- Email: [email protected]
Properties of LuAP:Ce scintillator containing intentional impurities
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 571 (2007) 325–328 www.elsevier.com/locate/nima
Properties of LuAP:Ce scintillator containing intentional impurities A.G. Petrosyana,, M. Derdzyana, K. Ovanesyana, P. Lecoqb, E. Auffrayb, J. Trummerb, M. Kronbergerb, C. Pedrinic, C. Dujardinc, P. Anfrec a
Institute for Physical Research, 378410 Ashtarak-2, Armenia b CERN, 1211 Geneva 23, Switzerland c LPCML UMR CNRS 5620, Universite Lyon1, 69622 Villeurbanne, France Available online 9 November 2006
Abstract Single crystals of LuAP:Ce and LuYAP(Lu*70%):Ce co-doped with tetravalent (Hf and Zr) and pentavalent (Ta) ions were grown from melts by the Bridgman process. Underlying absorption, slope of the optical edge and transmission in the range of emission were compared to those of LuAP:Ce crystals. Absorption coefficients at 260 nm less than 2 cm1 have been recorded in LuAP:Ce crystals containing tetravalent ions that are lower than the corresponding figures (5–6 cm1) measured in undoped LuAP. At high concentrations of added impurities, despite of suppression of the parasitic underlying absorption below 300 nm, the slope of the optical edge and transmission in the range of emission are seriously damaged. Scintillation parameters of crystals with added impurities are compared to those of LuAP:Ce. r 2006 Elsevier B.V. All rights reserved. PACS: 81.10.h; 61.72.S; 29.40.M Keywords: Scintillators; LuAP; Bridgman; Impurities
1. Introduction LuAP:Ce (LuAlO3:Ce3+) is well-recognized PET detector material with several attractive properties, such as high density and stopping power, fast decay, and good linearity of energy response. Scintillation parameters of LuAP:Ce crystals grown by different methods have been recently reported in Ref. [1]. To offer higher sensitivity of imaging instruments, there is a need to improve the light yield, which is the present limitation of LuAP:Ce. Several ways have been recognized as having potential to improve the light yield, such as (i) introduction of high concentrations of Ce3+, (ii) improvement of transparency in the range of emission, or (iii) introduction of impurities capable suppressing some of the traps and thus increasing the light intensity in the fast component at the expense of the slow component. The concentration of Ce3+ in so far studied LuAP:Ce crystals grown by the Bridgman process is up to Corresponding author. Tel.: +3 7410 28 8150; fax: +3 374 323 1132.
E-mail address: [email protected] (A.G. Petrosyan). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.10.093
0.4–0.45 at% and there is no clear saturation of the light yield in the corresponding correlation dependence; due to crystal chemistry reasons, it is however difficult to maintain high optical quality in more concentrated crystals requiring better control of growth conditions. The absorption coefficient at 260 nm, which is a measure of the underlying absorption, is in the range 6–10 cm1 in most of Bridgman crystals but shows a slight increase with Ce3+ concentration; a clear improvement of this parameter has been observed in crystals grown using lutetium oxides of better purity [2]. Improvement of the transparency in the range of emission has led to an increase of the light yield ratio measured in the long and short directions in 2 2 8 mm3 pixels from 50% to around 70% [3]. The role of traps in contribution of the slow component has been recognized; however, no systematic study is so far reported and it is therefore unclear whether the traps can be significantly modified. The first studies of the influence of non-isovalent impurities on properties of LuAP were reported in Ref. [3]; in particular, optical properties and radiation-induced effects in LuAP:Ce with intentional Hf or Zr ions were
ARTICLE IN PRESS A.G. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 325–328
2. Experimental details LuAP:Ce,Hf, LuYAP:Ce,Hf, LuYAP:Ce,Zr and LuYAP:Ce,Ta (with Y*30%) single crystals were grown by the Bridgman process from melts contained in molybdenum crucibles under an argon/hydrogen atmosphere. The purity of oxides (Lu2O3, Y2O3, CeO2, and Al2O3) was 99.99% or better. Intentional impurities were introduced as HfO2, ZrO2 or Ta2O5 in amounts of 30–100 ppm. Experimental conditions used in crystal growth provided for clear and single-phase crystals of +14 80 mm, which were cut to finished 0.2–2 mm thick plates and 2 2 8 mm3 scintillation elements. Optical absorption and transmission spectra were recorded at 300 K in the range 180–600 nm (Specord M40). Hfcontaining crystals were studied in more details. The actual concentration of Ce in grown crystals was determined by optical absorption means with an estimated accuracy of about 10%, as described in Ref. [4]. Polished plates 0.2–2 mm thick were used in optical studies to establish compositions leading to the smallest absorption at 260 nm and to the smallest overlap between the absorption and emission curves; elements of 2 2 8 mm3 size polished on all sides were used in the measurements of X-ray excited emission and scintillation using techniques as described in Ref. [1].
