AbstractID: 7166 Title: An Ideal Water-Equivalent Cavity Theory Liquid Ionization Chamber
We are attempting to produce an ideal water-equivalent cavity-theory dosimeter for
ionizing photon and electron radiation. The dosimeter design is guided by the
observation that: when a dosimeter material’s unrestricted mass collisional stopping
power and mass energy absorption coefficients are in a constant ratio with those of liquid
water over the clinically relevant photon and electron energy domains, then cavity theory
predicts a dosimeter response that is independent of radiation type and quality. The
hypothesis is that the chemical composition of the dosimeter can be modified without
significant deleterious effects on the dosimeter performance. A similar approach has
been applied in the recent development of plastic- and liquid-scintillators. In this study, a
novel liquid-ionization dielectric-fluid mixture with a response proportional to dose to
water within 2% total deviation for radiations between 10keV and 20MeV, was
developed. Its constituents were determined by selecting medium atomic-number
element substituted organic fluids with low dielectric constants and employing numerical
minimization to determine optimal concentrations. The major benefit of the resulting
dosimeter is that it obviates the need for complicated dosimetry protocols that correct for
the beam quality dependence of a dosimeter. As for air ionization chamber dosimetry
protocols where the measured signal suffers large relative deviations (up to 12% in the
megavoltage range and 70% below 100keV) as a function of radiation quality. A
commercial air ionization chamber was modified to accept the liquid dielectric provides
an acceptable signal. Preliminary measurements to validate beam-quality independence
for megavoltage and superficial beams are in progress.