| 释义 |
gamma|ˈgæmə|
[Gr. γάµµα.]
1. a. The third letter of the Greek alphabet, Γ, γ.
c1400Mandeville (1839) iii. 20 Here ȝee may seen hem [Lettres], with the Names..α Alpha, β Betha, γ Gamma [etc.]. 1775in Ash. 1885Athenæum 11 July 48/2 Whenever it occurs this intrusive gamma is hard.
b. An examiner's third-class mark. Also transf. and attrib.
1932A. Huxley Brave New World xi. 188 The fifty-six..machines were being manipulated by fifty-six aquiline and ginger Gammas. Ibid. xii. 204 A creature with a Gamma-Minus physique. 1956L. E. Jones Edwardian Youth iv. 98 A Second Class in Greats, thanks to ‘Gammas’ for Greek and Latin composition. 1958[see beta 3]. 1959E. Waugh R. Knox iv. 94 To qualify for a first it was necessary to get alphas in seven papers... A gamma cancelled an alpha. Betas were neutral. 1961M. Beadle These Ruins are Inhabited (1963) vi. 85 He's my gamma card... That's what the fellows call it. Because the lowest grade is gamma.
c. Used as a symbol for various quantities, etc., in science, etc.:
(i) Metallurgy. (a) Applied to one of a series of allotropic forms of a metal, as gamma iron, the allotrope of iron stable between 910°C and 1,403°C, characterized by a face-centred cubic crystal structure. (b) Applied to a solid solution in a range of alloys, as gamma brass, the third of a series of alloys of copper and zinc.
1896F. Osmond in Jrnl. Iron & Steel Inst. XLIX. 180 It was necessary to distinguish at least three molecular states of iron, which were respectively stable within certain limits of temperature: α below 700°, β between a range of 750° to 860°, γ above 860°. Ibid. 187 The distinction established between γ and β iron..is not an arbitrary, but an experimental one. 1902, etc. [see alpha 3 d]. 1904E. S. Shepherd in Jrnl. Physical Chem. VIII. 435 Copper and zinc unite to form six series of solid solutions, which have been distinguished [in this paper] as α, β, γ, δ, ε, and η. 1925Phil. Mag. L. 311 Some authors have supposed γ-brass to be CuZn2, while others believe that it is a solid solution of zinc in Cu2Zn3. 1956G. S. Brady Materials Handbk. (ed. 8) 116 Gamma brass, with the zinc above 45%,..is not easily worked either hot or cold. 1963W. H. Dennis Metallurgy Ferrous Metals i. 4 From 1,403°C to its melting point it again changes from the f.c.c. gamma iron to b.c.c. delta iron.
(ii) Physics. gamma rays or gamma radiation, electromagnetic radiation of very short wavelength emitted by radioactive substances, orig. regarded as the third and most penetrating kind of radiation emitted by radium but now known to be identical with very short X-rays. So gamma ray or gamma particle, a single photon of gamma radiation.
1903Rutherford in Phil. Mag. Feb. 177 The γ rays, which are non-deviable by a magnetic field, and which are of a very penetrating character. 1904R. J. Strutt Becquerel Rays iii. 83, I have succeeded in observing the γ-radiation from 10 milligrammes of radium bromide. 1926R. W. Lawson tr. Hevesy & Paneth's Man. Radioactivity iv. 49 A γ-‘particle’ produces 1·5 pairs of ions per unit of length. 1930J. Buckingham Matter & Radiation 31 The longest X-rays have the properties of the shortest ultra-violet rays and the longest Gamma-rays merge into the shortest X-rays. 1942S. Tolansky Introd. Atomic Physics xvi. 272 The nucleus, being usually left in an unstable condition after the departure of the β-particle, reverts back to a more stable state, the surplus energy being radiated as a γ-ray. 1957[see beta 2 f]. 1958Listener 30 Oct. 704/1 Food-preservation by gamma radiation. 1961Aeroplane C. 391/1 This will be the first attempt at ‘gamma ray astronomy’ from a satellite. Scientists are now limited in their study of extra-terrestrial gamma radiation because their measurements are affected by existing radiation in the Earth's atmosphere. 1969Times 29 Sept. 12/6 What is almost certainly the discovery of the first gamma ray star has now been reported.