35 30 Absorption coefficient, cm-1
studied and compared to those of LuAP:Ce. In the present work, which continues the studies of Ref. [3], new series of LuYAP:Ce, with intentional Hf, Zr and Ta impurities have been grown. In contrast to Ref. [3], low underlying absorption attained in currently grown LuAP crystals [1,2] enabled to follow fine variations in optical absorption spectra caused by intentional impurities, when introduced even in trace amounts. Scintillation measurements included light yield, decay time constants and else basic parameters.
LuAP:Ce, Hf (30ppm) LuAP:Ce
25 20 15 10 7.3 cm-1
5 0
1.4 cm-1 200
300
400
500
λ, nm Fig. 1. UV absorption spectra of LuAP:Ce (0.2 at%) and LuAP:Ce (0.2 at%), Hf (30 ppm); d ¼ 2 mm.
14 LuAP:Ce LuAP:Ce, Hf (30 ppm)
12 K at 260 nm (cm-1)
326
10 8 6 4 2 0
3. Results and discussion
0.20
0.15
0.25
0.30
0.35
Ce (at. %)
3.1. Absorption 12
LuYAP:Ce LuYAP:Ce, Hf (30 ppm) 10 K at 260 nm (cm-1)
Fig. 1 shows the UV absorption spectra measured in LuAP:Ce and LuAP:Ce,Hf (30 ppm) crystals containing similar Ce3+ concentrations. As compared to LuAP:Ce, a clear decrease of underlying absorption in Hf-containing crystal is seen. The absorption coefficient at 260 nm goes down to below 2 cm1, which is lower than the corresponding figures measured in LuAP:Ce (6–10 cm1) [2] and in un-doped LuAP (5–6 cm1) (see for example Ref. [5]). A possible role of tetravalent Hf ions is the reduction of oxygen vacancies, and thus of potential F-centers, while the opposite is true for divalent Ca ions, which may favor the formation of oxygen vacancies. The absorption spectrum of LuAP:Ce,Ca, as compared to LuAP:Ce, shows strong increase of intensities of bands peaking at 220, 260 and 275 nm [3], while the same bands are suppressed in LuAP:Ce,Hf. It can therefore be suggested that the low underlying absorption in the Hf-containing crystal (Fig. 1)
8
6
4 0.20
0.25
0.30
0.35
0.40
0.45
0.50
Ce (at. %) Fig. 2. Dependence of absorption coefficient at 260 nm on Ce3+ content in crystals: LuAP:Ce and LuAP:Ce,Hf (top); LuYAP:Ce and LuYAP: Ce,Hf (bottom).
ARTICLE IN PRESS A.G. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 325–328
is due to the reduction of oxygen vacancies. Note also that in the structurally related YAP crystal the absorption band at 275 nm has been also related to F-center [6]. Another observation is that, in contrast to the case of LuAP:Ce crystals, the absorption coefficient at 260 nm in LuAP:Ce,Hf does not exhibit any noticeable dependence on the Ce3+ concentration, (Fig. 2). The same is true for the concentration series of LuYAP:Ce,Hf, as compared to LuYAP:Ce. The slope of the optical edge depends on the concentration of added impurities: in LuAP:Ce the slope is commonly in the range 7.3–7.8%/nm [2]; in LuAP:Ce,Hf (30–40 ppm) the measured values are in the range 7.0–7.6%/nm; in LuAP:Ce,Zr (60 ppm) the values are even lower, about 5%/nm.
