1 Introduction
The application of the optical methods for detection of gamma radiation is a promising way for the radiation monitoring [1‐3]. These methods are high sensitive and selective, quick and may be used in the different environments.
Compared to the commonly used methods our new attempt: the application of the (Ga54.59In44.66Er0.75)2S300 crystal as the detector material along with photoinduced nonlinear optical method significantly improve the sensitivity to the dose of radiation.
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For this reason more important is to use several optical parameters. Normally it is used photoluminescence and one can expect the possible use of photoinduced nonlinear optics [4]. The latter is very sensitive because allows to monitor the higher excited states, take into account contribution of the photopolarized states [5].
Usually, for the radiation, the sensitivity is caused by the presence of the defects states, rare earth dopants, vacancies etc. [6].
New materials and the study of their properties is one of the main directions of modern radiation materials science. The introduction of dopants to binary and ternary compounds [7‐10], including rare earth metals [10‐12], is a prerequisite for the manufacture of novel optoelectronics radiation detectors. In addition, scientists are paying special attention to the properties of crystalline and amorphous media that are able to consistently show high-intensity photoluminescence and non-linear optical properties under the influence of irradiation [13, 14]. The mechanisms of fluorescent emission and non-linear optical effects are most sensitive to γ-rays due to their high penetrating ability. Therefore the development of radiation-resistant materials and their implementation in space technologies and optoelectronics is one of the main goals of radiation physics and materials science of optoelectronics radiation sensitive materials.
Our previous investigations of the Gа2S3–In2S3 system [15] found two ternary compounds, GaInS3 and Ga0.7In1.3S3. GaInS3 forms in a peritectic reaction L + In2S3 ↔ GaInS3 at 1190 K, crystallizes in the hexagonal symmetry, SG Р6
1
, a = 0.6655(4) nm, c = 1.7950(3) nm, and has a homogeneity region that stretches within 47–57 mol% In2S3 at 820 K. An Erbium-doped single crystal of the GaInS3 phase, (Ga54.59In44.66Er0.75)2S300, was grown using the technique described in detail in [15].
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We used X-ray photoelectron spectroscopy (XPS) to investigate core-level and valence-band spectra of pristine and Ar+ ion-bombarded surfaces of the (Ga54.59In44.66Er0.75)2S300 single crystal [16]. It was determined that the single crystal is very resistant to Ar+ ion-bombardment. For instance, such treatment did not cause any substantial changes of the binding energies of the core-level electrons, or of the shape of XPS core-level and valence-band spectra. It was suggested in [16] that the addition of Erbium into the (Ga55In45)2S300 lattice does not lead to significant changes in the particularities of the chemical bonding of the original (undoped) single crystal.
The objective of this work is to study the possibility to apply photoinduced third harmonic generation and to compare it with the photoluminescence spectra of non-irradiated and irradiated with different doses of γ-rays single crystal of (Ga54.59In44.66Er0.75)2S300. Finally, we compared two methods, first based on the investigations of luminescence intensity versus radiation dose and second THG versus radiation dose. We have shown that while both optical methods allow for the determination of dose, THG is definitely more sensitive.
2 Experimental
The (Ga54.59In44.66Er0.75)2S300 single crystal was grown by the solution-melt method that is described in detail in [15]. The diffraction pattern of the specimen that was cut from the middle part of the single crystal (scan step 0.05°, exposure time 27 s) have shown that the sample possesses single-phase and they were identified in space group P6
1
, a = 0.6657(3) nm, c = 1.7962(4) nm [15].
The photoluminescence spectra were investigated using an MDR-206 monochromator at room temperature with spectral resolution 1 nm. The signal was registered by Si and a PbS-based photodetector. The luminescence excitation was performed by a diode pumped second harmonic generated Nd:YAG laser was performed by a cw 150 mW laser (model LDM532U) at the wavelength 532 nm. The crystal was irradiated by a 60Co source at ambient conditions (Fig. 1). The average energy of the incident γ-rays was equal to about 1.25 MeV. Absorbed dose was controlled using a VDEG2-34 SP-1 device for the detection and measurement of γ-rays. The energy range of the γ-radiation detection was varied within the 0.05–3 MeV. The single crystal was irradiated by doses 420, 1260, 2520, and 5040 Gy.
Fig. 1
Irradiation of the (Ga54.59In44.66Er0.75)2S300 single crystal by a 60Co source
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Additionally, we measured the radial distribution of γ-quanta of the source (dependence of the number of γ-quanta on the distance from the source center) in two mutually perpendicular directions. The exposure time for the γ-quanta detection was 300 s. Each measurement was repeated fivefold, and the number of γ-quanta was averaged. The measurement results are presented in Table 1.
Table 1
Radial distribution of γ-quanta for the 60Co radioactive source
Distance from the center (mm) | |||||
|---|---|---|---|---|---|
Direction | −4 | −2 | 0 | +2 | +4 |
X | 6315 | 6523 | 6768 | 6627 | 6555 |
Y | 6200 | 6468 | 6589 | 6393 | |
The highest intensity of γ-quanta emission is found in the source center and it was decreased gradually upon distancing. This fact may cause the non-uniform distribution of the radiation-induced defects in the crystal which particularly affects its non-linear optical properties.
3 Results and discussion
3.1 Photoluminescence
We investigated the visible and NIR emission spectra (Fig. 2a, b) of the original single crystal of (Ga54.59In44.66Er0.75)2S300 and the specimen irradiated with different doses of γ-rays at ambient temperature under laser excitation with 532 nm wavelength.
