Dose depth and penetration of light dependence in the irradiated optical glass by reactor neutrons

Optical Materials 36 (2013) 489–494

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Dose depth and penetration of light dependence in the irradiated optical glass by reactor neutrons N. Baydogan ⇑, A.B. Tugrul ⇑ Istanbul Technical University, Energy Institute, Ayazaga Campus, Maslak, 34469 Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 23 July 2013 Received in revised form 22 September 2013 Accepted 9 October 2013 Available online 1 November 2013 Keywords: Absorbed dose Glass Optical properties Radiation effect

a b s t r a c t The effect of absorbed gamma dose of the optical glass containing B2O3 and BaO was examined at nine different dose levels, ranging between 1 and 36 kGy, in order to explain dose depth and penetration of light dependence. The specific light absorbance per unit area was examined to determine the changes in penetration ability of light photons into the irradiated glass structure due to the absorbed dose. Determining the increase in the absorbance of light photons is proposed as an approach to create the equivalent dose estimation. Examination of the penetration of light photon for the visible range presented an importance to evaluate absorbed dose depth in optical glass. The variations of light absorbance in the standardisation concept were controlled by the absorbed dose in a practical way. A calibration curve was determined depending on the variations of absorbed dose depth and optical density at 620 nm in the visible range. The effect of neutron and mixed gamma/neutron radiation in the tangential beam tube and the central thimble of the nuclear reactor, respectively were evaluated by means of equivalent gamma dose in order to explain the light absorbance in the irradiated glass. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Several types of glass are used in irradiation constructions, hot cell windows, and peeping holes during irradiation processes with direct and/or indirect ionizing radiation at high doses. The duty of optical material during its service life in the irradiation field is required to perform a considerable optical performance at the visible range (380–780 nm) [1–3]. Optical materials can subject to ionizing radiation during their missions at different absorbed dose levels in either terrestrial or space applications. Hence the changes of optical properties in the visible range present an importance with the increase of absorbed dose. Typical onboard sources of radiation in space vehicles include nuclear reactors as typical internal radiation sources. Ionizing radiations (such as neutrons, gamma rays and beta particles) are produced by the isotopes in the onboard sources of space vehicles [4]. When high refraction index is required in glass, it is usually necessary to have appreciable amounts of heavy metals such as lead and barium in the glass. However lead-free crystal has a similar refractive index to lead crystal, but it is lighter and it has less dispersive power. Lead-free crystal glass generally comprises 50–75 wt.% SiO2, 2–15 Na2O, 1–15 K2O, 3–12 CaO, 1–10 BaO, ⇑ Corresponding authors. Tel.: +90 212 285 34 92 (N. Baydogan), tel.: +90 212 285 38 84 (A.B. Tugrul). E-mail addresses: [email protected] (N. Baydogan), [email protected] (A.B. Tugrul). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.10.015

0–1- B2O3, 0–5 Al2O3. Barium oxide or potassium oxide are used in this crystal glass instead of lead oxide to increase the optical performance. B2O3 and BaO improve appearance and strength. But BaO provides less chromatic dispersion than B2O3 in lead-free glass. Hence, translation of colour in glass with BaO is more true and more distinctive with no fringing than translation of colour in glass with B2O3. BaO in the glass gives more vivid image than B2O3 in the glass. Besides, BaO has some advantages in lead-free crystal glass depending on the application purposes. The glass containing barium is lighter in weight than glass with lead but the barium gives comparable brilliance due to its high refractive index. Much of the published materials data concerned Co-60 gamma ray exposure in air environment, and is 50 years old in the space applications. Optical glass is shielded, but its outer surface can receive very high surface doses. Hence the optical changes of glass require evaluating dose–depth curves. Generally accepted: ‘‘equal dose gives equal damage’’, regardless of radiation type. Dose–depth profiles can be different depending on the type of ionizing radiation and the surface doses can be higher for ionizing radiation. It is difficult to simulate the irradiation effect realistically due to different dose–depth curves and different physics of interaction [4]. Differing penetration depths were investigated by using the changes in the penetration ability of the light photons in the irradiated optical glass to evaluate the changes of absorbed dose–depth in this study. The specific light absorbance per unit area was examined to determine the changes in the penetration ability of the light photon. The purpose of this study is to

