CHANGE OF OPTICAL PARAMETERS OF THE BK-7 ... - IFIN-HH

CHANGE OF OPTICAL PARAMETERS OF THE BK-7 GLASS SUBSTRATE UNDER GAMMA IRRADIATION M-R. IOAN1, I. GRUIA2,* 1

Horia Hulubei National Institute for Nuclear Physics and Engineering, P.O. Box MG-6, RO-077125 Bucharest-Magurele, Romania 2 Faculty of Physics, University of Bucharest, Romania * Corresponding author: [email protected] Received April 12, 2015 Optical components (lens, mirrors, filters, prisms, etc.) can be exposed to energetic photons such are gamma-rays when used in either outer space or nuclear applications. The study of the behavior is important to avoid failure of these samples, during operation. The optical spectral reflectance and transmittance for multi-layer coating substrates laser mirrors, exposed to ten 60Co gamma-rays doses ranging between (0.04÷21) kGy, have been investigated. The optical parameters of samples were measured, before and after gamma irradiation, to highlight the changes and the influence of gamma-absorption on spectral measurements. The ten samples have been investigated via spectral reflectance and transmittance, for the wavelengths range (400–800) nm. Some variations of optical properties have been detected in BK-7 glasses (substrate) after irradiation. Key words: reflectivity; laser mirror; gamma-rays.

1. INTRODUCTION

In several applications for low-orbit space and earth applications, laser dielectric mirrors are exposed to the influence of energetic photons. Gamma-rays resulting from the interaction of cosmic rays with the material of an orbiting satellite or an orbiting space station at an altitude of some few hundreds of kilometers, below the level of the radiations belt, have been calculated as a function of the geomagnetic latitude and solar activity level [1]. The total absorbed doses uptaken by an unshielded spaceman, satellites or space stations have also been calculated (1÷10) kGy/yr [2]. An induced change of the optical parameters of the substrates (absorption coefficient, refractive index, etc.) will influence the spectral reflectance and transmittance of a multi-layer laser mirrors and will lead to its optical performance degradations. In this work, we have studied the influence of gamma radiation to the optical properties of the substrates of lasers dielectric mirrors with nine coatings (Air(HL)(2x4+1)BK-7). Rom. Journ. Phys., Vol. 60, Nos. 9–10, P. 1515–1524, Bucharest, 2015

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M.-R. Ioan, I. Gruia

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In this paper, we considered only the normal incidence case, when the incident beam divides itself into reflected, absorbed, transmitted and scattered beams. The absorbed and scattered beams are generally very small compared with the reflectance and transmittance values, and we can consider these quantities as ignorable [3]. Thin film coatings have a wide range of applications, such as camera displays and lenses, laser and spectroscopy mirrors, filters, beam splitters and optical windows. Dielectric mirrors consist of similar alternating layers of high and low refractive indices, as shown in Figure 1.

Fig. 1 – Schematic drawing of the nine period Bragg mirror based on ZnS/MgF2 layers for the BK-7 substrate.

The optical thicknesses are typically chosen to be quarter-wavelength long,

λ 4

= nH × l H = nL × l L

(1)

λ – operating wavelength – nm, nH – high refractive index layer – first, nL – low refractive index layer – last, nS – BK-7 refractive index substrate, n0 – air refractive index (1.0), lS – substrate thickness (10 mm). The substrate, nS, can be chosen arbitrary. For the quarter-wavelength case the reflection response at λ [4] is: 2× N

2 ⎛ nH (λ , D) ⎞ nH (λ , D) ⎜ ⎟ 1− ⎜ × nL (λ , D) ⎟⎠ ΔnS (λ , D) , ⎝ r1 (λ , D) = 2× N 2 ⎛ n (λ , D ) ⎞ n (λ , D ) ⎟⎟ × H 1 + ⎜⎜ H ΔnS (λ , D) ⎝ nL ( λ , D ) ⎠

(2)

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Change of optical parameters of the BK-7 glass substrate under gamma irradiation

and the reflectance one:

R(λ, D) = (r1(λ, D)) . 2

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(3)

For the quarter-wavelength case, the reflection response to the wavelength λ and an absorbed dose, D, [5] is: 2

⎛ ⎛ n (λ , D) ⎞ 2× N n 2 (λ , D ) ⎞ ⎟ ⎜1− ⎜ H ⎟ × H ⎜ ⎜⎝ n L (λ , D ) ⎟⎠ Δn S ( λ , D ) ⎟ . R (λ , D ) = ⎜ ⎟ 2× N 2 ⎜ ⎛ n H (λ , D ) ⎞ n H (λ , D ) ⎟ ⎟⎟ × ⎟ ⎜ 1 + ⎜⎜ Δn S (λ , D) ⎠ ⎝ ⎝ n L (λ , D ) ⎠

(4)

