~ $.3.1 Counting Time A compromise was made as to length of counting time since there were many plates to be counted, on!y two counters were available, and it was desirable to keep the correction factor for decay toa minimum. The counting time for each plate was determined by its approximate counting rate, including a 50 c/m background, in accordance with the following schedule: Counting rate, c/m Countiag time, min <500 500 —1000 1000 —2000 20 10 & ~ | The counting period for practically all samples collected after the Mike test (November 1) was from November 24 to December 12. Most of the samples from the pretest collections were counted after this period but before the end of December. During this period a 24-hr-day 7day-week counting schedule was maintained. 3.3.2 Distribution af Counts The statistical distribution of sample counts appeared to be of a logarithmic or log-log nature. To further investigate the type of distribution, two series of counts of 100 samples each of unashed posttest Engebi sand were made. Sand, in a jar, was dried in the oven and mixed. Sampling cups for the two series held 6.1 + 0.05 mg (n = 100) and 2990 + 3 mg (n = 32), respectively (n is number of samples counted). The smali-sample series was counted in the Nucleometer for 10 min per sampie, and the large samples were counted in the end- window counter for 1 min each. The frequency distribution of the actual counts of the small samples was strongly skewed (Fig. 3.1a) but was approximately normai for the logarithms of the logarithms of these counts (Fig. 3.1b). For the large samples (Fig. 3.1c), the mode of the observed values was still to the left of the mean, but the distribution was more nearly normal. It would appear that the distribution of counts is strongly skewed to the left v-:en the chance of occurrence of speck contamination (Sec. 4.9.1) is small, but, as the numer of specks increases, the distribution approaches the normal curve. For biological samples, especially those with surface contamination, the distribution of counts could be expected to be similar to those of the sand samples. 3.3.3 Unit of Measvrement The unit of mez :ur-ment for recording radioactivity is disintegrations per minute per gram (d/m/g) of wet ...nple (unless otherwise noted), although it is realized that the actua! disintegration rate is not practically attained. The actual disinterration rate was approximated by correcting the gross sample counts for background, sample weight, geometry, backscatter, self-absorption, coincidence, and decay. Since it is desirable to express the amountof activity per sample in a weight unit comparable to that of living organism, wet weight was selected in preference to dried or ashed weight. Naturally the activity per unit of wet weight is lower than the activity per unit of dried or ashed weight. 3.3.4 Significant Figures Results have been recorded to two significant figures, aithough three figures generally were used in the computations. The number of significant figures in the final answer was lim- 20 UNCLASSIFIED, |