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determine a calibration curve in order to investigate the absorbed dose according to the standardisation concept in a practical way. A calibration curve of the specific absorbance of light photons per unit area, av(%)/d2 (mm) was used to control the absorbed dose. The effect of equivalent gamma dose of neutrons on glass was evaluated using dose depth and specific light absorbance relation. 2. Experimental studies 2.1. Optical characterisation Optical crown glass containing B2O3 and BaO was supplied in three different thicknesses; 3, 6, and 10 mm from the major glass company SISECAM in Turkey. Both surfaces of the glass samples were polished. Firstly, samples were rubbed with SiC abrasive paper at various grain sizes, and then polished with a CeO solution. Smooth and plane parallel surfaces were performed without optical distortion. Optical crown glass, a type of alkali-lime-silica glass, has low-dispersion and is used for making optical equipment. This type of colourless silica glass is manufactured generally from combinations of SiO2 with K2O, Na2O, CaO and other oxides (network modifiers) [5]. Soda-lime-silicate glass has a composition of in wt. 70% SiO2, 15% Na2O and 10% CaO [6]. Adding soda (Na2O), and sometimes potash (K2O), to silica lowers its softening point by 800–900 °C. Lime (CaO) and sometimes magnesia (MgO) and alumina (Al2O3) are added to improve its chemical resistance [7]. Boron provides good chemical resistance and high dielectric strength because of its low thermal expansion [5,6,8]. Modern optical glass usually contains barium oxide instead of lime. The element Barium is the hardest member of the alkaline earth group element [9]. Barium oxide is added to soda-lime-silicate glass in order to decolourise glass [10]. Besides, barium has the highest neutron-cross section in alkaline earth group [2,3,10–12]. Table 1 illustrates a Philips PW 1606 X-ray Fluorescence (XRF) Spectrometer, used to determine the chemical composition of glass. A Perkin Elmer, Lambda-9 UV/VIS/NIR spectrophotometer was used to measure the spectral transmittance and reflectance of samples between 280 nm and 2200 nm in order to examine the influence of radiation on glass. Optical measurements were recorded at different times during the gamma irradiation step. The direct light transmittance (sv), the direct light reflectance (qv), and the direct light absorbance (av) of the glass were calculated for the unirradiated and the irradiated states in the visible range according to European Standards. Details of sv, qv, and av are described elsewhere [10,12–15]. 2.2. Irradiation process using a Co-60 radioisotope A projection type Co-60 radioisotope with 362.6 GBq was used as the gamma source. The glass specimens with different thicknesses such as 3, 6, and 10 mm, were set up panoramically at several distances from the gamma ray source [16–18]. The collision possibility of photons with atoms increased due to the increase in thickness. Irradiated glasses can present an initial rapid fading in their optical response. Hence, the irradiation process was performed in dark at a hot cell to protect the induced colour changes by irradiation in case of the optical changes before expected. The optical

measurements were performed at the end of the gamma irradiation process to eliminate the early fading problem. The irradiation time intervals have taken hours for less doses (<5 kGy) and it has taken days for high doses (>5 kGy). The values of elements in glass are illustrated in Table 1. Nine different dose levels were selected in order to evaluate the characteristics of light absorbance and optical density of the glass; 1.0, 2.2, 4.5, 8.0, 12, 15, 21, 30, and 36.0 kGy. The maximum irradiation dose at 25 kGy helps reduce some radiation effects, such as decrease material strength or colour change in the material [19]. On the other hand, irradiation is used for special radiosterilisation purposes at 1–10 kGy, and specialised organisations of the United Nations Organization (FAO, IAEA, WHO) recommend that member states authorise these absorbed dose levels [20]. The absorbed dose at 35 kGy is the killing dose of several microorganisms for the medical sterilisation of products [21]. In this study, the maximum applied dose was selected as 36 kGy in order to examine changes in optical behaviour. A measure of how deep gamma radiation was evaluated to explain the changes of penetration depth for gamma ray in the irradiated glass by using Beer–Lambert law. The change of penetration depth of the absorbed dose was evaluated by using gamma transmission technique in the irradiated glass. Hence it can be possible to assess the changes of transmittance of gamma ray with the increase of glass thickness in the irradiated glass. For this purpose a certified Co-60 radioisotope was performed to investigate the changes of gamma transmittance in monochromatic conditions at 1.25 MeV. The increase of the absorbed dose affected the gamma transmittance of glass and the raise of glass thickness causes to decrease the gamma transmittance of glass. Gamma transmittances of glass were presented with the increase of glass thickness for unirradiated and irradiated states in Fig. 1. The background is detected as I0 = 10,332 ± 43. The variations on gamma transmittance indicate that the transmittance of gamma ray in the irradiated optical glass decreases with the increase of the absorbed dose.

