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Scintillation properties of Eu and alkaline metal co-doped LiCaAlF6
Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Scintillation properties of Eu and alkaline metal co-doped LiCaAlF6 Takayuki Yanagida a,n, Masanori Koshimizu b, Yutaka Fujimoto b, Kentaro Fukuda c, Kenichi Watanabe d, Go Okada a, Noriaki Kawaguchi a a
Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan Tohoku University, 6-6 Aramaki, Aoba, Sendai, Miyagi 980-8578, Japan c Tokuyama Corp., 1-1 Mikage-cho, Shunan-shi, Yamaguchi 745-8648, Japan d Nagoya University, Furocho, Chikusa, Nagoya, Aichi 464-8603, Japan b
art ic l e i nf o
a b s t r a c t
Article history: Received 14 October 2016 Received in revised form 13 January 2017 Accepted 23 January 2017
Very low amount of Eu and alkali metal (Na, K, Rb and Cs) co-doped LiCaAlF6 crystal scintillators were prepared by using the micro-pulling down method to investigate the origin of the light yield enhancement and the fast luminescence of LiCaAlF6-based scintillators. In X-ray and α-ray induced scintillation spectra, the host and Eu2 þ 5d-4f transition emission appeared around 300–500 nm as a broad shape and 370 nm as a sharp line, respectively. At a synchrotron facility (UVSOR), the excitation spectra under vacuum ultra violet (VUV) photons excitation were investigated, and the results confirmed new excitation bands around 80–90 nm as well as the band gap of LiCaAlF6 ( 113 nm). Fast luminescence with the lifetime of few ns was confirmed in K, Rb and Cs doped samples under VUV and X-ray excitations. The detector property was tested, and K co-doping improves the pulse shape discrimination performance. & 2017 Elsevier B.V. All rights reserved.
Keywords: Scintillator LiCaAlF6 Eu2 þ Alkali metal Synchrotron
1. Introduction Scintillators are one type of the luminescent materials, and they have been used in many kinds of ionizing radiation detectors in medical diagnostics [1], security [2], well-logging [3], astrophysics [4] and particle physics [5]. Generally, scintillation detectors consist of scintillators, which emit scintillation photons, and photodetectors, which convert scintillation photons to electrons. The functions of scintillators are to absorb the ionizing radiation, and convert the absorbed radiation energy to numerous low energy photons immediately [6–11]. When the scintillator absorbs high energy photons or particles, many secondary electrons (carriers) are created via interactions of radiations with matter, and a fraction of the energy can contribute to the scintillation. The common interest about the fundamental aspects in scintillation phenomenon is to describe how these electrons can reach the luminescence centers following by the recombination with holes or dissipate their energy. Although no common definition about the scintillation is confirmed, a key phenomenon is the generation of many careers by one incident photon or particle. In the case of the interaction of radiation photons, the energy threshold between scintillation and photoluminescence (PL) ranges from several tens to hundreds eV depending on the material. n
Corresponding author. E-mail address: [email protected] (T. Yanagida).
In scintillation detectors for practical applications, development of scintillators for thermal neurons has attracted much attentions in recent years due to the decreasing supply of 3He gas being as the fundamental of neutron detectors over the decades. As an alternative to 3He gas, many efforts have been paid to develop 6Li or 10B containing scintillators since these elements have high interaction probabilities against thermal neutrons. For this purpose, elpasolite [12–14], some solid state organic scintillators [15], liquid organic scintillators [16] and 10B-based crystals [17] have been introduced. We have also developed LiCaAlF6 scintillators [18,19] and LiCaAlF6-based neutron detectors [20,21], and these scintillators are now commercially available by Tokuyama Corp. During the R&D phase of LiCaAlF6, we found that Na co-doping enhanced the scintillation light yield of Eu-doped LiCaAlF6 [22], and similar effects were also observed by different co-doping. However, no investigations have been conducted to understand the reason of this enhancement while many studies have been done for the material search and detector applications. In addition, LiCaAlF6-based materials sometimes show a very fast luminescence [23,24], and this fast component is quite important for detector applications since we can conduct pulse shape discrimination by using the fast component [20,21]. The influences of alkali metal co-doping on this fast component were not considered at all. In this study, we investigated optical and scintillation properties of Eu and alkali metal co-doped LiCaAlF6 to understand the origins of the light yield enhancement and influences on the fast
http://dx.doi.org/10.1016/j.jlumin.2017.01.029 0022-2313/& 2017 Elsevier B.V. All rights reserved.