327
crystals with different concentrations of Zr (60, 80, and 100 ppm). The transmission spectrum of LuAP:Ce,Hf (30 ppm) crystal is close to that of LuAP:Ce. Fig. 3a shows the evolution of transmission spectra in LuYAP:Ce,Zr crystals; upon increasing the Zr concentration, a clear absorption band appears at 372 nm, which is responsible for the drop in emission intensity measured in the same series, as shown in Fig. 3b. The damage of transmission may be due to the formation of defects associated with cation vacancies, which form upon incorporation of high concentrations of tetravalent ions. In the difference absorption spectra between LuAP:Ce,Zr samples with various Zr concentrations, besides the center at 372 nm, there is also an intensive band peaking at about 335 nm (in related YAP crystal this band is assigned to a hole-type center [7]).
3.2. X-ray excited emission 3.3. Scintillation Emission spectra under X-ray excitation were measured in LuAP:Ce,Hf (30 ppm) and in a series of LuYAP:Ce,Zr 100
a 4 3
transmission (%)
80
2 60
1
40
20
The scintillation parameters that were measured include the light yield, light yield ratio, energy resolution and decay. The crystals were excited by g-rays emitted from a Cs-137 source (662 keV). The light output in horizontal geometry was carried out with the crystal optically coupled to a XP2020Q photomultiplier by means of silicon oil or grease (Rhodorsil Silicones paˆte 4) and wrapped with Teflon Tape. For the light yield measurement in the vertical position, the samples were wrapped with several layers of Teflon tape and tucked into a Teflon cylinder. The electronic chain consists of an attenuator, a bipolar shaping amplifier with the first peak after 1 ms and zero crossing after 2.1 ms, an ADC and MCA. Summary of scintillation parameters for three crystal batches (LuAP: Ce,Hf; LuYAP:Ce,Hf; LuYAP:Ce,Ta) is given in Table 1.
0 400
300
500
600
λ (nm) 8000
b
2 2 8 mm3
4
LuAP:CeHf
LuYAP:CeHf
LuYAP:CeTa
0.26
0.3
0.35
30067150 52257261 57.61
41127206 77417387 53.2
54877274 87547438 62.3
Energy resolution (%) Vertical Horizontal
18.1270.9 13.6470.68
15.3170.76 11.3670.56
15.0070.75 11.0470.55
Decay (ns) t1 t2
17.670.23 112714.8
21.2270.36 179.5712.5
20.5 172.4
Intensity (%) F1 F2
75.9 24.1
52.7 47.5
55 45
Ratio of light at (300/1600 ns)
98.4
91.04
91.9
Average %Ce Light yield (ph/MeV) Vertical Horizontal Ratio v/h (%)
7000 emission intensity (a.u.)
Table 1 Summary of scintillation dataa
6000 5000 4000 3 3000 2 2000 1
1000 0 250
300
350
400
450
500
λ (nm) Fig. 3. (a) Transmission spectra of LuYAP:Ce,Zr (curves 1–3 correspond to Zr concentrations of 100, 80 and 60 ppm, respectively) and LuAP:Ce,Hf (30 ppm) (curve 4); (b) X-ray excited emission measured in the same series.
a
Average values are given for batches of 10 pcs with 30 ppm nominal concentration of Hf and Ta and %Ce in the range 0.2–0.4.
ARTICLE IN PRESS A.G. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 325–328
328
Energy resolution (%)
22
a
LuAP:Ce,Hf (30ppm)
component gradually decreases from 23 ns (0.22% of Ce) to 20.5 ns (0.37% of Ce); the slow component decreases from 220 to 160 ns.
horizontal vertical
20
4. Conclusions
18
16
14
12 2000
3000
4000
6000
5000
LY (ph/MeV) 22
Energy resolution (%)
20
b
LuYAP:Ce,Hf (30ppm)
horizontal vertical
18 16 14 12 10 3000
4000
5000
6 000
7 000
800 0
9000
LY (ph/MeV) Fig. 4. Correlation between energy resolution and light yield in horizontal and vertical position in crystals: (a) LuAP:Ce,Hf (30 ppm), and (b) LuYAP:Ce,Hf (30 ppm).
