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Effect of gamma radiation on thermostimulated exoelectron emission from Gd2O3 films
Nuclear Inst. and Methods in Physics Research B 463 (2020) 21–26
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Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effect of gamma radiation on thermostimulated exoelectron emission from Gd2O3 films
T
Marina Romanovaa, , Regina Burveb, Stanislav Cichonc, Yuri Dekhtyara, Ladislav Feketec, Daniels Jevdokimovsd, Aija Kruminab, Kristaps Palskise, Vera Sergab ⁎
a
Institute of Biomedical Engineering and Nanotechnology, Riga Technical University, Kipsalas Str. 6B, Riga LV-1048, Latvia Institute of Inorganic Chemistry, Riga Technical University, Paula Valdena Str. 7, Riga LV-1048, Latvia c Institute of Physics of the Czech Academy of Sciences, Na Slovance, 1999/2, 182 00 Prague 8, The Czech Republic d Institute of Chemical Physics, University of Latvia, Jelgavas Str. 1, Riga LV-1004, Latvia e Riga East University Hospital, Clinic of Therapeutic Radiology and Medical Physics, Hipokrata Str. 2, Riga LV-1038, Latvia b
ARTICLE INFO
ABSTRACT
Keywords: Gadolinium oxide Exoelectron emission Gamma radiation Dosimetry
The effect of gamma irradiation on Gd2O3 films was studied using the thermostimulated exoelectron emission (TSEE) technique. The films were deposited on a glass and Si/SiO2 substrates using an extraction-pyrolytic method. Crystalline structure, chemical composition, film thickness and surface morphology were characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The films were irradiated by 10 MeV gamma photons and TSEE was measured from the irradiated films. It was found that gamma irradiation decreases TSEE intensity and the area below TSEE spectral curves. A linear correlation between the relative decrease in the area and the delivered dose was observed in a dose range of 0–10 Gy. These findings suggest that gamma radiation might decrease the density of trapped electrons present in the as-grown Gd2O3 films or create competing electron traps that inhibit TSEE from the films.
1. Introduction
absorbed in a cell monolayer [15], studies of dose build-up regions or steep dose gradient dosimetry for radiation therapy [13]. A dosimeter with a small sensitive volume has to absorb incident radiation very effectively. The material from which the dosimeter is made should be commercially available. Gamma photons exploited in radiation therapy have MeV energy range. Therefore, the material of the dosimeter should be stable in time and its constituting atomic number should be as high as possible to increase attenuation of gamma rays. Lead atoms (Pb, atomic number 82) are good candidates. However, it is known that MeV radiation is capable of producing neutrons [16–18]. Therefore, atoms of the dosimeter should also absorb neutrons. In this case, Pb is not the most suitable material. Instead, gadolinium (Gd) has an atomic number of 64 and is highly efficient in neutron capture [19–22]. Gadolinium oxide (Gd2O3) is also well known as a very radiation resistant material [23,24]. In this study, Gd2O3 films were exploited as a model for dosimetry of high energy (MeV) gamma radiation. It is well known that Gd2O3 is a wide band gap insulator (~5.2 eV) [25] and therefore can contain Ftype centers (oxygen vacancies with trapped electrons) [26], which could serve as a source of TSEE [27]. Radiation defects and their roles
The exoelectron emission (EE) is a non-stationary low-temperature (below 750 K) emission of electrons from the surface of a material. The EE is a highly surface-sensitive phenomenon with the electron escape depth of 1–10 nm [1,2], and therefore it can be used to study electric charges near the surfaces of solids. If a material has an energy gap, the electrons are emitted from electron traps [3]. Thus, some energy has to be supplied to the electrons in order to release them from the traps. If this energy is supplied by heating, the process is called thermostimulated exoelectron emission (TSEE) [2]. The TSEE current nonmonotonically depends on the sample temperature [4], unlike the current of thermionic emission [5]. The TSEE has found its application in studies of electron trap centers produced by ionizing radiation, radiation-induced electrostatic charging of solids [6,7] and development of radiation dosimetry systems. It has been shown in many studies that TSEE current depends on dose of ionizing radiation [8–14]. Due to its high surface sensitivity, TSEE is promising in cases where a small sensitive volume is required. Examples of important applications include determination of doses
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Corresponding author. E-mail address: [email protected] (M. Romanova).
https://doi.org/10.1016/j.nimb.2019.11.016 Received 2 October 2019; Received in revised form 5 November 2019; Accepted 12 November 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. XRD patterns: (1) glass substrate; (2) Si/SiO2 substrate; (3) Gd2O3 film on the glass substrate; (4) Gd2O3 film on Si/SiO2 substrate.
Fig. 2. Survey spectra of the as-grown Gd2O3 film and after sputtering. The binding energy calibration is approximately 30.0 eV. Individual core levels are marked. Spectral features below 50.0 eV are ignored. The origin of the undulated character of the spectrum is plentiful Gd Auger peaks.
in TSEE have been discussed more than once [28–30].
