76 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



60 



40 

 NORTH 



20 





 EQUATOR 



20 



40 

 SOUTH 



60 



Fig. 1 1 Atom ratio of ^ '*" Pu/^ ' Pu in worldwide fallout soil sample. 



the relatively higher deposition of atypical debris from the Nevada Test Site and from the 

 French testing site at Mururoa Atoll, respectively. 



Figure 11 also shows a slightly reduced ^^^Pu/^^^Pu mass ratio in the equatorial 

 region between 30°N and 30°S. This observation can be explained by the following 

 considerations. The neutrons generated in a nuclear detonation increase with the yield. 

 Therefore it seems reasonable that the ^'*°Pu/^^^Pu atom ratio will also increase with the 

 size of the nuclear detonation. Most of the nuclear test sites are located within the 30°N 

 to 30°S region where the tropopause height is at its maximum and only the debris clouds 

 from the larger yield tests in tliis region had sufficient momentum to penetrate the 

 stratosphere. Debris from these tests, which have an elevated ■^^^Pu/^^^Pu ratio, which 

 entered the stratosphere is largely deposited in the middle latitudes because of the greater 

 transfer rates from the stratosphere to the troposphere at these latitudes. By contrast, 

 debris from the smaller yield tests, which have lower ■^'*"Pu/^^^Pu mass ratios, remained 

 within the troposphere and are deposited on the earth's surface predominantly at the 

 latitude of the detonation. Therefore a relatively greater amount of fallout plutonium 

 from low-yield detonations is deposited in the equatorial regions, which gives rise to the 

 reduced ^''^Pu/^^^Pu mass ratio (Hardy, Krey, and Volchok, 1973). The average atom 

 ratios for ^^^Pu/^^^Pu, ^""'Puj^^^Pu, and ^^^Pu/^^^Pu corrected to January 1971 

 were 0.179 ± 0.014, 0.0083 ± 0.0017, and 0.0036 ± 0.0011, respectively. Samples con- 

 taining obvious contributions from the Nevada Test Site were excluded in the 

 determining of these ratios. 



The concentration of ^^^Pu from the SNAP-9 A device had been measured in surface 

 monthly air samples from northern to southern latitudes (Hardy, 1977). The yearly 

 average of these data is shown in Fig. 1 2. At most latitudes the ^^^Pu from the SNAP-9A 

 incident was barely detectable in 1966 but built up to a maximum in 1967 and 1968. The 