Tetravalent (Hf, Zr) and pentavalent (Ta) ions have been introduced in LuAP:Ce or LuYAP:Ce crystals. Optical observations confirm that these ions suppress some of the color centers located under absorption bands of Ce3+ ions, which possibly arise from oxygen vacancies. As a result, the underlying absorption is strongly reduced (to less than 2 cm1), as compared to that measured in LuAP:Ce (6–10 cm1) or un-doped LuAP (5–6 cm1). Modification of the trap ensemble is also reflected in somewhat shorter decay both for the fast and slow components. Despite of reduction of the underlying absorption, the figures for the light yield and the light yield ratio in LuAP:Ce,Hf and LuYAP:Ce,Hf are lower than those in LuAP:Ce and LuYAP:Ce studied previously, suggesting a stronger selfabsorption of the emitted light in the present series. This is confirmed by measurements of the slope of the optical absorption edge, which is shallower in Hf-containing crystals leading to larger overlap between absorption and emission. Another reason can be due to difference in purity of oxides used in crystal growth in Ref. [1] and in the present work, which is difficult to control. At high (X60 ppm) concentrations of added impurities, as shown for the case of Zr, transmission in the range of emission is further damaged due to appearance of additional absorption bands, which strongly reduce the emission intensity. Acknowledgments This work is performed in the frame of Crystal Clear Collaboration and supported by ISTC (project #A-1165).
As in Ref. [1], the light yield increases linearly with the Ce concentration. No double peaks have been seen in the light yield spectra in the present series of crystals and a clear correlation exists between the light yield and the energy resolution, when measured in the vertical and horizontal positions (Fig. 4). The figures for the light yield and light yield ratio are lower, as compared to those measured in LuAP:Ce [1], suggesting stronger self-absorption of the light in the present crystal series. Both the fast and slow decay constants show a stronger dependence on the Ce content: in concentration series of LuAP: (Ce,Hf) the fast component gradually decreases from 18.5 ns (0.18% of Ce) to 16.5 ns (0.42% of Ce); the slow component decreases from 130 to 90 ns. The same tendency is observed in LuYAP:Ce,Hf series: the fast
References [1] J. Trummer, E. Auffray, P. Lecoq, A. Petrosyan, P. Sempere-Roldan, Nucl. Instr. Meth. A 551 (2005) 339. [2] A.G. Petrosyan, M. Derdzyan, K. Ovanesyan, et al., in: Proceedings of Internternational Conference on Inorganic Scintillators and Their Applications ’SCINT’2005, Alushta, A. Gektin, B. Grinyov, (Eds.), Ukraine-Kharkov, 2006, pp. 442–445. [3] M. Derdzyan, A.G. Petrosyan, T.I. Butaeva, et al., Nucl. Instr. and Meth. A 537 (2005) 200. [4] A.G. Petrosyan, M. Derdzyan, P. Lecoq, et al., NSS and MIC Conference Records. Rome, 2004. [5] A. Lempicki, J. Glodo, Nucl. Instr. Meth. A 416 (1998) 333. [6] V.G. Baryshevski, M.V. Korzhik, B.I. Minkov, et al., J. Phys.: Condens. Matter 5 (1993) 7893. [7] O.F. Shirmer, K.W. Blazey, W. Berlinger, Phys. Rev. B 11 (1975) 4201.
Nuclear Instruments and Methods in Physics Research A 571 (2007) 325–328 www.elsevier.com/locate/nima
Properties of LuAP:Ce scintillator containing intentional impurities A.G. Petrosyana,, M. Derdzyana, K. Ovanesyana, P. Lecoqb, E. Auffrayb, J. Trummerb, M. Kronbergerb, C. Pedrinic, C. Dujardinc, P. Anfrec a
Institute for Physical Research, 378410 Ashtarak-2, Armenia b CERN, 1211 Geneva 23, Switzerland c LPCML UMR CNRS 5620, Universite Lyon1, 69622 Villeurbanne, France Available online 9 November 2006
Abstract Single crystals of LuAP:Ce and LuYAP(Lu*70%):Ce co-doped with tetravalent (Hf and Zr) and pentavalent (Ta) ions were grown from melts by the Bridgman process. Underlying absorption, slope of the optical edge and transmission in the range of emission were compared to those of LuAP:Ce crystals. Absorption coefficients at 260 nm less than 2 cm1 have been recorded in LuAP:Ce crystals containing tetravalent ions that are lower than the corresponding figures (5–6 cm1) measured in undoped LuAP. At high concentrations of added impurities, despite of suppression of the parasitic underlying absorption below 300 nm, the slope of the optical edge and transmission in the range of emission are seriously damaged. Scintillation parameters of crystals with added impurities are compared to those of LuAP:Ce. r 2006 Elsevier B.V. All rights reserved. PACS: 81.10.h; 61.72.S; 29.40.M Keywords: Scintillators; LuAP; Bridgman; Impurities
1. Introduction LuAP:Ce (LuAlO3:Ce3+) is well-recognized PET detector material with several attractive properties, such as high density and stopping power, fast decay, and good linearity of energy response. Scintillation parameters of LuAP:Ce crystals grown by different methods have been recently reported in Ref. [1]. To offer higher sensitivity of imaging instruments, there is a need to improve the light yield, which is the present limitation of LuAP:Ce. Several ways have been recognized as having potential to improve the light yield, such as (i) introduction of high concentrations of Ce3+, (ii) improvement of transparency in the range of emission, or (iii) introduction of impurities capable suppressing some of the traps and thus increasing the light intensity in the fast component at the expense of the slow component. The concentration of Ce3+ in so far studied LuAP:Ce crystals grown by the Bridgman process is up to Corresponding author. Tel.: +3 7410 28 8150; fax: +3 374 323 1132.
