STUDY OF THE OPTICAL ABSORPTION COEFFICIENT VARIATION IN SODA-LIME AND BOROSILICATE GLASSES DUE TO THEIR EXPOSURE TO GAMMA-RAYS

Transcription

1 STUDY OF THE OPTICAL ABSORPTION COEFFICIENT VARIATION IN SODA-LIME AND BOROSILICATE GLASSES DUE TO THEIR EXPOSURE TO GAMMA-RAYS M-R. IOAN Horia Hulubei National Institute of Physics and Nuclear Engineering IFIN HH, 30 Reactorului Str., Magurele Bucharest, Romania, Received September 7, 2016 Abstract. In this paper, the relative variation of the optical absorption for two types of glasses (soda-lime and borosilicate), due to their exposure to gamma-rays is studied. The optical absorptions were determined by measuring the transmission optical powers of a 633 nm continuous wave He-Ne laser beam passing thru the gamma-rays exposed glass samples. The optical powers were measured by using of a UV-VIS type power-meter. The relative optical absorption variations for the two types of glass, when were exposed to gamma-rays, were different. For the soda-lime type glass, the optical absorption variation was of about 2.4 % and for the borosilicate one was of about 16 %. The relative variations of the transmitted optical powers thru the glass samples as a function of the absorbed gamma-rays doses of 68.7 % in the first case, respectively 80.6 % in the second one, contributed to the results. Moreover, the analytical relation between the optical absorption coefficient variation and the magnitude of the absorbed doses which produced it, was determined. This method may be also used to determine the magnitude and the rate of change for the glass browning phenomenon induced by the gamma-rays to exposed optical transmission materials, such are the lens and the optical transmission windows operating in gamma rays environments, but also the glass materials used as dosimeters for estimating the gamma-rays absorbed doses. Key words: optical absorption coefficient, soda-lime glass, borosilicate glass, He-Ne laser, Beer-Lambert law. 1. INTRODUCTION Generally, for all types of glass materials after their exposure to ionizing gamma-rays (from a Co-60 source in this case), some changes, in a lower or higher manner, of their physical properties are occurring. The optical transmission is one of these changed physical properties. The main cause of it is the occurrence of the glass browning phenomenon by the activation of the color centers existing in the glass material or occurred during the exposure to gamma-rays fields. During the irradiation, these defects suffer agglomeration phenomena, leading to a decrease of the optical transmission. Romanian Journal of Physics 62, 206 (2017)

2 Article no. 206 M-R. Ioan 2 After a certain dose level, the glass browning magnitude and also the optical transmission as a function of the absorbed dose is no longer linear, the glass reaching its saturation limit. When operating in gamma-rays environments, instruments containing optical glass materials are starting losing their performances. In such instruments, the glass components are usually meant to serve as transmission media for the optical signal. Due to their exposure to ionizing radiation fields, the glass parts are starting to lose their transmission capabilities due to the increased optical absorption but also to the increased scattering phenomena of the incident signal. This means that the transmitted signal is weaker and the noise is increased, leading to higher uncertainties associated to the measurements performed with the instrument [1 7]. 2. THE EXPERIMENT To highlight the magnitude of the glass browning phenomenon caused by the exposure to gamma-rays, the concept of induced optical absorption, associated to glass samples having different up-taken doses (D 0 = 0 kgy; D 1 = = 29.5 kgy; D 2 = 31.4 kgy; D 3 = 33.5 kgy; D 4 = 35.6 kgy; D 5 = 37.9 kgy), was used. This task was fulfilled by measuring the optical transmission powers of a 633 nm He-Ne laser before and after passing the beam thru each gamma-ray exposed glass samples. The measurements were made by using an experimental set-up made of a 25-LHP type He-Ne laser (Melles Griot) and a PowerMax-USB UV-VIS type power-meter (Coherent), as it can be seen in Figure 1. The exposures of the glass samples to gamma-rays were performed at the Horia Hulubei National Institute of Physics and Nuclear Engineering (IFIN-HH), using a Co-60 source. The source provided a dose debit of 0.4 kgy/h and mean gamma-rays energy of 1.25 MeV. As it is known, the Lambert-Beer Law, in transmitted optical power terms, is expressed as: P P exp( α x), (1) tr. 0 where: P 0 is the optical power transmitted thru air; P tr. is the optical power transmitted. By analyzing the relation (1), it is seen the fact that small variations of the samples thicknesses x or of the optical absorption coefficients α, can lead to strong variations of the measured optical powers. Because the variation of the optical absorption coefficient due to the exposure of glass samples to ionizing gamma-rays was the main aspect of this study, the samples thickness was kept as a constant by using the same glass sample for all successive absorbed doses. This way, an application used to determine the variation of the optical absorption coefficient of a soda-lime type glass when it is exposed successively to different gamma-rays absorbed doses was shown. The experimental set-up is presented in the Figure 1.

