Radiation Statistics
Radioactive emission is a random process. The number of
emissions in a given time period is not constant but varies
statistically about an average value. The variation about the
true mean value is a Poisson distribution. In this distribution,
the standard deviation (σ) about the mean (n) is defined as:
σ = n
When the mean is greater than 100, the Poisson distribution can
be closely approximated by the normal distribution (Figure 12).
The normal distribution predicts the probability that any given
count rate will fall within a selected region about the mean.
Normal Distribution
Figure 13. Variation of Radioactive Emission
Using the mean of a larger number of counts to approximate the
true mean, the distribution shows that 68.3% of the time the
count rate obtained will be within 1 standard deviation of the
mean. Figure 14 shows the probabilities for three different
standard deviations. A statistical stability test may be
performed to compare the experimental standard deviation to
the theoretical standard deviation.
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