2.3. Irradiation processes at research reactor Samples were irradiated in a neutron beam collimator inserted inside the tangential beam tube, as illustrated in Fig. 2. The 10  15  25 mm3 unirradiated specimens with a 10 mm thickness were placed in the Tangential Beam Tube of ITU TRIGA Mark-II Reactor. A bismuth filter was inserted into the hole to absorb the gamma rays and the filter enabled the neutrons to pass through the collimator [22]. It was possible to obtain the thermal and epithermal neutron fluency and n/c ratio was 1.44  104 n cm2 s1 mR1 in that tube. The inlet diameter of the collimator, D, was 17 mm, and the length of the collimator hole, L, was 2407 mm. The thermal neutron fluency at the inlet of the tube, /1, was 1.67  1011 n cm2 s1 [22]. Measurements were taken using activation analysis (n, c) reaction, using gold foils. The specimen was irradiated in the tangential beam tube of the reactor for 30 min and the reactor was operated at a power of 250 kW. The specimen was placed at a distance of 407 mm to the filters system in the inlet of the collimator for the effective use of the thermal and epithermal neutron fluency. Hence, specimens were placed as near as possible to the reactor core.

Table 1 The chemical composition of optical crown glass containing K2O, Na2O, CaO, B2O3, and BaO. Chemical composition in wt (%) BaO

SO3

SiO2

K2O

Na2O

B2O3

SbO

Al2O3

Fe2O3

NiO

CaO

TiO2

1.00

0.25

70.442

9.00

9.00

1.00

0.18

0.10

0.01

0.00026

9.00

0.018

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1 2 3 4 5 6 7 8 9

-0.5 1 2 3 4 5 6 7 8

Ln (I/I0)

-1.0

- 1.5 -2.0

Table 2 The changes of the activity level of samples after the irradiation process at the nuclear reactor.

1.0 kGy 2.2 kGy 4.5 kGy 8.0 kGy 12.0 kGy 15.0 kGy 21.0 kGy 30.0 kGy 36.0 kGy

Irradiation places

After irradiation (lC/kg/ h)

Tangential beam tube Central thimble

0.0645 387

2 Months later (lC/kg/ h) <0.0258 0.0516

9

-2.5 -3.0

2

4

6

8

10

12

Glass Thickness (cm) Fig. 1. The changes of gamma transmittances with the increase of thickness in glass.

The glass samples were irradiated at the nearest place to the reactor core in order to gain the maximum neutron fluency in the central thimble of the reactor [22–24]. The maximum mixed radiation dose was combined with gamma and neutrons in order to investigate the maximum changes in the optical behaviour of the glass. The specimens had three different thicknesses at 3, 6, and 10 mm and were 15  25 mm2 in size. As the reactor was operated at a maximum power of 250 kW for 5 min, the glass samples were activated at the central thimble of the reactor. Table 2 illustrates activation levels on the surface of test samples. Activated samples were kept in a hot cell until handled. 3. Results and discussions Optical changes in the visible range were accompanied by the change in colour from clear to brown with the increase in the absorbed dose of glass. The reason of the colour change is that gamma irradiation causes electron ejection and trapping in glass [25,26]. These electronic defects are associated with both the existing structural defects, such as the intrinsic and the extrinsic defects [27–29]. The changes in valance electrons of the impurity atoms affect the colour of glass at the end of the irradiation [30–36]. There are few known facts about the effect neutrons have on glass. Particle radiation consisting of neutrons creates displacements due to a knock-on process whenever sufficient momentum and energy of

