Radiation quality discrimination by continuous and pulse ESR techniques

The biological damages produced by ionizing radiations in tissues and cells depend on the radiation quality, besides on the dose. The discrimination of the radiation quality, which is related to the linear energy transfer (LET), interests various fields such as radiobiology, astronautic space research, radiotherapy research and accidental dosimetry. In this work we have applied continuous wave ESR (cw-ESR) and pulse ESR techniques to ammonium tartrate samples with the aim of developing procedures able to discriminate radiation quality whose knowledge is fundamental for rabiobiological considerations. We have chosen the ammonium tartrate because it is a promising compound for the measurement of the absorbed ionizing radiation dose [1, 2, 3]. The compound is particularly competitive to standard alanine in the detection of ionizing radiation other than high energy gamma photons, such as low energy X photons, electrons, protons, thermal neutrons. At the same time cw-ESR and the Electron Spin Echo (ESE) decay techniques and Double Electron-Electron Resonance (DEER) can be used to obtain from average to local distributions of spins. CW-ESR is particular suited for the determination of total spin (macroscopic) concentration, whereas ESE is suited for the determination of local concentration. A new insight into the knowledge of the complex distribution of free radicals inside the dosimeters can be obtained by DEER. This technique is very useful for our purpose because it is able to measure distance between radicals in solids in the range of approximately 1.5-8 nm by analyzing the dipolar coupling between two electron spins. In this work we analyze the spatial distributions of the free radicals produced after exposure of ammonium tartrate dosimeters to various radiation beams (21 MeV protons, 60Co γ-photons, thermal neutrons). By measuring the differences between the local radical concentrations and the macroscopic one, and the distributions of radical-radical distances obtained with DEER, this study has given details on the differences between the distributions of radicals created by the radiation-matter energy transfer for the different ionizing radiations. Differences and analogies are discussed in terms of differences and analogies in the LET and type of particles involved. References [1] S. K. Olsson, S. Bagherian, E. Lund, G. A. Carlsson, A. Lund, Appl. Radiat. Isot. 1999, 50, 95565. [2] M. Brustolon, A. Zoleo and A. Lund, J. Magn. Reson., 1999, 137, 389-396. [3] A. Bartolotta, M. C. D'Oca, M. Brai, V. Caputo, V. De Caro, L. I. Giannola, Phys. Med. Biol., 2001, 46, 461-471

RADICAL DISTRIBUTIONS IN AMMONIUM TARTRATE SINGLE CRYSTALS EXPOSED TO PHOTON AND NEUTRON BEAMS

The radiation therapy carried out by means of heavy charged particles (such as carbon ions) and neutrons is rapidly becoming widespread worldwide. The success of these radiation therapies relies on the high density of energy released by these particles or by secondary particles produced after primary interaction with matter. The biological damages produced by ionising radiations in tissues and cells depend more properly on the energy released per unit pathlength, which is the linear energy transfer and which determines the radiation quality. To improve the therapy effectiveness, it is necessary to grasp the mechanisms of free radical production and distribution after irradiation with these particles when compared with the photon beams. In this work some preliminary results on the analysis of the spatial distributions of the free radicals produced after exposure of ammonium tartrate crystals to various radiation beams (Co-60 gamma photons and thermal neutrons) were reported. Electron spin resonance analyses were performed by the electron spin echo technique, which allows the determination of local spin concentrations and by double electron-electron resonance technique, which is able to measure the spatial distance distribution (range 1.5-8 nm) among pairs of radicals in solids. The results of these analyses are discussed on the basis of the different distributions of free radicals produced by the two different radiation beams used

Selective detection of EchoEPR signals due to naturally substituted 13C radicals in a single crystal of ammonium tartrate

Echo-detected electron paramagnetic resonance (echoEPR) profiles for irradiated deuterated ammonium tartrate single crystals depend strongly on the delays between pulses of the echo sequence. This is mainly due to instantaneous and spectral diffusion that plays a crucial role in determining the decay of the echo at every field position: the dephasing rate 1/T-M depends on the number of spins excited by the pulses and on the total number of interacting spins. A rigorous simulation of the echoEPR profiles at different delays requires the evaluation of the modulation pattern (ESEEM) and of the dephasing processes at every field position. From the simulations, information on the microscopic radical concentration, and on the electron-electron flip-flop rates of the single radical species can be obtained. Natural isotope C-13 substitution generates low-concentration radicals with relaxation properties different from the equivalent C-12-substitued radicals. The different behavior is discussed

RADICAL DISTRIBUTIONS IN AMMONIUM TARTRATE SINGLE CRYSTALS EXPOSED TO PHOTON AND NEUTRON BEAMS

The radiation therapy carried out by means of heavy charged particles (such as carbon ions) and neutrons is rapidly becoming widespread worldwide. The success of these radiation therapies relies on the high density of energy released by these particles or by secondary particles produced after primary interaction with matter. The biological damages produced by ionising radiations in tissues and cells depend more properly on the energy released per unit pathlength, which is the linear energy transfer and which determines the radiation quality. To improve the therapy effectiveness, it is necessary to grasp the mechanisms of free radical production and distribution after irradiation with these particles when compared with the photon beams. In this work some preliminary results on the analysis of the spatial distributions of the free radicals produced after exposure of ammonium tartrate crystals to various radiation beams (60Co gamma photons and thermal neutrons) were reported. Electron spin resonance analyses were performed by the electron spin echo technique, which allows the determination of local spin concentrations and by double electron–electron resonance technique, which is able to measure the spatial distance distribution (range 1.5–8 nm) among pairs of radicals in solids. The results of these analyses are discussed on the basis of the different distributions of free radicals produced by the two different radiation beams used

Pulsed EPR analysis of tooth enamel samples exposed to UV and gamma-radiations

The electron paramagnetic resonance (EPR) spectroscopy is widely applied for retrospective dosimetric purposes by means of quantitative detection of radicals in tooth enamel and bone samples. In this work we report a study by cw and pulsed EPR on two samples of human tooth enamel respectively irradiated by UV (254 nm) and gamma-exposed. The continuous wave (cw) EPR spectra have shown the usual presence in both samples of two types of CO_(2)^(-) radicals, with axial and orthorombic g tensors. We have obtained the electron spin echo detected EPR (ED-EPR) spectra at 80 K of the two samples, and we have shown that they are suitable to mark the difference between the effects produced by the different irradiations. At low temperature the contribution to the ED-EPR spectrum of the mobile radical with the axial g tensor is still present in the UV irradiated sample, but not in the gamma-irradiated one, where its dynamics is too slow to average the g tensor. We have moreover studied the two-pulse electron spin echo decay on varying the microwave power, a well established method for measuring the Instantaneous Diffusion. We have found that the spectral diffusion parameter is almost the same for both radiation types, whereas the Instantaneous Diffusion is significantly larger for gamma-exposed samples than for UV irradiated ones. This difference is due to a higher local microscopic concentration of free radicals for samples irradiated with gamma photons