Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

694 CHAPTER 13
The DAC for 210 Pb is 1 × 10 −4 μCi/m 3 (3.7 Bq/m 3 ), and the specific activity of 210 Pb
is 79.2 Ci/g (2.93 × 10 12 Bq/g). The activity of a 1-μm 210 Pb particle is
A = 1.6 × 10 −13
= 1.27 × 10 −5
g Ci
Ci
× 79.2 = 1.27 × 10−11
particle g
μCi
particle .
particle
The particle concentration corresponding to the radiological DAC is
−4 μCi
1 × 10
C (DAC) =
m3 1.27 × 10−5 μCi
particle
= 8 particles
m3 .
The influence of this very small particle concentration on sample size may be illustrated
by the following example.
W EXAMPLE 13.8
Particles of 239 PuO2 will be sampled with a membrane filter whose collection efficiency
is given as 99%. What minimum sample size is needed to determine whether
the Pu concentration exceeds 10% of the DAC (DAC = 7 × 10 −12 μCi/mL)? The
MDA for the counting system is 5 × 10 −7 μCi (0.0185 Bq).
Solution
The minimum sample size, if we did not have to consider the particle concentration
is calculated from Eq. (13.21):
MDA
C =
V (min) × ε ,
5 × 10
V =
−7μCi 0.1 × 7 × 10−12 μCi/mL × 0.99 = 7.2 × 105 mL = 0.72 m 3 .
The PuO2 particles, 1-μm AMAD (0.3 μm actual size), have a mean activity of 3.5 ×
10 −7 μCi/particle. The DAC for class Y 239 Pu is 7 × 10 −12 μCi/mL (0.3 Bq/m 3 ). To
meet the DAC means a particle concentration of
C (DAC) =
7 × 10−12 μCi/mL
3.5 × 10 −7 μCi/particle = 2 × 10−5 particle/mL = 20 particle/m 3 .
For health physics purposes, we wish to detect 10% of the DAC (i.e.,
2 particles/m3 ) within ±20% at the 95% confidence level (or confidence interval,
CI). That is, we want to collect 2 ± 0.4 particles at the 95% confidence level.
95%CI = 0.4 =±1.96σC,
σC = 0.4
= 0.2
1.96