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scintillation decay. The samples were synthesized by the micropulling down method. Here, doping concentration of Eu was substantially small compared to that of alkali metals. Subsequently, the samples were mechanically polished to conduct optical transmittance measurements. X-ray and α-ray induced scintillation spectra were evaluated under different LETs (linear energy transfers). Then, excitation spectra and PL decay times under vacuum ultraviolet (VUV) photon excitations were investigated at a synchrotron facility (UVSOR). In addition to the PL decay, wavelength resolved X-ray induced scintillation decay times were evaluated since the previous work showed a disappearance of fast component in high LET excitations [24].
2. Experimental procedures In the sample preparation, nominally Eu 0.01 mol% with respect to Ca and 1% alkali metal (Na, K, Rb and Cs) with respect to Li doped LiCaAlF6 crystals were synthesized by the micro-pulling down method at Tokuyama Corp. The all raw materials were fluoride powders with 3N or 4N purities. Our aim was to study the influence of alkali metal co-doping to the light yield enhancement and fast component in Eu:LiCaAlF6 scintillator. In addition, the concentration of Eu was fixed to 0.01% since fast scintillation cannot be observed when doped with large amount of Eu. To our knowledge based on the ICP analysis, actual Eu concentration in LiCaAlF6 crystal was around few% of nominal amount so it would not prevent the observations. After the synthesis, optical in-line transmittance spectra were collected by JASCO V670 spectrometer from 190 to 2700 nm with 1 nm intervals. Then, X-ray [25] and α-ray [26] induced radioluminescence spectra were evaluated, and experimental geometries were described previously. The detectors used for measuring X-ray and α-ray scintillation emission spectra were DU920-BU2NC CCD (Andor) and FP8600 (JASCO), respectively. The X-ray source was a conventional X-ray tube supplied with 80 kV bias voltage and 1 mA tube current, and the α-ray source was 4 MBq 241Am which emitted 5.5 MeV α-rays. Scintillation decay time profiles were evaluated by using pulse X-ray equipped streak camera system [27]. The monitoring wavelength of the scintillation decay was 420 nm. At UVSOR, a sample in a vacuum chamber was excited with photons in the UV–VUV region monochromated with a 3-m normal incidence monochromator at the Beamline 7B. The measurements were performed at room temperature. The luminescence spectra were measured with an optical multichannel analyzer, and the blaze wavelength of the grating was 500 nm. The time profile of the luminescence intensity at each emission wavelength was obtained by single-photon counting technique. The period of successive excitation light pulses was 176 ns. We investigated VUV excitation spectra monitoring at 300–500 nm emission range, and decay time monitoring at 420 nm under 110 nm excitation. In the excitation spectra, the excitation wavelength was scanned from 50 to 200 nm with 2 nm intervals. The host luminescence appeared with a broad shape from 300 to 500 nm and Eu2 þ 5d-4f emission appeared at 370 nm. Although we analyzed the excitation spectra of only Eu2 þ emission, no significant difference was observed. The excitation spectra were measured at the multi bunch mode in UVSOR while the decay was in the single bunch mode. In order to examine the detector performance, the pulse shape discrimination capability was tested. For this test, nominally Eu 2% doped LiCaAlF6 and Eu and K 2% co-doped LiCaAlF6 crystals were used, which were prepared by the same manner as described above. In commercial LiCaAlF6, nominal concentration of Eu is 2–4%, and it is important to investigate the application property in densely Eu-doped sample. Neutron and gamma-ray sources were
252 Cf and 60Co, respectively. The anode signal of PMT was fed into high-speed digitizer (Agilent U1071A, 1 GHz, 1 GS/s). Digitized waveforms were processed with PC in order to discriminate neutron and gamma-ray induced events.
