1 14 TRANS URANIC ELEMENTS IN THE ENVIRONMENT 



alpha-particle tracks are counted by using transmitted light and a magnification of 1 000 X . 

 Epiplan, flat-field objectives are used because they are corrected for uncovered specimens 

 and do not require cover glasses. The numbers of alpha-particle tracks in the lower and 

 upper emulsions are added to give the total number of alpha tracks observed. 



Three Polaroid pictures of tracks from a single particle — one with the focal plane in 

 the lower emulsion, one in the polycarbonate film, and one in the upper emulsion — are 

 shown in Fig. 3. 



Identification of Fissionable Materials 



Table 1 gives the theoretical ratios of alpha-particle to fission-fragment tracks which 

 would be produced from particles irradiated with a fluence of 8.64 x 10''* thermal 

 neutrons/cm^ when there is a 7-day interval between film casting and etching and during 

 exposure to nuclear track emulsion. The stipulation that etching follows film casting by 7 

 days is included because spontaneous fissions will add to the number of fission-fragment 

 tracks during this period. 



The atom percents in the uranium mixtures in Table 1 are given in Table 2, and those 

 in the plutonium mixtures are given in Table 3. 



The isotopic mixtures of plutonium contain ^"^'Pu. All but 0.0023% of this nuclidp 

 decays by beta emission to ^'*'Am, which has a 433-yr half-life. The amount of mis 

 americium nuclide in a mixture will reach a maximum of 0.887 of the initial ^"^ ' Pu atom 

 percent in 74.6 yr. Americium-241 will add additional alpha tracks to those from 

 plutonium. Thus two ratios are given in Table 1 for each plutonium mixture, one for 

 freshly purified plutonium and one for 75-yr-old plutonium containing the maximum 

 ^^ ^ Am activity. In two of the three mixtures, this caused an increase in alpha-particle-to- 

 fission-fragment ratios. However, with heat-source plutonium, the decrease in *^^Pu 

 activity was not compensated for by the increase in "'^ ' Am activity. 



This identification procedure can be used to distinguish particle-bound plutonium 

 from uranium. Table 2 shows that, of the six isotopic mixtures of uranium, only the 

 higWy enriched uranium mixture will give a number of fission-fragment tracks 

 comparable to that of the plutonium mixtures. Even if there should be enough uranium 

 to produce fission-fragment tracks, mixtures of these isotopes would not produce 

 alpha-particle tracks. 



This procedure can be used not only to identify plutonium but also to identify the 

 plutonium isotopic composition in a particle. For example, a particle having 10 

 fission-fragment tracks would also have 5 alpha-particle tracks if the mixture were 

 low-irradiation plutonium, 640 alpha-particle tracks if it were higli-irradiation plutonium, 

 and 5080 alpha-particle tracks if it were heat-source plutonium. 



Table 1 includes (in addition to uranium and plutonium track data) a number of 

 curium and californium nuclides that could mimic the plutonium mixtures. Some of these 

 nuclides decay by spontaneous fission. The polycarbonate film should be allowed to 

 stand several weeks after casting and then be etched both before and after thermal 

 neutron irradiation to detect spontaneous fissioning. Under these conditions tracks due to 

 spontaneous fissioning will appear in unirradiated films. 



Measurement of Plutonium and Uranium Ratios 



For a demonstration of the effectiveness of this identification method in distinguishing 

 between plutonium and uranium, samples of particles were obtained from two sources of 



