Optical Materials

In the optical materials that are used as substrates for mirrors and laser output windows, under the influence of the reactor radiation, additional losses of light can occur, associated with the reduction in their transparency. As was demonstrated by the results of studies [4042], the absorption coefficient depends on the type of optical material and the concentration of impurities in it, the temperature of the sample, the wavelength of the light radiation, the absorbed dose, and absorbed dose rate. Absorption of light radiation in optical materials occurs as a result of forma­tion of color centers, which arise with the capture of charge carriers (electrons and holes) on structural defects of the material (for example, see [43, 44]). At the same time, the color centers are formed both as a result of the change of state of already existing defects and as a result of the onset of new defects.

In the majority of studies, materials were irradiated for a long time using stationary reactors, high power isotopic y-irradiating sources, and their coefficients
of absorption were measured before and after irradiation. The data obtained from stationary irradiation cannot be used to predict the value of the induced coefficient of absorption in the process of pulsed irradiation [41, 42], since frequently a large contribution to absorption is made by the color centers with short lifetimes. The coefficients of induced absorption for these two variants of irradiation can differ by a factor of 10 [42].

At VNIIEF, studies into the radiation resistance of optical materials to pulsed radiation have been carried out since the early 1970s. The methods of measuring induced absorption coefficients and radioluminescence of optical materials under the effects of reactor radiations are cited in study [40]. Basic attention was given to the change in optical properties of materials as a function of time during the reactor pulse.

The methods [4042] used to measure the induced absorption coefficient in the process of pulsed irradiation (and for any time interval after it) are based on a very simple principle of measurement of the intensity of light radiation passed through the specimen prior to irradiation and any subsequent moment of time. CW lasers were used as the sources of light radiation; for example, a helium-neon or helium — cadmium laser, radiating at individual lines in the visible and IR regions of the spectrum, or a lamp with a continuous spectrum. In the latter case, light filters or a monochromator were used to isolate the probe light radiation in the narrow spectral range. To extract the useful signal against the background of various types of noise (including radiation noise in the photodetectors), the probe light signal was modulated.

Such methods make it possible to perform measurements of absorption coeffi­cients simultaneously in several wavelengths. For example, Figure 2.4 shows an oscillogram of one of the experiments with the VIR-2 reactor to measure induced absorption coefficients simultaneously at three wavelengths [40]. The absorbed dose rate of у radiation at the maximum of the reactor pulse was around 1 x 106 Gy/s. The contribution of the neutron radiation to the absorbed dose did not exceed 10 %. Induced absorption coefficients at wavelengths 633, 1,150, and

image013Fig. 2.4 Change in the transmission coefficient of a BaF2 crystal 10 mm thick when irradiated by a pulse of n, Y radiation of a VIR-2 reactor [40]: (1) reactor pulse, (2-4) signals of modulated probe light at wavelengths of 3,390, 1,150 and 633 nm, respectively. Scale division is 5 ms

3,390 nm in the pulse maximum were 0.65, 0.27, and <0.01 cm-1 respectively. It is clear from the data of Fig. 2.4 that the induced absorption coefficient decreases with an increase in light wavelength. This principle is observed for all optical materials.

The results of measuring induced absorption coefficients for certain optical materials obtained in experiments [42] with the pulsed TRIGA reactor are shown in Table 2.2.

Experimental investigations of various optical materials showed that the greatest radiation resistance is possessed by silica glass. Detailed research on the common types of silica glass KU-1, KV, and KI were carried out in experiments with the VIR-2 reactor [41]. In the absorption spectra measured 1 h after pulse irradiation, all the types of quartz showed the band with a maximum at the wavelength of 215 nm, which is characteristic for SiO2. Apart from the indicated band, the KI quartz (which lacks the hydroxyl group OH) had broad bands with maximums at wavelengths of 300 and 550 nm, while the KV quartz had a band with a maximum at a wavelength of 300 nm. The induced absorption coefficients measured in study [41] are shown in Table 2.3.

The value of the induced absorption coefficient depends greatly on the concen­tration of coloring impurities and hydroxyl OH. For the purest commercial glass, KU-1 (concentration of coloring impurities ~10 ppm, hydroxyl concentration <2,000 ppm), the induced absorption coefficient is minimal, which may be explained not only by the low concentration of coloring impurities, but also by the protective properties of the ОН+ ion.

An additional effect that can influence the recording instruments and the accu­racy of measurement of the light intensity is the radioluminescence of optical

Table 2.2 Maximum induced absorption coefficients (cm ‘) at an absorbed dose rate of 7 x 104 Gy/s [42]

Wavelength of probe light, nm

Material

325

633

3,390

Fused quartz (Corning 7940)

0.0073

Pyrex (Corning 7740)

0.58

CsI (monocrystal)

0.067

<0.009

Sapphire

0.014

<0.008

Spinel

0.58

0.011

ALON

4.9

0.05

Table 2.3 Maximum induced absorption coefficients (cm ‘) at absorbed dose rate of around 1 x 106 Gy/s [41]

Wavelength of probe light, nm

Type of silica glass

400

500

600-700

900

1,150

KI

0.6

0.28

0.022

0.015

KV

0.36

0.18

<0.01

<0.01

KU-1

0.07

0.045

<0.003

<0.005

<0.005

7 x 104 Gy/s [42]

Подпись:Подпись:materials [40, 42]. The intensity of radioluminescent radiation depends on the type of optical material, its temperature, and the spectral range. Table 2.4 shows the specific powers of radioluminescent radiation for certain materials [42].

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