3. Experimental results and discussion Fig. 1 represents the picture of Na, K, Rb and Cs co-doped Eu: LiCaAlF6 crystals. They looked transparent, and no inclusions or cracks were observed. The sample size was fixed to 2 10 1 mm3, and side surfaces were polished for optical characterizations. In Fig. 2, optical in-line transmittance spectra are presented. The transmittance on 1 mm thickness was 70–85% in each sample from ultra violet to near infrared wavelengths. The dip around 850 nm was an instrumental artifact due to the change of the light source. In the observed wavelengths, no particular absorption bands were detected, so it was confirmed that actual Eu concentration was very low since the absorption due to Eu2 þ was not detected and no unexpected contamination was occurred in the synthesis. Fig. 3 shows X- and α-ray induced scintillation emission spectra. In X-ray excitation, main emission appeared at 370 nm, and the origin of this emission was Eu2 þ 5d-4f transition [28]. A broad emission from 300 to 500 nm was observed in all the samples, and this emission band would consist of several emission bands. These emissions were also observed in the nondoped LiCaAlF6, and the
Na
K
Rb
Cs
Fig. 1. From left to right, Na, K, Rb and Cs co-doped Eu:LiCaAlF6 crystals.
Fig. 2. Optical in-line transmittance spectra of Na, K, Rb and Cs co-doped Eu: LiCaAlF6 crystals.
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Fig. 3. X-ray (left) and α-ray (right) induced scintillation spectra of Na, K, Rb and Cs co-doped Eu:LiCaAlF6 crystals. The inset of the left panel expands the STE and defect emissions.
Fig. 4. From left to right, X-ray induced scintillation decay time profiles of K, Rb and Cs co-doped Eu:LiCaAlF6 crystals monitoring at 420 7 15 nm.
origin of the shorter emission band was interpreted as the selftrapped exciton (STE) [29]. The decay time of the STE was on the μs order, so it would not be related to the very fast decay. In the recent work, the origin of 420 nm emission was interpreted as defects of the host matrix [24], and it was not observed clearly in the nondoped LiCaAlF6 with the same experimental condition by us. So, co-doping with alkali metal enhances the defect-related emission around 420 nm, and K-doping seems to be the most effective. Although scintillation spectra have generally qualitative meaning, we could roughly compare the intensities in this work since the stopping power against X-rays of present samples were almost the same. On the other hand, in α-ray induced luminescence spectra, the relative intensity ratios between STE, Eu2 þ 5d4f transition and defects are different, and this phenomenon can be explained as the LET effect. The difference of Eu2 þ emission intensity by co-doping follow the same order as those in X-ray induced spectra, and in practical applications, Na co-doping was the most effective from this result. This result was consistent with our previous study of 252Cf neutron induced scintillation light yield in the pulse height (PH) analysis [22] since the Eu2 þ emissions mainly contribute to the signal of PH. In Fig. 4, X-ray induced scintillation decay time profiles of K, Rb and Cs co-doped Eu:LiCaAlF6 monitoring at 420 7 15 nm are demonstrated. In the same experimental condition, we could not observe a clear signal from Na co-doped sample as well as the nondoped LiCaAlF6, and this results was consistent with our observations at synchrotron described later. On the other hand, by K, Rb and Cs doping, the fast emission was clearly observed. In the practical point of view, the fast emission can be realized by dense K-doping in Eu-doped LiCaAlF6. Up to now, pulse shape
discrimination is only available by Ce:LiCaAlF6 since the rise part of the scintillation decay in this material is slow so it is technically easy to distinguish Ce (slow) and defect (fast) emissions [30]. If the pulse shape discrimination becomes possible in Eu:LiCaAlF6, the technical merit is quite high because the light yield of Eu:LiCaAlF6 is one digit higher than Ce:LiCaAlF6. In Fig. 5, excitation spectra with the monitoring wavelength of 300–500 nm of Na, K, Rb and Cs co-doped Eu:LiCaAlF6 are shown. We observed the excitation band around 110–120 nm which corresponded to the band gap energy ( 113 nm) of LiCaAlF6. The large difference of the intensity at the band gap was due to the resolution and the step wavelength, and some spike-like peaks were ascribed to noise. In addition to the band gap, a broad excitation band around 80–90 nm was observed, and this excitation band was also observed in the nondoped LiCaAlF6 at very low temperature [29]. A clear difference with the past study [29] was that our experiments were conducted at room temperature, and only the band gap peak was detected at room temperature in the nondoped LiCaAlF6 [29]. So, we could conclude that 80–90 nm excitation bands were enhanced by alkali metal co-doping. In halide crystals, the valence band are composed of np orbitals of halogen, while the conduction band is composed of the lowest unoccupied state of cations. Thus, the appearance of the excitation band at 80–90 nm suggests that the conduction band structure is altered by the co-doping. Fig. 6 shows decay time profiles of alkali metal co-doped Eu: LiCaAlF6 crystals monitoring at 420 nm under 110 nm VUV excitation. Although the integration times were the same, clear difference was observed. As in the case of X-ray induced scintillation decay, we could not detect significant signals from the Na co-doped sample
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Absorption of Eu Band gap
Na
K
Cs Rb
Fig. 5. Excitation spectra monitoring at 300–500 nm emission bands of Na, K, Rb and Cs co-doped Eu:LiCaAlF6 crystals.
