706 TABLES 785-793.— FISSION 



Artificial disintegration is generally considered in two parts : the first when 

 the bombarded atom suffers a change not greater than the loss (or gain) of an 

 alpha particle, and the second when the change in the bombarded atom is much 

 greater — the bombarded atom being at times split into two nearly equal parts. 

 This latter is called fission ; the former, artificial disintegration. Fission was at 

 first brought about by bombardment with neutrons but it can be caused by 

 bombardment by almost any particle with the proper energy (see Table 726). 

 This effect can be produced in a number of isotopes of the heavier atoms such 

 as Np, U, Pa, Th, Pb, Sn, Eu, and Ni. Some other atoms such as Bi, Rb, Tl, 

 Hg, Au, Pt, W, and many others show no fission ; at least if such an effect 

 exists it is less than 1/1000 that of Th. There are a great many products of 

 fission as shown by a paper by scientists of the Plutonium Project. 240 One 

 example of fission is 



92 U 235 + n 1 ^4oZr 97 + 52 Te 137 + oW 1 + on 1 



There is a considerable release of energy when fission takes place. Complete 

 data are not available but such as are available give values of about 200 Mev 

 per fission per atom of the heavier elements. (See Table 790.) It is also to 

 be noted that there are two neutrons given as a result of the above reaction ; 

 thus, it is self-sustaining. 



240 Journ. Amer. Chem. Soc, vol. 68, p. 2411, 1946. 



TABLE 785.— FISSION DATA* 



Target substance ozU 205 kjPu 239 9oTh 282 82 U 238 mPa 231 02U 283 



Compound nucleus „ 2 U 236 93PU 240 aoTh 233 82 U 239 „Pa 212 ^U 234 



Threshold energy for fast neutron fis- 

 sion, in Mev 1.0 ± .1 1.1 ± .1 ~ 1 



Energy released per fission, in Mev. . . 200 



Energy of fission neutrons, in Mev... <3.5 



(Ave~l) 



Average number of neutrons released 



per fission 2.3 r Estimated to be same as for 92U 2 



(2 to 3.5) 



Average number of neutrons released 

 per thermal neutron absorbed, 77. . . . 1.4 



For reference, see footnote 226, p. 667. 



TABLE 786.— FISSION THRESHOLDS* 



Revised by J. L. Rhodes, University of Pennsylvania. For reference, see footnote 226, p. 667. 



SMITHSONIAN PHYSICAL TABLES 