the particle are transferred to an atom in the glass network. If the energy is sufficient to break the bond, that is, 4–25 eV, the struck atom is displaced into an interstitial position [37]. After the irradiation processes, changes in sv, qv, and av were determined according to European Standards-EN 410 [13,14] and their results were presented for gamma irradiation in Table 3, and for the irradiation processes at the reactor in Table 4. sv decreased as the glass thickness increased. av increased due to the absorbed dose and the penetration of light photons through the irradiated glass structure. Irradiation caused a slight decrease in qv after the irradiation treatment. The changes in av of irradiated glass with a 10 mm thickness were determined more strongly than the ones with 3 and 6 mm thicknesses at the end of thermal neutron irradiation in the tangential beam tube of the reactor. Therefore, optical changes were investigated at 10 mm glass thickness at the end of the irradiation process in the tangential beam tube. The dose interval was between 102 and 102 kGy for the coloration of glass [5]. The dose level at 0.2 kGy was convenient to the expecting dose for the first colouration at the glass. Boron is a near unique element that absorbs thermal neutrons strongly as it has the highest neutron-cross sections for (n, c) reaction. There are two stable isotopes: B-10 and B-11. B-10, with an abundance of 20%, has a large cross-section. There is a considerable attribution of B-10 to the knock-on process. Barium in alkali-earth group elements has a high attenuation coefficient for gamma and a high neutron cross section. Hence it is another important element for the neutron and the mixed dose evaluation in the glass. The equivalent gamma dose effect was similar to the gamma absorbed dose effect on the glass structure. There is the contribution of neutron irradiation to av due to the knock-on process and the (n, c) reaction. av of the irradiated specimen with 10 mm thickness was investigated with the increase in the gamma absorbed dose, as illustrated in Fig. 3. The estimated uncertainty in absorbed dose was

Fig. 2. The irradiation position of the glass specimen at the tangential beam tube of nuclear reactor.

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Table 3 The changes of sv, qv, and av of glass with the increase of the absorbed dose. Light transmittance, sv (%)

Absorbed dose (kGy)

0.0 1.0 1.8 2.25 4.5 36.0

Light absorbance, av (%)

Light reflectance, qv (%)

3 mm

6 mm

10 mm

3 mm

6 mm

10 mm

3 mm

6 mm

10 mm

91.55 74.88 69.62 64.53 63.83 27.79

90.89 68.96 56.87 53.86 38.03 9.19

89.91 47.35 41.47 27.38 19.05 0.63

7.62 6.60 6.26 5.88 5.62 4.52

6.91 6.14 5.45 4.98 4.55 4.13

6.31 4.61 4.57 4.13 4.09 4.02

0.83 18.52 24.13 29.59 35.00 67.70

2.20 24.90 37.67 41.16 52.41 74.00

3.78 48.04 53.96 68.49 73.00 87.00

Table 4 Optical changes via the standardization concept as a result of the increase in glass thickness and irradiation time at the nuclear reactor. Irradiation process

Glass thickness (mm)

Irradiation time (min)

sv (%)

qv (%)

av (%)

Tangential beam tube Central thimble

10 3 6 10

30 5 5 5

80.52 22.67 6.52 1.12

6.16 4.14 4.01 3.88

13.32 73.18 84.09 95.00

100 80 60

Co-60 radioisotope Tangential beam tube Central thimble

40 20 0 0

5

10

15

20

25

30

35

40

45

Absorbed Dose (kGy)

area, [av (%)/d2 (mm2)] as a function of the absorbed dose, is a kind approach to explain the absorbed dose. This approach is defined as the absorbed light photons per unit mass with the increase of the absorbed dose in semi log scale. Changes in av (%)/d2 (mm) have explained the specific absorption of light as the area from the glass surface to the depth of the glass with the increase in the absorbed dose. Changes in av (%)/d2 (mm) indicated that the glass thickness was an important parameter in the glass, as illustrated in Fig. 6. The slop of the curve was 0.54. The provided curve was used to

Light Absorbance, αv (%)

100

75

10 mm

6 mm

50

Mixed Dose Gamma Dose

3 mm

25

0 1

10

100

Absorbed Dose, D (kGy) Fig. 4. Changes in av with the increase in absorbed dose in semi log. scale for three different glass thicknesses, and assessment of the equivalent gamma dose for mixed gamma/neutron doses.