nanopowders of cobalt and nickel ferrites, platinum nanoparticles on the carrier, MgO nanoparticles, ZnO, ZnO-CdO thin films and some others [32–35]. The method of fabricating Gd2O3 films consisted of the following steps: preparation of a Gd-based precursor (the extract), deposition of the extract on a substrate by a self-spreading method, thermal treatment (pyrolysis). Gd-based precursor was prepared by a liquid–liquid extraction method. Gd cations were extracted from an aqueous solution of Gd(NO3)3 using valeric acid (C4H9COOH) without diluent by adding
2. Materials and methods Gd2O3 films were deposited on insulating substrates by an extraction-pyrolytic method [31]. This method involves the use of extractive systems for production of oxide materials for various applications. Due to several advantages, such as simplicity, low cost and no need for sophisticated equipment, this method was successfully used to produce 22
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Fig. 3. High resolution spectra of the as-grown Gd2O3 film. The binding energy calibration is approximately 30.0 eV. Individual core levels are marked. Vertical lines denote peak maxima as a guide to the eye. Gd : O concentration ratio was calculated in CasaXPS using Gd 3d and O 1s peaks with Shirley background and Scofield’s relative sensitivity factors.
stoichiometric amounts of alkali. The resulting extract was diluted 4 times with decane, shaken up, and then a drop of the obtained emulsion was deposed on a substrate (0.010 mL per 1 cm2 of the substrate area). This was followed by thermal treatment of the sample in a muffle furnace: heating from 50° to 650 °C at a rate of 10 °C/min in atmospheric air, and then annealing for 15 min at 650 °C. After annealing, the sample was allowed to cool to room temperature in atmospheric air in the muffle furnace. Two types of the substrates were used for the film deposition – 0.35 mm thick oxidized silicon wafer (Si/SiO2) and 1 mm thick Chemland microscope glass slides. The substrates were cut into squares with a side of about 1 cm. In the case of Si/SiO2 substrate, 1 µm thick SiO2 was thermally grown on Si wafer surface at 1130 °C. The insulating materials were selected for the substrates in order to prevent possible leakage of the electric charge which could form in Gd2O3 films during gamma irradiation. The deposited films were analyzed by XRD technique using a D-8 Advance (Bruker AXS) diffractometer with CuKα radiation (λ = 1.5418 Å) in a wide range of Bragg angles (10° < 2θ < 75°), with a scanning rate of 0.005°/s and at room temperature. The XRD
Fig. 4. A cross-sectional SEM image of Gd2O3 film on Si/SiO2 substrate.
Fig. 5. AFM image of Gd2O3 film on Si/SiO2 substrate and its diagonal cross-section. 23
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in such a way that the bulk part of the sample material could be probed and compared with the surface. Roughly, a thickness of 10 nm was sputtered. The Ar sputter gun is located in the preparation chamber of the NanoESCA instrument. The parameters of the sputtering were as follows: energy of ions 3 keV, Ar pressure 2 × 10−4 Pa, the angle between the ion beam and the surface normal was 15°. We were aware of the fact that the sputtering may induce damage, therefore the samples used in the XPS analysis were not used in further TSEE experiments. The thickness of the films was measured using a Hitachi S-4800 scanning electron microscope. To measure the thickness, the sample was cut in half using a scriber with a tungsten carbide tip and mounted perpendicular to the electron beam. The thickness was measured at several positions along the cut line. The surface morphology of the films was studied by the AFM technique using a Bruker Dimension Icon microscope and Bruker ScanAsyst-Air probes with a tip radius of 2 nm. The processing of the obtained AFM data was performed in Gwyddion software. The films were irradiated with 10 MeV gamma radiation provided by medical linear accelerators (LINAC). The films were positioned on a LINAC’s table at a source-to-skin distance of 100 cm. The radiation field size was 10 × 10 cm. The delivered doses ranged from 5 to 100 Gy at the film surface. The films deposited on Si/SiO2 substrate were irradiated with a dose rate of 6 Gy/min using a Varian Clinac 2100 C/D linear accelerator. The films deposited on the glass substrate were irradiated with a dose rate of 24 Gy/min using a Varian TrueBeam linear accelerator in a 10×FFF (flattening filter free) mode. After irradiation, the samples were placed in a plastic box, the box was wrapped into aluminium foil and delivered for TSEE measurements. TSEE measurements of the unirradiated and gamma-irradiated films were carried out in a home-made EE spectrometer in a vacuum 10−3 Pa. During measurements, the samples were heated from the room temperature to 550 °C with the rate of 10 °C/min. The emitted electrons were detected using a SEM-6M (VTC Baspik, Russia) secondary electron multiplier connected to a Robotron 20046 radiometer and a Hamamatsu Photonics M8784 counting board.
Fig. 6. TSEE spectra of the unirradiated and with 100 Gy of gamma rays irradiated Gd2O3 films on Si/SiO2 and glass substrate.
3. Results and discussion XRD (Fig. 1) demonstrated that Gd2O3 in the deposited films had predominant orientation along the (4 0 0) crystallographic plane and cubic structure at room temperature. The average size of the crystallites was about 9 nm on Si/SiO2 substrate and 11 nm on the glass substrate. Fig. 2 presents an XPS survey spectrum of the as-grown sample surface and also a spectrum after the sample was sputtered with Ar+ ions. We note that as-measured spectra were shifted roughly by 30.0 eV to higher binding energies. This was caused by film charging due to insufficient electrical conductivity of the sample. The instrument does not provide electrical compensation. The value of the shift was estimated based on the expected positions for O 1s core levels in Gd oxides (531.0 eV). Fortunately, the weak charging did not completely impede reasonable identification of the principal spectral features. The spectral features of Gd and O were identified. However, full width at half maxima (FWHM) of the individual core levels reaching unnaturally high values and distorted peak shapes prevented deeper analysis of the chemical shifts and states. For the sputtered samples, the ratio between Gd 3d and O 1s peak areas slightly changed in comparison with the surface, also the peaks appeared to reach lower FWHM. High resolution spectra of the as-grown sample surface are presented in Fig. 3. We were not able to identify carbon, usually found as adventitious contamination on as-grown surfaces [36,38,39]. Its core level, with the maximum typically at 285.0 eV, is located within the spectral region of a strong Gd 4p signal but this cannot represent a condition of C 1s being overshadowed by Gd 4p. A likely explanation is that its signal is weak and smeared due to the high FWHMs observed. Regarding the positions or the intensity maxima of Gd 4d core levels, these are relatively close to the positions reported for Gd oxides in
Fig. 7. Relative decrease in the area under TSEE curves of Gd2O3 films deposited on different substrates and irradiated with different dose rates. The decrease in the area was calculated in the temperature range above 400 °C for the films on Si/SiO2 substrate, and above 450 °C for the films on the glass substrate.