Fig. 2
Visible and NIR emission spectra of original and γ-irradiated (Ga54.59In44.66Er0.75)2S300 single crystal in a 600‒1000 nm, and b 1450‒1650 nm wavelength regions
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Strong emission bands are observed in the visible and near-IR regions, with maxima at 810 and 1540 nm which correspond to the radiation transitions of Erbium ions, 4I9/2 → 4I15/2, 4І13/2 → 4I15/2 respectively. The location and the shape of the emission spectra in all samples are the same, the intensities of both peaks increase at radiation dose 2500 Gy. With increasing the dose of irradiation till 5060 Gy, the intensity of the peak at 810 nm is almost unchanged (comparatively with the intensity of the photoluminescence of the single crystal specimen irradiated by dose 2500 Gy) (Fig. 2a), and the intensity of the maximum at 1540 nm slightly decreases (Fig. 2b).
The diagram of energy levels of the Er3+ ions (Fig. 3) was employed to understand the emission mechanism. The excitation with 532 nm wavelength boosts erbium ions from the ground state to 2H11/2.
Fig. 3
Diagram of energy levels in erbium ions
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Erbium ions cannot undergo non-radiative relaxation to 4І9/2 state which is responsible for the 810 nm emission due to high energy distance between the states 4S3/2–4F9/2 and 4F9/2–4І9/2. Therefore, the excitation of states 4I9/2 is resulted from the cross-relaxation process.
The cross-relaxation involves adjacent erbium ions in the states 2H11/2 and 4І15/2, after which the former ion transits to 4І9/2 state, and the other to 4І13/2 state according to the formula:
$$^{2}{\text{H}_{11/2}}{+^4}{\text{I}_{15/2}}{ \to ^4}{\text{I}_{9/2}}{+^4}{\text{I}_{13/2}}$$
(1)
The diagram (Fig. 3) shows that such mechanism results in large numbers of erbium ions in excited states 4I9/2, 4I13/2 which are involved in the luminescent emission. Additionally, we suggest that high radiation doses (2500, 5060 Gr) generate a substantial concentration of phonons with energy and momentum that are necessary for the cross-relaxation of adjacent erbium ions. Therefore luminescence intensity would increase at higher radiation doses, as observed in Fig. 2.
Unlike the single crystal that was investigated in this work, the glassy sulfide alloys [11, 12] exhibit many emission bands but with lower intensity. This is because erbium ions in glasses may occupy several positions, and each may provide photoluminescent radiation. Also, γ-irradiation of the glasses [17] changes the mechanism of the luminescence emission. Whereas, no significant changes in the luminescence spectra were found after γ-irradiation of the (Ga54.59In44.66Er0.75)2S300 single crystal, which determines its advantage in radiation resistance for the possible applications in optoelectronics.
3.2 Non-linear optical properties (third harmonic generation)
Contrary to the materials, which use only the photoluminescence in the present work we will try to propose some multi-functional materials. For instance, a material, which possesses additionally the high photoinduced nonlinear optical sensitivity. That means—it may be used at least two independent methods. The first tone was more traditional PL described above and the second one was connected with the photoinduced nonlinear optics using a photoinducing beams of the coherent beams with two coherent wavelengths at 1540 and 770 nm. The beams correspond to the fundamental and doubled frequency (due to second harmonic generation) of Er: glass 20 ns laser beam diameter about 3 mm and of the Gaussian like form with frequency repletion 10 Hz and wavelength 1540 nm using a scheme similar to the described in the Ref. [18]. We have found that for the titled crystal the optimal angle was varied within the degree and the process of the duration treatment up to 2–3 min and additionally the dc-electric field with frequency 50 Hz and voltage about 2 kV was applied.
As a consequence after the treatment there occurred the space grating. For the different radiation doses, they are shown in the Figs. 4 and 5. One can see that depending on the gamma radiation doses the general view of this picture were substantially different. First of all the space distribution of the diffracted beams were quite different. The shapes of the optical quasi-interferometer reflexes as well as their space frequencies also were quite different and the studied third harmonic generation for fundamental wavelengths 1540 nm have shown some correlation with the dose dependence the third harmonic maxima. So the laser induced bicolor coherent gratings may be very crucial here. And as for the case of photoluminescence (Fig. 2) the maximal value are obtained for the dose 2520 Gy. So one can say about the existence of some maxima in optoelectronic parameters versus the gamma radiation doses.
Fig. 4
Computationally reconstructed changes of the bicolor induced gratings at the same conditions for the 1540/770 nm bicolor coherent laser treatment for different doses: a 1250 Gy; b 2520 Gy; and c 5040 Gy. The detection was performed using the cw He–Ne laser beam at 1150 nm
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Fig. 5
Dependent of the third order optical susceptibilities versus the power density of the bicolor Er: glass laser excitations
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4 Conclusions
The novel type of the optoelectronics materials for sensing of gamma-radiation is proposed based on (Ga54.59In44.66Er0.75)2S300 single crystal. The strong emission bands detected at 810 and 1540 nm correspond to the radiation transitions 4I9/2 → 4I15/2, 4І13/2 → 4I15/2 of Erbium ions, respectively. At the same time use of the two bicolor treatment by Er: glass laser at 1540/770 nm of the THG have shown that the photoinduced nonlinear optical method gives higher sensitivity. The obtained data allow proposing the bicolor treated methods for detection of the gamma radiation as more efficient tools for radiation detection with respect to the traditional photoluminescent methods.
Acknowledgements
This research was supported by Ministry of Education and Science of Ukraine under project number 0116U004569.
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