154 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 6 



elastic collisions with nuclei take a relatively more important part in the 

 stopping of fission fragments than in the stopping of lighter charged particles. 



Despite the great kinetic energy (160 mev total) with which the fragments 

 recoil at the moment of separation in the fission process their velocities are 

 relatively low. The most probable initial velocities of the light and heavy 

 groups are found to be about 13.3 X 10 8 cm per sec and 9.0 X 10 8 cm per sec, 

 respectively [33]. These velocities are lower than the orbital velocities of 

 electrons in the innermost shells of the fragments. Such electrons with 

 velocities greater than that of the fragment are most likely to be retained as 

 a permanent core from the instant the two fragments separate. Although 

 the fragments are never completely stripped of electrons, the initial deficiency 

 at the moment of separation has been found on the average to be about 20 

 electrons for the light group and about 22 for the heavy group of fragments. 

 As a fragment is slowed down by an absorbing medium, additional electrons 

 are captured and the charge of the particle rapidly decreases until the full 

 complement of electrons is attained when the fragment is brought to rest. 



The specific ionization along the path of the fragment does not follow the 

 Bragg curve which describes the ionization produced by protons and alpha 

 particles. Instead, the ionization is most intense at the beginning of the 

 path where the fragment has its greatest charge and then diminishes rapidly 

 along the path owing to the decrease in charge resulting from electron capture 

 as the particle loses momentum. The fragment continues to ionize until 

 brought to rest, but below a velocity of about 2.5 X 10 8 cm per sec the 

 electron shells remain nearly filled and energy loss by ionization is negligible. 



Energy loss at low velocities, < 2.5 X 10 s cm per sec, is due almost 

 entirely to elastic collisions with nuclei. Large fractions of the fragments' 

 residual kinetic energy and momentum may be lost in single collisions, and 

 relatively few collisions are necessary to stop the fragment. The average 

 number of collisions depends on the atomic weight of the absorbing medium, 

 but even in light elements it is probably less than ten. The recoil nuclei are 

 observed in cloud chambers as short branches of varying length extending 

 from the track of the fission fragment. In light gases such as hydrogen the 

 fragment track remains relatively straight and shows many branches, whereas 

 in gases of medium atomic weight such as zenon, collisions are fewer and the 

 fragment track is more strongly deflected. 



The stopping of fission fragments in the first part of the range where 

 ionization is most important results in a nearly linear relation between 

 velocity and range, as shown in Fig. 46. In the second part, below a velocity 

 of 2.5 X 10 8 cm per sec, the range is influenced solely by elastic collisions and 

 decreases rapidly to zero. The great variation in the number of collisions 

 that fragments undergo strongly influences the shape of the second part of 

 the velocity-range curve and makes straggling large. The straggling is 



