Comparative analysis of scintillation characteristics derived from different emission mechanisms in BaCl 2. Akihiro Fukabori *

Transcription

1 Comparative analysis of scintillation characteristics derived from different emission mechanisms in BaCl 2 Akihiro Fukabori * Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA *Corresponding author: akihirofukabori@yahoo.co.jp Abstract Crack-free La 3+ -substituted BaCl 2 single crystals without and with Eu 2+ as an activator were successfully grown using the micro-pulling-down method. Luminescence bands from BaCl 2 without and with the Eu 2+ activator were assigned as mainly intrinsic and extrinsic, respectively. Therefore, BaCl 2 as a host material is suitable for investigating the effects of the emission mechanism on the non-proportionality curves and energy resolution plots. First, the scintillation characteristics of BaCl 2 :La 3+ (11 at%) and BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) single crystals were determined along with those of BaCl 2 and BaCl 2 :Eu 2+ (0.5 at%) crystals. Second, the non-proportionality curves from the different emission origins in BaCl 2 were confirmed to be consistent with phenomenological models. Third, the non-proportionality curves and energy resolution plots originating from intrinsic (without Eu 2+ ) and extrinsic (with Eu 2+ ) luminescence were compared. The results experimentally demonstrated that the emission mechanism influenced scintillator non-proportionality curves. 1

2 I. Introduction To date, studies comparing scintillation properties from different emission origins have been insufficient. The origins of scintillators are generally categorized as intrinsic (i.e. excitonic) or extrinsic (i.e. non-excitonic) or mixed (i.e. intermediate) emissions. Intrinsic emissions include self-trapped exciton (STE) emission and Auger-free luminescence (AFL) 1 (AFL, core-valence luminescence, and cross luminescence are the same). AFL is a unique phenomenon for alkali halide scintillators. On the other hand, emissions originating from trapped excitons (TEs, categorized as excitonic) and activators, such as Eu 2+, Ce 3+, and Tl +, are considered to be extrinsic. The radioluminescence spectrum of pure BaCl 2 comprises two luminescent bands at ev ( nm) and 4.13 ev (300 nm) 2-5. The luminescence band near 4.13 ev is regarded as an STE-derived emission and not an AFL-derived emission 2-5. Because, first, AFL is observed when the energy gap (E vc ) between the top of the valence band and the uppermost core band is less than the band gap (E g ): E vc < E 1 g. However, E g values of 7.0, 7.5, and approximately 9.0 ev 6-8 have been reported for pure BaCl 2, which are less than the E vc value ( ev) 7. Second, the intensity of AFL does not change with temperature 9 but the light yield for BaCl 2 does 10. Therefore, considering these two facts, the luminescence band near 4.13 ev was determined to be an STE-derived emission. On the other hand, the origin of the emission band near ev for pure BaCl 2 has not yet been definitively determined. This emission band may be assigned to an extrinsic emission derived from defects and/or impurities 5. The other, Eu 2+ activator compounds exhibit typical Eu 2+ luminescent bands from t 2g and e g to 8 S 7/2 orbitals 11. Thus, BaCl 2, which emits without and with Eu 2+ activator, is a very rare compound and an ideal material for comparing the scintillation properties from intrinsic and extrinsic origins. Non-proportionality indicates a situation where the light output from a scintillator is not exactly proportional to the irradiated energy. Several researchers have reported theoretical studies of this phenomenon 12, 13. The shapes of non-proportionality curves have been predicted by categorizing scintillators into three groups: excitonic scintillators, non-excitonic scintillators, and intermediate scintillators. However, only the behavior of four representative scintillators (NaI:Tl, BaF 2, GSO:Ce, and LaCl 3 :Ce) were discussed. Thus, it should be confirmed whether this model is applicable to other scintillators. Additionally, the dependence of the non-proportionality curves in the same host on the emission origins has not yet been well studied 14, 15. The shapes of non-proportionality curves have been phenomenologically predicted considering both linear and non-linear processes including bimolecular (second order) and Auger (third order) processes in terms of the rates of radiative and non-radiative (quenching) recombination and the fractional concentration (f x ) of excitons within the track 12, 13. 2

