Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin TL in green tourmaline: Study of the centers responsible for the TL emission by EPR analysis Nilo F. Cano a, ⁎ , T.K. Gundu Rao b , Jorge S. Ayala-Arenas c, ⁎ , Carlos D. Gonzales-Lorenzo b , Letícia M. Oliveira d , Shigueo Watanabe b, ⁎ a Instituto do Mar, Universidade Federal de São Paulo, Rua Doutor Carvalho de Mendonça, 144, CEP 11070-100 Santos, SP, Brazil b Instituto de Física, Universidade de São Paulo, Rua do Matão, Travessa R, 187, CEP 05508-090 São Paulo, SP, Brazil c Escuela Profesional de Física, Facultad de Ciencias Naturales y Formales, Universidad Nacional de San Agustín (UNSA), Av. Independencia S/N, Arequipa, Peru d Instituto de Pesquisas Energéticas e Nucleares, IPEN-CNEN/SP, Av. Prof. Lineu Prestes, 2242, Cidade Universitária, 05508-000 São Paulo, SP, Brazil ARTICLE INFO Keywords: TL EPR Radiation dosimetry Silicate Tourmaline ABSTRACT Electron paramagnetic resonance (EPR) studies have been carried out to identify the defect centers responsible for the thermoluminescence (TL) peaks in the mineral tourmaline. The mineral exhibits three TL peaks ap- proximately at 170, 250 and 310 °C. The EPR spectrum of the green tourmaline sample pre-heated to 500 °C presented a large signal around g = 4.3 due to Fe 3+ ion. Room temperature EPR spectrum of irradiated green tourmaline shows the formation of two defect centers in the region of g = 2.0. One of the centers (center II) with a g factor equal to 1.96 is identified as an F + -center and is related to the observed high temperature 250 and 310 °C TL peaks. Center I exhibiting a doublet is due to hydrogen atoms (H 0 ), stable in the crystal lattice at room temperature and this center correlates with the TL peak at 170 °C of the green tourmaline. An optical absorption measurement also was carried out. Bands at around 430, 730 and 1100 nm have been observed. 1. Introduction The silicate minerals are, in general, excellent thermoluminescent materials, some of them with high sensitivity to low as well as high radiation doses [1–7]. Hence, they are candidates for radiation dosi- metry. There is one group of silicates called ring silicates or cyclosilicate to which belong beryl, cordierite and tourmaline. The tourmaline struc- ture is typically rhombohedral with space-group R3m [8–10], although some studies report lower symmetry such as orthorhombic, monoclinic or triclinic [11–13]. The structure is characterized by groups of XO 9 , YO 6 , TO 4 , and BO 3 polyhedra connected to each other through ZO 6 octahedra. The latter are arranged in a 3-D framework and are linked to the YO 6 octahedron through the O3–O6 edge. The tourmaline has a complex formula, XY 3 Z 6 (T 6 O 18 )(BO 3 ) 3 V 3 W, where X, Y and Z sites can be occupied by different ions [8–10]. Therefore, about 12 varieties of tourmaline are formed in nature. According to several authors [14–17], the following ions fit into the following structural sites: X = Na, Ca, ο (= vacancy), K; Y = Al, Fe 3+ , Cr 3+ ,V 3+ , Mg, Fe 2+ , Mn 2+ , Cu 2+ , Zn, Li, Ti 4+ , ο; Z = Al, Fe 3+ , Cr 3+ ,V 3+ , Mg, Fe 2+ ; T = Si, Al, B, Be; B = B, (ο); W(O1) = OH, F, O; V(O3) = OH, O. Tourmaline is a well known silicate mineral because some of its varieties have high gemological value [18,19]. The tourmaline crystal has been widely investigated by many au- thors through spectroscopic methods such as Mössbauer spectroscopy, UV–Vis spectroscopy, Raman spectroscopy and other spectroscopic techniques due to its color and gemological value [20–36]. Several authors [37–42] measured the effects of thermal treatments and irradiation on optical absorption spectra of natural tourmaline of different color and discussed the crystal field effect on the energy levels of transition ions, mostly Mn 2+ , Mn 3+ , Fe 2+ , Fe 3+ and Ti 4+ , usually responsible for the coloration of the crystal. In contrast, studies on lu- minescence properties and the identification of the defects responsible for emission of the TL peaks of the tourmalines crystal so far are few. The process of irradiation and thermal treatment can change some physics properties of the mineral that are dependent on point defects, such as luminescence and electron paramagnetic resonance. These properties make tourmaline crystal an interesting material for some applications like dosimetry. However, although it has been a subject of some experimental studies, an investigation of defect centers created by ionizing radiation responsible for TL properties of tourmaline is still lacking. The identification and characterization of these centers form an essential step in understanding the mechanisms of TL emission. In this context, EPR provides a convenient and sensitive technique for such a https://doi.org/10.1016/j.jlumin.2018.09.034 Received 2 May 2018; Received in revised form 13 September 2018; Accepted 15 September 2018 ⁎ Corresponding authors. E-mail addresses: nilocano@if.usp.br, nilo.cano@unifesp.br (N.F. Cano), jayala@unsa.edu.pe (J.S. Ayala-Arenas), watanabe@if.usp.br (S. Watanabe). Journal of Luminescence 205 (2019) 324–328 Available online 15 September 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved. T