patterns were referenced to the PDF 00-012-0797 for identification of Gd2O3. The mean size of Gd2O3 crystallites in the films was defined from the width of (4 0 0) peak by the Scherrer method. Chemical composition of the samples was characterized by XPS technique using a NanoESCA instrument from Omicron Nanotechnology. An Al anode K alpha radiation was used as the X-ray source. The size of the analyzed spot was 100 × 300 µm and several positions on the sample surface were probed. The energy resolution was 0.5 eV for the survey spectra and 0.1 eV for the high resolution spectra. The electron transmission function of the instrument is unknown. Immediately before insertion into the XPS chamber, the samples were rinsed with acetone and blown dry with nitrogen. The obtained spectra were processed in CasaXPS software. Several XPS databases were used for the identification and interpretation of the observed spectral features [36–38]. Furthermore, a single Ar+ sputtering step was performed 24
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literature [40–42]. On the other hand, Gd 3d core levels are found about 2 eV higher. In literature, O 1s core level is reported to be composed of more than one component; there are components attributed to lattice oxygen in the Gd oxide, surface hydroxyl groups and Gd carbonates [40–42]. In our study, O 1s forms a single broad component of questionable elucidation. As we stated in the earlier paragraphs, these effects arise from the charging. Calculation of the relative concentration for Gd:O gives 2:3, corresponding to the composition Gd2O3. The film thickness estimated by the SEM technique ranged from 700 to 900 nm. A typical cross-sectional SEM image of the film on Si/SiO2 substrate is shown in Fig. 4. The AFM image of the surface of Gd2O3 film on Si/SiO2 substrate and its diagonal cross-section is shown in Fig. 5. The average surface roughness (Ra) and the root mean square roughness (Rq) were calculated from AFM images of 1 × 1 µm size and 512 × 512 data points using the Gwyddion software, and were equal to 0.9 nm and 1.1 nm, respectively. The AFM results demonstrate that the extraction-pyrolytic method allows to obtain Gd2O3 films with a relatively smooth surface if the surface of the substrate is smooth (Ra value of Si/SiO2 substrate measured by AFM was equal to 0.15 nm). TSEE spectra of the unirradiated and with 100 Gy of gamma rays irradiated Gd2O3 films on Si/SiO2 and glass substrates are shown in Fig. 6. The TSEE detected from the unirradiated films evidence that the as-grown films have trap centers filled with electrons. The appearance of new TSEE peaks after the irradiation was not observed. However, gamma irradiation decreased TSEE intensity in the temperature range above 400 °C in the case of the films on Si/SiO2 substrate and in the range above 450 °C in the case of the films on the glass substrate. It should also be noted that insignificant TSEE peaks were observed at 330 °C for the irradiated films on Si/SiO2 substrate and at 420 °C for the irradiated films on the glass substrate. We associate these peaks with the measurement noise of the equipment since the intensity of these peaks is low (3 electrons/sec) and there is always a probability that some noise pulses can cluster, forming insignificant peaks. Also, the electron emission process is described using the Poisson statistics [43]. In this case, the standard deviation of the measured emission current is equal to the square root of its intensity [44]. For the TSEE intensity of 3 electrons/sec, this gives a relative error of 57.7%, which does not allow us to say that the observed peaks appeared as a result of gamma irradiation. Next, the area under TSEE curves was analysed. The area represents the total number of the exoelectrons emitted from the traps in Gd2O3. The area under the TSEE curves decreases for the gamma-irradiated films compared to the unirradiated films. For the films on Si/SiO2 substrate, the relative decrease was calculated in the temperature range above 400 °C. For the films on the glass substrate, the relative decrease was calculated in the range above 450 °C. The relative decrease as a function of the irradiation dose is shown in Fig. 7 for the films on both substrates irradiated with different dose rates. The area decreased for all irradiation doses. The character of the decrease was similar for the films on different substrates and did not depend on the dose rate. The linear decrease in the area was observed in the range of 0–10 Gy. The decrease in the area suggests that gamma radiation decreases the density of trapped electrons present in the as-grown Gd2O3 films or creates competing electron traps that inhibited TSEE from the films. The observed results relate to the near-surface effects of gamma irradiation, since the exoelectrons are emitted only from a thin surface layer. The TSEE occurs only from a thin surface layer due to the fact that to overcome the potential barrier, the exoelectrons cannot lose their energy on phonon excitation.
