Other Types of Detectors

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Many other types of interactions between matter and elementary particles are used in detectors. Thus in semiconductor detectors, electron-hole pairs that elementary particles produce in a semiconductor junction momentarily increase the electric conduction across the junction. The Cherenkov detector, on the other hand, makes use of the effect discovered by the Russian physicist Pavel Alekseyevich Cherenkov in 1934: A particle emits light when it passes through a nonconducting medium at a velocity higher than the velocity of light in that medium (the velocity of light in glass, for example, is lower than the velocity of light in vacuum). In Cherenkov detectors, materials such as glass, plastic, water, or carbon dioxide serve as the medium in which the light flashes are produced. As in scintillation counters, the light flashes are detected with photomultiplier tubes.

Neutral particles such as neutrons or neutrinos can be detected by nuclear reactions that occur when they collide with nuclei of certain atoms. Slow neutrons produce easily detectable alpha particles when they collide with boron nuclei in borontrifluoride. Neutrinos, which barely interact with matter, are detected in huge tanks containing perchloroethylene, a dry-cleaning fluid). The neutrinos that collide with chlorine nuclei produce radioactive argon nuclei. The perchloroethylene tank is flushed at regular intervals, and the newly formed argon atoms, present in minute amounts, are counted. This type of neutrino detector, placed deep underground to shield against cosmic radiation, is currently used to measure the neutrino flux from the sun. Neutrino detectors may also take the form of scintillation counters, the tanks in this case being filled with an organic liquid that emits light flashes when traversed by electrically charged particles produced by the interaction of neutrinos with the liquid's molecules.

The detectors now being developed for use with the storage rings and colliding particle beams of the most recent generation of accelerators are bubble-chamber types known as time-projection chambers. They can measure three-dimensionally the tracks produced by particles from colliding beams, with supplementary detectors to record other particles resulting from the high-power collisions. The Fermi National Accelerator Laboratory's CDF (Collision Detector Fermilab) is used with its colliding-beam accelerator (see Particle Accelerators) to study head-on particle collisions. CDF's three different systems can capture or account for nearly all of the subnuclear fragments released in such violent collisions.

Michelson, Albert Abraham

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Michelson, Albert Abraham (1852-1931), German-born American physicist, known for his famous experiment to measure the velocity of the earth through the ether, a substance that scientists believed filled the universe. This experiment helped prove that the ether does not exist. In 1907 he was awarded the Nobel Prize in physics for developing extremely precise instruments and conducting important investigations with them, becoming the first American citizen to earn a Nobel Prize in the sciences.

Michelson was born in Strelno (now Strzelno, Poland), brought to the United States as a child, and educated at the United States Naval Academy and at the universities of Berlin, Heidelberg, and Paris. He was professor of physics at Clark University from 1889 to 1892, and from 1892 to 1929 was head of the department of physics at the University of Chicago. He determined the velocity of light with a high degree of accuracy, using instruments of his own design.

In 1887 Michelson invented the interferometer, which he used in the famous experiment, performed with the American chemist Edward Williams Morley. At that time, most scientists believed that light traveled in waves through the ether. They also believed that the earth traveled through the ether. The Michelson-Morley experiment showed that two beams of light sent in separate directions from the earth were reflected at the same speed. According to the theory of ether, the beams would have been reflected in waves of different speeds, in relation to the velocity of the earth. In this way, the experiment proved that the ether did not exist. The negative results of the experiment were also useful in the development of the theory of relativity. Michelson's major works include Velocity of Light (1902) and Studies in Optics (1927).

Einstein, Albert

Einstein, Albert (1879-1955), German-born American physicist and Nobel laureate, best known as the creator of the special and general theories of relativity and for his bold hypothesis concerning the particle nature of light. He is perhaps the most well-known scientist of the 20th century.

Einstein was born in Ulm on March 14, 1879, and spent his youth in Munich, where his family owned a small shop that manufactured electric machinery. He did not talk until the age of three, but even as a youth he showed a brilliant curiosity about nature and an ability to understand difficult mathematical concepts. At the age of 12 he taught himself Euclidean geometry.

Einstein hated the dull regimentation and unimaginative spirit of school in Munich. When repeated business failure led the family to leave Germany for Milan, Italy, Einstein, who was then 15 years old, used the opportunity to withdraw from the school. He spent a year with his parents in Milan, and when it became clear that he would have to make his own way in the world, he finished secondary school in Aarau, Switzerland, and entered the Swiss Federal Institute of Technology in Zürich. Einstein did not enjoy the methods of instruction there. He often cut classes and used the time to study physics on his own or to play his beloved violin. He passed his examinations and graduated in 1900 by studying the notes of a classmate. His professors did not think highly of him and would not recommend him for a university position.

For two years Einstein worked as a tutor and substitute teacher. In 1902 he secured a position as an examiner in the Swiss patent office in Bern. In 1903 he married Mileva Marić, who had been his classmate at the polytechnic. They had two sons but eventually divorced. Einstein later remarried.

See: EINSTEIN’S SPECIAL THEORY OF RELATIVITY .

Maxwell, James Clerk

Maxwell, James Clerk (1831-1879), British physicist, best known for his work on the connection between light and electromagnetic waves (traveling waves of energy). Maxwell discovered that light consists of electromagnetic waves (see Electromagnetic Radiation) and established the kinetic theory of gases. The kinetic theory of gases explains the relationship between the movement of molecules in a gas and the gas’s temperature and other properties. He also showed that the rings of the planet Saturn are made up of many small particles and demonstrated the principles governing color vision.