E-mail address: [email protected] (A.G. Petrosyan). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.10.093
0.4–0.45 at% and there is no clear saturation of the light yield in the corresponding correlation dependence; due to crystal chemistry reasons, it is however difficult to maintain high optical quality in more concentrated crystals requiring better control of growth conditions. The absorption coefficient at 260 nm, which is a measure of the underlying absorption, is in the range 6–10 cm1 in most of Bridgman crystals but shows a slight increase with Ce3+ concentration; a clear improvement of this parameter has been observed in crystals grown using lutetium oxides of better purity [2]. Improvement of the transparency in the range of emission has led to an increase of the light yield ratio measured in the long and short directions in 2 2 8 mm3 pixels from 50% to around 70% [3]. The role of traps in contribution of the slow component has been recognized; however, no systematic study is so far reported and it is therefore unclear whether the traps can be significantly modified. The first studies of the influence of non-isovalent impurities on properties of LuAP were reported in Ref. [3]; in particular, optical properties and radiation-induced effects in LuAP:Ce with intentional Hf or Zr ions were
ARTICLE IN PRESS A.G. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 325–328
2. Experimental details LuAP:Ce,Hf, LuYAP:Ce,Hf, LuYAP:Ce,Zr and LuYAP:Ce,Ta (with Y*30%) single crystals were grown by the Bridgman process from melts contained in molybdenum crucibles under an argon/hydrogen atmosphere. The purity of oxides (Lu2O3, Y2O3, CeO2, and Al2O3) was 99.99% or better. Intentional impurities were introduced as HfO2, ZrO2 or Ta2O5 in amounts of 30–100 ppm. Experimental conditions used in crystal growth provided for clear and single-phase crystals of +14 80 mm, which were cut to finished 0.2–2 mm thick plates and 2 2 8 mm3 scintillation elements. Optical absorption and transmission spectra were recorded at 300 K in the range 180–600 nm (Specord M40). Hfcontaining crystals were studied in more details. The actual concentration of Ce in grown crystals was determined by optical absorption means with an estimated accuracy of about 10%, as described in Ref. [4]. Polished plates 0.2–2 mm thick were used in optical studies to establish compositions leading to the smallest absorption at 260 nm and to the smallest overlap between the absorption and emission curves; elements of 2 2 8 mm3 size polished on all sides were used in the measurements of X-ray excited emission and scintillation using techniques as described in Ref. [1].
35 30 Absorption coefficient, cm-1
studied and compared to those of LuAP:Ce. In the present work, which continues the studies of Ref. [3], new series of LuYAP:Ce, with intentional Hf, Zr and Ta impurities have been grown. In contrast to Ref. [3], low underlying absorption attained in currently grown LuAP crystals [1,2] enabled to follow fine variations in optical absorption spectra caused by intentional impurities, when introduced even in trace amounts. Scintillation measurements included light yield, decay time constants and else basic parameters.
LuAP:Ce, Hf (30ppm) LuAP:Ce
25 20 15 10 7.3 cm-1
5 0
1.4 cm-1 200
300
400
500
λ, nm Fig. 1. UV absorption spectra of LuAP:Ce (0.2 at%) and LuAP:Ce (0.2 at%), Hf (30 ppm); d ¼ 2 mm.