3 Non-proportionality curves for excitonic scintillators decrease from 662 kev, and for these scintillators, f x = 1, indicating that only excitons are generated within the track. In non-excitonic scintillators, the excited e h pairs remain unbound and do not form excitons and f x ~ 0. The curves in this category show a hump below 662 kev that is unique to alkali halide scintillators. Most scintillators are considered to be intermediate scintillators, for which f x ranges from 0 to 1, indicating that both excitons and e h pairs are created within the track. The shapes of the curves of compounds in this category are intermediate between those for f x = 1 and f x ~ 0. Namely, the curves for compounds with f x close to 1 gradually decline, while those for compounds with f x close to 0 show a tiny hump. Strictly speaking, when predicting curve shapes, not only the fraction concentration f x but also the rate of radiative and non-radiative recombination should be considered. However, one can roughly predict the shape of 12, non-proportionality curves using f 13 x. The energy resolution is defined by dividing the full width at half maximum (FWHM) by the centroid peak channel of the photo-peak. Theoretically, the ideal energy resolution is represented only by statistical fluctuation and must obey Poisson distribution. However, actual energy resolution plots do not strictly follow only the Poisson distribution. This difference is caused by non-uniformity including the inhomogeneous distribution of the emission centers and poor crystal quality, and non-proportional contribution which is defined as the square root of the sum of the squares of multi-compton scatter contribution and delta-ray contribution 16. The distribution of the emission centers depends on their types. Therefore, the effects of different emission centers to the energy resolution should be investigated. The melt growth of hygroscopic chloride crystals including BaCl 2 is difficult because of the need to prevent the exposure of both the starting materials and grown crystals to the surrounding atmosphere. Furthermore, BaCl 2 crystal growth is challenging because a phase transition occurs when the material is cooled from its melting point to room temperature 17, 18. Specifically, a cubic orthorhombic phase transition is observed at approximately 920 C 2 in 17, pure BaCl According to the phase diagram 18, 10 atomic% (at%) La-substituted BaCl 2 may be cooled to room temperature without the occurrence of the cubic (high temperature) orthorhombic (room temperature) phase transition. However, an accurate La concentration could not be identified from the phase diagram 18. The melting point of BaCl 2 is 960 C, its density is 3.89 g/cm 3 19, and its effective atomic number Z eff (50.5) is relatively high; additionally, BaCl 2 is slightly hygroscopic. The scintillation decay for BaCl 2 was reported to be very fast with components at 1.6 ns and 34.8 ns 4. In this case, the authors assigned the faster component to STE emission and the slower component to an extrinsic origin 4. Different researchers also reported scintillation decays with 137 Cs excitation that had either three components at 2.54 (18.5%), (79.7%), and 520 3

4 (1.82%) ns 10 or one component at 980 ns 2, 3. The light yields for BaCl 2 have been reported to be 1,500 (0.5 s shaping time) and 1,700 (10 s shaping time) ph/mev with an energy resolution of 17.4% 2, 3. For Eu 2+ -doped samples, the typical Eu 2+ decay time was reported to be 390 ns with 100% contribution, and the light yields for Eu 2+ (0.1 at%)-activated BaCl 2 were found to be 14,400 (0.5 s shaping time) and 19,400 (10 s shaping time) ph/mev with an energy resolution of 8.8% 2, 3. The energy resolution for 5% Eu-doped BaCl 2 has also been reported to be 3.5% 3.8% 20. The aim of the present study is to determine the scintillation properties derived from intrinsic and extrinsic sources for BaCl 2, La 3+ -substituted BaCl 2, Eu 2+ -activated BaCl 2, and Eu 2+ -activated and La 3+ -substituted BaCl 2. Furthermore, the non-proportionality curves and energy resolution plots for intrinsic (without Eu 2+ ) and extrinsic (with Eu 2+ activator) emissions are evaluated to determine if they follow theoretical models. The non-proportionality curves and energy resolution plots derived from the different emission mechanisms are also compared. II. Experimental procedures Non-activated