Chapter 24. -NUCLEAR POWER PLANTo 



many unstable nuclei emit an alpha particle. 

 The results of alpha emission can be seen 

 from the following equation: 



92 



U' 



238 



1"^* 



90 



Th 



234 



In the above equation, the parent isotope 

 of uranium (goU ) is a naturally occurring, 

 radioactive isotope which decays by alpha 

 emission. Since the A and Z numbers must 

 balance in a nuclear equation, and since an 

 alpha particle contains two protons, we see 

 that the uranium has changed to an entirely 

 new element. 



The radioactive isotope of thorium 

 (QQTh234) produced in the above reaction further 

 (fecays by the emission of a beta particle sym- 

 bol ^) as indicated in the following equation: 



90 



Th234__^^+g^Pa234 



The beta particle has properties similar 

 to an electron.3 However, the origin of the 

 beta particle is within the nucleus rather 

 than the orbital shells of an atom. It is postu- 

 lated that a beta particle is emitted at an 

 extremely high energy level when a neutron 

 within the nucleus decays to a proton and an 

 electron (beta particle). When this phenomenon 

 occurs, the proton stays within the nucleus 

 forming an isotope of a different element 

 having the same mass. 



A radioactive isotope may go through several 

 transformations of the above types before reach- 

 ing a stable state. In the case of 92U238 

 there are a total of eight alpha particles and 

 six beta particles emitted prior to reaching 

 a stable isotope of lead (82Pb206), 



The third manner in which a naturally 

 radioactive isotope may reach a more stable 

 configuration is by the emission of gamma 

 rays (symbol y). The gamma ray is an electro- 

 magnetic type of radiation having frequency, 



high energy, and a short wave length. Gamma 

 rays are similar to X-rays in that the prop- 

 erties are the same. The distinguishing factor 

 between the two is the fact that gamma rays 

 are originated in the nucleus of an atom, 

 whereas the X-ray originates from the orbital 

 electrons. In general it can be said that a 

 gamma ray is of higher energy, higher fre- 

 quency, and shorter wave length than an X- 

 ray. 



Frequently an isotope which emits an alpha 

 or beta particle in the decay process will 

 emit one or more gamma rays at the same 

 time, as in the case of 27^'^ > ^^ isotope 

 that decays by beta emission and at the same 

 time emits two gamma rays of different energy 

 levels. Some radioactive isotopes reach a stable 

 state by the emission of gamma rays only. In 

 the latter case, since gamma rays have neither 

 mass nor electrical charge, the A and Z 

 numbers of the isotope remain unchanged but 

 the energy level of the nucleus is reduced. 



An important property of any radioactive 

 isotope is the time involved in radioactive 

 decay. To understand the time element, it is 

 necessary to understand the concept of half- 

 life . Half-life may be defined as the time 

 required for one-half of any given number 

 of radioactive atoms to disintegrate, thus re- 

 ducing the radiation intensity of that particular 

 isotope by one-half. Half lives may vary from 

 microseconds to billions of years. At times 

 an isotope may be said to be "short-lived" 

 or "long-lived", depending upon its peculiar 

 radio-active half-life. Some half-lives of typical 

 elements are: 



ggU^^S ^ ^ gj ^ jq9 yg^j.g 



ggU^^^ = 7.13 X 10^ years 



88 



Ra226 = 1620 years 



Francis W. Sears and Mark W. Zemansky, Uni - 

 versity Physics (3d ed.; Reading, Mass.: Addlson- 

 Wesley Publishing Company, Inc., 1964), p.986. 



53 



I^^^ = 6.7 hours 



84 



Po214 ^ jo-6 seconds 



617 