14 LuAP:Ce LuAP:Ce, Hf (30 ppm)
12 K at 260 nm (cm-1)
326
10 8 6 4 2 0
3. Results and discussion
0.20
0.15
0.25
0.30
0.35
Ce (at. %)
3.1. Absorption 12
LuYAP:Ce LuYAP:Ce, Hf (30 ppm) 10 K at 260 nm (cm-1)
Fig. 1 shows the UV absorption spectra measured in LuAP:Ce and LuAP:Ce,Hf (30 ppm) crystals containing similar Ce3+ concentrations. As compared to LuAP:Ce, a clear decrease of underlying absorption in Hf-containing crystal is seen. The absorption coefficient at 260 nm goes down to below 2 cm1, which is lower than the corresponding figures measured in LuAP:Ce (6–10 cm1) [2] and in un-doped LuAP (5–6 cm1) (see for example Ref. [5]). A possible role of tetravalent Hf ions is the reduction of oxygen vacancies, and thus of potential F-centers, while the opposite is true for divalent Ca ions, which may favor the formation of oxygen vacancies. The absorption spectrum of LuAP:Ce,Ca, as compared to LuAP:Ce, shows strong increase of intensities of bands peaking at 220, 260 and 275 nm [3], while the same bands are suppressed in LuAP:Ce,Hf. It can therefore be suggested that the low underlying absorption in the Hf-containing crystal (Fig. 1)
8
6
4 0.20
0.25
0.30
0.35
0.40
0.45
0.50
Ce (at. %) Fig. 2. Dependence of absorption coefficient at 260 nm on Ce3+ content in crystals: LuAP:Ce and LuAP:Ce,Hf (top); LuYAP:Ce and LuYAP: Ce,Hf (bottom).
ARTICLE IN PRESS A.G. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 325–328
is due to the reduction of oxygen vacancies. Note also that in the structurally related YAP crystal the absorption band at 275 nm has been also related to F-center [6]. Another observation is that, in contrast to the case of LuAP:Ce crystals, the absorption coefficient at 260 nm in LuAP:Ce,Hf does not exhibit any noticeable dependence on the Ce3+ concentration, (Fig. 2). The same is true for the concentration series of LuYAP:Ce,Hf, as compared to LuYAP:Ce. The slope of the optical edge depends on the concentration of added impurities: in LuAP:Ce the slope is commonly in the range 7.3–7.8%/nm [2]; in LuAP:Ce,Hf (30–40 ppm) the measured values are in the range 7.0–7.6%/nm; in LuAP:Ce,Zr (60 ppm) the values are even lower, about 5%/nm.
327
crystals with different concentrations of Zr (60, 80, and 100 ppm). The transmission spectrum of LuAP:Ce,Hf (30 ppm) crystal is close to that of LuAP:Ce. Fig. 3a shows the evolution of transmission spectra in LuYAP:Ce,Zr crystals; upon increasing the Zr concentration, a clear absorption band appears at 372 nm, which is responsible for the drop in emission intensity measured in the same series, as shown in Fig. 3b. The damage of transmission may be due to the formation of defects associated with cation vacancies, which form upon incorporation of high concentrations of tetravalent ions. In the difference absorption spectra between LuAP:Ce,Zr samples with various Zr concentrations, besides the center at 372 nm, there is also an intensive band peaking at about 335 nm (in related YAP crystal this band is assigned to a hole-type center [7]).
3.2. X-ray excited emission 3.3. Scintillation Emission spectra under X-ray excitation were measured in LuAP:Ce,Hf (30 ppm) and in a series of LuYAP:Ce,Zr 100
a 4 3
transmission (%)
80
2 60
1
40
20
The scintillation parameters that were measured include the light yield, light yield ratio, energy resolution and decay. The crystals were excited by g-rays emitted from a Cs-137 source (662 keV). The light output in horizontal geometry was carried out with the crystal optically coupled to a XP2020Q photomultiplier by means of silicon oil or grease (Rhodorsil Silicones paˆte 4) and wrapped with Teflon Tape. For the light yield measurement in the vertical position, the samples were wrapped with several layers of Teflon tape and tucked into a Teflon cylinder. The electronic chain consists of an attenuator, a bipolar shaping amplifier with the first peak after 1 ms and zero crossing after 2.1 ms, an ADC and MCA. Summary of scintillation parameters for three crystal batches (LuAP: Ce,Hf; LuYAP:Ce,Hf; LuYAP:Ce,Ta) is given in Table 1.
