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BLOCH (Bloch), Felix

( Swiss-American physicist, Nobel Prize in Physics, 1952)

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Biography BLOCH (Bloch), Felix
October 23, 1905, Mr.. - September 10, 1983
Swiss-American physicist Felix Bloch was born in Zurich, the son of Gustave Bloch, a wholesale grain merchant, and Agnes (nee Mayer) Bloch. He studied at the gymnasium of the canton of Zurich, graduating in 1924. Boy, interested in mathematics and astronomy, recorded on the engineering expertise at the Federal Institute of Technology in Zurich. However, having listened to the first physics course, B. decided to become a theoretical physicist, not an engineer. From 1924 to 1927. He studied at the Federal Institute, where among his teachers were Peter Debye and Erwin Schrodinger. Then he studied at the University of Leipzig in Werner Heisenberg. Doctoral degree he received in 1928. in Leipzig for his thesis on the conduction electrons in metals. In this thesis, which is now recognized as laid the foundations of solid state physics, he formulated the theory, for determining the shape of wave functions of electrons in metals (Bloch function).
After completing his doctoral dissertation B. became the owner of several scholarships that allowed him to work with Heisenberg, Niels Bohr, Enrico Fermi and Wolfgang Pauli, during which period he made his major contribution to theoretical physics. B. theoretically derived empirical law of the German physicist Edward Gryuneyzena on the dependence of the conductivity of metals on the temperature, which is now known as the ratio of the Bloch - Gryuneyzena. Thanks to his contribution to the theory of superconductivity and in the theoretical understanding of magnetic systems, a number of theorems and effects named after him: the Bloch theorem in the theory of superconductivity, . Bloch law, . Depending on the magnetization of ferromagnetic materials on the temperature (materials such as iron, . whose atomic structure allows them to easily magnetized), . Bloch wall (zone of transition between the regions of ferromagnetic materials with different magnetic orientations),
. In 1932, Mr.. B. developed the work of Bohr and Hans A. Bethe inhibition of moving charged particles in a substance having the formula Bethe - Bloch for this effect.
When Hitler in 1933. came to power, Boris, who was Jewish, left Germany and settled in the United States. He became an associate professor at Stanford University in 1934 and two years later they took the post of full professor. During this time he performed a number of important works on the quantum theory of electromagnetic field. Then he studied the recently discovered neutron, . predicted, . that its magnetic moment (a measure of the magnetic field) can be determined by the scattering of slow neutrons on iron and that the neutron beam will be polarized after scattering on an iron target,
. These predictions were confirmed in the next year. Then B. returned to the experimental studies. In 1939, Mr.. he, together with Luis Y. Alvarez measured the magnetic moment of the neutron, using the cyclotron at UC Berkeley as a source of neutrons. During the Second World War as a member of the Manhattan Project to build atomic bombs, B. investigated the properties of isotopes of uranium. Later he became assistant head of the group engaged in military protivoradarnymi developments in the radio laboratory research at Harvard University.
After the war, B. returned to Stanford University. Here, he used a radio-wave technology, researched them while working in the war on the radar to study the magnetic moments of nuclei. Physicists who tried to understand the behavior of atomic nuclei, it was necessary to know the relative magnetic moments of different types of nuclei with a high degree of accuracy. In 30-ies. IA. Rabi developed a technique for measuring nuclear magnetic moments, but his method required to vaporize the sample and the method itself was not very accurate. In 1946, Mr.. B. proposed method, which is highly accurate and totally not damage the sample. Although B. known for many achievements in the field of physics, namely for the development of this technique, he was awarded the Nobel Prize.
. When an atom is in a magnetic field, the magnetic moment of its nucleus is forcing the core precess (an effect similar to the effect of gravity on a rotating gyroscope, forcing him to swing axle)
. Frequency, or speed, precession of the nucleus depends on the magnitude of the magnetic field and the magnetic moment of the nucleus. Thus, if we know the strength of the field and were able to determine the frequency of precession, we can calculate the magnetic moment. To determine the frequency of precession, B. sample with the test material in a magnetic field of a powerful electromagnet, causing the core sample to precess at a constant speed. Then he roused the sample with a much weaker magnetic field, controlled by radio signals, a second field to fluctuate (changed direction) with a frequency corresponding to the frequency control of radio waves. When the frequency of the exciting field becomes equal to the precessional frequency of the nuclei, nuclear spin orientation suddenly changed to the opposite - this is easily detectable effect is called nuclear magnetic resonance (NMR). Known frequency radio signals corresponding to this resonance, the precession frequency is equal to the kernel. Knowing the exact frequency of the precession of the nucleus in a given field strength, we can determine the magnetic moment of this nucleus with remarkable precision. The method of Bloch gave a nuclear physicist and a very precise desired information, with absolutely no damage to the sample. Moreover, using this method made possible a completely new and very simple to measure magnetism: once it becomes known to the magnetic moment of a given nucleus, it can be used to determine the magnetic field.
. At the same time, Edward M
. Purcell (who also worked on radar during the war) studied this same problem. Simultaneously and independently, he thought through the method of measuring nuclear magnetic moments, which was almost identical to the method of Bloch. Using NMR, Purcell discovered that hydrogen emits radio frequency signal (a discovery which led to the development of radio astronomy).
. Researchers using MRI found that the resultant magnetic moment of atomic nuclei in the molecule changes under the influence of magnetic fields surrounding the electron
. It is in these changes is the key to the structure of molecules. MRI has quickly become one of the most important analytical tools of chemistry. Moreover, measurements using NMR does not affect the sample, and they can be carried out with living organisms, without damaging them. Techniques and methods of computation applied in computer tomography (tomographs were developed by Allan Cormac and Godfrey Hounsfield), began in the 70-ies. combined with observations of the NMR technique and the result appeared NMR scanning devices to monitor specific chemical reactions inside the human body. It turned out that these devices are of great importance to research and represent a powerful tool for medical diagnostics. Diagnostic MRI scanning devices have become available to doctors to work in the mid 80's.
B. and Purcell were awarded in 1952. Nobel Prize in Physics "for the development of new methods for nuclear magnetic precision measurements and related discovery '. When presenting the winners of Eric Hultц╘n, a member of the Royal Swedish Academy of Sciences, noted that 'methods of Purcell and B. give a huge simplification and generalization of the 'method of molecular beams IA. Rabi 'that allows you to apply them to solid, liquid and gaseous substances'. Hultц╘n continued: 'Because each type of atom and its isotopes have a strictly defined and the characteristic frequency of nuclear, . we can in any facility, . placed between the poles of an electromagnet, . seek and explore with the help of radio waves all kinds of atoms and isotopes, . present in the test object ..,
. without exerting any detectable effect on the sample '. The use of physical research in astronomy, chemistry and medicine is an outstanding example of how fundamental research has an impact that goes far beyond the area where it was held.
Most research B. after 1946. involves the use of NMR or, as he originally called it, 'nuclear induction'. In 1954 ... 1955. He took a two-year leave at Stanford to become Director General of CERN (European Organization for Nuclear Research) in Geneva (Switzerland). In 1963, Mr.. he was appointed professor of physics at Stanford. After retiring to resign in 1971, B. returned to Zurich, where he died Sept. 10, 1983
In 1940. B. married to Laura K. Mish, physics, and also a refugee from Germany, they had three sons and a daughter. He became a U.S. citizen in 1939
B. was a member of the U.S. National Academy of Sciences. American Academy of Arts and Sciences. Swiss Academy of Natural Sciences and the American Physical Society, whose president he was in 1965

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