Luminescence of X-ray-Irradiated Single Crystals under the Influence of Compression

Abstract

Analyzing the data on bleaching with the F-light at the excitation bandpass 5 or 20 nm, it was found that the absorption spectrum of KCl:Eu²⁺ (0.1 mol%) single crystals exposed to X-ray has a peak due to F_z-centre within the wavelength 20 nm near F-centre peak. Deforming the X-ray-irradiated KCl:Eu²⁺ (0.02 mol%) single crystal by the compression at 100 𝜇m min⁻¹, a new peak is observed around 330 K on the thermoluminescence (TL) glow curve. Its color centre has a new energy band near F-band, which is based on the F- and thermal-bleaching effects of the compressed crystal.

1. Introduction

KCl:Eu²⁺ is one of the efficient photostimulated luminescence (PSL) phosphors and has an excellent sensitivity as a X-ray imaging sensor utilizing an optically stimulated luminescence (OSL) phenomenon in comparison with pure KCl [1]. It is well known that the rare-earth dopants, Eu²⁺ ions, are fluorescent and cause strong luminescence. KCl:Eu²⁺ exhibits excellent storage performance and is reusable material for radiation therapy dosimetry [2, 3].

Co-doped KCl single crystal (i.e. KCl:Eu²⁺, Ce³⁺) acts as a potential thermoluminescence (TL) and OSL dosimeter due to its high sensitivity to ionizing radiation because of the enhanced intensity of TSL (thermally stimulated luminescence) and OSL as against single doping (i.e. KCl:Eu²⁺ and KCl:Ce³⁺) [4]. It has been also reported that the occurrence of different states of dopants Eu²⁺ due to the heat treatment influences the absorption and fluorescence optical properties for KCl-KBr-RbBr:Eu²⁺ crystals [5]. Furthermore, it was recently found that BaFBr_0.85I_0.15:Eu²⁺ demonstrates reusable and excellent dosimetry such as KCl:Eu²⁺ [6].

By irradiating KCl:Eu²⁺ crystal with the X-ray, electron–hole pairs are made in the crystal and chlorine vacancies store the electrons. This process generates F-centres (trapped electrons) and F_z-centres (F-centres near Eu²⁺-cation vacancy dipoles). The electrons are liberated from the F-centres under a photostimulation of 560 nm, which agrees with the absorption wavelength of F-centre in KCl:Eu²⁺ [7], and recombine with the trapped holes. Then, characteristic Eu²⁺ luminescence results from the energy released in the PSL process of which the intensity becomes large with the irradiation dose of X-ray [8, 9].

The free electrons are released from the F_z-centre at 370 K and F-centre at 450 K and combine with Eu³⁺ ions. Then, the two peaks are created on TL glow curve, which show the 420 nm emission due to the 4f⁶5d → 4f⁷ transition of Eu²⁺ ions [10].

A number of studies on OSL and TL processes have been carried out for X-ray-irradiated KCl:Eu²⁺ and have proved that F_z- and F-centres are directly involved as the trapping sites for the electrons in their processes. But, to my knowledge, the details seem not to be clear (e.g. [11, 12]). In this chapter, a further insight into this situation is described with the help of optical absorption and TL glow measurements for the KCl:Eu²⁺ single crystals after X-ray irradiation. KCl single crystals doped with Eu²⁺ (0.02, 0.04, 0.1 mol% in melt) were prepared by cleaving the ingot, which was grown by the vertical Bridgman method, to the size of 2 × 3 × 5 mm³. The crystals were thereafter heat-treated in order to remove internal strains as much as possible and disperse the dopants in the crystal. The X-ray irradiation was conducted with W-target operated at 40 kV and 30 mA by Shimadzu XD-610. The absorption spectra were measured with a Hitachi U-3500 spectrophotometer. TL was observed using a photomultiplier (Hamamatsu R928) with a sensitive region from 185 to 900 nm. These were conducted at 300 to 540 K, and the irradiation and measurements were performed in the dark to avoid the unwanted effect of environmental light.

In the Section 4 of this chapter, absorption spectrum and TL glow curve are also reported for the crystals under the influence of compressive deformation after the irradiation. The crystals were plastically deformed by compression along the <100> axis direction at the cross-head speed of 100 μm min⁻¹ using an Instron 4465 testing machine at room temperature.

2. Absorption spectrum and TL glow curve for X-ray-irradiated crystals

Variations of absorption spectra and TL glow curves with X-ray irradiation times are shown in Figure 1a and b for the irradiated KCl:Eu²⁺ (0.04 mol%) single crystals, which were irradiated by the X-ray up to the time of 120 min at room temperature.

Figure 1.

Variations of (a) absorption spectrum and (b) TL glow curve with the irradiation time for X-ray-irradiated KCl:Eu²⁺ (0.04 mol%) single crystals.

