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.