We study the effect of intense laser radiation on the surface of silicate glass prior irradiated by gamma radiation. Experimental results show that the gamma radiation with dose 5 × 104 R leads to the degradation of the surface resistance of this optical dielectric to electromagnetic radiation. Depending on the dose of the radiation the laser radiation can result in either surface erosion or its pronounced cracking. It is also found that the efficiency of the degradation process is determined not only by the radiation dose, but also by the presence of different impurities in the glass.
Optical materials, in particular silicate glass, are used in different experimental conditions with the increased dose of radiation [
In this work we study the effect of gamma-radiation-created-defects on the interaction of laser radiation with the surface of silicate glass. The intensity of the neodymium glass laser, working in single shoot mode, was calibrated in the interval of q = 108 − 1011 W/cm2 using optical filters. We found that the presence of a layer of adsorbed gas atoms, water and dust results in error in determining the optical resistance of the material. To avoid this we have placed the sample in a vacuum condition ~10−6 Torr. We first cleaned the surface of the sample using the laser radiation with small intensity. We used optical microscopy and spectroscopy to study the processes of the surface damage.
It is known that the damage threshold of the optical dielectrics is above 109 W/cm2. Since this value exceeds the threshold for the formation of laser-produced plasma (~108 W/cm2), we combined the results of optical micro- scopy with the results of laser mass-spectrometry, where we study the properties of the plasma ions. This allows us to study not only the morphology of the surface destruction, but also to determine the target composition, as well as the inclusions which results in degradation of the optical resistance of the glass. Schematics of our experimental setup is shown in
The radiation from the Nd3+ glass laser with wavelength 1060 nm and pulse duration 50 nm has been focused on the surface of the target within the area of 250 mm in diameter. As the threshold intensity qc for destruction of the target was used the value of q which corresponds to high-temperature shining and formation of dense plasma. To avoid the effect of accumulation [
Microscopic analysis of the target surface shows that the damage of the surface by the laser radiation occurs in
Schematics of the experimental setup. 3: multifunctional target chamber, 7: time- of-flight mass-analyzer, 12: electrostatic analyzer, 13: ions detector, 17: Nd:YAG laser, 18: mo- dulator, 11: parallel plates, 22: mirrors, 4: focusing lenses, 16: calorimeter, 14: scillograph
the form of craters with molten edges, inside of which there are small cavities. The reason for the formation of these microcraters is the presence of impurity inclusions and optical inhomogeneities, which result in effective adsorption of laser energy [
Similar crater formation has been reported previously by one of the authors [
We start with determining the discharge of impurity inclusions during the laser interaction. We found that the amplitude of the signal in the spectra corresponding to particular element depends on the intensity of the laser radiation. For example, peaks corresponding to C and Fe appear in the mass-spectra starting at q = qc together with the elements of the crystal lattice of the target [see
Next, we consider the effect of Nd doping of GLS glass on its resistance to laser radiation. We found experimentally that qc decreases monotonically with increasing the concentration of Nd ions (see
We also study the resistance of GLS glasses on laser radiation when the sample irradiated by gamma radiation from Co60 sources with dose in the range of 104 - 5 × 1010 R. Experimental results on the effect of gamma radiation show the followings (see
Thus, the critical intensity of the laser radiation qc depends on both the concentration of Nd ions and the dose of the gamma radiation, as shown in
In order to increase optical resistance of the glasses for the interacting ionizing radiation, we included Ce3+ ions into the sample. Experimental results show that co-activation of clean GLS glasses with these ions increases optical resistance of the sample with our without gamma radiation. We also found out that cerium inclusion for the Nd doped samples does not affect the value of qc. In gamma radiated samples the size of the cracks is smaller in Ce doped samples that the ones for Nd included samples. For example, the size of the surface destruction of 2% Nd doped samples at q = 109 W/cm2 decreases from 30% to 10% with including Ce with concentration in the range of 0.3% to 3.2%.
