核工程专业英语Chapter3- Radioactivity.ppt

1、1,NUCLEAR ENERGY,PROFESSIONAL(SPECIAL) ENGLISH COURSE,CHAPTER 3 Radioactivity,2,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 particl

2、e. 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.,3,3.1 RADIOACTIVE DECAY（放射性衰变 ）,Many elements that are found in nature or man-made are radioactive(

3、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:,4,3.2 THE DECAY LAW(衰变规律),The rate at which a radioactive substance disintegrates (and thus the rate of release of particles) depends

4、 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), o

5、r a few days later, or even hundreds of years later.,5,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

6、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.,6,7,

7、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 radioactive isotope (放射性同位素) having a definite half-life.,8,The formula for N/N0

8、 is not very convenient for calculations except when t is some integer multiple of tH. Defining the decay constant (lambda), as the chance of decay of a given nucleus each second, an equivalent (同意义的) exponential (指数的) formula for decay is We find that = 0.693/tH,9,The number of disintegrations per

9、second (dis/sec) of a radioisotope is called the activity, A. Since the decay constant is the chance of decay each second of one nucleus, for N nuclei the activity is the product (乘积) A = N The unit dis/sec is called the becquerel (Bq), honoring the scientist who discovered radioactivity (放射性活度). An

10、other older and commonly used unit of activity is the curie (Ci) named after the French scientists Pierre and Marie Curie who studied radium (镭). The curie is 3.7 1010 dis/sec, which is an early measured value of the activity per gram of radium.,10,3.3 MEASUREMENT OF HALF-LIFE(半衰期的测量),Finding the ha

11、lf-life of an isotope provides part of its identification (鉴定), needed for beneficial use or for protection against radiation hazard. Let us look at a method for measuring the half-life of a radioactive substance. As in Fig. 3.2, a detector 探测器 (that counts the number of particles striking it ) is p

12、laced near the source of radiation (放射源).,11,12,From the number of counts observed in a known short time interval, the counting rate (计数率)is computed. It is proportional to (成比例的)the rates of emission of particles or rays from the sample and thus to the activity A of the source. The process is repea

13、ted after an elapsed time for decay. The resulting values of activity are plotted on semi-log graph paper (半对数坐标纸) as in Fig. 3.3, and a straight line drawn through the observed points. From any pairs of points on the line and tH = 0.693/ can be calculated (see Exercise 3-8).,13,14,The measurement o

14、f the activity of a radioactive substance is complicated by the presence (存在) of background radiation (本底辐射), which is due to cosmic rays (宇宙射线) from outside the earth or from the decay of minerals in materials of construction (建筑物) or in the earth. It is always necessary to measure the background c

15、ounts (本底计数) and subtract (减去) them from those observed in the experiment.,15,3.4 SUMMARY(总结),Many elements that are found in nature or are man-made are radioactive, emitting alpha particles, beta particles, and gamma rays. The process is governed by an exponential (指数的）relation, such that half of s

16、ample decays in a time called the half-life tH. Values of tH range from fractions (小部分, 片断) of a second to billions of years among the hundreds of radioisotopes (放射性同位素) known. Measurement of the activity (放射性测量), as the disintegration rate of a sample, yields half-life values, of importance in radiation use and protection.,16,补充与拓展,radioactivity n. 放射性; 放射现象; 放射能力; 放射学 artificial radioactivity 人工放射性 atmospheric radioactivity 大气放射性 natural radioactivity 天然放射性 specific radioactivity 放射性比度, 比放射性 a