Biomedical Engineering Reference
In-Depth Information
proximately,
B
2
= 12. The exposure rate at 6 m for these photons without shielding
is
0.5
×
10
×
1.37
X
1.37
=
= 0.190 R h
-1
.
(15.9)
6
2
With the shield it is
= 3.8 mR h
-1
.
X
1.37
= 12 × 190 × e
-6.4
(15.10)
The total shielded exposure rate is
X
=
X
2.75
+
X
1.37
=
23 mR h
-1
.
19 + 4
=
(15.11)
Some additional thickness is needed. One can proceed simply by incrementing the
relaxation length until the desired value of the total exposure rate is obtained. In place
of 4.5, for example, one can try
µ
1
x
=
4.7, for which
B
1
=
4.8,
µ
2
x
=
6.7, and
B
2
=
15.
One finds then that
X
=
X
2.75
+
X
1.37
=
20 mR h
-1
.
16.7 + 3.5
=
20.2
=
(15.12)
4.7/(0.043 cm
-1
)
It follows that the required depth of water is
x
=
4.7/
µ
1
=
=
109 cm.
The exposure rate at the surface of the water is 20(600/109)
2
= 610 mR h
-1
.
In principle, different sets of buildup factors are needed to compute different
transmitted quantities, such as dose, exposure, kerma, and energy fluence. How-
ever, values given in Figs. 15.1-15.6 specifically for exposure are not very different
from those for the others. We shall make no distinction in using the single set of
figures for all computations.
1)
15.3
Shielding in X-Ray Installations
X-ray machines have three principal uses—as diagnostic, therapeutic, and non-
medical radiographic devices. An X-ray tube is usually housed in a heavy lead cas-
ing with an aperture through which the primary, or useful, beam emerges. Typi-
cally, the beam passes through metal filters (e.g., Al, Cu) to remove unwanted, less
penetrating radiation and is then collimated to reduce its width. The housing, sup-
plied by the manufacturer, must conform to certain specifications in order to limit
the leakage radiation that emerges from it during operation. For diagnostic X-ray
tubes, regulations require that manufacturers limit the leakage exposure rate at a
distance of 1 m from the target of the tube to 0.1 R h
-1
when operated continuously
at its maximum rated current and potential difference.
1
Figures 15.1-15.5 are based on Table 6.5.1
from B. Schleien, L. A. Slayback, Jr., and
B. K. Birky,
Handbook of Radiological Health,
3rd Ed.
, Williams and Wilkins (1998).
Figure 15.6 is plotted from data on p. 147,
Radiological Health Handbook, Revised Ed.
,
Jan. 1970, Bureau of Radiological Health,
Rockville, MD.