suggests that gamma radiation decreases the density of trapped electrons present in the as-grown Gd2O3 films or creates competing electron traps that inhibit TSEE from the films. The area below TSEE curves was analysed in the temperature range above 400 °C for the films on Si/SiO2 substrate and above 450 °C for the films on glass substrate. Linear correlation takes place between the relative decrease in the area after irradiation and the dose of gamma photons, at least in the dose range of 0–10 Gy. The observed results are attributed to the near-surface effects of the gamma irradiation since the exoelectrons are emitted only from a near-surface area. The findings of this study suggest that Gd2O3 films are promising for use in dosimetry of small sensitive volumes; however, additional metrological calibration is still required. Acknowledgements This work was supported by the European Regional Development Fund within the project No.1.1.1.2/VIAA/1/16/167 “Thin films with embedded nanoparticles for dosimetry of ionizing radiation”. References [1] W.J. Baxter, Exoelectron emission, in: H. Herman (Ed.), Treatise on Materials Science & Technology: Experimental Methods Part B, Vol.19 Academic Press Inc., New York, 1983, p. 274. [2] G. Holzapfel, Thermionic emission from electron traps, Vacuum 22 (1972) 467. [3] A.F. Zatsepin, D.Y. Biryukov, V.S. Kortov, Method for the analysis of nonselective spectra of optically stimulated electron emission from irradiated dielectrics, Phys. Status Solidi A 202 (2005) 1935. [4] Yu.D. Dekhtyar, Yu.A. Vinyarskaya, Exoelectron analysis of amorphous silicon, J. Appl. Phys. 75 (1994) 4201. [5] O.W. Richardson, Thermionic phenomena and the laws which govern them, Nobel Lecture, December, 12, 1929, https://www.nobelprize.org/prizes/physics/1928/ richardson/lecture/. [6] E. Savchenko, I. Khyzhniy, S. Uyutnov, M. Bludov, A. Barabashov, G. Gumenchuk, V. Bondybey, Radiation effects in nitrogen and methane “ices”, Nucl. Instrum. Methods Phys. Res. Sect. B 435 (2018) 38. [7] E. Savchenko, I. Khyzhniy, S. Uyutnov, M. Bludov, G. Gumenchuk, V. Bondybey, Effects induced by electron beam in methane ices, Nucl. Instrum. Methods Phys. Res. Sect. B (2018) in press. [8] A. Scharmann, W. Kriegseis, Exoelectron emission, Tribol. Ser. 7 (1981) 489. [9] G.G. Eichholz, Principles of Nuclear Radiation Detection, CRC Press, New York, 2017, p. 391. [10] E.V. Savchenko, O. Kirichek, C.R. Lawson, I.V. Khyzhniy, S.A. Uyutnov, M.A. Bludov, Relaxation processes in solid methane pre-irradiated with an electron beam, Nucl. Instrum. Methods Phys. Res. Sect. B 433 (2018) 23. [11] M. Autzen, N.R.J. Poolton, A.S. Murray, M. Kook, J.-P. Buylaert, A new automated system for combined luminescence and exo-electron measurements, Nucl. Instrum. Methods Phys. Res. Sect. B 443 (2019) 90. [12] P.L. Antonio, L.V.E. Caldas, Applying the TSEE technique to spectrolite and opal pellets irradiated with high doses of gamma radiation, Radiat. Meas. 106 (2017) 538. [13] Yu. Dekhtyar, Emission of weak electrons: dosimetry of nanovolumes, Radiat. Meas. 55 (2013) 34. [14] F.D.G. Rocha, S.G.P. Cecatti, L.V.E. Cadas, Dosimetric characterisation of Brazilian natural stones using the thermally stimulated exoelectron emission technique, Radiat. Prot. Dosim. 100 (2002) 417. [15] A. Lehnert, E. Beyreuther, E. Lessmann, J. Pawelke, Investigation of a TSEE dosimetry system for determination of dose in a cell monolayer, Radiat. Meas. 42 (2007) 1530. [16] A. Naseri, A. Mesbahi, A review on photoneutrons characteristics in radiation therapy with high-energy photon beams, Rep. Pract. Oncol. Radiother. 15 (2010) 138. [17] F. Horst, D. Czarnecki, D. Harder, K. Zink, The absorbed doses to water and the TLD-100 signal contributions associated with the neutron contamination of a clinical 18 MV photon beam, Radiat. Meas. 106 (2017) 331. [18] K. Polaczek-Grelik, A. Kawa-Iwanicka, M. Rygielski, and Ł. Michalecki, Gamma Radiation in the Vicinity of the Entrance to Linac Radiotherapy Room. In Use of Gamma Radiation Techniques in Peaceful Applications (IntechOpen, 2019). [19] J. Kopecky, J.C. Sublet, J.A. Simpson, R.A. Forrest, and D. Nierop, Atlas of neutron capture cross sections, No. INDC (NDS)–362. (International Atomic Energy Agency, 1997). [20] A.C. Rivera, N.N. Glazener, N.C. Cook, B.A. Akins, J.B. Plumley, N.J. Withers, K. Carpenter, G.A. Smolyakov, R.D. Busch, M. Osinski, Detection of thermal neutrons using gadolinium-oxide-based nanocrystals, Proc. SPIE-Int. Soc. Opt. Eng. 8018 (2011) 80180F. [21] D.R. McAlister, Neutron Shielding Materials, Revision 2.1 (2016), https://www. eichrom.com/wp-content/uploads/2018/02/neutron-attenuation-white-paper-byd-m-rev-2-1.pdf. [22] A.I. Popov, J. Zimmermann, G.J. McIntyre, C. Wilkinson, Photostimulated
4. Conclusions The extractive-pyrolytic method makes it possible to deposit nanocrystalline Gd2O3 films with the average crystallite size of 9–11 nm on insulating substrates. TSEE current from the films decreases after irradiation with 10 MeV gamma photons. The decrease in TSEE current 25
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Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effect of gamma radiation on thermostimulated exoelectron emission from Gd2O3 films
T
Marina Romanovaa, , Regina Burveb, Stanislav Cichonc, Yuri Dekhtyara, Ladislav Feketec, Daniels Jevdokimovsd, Aija Kruminab, Kristaps Palskise, Vera Sergab ⁎
a
Institute of Biomedical Engineering and Nanotechnology, Riga Technical University, Kipsalas Str. 6B, Riga LV-1048, Latvia Institute of Inorganic Chemistry, Riga Technical University, Paula Valdena Str. 7, Riga LV-1048, Latvia c Institute of Physics of the Czech Academy of Sciences, Na Slovance, 1999/2, 182 00 Prague 8, The Czech Republic d Institute of Chemical Physics, University of Latvia, Jelgavas Str. 1, Riga LV-1004, Latvia e Riga East University Hospital, Clinic of Therapeutic Radiology and Medical Physics, Hipokrata Str. 2, Riga LV-1038, Latvia b
