核工程专业英语ChapterRadioactivity.pptVIP

NUCLEAR ENERGY PROFESSIONAL(SPECIAL) ENGLISH COURSE CHAPTER 3 Radioactivity（放射性） Many naturally occurring and man-made isotopes have the property of radioactivity(放射性), which is the spontaneous(自发的) disintegration (decay) (衰变) of the nucleus with the emission of a particle. The process takes place in minerals (矿物质) of the ground, in fibers (纤维) of plants, in tissues (组织) of animals, and in the air and water, all of which contain traces (痕迹,微量) of radioactive elements. 3.1 RADIOACTIVE DECAY（放射性衰变 ） Many elements that are found in nature or man-made are radioactive(放射性的), emitting alpha partic1es(?), beta particles (?) (including electron or negatron and positron), neutrinos(中微子) , and gamma rays(?). Some examples are as following: 3.2 THE DECAY LAW(衰变规律) The rate at which a radioactive substance disintegrates (and thus the rate of release of particles) depends on the isotopic species(种类，核素), but there is a definite (一定的) “decay law” that governs(管理,支配) the process. In a given time period, say one second, each nucleus of a given isotopic species has the same chance of decay. If we were able to watch one nucleus, it might decay in the next instant (立即的), or a few days later, or even hundreds of years later. Such statistical(统计学的) behavior is described by a constant property of the atom called half-life (半衰期). This time interval(间隔), symbolized by tH is the time required for half of the nuclei to decay, leaving half of them intact(完整无缺的). We should like to know how many nuclei of a radioactive species remain at any time. If we start at time zero with N0 nuclei, after a length of time tH there will be N0/2; by the time 2tH has elapsed (流逝), there will be N0/4; etc. A graph of the number of nuclei as a function of time is shown in Fig. 3.l. For any time t on the curve (曲线图), the ratio of the number of nuclei present to the initial (初始的) number is given by Half-lives range from very small fractions (小部分, 片断) of a second to billions of (数十亿) years, with each ra