In the nuclear energy industry, in many situations, the skin is subjected to low energy beta radiation. Therefore, the dose may be limited [1[1]. LAKSHMANAN A. Development and application of solid forms of CaSO4:Dythermoluminescentdosemeters in radiation protection dosimetry-A review. Radiat. Prot. Dosim. 2018; 181(2):57-99.]. The extrapolation chamber is the primary instrument for measurements in beta radiation beams [2[2]. BÖHM J. The national primary standard of the PTB for realizing the unit of the absorbed dose rate to tissue for beta radiation. Braunschweig, Germany: Physikalisch-TechnischeBundesanstalt, PTB-Dos-13. 1986.]. These measurements should be taken under laboratory reference conditions of temperature, pressure and humidity. In addition, the extrapolation chamber must be handled with extreme care, because it is very heavy and its entrance window is very delicate. Thermoluminescence may be an alternative method for dosimetry to constitute a beta-radiation transfer system.
Thermoluminescence has already been used in applications of beta radiation [1[1]. LAKSHMANAN A. Development and application of solid forms of CaSO4:Dythermoluminescentdosemeters in radiation protection dosimetry-A review. Radiat. Prot. Dosim. 2018; 181(2):57-99.-6[6]. KUMAR M, RAKESH RB, SNEHA C, RATNA P,et.al. Beta response of CaSO4:Dy based thermoluminescent dosimeter badge and its angular dependence studies for personnel monitoring applications. Radiat. Prot. Environ. 2016; 39: 132-137.]. This kind of radiation has a low penetration power; therefore, the choice of the dosimeter material is very important [1[1]. LAKSHMANAN A. Development and application of solid forms of CaSO4:Dythermoluminescentdosemeters in radiation protection dosimetry-A review. Radiat. Prot. Dosim. 2018; 181(2):57-99., 7[7]. INTERNATIONAL ATOMIC ENERGY AGENCY(IAEA). Calibration of Photon and Beta Ray Sources used in Brachytherapy. IAEA-TECDOC1274.Vienna: IAEA, 2002.-10[10]. ANTONIO PL, OLIVEIRA ML, CALDAS LVE. Thin CaSO4:Dythermoluminescent dosimeters for calibration of 90Sr+90Y applicators.Appl. Radiat. Isot. 2012; 70: 790-793.]. The dosimeter response depends on the energy range of the beta radiation. However, dosimeters intended to measure doses of this radiation should be as thin as possible compared to the ranges of all the electron energies of interest [6[6]. KUMAR M, RAKESH RB, SNEHA C, RATNA P,et.al. Beta response of CaSO4:Dy based thermoluminescent dosimeter badge and its angular dependence studies for personnel monitoring applications. Radiat. Prot. Environ. 2016; 39: 132-137., 11[11]. KUMAR M, DHABEKAR B, MENON S, BAKSHI S,et.al. Beta response of LiMgPO4:Tb,B based OSL discs for personnel monitoring applications.Radiat. Prot. Dosim. 2013:155(4): 410-417.].
For the calibration of beta radiation fields, thin thermoluminescent (TL) dosimeters of materials with low atomic number, such as LiF, Li2B4O3, Mg2B4O7, Al2O3, among others, may be used [12[12]. International Commissionon Radiation Units and Measurements(ICRU).Dosimetry of External Beta Rays for Radiation Protection.ICRU Report 56. Bethesda: ICRU, 1997.]. Some types of fine detectors have been prepared with TL high-sensitivity phosphors such as Mg2B4O7:Dy, CaSO4:Dy, Al2O3:C and LiF:Mg,Cu,P. For beta radiation, the high sensitivity of the phosphor is very important. In the case of low energy beta radiation, for example 204Tl (Emax = 0.77 MeV), the TL sensitivity decreases with increasing dosemeter thickness much faster as compared to that of high-energy beta rays, for example 32P (Emax = 1.71 MeV) [1[1]. LAKSHMANAN A. Development and application of solid forms of CaSO4:Dythermoluminescentdosemeters in radiation protection dosimetry-A review. Radiat. Prot. Dosim. 2018; 181(2):57-99.].