ARTICLE INFO
ABSTRACT
Keywords: Gadolinium oxide Exoelectron emission Gamma radiation Dosimetry
The effect of gamma irradiation on Gd2O3 films was studied using the thermostimulated exoelectron emission (TSEE) technique. The films were deposited on a glass and Si/SiO2 substrates using an extraction-pyrolytic method. Crystalline structure, chemical composition, film thickness and surface morphology were characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The films were irradiated by 10 MeV gamma photons and TSEE was measured from the irradiated films. It was found that gamma irradiation decreases TSEE intensity and the area below TSEE spectral curves. A linear correlation between the relative decrease in the area and the delivered dose was observed in a dose range of 0–10 Gy. These findings suggest that gamma radiation might decrease the density of trapped electrons present in the as-grown Gd2O3 films or create competing electron traps that inhibit TSEE from the films.
1. Introduction
absorbed in a cell monolayer [15], studies of dose build-up regions or steep dose gradient dosimetry for radiation therapy [13]. A dosimeter with a small sensitive volume has to absorb incident radiation very effectively. The material from which the dosimeter is made should be commercially available. Gamma photons exploited in radiation therapy have MeV energy range. Therefore, the material of the dosimeter should be stable in time and its constituting atomic number should be as high as possible to increase attenuation of gamma rays. Lead atoms (Pb, atomic number 82) are good candidates. However, it is known that MeV radiation is capable of producing neutrons [16–18]. Therefore, atoms of the dosimeter should also absorb neutrons. In this case, Pb is not the most suitable material. Instead, gadolinium (Gd) has an atomic number of 64 and is highly efficient in neutron capture [19–22]. Gadolinium oxide (Gd2O3) is also well known as a very radiation resistant material [23,24]. In this study, Gd2O3 films were exploited as a model for dosimetry of high energy (MeV) gamma radiation. It is well known that Gd2O3 is a wide band gap insulator (~5.2 eV) [25] and therefore can contain Ftype centers (oxygen vacancies with trapped electrons) [26], which could serve as a source of TSEE [27]. Radiation defects and their roles
The exoelectron emission (EE) is a non-stationary low-temperature (below 750 K) emission of electrons from the surface of a material. The EE is a highly surface-sensitive phenomenon with the electron escape depth of 1–10 nm [1,2], and therefore it can be used to study electric charges near the surfaces of solids. If a material has an energy gap, the electrons are emitted from electron traps [3]. Thus, some energy has to be supplied to the electrons in order to release them from the traps. If this energy is supplied by heating, the process is called thermostimulated exoelectron emission (TSEE) [2]. The TSEE current nonmonotonically depends on the sample temperature [4], unlike the current of thermionic emission [5]. The TSEE has found its application in studies of electron trap centers produced by ionizing radiation, radiation-induced electrostatic charging of solids [6,7] and development of radiation dosimetry systems. It has been shown in many studies that TSEE current depends on dose of ionizing radiation [8–14]. Due to its high surface sensitivity, TSEE is promising in cases where a small sensitive volume is required. Examples of important applications include determination of doses
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Corresponding author. E-mail address: [email protected] (M. Romanova).
https://doi.org/10.1016/j.nimb.2019.11.016 Received 2 October 2019; Received in revised form 5 November 2019; Accepted 12 November 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. XRD patterns: (1) glass substrate; (2) Si/SiO2 substrate; (3) Gd2O3 film on the glass substrate; (4) Gd2O3 film on Si/SiO2 substrate.
Fig. 2. Survey spectra of the as-grown Gd2O3 film and after sputtering. The binding energy calibration is approximately 30.0 eV. Individual core levels are marked. Spectral features below 50.0 eV are ignored. The origin of the undulated character of the spectrum is plentiful Gd Auger peaks.
in TSEE have been discussed more than once [28–30].
nanopowders of cobalt and nickel ferrites, platinum nanoparticles on the carrier, MgO nanoparticles, ZnO, ZnO-CdO thin films and some others [32–35]. The method of fabricating Gd2O3 films consisted of the following steps: preparation of a Gd-based precursor (the extract), deposition of the extract on a substrate by a self-spreading method, thermal treatment (pyrolysis). Gd-based precursor was prepared by a liquid–liquid extraction method. Gd cations were extracted from an aqueous solution of Gd(NO3)3 using valeric acid (C4H9COOH) without diluent by adding
2. Materials and methods Gd2O3 films were deposited on insulating substrates by an extraction-pyrolytic method [31]. This method involves the use of extractive systems for production of oxide materials for various applications. Due to several advantages, such as simplicity, low cost and no need for sophisticated equipment, this method was successfully used to produce 22
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Fig. 3. High resolution spectra of the as-grown Gd2O3 film. The binding energy calibration is approximately 30.0 eV. Individual core levels are marked. Vertical lines denote peak maxima as a guide to the eye. Gd : O concentration ratio was calculated in CasaXPS using Gd 3d and O 1s peaks with Shirley background and Scofield’s relative sensitivity factors.
