Mathematical Methods for Physicists: A concise introduction - Site Map

SPECIAL PROBABILITY DISTRIBUTIONS
The Poisson distribution is very important in nuclear physics. Suppose that we
have n radioactive nuclei and the probability for any one of these to decay in a
given interval of time T is p, then the probability that m nuclei will decay in the
interval T is given by the binomial distribution. However, n may be a very large
number (such as 10 23 ), and p may be the order of 10 20 , and it is impractical to
evaluate the binomial distribution with numbers of these magnitudes.
Fortunately, the Poisson distribution can come to our rescue.
The Poisson distribution has its own signi®cance beyond its connection with the
binomial distribution and it can be derived mathematically from elementary considerations.
In general, the Poisson distribution applies when a very large number
of experiments is carried out, but the probability of success in each is very small,
so that the expected number of successes is a ®nite number.
The Gaussian (or normal) distribution
The second limit of the binomial distribution that is of interest to us results when
both n and pn are large. Clearly, we assume that
p
m, n, andn m are large enough
to permit the use of Stirling's formula (n!
2n n n e n ). Replacing m!, n!, and
(n m)! by their approximations and after simpli®cation, we obtain
np
m
nq
nm
r
n
P…X ˆ m†
: …14:29†
m n m 2m…n m†
The binomial distribution has the mean value np (see Eq. (14.20). Now let
denote the deviation of m from np; that is, ˆ m np. Then n m ˆ nq ;
and Eq. (14.29) becomes
or
1
P…X ˆ m† ˆp
1 ‡ …np‡†
1
2npq…
1 ‡ =np† … 1 =np†
np
nq
P…X ˆ m†A ˆ 1 ‡ …np‡†
1 …nq†
;
np
nq
…nqˆ†
where
s
A ˆ 2npq 1 ‡
1
:
np nq
Then
log… P…X ˆ m†A† …np ‡ † log…
1 ‡ =np†…nq † log…1 =nq†:
497