Fig. 6. Decay time profiles monitoring at 420 nm under 84 nm VUV excitation of Na, K, Rb and Cs co-doped Eu:LiCaAlF6 crystals.
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Fig. 7. Two-dimensional histogram of Eu 2% doped (left) and Eu and K 2% doped (right) LiCaAlF6 crystals on a plane of the pulse height with fast integrated time window and the ratio of the pulse height under fast and slow integrated time windows. Fast/slow integrated time windows of the Eu singly doped and co-doped samples were 7.5–17.5 ns and 17.5–3500 ns, respectively. The irradiation sources were 60Co γ-rays and 252Cf neutrons.
Eu 2% doped Count rate (cps)
Count rate (cps)
10
5
0
0
50
100 Fast/Slow
Eu & K 2% co-doped
6
150
5 4 3 2 1 0
0
50
100 Fast/Slow
150
Fig. 8. PSD index spectra of Eu-doped and Eu and K co-doped LiCaAlF6.
while the other samples clearly showed the fast emission. In the PH analysis, the light yield enhancement occurred in Na co-doping [22] while co-doping of other alkali metals (K, Rb and Cs) did not improve the light yield. In the present study, the enhancement of the fast emission occurred in K, Rb and Cs co-doped Eu:LiCaAlF6 while Na co-doped sample did not show the fast emission. From VUV spectroscopy, it was confirmed that the excitation spectra of Na, K, Rb and Cs co-doped LiCaAlF6 were similar. Therefore, the enhancement of the light yield and the fast emission would be competitive in Eu:LiCaAlF6. Fig. 7 compares the pulse shape discrimination capabilities on the two dimensional plots for Eu-doped and Eu and K co-doped samples as a demonstration of detector applications under 60Co γrays and 252Cf neutrons mixed irradiation. The horizontal and vertical axes stand for the slow and fast components of signals, respectively. In order to avoid Cherenkov radiations, the region from the starting point for 7.5 ns in each signal was eliminated. The regions from 7.5 ns to 17.5 ns and from 17.5 ns to 3500 ns were integrated for the fast and slow components, respectively. As an index of the pulse shape discrimination (PSD), the ratio of fast and slow components is used. Fig. 8 shows the PSD index spectra for the Eu-doped and Eu and K co-doped samples. The spectrum shape is considered as a double Gaussian function, which consists of neutron and gamma contribution. We also evaluated the figure of merit (FOM) for PSD [31], and the FOM is a typical index to evaluate the effect of PSD quantitatively. The FOM is defined as follows:
FOM =
S , δneutron + δgamma
where S represents the distance between neutron and gamma peaks in this figure, and δneutron and δgamma mean the full width at half maximum of each peak. The FOMs were evaluated to be 0.104 and 0.110 for Eu-doped and Eu and K co-doped samples, respectively. As a result, K co-doped sample showed a slightly higher FOM than Eu singly doped LiCaAlF6, and we confirmed that the alkali metal co-doping can be useful from the point of view of PSD. Although the separation is not clearer than Ce-doped LiCaAlF6 [30] and some organic scintillators [15], higher separation would be achieved by optimizing the Eu and K concentrations.
4. Conclusion Na, K, Rb and Cs co-doped Eu:LiCaAlF6 crystals were prepared by the micro-pulling down method, and examined on their optical and scintillation properties as a aim of studying the origins and possibilities of enhancement of the light yield and the fast decay time. By alkali metal co-doping, 80–90 nm excitation bands were enhanced when compared with the nondoped LiCaAlF6. K, Rb and Cs co-doped Eu:LiCaAlF6 showed the enhancement of the fast decay time while Na exhibited the light yield enhancement. Finally, Eu 2% and K 2% co-doped LiCaAlF6 was examined on the pulse shape discrimination capability, and the detection performance can be improved by K co-doping.