CaSO4 doped with Rare Earths (RE) has been widely studied in some works as a TL material [1[1]. LAKSHMANAN A. Development and application of solid forms of CaSO4:Dythermoluminescentdosemeters in radiation protection dosimetry-A review. Radiat. Prot. Dosim. 2018; 181(2):57-99.,13[13]. YAMASHITA T, NADA N, ONISHI H, KITAMURA S. Calcium sulphate phosphor activated by thulium or dysprosium for thermoluminescencedosimetry.Health Phys. 1971; 21: 295-300.-14[14]. KÁSAI, CHOBOLAR, MELLP, SZAKÁCSS et al.Preparation and investigation of thermoluminescence properties of CaSO4:Tm,Cu.Radiat. Prot. Dosim. 2007; 123: 32-35.]. Calcium sulfate doped with dysprosium (CaSO4:Dy) is a material already well studied in beta radiation beams [1[1]. LAKSHMANAN A. Development and application of solid forms of CaSO4:Dythermoluminescentdosemeters in radiation protection dosimetry-A review. Radiat. Prot. Dosim. 2018; 181(2):57-99.,4[4]. CONTENO G, MALISAN MR, PADOVANI R. Response of thermoluminescence dosimeters to beta radiation and skin dose measurement. Phys. Med. Biol. 1984;29: 661-678., 6[6]. KUMAR M, RAKESH RB, SNEHA C, RATNA P,et.al. Beta response of CaSO4:Dy based thermoluminescent dosimeter badge and its angular dependence studies for personnel monitoring applications. Radiat. Prot. Environ. 2016; 39: 132-137., 10[10]. ANTONIO PL, OLIVEIRA ML, CALDAS LVE. Thin CaSO4:Dythermoluminescent dosimeters for calibration of 90Sr+90Y applicators.Appl. Radiat. Isot. 2012; 70: 790-793., 15[15]. CAMPOS LL, LIMA MF. Thermoluminescent CaSO4:Dyteflon pellets for beta radiation detection.Radiat. Prot. Dosim. 1987; 18 (2): 95-97.]. Calcium sulfate doped with europium (CaSO4:Eu) presents a suitable TL response [16[16]. DENG W, GOLDYS EM. Plasmonic approach to enhanced fluorescence for applications in biotechnology and the life sciences.Langmuir 2012; 28 (27):10152-10163.]. The addition of silver to CaSO4:Eu allows the increase of the TL intensity [17[17] JUNOT DO, COUTO DOS SANTOS M A, SOUZA DN, VASCONCELOS et. al. Silver addition in CaSO4:Eu, TL and TSEE properties.Radiat. Meas. 2011; 46: 1500-1502.-18[18]. JUNOT DO, COUTO DOS SANTOS MA, ANTONIO PL, CALDAS LVE, et.al.Feasibilitystudy of CaSO4:Eu, CaSO4:Eu,Ag and CaSO4:Eu,Ag(NP) as thermoluminescent dosimeters.Radiat. Meas. 2014; 71: 99-103.].
The objective of the present work was to perform a TL response analysis of CaSO4:Eu,Ag detectors in 90Sr/90Y beams, for the establishment of a transfer system or alternative/complementary method for beta radiation dosimetry.
For the dosimetric characterization of the detectors, the 90Sr/90Y source of the Beta Secondary Standard BSS2 of the Calibration Laboratory (LCI) of the IPEN/CNEN in Brazil was used. The main characteristics of this source are the following: 460 MBq of nominal activity; 0.8 MeV of average beta energy and 10,483 days of half-life. The calibration date was 11/19/2004. The calibration distance was11 cm without the use of the beamflattening filter [19[19]. BSS2. Beta secondary standard 2. Operation manual. Isotrak. QSA Global GmbH. 2005.].
The dosimetric system consists of the detectors, the TL reader, the thermal treatment system and auxiliary materials that allow performing the luminescent dosimetry.
The CaSO4:Eu:Ag detectors were produced by stages in the Laboratory of Medical Physics (LFM) of the Department of Physics (DFI) of the Federal University of Sergipe (UFS) and in the Laboratory of Dosimetric Materials (LMD) of the Radiation Metrology Center (CMR) of IPEN. The crystals of CaSO4:Eu:Ag were produced by a route based on the mixture of Calcium Carbonate (CaCO3), Sulfuric Acid (H2SO4), Europium Oxide (Eu2O3) and silver metal particles Ag0. The dopants were incorporated in the proportion of 0.1 mol %. For the production of the pellets, powdered Polytetrafluoroethylene (Teflon) was added in the proportion of 1:1 for the mass of the phosphor and the mass of Teflon. The detectors have 6 mm in diameter, 1 mm in thickness and 40 mg in mass [17[17] JUNOT DO, COUTO DOS SANTOS M A, SOUZA DN, VASCONCELOS et. al. Silver addition in CaSO4:Eu, TL and TSEE properties.Radiat. Meas. 2011; 46: 1500-1502.].
