Biomedical Engineering Reference
In-Depth Information
Fig. 12.4
Cross section of graphite-walled CO
2
chamber for measuring photon dose.
Fig. 12.5
Ratio of absorbed doses in bone, air, and carbon to that in soft tissue,
D
t
.
∼
0.1 MeV to
∼
10 MeV, the ratios for all materials of low atomic number are near
unity, because Compton scattering dominates. The curve for bone, in contrast to
the other two, rises at low energies due to the larger cross section of photoelectric
absorption in the heavier elements of bone (e.g., Ca and P).
12.6
Neutron Dosimetry
An ionization device, such as that shown in Fig. 12.4, used for measuring gamma-
ray dose will show a reading when exposed to neutrons. The response is due to
ionization produced in the gas by the charged recoil nuclei struck by neutrons
in the walls and gas. However, the amount of ionization will not be proportional