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Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (A) 26249147 from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) and partially by JST A-step. The Cooperative Research Project of Research Institute of Electronics, Shizuoka University, KRF foundation and Inamori foundation are also acknowledged.
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Scintillation properties of Eu and alkaline metal co-doped LiCaAlF6 Takayuki Yanagida a,n, Masanori Koshimizu b, Yutaka Fujimoto b, Kentaro Fukuda c, Kenichi Watanabe d, Go Okada a, Noriaki Kawaguchi a a
Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan Tohoku University, 6-6 Aramaki, Aoba, Sendai, Miyagi 980-8578, Japan c Tokuyama Corp., 1-1 Mikage-cho, Shunan-shi, Yamaguchi 745-8648, Japan d Nagoya University, Furocho, Chikusa, Nagoya, Aichi 464-8603, Japan b
art ic l e i nf o
a b s t r a c t
Article history: Received 14 October 2016 Received in revised form 13 January 2017 Accepted 23 January 2017
Very low amount of Eu and alkali metal (Na, K, Rb and Cs) co-doped LiCaAlF6 crystal scintillators were prepared by using the micro-pulling down method to investigate the origin of the light yield enhancement and the fast luminescence of LiCaAlF6-based scintillators. In X-ray and α-ray induced scintillation spectra, the host and Eu2 þ 5d-4f transition emission appeared around 300–500 nm as a broad shape and 370 nm as a sharp line, respectively. At a synchrotron facility (UVSOR), the excitation spectra under vacuum ultra violet (VUV) photons excitation were investigated, and the results confirmed new excitation bands around 80–90 nm as well as the band gap of LiCaAlF6 ( 113 nm). Fast luminescence with the lifetime of few ns was confirmed in K, Rb and Cs doped samples under VUV and X-ray excitations. The detector property was tested, and K co-doping improves the pulse shape discrimination performance. & 2017 Elsevier B.V. All rights reserved.
Keywords: Scintillator LiCaAlF6 Eu2 þ Alkali metal Synchrotron
1. Introduction Scintillators are one type of the luminescent materials, and they have been used in many kinds of ionizing radiation detectors in medical diagnostics [1], security [2], well-logging [3], astrophysics [4] and particle physics [5]. Generally, scintillation detectors consist of scintillators, which emit scintillation photons, and photodetectors, which convert scintillation photons to electrons. The functions of scintillators are to absorb the ionizing radiation, and convert the absorbed radiation energy to numerous low energy photons immediately [6–11]. When the scintillator absorbs high energy photons or particles, many secondary electrons (carriers) are created via interactions of radiations with matter, and a fraction of the energy can contribute to the scintillation. The common interest about the fundamental aspects in scintillation phenomenon is to describe how these electrons can reach the luminescence centers following by the recombination with holes or dissipate their energy. Although no common definition about the scintillation is confirmed, a key phenomenon is the generation of many careers by one incident photon or particle. In the case of the interaction of radiation photons, the energy threshold between scintillation and photoluminescence (PL) ranges from several tens to hundreds eV depending on the material. n
Corresponding author. E-mail address: [email protected] (T. Yanagida).
In scintillation detectors for practical applications, development of scintillators for thermal neurons has attracted much attentions in recent years due to the decreasing supply of 3He gas being as the fundamental of neutron detectors over the decades. As an alternative to 3He gas, many efforts have been paid to develop 6Li or 10B containing scintillators since these elements have high interaction probabilities against thermal neutrons. For this purpose, elpasolite [12–14], some solid state organic scintillators [15], liquid organic scintillators [16] and 10B-based crystals [17] have been introduced. We have also developed LiCaAlF6 scintillators [18,19] and LiCaAlF6-based neutron detectors [20,21], and these scintillators are now commercially available by Tokuyama Corp. During the R&D phase of LiCaAlF6, we found that Na co-doping enhanced the scintillation light yield of Eu-doped LiCaAlF6 [22], and similar effects were also observed by different co-doping. However, no investigations have been conducted to understand the reason of this enhancement while many studies have been done for the material search and detector applications. In addition, LiCaAlF6-based materials sometimes show a very fast luminescence [23,24], and this fast component is quite important for detector applications since we can conduct pulse shape discrimination by using the fast component [20,21]. The influences of alkali metal co-doping on this fast component were not considered at all. In this study, we investigated optical and scintillation properties of Eu and alkali metal co-doped LiCaAlF6 to understand the origins of the light yield enhancement and influences on the fast
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scintillation decay. The samples were synthesized by the micropulling down method. Here, doping concentration of Eu was substantially small compared to that of alkali metals. Subsequently, the samples were mechanically polished to conduct optical transmittance measurements. X-ray and α-ray induced scintillation spectra were evaluated under different LETs (linear energy transfers). Then, excitation spectra and PL decay times under vacuum ultraviolet (VUV) photon excitations were investigated at a synchrotron facility (UVSOR). In addition to the PL decay, wavelength resolved X-ray induced scintillation decay times were evaluated since the previous work showed a disappearance of fast component in high LET excitations [24].