3 3 Optical absorption coefficient variation due to exposure to gamma-rays Article no. 206 Fig. 1 The experimental set-up. Further, the analytical algorithm providing the relation between the optical absorption coefficient and the magnitude of the absorbed dose is presented. By moving the laser beam on the sample surface in different i points, the transmitted optical powers are measured in each case. As a general case, a number of i measuring points corresponding to different absorbed dose values were performed, for each one obtaining (P i ) transmitted optical powers, where i = 1,n is the number of points measured on the same irradiated glass sample (i.e., n = 10), = 0,m represents the number of the successive irradiation of the samples (i.e., m = 5). For each case, the mean transmitted optical power is calculated according to Relation (2): 1 P = P (2) i n i 1 and the associated standard deviation expressed as variance (3): n 2 n 1 var( P ) P P. (3) i n 1 i 1 From the Relation (1), the expression of the mean transmitted optical power, (P ), will have the form (4): where: P 0 is the optical power transmitted thru air; α is the mean optical absorption coefficient; x is the sample thickness. P P0 exp( α x), (4) For each set of measurements, an optical absorption coefficient deviated from the mean value α with the uncertainty of var(α) is obtained. This way, the Relation (4) can be rewritten as (5): ' P = P exp α var α 1 var(α ) 0 + x = P x (5)

4 Article no. 206 M-R. Ioan 4 considering: var(α ) x 1. Taking into account the Relation (5), the dispersion of each measured transmitted optical power from the mean value is given by the Relation (6): (Δ P) ( P' ) P P var(α ) x. (6) By rewriting the Relation (6), the Relation (7) is obtained: 1 (Δ ) var(α ) P. x P (7) From the Relations (4) and (7), the expression of the variation of the optical absorption coefficient it is obtained (8), expressed in percent (%): var(α ) (Δ P) 1 (%) 100. (8) α P P 0 ln P The Relation (8) is used to determine the analytical relation between the absorbed dose and the variation of the optical absorption coefficient, by fitting the experimental data. Its general form is linear (9): where: a, b fitting parameters; D the absorbed dose. var(α ) (%) a +b D, (9) α In this paper, two types of glass samples [8] were exposed to different gamma-rays fields. One of them was soda-lime type, having a circular shape, with a 100 mm diameter and a thickness of 10 mm and having the following chemical composition: (%) wt. SiO 2 Fe 2 O 3 CaO Na 2 O K 2 O Al 2 O 3 MgO TiO and the other one was BK-7 borosilicate type, having the same geometry as the first one, and the following chemical composition: wt. (%) SiO 2 B 2 O 3 BaO Na 2 O K 2 O As 2 O

5 5 Optical absorption coefficient variation due to exposure to gamma-rays Article no. 206 After the exposures to the Co-60 source, the optical glass samples showed an instable thermodynamic character, meaning the fact that the defects induced by the ionizing radiation into the glass structure can be reverted by using of a controlled heat treatment, which provides a supplementary energy to the system [9 10]. 3. RESULTS AND DISCUSSIONS The most important method of testing the optical transmission performance of an optical glass is to measure its power output. Output power directly affects laser s ability to perform a certain process after passing through an irradiated glass. These types of measurement systems are composed by a sensor that is placed into the laser beam after passing through samples and provides an output signal proportional to the initial one of laser beam and a meter attached to it, which is an analyzer that will display and interpret the signal detected by the sensor. Figure 2 shows the measurement of laser s output power after passing through the irradiated glass samples. Fig. 2 The variation of the laser s output power after passing through the irradiated glasses as a function of the absorbed dose. In Fig. 2, the nonlinear transmitted optical power variation as a function of the absorbed doses, for the two types of glass, can be seen. An increased absorption of the laser beam for the BK-7 type glass than the one for the case of the soda-lime can be also seen. This aspect leads to a relative variation of the transmitted optical power of 68.7 % for the soda-lime glass and respectively 80.3 % in the case of

6 Article no. 206 M-R. Ioan 6 BK-7. From the relations showing the dependence of transmitted optical powers to the absorbed doses, the inverse relations, meaning the variation of the absorbed doses as a function of the transmitted powers. These variations are presented in the Fig. 3, and serve as a calibration system for the two types of glass (10 mm thickness each one). Fig. 3 The absorbed dose as a function of the transmitted optical power for the soda-lime, respectively BK-7 glass types. This way, a linear relation between the absorbed doses and the variation of the transmitted optical power for each glass type was obtained. These linear relations show the possibility of using this glass types as instruments for gammarays doses estimations, on a range between 0 and 40 kgy, by measuring the optical powers transmitted thru other samples of the same types, after their exposure in ionizing gamma-rays fields. For obtaining the values of the absorbed doses, the calibration linear relations from Fig. 3 are used. It these types of determinations, it is very important to use glass samples from the same manufacturer and for the same batch as in the case of the ones used for calibration, to avoid the differences between individual recipes of each producer, which can be slightly different between them and also to avoid differences occurring in different producing batches. Otherwise, these differences may influence the precision of the determinations and lead to higher associated uncertainties. It is also important to double-check the results with classical methods such are the ethanol-chlorinebenzene based absorbed doses detectors (about 2.5 % uncertainty for k = 2) to take into account the influences of the compositional and thicknesses dispersions of the used samples on the uncertainties budget [5]. Further, using the obtained results, the analytical relation describing the dependence of the absorbed doses as a function of the optical absorption coefficients, is represented, as it can be seen in Figure 4.