and non-substituted, non-activated and La 3+ -substituted, Eu 2+ -activated and non-substituted, Eu 2+ -activated and La 3+ -substituted BaCl 2 single crystals ( = 4 5 mm) were grown via the micro-pulling-down method 21. Specially, La 3+ -substituted single crystal rods were grown without cracks for the first time. EuCl 2 (4N), LaCl 3 (4N), and BaCl 2 (5N) were used as the starting materials. These ingredients were mixed well using an agate mortar and pestle before growth in a glove box with a controlled atmosphere containing less than 0.1 ppm H 2 O and 0.1 ppm O 2. Crystal growth was performed using a Pt crucible ( : mm, height: mm, and square die: 5 mm 2 ), a Pt after-heater, and Al 2 O 3 heat insulators. The edge of the die for the crucible was spherical. The starting ingredients were exposed to air briefly when placing them in the growth chamber. However, exposing the powders to air for a few minutes was not an issue, because BaCl 2 is slightly hygroscopic. All growths were performed after heating for one night (more than 8 h) in order to evaporate any moisture attached on the raw powders, the chamber, the crucible, the after-heater, and the heat insulators. All growths were carried out under the ambient pressure. Slow and long cool-down were performed for over 48 h after growth. The pulling down rate was mm/min. High purity Ar (5N) was used as the growth atmosphere. BaCl 2 ceramics and crystals were used as seeds. All the grown specimens were stored in the glove box to prevent them from absorbing moisture from the air. Details regarding the crystal growth process have been previously reported 22. Powder XRD patterns were obtained under excitation using X-rays from a Nonius FR591 water-cooled rotating copper-anode X-ray generator (50 kv, 60 ma). Samples were sealed with two pieces of plastic tape, attached to a nylon washer, and set in a special rotating chamber. The 4

5 X-rays passed through the samples and were absorbed by a Mar detector (Mar USA, Inc., Evanston, IL). Fit-2D software was used to transform the two-dimensional diffraction patterns (Debye Scherrer ring) to a one-dimensional pattern 23. A scanning electron microscope (Hitachi S-4300SE/N) with an energy-dispersive X-ray spectrometer (NORAN System SIX X-ray Microanalysis System, Thermo Electron Corporation) (SEM EDX) was used to investigate the La 3+ and Ba 2+ concentrations. Secondary electrons from an electron gun were accelerated at 15 kv for irradiating the samples. The characteristic X-rays from each element were detected for 120 s to reduce the measurement inaccuracy. Polished pellets cut from grown rods and residual solids in the crucible were evaluated. The analyses were performed using points and the line scan mode; the estimated results are summarized in Table I. Nominal chemical compositions were used as sample notations in this study. Photoluminescence spectra under Xe lamp excitation and radioluminescence spectra under X-ray excitation (50 kv, 60 ma) were observed using a SpectraPro-2150i (Acton Research Corp., Acton, MA) connected to a PIXIS:100B charged coupled device (CCD) (Princeton Instruments, Inc., Trenton, NJ) cooled at 70 C. The measurements were performed using three different spectrometers equipped with three gratings (red, green, and blue). Finally, the emission spectra were recorded by combining these three spectra into a single spectrum on a PC. The emission spectra were corrected by considering the background, grating efficiency, and CCD quantum efficiency 23. The discontinuity around 3.44 ev (360 nm) in the spectra was caused by a grating change in the SpectraPro-2150i monochromator. A pulse X-ray system was used to examine scintillation rise and decay times. This facility consists of a Nd:YAG laser that drives a cavity-dumped Ti-sapphire laser to produce 200 fs pulses at 800 nm (Mira 900 and Verdi V5, Coherent, Inc.). A part of the 800 nm beam was sent to a fast diode (Electro-Optics Technology, Inc.) with a 10 ps time jitter and its output was used to start an ORTEC 9308 picosecond time analyzer. The other part of doubled 400 nm beam was sent to an N5084 light-excited X-ray tube (Hamamatsu Photonics). The X-ray tube was operated with a grounded cathode and an anode at +40 kv. An R3809U-50 micro-channel phototube with a 35 ps FWHM time jitter (Hamamatsu Photonics) and an ORTEC 9307 pico-timing discriminator were used to generate stop pulses from the individual fluorescent photons. The impulse response of the system was 100 ps FWHM. An excitation rate of 165 khz was used and the data were digitized into 78 ps time bins over a range of 5,000 ns 24. To estimate the light yield and energy resolution using an excited 137 Cs 662 kev gamma source, pulse height spectra were examined with a bias photo multiplier tube (PMT, Hamamatsu Photonics R ) under an accelerating voltage of 700 V at room temperature. The specimens were optically coupled to the PMT window using optical grease (Viscasil 60,000, GE 5