2. Experimental procedures In the sample preparation, nominally Eu 0.01 mol% with respect to Ca and 1% alkali metal (Na, K, Rb and Cs) with respect to Li doped LiCaAlF6 crystals were synthesized by the micro-pulling down method at Tokuyama Corp. The all raw materials were fluoride powders with 3N or 4N purities. Our aim was to study the influence of alkali metal co-doping to the light yield enhancement and fast component in Eu:LiCaAlF6 scintillator. In addition, the concentration of Eu was fixed to 0.01% since fast scintillation cannot be observed when doped with large amount of Eu. To our knowledge based on the ICP analysis, actual Eu concentration in LiCaAlF6 crystal was around few% of nominal amount so it would not prevent the observations. After the synthesis, optical in-line transmittance spectra were collected by JASCO V670 spectrometer from 190 to 2700 nm with 1 nm intervals. Then, X-ray [25] and α-ray [26] induced radioluminescence spectra were evaluated, and experimental geometries were described previously. The detectors used for measuring X-ray and α-ray scintillation emission spectra were DU920-BU2NC CCD (Andor) and FP8600 (JASCO), respectively. The X-ray source was a conventional X-ray tube supplied with 80 kV bias voltage and 1 mA tube current, and the α-ray source was 4 MBq 241Am which emitted 5.5 MeV α-rays. Scintillation decay time profiles were evaluated by using pulse X-ray equipped streak camera system [27]. The monitoring wavelength of the scintillation decay was 420 nm. At UVSOR, a sample in a vacuum chamber was excited with photons in the UV–VUV region monochromated with a 3-m normal incidence monochromator at the Beamline 7B. The measurements were performed at room temperature. The luminescence spectra were measured with an optical multichannel analyzer, and the blaze wavelength of the grating was 500 nm. The time profile of the luminescence intensity at each emission wavelength was obtained by single-photon counting technique. The period of successive excitation light pulses was 176 ns. We investigated VUV excitation spectra monitoring at 300–500 nm emission range, and decay time monitoring at 420 nm under 110 nm excitation. In the excitation spectra, the excitation wavelength was scanned from 50 to 200 nm with 2 nm intervals. The host luminescence appeared with a broad shape from 300 to 500 nm and Eu2 þ 5d-4f emission appeared at 370 nm. Although we analyzed the excitation spectra of only Eu2 þ emission, no significant difference was observed. The excitation spectra were measured at the multi bunch mode in UVSOR while the decay was in the single bunch mode. In order to examine the detector performance, the pulse shape discrimination capability was tested. For this test, nominally Eu 2% doped LiCaAlF6 and Eu and K 2% co-doped LiCaAlF6 crystals were used, which were prepared by the same manner as described above. In commercial LiCaAlF6, nominal concentration of Eu is 2–4%, and it is important to investigate the application property in densely Eu-doped sample. Neutron and gamma-ray sources were
252 Cf and 60Co, respectively. The anode signal of PMT was fed into high-speed digitizer (Agilent U1071A, 1 GHz, 1 GS/s). Digitized waveforms were processed with PC in order to discriminate neutron and gamma-ray induced events.
3. Experimental results and discussion Fig. 1 represents the picture of Na, K, Rb and Cs co-doped Eu: LiCaAlF6 crystals. They looked transparent, and no inclusions or cracks were observed. The sample size was fixed to 2 10 1 mm3, and side surfaces were polished for optical characterizations. In Fig. 2, optical in-line transmittance spectra are presented. The transmittance on 1 mm thickness was 70–85% in each sample from ultra violet to near infrared wavelengths. The dip around 850 nm was an instrumental artifact due to the change of the light source. In the observed wavelengths, no particular absorption bands were detected, so it was confirmed that actual Eu concentration was very low since the absorption due to Eu2 þ was not detected and no unexpected contamination was occurred in the synthesis. Fig. 3 shows X- and α-ray induced scintillation emission spectra. In X-ray excitation, main emission appeared at 370 nm, and the origin of this emission was Eu2 þ 5d-4f transition [28]. A broad emission from 300 to 500 nm was observed in all the samples, and this emission band would consist of several emission bands. These emissions were also observed in the nondoped LiCaAlF6, and the
Na
K
Rb
Cs
Fig. 1. From left to right, Na, K, Rb and Cs co-doped Eu:LiCaAlF6 crystals.
