652 Chapter 11. Dosimetry and Radiation Protection
equivalence can be approached by assuring that the energy losses of charged particles
in the gas is the same as in the tissue, that is
E ̄g=E ̄t, (11.5.11)whereE ̄gandE ̄trepresent the mean energy losses in the active gas volume of the
chamber and the tissue. Now, let us suppose the mean distance traveled by a charged
particle in the gas isdg. For a small spherical chamber, this could be the diameter
of the sphere. If the mass stopping power for the particles in the gas volume isSρ,g
(see chapter 2 for discussion on mass stopping power) and the density of the gas is
ρg, the mean energy deposited will be given by
E ̄g=Sρ,gρgdg. (11.5.12)Similarly the mean energy deposited in the tissue can be evaluated from
E ̄t=Sρ,tρtdt, (11.5.13)whereSρ,tis the stopping power of same particles in the tissue having densityρt.
The equivalence of absorbed energy then requires that
Sρ,gρgdg=Sρ,tρtdt. (11.5.14)Now, suppose that the atomic composition of the gas and the tissue are identical.
In this case the mass stopping powers in the two median will be the same, that is
Sρ,g=Sρ,t. With this condition the above equation becomes
ρgdg=ρtdt. (11.5.15)This condition for TEPCs is generally written as
ρg=kρg, (11.5.16)wherek≡dt/dgis equal to the ratio of the tissue diameter to the chamber diameter
for spherical chambers. Looking closely at the above relation we find that, for a fixed
chamber dimension, the only parameter that can be varied to assure the equivalence
of energy deposition is the gas density. This is the basic working principle of a
TECP, that is, the density of the gas is adjusted such that the above relation is
satisfied. In such a chamber, the absorbed dose is equivalent to the dose absorbed
in the tissue.
Adjusting the gas density is generally done through adjusting the gas pressure.
This simple principle gives fairly accurate measure of the dose in tissues. The low
pressure TEPCs are therefore the foremost microdosimeters constructed and are still
widely used. A typical TEPC has a spherical or cylindrical geometry. Except for the
lower gas density, its working principle is the same as the conventional proportional
counters. Such detectors have already been discussed at length in the chapter on
gas filled detectors.
Design and construction of a TEPC requires special attention to the uncertainties
that can be caused by the walls of the chamber. These uncertainties can be due to
several effects. One such effect is due to the so calledre-entrymechanism. What
happens is that an electron produced by the incident radiation can move out of the
chamber’s active volume and then re-enter it at a later time. In a real tissue, the