Absorption coefficient is obtained from $\left(\log I_0/I\right)/\left(d\log e\right)$, where $\log I_0/I$ is the absorbance, and d is the width of specimen. The unit of absorption coefficient is cm⁻¹ according to the above description. This is theoretically based on the assumption that defects in solid (absorption source) are equally distributed in the solid. KCl:Eu²⁺ single crystal before and after the X-ray irradiation is shown in Figure 2a and b.

Figure 2.

KCl:Eu²⁺ single crystal (a) before and (b) after the X-ray irradiation.

In this study, the defects as absorption source were made by the X-ray irradiation at room temperature. This method cannot make the uniform distribution of defects in the specimen exposed to X-ray on the wide surfaces. The concentration distribution of F-centres (trapped electrons) was observed also for the X-ray-irradiated KBr single crystal in the article [13]. Figure 3 and Table 1 shows the variation of F-centre concentration with the distance from the surface of X-ray-irradiated KBr single crystal. As illustrated in Figure 3, the sample was exposed to X-ray for 3 h on each of the pair wide surfaces at room temperature and was cleaved in four thin crystal plates (a) ~ (d) at regular intervals (a thickness of about 1 mm). The F-centre concentration is smaller as the position of the specimen becomes centrally. Although the unit of absorption coefficient is supposed to be cm⁻¹ for an unirradiated specimen, the unit of it (i.e. the vertical scale of Figure 1) for the X-ray-irradiated specimen is accordingly represented by a.u. (arbitrary unit).

Figure 3.

Four thin crystal plates (a)–(d) cleaved from X-ray-irradiated KBr single crystal.

Crystal plate	(a)	(b)	(c)	(d)
Concentration of F-centres ($\times {10}^{16}c{m}^{-3}$)	25	3.0	3.3	28

Table 1.

F-centres concentration in each crystal plate (a)–(d) cleaved from the X-ray-irradiated KBr single crystals.

Reproduced form Ref. [13].

In Figure 1a, the absorption peaks at the wavelength of 243 and 343 nm are due to Eu²⁺, and the peak at 560 nm is due to F-centre [7, 10]. A small absorption peak at 825 nm is ascribed to the M-centre, which is a complex centre comprising two neighboring Cl vacancies and a trapped electron [7]. Figure 1b shows TL glow curve of X-ray irradiated KCl:Eu²⁺ (0.04 mol%), which was obtained by the linear heating at 24 K min⁻¹. The TL glow peak at the temperature of 370 K is related to F_z-centre (F-centre perturbed by neighboring a Eu²⁺ ion-cation vacancy dipole), while that at 450 K seems to be due to F-centre [14, 15]. The former low-temperature peak and the later high one are termed LTP and HTP in this chapter, respectively. The height of absorption peak due to F-centre at 560 nm becomes larger as the irradiation time is longer in Figure 1a. This is caused by the increase in F-centre concentration with X-ray irradiation dose. The production of F-centre leads to the decrease of the concentration of the Eu²⁺ ions in the crystal (because Eu²⁺ → Eu³⁺+e⁻). But the peak height at the wavelength of 343 nm due to the Eu²⁺ ion seems mostly unchanged taking account of the baseline, as shown in Figure 1a. This is because the F-centre concentration is much higher as against Eu²⁺ ion in the crystals. As an instance, the concentration (N) of Eu²⁺ is 105.6 ppm for 40 min X-ray-irradiated KCl:Eu²⁺ (0.04 mol% in melt) single crystal according to Hernandez et al. [16] from the absorption peak α_m = 3.0 at 243 nm in Figure 1a, namely.

$$N=35.2{\alpha }_{\text{m}}$$E1

While, the concentration (N₀) of F-centre is 2.57 ppm for the above-mentioned specimen using Smakula’s formula given by [17].

$$N_0=\frac{1}{2.06}{10}^{16}\frac{{\alpha }_m}{f}{\left(\frac{r}{{10}^8}\right)}^3$$E2

where f = 0.8 is the oscillator strength, absorption peak α_m = 17.479 at 560 nm in Figure 1a, and r = 3.1464 Å [18] is the interionic distance of KCl at room temperature. While, the absorption peak at about 825 nm due to the M-centre is insensitive to the increment of X-ray irradiation dose.

In Figure 1b, the heights of both LTP (due to F_z centre) and HTP (due to F-centre) increase with the irradiation time. And also, LTP shifts to slightly higher temperature with it. This may suggest that the excitonic electrons are trapped into deeper states when the irradiation dose increases. HTP lies a little towards high-temperature side below 6 min of the time.

