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Do neutrinos change?. An invisible firmament
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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. The Standard Model for elementary particles will have to be modified if neutrinos have mass, and this mass can be highly significant for the collected mass of the universe. Studies designed to confirm or disprove the neutrino oscillation theory are in progress at many laboratories around the world.
An invisible firmament
The X-rays Wilhelm Rö ntgen discovered in 1895 were quickly put to use by physicists and doctors at laboratories and clinics all over the world. In contrast it took half a century for astronomers to study this type of radiation. The main reason was that X-ray radiation, which can so easily penetrate human tissue and other solid material, is almost entirely absorbed by the air in the Earth's thick atmosphere. It was not until the 1940 s that rockets had been developed that could send instruments high enough up in the 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.
Fig. 3. The instrument in the nose of the Aerobee rocket that was launched in June 1962 by Giacconi and his group and which was the first to record a source of X-rays outside the solar system. The instrument, about one metre long, contained three Geiger counters (indicated by arrows), provided with windows of varying thickness so that the energy of the radiation could be determined
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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 (Fig. 3) 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 time the way in which the direction of the radiation could be determined was improved and the sources could be identified with observations made in normal light. The source discovered in the first successful experiment was a distant ultraviolet star in the Scorpio constellation, Scorpius X-1 (X for X-ray, 1 for the first). Other important sources were stars in the Swan constellation (Cygnus X-1, X-2 and X-3). Most of the newly-discovered sources were double stars, in which one star circles in a narrow orbit around another object which is very compact — a neutron star or perhaps a black hole (Fig. 4). However, it was difficult to carry out these studies because the possible observation times from the balloons and rockets were too short.
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