Hence ellipt. for gamma rays in gamma counter, gamma emitter, gamma irradiation, etc.; so gamma emitting adj.
1929Brit. Med. Jrnl. 14 Sept. 508/1 (heading) The effect of gamma irradiation on cell division. 1949W. S. Eastwood Surv. Pile Made Isotopes 1 Sources of this kind have previously been confined to the natural gamma emitting elements. 1949Non-Destructive Testing VIII. ii. 23/1 It is standard practice in gamma radiography to place the film between sheets of lead when making an exposure. 1951Nucleonics IX. i. 50 British industry has never made wide use of the natural gamma emitters radium and radon for radiography. 1953Ann. Reg. 1952 403 Gamma radiography (the method of photographing through solid objects such as metals). 1955Gloss. Terms Radiology (B.S.I.) 6 Gamma emitter, an atom whose radioactive decay process is associated with the emission of gamma rays. 1961Lancet 16 Sept. 633/2 Rb is a hard gamma emitter. 1971Nature 4 June 324/1 Radioactivity was measured in a well-type gamma counter.
(iii) Photogr. The gradient of the straight-line portion of the characteristic curve of a photographic emulsion, taken as measuring the contrast of the developed image compared with that of the scene photographed; hence loosely, contrast. Similarly in Television, a measure of the contrast of the transmitted picture compared with that of the scene televised.
1903Photogr. Jrnl. XLIII. 48 The relation between the development factor (afterwards called γ) and the time of development (t). 1935Discovery Mar. 85/1 When the plate is removed from the [compound] developer it generally has a lower contrast (gamma) than if it had been normally developed. 1937A. T. Witts Telev. Cycl. 65 Intensity contrast, the contrast in detail in a picture, sometimes referred to as the gamma of the picture. 1961G. Millerson Technique Telev. Production iii. 48 From the angle of the slope, the curve's gamma can be deduced (tan α)... A low gamma device accepts a wide contrast range, but compresses it to fit reproduction limits. 1962Unesco Bull. Libr. XVI. 16 A high value of gamma is desirable in photographic reproduction of lines, as in the pages of a printed book.
(iv) A unit of magnetic field strength, 10—5 oersted.
1903Nature 5 Nov. 6/1 There was a movement of the declination needle to the west through about 34′, and a diminution of 240 γ in the horizontal force. 1967New Scientist 7 Dec. 617/2 The lunar magnetic field is no bigger than two gammas.
(v) A unit of mass, one millionth part of a gramme.
1931Industrial & Engin. Chem. (Analytical Ed.) 15 July 314/2 The value obtained was 4·0 γ copper per gram (1 γ = 0·001 mg.) for the combined filtrates. 1940Ibid. 15 June 359/2 The Committee on Nomenclature, Spelling, and Pronunciation of the American Chemical Society has approved the following recommendation of the Division of Microchemistry: For 0·001 milligram the term ‘micro⁓gram’, designated by the symbol γ (the word ‘gamma’ should not be used as a substitute for ‘microgram’). 1956A. Huxley Let. 23 Sept. (1969) 807 Fifty gamma of LSD were sufficient to produce in me virtually the full effect of the standard dose. 1963Jerrard & McNeill Dict. Sci. Units 55 In 1937 it was claimed that the gamma was being superseded by the microgramme in the United Kingdom whereas the gamma reigned supreme in the U.S.A. and continental Europe.
(vi) gamma globulin (see quot. 1957).
1937A. Tiselius in Biochem. Jrnl. XXXI. 1466 The fastest of these components could be identified with serum albumin. The other three..are more or less completely precipitated by half saturation with ammonium sulphate. They will therefore be named α, β, and γ serum-globulin. Ibid. 1473 The β- and γ- globulins gave rather diffuse boundaries. 1957Dorland's Med. Dict. (ed. 23) 563/2 Gamma g[lobulin]s, globulins of plasma which in neutral or alkaline solutions have the slowest electrophoretic mobility... Most antibodies are gamma globulins. 1960M. E. Florey Clin. Appl. Antibiotics II. vii. 202 Gamma globulin has been considered to play a part in enhancing the effect of antibiotic therapy. 1970Nature 7 Nov. 509/2 ‘Immunoglobulin’ thus superseded ‘γ-globulin’.