stoichiometric amounts of alkali. The resulting extract was diluted 4 times with decane, shaken up, and then a drop of the obtained emulsion was deposed on a substrate (0.010 mL per 1 cm2 of the substrate area). This was followed by thermal treatment of the sample in a muffle furnace: heating from 50° to 650 °C at a rate of 10 °C/min in atmospheric air, and then annealing for 15 min at 650 °C. After annealing, the sample was allowed to cool to room temperature in atmospheric air in the muffle furnace. Two types of the substrates were used for the film deposition – 0.35 mm thick oxidized silicon wafer (Si/SiO2) and 1 mm thick Chemland microscope glass slides. The substrates were cut into squares with a side of about 1 cm. In the case of Si/SiO2 substrate, 1 µm thick SiO2 was thermally grown on Si wafer surface at 1130 °C. The insulating materials were selected for the substrates in order to prevent possible leakage of the electric charge which could form in Gd2O3 films during gamma irradiation. The deposited films were analyzed by XRD technique using a D-8 Advance (Bruker AXS) diffractometer with CuKα radiation (λ = 1.5418 Å) in a wide range of Bragg angles (10° < 2θ < 75°), with a scanning rate of 0.005°/s and at room temperature. The XRD
Fig. 4. A cross-sectional SEM image of Gd2O3 film on Si/SiO2 substrate.
Fig. 5. AFM image of Gd2O3 film on Si/SiO2 substrate and its diagonal cross-section. 23
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in such a way that the bulk part of the sample material could be probed and compared with the surface. Roughly, a thickness of 10 nm was sputtered. The Ar sputter gun is located in the preparation chamber of the NanoESCA instrument. The parameters of the sputtering were as follows: energy of ions 3 keV, Ar pressure 2 × 10−4 Pa, the angle between the ion beam and the surface normal was 15°. We were aware of the fact that the sputtering may induce damage, therefore the samples used in the XPS analysis were not used in further TSEE experiments. The thickness of the films was measured using a Hitachi S-4800 scanning electron microscope. To measure the thickness, the sample was cut in half using a scriber with a tungsten carbide tip and mounted perpendicular to the electron beam. The thickness was measured at several positions along the cut line. The surface morphology of the films was studied by the AFM technique using a Bruker Dimension Icon microscope and Bruker ScanAsyst-Air probes with a tip radius of 2 nm. The processing of the obtained AFM data was performed in Gwyddion software. The films were irradiated with 10 MeV gamma radiation provided by medical linear accelerators (LINAC). The films were positioned on a LINAC’s table at a source-to-skin distance of 100 cm. The radiation field size was 10 × 10 cm. The delivered doses ranged from 5 to 100 Gy at the film surface. The films deposited on Si/SiO2 substrate were irradiated with a dose rate of 6 Gy/min using a Varian Clinac 2100 C/D linear accelerator. The films deposited on the glass substrate were irradiated with a dose rate of 24 Gy/min using a Varian TrueBeam linear accelerator in a 10×FFF (flattening filter free) mode. After irradiation, the samples were placed in a plastic box, the box was wrapped into aluminium foil and delivered for TSEE measurements. TSEE measurements of the unirradiated and gamma-irradiated films were carried out in a home-made EE spectrometer in a vacuum 10−3 Pa. During measurements, the samples were heated from the room temperature to 550 °C with the rate of 10 °C/min. The emitted electrons were detected using a SEM-6M (VTC Baspik, Russia) secondary electron multiplier connected to a Robotron 20046 radiometer and a Hamamatsu Photonics M8784 counting board.
Fig. 6. TSEE spectra of the unirradiated and with 100 Gy of gamma rays irradiated Gd2O3 films on Si/SiO2 and glass substrate.
3. Results and discussion XRD (Fig. 1) demonstrated that Gd2O3 in the deposited films had predominant orientation along the (4 0 0) crystallographic plane and cubic structure at room temperature. The average size of the crystallites was about 9 nm on Si/SiO2 substrate and 11 nm on the glass substrate. Fig. 2 presents an XPS survey spectrum of the as-grown sample surface and also a spectrum after the sample was sputtered with Ar+ ions. We note that as-measured spectra were shifted roughly by 30.0 eV to higher binding energies. This was caused by film charging due to insufficient electrical conductivity of the sample. The instrument does not provide electrical compensation. The value of the shift was estimated based on the expected positions for O 1s core levels in Gd oxides (531.0 eV). Fortunately, the weak charging did not completely impede reasonable identification of the principal spectral features. The spectral features of Gd and O were identified. However, full width at half maxima (FWHM) of the individual core levels reaching unnaturally high values and distorted peak shapes prevented deeper analysis of the chemical shifts and states. For the sputtered samples, the ratio between Gd 3d and O 1s peak areas slightly changed in comparison with the surface, also the peaks appeared to reach lower FWHM. High resolution spectra of the as-grown sample surface are presented in Fig. 3. We were not able to identify carbon, usually found as adventitious contamination on as-grown surfaces [36,38,39]. Its core level, with the maximum typically at 285.0 eV, is located within the spectral region of a strong Gd 4p signal but this cannot represent a condition of C 1s being overshadowed by Gd 4p. A likely explanation is that its signal is weak and smeared due to the high FWHMs observed. Regarding the positions or the intensity maxima of Gd 4d core levels, these are relatively close to the positions reported for Gd oxides in
Fig. 7. Relative decrease in the area under TSEE curves of Gd2O3 films deposited on different substrates and irradiated with different dose rates. The decrease in the area was calculated in the temperature range above 400 °C for the films on Si/SiO2 substrate, and above 450 °C for the films on the glass substrate.
