642 Chapter 11. Dosimetry and Radiation Protection
withμm.,enbeing the mass energy absorption coefficient. This coefficient should be
computed from
μm,en=
μm,tr
1 −g, (11.4.12)
whereμm,tr is the mass energy transfer coefficient andg is the fraction of loss
of energy of the secondary charged particles through radiative processes, such as
Bremsstrahlung.
So far we have explicitly used Bragg-Gray cavity theory to derive expressions for
dose equivalence. However, as we saw earlier in the chapter, even with all the cor-
rections made, this theory does not give very accurate results specially in situations
where the wall is thick and is made of high Z elements. Since Spencer-Attix theory
gives better results than the Bragg-Gray theory, general practice is to start with the
Spencer-Attix equation and then apply some correction factors to it. The modified
form of this equation for an ion chamber dosimeter can be written as
Dmed
Dcav=
( ̄
L
ρ)med,cav,kt, (11.4.13)where, as before,ktrepresents the total correction factor and
( ̄
L
ρ)med,cav,≡
(
L/ρ ̄)
( med,^
L/ρ ̄)
cav,Using the expression forDcav=Dairas derived earlier for the Bragg-Gray cavity,
the computational form of the Spencer-Attix equation becomes
Dmed=QairWair
eMair( ̄
L
ρ)med,cav,kt. (11.4.14)As noted earlier, there are uncertainties associated with various quantities in
the expressions we have derived so far. Fortunately, in well designed chambers the
combined uncertainty is typically less than 1%. Still, it is a good practice to care-
fully analyze all sources of uncertainties before inferences from the measurements
are drawn. Table.11.4.1 lists some of the parameters and their typical uncertain-
ties. The reader should note that these values can not be used for any type of
chamber, and uncertainties for specific dosimeters should either be experimentally
determined or, in case of a commercial product, evaluated from the data provided
by the manufacturer.
11.4.BSolidStateDosimetry
We have already seen a few examples of solid state dosimeters, namely the TL,
OSL, and film dosimeters. Apart from these so called passive dosimeters, there
are also active solid state dosimeters that are being extensively used specially in
the field of radiation therapy. Perhaps the most commonly used active solid state
dosimeters are based on silicon diodes. The silicon detectors used in dosimetry are
similar to the usual silicon diode detectors we studied in the chapter on solid state
detectors. These mostly single channel devices have many advantages over their gas