6 silicones). The output signal was transmitted to a pre-amplifier (Canberra model 2005), a shaping amplifier (ORTEC 672) with 0.5 s and 10 s shaping times, a multi-channel analyzer (Ortec easy MCA 8K), and finally to a PC. NaI:Tl(0.2 at%) (12 cm cm height encapsulated in Al case) was used as a standard. Several layers of Teflon tape were used as ultraviolet light-collective materials. Small pieces of crystals were harvested from grown rods for pulse height measurements. All the small pieces were similar in size (approximately mm 3 ) and were transparent. The cleavage faces of all the pieces were attached to the PMT window to maximally collect the generated scintillation light. To evaluate the non-proportionality curves and energy resolution plots as a function of the photon energy, 133 Ba (32, 81, 276, 303, 356, and 384 kev), 241 Am (59.5 kev), 57 Co (122 kev), 60 Co (1333 kev), 22 Na (511 and 1275 kev), 137 Cs (33 and 662 kev), and 109 Cd (22 and 88 kev) radiation sources were used as excitation sources. To determine the photoabsorption peaks due to excitation by these various sources, multiple Gaussian fittings were performed considering multiple Compton scattering (i. e. background) and the characteristic X-ray escape peaks. All the pulse height measurements were performed at room temperature in the glove box with a controlled atmosphere containing less than 0.1 ppm H 2 O and 0.1 ppm O 2 to prevent the specimens from absorbing moisture in the air. III. Results and Discussions A. Phase identification, crystal growth, and composition analysis As exhibited in Fig. 1, most of the peaks in the XRD patterns for all four samples were identified as belonging to an orthorhombic BaCl 2 phase (PDF ). Cubic phase peaks were not identified in the patterns; this is not in agreement with previous reports for ceramics samples 11, 25. Furthermore, hexagonal BaCl 2 peaks were not observed at room temperature. Although heating for one night was performed prior to the growth of each sample to dry the starting powders, BaCl 2 (H 2 O) and/or BaCl 2 (H 2 O) 2 peaks appeared in all the XRD patterns. Growths were performed with careful handling to prevent contact with the starting powders and moisture in the air. Therefore, it is possible that moisture was absorbed from the air during the XRD analyses after growths. The XRD peak intensity depends only on the arrangement and types of atoms. Therefore, the actual concentrations of the BaCl 2 hydrates in the investigated samples could not be clarified using the XRD peak intensity. However, H 2 O dose not interact with X- and -rays. Therefore, the effect of H 2 O in BaCl 2 hydrates to the scintillation properties is not considered in the present study. Single crystal growth was attempted using the micro-pulling-down technique 21. As presented in Fig. 2, crack-free BaCl 2 :La 3+ (7.7 at%) and BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) single crystals ( = 4 5 mm, length = mm) were grown 22. However, the BaCl 2 and 6

7 BaCl 2 :Eu 2+ (0.5 at%) crystals had many cracks due to the occurrence of phase transition in 17, BaCl It was difficult to control the crystal shape for all the rods because of the volatilization of the starting powders and grown rods during growth at temperatures near 960 C and the nearly spherical configuration of the die of the crucible. Careful operation was required to avoid exposure of the starting materials and crystals to the atmosphere. Appropriate La substitution led to the growth of single crystals without cracks. These results confirmed that La substitution suppresses the phase transition in BaCl 2. Notably, the actual La 3+ concentrations were different from the nominal concentrations, because white solids always remained at the bottom of the crucible after crystal growths. Therefore, the actual La concentrations in each of the polished pellets and residual solids in the crucible were examined. The actual La 3+ concentrations were estimated using the Ba weight% and La weight% as determined via