However, noncontrollabe Fe inclusions have opposite effect: optical resistance of non Ce activated targets decreases with increasing the concentration of Fe. It also increases the size of the main crater and the cavities as well. Gamma irradiation further increases the effect of Fe inclusions. The analysis of adsorption spectra of the glasses shows that gamma radiation results in strong coloring of the samples, increased background adsorption in the range of 850 - 1200 nm and appearance of additional intense gap in the absorption with a maximum at
Mass-spectra of plasma generated on the surface of silicate glass during the interaction of laser radiation with intensities q = 7 × 109 (а)-(c) and q = 9 × 109 W/cm2 (d)-(f). The energies of the ions are Е/z = 50 eV ((a), (d)), 800 eV ((b), (e)) and 2000 eV ((c), (f))
Dependence of the intensity of laser radiation (´109) corresponding to the surface destruction of the glass samples on the concentration of Nd ions
The critical intensity of the laser radiation (´109) corresponding for the surface destruction of the glass sample as a function of Nd concentration. The results are shown for different values of the gamma-radiation dose
1100 nm. The gap at 1100 nm increases with increasing the concentration of Fe. For example, the absorption in the range of 1100 nm increases 1.5 and 1.8 times with including 10−3% and 10−2% of Fe into the sample after gamma irradiation. With increases the dose of the irradiation above D > 108 R additional gap appeared in the optical spectra with maximum at 215 nm. Its intensity increases with increasing the radiation dose.
Analysis of the data about the effect of gamma irradiation on the critical laser intensity qc shows one characteristical feature for both clean and doped samples: monotonic decrease of qc and increase of the size of the craters occur only for doses D = 108 R. At higher doses the intensity of surface destruction and degradation of qc further increases. The destruction is correlated with the appearance of additional adsorption gaps in the optical spectrum of gamma-irradiated samples with maximum at 215 nm. Intensity of this gap is proportional to the dose of radiation and becomes more pronounced for the does above 109 R.
Strong cracking of the sample surface is observed for the doses above 1010 R. The destruction occurs in the form of a large crater with smaller ones inside it. The main crater is surrounded by a set of cracks of different width and depth. The critical laser intensity of the glasses containing Fe is always smaller than the one for Ce doped samples. For example, for the sample including 5.6% Nd and 10−2% Fe the cracking starts at qc = 1010 W/cm2 and decrease of the Fe concentration to 10−3% increases the qc up to 2.8 × 1010 W/cm2. With including Ce and increasing its concentration from 0.3% to 2% results in increase of qc from 4 × 1010 tо 8 × 1010 W/cm2. In these experimental conditions qc of the clean sample was 1011 W/cm2.
To the best of our knowledge, the negative role of Fe ions on the resistance and optical absorption of gamma radiated glasses can be explained as follows. It is well knows that Fe can exists in glasses in different forms. The appearance of additional gap in the adsorption spectra of the sample with maximum at 1100 nm and increase of its intensity with increasing the radiation indicates to the fact that gamma radiation results in inovalent transition of Fe ions, i.e., Fe3+ ions capture additional electron during the irradiation and forms Fe2+ [
It was found experimentally that the degradation process of light resistance of gamma-irradiated glasses can be slowed down by including Ce ions. The analysis of optical adsorption spectra of non Ce activated sample show that gamma radiation results in formation of defects which absorb the electromagnetic radiation in UV, optical and infrared range of the spectra. The high temperature plasma, generated during the interaction of laser radiation is an effective source of optical radiation covering all the optical part of the spectra. The formation of defects in gamma-radiated sample results in addition heating of the sample and promotes initial destruction. Introducing of Ce during the gamma radiation affects the electron capturing process of the other defects. This results in decrease of the defect formation process, which can absorb the laser radiation in addition to plasma radiations. To our knowledge, this can explain the experimental fact that the presence of Ce ions results in reduction f the surface destruction.
We also found that independently on the type and concentration of the inclusions, there is a change in the dependence of qc on the dose of radiation for D ≥ 3 × 108 R. This change is correlated with the formation of additional gaps in the optical spectra of adsorption of gamma-irradiated sample with maximum of 215 nm. This is connected by the formation of additional centers which adsorbs in this range (i.e., E-centers) [
Using the mass-spectrometric method we study the effect of gamma radiation, as well as the effect of metallic inclusions on the absorption properties and light resistance of silicate glasses. Depending on the dose of the gamma radiation, the surface resistance of the samples to the laser radiation decreases considerably due to the surface erosion or crack formation. Different metallic inclusions (Fe or Ce) further decrease the resistance of the glasses as these inclusions become centers of intense light absorption.