Biomedical Engineering Reference
In-Depth Information
Table 15.3
Average Densities of Commercial
Building Materials
Material
Density
(g cm
-3
)
Barytes concrete
3.6
Brick (soft)
1.65
Brick (hard)
2.05
Earth (packed)
1.5
Granite
2.65
Lead
11.4
Lead glass
6.22
Sand plaster
1.54
Concrete
2.35
Steel
7.8
Tile
1.9
Table 15.4
Half-Value Layers for X Rays (Broad Beams) in Lead and Concrete
Peak Voltage (kVp)
HVL Lead (mm)
HVL Concrete (cm)
50
0.06
0.43
70
0.17
0.84
100
0.27
1.6
125
0.28
2.0
150
0.30
2.24
200
0.52
2.5
250
0.88
2.8
300
1.47
3.1
400
2.5
3.3
concrete, which has an average density of 2.35 g cm
-3
. Table 15.3 gives the aver-
age densities of some common commercial materials. X-ray attenuation in similar
materials can be obtained from the curves in Fig. 15.10 for concrete of equivalent
thickness. For example, the attenuation provided by 2 in. of tile (average density
1.9 g cm
-3
) is equivalent to that of
2(1.9/2.35)
1.62
in. of concrete. (If the building
materials are of significantly higher atomic number than concrete, this procedure
tends to overestimate the amount of shielding needed.) Layers of lead are com-
monly used with building materials to provide protective barriers. For computing
the attenuation it is convenient to have half-value layers for both lead and concrete
at different tube operating potentials. These are given in Table 15.4. As stressed in
the last paragraph, the shielding properties of concrete and lead for X rays cannot
be accurately compared on the basis of the ratio of their densities.
=
