Maria Cristina D'Oca

Sensitivity of Alanine Dosimeters with Gadolinium Exposed to 6 MV Photons at Clinical Doses

In this study we analyzed the ESR signal of alanine dosimeters with gadolinium exposed to 6 MV linear accelerator photons. We observed that the addition of gadolinium brings about an improvement in the sensitivity to photons because of its high atomic number. The experimental data indicated that the addition of gadolinium increases the sensitivity of the alanine to 6 MV photons. This enhancement was better observed at high gadolinium concentrations for which the tissue equivalence is heavily reduced. However, information about the irradiation setup and of the radiation beam features allows one to correct for this difference. Monte Carlo simulations were carried out to obtain information on the expected effect of the addition of gadolinium on the dose absorbed by the alanine molecules inside the pellets. These results are compared with the experimental values, and the agreement is discussed.

Improvement of ESR dosimetry for thermal neutron beams through the addition of gadolinium

In this paper, the addition of gadolinium is proposed as a useful tool to enhance the electron spin resonance (ESR) sensitivity of organic compounds to thermal neutrons. The target of this work is the detection, through the ESR technique, of the thermal neutron fluence in a mixed field of photons and neutrons. Gadolinium was chosen because it has a very high capture cross section to thermal neutrons; its nuclear reaction with thermal neutrons induces complex inner shell transitions that generate, besides other particles, Auger electrons, which in turn release their energy in the neighborhood (only several nanometers) of the place of reaction. Gadolinium was added to two organic molecules: alanine and ammonium tartrate. The main result obtained was a greater neutron sensitivity for dosimeters with gadolinium than for those without gadolinium for both organic molecules used. Since a dosimeter pair is required to discriminate between the two components of a mixed field, we studied the response of each dosimeter pair irradiated in a mixed field. Through a blind test we verified the usefulness of this dosimetric system and we obtained an estimate of the fluence in the mixed field with a relative uncertainty of 3%, when the pair composed of an alanine dosimeter and a dosimeter with alanine and gadolinium is used.

Response characterization of ammonium tartrate solid state pellets for ESR dosimetry with radiotherapeutic photon and electron beams

Solid state pellets (1 mm thick) for electron spin resonance (ESR) dosimetry were made using ammonium tartrate as the radiation-sensitive substance. Their behaviour was experimentally investigated as a function of dose with 60Co gamma rays. The calibration function obtained permits measurements of absorbed dose in the 2-50 Gy range, with a combined uncertainty of +/-4%. The lowest detectable dose was about 0.5 Gy. These properties are comparable with or even better than those of ESR dosimeters made from other materials. The time stability of the ESR signal of ammonium tartrate dosimeters at different storage conditions after irradiation was studied. A rather complex behaviour was observed, which suggests that more species of free radicals are produced by radiation and that migration processes may be effective. No dependence of the response on beam quality was found for high-energy photon and electron beams produced by a linear accelerator used in radiotherapy, whereas dose was underestimated with low-energy x-rays.

Investigation on dose response and fading behavior of ammonium tartrate-polyethylene solid state ESR dosimeters

The results of the experimental investigation presented in this paper show that the solid state ESR dosimeters based on ammonium tartrate realized at the University of Palermo have dose response characteristics that render them suitable for application in /sup 60/Co radiotherapy, for doses higher than about 1 Gy. The main drawback up to now is a certain instability of the ESR signal, even at laboratory conditions, that calls for further investigation. Other studies are underway to investigate the dependence of the response of these dosimeters on photon energy and the response to charged particle beams. The overall accuracy in the evaluation of dose is estimated to be between 4.5 % and 7 %, depending on dose range, provided measurements are carried out always one day after irradiation to avoid any fading contribution to the uncertainty, and more than one dosimeter is used, especially in the low dose range.