EDX, and the estimated concentrations at all points were averaged; the results are listed in Table I. The Eu 2+ peaks were not detected because the signals for Eu 2+ were below the detection limit of the EDX and/or buried in noise. Therefore, the La 3+ concentrations at the Ba sites were calculated without considering the Eu 2+ concentrations. The characteristic X-ray energies of La K kev) and K (37.8 kev) are adjacent to those of Ba K kev) and K kev). Thus, measurements were performed using a 120 s dwell time to avoid overlap of the peaks. The large discrepancies between the nominal and actual La 3+ concentrations revealed that the limit of La substitution into the Ba sites in BaCl 2 hosts during melt growth is approximately 3 at%. According to the phase diagram in the database 18, 10 at% La substitution eliminate the formation of cubic phase. However, this is not the diagram in the database is different from that in the original report 17. From the results in this study, it is most probable that approximately 3 at% La substitution (not 10 at%) eliminates the formation of cubic phase. As a result, crack free-single crystals for La-substitued samples could be obtained, as shown in the Fig. 2 inset pictures. The La concentration (17 at%) in the residual solids supported this interpretation. This limit is associated with the difference in the effective ionic radii 26 and charge compensation between Ba 2+ and La 3+. The inhomogeneity of the distribution (segregation) of Ba 2+ and La 3+ in the crystals was not observed, because only small areas near the edges of the crystals were selected for the analyses. However, segregation must have occurred in all specimens because it is a known phenomenon of crystals grown via melt growth using the micro-pulling-down method 27. B. Luminescence and scintillation properties Radioluminescence spectra as a function of energy obtained following X-ray excitation are shown in Fig. 2. To assign the emission origins, the spectra of the samples without and with Eu 2+ activators were fitted with double (Fig. 2 (a)) and triple (Fig. 2 (b)) Gaussian functions, respectively. Initially, double Gaussian functions were also used for the samples with Eu 2+ 7

8 activators; however, the fitting curves did not adequately match the experimental curves. The triple Gaussian functions, on the other hand, matched the experimental curves fairly well. It can be clearly seen that the peaks near 3.1 ev (400 nm) are not symmetrical. The peak shape and fitting results with triple Gaussian indicates that the emission around 3.1 ev is affected by crystal field splitting 11. Vertical axis doesn t have physical meaning because luminescence intensity is generally affected by the sample size and shape, and the X-ray intensity. Therefore, luminescence intensities were normalized. As previously reported 2-5, two luminescence bands at 4.2 ev (300 nm) and 3.2 ev (390 nm) were observed from BaCl 2 and BaCl 2 :La(11 at%) in Fig. 2 (a). However, the emission intensity at 4.2 ev is more intense than that at 3.2 ev in our samples, unlike in most of the previous studies 2, 3, 5. Only ref. 4 reported that the emission band around 4.2 ev was more intense than around ev. The authors concluded that the result was due to the use of protons with high linear energy transfer (LET) as the excitation source (X-rays and -rays, which have low LETs, were used in other studies 2, 3, 5 ). This explanation is not applicable to the present study. It was concluded that the single crystals used for the current analyses were of higher crystal quality than those used in previous studies 2, 3, 5, and as a result the luminescence band around 4.2 ev derived from STE was more intense than the emission band around 3.2 ev derived from defects. Additionally, luminescence at 2.61 ev (475 nm), which has been previously reported 2, 3, was not observed in the samples evaluated in the present study. On the other hand, the Eu 2+ -activated Ba compounds show typical Eu 2+ 5d 4f spin forbidden and parity allowed transition around 3.1 ev (400 nm), as shown in Fig. 2 (b). The luminescence band around 3.1 ev was assigned to two bands (4f 6 )(5d 1 2 D 5/2, 7/2 )(t 2g and e g ) (4f 7 8 S 7/2 ) due to the effect of crystal field splitting 11, 28, as mentioned above. Furthermore, unidentified weak peaks around ev (450 nm) were observed. The energy, energy width (FWHM), and contribution of each emission band to the total emission after decomposition using multi-gaussian functions