180 DREVINSKY AND PECCI The average »Zr/*Sr activity ratios for r< 0.02 u particles over all altitudes in the May and August 1963 collections are about 35 and 14, respectively. The corresponding average “Zr/*Sr activity ratios for 0.02 u<r<0.15 uparticles, excluding the 27- to 30-km collection of May 1963, are 18 and 6. The ratio in both particle size ranges has decreased in accord with the 65-day half-life of Zr, 9577/1449 Activity Ratios In Figs. 7 and 8, 57 r/44Ce@ activity ratios are also plotted as a function of size and altitude for the collections of May and August 1963, respectively. One advantage of considering this ratio is that it is thought to be more accurate than those involving Sr whose sample concentrations were frequently very low and difficult to measure. Another advantage is that the ratio should be less sensitive to older debris than '4Ce/™Sr and ®Zr/™Sr activity ratios. There is little similarity in the trends of the ®Zr/“‘Ce and 4Ce/%Sr activity ratios with size. Our data suggest that there is chemical fractionation between Zr and ‘Ce although additional data would be helpful, The smallest amount (about 10% or less) of the total *Zr and “4Ce is carried by particles with r> 0.15 yu, and the largest amount is gen- erally carried by particles with 0.02 u< r< 0.15 uy. It is notable that the altitudinal variations of 87 r/M4Ce activity ratios for these two size ranges are nearly identical. The sameratio for the smallest particles, however, is highly variable with altitude. Activity Ratios Involving 54Min and 14%, Since data for “Mn and '°Ba were far less complete than for the Sr, *Zr, and ‘Ce, the concentrations and the activity-ratio data are merely listed in Tables 3 to 6. As mentioned earlier, we can only say from the March 1962 collection that most of the °Ba appears to be carried on particles with 0.02 u< r< 0.15 u and that there seems to be a peak concentration in the 15- to 21-km band. The “Mn concentrations for the May and August 1963 collections also show this same general behavior although the largest particles in May seem to contain an unusually high amount. A comparison of the three activity ratios involving 4vin (see Table 6) indicates that its size distribution is quite variable relative to “Sr, ®Zr, and '4Ce. There seems to be closer agreementin the May and August 1963 collections in the 9- to 27-km region between the 44Ce/Mn activity ratio and particle size, however, and it is possible that these two nuclides have more similar distributions in debris of this age.