0 400
300
500
600
λ (nm) 8000
b
2 2 8 mm3
4
LuAP:CeHf
LuYAP:CeHf
LuYAP:CeTa
0.26
0.3
0.35
30067150 52257261 57.61
41127206 77417387 53.2
54877274 87547438 62.3
Energy resolution (%) Vertical Horizontal
18.1270.9 13.6470.68
15.3170.76 11.3670.56
15.0070.75 11.0470.55
Decay (ns) t1 t2
17.670.23 112714.8
21.2270.36 179.5712.5
20.5 172.4
Intensity (%) F1 F2
75.9 24.1
52.7 47.5
55 45
Ratio of light at (300/1600 ns)
98.4
91.04
91.9
Average %Ce Light yield (ph/MeV) Vertical Horizontal Ratio v/h (%)
7000 emission intensity (a.u.)
Table 1 Summary of scintillation dataa
6000 5000 4000 3 3000 2 2000 1
1000 0 250
300
350
400
450
500
λ (nm) Fig. 3. (a) Transmission spectra of LuYAP:Ce,Zr (curves 1–3 correspond to Zr concentrations of 100, 80 and 60 ppm, respectively) and LuAP:Ce,Hf (30 ppm) (curve 4); (b) X-ray excited emission measured in the same series.
a
Average values are given for batches of 10 pcs with 30 ppm nominal concentration of Hf and Ta and %Ce in the range 0.2–0.4.
ARTICLE IN PRESS A.G. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 325–328
328
Energy resolution (%)
22
a
LuAP:Ce,Hf (30ppm)
component gradually decreases from 23 ns (0.22% of Ce) to 20.5 ns (0.37% of Ce); the slow component decreases from 220 to 160 ns.
horizontal vertical
20
4. Conclusions
18
16
14
12 2000
3000
4000
6000
5000
LY (ph/MeV) 22
Energy resolution (%)
20
b
LuYAP:Ce,Hf (30ppm)
horizontal vertical
18 16 14 12 10 3000
4000
5000
6 000
7 000
800 0
9000
LY (ph/MeV) Fig. 4. Correlation between energy resolution and light yield in horizontal and vertical position in crystals: (a) LuAP:Ce,Hf (30 ppm), and (b) LuYAP:Ce,Hf (30 ppm).
Tetravalent (Hf, Zr) and pentavalent (Ta) ions have been introduced in LuAP:Ce or LuYAP:Ce crystals. Optical observations confirm that these ions suppress some of the color centers located under absorption bands of Ce3+ ions, which possibly arise from oxygen vacancies. As a result, the underlying absorption is strongly reduced (to less than 2 cm1), as compared to that measured in LuAP:Ce (6–10 cm1) or un-doped LuAP (5–6 cm1). Modification of the trap ensemble is also reflected in somewhat shorter decay both for the fast and slow components. Despite of reduction of the underlying absorption, the figures for the light yield and the light yield ratio in LuAP:Ce,Hf and LuYAP:Ce,Hf are lower than those in LuAP:Ce and LuYAP:Ce studied previously, suggesting a stronger selfabsorption of the emitted light in the present series. This is confirmed by measurements of the slope of the optical absorption edge, which is shallower in Hf-containing crystals leading to larger overlap between absorption and emission. Another reason can be due to difference in purity of oxides used in crystal growth in Ref. [1] and in the present work, which is difficult to control. At high (X60 ppm) concentrations of added impurities, as shown for the case of Zr, transmission in the range of emission is further damaged due to appearance of additional absorption bands, which strongly reduce the emission intensity. Acknowledgments This work is performed in the frame of Crystal Clear Collaboration and supported by ISTC (project #A-1165).
As in Ref. [1], the light yield increases linearly with the Ce concentration. No double peaks have been seen in the light yield spectra in the present series of crystals and a clear correlation exists between the light yield and the energy resolution, when measured in the vertical and horizontal positions (Fig. 4). The figures for the light yield and light yield ratio are lower, as compared to those measured in LuAP:Ce [1], suggesting stronger self-absorption of the light in the present crystal series. Both the fast and slow decay constants show a stronger dependence on the Ce content: in concentration series of LuAP: (Ce,Hf) the fast component gradually decreases from 18.5 ns (0.18% of Ce) to 16.5 ns (0.42% of Ce); the slow component decreases from 130 to 90 ns. The same tendency is observed in LuYAP:Ce,Hf series: the fast
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