| 释义 |
neutron Physics.|ˈnjuːtrɒn|
[f. neutral a. and n. + -on1.]
An electrically uncharged sub-atomic particle whose mass (939·6 MeV) is very slightly greater than that of the proton, which can decay into a proton, an electron, and an antineutrino (as in beta decay), and which is a constituent (with the proton) of all atomic nuclei except that of the common isotope of hydrogen; it is now usu. regarded as a particular state of a nucleon.
Before its discovery in 1932 it was conceived as a close association of a proton and an electron. Rutherford (who communicated Glasson's 1921 paper to the Royal Society) discusses this concept in a paper of 1920 cited by Glasson, but without using the word neutron. Harkins (of Chicago) seems to have coined the term independently. (The use in quot. 1899 is unrelated to the current use.)
[1899W. Sutherland in Phil. Mag. 5th Ser. XLVII. 273 If the electrons are distributed through the æther, we must suppose that in æther showing no electric charge each negative electron is united with a positive electron to form the analogue of a material molecule, which might conveniently be called a neutron.] 1921W. D. Harkins in Ibid. 6th Ser. XLII. 309 Any complex atom has a mass and weight 0·76 per cent. less than the hydrogen atoms (neutrons) from which it may be assumed to be built. Ibid. 315 The term neutron represents one proton plus one electron. 1921J. L. Glasson in Ibid. 597 In the ordinary atom of hydrogen we have a single electron separated from the nucleus by a distance of the order of 10-8 cm. It is here contemplated that a more intimate union of the two is possible... Such a particle, to which the name neutron has been given by Prof. Rutherford, would have novel and important properties. It would, for instance, greatly simplify our ideas as to how the nuclei of the heavy elements are built up. 1930E. Rutherford et al. Radiations from Radio-Active Substances xvii. 523 The existence of a neutron, i.e. a close combination of a proton and electron, has been suggested. 1932J. Chadwick in Nature 27 Feb. 312/1 (heading) Possible existence of a neutron. Ibid., These results..are very difficult to explain on the assumption that the radiation from beryllium is a quantum radiation, if energy and momentum are to be conserved in the collisions. The difficulties disappear, however, if it be assumed that the radiation consists of particles of mass 1 and charge 0, or neutrons. The capture of the α-particle by the Be9 nucleus may be supposed to result in the formation of a C12 nucleus and the emission of the neutron. 1938R. W. Lawson tr. Hevesy & Paneth's Man. Radioactivity (ed. 2) ix. 94 We must regard these protons and neutrons as the ultimate constituents of the nuclei, and consider the α-particle as a kind of ‘molecule’ of the ‘nuclear atoms’ possessing especial stability, and arising from the union of 2 protons and 2 neutrons. 1950Glasstone Sourcebk. Atomic Energy xi. 287 One of the most striking arguments for the wave-particle duality of matter..has been provided by the diffraction of neutrons. 1951S. Dushman Fund. Atomic Physics xiii. 227 The U-235 nucleus, in splitting, emits high-speed neutrons which may produce fission in other U-235 nuclei, thus initiating a chain reaction. 1968M. S. Livingstone Particle Physics ii. 25 The nucleus could now be conceived as a closely packed assemblage of protons and neutrons, with the atomic charge number Z given by the number of protons and the atomic weight number A by the total of protons and neutrons. 1972Physics Bull. June 349/1 Although it is comforting and often convenient to consider the proton and neutron as elementary particles with no internal substructure, they are in fact particles in a state of continual change... The neutron divides its time between being a neutron and a composite proton-negative pion system, while remaining electrically neutral. 1973A. J. MacLeod Instrumental Methods Food Analysis vii. 684 Isotopes of an element possess different numbers of neutrons in their nuclei, but are otherwise identical.
2. attrib. and Comb., as neutron absorption, neutron bombardment, neutron flux; neutron-absorbing adj.; neutron activation, the process of making a substance radioactive by irradiating it with neutrons; freq. attrib., esp. in neutron activation analysis, activation analysis in which this is employed; neutron bomb, a bomb that would produce large numbers of neutrons but little blast, and would consequently be harmful to life but not destructive of property; neutron capture, the absorption of a neutron by an atomic nucleus; neutron chopper, a device for converting a continuous beam of neutrons into a pulsed beam by passing it through a rotating slotted disc or cylinder; neutron diffraction, diffraction of a beam of neutrons; neutron excess, the excess of the number of neutrons in an atomic nucleus over the number of protons; neutron number, the mass number of a nucleus minus its atomic number, taken as being the number of neutrons it contains; neutron radiography, radiography in which the radiation employed is a beam of neutrons; neutron star Astr., a hypothetical extremely dense kind of star composed predominantly of closely packed neutrons, which is believed to have a mass similar to that of the sun but a diameter of only a few kilometres; cf. pulsar; neutron therapy, radio-therapy in which the radiation employed is a beam of neutrons.
1947Physical Rev. LXXII. 16 Introduction of a neutron-absorbing substance into a pile decreases the reactivity. 1964M. Gowing Britain & Atomic Energy 29 The Paris group also worked out the idea of controlling the reaction by introducing neutron-absorbing material to limit the multiplication of neutrons.
