Particle size was described by either of two size parameters, equi-
valent projected area diameter (D,) or maximum diameter (Dm).
The pro-
jected area method is conventional, and the area can be related to particle falling velocity - an important quantity in any fallout model. No
significant physical relation can be attached to maximum diameter; however, it does offer ease of measurement and reproducibility. Diameters
were measured with ocular micrometers having either a linear scale or a
Fairs graticule (sizing circles). The linear scale was employed exclu-
sively to size YAG-40-Shot B particles in terms of Dy.
All other Shot B
particles and the YAG-40-Shot A particles* were sized by the graticule
system in terms of Dz, whereby particle area was compared with graticule
area. Measurements were generally taken under a total magnification of
kx, although 19.5X and 9.9K were used occasionally by changing micro-
scope objectives. In general, interpolation between scale units for
either micrometer was not attempted, except in the case of YAG-40-Shot
A particles.
Particle Activity Measurements
Particles were individually counted for relative gamma activity in
@ well scintillation counter** employing a 1-3/h-in. dia. x 2-in. thick
Tl-activated NaI crystal detector. In the case of measurements made at
the site (YAG-40 collections), three l-min counts were taken and, for
convenience, the median was selected as the representative value. Where gross counting rates were less than twice background, single l-min counting was done. Particles analyzed at NRDL were all counted for single
l-min periods.
Backgrounds were on the order of 300 c/m.
Experimentally
determined coincidence loss corrections were applied when activities
exceeded 106 c/m. Response characteristics of the several well counters
employed were normalized, where necessary, through laboratory gamma stan-
dards.
Field measurements were taken from about H + 5 to 4O hr and NRDL
measurements from about H + 300 to 650 hr. Activities were corrected to
an appropriate reference time for analysis, and experimental decay curves
were determined by following the decay characteristics of selected particles and aliquots of particle solutions.
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*
A small group of YAG-40-Shot A particles was sized in terms of cir-
cumsecribed diameter (D,) by utilizing graticule circumference.
Be-
cause of its small number, this group was not included in this study
but is listed in Appendix A.
*#* An end-on 1-1/2-in. dia. x 1/2-in. thick NaI scintillator was occa-
sionally used to count exceptionally active particles. A conversion
factor of 10 from shelf 1 to well was obtained from lower-activity
particles counted in both counters, and has been applied to all results.
4