To

quantitatively results, we use the following approximations: n H (λ , D) ≈ n H (λ ) , n L (λ , D) ≈ n L (λ ) and Δn S = β S (λ ) × DS × l S . In this case, the change in refraction coefficient, Δn S , will lead to a reflection response at a wavelength, λ, and an absorbed dose, D, expressed as: 2 ⎛ ⎛ n (λ ) ⎞ 2× N n H (λ ) ⎜1− ⎜ H ⎟ × ⎜ ⎜ n (λ ) ⎟⎠ β S (λ ) × D S × l S R (λ , D ) = ⎜ ⎝ L 2× N 2 ⎜ ⎛ n H (λ ) ⎞ n H (λ ) ⎟⎟ × ⎜ 1 + ⎜⎜ β S (λ ) × D S × l S ⎝ ⎝ n L (λ ) ⎠

where

β S (λ ) =

2

⎞ ⎟ ⎟ ⎟ , ⎟ ⎟ ⎠

α S (λ ) D

(5)

(6)

β S (λ ) – dose coefficient (mm-1kGy-1) [6], D – absorbed dose (kGy), α S (λ ) – BK-7 absorption coefficient (mm-1), N – number of pairs of high and low refractive index layers. The above approximations refer to the fact that thin layers absorb small amounts of gamma radiation, which lead to a reduced number of defects created in layers compared to similar irradiation conditions of the same but thicker materials (5). The reflectivity of the stack increases with the number of layers in the stack and with the spectral bandwidth, for a certain

nH ratio. nL

In the actual case, the relationship (5) takes on forms (7) and (8) may:

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4 2

32.66 ⎛ ⎜1 − β S (532) × DS R( DS , β S (532)) = ⎜ ⎜ 32.66 ⎜1 + β S (532) × DS ⎝

⎞ ⎟ ⎟ , ⎟ ⎟ ⎠

39.81 ⎛ ⎜1− β S (633) × DS R ( DS , β S (633)) = ⎜ ⎜ 39.81 ⎜1 + β S (633) × DS ⎝

⎞ ⎟ ⎟ . ⎟ ⎟ ⎠

(7)

2

(8)

The reflectivity is described by the wavelength, substrate dose coefficient and absorbed dose. 2. EXPERIMENTAL SET-UP SAMPLES IRRADIATION

The exposure to the gamma rays emitted by a 60Co source has been performed at IRASM-IFIN-HH center. A maximum total absorbed dose of 21 kGy was reached at a dose rate of about 5.3 kGy/h. Irradiation was performed at room temperature, using a gamma irradiation chamber and a source of 60Co located at IFIN-HH, Bucharest. A source emits two gamma quanta with the energies of 1.173 MeV (99.85%) and 1.332 MeV (99.98%) and a beta particles of 0.31 MeV (99.88%) and 1.49 MeV (0.12%). The effects of beta radiations are completely eliminated due to their strong attenuation in air and 0.5 mm thick polyethylene that coated our samples. Total absorbed doses, kGy, of gamma radiation accumulated in our samples were estimated by an ECB dosimeter system with an average measurement uncertainty of 2.5%. Absorbed dose values that were achieved for BK-7 glass cylinders were: (0.04; 0.08; 0.17; 0.33; 0.66; 1.3; 2.7; 5.3; 10.6 and 21.2) kGy. The dimensions of our Schott BK-7 glass cylinders were 25 mm diameter and 10 mm thickness. The used samples had relatively large thickness because of the low absorption in the visible region of the spectrum, for doses below 10 kGy. At the average energy of 1.25 MeV of a gamma radiation 60Co source, there is no possibility of radioactive activation of any chemical elements contained by our samples. In terms of radioactive contamination the glass samples were coated in protective film with the purpose that the measurements could be performed shortly after the end of irradiation [7].

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3. RESULTS AND DISCUSSIONS

We calculated the high-reflectivity of two different coating materials for two different wavelengths within VIS spectrum. The results depend on refractive index of materials, number of layers, incident angle and the absorbed dose of substrate [8]. Figure 1 shows that coating consists of MgF2 as low refraction index (1.3789; 1.3770), ZnS as high refraction index (2.3069; 2.3504), BK-7 as substrate with the refractive index (1.5195; 1.5151) for two laser wavelength (532; 633) nm and N = 4 [9]. Geometric shape of the BK-7 substrate is similar to plan concave spherical lens. The results of (7), (8) relations are shown in Table 1. Table 1 Laser beam reflection variation for a laser mirror with nine dielectric layers

Dose (kGy)

Reflectivity (%) R(633nm)

R(532nm)

0

99.89088

99.89648

0.04

99.89867

99.87293

0.08

99.88667

99.85485

0.17

99.86006

99.79227

0.33

99.81594

99.73017

0.66

99.74429

99.59816

1.3

99.63869

99.41014

2.7

99.51735

99.22992

5.3

99.36549

99.06183

10.6

99.24062

98.96434

21.2

99.18066

98.91979

The reflectivity (7, 8) of the mirror presented in Fig. 1 is plotted as a function of absorbed dose in Figure 2. The maximum (non-irradiated) theoretical reflectivity for the nine period laser mirrors is 99.89% (λ = 633 nm) and 99.90% (λ = 532 nm). The spectral transmittance and reflectance of samples were measured, before and after irradiation, using a SM242-CM0P1723-EU Spectral Products type spectrophotometer (range of (400÷750) nm). A special sample holder was used to ensure a correct positioning of laser mirrors.