For the TL measurements, the RISÖ TL/OSL-DA20 system was used. The system allows up to 48 samples to be individually heated at any temperature up to 700 °C. The measurements were performed in a vacuum chamber. The emitted luminescence was measured by a light detection system, composed of a photomultiplier valve and suitable detection filters. TL measurements were performed under N2 atmosphere. The luminescence was detected by a bialkali EMI 9235QB photomultiplier tube (PMT) which has a maximum detection efficiency between 200 nm and 400 nm. The 7 mm band pass filter Hoya U-340 (transmission band 250-390 nm Full width at half maximum (FWHM)) was utilized [20[20]. DTU Nutech. Guide to “The Risö TL/OSL Reader”. Denmark: DTU Nutech. 2015.]. The heating rate was 10°C/s, and the maximum temperature was 350°C. Figure 1 shows the RISÖ TL/OSL-DA20 measuring system of LCI/IPEN.
The thermal treatment was performed in a furnace manufactured by the Institute of Radioprotection and Dosimetry (IRD), CNEN, in Rio de Janeiro. The samples were thermally treated at 400 °C for 1h under air atmosphere.
Figure 2 shows the holder for irradiation of the detectors and the support for performing the thermal treatment. The holder for the irradiation is made of polymethylmethacrylate (PMMA), and it has the following dimensions: 110 mm in width and length, and 18 mm in depth. It allows the irradiation of 25 detectors. The holder cover is a 0.015 mm Hostaphan sheet. The support for the heat treatment is a circular aluminum plate.
For the evaluation of the response reproducibility of the detectors, thirty detectors were used. They were irradiated with a dose of 1 Gy of the RISÖ system 90Sr/90Y source. The TL response was considered as the integral under the whole glow curve. After irradiation, the TL reading, the thermal treatment and the reading of the background were performed. This procedure was repeated for 5 cycles. Twelve detectors of this batch were chosen for the study of TL response. Table 1 shows the mean values of the TL response, the standard deviation and the coefficient of variation (C.V.) of the chosen detectors.
In order to consider that the detectors have a good response reproducibility, the recommended coefficient of variation should be less than 5% [21[21]. FURETTA C. Questions and answers on thermoluminescence(TL) and optically stimulated luminescence (OSL). London: World Scientific Publishing, 2008.]. The results of Table 1 are in agreement with these recommendations.
Figure 3 shows the TL glow curves of CaSO4:Eu:Ag detectors. Detector 1 was chosen arbitrarily for the representation. The dose range considered was 30 mGy up to 500 mGy.
To obtain the TL response curves as a function of the absorbed dose of the CaSO4:Eu:Ag detectors, they were irradiated in the BSS2 90Sr/90Y radiation beam with doses of 30 mGy up to 500 mGy. The dose range for the study was established, considering the irradiation times of the BSS2 system [19[19]. BSS2. Beta secondary standard 2. Operation manual. Isotrak. QSA Global GmbH. 2005.]. Figure 4 shows the dose-response curve of the CaSO4:Eu:Ag detectors in the range of the aforementioned doses.
The curve was fitted linearly by a computational program, and the R2 correlation coefficient was 0.99828. The detectors showed a linear behavior in the tested dose range. The calibration factor was determined by linear fitting, and it was determined by Equation 1:
where c is the slope of the fitted line.
The calibration factor can also be determined as the ratio between the absorbed dose and the mean value of the TL measurements at each point of the calibration curve [21[21]. FURETTA C. Questions and answers on thermoluminescence(TL) and optically stimulated luminescence (OSL). London: World Scientific Publishing, 2008.].
The determined calibration factor was (1.166 ± 0.024) x10-3mGy/a.u. and the factor determined by linear fit was (1.120 ± 0.014) x10-3mGy/a.u. The calibration factors obtained by both methods are in good agreement. The difference between them was only 3.9%.
The lower detection limit (LDL) is the minimum dose that can be detected by luminescent material [20[20]. DTU Nutech. Guide to “The Risö TL/OSL Reader”. Denmark: DTU Nutech. 2015.]. The LDL is important in low dose measurements where the dosimeter signal is almost equal to the background signal. The LDL was determined by Equation 2:
where σBKG is the standard deviation of the zero dose reading [21[21]. FURETTA C. Questions and answers on thermoluminescence(TL) and optically stimulated luminescence (OSL). London: World Scientific Publishing, 2008.].
Table 2 shows the values of the material TL sensitivity for each detector. For this test, a dose of 50 mGy was chosen.
The detectors show an appropriate sensitivity for 90Sr/90Y beta radiation, suggesting a potential use of these detectors for beta dosimetry with also the other BSS2 radiation sources (85Kr and 147Pm).
The analysis of the TL response of the CaSO4:Eu:Ag detectors in 90Sr/90Y radiation beams was performed. The TL glow curves for doses between 30 mGy and 500 mGy were obtained. The reproducibility of the response, the calibration factor, the lower limit of detection and the intrinsic sensitivity of the detectors were determined.
The preliminary results of the tests carried out show suitable dosimetric characteristics for the establishment of a transfer system or alternative/ complementary method for dosimetry of beta radiation.