11.4. Active Dosimetry 639
where
kd is the correction due to variation in density of air.kh is the correction for the dependence of humidity onWair,
kr is the correction for the loss of charges due to recombination.
ks is the correction for scattering of electrons outside the region of interest,kf is the correction for the non-uniformity in the electric field,
kp is the correction for penetration of radiation through the collimator material,
ke is the correction for loss of secondary electrons on electrodes,
ka is the correction for attenuation before the measurement volume, and
ko is the correction for other design specific error sources.The uncertainties due to these factors in well designed chambers lies well below 1%.
For example,kdfor 10-100keV x-rays is only 0.03%. In fact the uncertainties asso-
ciated with other factors in equation 11.4.2 are much larger. The error introduced
byW-value, for example, can be as high as 0.25%. The same is true for thegfactor.
A.2 CavityIonChamberDosimetry................
The free air ionization chamber technique we just studied is only good for moderate
photon energies up to about 400keV. As the mean energy of the incident photons
increases, the average energy of the secondary electrons also increases. This results
in secondary electrons traveling farther in the chamber before being absorbed and
even escaping from the active volume. One could argue that a solution would be
to increase the volume of the chamber. However, this has associated engineering
difficulties. For example, as the chamber size becomes larger the electrode distance
also widens and requires higher voltage to achieve ionization chamber plateau. Re-
call that ionization chamber plateau is the flat region in the voltage-pulse height
curve that corresponds to the minimum recombination and collection of almost all
charge pairs created by the incident radiation. Now, the higher the voltage the more
probable it becomes that a electrical discharge between one of the electrodes and a
nearby metal occurs. Hence one can not indefinitely increase the bias voltage.
The solution to this problem is to use a Bragg-Gray cavity, which has been
described earlier in the chapter. Since we are now not required to assure secondary
electron equilibrium, there is no need to construct a large chamber. On the contrary,
now the Bragg-Cavity theory requires the chamber to be as small as permissible by
the following two conditions.
1.The fluence of the primary, secondary, and all subsequent generations of elec-
trons should be uniform throughout the detector’s active volume.
2.The total energy delivered by the radiation to the air molecules should be much
larger than the energy delivered to the secondary electrons.
A simple ion chamber dosimeter that fulfills the Bragg-Gray cavity conditions
is shown in Fig.11.4.3. The chamber consists of a cylindrical cathode with a thin
anode wire stretched across its axis. The chamber is filled with air under standard
atmospheric conditions. The main problem with this chamber is that it has walls
with a material that is very different from air. Therefore, the dose measured from