Spesific Light Absorbance per unit thickness, α (%) / d (mm)

Light Absorbance, αv (%)

acceptable. Calculated standard deviations of av were <2%. Hence the error bars were resolved in the graph. After irradiation treatments in the tangential beam tube and central thimble of the reactor, av values of irradiated specimens settled on the curve. The equivalent gamma dose in the tangential beam tube of the reactor was 0.2 kGy as the minimum absorbed dose at the end of the thermal neutron irradiation. It was convenient to expect this dose for the light brown colouration of the glass. The equivalent gamma dose in the central thimble of the reactor was determined as the mixed neutron/gamma radiation at 42 kGy. The mixed dose of specimens was determined using the equivalent gamma dose approach based on the av measurements illustrated in Fig. 3. According to this approach, the absorbed dose was determined at the semi log scale illustrated in Fig. 4 and the validity of this values was confirmed using 3, 6, and 10 mm thicknesses after the irradiation processes of the glass samples. Changes in specific light absorbance per unit thickness, [a(%)/ d (mm)] were evaluated with the increase in glass thickness for different absorbed doses, as illustrated in Fig. 5. a(%)/d (mm) reached a maximum decrease at 36 kGy. The a(%)/d (mm) of the sample, irradiated in the central thimble, had a maximum slope with 2.05. When the material is used in either terrestrial or space applications, it is exposed to radiation fields that are non-uniform. Irradiation fields may vary with geometry or with time. Typically, the laboratory radiation impinges on the sample on one side and in this case, exhibits a decreasing absorbed dose through the material [38]. Therefore, the cumulative dose of materials as the absorbed dose is an important parameter at the place of interest during its mission. Using the specific absorbance of light photons per unit

40

30 central thimble 36 kGy

20 4.5 kGy 2.25 kGy 1.5 kGy 1 kGy

10

0 kGy

0 0

2

4

6

8

10

12

Glass Thickness (mm) Fig. 3. Changes in av with the increase in absorbed dose for a 10 (mm) glass thickness, and assessment of the equivalent gamma dose for neutrons and mixed gamma/neutron doses.

Fig. 5. The changes in av (%)/d (mm) with the increase in the glass thickness for the different absorbed dose levels.

N. Baydogan, A.B. Tugrul / Optical Materials 36 (2013) 489–494

determine the absorbed dose. The obtained specific light absorbance value for the mixed absorbed dose was settled on the curve as the equivalent gamma dose to mixed absorbed dose. The equivalent gamma dose on this curve was 42 kGy. Deviation of equivalent gamma dose was below 5%. It is suggested that this curve is used for a practical and rapid evaluation of the equivalent gamma dose of the high absorbed dose for an accident state in the area. 3.1. Optical density Optical density consists of both absorption and reflection [39,40]. The use of optical density is preferable for the dosimetric evaluation of glass [1,41]. In Eq. (4), D is optical density; t is glass thickness; a is absorbance, and q is reflectance. The optical density was calculated based on reflectance and absorbance [14,15].

D ¼ atlog10 e  log10 e ð1  qÞ2

ð4Þ

Spesific Light Absorbance per unit area, α (%) / d2

Transition elements such as Fe and Ti are mainly responsible for the colouration of the glass [1,32,33]. Fe3+ ion is a tetrahedral coordination and it takes place at absorption near 380, 420, and 435 nm. On the other hand, trivalent titanium exhibits colouration in glass with a single band centred at 570 nm [32]. The near infrared absorption of ferrous iron (Fe2+) occurred at 1000 nm in all glass systems although the particular character in terms of shape, intensity, and wavelength position varied with the glass composition. Fe2+ ion is an octahedral coordination and it has a position at 1050 nm in soda-lime-silicate glass [32]. Both iron and titanium are substitutes for aluminium for the Al2O3 in the glass structure. Iron itself is presented either as Fe3+ or as Fe2+, while titanium itself is usually present as Ti4+. In the event that both Fe2+ and Ti4+ are present it is possible that they will interact with each other if they are located adjacent to Al sites. Fe2+ is converted to Fe3+ by losing one electron and in gaining the electron Ti4+ is converted to Ti3+. Two characteristic optical density bands occurred at 460 and 620 nm related to the aluminium centre due to the substitution aluminium with a trapped hole, while an alkali ion traps the corresponding electron [42,43]. It consists of hole trapped on the oxygen bonded to aluminium substituted for the silicon in the lattice. Since Al is often present as an impurity in alkali-silicate glasses, it is important to determine its effect based on the radiation-induced optical properties at the spectrum [42]. Fe and Ti atoms, as transition elements, and Al atoms, as the post-transition element, are effective on the colouration of glasses as the impurity atoms in the glass structure [42,44]. The colouration of glass is related with the redistribution of electrons using gamma irradiation. Therefore, the addition or removal of one or more electrons from

