Production of synthetic polycrystals of CaSiO3 and their study as new detectors of gamma radiation, thermal neutrons and accelerated ions.

• Autor(a) principal: Lorenzo, Carlos David Gonzales
• Data de Publicação: 2019
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/43/43134/tde-28012020-144614/
• topic: Calcium Silicate; Centros de Defeitos; Defect center; Devitrificação; Devitrification Method; Dosimetria de íons acelerados; Dosimetria de Nêutrons; Dosimetria Gama; Electron Spin Resonance; Gamma dosimetry; Ion beam dosimetry; Irradiação de prótons e íons de carbono; Neutron dosimetry; Nêutrons Térmicos; Pastilhas de CaSiO3.; Pellets of CaSiO3.; Policristais sintéticos; Polycrystal; Proton and Carbon beam irradiation; Ressonância Paramagnética Eletrônica; Silicato de Cálcio; Termoluminescência; Thermal Neutrons; Thermoluminescence

In this work, synthetic polycrystals of calcium silicate (CaSiO3) are prepared. The objective is to expand the current knowledge of low cost and easy production thermoluminescent detectors for doses of gamma radiation, thermal neutrons and accelerated ions beam, as well as, the possible applications in nuclear medicine or monitoring actual radiotherapy treatment. For that reason, calcium silicate doped with Boron, Cadmium, Dysprosium, and Europium were produced keeping in mind gamma and neutron detection. For the production of synthetic calcium silicate polycrystals, the devitrification method was used. In this process, two polytypes of undoped CaSiO3 were identified, alpha- and beta- polytypes. alpha-CaSiO3 is a very sensitive gamma radiation detector with main prominent thermoluminescence (TL) peak occurring at about 250-270 ºC and a low-temperature peak at 120 ºC, using 4 ºC/s heating rate. The TL response of alpha-CaSiO3 is linear for dose < 10 Gy and then has a supralinear behavior up to about 7 kGy and saturating beyond. The TL behavior changes very little by doping with B, Cd, and Dy while Eu doping brings changes. Additionally, beta-CaSiO3 polycrystal exhibits three TL peaks at 124, 250, and 306 ºC. All peaks at 124, 250 and 306 ºC for beta-CaSiO3 have a linear behavior in the dose range from cGy to 1 Gy. After that, the TL responses are supralinear up to about 3 kGy and then they are saturated. Synthetic polycrystals of CaSiO3 (doped or not) were also irradiated with neutrons from a nuclear research reactor. This thermal neutron reacts with Ca, Si and O through (n, gamma) process, all or part of the gamma emitted in this reaction is absorbed by the sample and is responsible for the induction of TL. The total energy emitted by the (n, gamma) reaction on the sample of CaSiO3 was calculated analytically. Furthermore, Monte Carlo simulations using the MCNP5 radiation transport code was carried out at the Nuclear Engineering Center in IPEN to calculate the deposited dose on CaSiO3 by the neutron interaction finding doses ranging from 42 Gy to 21 kGy. CaSiO3 TL glow curves, after radiation exposure from the reactor, display the main prominent TL peak around 234 - 259 ºC. Electron paramagnetic resonance (EPR) spectroscopy was used to study the defect centers induced in the polycrystals of alpha- and beta-CaSiO3 by gamma irradiation and to identify the centers responsible for the TL process. For beta-CaSiO3, three defect centers contribute to the observed spectrum at room temperature. Center I with principal g-values = 2.0135, 2.0094 and 2.0038 is attributed to O- ion and the center appears to be the recombination center for 124, 147, and 306 ºC TL peaks. Center II exhibiting an isotropic g-value of 2.00025 is identified as an F+-center. F+-center is also observed to be a recombination center for several TL peaks. Center III is assigned to a Ti3+ center displaying an orthorhombic g-tensor with principal values g1 = 1.9830, g2 = 1.9741 and g3 = 1.9046. This center is associated with 124 ºC and 147 ºC TL peaks. TL emission spectrum of beta-CaSiO3 shows two emission bands at 370 and 520 nm. In addition, pellets of CaSiO3 were irradiated at 160 MeV proton beam and at 290 MeV/n carbon ion beam from an upper synchrotron of HIMAC (Heavy Ion Medical Accelerator in Chiba, Japan) at NIRS (National Institute of Radiological Sciences). These pellets of calcium silicate presented a prominent TL peak at 120 ºC and at 270 ºC when irradiated with proton and carbon ion beam. The dose read out in Harshaw TL reader presented a good agreement with doses found using ion chamber in the case of proton beams and slightly less in the case of carbon beam due to the linear energy transference (LET).