†2. = gamut. [See ] Obs.
1622Peacham Compl. Gent. xi. (1634) 104 Two Lutes of equall size being.. tuned Vnison, or alike in the Gamma, G sol re vt. 1724Explic. For. Words Mus., Gama or Gamma, is what we call the Gamut. 1825Danneley Encycl. Mus., Gamme..Gamma, Gamut or Gammut.
†3. Surg. (See quot.) Obs. Cf. gammot.
1848Craig, Gamma..a surgical instrument used for cauterising a hernia—so called from its shape resembling that letter. 1854in Mayne Expos. Lex.
4. A common moth, Plusia gamma. In full gamma moth.
1869Eng. Mech. 24 Dec. 345/2 The..caterpillar of the Gamma moth is an instance. 1882Cassell's Nat. Hist. VI. 65 Several of the Plusidæ are also day-flying Moths. The well-known Gamma Moth or Silver Y (Plusia gamma) is one of these.
5. Comb., as gamma-shaped adj.; gamma-function Math. (see quot. 1865).
1865B. Price Infinit. Calc. (ed. 2) II. 155 The symbol Γ(n), devised by Legendre, has been of late ordinarily employed to denote it; so that we have Γ(n) = ∫∞0 e-xxn-1dx. For this reason and for the sake of a distinctive name, the definite integral has been called the Gamma-function. 1875B. Williamson Integral Calc. 150 All definite integrals which are reducible to Gamma-functions. 1893W. M. Ramsay Ch. in Rom. Emp. xii. 262 A gamma-shaped crypt, attached to a small chapel.
Add:[1.] [c.] (vii) Comm. gamma stock now Hist., on the U.K. Stock Exchange: stock in the third category of a system which classifies stocks available through SEAQ (see *S I. 4 a) according to the intensity with which they are traded; also ellipt., and as gamma security, gamma share.
1986Observer 14 Sept. 34/8 Investment clients are as concerned about the future of gamma securities as corporate clients. 1986Times 29 Sept. 19/1 From next month all stocks..with a market capitalization of less than {pstlg}100 million will be traded as gamma stocks. 1986Daily Tel. 28 Oct. 21 For beta and gamma shares it [sc. the Prudential] expects to use brokers. 1987Ibid. 26 Oct. 27/8 They have avoided the leaders, concentrating instead on gammas. 1987Investors Chron. 27 Nov. 23/3 With a thinly-traded gamma stock like Boosey & Hawkes..the minimum spread is some 10p. 1989Financial Times 11–12 Feb. 4/8 Its chief market-maker said the band of sizes it would quote for..the less actively traded betas and gammas would be ‘anything from 5,000 to 100,000’.
▸ gamma-aminobutyric acid n. Biochem. and Physiol. an amino acid (one of the isomers of aminobutyric acid) which is found principally in the central nervous system of vertebrates, where it acts as an inhibitory neurotransmitter. Abbreviated GABA.
Chemical name: 4-aminobutanoic acid; H2N(CH2)3COOH
[1890Jrnl. Chem. Soc. 58 i 360 When digested for three hours with 27 per cent. hydrochloric acid, it is resolved into phthalic acid and γ-amidobutyric acid, NH2·CH2·CH2·CH2·COOH.] 1900Jrnl. Chem. Soc. 78 i 557 When pyrrolidone is boiled with alkalis, concentrated hydrochloric acid, or barium hydroxide, it is converted into *γ-aminobutyric acid. 1938C. L. A. Schmidt Chem. Amino Acids & Proteins v. 245 In putrefying meat a betaine has been isolated whose composition is that of..a betaine of γ-aminobutyric acid. It has an action on nerve-endings resembling that of curare. 1975W. R. Uttal Cellular Neurophysiol. ix. 196 Substances that have classically been considered to be solely inhibitory in mammalian brains include the amino acids glycine and GABA (gamma aminobutyric acid). 1995Independent 12 May 3/2 Benzodiazepines..promote the action of a chemical, gamma-aminobutyric acid, which blocks the transmission of messages between brain cells, and reduces the excess brain activity associated with anxiety and insomnia. |