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Fig. 2 – Reflectivity of a nine-period Bragg mirror as a function of absorbed dose, for two VIS wavelengths.

In Figure 3 it is shown the transmitted intensity of the BK-7 substrate as a function of VIS wavelengths range, for all ten absorbed doses. We can observe a decrease in the transmission which depends on the absorbed dose and which will be saturated for higher values of about 10 kGy. This parameter is relevant for beam splitters and semitransparent mirrors.

Fig. 3 – Transmitted intensity of BK-7 substrate as a function of VIS wavelengths range.

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For laser mirrors with high reflectivity the relevant parameter is the reflectivity. In Fig. 4 it is shown the reflectivity of a BK-7 substrate (concave side – a) and (flat side – b) as a function of VIS wavelengths range.

Fig. 4 – Reflectivity of a BK-7 substrate (concave side – a) and (flat side – b) as a function of VIS wavelengths range.

In Fig. 5 is presented the relative variation of reflectivity (a) and of transmittance (b), for the BK-7 substrate versus absorbed dose.

(a)

(b)

Fig. 5 – The relative variation of reflectivity (a) and transmittance (b), for the BK-7 substrate versus absorbed dose.

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In Fig. 6 it is an example of spectral reflectivity of the irradiated and nonirradiated laser mirror.

Fig. 6 – Spectral reflectivity of the irradiated and non-irradiated (Nir) laser mirror as a function of VIS wavelengths range.

Figure 7 shows an example of laser mirror transmitted intensity versus VIS wavelengths range. The variation of the transmission for the same laser mirror as the one in Fig. 6 is presented.

Fig. 7 – Transmitted intensity of the laser mirror versus VIS wavelengths range.

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The reflectance is affected to a lesser extent than the degree of transmission. This fact can be explained by the different degrees of optical degradation of the thin dielectric layers versus the thick BK-7 substrate. 4. CONCLUSIONS

A nine-layer laser mirror with BK-7 substrate (10 mm thickness) was analyzed. We highlighted the influence of ten doses of gamma radiation produced to the laser mirror substrate. In addition, we showed the influence on the laser mirrors behavior (lowering their high reflectance) when used in environments affected by gamma radiation. Initial (non-irradiated), highest reflectivity was of 99.89% (633 nm) and 99.90% (532 nm), but after the irradiation of the BK-7 type glass substrate, these values suffer a decrease of about 0.7% (633 nm) or 0.9% (532 nm). All thin layers were able to resist better than the thicker BK-7 substrate, when were exposed to high doses of gamma rays. It is relevant that the reflectance is strongly influenced by the wavelength at which the optical devices operate. Depending on the purpose of use of laser mirrors, relevant parameter can be considered either transmission or reflection. The relative variation of the transmission is about 66%. The reflectance is affected to a lesser extent than the transmission. This fact can be explained by the different degrees of optical degradation of the thin dielectric layers versus the thick BK-7 substrate. As a final conclusion, if these optical components are operating in hostile radiation environments, it should be taken into account that the radiation induced changes will affect the general dynamic of use. REFERENCES 1. H. A. Sandmeier, G. E. Hansen, M. E. Battat, K. O'Brien, The Cosmic-Ray-Induced Radiation Environment and Dose to Man for Low-Orbit Space Applications, LA-8975-MS, UC-34b, September 1981. 2. Ilaria Di Sarcina, M. Luisa Grilli, F. Menchini, A. Piagari, S. Scaglione, A. Sytchkova, D. Zola, Behavoir of optical thin-film materials and coatings under proton and gamma irradiation, Applied Optics, 53, 4, 2014. 3. Z. E. Salaiby, S. N. Turki, Study the reflectance of dielectric coating for the visible spectrum, International Journal of Emerging Trends and Technology in Computer Science, 3, 6, 2014. 4. S. Mohammmed Abed, S. Naif Turki Al-Rashid, Designig High Reflectivity Omnidirectional Coating of Mirrors for Near Infrared Spectrum (700-2500nm), Applied Physics Research, 5, 1, 2013. 5. A.I.Gusarov, D.Doyle, A. Hermanne, F. Berghmans, M. Fruit, G. Ulbrich, M.Blondel, Refractiveindex changes caused by proton radiation in silicate optical glasses, Applied Optics, 41, 4, 2002. 6. M-R. Ioan, I. Gruia, P. Ioan, I.L. Cazan, C. Gavrila, Investigation of optical effects induced of gamma radiation in refractory elements, JOAM, 4, 254-263, 2013.

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7. M.-R. Ioan, I. Gruia, G.-V. Ioan, L. Rusen, C. D. Negut, P. Ioan, The influence of gamma rays and protons affected optical media on real Gaussian laser beam parameters, Romanian Reports in Physics, 67, 508-522, 2015. 8. I. Di Sarcina, M. L. Grilli, F. Menchini, A. Piegari, S. Scaglione, A. Sytchkova, D. Zola, Behavior of optical thin-film materials and coatings under proton and gamms irradiation, Applied optics, 53, 4, 2014. 9. http://refractiveindex.info.