3. F-bleach and thermal-bleach effects on F-centre peak, LTP, and HTP

The absorption spectra and the TL intensity of X-ray irradiated KCl:Eu²⁺ were measured after F-light (560 nm wavelength) bleach by using a Hitachi U-3010 fluorescence spectrometer after the irradiation. The F-light source was a xenon lamp. Figure 4a and b represents the absorption coefficient α_m at 560 nm wavelength, LTP, and HTP of dependence on the F-light exposure time for 8 min X-ray-irradiated KCl:Eu²⁺ (0.1 mol%) single crystals. The F-light bleach was carried out with an optical bandpass filter of 5 nm bandwidth. The F-bleach was done for the purpose of removing the F-centre peak within the bandwidth from the TL signal. As a result, the F_z-centre peak is enhanced. This may lead to resolve the F_z-centre.

Figure 4.

Relations between F-light exposure time versus the absorption coefficient and each peak (LTP, HTP) of TL glow curve for X-ray- irradiated KCl:Eu²⁺ (0.1 mol%) single crystals. The X-ray irradiation time and the excitation bandpass: (a) (b) 8 min and 5 nm; (c) (d) 6 min and 20 nm [19].

In Figure 4a and b, the values of α_m at 560 nm and HTP decrease with increasing F-light exposure time. By the F-light bleaching, the F-centre peak is removed within 5 nm bandwidth from the TL signal. But LTP is almost constant independently of the exposure time as shown in Figure 4b.

In addition, the X-ray irradiated crystals were similarly done using F-light at the excitation bandpass of 20 nm. The result of F-bleach is shown in Figure 4c and d for the irradiated crystals for 6 min. Not only the values of α_m at 560 nm and HTP but also LTP decreases with the F-light exposure time. This means the hidden peak due to the F_z-centre is either on the lower or on the higher side of 560 nm in the absorption spectrum. In Figure 4b and d, LTP initially increases in intensity below the exposure time 1 min. This is because F_z-centres are formed by the dynamical rearrangement of F-centres to Eu²⁺ as described in the papers [12, 15]. That is to say, F-centres transform partially to F_z-centres during the F-bleach.

The thermal bleach was further carried out for 10 min X-ray-irradiated KCl:Eu²⁺ (0.02 mol%) single crystals. The absorption spectra and the TL intensity of the irradiated KCl:Eu²⁺ were also measured by using a Hitachi U-3010 fluorescence spectrometer after the thermal bleach. The thermal bleach was carried out by heating at the rate of 24 K min⁻¹ up to 523 K after the irradiation. The bleach temperature dependence of F-centre peak in absorption spectrum, LTP, and HTP is shown in Figure 5.

Figure 5.

Relations between thermal-bleach temperature versus F-centre peak in absorption spectrum and each peak (LTP, HTP) of TL glow curve for X-ray-irradiated (irradiation time: 10 min) KCl:Eu²⁺ (0.02 mol%) single crystals [19].

The LTP height (blue solid line) decreases with increasing the bleach temperature above 360 K and gradually approaches to zero above 400 K. F-centre peak (red solid one) also begins to become small with it at the same thermal-bleach temperature (360 K). This suggests that F_z- and F-centres peaks are at nearly equal wavelength, as the above-mentioned results of F-light bleach. F-centre peak continually decreases with the bleach temperature at 400–473 K in accordance with HTP (green solid line) of dependence on the bleach temperature. On the basis of the results of F-light and thermal bleaches for the crystals, it is considered that electrons trapped at shallow traps (i.e. F_z-centres) begin to be set free at about 370 K and end detrapping from the traps around 400 K. The electrons trapped at deep traps (i.e. F-centres) begin to detrap continuously at 400–473 K. F_z-centre peak height is so low that it cannot be observed here. The two absorption peaks due to F- and F_z-centres are considered to be overlapped around the peak at wavelength 560 nm in the absorption spectrum for the crystals.

4. Optical absorption and TL glow peaks under the influence of compressive deformation

Figure 6a and b shows the absorption spectra and TL glow curves for 10 min X-ray-irradiated KCl:Eu²⁺ (0.02 mol%) single crystal before (blue solid line) and after (red solid one) the compression (applied stress: 20.8 MPa), where luminescence intensity and load signals were recorded on a computer and then the data were processed.

Figure 6.

(a) Absorption spectra and (b) TL glow curves for X-ray-irradiated KCl:Eu²⁺ (0.02 mol%) single crystals after the following treatments: () X-ray irradiation; () X-ray irradiation → compression (reproduced from Ref. [20] with permission from the publisher).