the defects or the impurity centres causes the formation of colour centres [32–35]. Fe2+ as impurity atom is oxidized to Fe3+ as a result of irradiation. Optical density increased considerably, and two distinct characteristic bands occurred slightly with the increase in the absorbed dose, as illustrated in Fig. 7. One of optical density bands centred between 380–460 nm, and the other one placed between 570 and 620 nm. A broad characteristic optical density band between 380 and 700 nm was created by gamma irradiation at 31 kGy and over of this dose level. Two characteristic bands were detected after neutron irradiation in the tangential beam tube. The positions of these bands were similar to the position of those for gamma irradiation. The mixed radiation at the central thimble provided a considerable change in the optical density of glass. The effect of mixed dose was similar to the optical density of gamma dose. The characteristic optical density bands shifted to longer wavelengths with the increase of absorbed dose. As optical changes are illustrated clearly at 620 nm in Fig. 7, the changes in the optical density were evaluated with the increase of the absorbed dose at 620 nm in Fig. 8. The induced optical density rapidly increased to 8 kGy and it was more sensitive to irradiation up to 8 kGy. The optical density increased gradually between 8 and 20 kGy. The optical density reached saturation at 30 kGy, and the saturation condition of the optical density covered between 30 and 35 kGy. The equivalent gamma dose of the irradiated glass at the tangential beam tube of the reactor was settled on the optical density curve and was 0.2 kGy, as illustrated in Fig. 8. On the other hand, the equivalent gamma dose of the irradiated glass at the central thimble of the reactor was settled on this curve and was 42 kGy. A glass specimen, as a test sample, was irradiated to 2.8 kGy using a Co-60 radioisotope and its optical density was settled on the curve. The determined dose of the test sample deviated by approximately 5% compared to the deliver dose. The optical density values of irradiated glass samples at the tangential beam tube and the central thimble of the reactor were settled on the curve. The neutron and mixed radiation dose were determined with respect to the equivalent gamma dose deviated by approximately 5% compared to the delivered dose. 5% difference between gamma and mixed/neutron radiation were determined due to measurements error in radiation measurement set.

4 Central Thimble Co-60 Radioisotope

3

2

1

0 1

10

100

Absorbed Dose, D (kGy) Fig. 6. Changes in av (%)/d2 (mm2) with the increase in absorbed dose in semi log. scale for a 10 (mm) glass thickness, and assessment of the equivalent gamma dose of mixed gamma/neutron dose.

493

Fig. 7. Changes in optical density for the different levels of absorbed dose.

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Optical Density at 620 nm

0.60 0.50 0.40 0.30 Irradiated samples by Co-60 Irradiated samples by neutrons Irradiated samples by mixed radiation Test sample for Co-60 radioisotope

0.20 0.10 0.00 0

5

10

15

20

25

30

35

40

45

Absorbed Dose (kGy) Fig. 8. Changes in optical density with the increase in absorbed dose at a 620 (nm) wavelength, and assessment of the equivalent gamma dose for neutrons and mixed gamma/neutron doses.

4. Conclusions The following can be concluded based on the results of this study:  A calibration curve determined by standardization concept was performed considering the specific absorbance of light photons per unit area, av (%)/d2 (mm). The use of this curve was practical in the visible range to evaluate the absorbed dose and penetration of light dependence of the irradiated optical glass at the high dose areas. The absorbed dose of neutrons was evaluated as the equivalent gamma dose. The penetration ability of light photons into the irradiated glass structure decreased with the increase of specific light absorbance after gamma and neutron irradiation.  The increase in direct light absorbance, av was related with the increase in optical density. Optical density at 620 nm and direct light absorbance increased linearly up to 8 kGy due to the creation of the induced colour centres. Hence, this glass was more sensitive up to 8 kGy as a dosimeter. There was a saturation condition of optical density between 30 and 35 kGy. Neutron irradiation was expected to contribution to av due to the creation of the defect centres in the glass. As neutron irradiation indirectly generated colour centres in the glass with the knock on process, optical density, and direct light absorbance improved with neutron and mixed gamma/neutron irradiation.  Two characteristic optical density bands appeared clearly and shifted to longer wavelengths with the increase in absorbed dose. These bands combined together over 30 kGy. Changes in optical density appeared clearly at 620 nm. Hence, an optical density curve was determined to explain the optical characteristics at this wavelength in order to evaluate the dose depth and penetration of light dependence of the optical glass.

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