Maxwell was born in Edinburgh, Scotland. He was educated at Edinburgh Academy from 1841 to 1847, when he entered the University of Edinburgh. He then went on to study at the University of Cambridge in 1850, graduating with a bachelor’s degree in mathematics in 1854. He became a professor of natural philosophy at Marischal College in Aberdeen in 1856. Then in 1860 he moved to London to become a professor of natural philosophy and astronomy at King's College. On the death of his father in 1865, Maxwell returned to his family home in Scotland and devoted himself to research. In 1871 he moved to Cambridge, where he became the first professor of experimental physics and set up the Cavendish Laboratory, which opened in 1874. Maxwell continued in this position until 1879, when illness forced him to resign.

COLOR VISION

Maxwell’s first important contribution to science began in 1849, when he applied himself to examining how human eyes detect color. He built on the ideas of British physicist Thomas Young and German scientist Hermann Helmholtz on color vision. Maxwell spun disks painted with sectors of red, green, and blue to mix those primary colors into other colors. He confirmed Young's theory that the eye has three kinds of receptors sensitive to the primary colors and showed that color blindness is due to defects in the receptors. He also fully explained how the addition and subtraction of primary colors produces all other colors. He crowned this achievement in 1861 by producing the first color photograph. Maxwell took this picture, the ancestor of all color photography, printing, and television, of a tartan-patterned ribbon. He used red, green, and blue filters to expose three frames of film. He then projected the images through the appropriate filters to project a colored image.

ELECTROMAGNETIC THEORY OF LIGHT

Maxwell's development of the electromagnetic theory of light took many years. It began with the paper “On Faraday's Lines of Force” (1855–1856), in which Maxwell built on the ideas of British physicist Michael Faraday. Faraday explained that electric and magnetic effects result from lines of force that surround conductors and magnets. Maxwell drew an analogy between the behavior of the lines of force and the flow of a liquid, deriving equations that represented electric and magnetic effects. The next step toward Maxwell’s electromagnetic theory was the publication of the paper “On Physical Lines of Force” (1861–1862). Here Maxwell developed a model for the medium that could carry electric and magnetic effects. He devised a hypothetical medium that consisted of a fluid in which magnetic effects created whirlpool-like structures. These whirlpools were separated by cells created by electric effects, so the combination of magnetic and electric effects formed a honeycomb pattern.

KINETIC THEORY OF GASES

Maxwell's other major contribution to physics was to provide a mathematical basis for the kinetic theory of gases, which explains that gases behave as they do because they are composed of particles in constant motion. Maxwell built on the achievements of German physicist Rudolf Clausius, who in 1857 and 1858 had shown that a gas must consist of molecules in constant motion colliding with each other and with the walls of their container. Clausius developed the idea of the mean free path, which is the average distance that a molecule travels between collisions.

X Ray

X Ray, penetrating electromagnetic radiation, having a shorter wavelength than light, and produced by bombarding a target, usually made of tungsten, with high-speed electrons. X rays were discovered accidentally in 1895 by the German physicist Wilhelm Conrad Roentgen while he was studying cathode rays in a high-voltage, gaseous-discharge tube. Despite the fact that the tube was encased in a black cardboard box, Roentgen noticed that a barium-platinocyanide screen, inadvertently lying nearby, emitted fluorescent light whenever the tube was in operation. After conducting further experiments, he determined that the fluorescence was caused by invisible radiation of a more penetrating nature than ultraviolet rays (see Ultraviolet Radiation). He named the invisible radiation “X ray” because of its unknown nature. Subsequently, X rays were known also as Roentgen rays in his honor.

X rays are electromagnetic radiation ranging in wavelength from about 100 A to 0.01 A (1 A is equivalent to about 10-8 cm/about 4 billionths of an in.). The shorter the wavelength of the X ray, the greater is its energy and its penetrating power. Longer wavelengths, near the ultraviolet-ray band of the electromagnetic spectrum, are known as soft X rays (see Spectrum). The shorter wavelengths, closer to and overlapping the gamma-ray range, are called hard X rays (see Radioactivity). A mixture of many different wavelengths is known as “white” X rays, as opposed to “monochromatic” X rays, which represent only a single wavelength. Both light and X rays are produced by transitions of electrons that orbit atoms, light by the transitions of outer electrons and X rays by the transitions of inner electrons. X rays are produced by the retardation or deflection of free electrons passing through a strong electrical field. Gamma rays, which are identical to X rays in their effect, are produced by energy transitions within excited nuclei. See Atom.

X rays are produced whenever high-velocity electrons strike a material object. Much of the energy of the electrons is lost in heat; the remainder produces X rays by causing changes in the target's atoms as a result of the impact. The X rays emitted can have no more energy than the kinetic energy of the electrons that produce them (see Energy). Moreover, the emitted radiation is not monochromatic but is composed of a wide range of wavelengths with a sharp, lower wavelength limit corresponding to the maximum energy of the bombarding electrons. This continuous spectrum is referred to by the German name bremsstrahlung, which means “braking,” or slowing down, radiation, and is independent of the nature of the target. If the emitted X rays are passed through an X-ray spectrometer, certain distinct lines are found superimposed on the continuous spectrum; these lines, known as the characteristic X rays, represent wavelengths that depend only on the structure of the target atoms. In other words, a fast-moving electron striking the target can do two things: It can excite X rays of any energy up to its own energy; or it can excite X rays of particular energies, dependent on the nature of the target atom.