1947Physical Rev. LXXII. 16 (heading) Method for measuring neutron-absorption cross sections by the effect on the reactivity of a chain-reacting pile. 1951Analyst LXXVI. 644 (heading) The estimation of tantalum in mixtures by neutron activation analysis. 1960Nature 16 Jan. 196/2 (heading) Neutron activation analysis of ancient Roman potsherds. 1965D. Gibbons in Lenihan & Thomson Activation Analysis x. 68 Phosphorus and barium can be determined, rapidly and non-destructively, in lubricating oil in the range 0·01–1% and 0·05–2% using 14 MeV neutron activation and γ-ray spectrometry. 1966Encycl. Industr. Chem. Analysis I. 52 With the high neutron flux of a nuclear reactor, neutron activation analysis (NAA) provides the most sensitive means of detection of low concentrations known today for most of the elements of the periodic system. A typical limit of detection is about one part per billion (ppb) in a 1-g sample. 1973A. J. MacLeod Instrumental Methods Food Analysis vii. 695 Of the three methods described for separation and analysis of the products of neutron activation, gamma ray spectrometry is by far the most common.
1960Congress. Rec. 12 May 10138/3 Although there have been a few fragmentary references to the neutron bomb in the press, I was told..that the matter was classified. 1962L. Deighton Ipcress File xxiv. 155, I guessed that it was a neutron bomb that they were about to explode. 1967New Scientist 14 Sept. 534/1 Whenever the possibility of a neutron bomb was discussed in the late 'fifties and early 'sixties it was with particular reference to the military interest in the application of tactical nuclear firepower in a controlled and highly selective fashion.
1937G. Gamow Struct. Atomic Nuclei x. 193 These phenomena (of induced activity by neutron bombardment) were observed by Fermi for many very heavy elements also. 1964M. Gowing Britain & Atomic Energy ii. 55 It was known that in some nuclei fission occurred spontaneously, without neutron bombardment.
1945H. D. Smyth Gen. Acct. Devel. Atomic Energy Mil. Purposes i. 17 [By 1940] neutron-capture cross-sections had been measured. 1959Listener 19 Nov. 872/1 At slow or ‘thermal’ speeds neutron capture by nuclei of Uranium 238 is less important. 1966Phillips & Williams Inorg. Chem. II. xxxv. 635 Neutron-capture efficiencies are usually measured in terms of nuclear cross⁓sections, for which the whimsical unit the ‘barn’ has been devised.
1950Glasstone Sourcebk. Atomic Energy xi. 306/2 Three main techniques have been developed for making measurements with neutrons of specific energies; these are the time-of-flight velocity selector..; the mechanical selector, sometimes referred to colloquially as a ‘neutron chopper’..; and the crystal spectrometer. 1971New Scientist 8 July 72/1 Another rotating component developed at Harwell has been the so-called ‘neutron chopper’... When it rotates, the beam is chopped into pulses of neutrons of the same energy level.
1949Physical Rev. LXXVI. 1256/2 (heading) Detection of antiferromagnetism by neutron diffraction. 1973J. Yarwood Atomic & Nuclear Physics xiii. 381 Whereas X-ray diffraction cannot readily lead to a knowledge of the positions of hydrogen atoms in a crystal because the scattering is dependent on the number of orbital electrons.., neutrons are strongly scattered by hydrogen nuclei. The technique of neutron diffraction therefore supplements that of X-ray diffraction.
1955R. D. Evans Atomic Nucleus iii. 99 (N–Z) is called..the ‘neutron excess’.
1947Science 9 May 491/1 Facilities will include..another pile with 100 times the neutron flux of the first. 1971Engineering Apr. 34/2 The neutron flux reaching the sample is in excess of 107 neutrons cm-2 sec-1.
1955R. D. Evans Atomic Nucleus iii. 99 Nuclei having the same neutron number are isotones. 1973McGraw-Hill Yearbk. Sci. & Technol. 181/1 Theoretical calculations..have shown that the same r-process which synthesizes the actinides..may also synthesize the predicted superheavy elements with atomic number Z ∼ 114 and neutron number N ∼ 184.
1948H. Kallmann in Research I. 254/1 The first successful experiments in neutron radiography are due to the author and his collaborator E. Kuhn, who in the years 1935–39 developed and partly tried out the fundamental methods of neutron radiography. 1962Sci. Amer. Nov. 107/2 Neutron radiography has become practical with the advent of nuclear reactors and particle accelerators, which can provide a source of neutrons of the required intensity. 1971Engineering Apr. 37/1 An advantage of using neutron radiography is the possibility of detecting small amounts of low density materials, within sections of high density materials such as lead or steel.
1934Baade & Zwicky in Physical Rev. XLV. 138/2 We advance the view that supernovae represent the transitions from ordinary stars into neutron stars, which in their final stages consist of extremely closely packed neutrons. 1968New Scientist 16 May 331/1 Neutron stars, if they exist, could have densities as much as ten million times higher even than a white dwarf. 1970B. Lovell in Times 19 Aug. 9/7 It is now generally accepted that the pulsars are the remnants of stars which have collapsed to form neutron stars. 1974Nature 13 Sept. 99/3 The neutron star represents the most extreme density of matter in the observable Universe (greater densities lead to unobservable black hole matter).
1947Radiology XLVIII. 431/1 (heading) Possible progress in the radiotherapy of cancer (neutron therapy, Joliot therapy, alpha therapy, beta therapy..). 1974Nature 25 Jan. 173/1 Because of Hammersmith Hospital's success in the use of neutron therapy for the treatment of cancer a second British hospital is to be supplied with a compact cyclotron. Ibid., The effectiveness of neutron therapy depends on the ability of the neutrons to destroy cancer cells even in the absence of oxygen. |