are summarized in Table II. The contributions of each luminescence band were estimated from the areas of the Gaussian functions. Fig. 3 shows the photoluminescence spectra for Eu 2+ (0.5 at%)-activated BaCl 2 and Eu 2+ (0.5 at%)-activated and La 3+ (7.7 at%)-substituted BaCl 2. Typical wide excitation bands, including two absorption transitions corresponding to 4f ( 8 S 7/2 ) 5d (t 2g and e g ) 11, 28, were observed, although they were not clearly separated. Typical Eu 2+ emission bands at 3.1 ev (400 nm) for both spectra were also observed. Similarity radioluminescence spectra at 3.1 ev, these peaks were fitted using double Gaussian functions (not shown) that were assigned to Eu 2+ (4f 6 )(5d 1 2 D 5/2, 7/2 ) (t 2g and e g ) (4f 7 8 S 7/2 ) transitions 11, 28. Unidentified weak peaks around 2.76 ev (450 nm) next to 3.1 ev also appeared. Photoluminescence spectra for the Ba compounds without Eu 2+ activators were not observed. 8

9 There are several possible explanations for the unclarified emission near 2.76 ev (450 nm) in the photoluminescence and radioluminescence spectra of the Eu 2+ -activated specimens. Because of charge compensation between Ba 2+ and La 3+, extra electrons released from La ions may occupy the halogen sites (anion sites) 3, Because of these electrons, the halogen ions may not be able to occupy the anion sites and thus may become interstitial halogen atoms. Next, electrons that occupy the anion sites may capture interstitial halogen atoms and nearly halogen ions. And then, after ejection of the halogen atoms fragment, F centers may be formed 32. Moreover, interstitial halogen atoms should be able to move around in the BaCl 2 host. H centers may also be formed by combining such interstitial halogen atoms and halogen ions 32. The other, the difference in the effective ionic radii for the Ba 2+ and La 3+ ions 26 may induce lattice defects at which extra electrons can be trapped. Also, these trapped electrons may result in extrinsic emission. The scintillation time responses were measured under X-ray excitation using the custom-made pulse X-ray facilities, as shown in Fig.4. Scintillation rise and decay times are described as follows: di ri I( t) A e B e const, (1) i t / t / i i i where I(t) denotes the signal intensity, A i and B i denote constants, and di and ri denote the decay and rise times, respectively. The first and second terms on the right-hand side of the equation represent the decay and rise times, respectively. Using eq. (1), fittings were performed, and the results are summarized in Table III. The rise time for BaCl 2 was 0.2 ns. Unfortunately, the rise time of the other samples could not be measured. The rise times for other three samples were shorter than the impulse response of 100 ps FWHM. The scintillation decay times for the BaCl 2 samples without Eu 2+ activators (BaCl 2 and BaCl 2 :La(11 at%)) were ultrafast at ns (42% 43%) for the 1st component, and are described by the sharp spikes in Fig. 4 (a). These results suggest that BaCl 2 may be a possible alternative material for BaF 2 in time-of-flight positron emission tomography applications, which require ultrafast speeds for timing resolution 4, 33, 34. The decay times of the 2nd (26 ns) and 3rd (85 ns) decay components were similar to the reported values (34.8 ns and ns, respectively) 4, 10. The longest components of the decay times for the samples without Eu 2+ activators were found to be 1430 ns, which is longer than previously reported values (980 and 520 ns) 2, 3, 10. On the other hand, the decay times for the Ba compounds with Eu 2+ activators consisted of very fast components at 0.6 ns and two typical Eu 2+ 5d 4f transitions at approximately ns and ns. To estimate the light yields, the pulse height spectra were obtained following excitation using 662 kev -rays from a 137 Cs source; these spectra are displayed in Fig. 5. The photoabsorption peaks for the BaCl 2 compounds without Eu 2+ activators were fitted using a 9

