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

Evaluation of Radiation Safety Measures 687
Each of these has its own flow rate, filtering, and physical or chemical characteristics.
Filters trap particles mainly by two mechanisms: (1) sieving, which captures particles
that are larger than the pore size, and (2) impaction, which captures particles
smaller than the pore size. In impaction, because of the inertia of the particles in the
airstream, the particles tend to move in straight lines. As the air bends in its tortuous
path through the filter’s pores, the particles tend to continue in straight lines and
thus strike the filter matrix, where they are captured. For this reason, the filtration
efficiency of a filter increases as the flow rate through the filter increases. Filters
made of glass fiber or paper trap particles within the matrix; membrane filters trap
particles on the filter surface. This point is important when one is sampling for alpha
or for low-energy beta particles because of the corrections for self-absorption that
must be used. For this reason as well as for their very high retention efficiency for
particles of respirable size, membrane filters are very widely used for the sampling
of radioactive aerosols. Membrane filters may be made transparent with immersion
oil, thus allowing direct microscopic observation of the particles for sizing or for
particle concentration measurements. However, these filters retain a strong electrostatic
charge. If the filter is placed into a windowless counter, this electrostatic charge
would distort the electric field around the anode and thus would introduce a counting
error. To prevent this, the membrane filter is treated with a mixture of dioxane
and petroleum ether, which eliminates the static charges from the filter without
interfering with the dust particles on it. The filtration efficiencies, at a sampling
velocity of 0.61 m/s (2 ft/s), are shown in Table 13-3.
Airborne gases may be collected by a number of different techniques, including
the following:
1. Adsorption is the process in which a monolayer of the gas binds to the surface
of certain granular substances called adsorbents. Commonly used adsorbents include
activated charcoal, silica gel (a hard, glassy form of SiO2), activated alumina
(a very porous form of Al2O3 · 3H2O), and molecular sieves (synthetic Na
or Ca aluminosilicates of high porosity). The capacity of each adsorbent depends
on its specific surface area (square meters per gram), the partial pressure
of the gas, and the temperature. This binding capacity is represented by a curve
called an adsorption isotherm (Fig. 13-5), in which the amount of gas adsorbed
per gram of adsorbent is plotted against the equilibrium pressure at constant
temperature. In use, the adsorbent is packed into a suitable container, and the
air to be sampled is drawn through the adsorbent. If desired, the adsorbed gas
can be driven off in the laboratory by the application of heat, or it may be chemically
desorbed by passing an appropriate chemical absorber through the adsorbent
bed.
TABLE 13-3. Collecting Efficiency of Certain Filters (Percent)
2
Whatman 41 23 28 64 74 80 100
Whatman 4 23 32 38 79 84 100
MSA S 48 47 77 92 94 100
H-70 99.3 99.3
Glass fiber 99.9 99.9
Membrane 99.9 99.9