DOSE-RESPONSE OF VARIOUS RADIATION DETECTORS TO SYNCHROTRON-RADIATION

Accurate dosimetry is particularly difficult for low- to medium-energy x-rays as various interaction processes with different dependences on material properties determine the dose distribution in tissue and rad iation detectors. Monoenergetic x-rays from synchrotron radiation offe r the unique opportunity to study the dose response variation with pho ton energy of radiation detectors without the compounding effect of th e spectral distribution of x-rays from conventional sources. The varia tion of dose response with photon energies between 10 and 99.6 keV was studied for two TLD materials (LiF:Mg,Ti and LiF:Mg,Cu, P), MOSFET se miconductors, radiographic and radiochromic film. The dose response at synchrotron radiation energies was compared with the one for several superficial/orthovoltage radiation qualities (HVL 1.4 mm Al to 4 mm Cu ) and megavoltage photons from a medical linear accelerator. A calibra ted parallel plate ionization chamber was taken as the reference dosim eter. The variation of response with x-ray energy was modelled using a two-component model that allows determination of the energy for maxim um response as well as its magnitude. MOSFET detectors and the radiogr aphic film were found to overrespond to low-energy x-rays by up to a f actor of 7 and 12 respectively, while the radiochromic film underestim ated the dose by approximately a factor of 2 at 24 keV. The TLDs showe d a slight overresponse with LiF:Mg,Cu, P demonstrating better tissue equivalence than LiF:Mg, Tr (maximum deviation from water less than 25 %). The results of the present study demonstrate the usefulness of mon oenergetic photons for the study of the energy response of radiation d etectors. The variations in energy response observed for the MOSFET de tectors and GAF chromic film emphasize the need for a correction for i ndividual dosimeters if accurate dosimetry of low- to medium-energy x- rays is attempted.