| 释义 |
half-value
A value of a physical property that is half an earlier value; used esp. attrib., as half-value period = half-life 2; half-value thickness (see quot. 1938).
1903Phil. Mag. V. 578 The activity of the radium emanation decays to half-value in four days. 1908Amer. Jrnl. Sci. XXV. 506 The half-value period of radium is..about 2000 years. 1910Nature 6 Oct. 431/1 It is proposed that the term ‘half-value period’ should be used in all cases to represent the term required for a substance to be transformed to half its original value. 1922Times Lit. Suppl. 18 May 318/2 The rate of decay of an element is measured by the ‘half-value’ period, which may vary from 1010 years to 10-11 of a second. 1938R. W. Lawson tr. Hevesy & Paneth's Man. Radioactivity (ed. 2) iii. 37 The intensity of the radiation is reduced to half its value in its passage through a layer of thickness o·16 mm. Al... Accordingly, the ‘half-value thickness’ of aluminium for the β-rays from RaE has the value of 0·16 mm. 1955C. Croxson in W. C. Newell Casting of Steel xii. 531 Some half-value thicknesses for steel are shown in the following table. 1962F. I. Ordway et al. Basic Astronautics xii. 500 A useful concept in physical shielding is that of the half value layer (HVL) or the thickness of a specific material required to reduce a particular quantity of radiation by half. 1963W. E. Burcham Nucl. Physics ii. 30 The interval..during which half the atoms disappear by decay (half-value period or half-life). |