patterns were referenced to the PDF 00-012-0797 for identification of Gd2O3. The mean size of Gd2O3 crystallites in the films was defined from the width of (4 0 0) peak by the Scherrer method. Chemical composition of the samples was characterized by XPS technique using a NanoESCA instrument from Omicron Nanotechnology. An Al anode K alpha radiation was used as the X-ray source. The size of the analyzed spot was 100 × 300 µm and several positions on the sample surface were probed. The energy resolution was 0.5 eV for the survey spectra and 0.1 eV for the high resolution spectra. The electron transmission function of the instrument is unknown. Immediately before insertion into the XPS chamber, the samples were rinsed with acetone and blown dry with nitrogen. The obtained spectra were processed in CasaXPS software. Several XPS databases were used for the identification and interpretation of the observed spectral features [36–38]. Furthermore, a single Ar+ sputtering step was performed 24
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literature [40–42]. On the other hand, Gd 3d core levels are found about 2 eV higher. In literature, O 1s core level is reported to be composed of more than one component; there are components attributed to lattice oxygen in the Gd oxide, surface hydroxyl groups and Gd carbonates [40–42]. In our study, O 1s forms a single broad component of questionable elucidation. As we stated in the earlier paragraphs, these effects arise from the charging. Calculation of the relative concentration for Gd:O gives 2:3, corresponding to the composition Gd2O3. The film thickness estimated by the SEM technique ranged from 700 to 900 nm. A typical cross-sectional SEM image of the film on Si/SiO2 substrate is shown in Fig. 4. The AFM image of the surface of Gd2O3 film on Si/SiO2 substrate and its diagonal cross-section is shown in Fig. 5. The average surface roughness (Ra) and the root mean square roughness (Rq) were calculated from AFM images of 1 × 1 µm size and 512 × 512 data points using the Gwyddion software, and were equal to 0.9 nm and 1.1 nm, respectively. The AFM results demonstrate that the extraction-pyrolytic method allows to obtain Gd2O3 films with a relatively smooth surface if the surface of the substrate is smooth (Ra value of Si/SiO2 substrate measured by AFM was equal to 0.15 nm). TSEE spectra of the unirradiated and with 100 Gy of gamma rays irradiated Gd2O3 films on Si/SiO2 and glass substrates are shown in Fig. 6. The TSEE detected from the unirradiated films evidence that the as-grown films have trap centers filled with electrons. The appearance of new TSEE peaks after the irradiation was not observed. However, gamma irradiation decreased TSEE intensity in the temperature range above 400 °C in the case of the films on Si/SiO2 substrate and in the range above 450 °C in the case of the films on the glass substrate. It should also be noted that insignificant TSEE peaks were observed at 330 °C for the irradiated films on Si/SiO2 substrate and at 420 °C for the irradiated films on the glass substrate. We associate these peaks with the measurement noise of the equipment since the intensity of these peaks is low (3 electrons/sec) and there is always a probability that some noise pulses can cluster, forming insignificant peaks. Also, the electron emission process is described using the Poisson statistics [43]. In this case, the standard deviation of the measured emission current is equal to the square root of its intensity [44]. For the TSEE intensity of 3 electrons/sec, this gives a relative error of 57.7%, which does not allow us to say that the observed peaks appeared as a result of gamma irradiation. Next, the area under TSEE curves was analysed. The area represents the total number of the exoelectrons emitted from the traps in Gd2O3. The area under the TSEE curves decreases for the gamma-irradiated films compared to the unirradiated films. For the films on Si/SiO2 substrate, the relative decrease was calculated in the temperature range above 400 °C. For the films on the glass substrate, the relative decrease was calculated in the range above 450 °C. The relative decrease as a function of the irradiation dose is shown in Fig. 7 for the films on both substrates irradiated with different dose rates. The area decreased for all irradiation doses. The character of the decrease was similar for the films on different substrates and did not depend on the dose rate. The linear decrease in the area was observed in the range of 0–10 Gy. The decrease in the area suggests that gamma radiation decreases the density of trapped electrons present in the as-grown Gd2O3 films or creates competing electron traps that inhibited TSEE from the films. The observed results relate to the near-surface effects of gamma irradiation, since the exoelectrons are emitted only from a thin surface layer. The TSEE occurs only from a thin surface layer due to the fact that to overcome the potential barrier, the exoelectrons cannot lose their energy on phonon excitation.
