672 TABLES 731-758.— RADIOACTIVITY * 



A number of elements (12 ; 43 isotopes) of high atomic weight, now found 

 in the earth, and one of the isotopes of each of six lighter elements (Table 732) 

 are unstable in that they spontaneously break down into other elements, emit- 

 ting a, /? or y rays. The study of artificial radioactivity shows some other 

 types of breakdown. Some of the artificial radioactive nuclei break down by 

 the emission of positive electrons or of neutrons ; a K electron may be captured 

 (designated by K) ; some internal conversion of electrons may take place {e~) 

 or there may be some isomeric transition of the nucleus (I.T.). 



The characteristics of the three rays — a, /?, and y — are quite different. A 

 3 Mev a-particle has a velocity of about 1/25 that of light, a range in air of 

 1.7 cm, and produces some 4,000 ion pairs per mm in air at 760 mmHg at 

 15°C. A 3 Mev /3-ray has a velocity of nearly 99 percent of that of light and 

 a range in air of about 13 meters, and produces only about 4 ion pairs per mm 

 in air. The energy of a y-ray, which is very short-wavelength radiant energy, 

 is E = hv, and it has the velocity of light. Thus a 3 Mev y-ray has a wave- 

 length of 4.1 XU. However, the y-rays given by the natural radioactive mate- 

 rials have much less energy than this (4 Mev), generally about 1 Mev. [Some 

 artificial radioactive materials emit y-rays with very high energy (See Tables 

 750-752.).] The wavelengths of the y-rays from natural radioactivity particles 

 range from about 4.5 to about 4,000 XU. y-rays have a very long range. 

 A y-ray produces directly no ions along its path but spends almost its entire 

 energy in producing a photoelectron. Rutherford 230m says that the /8-rays are 

 about 100 times as penetrating as the a-rays, and the y-rays 10 to 100 times as 

 penetrating as the /?-rays. 



Today it should be stated that, in general, the radioactive isotopes (about 

 43 in number) of these 12 elements change into other isotopes, either smaller 

 or of the same weight, depending upon the type of breakdown. The nucleus 

 of the resulting isotope may be smaller in weight by about four units and have 

 a charge two units smaller than the parent due to the emission of an a-particle, 

 or it may be of almost the same weight and have a charge one unit greater due 

 to the emission of a /3-ray. There are several changes in both the weight and 

 charge that may take place for some of the artificial radioactive nuclei. 



The character of these changes varies with the element and seems to be 

 determined by some probability law. It does not seem possible, by any ordinary 

 physical or chemical means, to change these characteristics. (See artificial dis- 

 integration, Table 726.) 



* For reference, see footnote 199, p. 618. 



2:o * Rutherford, E., Chadwick, J., and Ellis, C. D., Radiation from radioactive substances, 

 Cambridge Univ. Press, 1930. 



TABLE 731.— UNITS FOR THE RATE OF RADIOACTIVE DISINTEGRATION 



The curie, the adopted unit of the rate of radioactive decay, is denned as the number of 

 disintegrations of 1 gram of radium (3.61 XlO 10 ) in 1 second. As a working value for the 

 curie the National Bureau of Standards some years ago adopted the value 3700X10 10 

 disintegrations per second. 



The rutherford (abbreviated rd) = 10" disintegrations per second, has been suggested 

 as a smaller working standard. Then, 1 millirutherford (mrd) = 10 s disintegrations per 

 second and 1 microrutherford (fird) = 1 disintegration per second. 



The rate of disintegration of an isotope that emits gamma-rays may be determined by a 

 measure of the 7-ray emission in roentgens. 



A committee of the National Research Council W1 recommended that the curie be denned 

 as 3.70X10 10 disintegrations per second; the rutherford (rd) as just given. For quantita- 

 tive comparison of radioactive sources emitting gamma-rays, for which disintegration 

 rates cannot be determined, the roentgen per hour at 1 meter (rhm) is recommended. 



231 Physics Today, vol. 3, p. 5, 1950. 



SMITHSONIAN PHYSICAL TABLES 