The peak at the wavelength of 560 nm due to F-centre [7, 10] appears on the absorption spectra irrespectively of the compression. As described in Section 3, it is considered that the peak due to F_z-centre is approximately within 20 nm of the wavelength 560 nm in the absorption spectrum of X-ray-irradiated KCl:Eu²⁺ single crystal. The height of the peak around 560 nm becomes larger under the influence of compression, as can be seen in Figure 6a. This is assumed to be the phenomenon that many F- and F_z-centres in the crystal are expected to be formed increasing vacancy during the process of compressive deformation.

On the other hand, the TL glow curve has two peaks (i.e. LTP and HTP) at the temperatures of 370 and 450 K for the X-ray-irradiated crystal before the compression. After the compressive deformation for the irradiated crystal, a new peak appears around 330 K on the TL glow curve in Figure 6b. This peak is termed 330 K peak in this chapter.

As for pure KCl single crystal after the deformation, there are no additional bands characteristic of local radiation defects on absorption spectrum [21].

Figures 7 and 8 show the influence of 330 K thermal bleaching on the absorption spectra and TL glow curves for the compressed and subsequent X-ray-irradiated KCl:Eu²⁺ (0.02 mol%) crystal. Blue and red solid lines in these figures are related to the crystals before and after the thermal-bleaching treatment, respectively. The 330 K thermal-bleach time was 35 min and X-ray irradiation time 10 min. In addition, the absorption spectrum for the irradiated crystal before the compression is represented by a green solid line in Figure 7.

Figure 7.

Absorption spectra for KCl:Eu²⁺ (0.02 mol%) single crystals after the following treatments: () X-ray irradiation; () X-ray irradiation→compression→X-ray irradiation; () X-ray irradiation→compression→X-ray irradiation→thermal-bleach at 330 K for 35 min. X-ray-irradiation time was 10 min (reproduced from Ref. [20] with permission from the publisher).

Figure 8.

TL glow curves for KCl:Eu²⁺ (0.02 mol%) single crystals after the following treatments: () X-ray irradiation→compression→X-ray irradiation; () X-ray irradiation→compression→X-ray irradiation→thermal-bleach at 330 K for 35 min. X-ray-irradiation time was 10 min. (reproduced from Ref. [20] with permission from the publisher).

By the 330 K thermal bleaching with 5 nm bandpass filter, the peak at the wavelength of 560 nm due to F-centre decays until near height of the irradiated crystal before the compression in the absorption spectra, as shown in Figure 7. Then, the 330 K peak disappears on the TL glow curve in Figure 8. TL intensities of LTP and HTP are almost constant as expected. On the basis of these results on 330 K thermal bleach, it is considered that the increase in F-centre peak after the compression in Figure 7 is attributable to the production of the color centres (330 K color centre) caused the 330 K peak for the compressed crystal. The absorption wavelength at F_z-centre peak is supposed to be approximately within 20 nm of that (560 nm) at F-centre peak (see Section 3). And further, the 330 K color centre may be also overlapped near 560 nm in the absorption spectrum of compressed crystal (blue solid line) from Figure 7. Here, it was investigated whether the height of 330 K peak is influenced by bleaching with F-light (bandpass 5 nm) for 10 min X-ray-irradiated KCl:Eu²⁺ (0.02 mol%) single crystals after the compression (applied stress: 20.8 MPa). Figure 9 shows the F-bleaching results, where red and blue solid lines represent the 330 K peak of TL glow curve and F-peak in the absorption spectrum, respectively.

Figure 9.

Relations between F-bleach time (excitation bandpass: 5 nm) versus () 330 K peak of TL glow curve and () F-centre peak in absorption spectrum for KCl:Eu²⁺ (0.02 mol%) single crystals previously compressed and subsequently X-ray-irradiated (irradiation time: 10 min). (reproduced from Ref. [20] with permission from the publisher).

The TL intensity of 330 K peak rapidly decreases with increasing F-light exposure time until about 5 min and gradually approaches to zero in accordance with the variation of F-peak. Therefore, it is considered that the absorption wavelength due to the 330 K color centre is near 560 nm in the absorption spectrum.

5. Conclusion

The X-ray irradiation induces F_z- and F-centres in KCl:Eu²⁺ crystal, of which the wavelengths are almost equal with each other in the absorption spectrum. The absorption peak due to the F_z-centre is either on lower or higher side within 20 nm of the wavelength 560 nm in the absorption spectrum.

The changes in optical absorption and TL glow curves induced by compressive deformation of KCl:Eu²⁺ were presented here. That is, the new peak (i.e. 330 K peak) appears around 330 K on TL glow curve. The analyses of F-light bleach and 330 K thermal bleach reveal that the absorption wavelength due to the 330 K peak is near the F-centre peak in the absorption spectrum for the crystal.

Acknowledgments

The author is pleased to give thanks to Dr. T. Ohgaku for helpful and constructive discussions on the article and to Y. Igari for his experimental assistance.

Conflict of interest

The author declares no conflict of interest.