7 7 Optical absorption coefficient variation due to exposure to gamma-rays Article no. 206 Fig. 4 The relative variation of the optical absorption coefficient as a function of the absorbed doses. In Fig. 4, the relative variations of the optical absorption coefficient as a function of the absorbed doses, for both glass type cases, are presented. By the analyzing the results, a variation of the optical absorption coefficient of about 2.4 % for the soda-lime glass sample and respectively 16 % for the BK-7 one are obtained. These results show a faster reaching speed of the saturation limit for the BK-7 glass comparing to the soda-lime one, meaning also a larger operating interval for the second one. The variation of the optical absorption coefficient shows also a higher rate of change in the case of the BK-7 glass type, as it can be seen from the slopes of the fitting equation (0.14 for BK-7 vs. 4.9*10-4 for sodalime). 4. CONCLUSIONS Two types of optical glasses were exposed to different gamma-rays ionizing fields to check their optical parameters resistance to ionizing radiation when operate as transmission optical windows in the reaction chambers of the particles accelerators for monitoring the reaction products, in the hot cells of the nuclear facilities and so on. Their resistance is limited and must be very well determined to find if there are suitable for a certain activity or not and to find the best solutions. In this paper, by characterizing two types of glass before and after their exposure to gamma-rays fields, it was show that the exposure to ionizing radiation strongly affects the optical properties of the different types of glass materials, but in a

8 Article no. 206 M-R. Ioan 8 different manner for each individual type. In the case of the soda-lime glass, a 68.7 % relative variation of the transmitted power was obtained. For the BK-7 glass, the determined relative variation was of about 80.3 %. A variation of the optical transmission for the soda-lime of approximatively 2.4 % glass was obtained. In the BK-7 glass case, this variation was of about 16 %. These values show a faster optical saturation in the case of the BK-7 glass comparing to the soda-lime one. Moreover, the speeds of change in the variation of the optical absorption coefficients as a function of the absorbed doses are different for each case (faster in the BK-7 case). This aspect can be seen from the values of slopes of the two fitting equations from Figure 4. These differences between the two types of glass are given by the different damage magnitudes for each case when exposed to the same gamma-rays field. Since the irradiation conditions were the same in both cases, the different damage for each case must be due to the intrinsic differences between the two types of glass, to the impurities content from the manufacturing processes (wanted or not), to the compositional stoichiometric content of each type and also due to the processing of the samples surfaces before the measurements. REFERENCES 1. A. Gusarov, D. Doyle, L. Glebov, F. Berghamans, Comparison of radiation-induced transmission degradation of borosilicate crown optical glass from four different manufactures, Photonics for Space Environments X, Proceeding of SPIE 5897 (2005). 2. W. Xu, Y. Zhang, G. Cheng, W. Jian, P.C. Millett, C.C. Koch, S.N. Mathaudhu, Y. Zhu, In-situ atomic-scale observation of irradiation-induced void formation, Nature Communications 4, 1 5, doi: /ncomms3288 (2013). 3. L.F. Santos, M.A. Stefani, Influence of Ionizing Radiation on Optical Glasses for Space Applications, ENFNC, Annals of Optics, S.P. Brazil (2006). 4. S. Baccaro, A. Piegari, I. Di Sarcina, A. Cecilia, Effect of gamma irradiation on optical components, IEEE Transactions on Nuclear Science 52 (N5), , doi: /tns (2005). 5. M-R. Ioan, Amorphous and crystalline optical materials used as instruments for high gamma radiation doses estimations, Nucl. Instrum. Meth. B 377, 43 49, doi: /.nimb (2016). 6. M-R. Ioan, Investigation of RGB spectral components in the images captured through gamma rays affected optical focusing lens, Rom. J. Phys. 61, (2016). 7. M-R. Ioan, LIDT test coupled with gamma radiation degraded optics, Opt. Commun. 369, 94 99, doi: /.optcom (2016). 8. SCHOTT Technical Information, TIE-42: Radiation resistant optical glasses (2007), 9. M-R. Ioan, Analyzing of the radiation induced damage to optical glasses by using online heating laser measurements, Rom. J. Phys. 61, (2016). 10. M-R. Ioan, Study of the optical materials degradation caused by gamma radiation and the recovery process by controlled heat treatment, Rom. J. Phys. 61, (2016).