Fig. 2. Optical in-line transmittance spectra of Na, K, Rb and Cs co-doped Eu: LiCaAlF6 crystals.
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Fig. 3. X-ray (left) and α-ray (right) induced scintillation spectra of Na, K, Rb and Cs co-doped Eu:LiCaAlF6 crystals. The inset of the left panel expands the STE and defect emissions.
Fig. 4. From left to right, X-ray induced scintillation decay time profiles of K, Rb and Cs co-doped Eu:LiCaAlF6 crystals monitoring at 420 7 15 nm.
origin of the shorter emission band was interpreted as the selftrapped exciton (STE) [29]. The decay time of the STE was on the μs order, so it would not be related to the very fast decay. In the recent work, the origin of 420 nm emission was interpreted as defects of the host matrix [24], and it was not observed clearly in the nondoped LiCaAlF6 with the same experimental condition by us. So, co-doping with alkali metal enhances the defect-related emission around 420 nm, and K-doping seems to be the most effective. Although scintillation spectra have generally qualitative meaning, we could roughly compare the intensities in this work since the stopping power against X-rays of present samples were almost the same. On the other hand, in α-ray induced luminescence spectra, the relative intensity ratios between STE, Eu2 þ 5d4f transition and defects are different, and this phenomenon can be explained as the LET effect. The difference of Eu2 þ emission intensity by co-doping follow the same order as those in X-ray induced spectra, and in practical applications, Na co-doping was the most effective from this result. This result was consistent with our previous study of 252Cf neutron induced scintillation light yield in the pulse height (PH) analysis [22] since the Eu2 þ emissions mainly contribute to the signal of PH. In Fig. 4, X-ray induced scintillation decay time profiles of K, Rb and Cs co-doped Eu:LiCaAlF6 monitoring at 420 7 15 nm are demonstrated. In the same experimental condition, we could not observe a clear signal from Na co-doped sample as well as the nondoped LiCaAlF6, and this results was consistent with our observations at synchrotron described later. On the other hand, by K, Rb and Cs doping, the fast emission was clearly observed. In the practical point of view, the fast emission can be realized by dense K-doping in Eu-doped LiCaAlF6. Up to now, pulse shape
discrimination is only available by Ce:LiCaAlF6 since the rise part of the scintillation decay in this material is slow so it is technically easy to distinguish Ce (slow) and defect (fast) emissions [30]. If the pulse shape discrimination becomes possible in Eu:LiCaAlF6, the technical merit is quite high because the light yield of Eu:LiCaAlF6 is one digit higher than Ce:LiCaAlF6. In Fig. 5, excitation spectra with the monitoring wavelength of 300–500 nm of Na, K, Rb and Cs co-doped Eu:LiCaAlF6 are shown. We observed the excitation band around 110–120 nm which corresponded to the band gap energy ( 113 nm) of LiCaAlF6. The large difference of the intensity at the band gap was due to the resolution and the step wavelength, and some spike-like peaks were ascribed to noise. In addition to the band gap, a broad excitation band around 80–90 nm was observed, and this excitation band was also observed in the nondoped LiCaAlF6 at very low temperature [29]. A clear difference with the past study [29] was that our experiments were conducted at room temperature, and only the band gap peak was detected at room temperature in the nondoped LiCaAlF6 [29]. So, we could conclude that 80–90 nm excitation bands were enhanced by alkali metal co-doping. In halide crystals, the valence band are composed of np orbitals of halogen, while the conduction band is composed of the lowest unoccupied state of cations. Thus, the appearance of the excitation band at 80–90 nm suggests that the conduction band structure is altered by the co-doping. Fig. 6 shows decay time profiles of alkali metal co-doped Eu: LiCaAlF6 crystals monitoring at 420 nm under 110 nm VUV excitation. Although the integration times were the same, clear difference was observed. As in the case of X-ray induced scintillation decay, we could not detect significant signals from the Na co-doped sample
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Absorption of Eu Band gap
Na
K
Cs Rb
Fig. 5. Excitation spectra monitoring at 300–500 nm emission bands of Na, K, Rb and Cs co-doped Eu:LiCaAlF6 crystals.
