the dependence of the concentration or activity ratio, R, of plutonium to a refractory nuclide, such as 147pm or !4*4Ce, on the particle diameter, has the form: R = ad™ ; (a) where a and m are constants, ameter of the size fraction. and d is the median (geometric mean) di- The justification of our assumption, above, is derived from the radial distribution theory of Freiling (196la). According to this theory, most radionuclides are volume-distributed, surface-distributed, or distributed with a concentration gradient in the particles, so that the radionuclide content of the particles is described by: A = bd" (b) where A is the activity of the nuclide, or the number of atoms in a particle, d is the particle diameter, and b and n are constants. The value of n lies usually between 2 and 3. Consequently, the ratio of the activities of two nuclides, i and j, has the following particle size dependence: (A,/A;) =R i,j = (b;/b,)d n, - 7, i 4 = ad ™m We admit that for surface and subsurface bursts, (c) these relationships are too simplistic, and the description (b) is not valid for the entire size range of radioactive particles, which may extend from less than 0.1 um to 1 mm or above. No other resource can be had, however. Twelve sets of data from the events have thus been analyzed. These events include one subsurface shot in alluvium at shallow depth, seven shots on or above coral, and three shots above alluvium. The scaled heights of burst ranged from -21 feet to almost 400 feet, yields ranged from about 1 kiloton to more than 10 megatons. Lower limits of the particle size ranges varied from less than 0.1 um to about 12 un, upper limits ranged from 1 pm to almost 1 mm. The smaller ranges were associated with the greater scaled heights of burst. The data were correlated statistically by least-squares regression in the log-log plane. Values for the exponent, m, were thus obtained as slopes of the log R-log d correlation. The significance of the correlations were tested by means of the correlation coefficient. The hypothesis that m # 0 was tested by means of the t-test. The results are shown in Table 5. It is expected that the 739Pu/!47pm and 239Pu/!4*?pm ratios are indepen- dent of the particle size, if plutonium behaves like a refractory species. In that case, therefore, the expectation is that m= 0. If plutonium is more volatile than the rare earths, its concentration decreases more rapidly with increasing particle size, and m< 0. If plutonium behaves in a more refractory fashion than the rare earths, m > 0, 565