suggests that gamma radiation decreases the density of trapped electrons present in the as-grown Gd2O3 films or creates competing electron traps that inhibit TSEE from the films. The area below TSEE curves was analysed in the temperature range above 400 °C for the films on Si/SiO2 substrate and above 450 °C for the films on glass substrate. Linear correlation takes place between the relative decrease in the area after irradiation and the dose of gamma photons, at least in the dose range of 0–10 Gy. The observed results are attributed to the near-surface effects of the gamma irradiation since the exoelectrons are emitted only from a near-surface area. The findings of this study suggest that Gd2O3 films are promising for use in dosimetry of small sensitive volumes; however, additional metrological calibration is still required. Acknowledgements This work was supported by the European Regional Development Fund within the project No.1.1.1.2/VIAA/1/16/167 “Thin films with embedded nanoparticles for dosimetry of ionizing radiation”. References [1] W.J. Baxter, Exoelectron emission, in: H. Herman (Ed.), Treatise on Materials Science & Technology: Experimental Methods Part B, Vol.19 Academic Press Inc., New York, 1983, p. 274. [2] G. Holzapfel, Thermionic emission from electron traps, Vacuum 22 (1972) 467. [3] A.F. Zatsepin, D.Y. Biryukov, V.S. Kortov, Method for the analysis of nonselective spectra of optically stimulated electron emission from irradiated dielectrics, Phys. Status Solidi A 202 (2005) 1935. [4] Yu.D. Dekhtyar, Yu.A. Vinyarskaya, Exoelectron analysis of amorphous silicon, J. Appl. Phys. 75 (1994) 4201. [5] O.W. Richardson, Thermionic phenomena and the laws which govern them, Nobel Lecture, December, 12, 1929, https://www.nobelprize.org/prizes/physics/1928/ richardson/lecture/. [6] E. Savchenko, I. Khyzhniy, S. Uyutnov, M. Bludov, A. Barabashov, G. Gumenchuk, V. Bondybey, Radiation effects in nitrogen and methane “ices”, Nucl. Instrum. Methods Phys. Res. Sect. B 435 (2018) 38. [7] E. Savchenko, I. Khyzhniy, S. Uyutnov, M. Bludov, G. Gumenchuk, V. Bondybey, Effects induced by electron beam in methane ices, Nucl. Instrum. Methods Phys. Res. Sect. B (2018) in press. [8] A. Scharmann, W. Kriegseis, Exoelectron emission, Tribol. Ser. 7 (1981) 489. [9] G.G. Eichholz, Principles of Nuclear Radiation Detection, CRC Press, New York, 2017, p. 391. [10] E.V. Savchenko, O. Kirichek, C.R. Lawson, I.V. Khyzhniy, S.A. Uyutnov, M.A. Bludov, Relaxation processes in solid methane pre-irradiated with an electron beam, Nucl. Instrum. Methods Phys. Res. Sect. B 433 (2018) 23. [11] M. Autzen, N.R.J. Poolton, A.S. Murray, M. Kook, J.-P. Buylaert, A new automated system for combined luminescence and exo-electron measurements, Nucl. Instrum. Methods Phys. Res. Sect. B 443 (2019) 90. [12] P.L. Antonio, L.V.E. Caldas, Applying the TSEE technique to spectrolite and opal pellets irradiated with high doses of gamma radiation, Radiat. Meas. 106 (2017) 538. [13] Yu. Dekhtyar, Emission of weak electrons: dosimetry of nanovolumes, Radiat. Meas. 55 (2013) 34. [14] F.D.G. Rocha, S.G.P. Cecatti, L.V.E. Cadas, Dosimetric characterisation of Brazilian natural stones using the thermally stimulated exoelectron emission technique, Radiat. Prot. Dosim. 100 (2002) 417. [15] A. Lehnert, E. Beyreuther, E. Lessmann, J. Pawelke, Investigation of a TSEE dosimetry system for determination of dose in a cell monolayer, Radiat. Meas. 42 (2007) 1530. [16] A. Naseri, A. Mesbahi, A review on photoneutrons characteristics in radiation therapy with high-energy photon beams, Rep. Pract. Oncol. Radiother. 15 (2010) 138. [17] F. Horst, D. Czarnecki, D. Harder, K. Zink, The absorbed doses to water and the TLD-100 signal contributions associated with the neutron contamination of a clinical 18 MV photon beam, Radiat. Meas. 106 (2017) 331. [18] K. Polaczek-Grelik, A. Kawa-Iwanicka, M. Rygielski, and Ł. Michalecki, Gamma Radiation in the Vicinity of the Entrance to Linac Radiotherapy Room. In Use of Gamma Radiation Techniques in Peaceful Applications (IntechOpen, 2019). [19] J. Kopecky, J.C. Sublet, J.A. Simpson, R.A. Forrest, and D. Nierop, Atlas of neutron capture cross sections, No. INDC (NDS)–362. (International Atomic Energy Agency, 1997). [20] A.C. Rivera, N.N. Glazener, N.C. Cook, B.A. Akins, J.B. Plumley, N.J. Withers, K. Carpenter, G.A. Smolyakov, R.D. Busch, M. Osinski, Detection of thermal neutrons using gadolinium-oxide-based nanocrystals, Proc. SPIE-Int. Soc. Opt. Eng. 8018 (2011) 80180F. [21] D.R. McAlister, Neutron Shielding Materials, Revision 2.1 (2016), https://www. eichrom.com/wp-content/uploads/2018/02/neutron-attenuation-white-paper-byd-m-rev-2-1.pdf. [22] A.I. Popov, J. Zimmermann, G.J. McIntyre, C. Wilkinson, Photostimulated
4. Conclusions The extractive-pyrolytic method makes it possible to deposit nanocrystalline Gd2O3 films with the average crystallite size of 9–11 nm on insulating substrates. TSEE current from the films decreases after irradiation with 10 MeV gamma photons. The decrease in TSEE current 25
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