Fig. 6. Decay time profiles monitoring at 420 nm under 84 nm VUV excitation of Na, K, Rb and Cs co-doped Eu:LiCaAlF6 crystals.
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Fig. 7. Two-dimensional histogram of Eu 2% doped (left) and Eu and K 2% doped (right) LiCaAlF6 crystals on a plane of the pulse height with fast integrated time window and the ratio of the pulse height under fast and slow integrated time windows. Fast/slow integrated time windows of the Eu singly doped and co-doped samples were 7.5–17.5 ns and 17.5–3500 ns, respectively. The irradiation sources were 60Co γ-rays and 252Cf neutrons.
Eu 2% doped Count rate (cps)
Count rate (cps)
10
5
0
0
50
100 Fast/Slow
Eu & K 2% co-doped
6
150
5 4 3 2 1 0
0
50
100 Fast/Slow
150
Fig. 8. PSD index spectra of Eu-doped and Eu and K co-doped LiCaAlF6.
while the other samples clearly showed the fast emission. In the PH analysis, the light yield enhancement occurred in Na co-doping [22] while co-doping of other alkali metals (K, Rb and Cs) did not improve the light yield. In the present study, the enhancement of the fast emission occurred in K, Rb and Cs co-doped Eu:LiCaAlF6 while Na co-doped sample did not show the fast emission. From VUV spectroscopy, it was confirmed that the excitation spectra of Na, K, Rb and Cs co-doped LiCaAlF6 were similar. Therefore, the enhancement of the light yield and the fast emission would be competitive in Eu:LiCaAlF6. Fig. 7 compares the pulse shape discrimination capabilities on the two dimensional plots for Eu-doped and Eu and K co-doped samples as a demonstration of detector applications under 60Co γrays and 252Cf neutrons mixed irradiation. The horizontal and vertical axes stand for the slow and fast components of signals, respectively. In order to avoid Cherenkov radiations, the region from the starting point for 7.5 ns in each signal was eliminated. The regions from 7.5 ns to 17.5 ns and from 17.5 ns to 3500 ns were integrated for the fast and slow components, respectively. As an index of the pulse shape discrimination (PSD), the ratio of fast and slow components is used. Fig. 8 shows the PSD index spectra for the Eu-doped and Eu and K co-doped samples. The spectrum shape is considered as a double Gaussian function, which consists of neutron and gamma contribution. We also evaluated the figure of merit (FOM) for PSD [31], and the FOM is a typical index to evaluate the effect of PSD quantitatively. The FOM is defined as follows:
FOM =
S , δneutron + δgamma
where S represents the distance between neutron and gamma peaks in this figure, and δneutron and δgamma mean the full width at half maximum of each peak. The FOMs were evaluated to be 0.104 and 0.110 for Eu-doped and Eu and K co-doped samples, respectively. As a result, K co-doped sample showed a slightly higher FOM than Eu singly doped LiCaAlF6, and we confirmed that the alkali metal co-doping can be useful from the point of view of PSD. Although the separation is not clearer than Ce-doped LiCaAlF6 [30] and some organic scintillators [15], higher separation would be achieved by optimizing the Eu and K concentrations.
4. Conclusion Na, K, Rb and Cs co-doped Eu:LiCaAlF6 crystals were prepared by the micro-pulling down method, and examined on their optical and scintillation properties as a aim of studying the origins and possibilities of enhancement of the light yield and the fast decay time. By alkali metal co-doping, 80–90 nm excitation bands were enhanced when compared with the nondoped LiCaAlF6. K, Rb and Cs co-doped Eu:LiCaAlF6 showed the enhancement of the fast decay time while Na exhibited the light yield enhancement. Finally, Eu 2% and K 2% co-doped LiCaAlF6 was examined on the pulse shape discrimination capability, and the detection performance can be improved by K co-doping.
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Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (A) 26249147 from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) and partially by JST A-step. The Cooperative Research Project of Research Institute of Electronics, Shizuoka University, KRF foundation and Inamori foundation are also acknowledged.
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