of sand in the whole of the Sahara desert!
This experiment gathered data until 1994 and all in all approximately 2,000 argon atoms were extracted. However, this was fewer than expected.
By means of control experiments Davis was able to show that no argon atoms were left in the tank of chlorine, so it seemed as though our understanding of these processes in the Sun was incomplete or that some of the neutrinos had disappeared on their way to the Earth.
Masatoshi Koshiba of the International Center for Elementary Particle Physics, University of Tokyo, Japan has done pioneering work in astrophysics, in particular for the detection of cosmic neutrinos.
Neutrinos from space
While Davis's experiment was running, the Japanese physicist Masatoshi Koshiba and his team constructed another detector, which was given the name Kamiokande. It was placed in a mine in Japan and consisted of an enormous tank filled with water. When neutrinos pass through this tank, they may interact with atomic nuclei in the water. This reaction leads to the release of an electron, creating small flashes of light.
The tank was surrounded by photomultipliers that can capture these flashes. By adjusting the sensitivity of the detectors the presence of neutrinos could be proved and Davis's result was confirmed.
Decisive differences between Davis's and Koshiba's experiments were that the latter registered the time for events and was sensitive to direction. It was therefore possible for the first time to prove that neutrinos come from the Sun .
This lies at about 170,000 light years from the Earth (one light year corresponds to 1016 metres). If a neutron star is formed when a supernova explosion takes place, most of the enormous amount of energy released will be emitted as neutrinos. A total of about 1058 neutrinos is estimated to have been emitted from supernova 1987A, of which Koshiba's research group observed twelve of the approximately 1016 that passed through the detector. A similar experiment in the United States confirmed this discovery.
Do neutrinos change?
In order to increase sensitivity to cosmic neutrinos, Koshiba constructed a larger detector, Super Kamiokande, which came into operation in 1996. This experiment has recently observed effects of neutrinos produced within the atmosphere, indicating a completely new phenomenon, neutrino oscillations, in which one kind of neutrino can change to another type.
This implies that neutrinos have a non-zero mass, which is of great significance for the Standard Model of elementary particles and also for the role that neutrinos play in the universe. It could also explain why Davis did not detect as many neutrinos as he had expected. Davis's and Koshiba's discoveries and their development of instruments have created the foundation for a new field, neutrino astronomy, which is of great importance for elementary particle physics, astrophysics and cosmology.
University of Pennsylvania and Brookhaven National Laboratory Physicist Dr. Raymond Davis Jr proved the existence of neutrinos and thus defied the notion that directing neutrinos was practically impossible.
An invisible firmament
The X-rays Wilhelm Röntgen discovered in 1895 were quickly put to use by physicists and doctors at laboratories and clinics all over the world. Astronomers took half a century to study x-ray radiation. The main reason was that X-ray radiation, is almost entirely absorbed by the air in the Earth's thick atmosphere.
The first X-ray radiation outside the Earth was recorded in 1949 by instruments placed on a rocket by the late Herbert Friedman and his colleagues. It was shown that this radiation came from areas on the surface of the Sun with sunspots and eruptions and from the surrounding corona, which has a temperature of several million degrees Celsius. But this type of radiation would have been very difficult to record if the Sun had been as far away as other stars in the Milky Way.
In 1959 the then 28-year-old Riccardo Giacconi was recruited to build up a space-research program for a company that was to make it easier for young researchers to get commissions from e.g. NASA.
Together with the man who took this initiative, the late Bruno Rossi, Giacconi worked out principles for how an X-ray telescope should be constructed. Giacconi and his newly-formed group also carried out rocket experiments to try to prove the presence of X-ray radiation from the universe, primarily to see whether the moon could emit X-ray radiation under the influence of the Sun. In one experiment a rocket flew at a high altitude for six minutes.
No radiation from the moon could be detected, but a surprisingly strong source at a greater distance was recorded since the rocket was rotating and its detectors swept the sky. In addition, a background of X-ray radiation was discovered evenly distributed across the sky. These unexpected discoveries gave an impetus to the development of X-ray astronomy.
In order to extend observation times, Giacconi initiated the construction of a satellite to survey the sky for X-ray radiation.
This satellite was launched in 1970 from a base in Kenya. It was ten times more sensitive than the rocket experiments and every week it was in orbit it produced more results than all the previous experiments put together.
However, so far no high-definition X-ray telescope had been sent into space that could provide sharp images. Giacconi constructed one, which was ready for use in 1978. It was called the Einstein X-ray Observatory and was able to provide relatively sharp images of the universe at X-ray wavelengths. Its sensitivity had been improved and objects a million times weaker than Scorpius X-1 could be recorded.
This telescope made a large number of discoveries. Many X-ray double stars were studied in detail, not least a number of objects that were thought to contain black holes. More normal stars could also be studied for the first time in X-ray radiation. Remnants of supernovas were analysed, X-ray stars in galaxies outside the Milky Way were discovered and eruptions of X-ray radiation from distant active galaxies could be examined more closely.
The X-ray radiation from the gas between galaxies in galaxy groups helped scientists draw conclusions about the dark matter content of the universe.In 1976 Giacconi initiated the construction of an improved, even larger X-ray observatory.
It was not launched until 1999, and was named Chandra after the astrophysicist and Nobel Prize Laureate Subrahmanyan Chandrasekhar. Chandra has provided extraordinarily detailed images of celestial bodies in X-ray radiation corresponding to those from the Hubble Space telescope or the new Earth-based telescopes using visible light. Thanks to X-ray astronomy and its pioneers, in particular Giacconi, our picture of the universe has been changed in decisive ways.
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