10 single Gaussian function with subtraction of the multiple Compton scattering, while those for the BaCl 2 compounds with Eu 2+ activators were fitted using double Gaussian functions, including the characteristic X-rays, also with subtraction of the multiple Compton scattering. Essentially, the fitting of the spectra for the BaCl 2 compounds without Eu 2+ doping should be also performed by double Gaussian functions, but that approach was not applicable because the X-ray escape peaks were very weak. The light yields were calculated using integral quantum efficiency (QE) derived from radio-luminescence spectra and the PMT QE curve. Eventually, the light yield values were decided using the ratio of the most intense peak channels of the investigated samples to that of the standard. The QE for R PMT disclosed by Hamamatsu Photonics range from 1.8 ev (690 nm) to 4.6 ev (270 nm). Therefore, the light yields reported in this study were estimated within this range. Specially, light yields for BaCl 2 without Eu 2+ doping, which were emitted over 4.6 ev, are underestimation. The maximum light yield of NaI:Tl(0.2 at%) was previously reported to be 45,000 ph/mev 35. However, the light yield of the NaI:Tl(0.2 at%) standard sample used in the present study was decided to be approximately 25,000 ph/mev after comparing to those of other standards including NaI:Tl, CsI:Tl, YAP:Ce, and BGO. This considerable discrepancy in the previously reported 35 values and the light yield determined for the present reference compound may be due to the different sizes and shapes of the samples. When estimating the light yields, effect of reflection on the PMT window was not considered. The light yields for BaCl 2 and BaCl 2 :La(11 at%) with 0.5 and 10 s shaping times were estimated to be 700 ± 40 and 730 ± 40 ph/mev and 790 ± 40 and 790 ± 40 ph/mev, respectively. The error for each value was estimated to be 5% considering the statistical fluctuation, drift properties of the PMT within the 100 h measurement time, mismatch of the optical coupling, and non-uniformity of the photocathode in the PMT. An increase in the shaping time from 0.5 s to 10 s led to an increase of approximately 8 13% in the light yield. The contribution of the longest components at approximately 1430 ns caused this phenomenon. The light yields for BaCl 2 were smaller than (0.5 s) and similar with (10 s) those of BaCl 2 :La(11 at%) though intense peak positions for BaCl 2 were larger in Fig. 5 (a) and (b). This was caused by the difference of shape of radioluminescence spectra and different measurement date. The estimated light yields for the Ba compounds without Eu 2+ activators were lower than previously reported values ( ph/mev 10 and ph/mev 2, 3 ), while the energy resolution for these Ba compounds was approximately 18% that were similar with 17.4% 2, 3 and better than 24.4% 10. For the BaCl 2 :Eu(0.5 at%) and BaCl 2 :La(7.7 at%):eu(0.5 at%) compounds, the light yields were estimated to be 13,400 ± 670 and 15,000 ± 750 ph/mev and 18,300 ± 920 and 19,900 ± 1,000 ph/mev with 0.5 and 10 s shaping times, respectively. The longest decay components also led to an approximately 33 37% increase in the light yields as the shaping time increased. The energy resolutions of these Ba compounds with Eu 2+ activators were 10

11 evaluated to be approximately 8.1% 12.4% that should be improved compared to 3.5% 3.8% 20 and 8.8% 2, 3. In the present study, the focus was not on the production of Eu 2+ -activated BaCl 2 scintillators with highest light yields. The estimated light yields and energy resolutions are summarized in Table IV. C. Scintillator non-proportionality and energy resolution Non-proportionality refers to the ratio of the light yield obtained when irradiating a sample with a given photon energy to that obtained when irradiating the sample with 137 Cs 662 kev reference radiation and can be described as follows: NP LY energy, (2) LY662keV where NP is the scintillator non-proportionality value and LY energy and LY 662keV are the light yields following irradiation with various -ray energies and 662 kev from a 137 Cs source, respectively. As presented in Fig. 6, the non-proportionality curves were plotted for various -ray radiation source excitations. The fitting curves are guides for the eye. To fit the pulse height spectra under 133 Ba excitation, multi-gaussian function fittings were also performed along with subtraction of the background. The fittings for the compounds without Eu 2+ activators were performed using 303 and 356 kev peaks along with background, because each peak were not clearly distinguished due to overlap and weak signals from 273 and 384 kev. 303 kev data were not plotted in Figs. 6 and 8 because the peak intensities were also weak. On the other hand, as shown in Fig. 7 (a, b), multiple fittings for the Ba compounds with Eu 2+ activators were performed using peaks at four energies (276, 303, 356, and 384 kev) together with background, although the 384 kev peaks were buried into other peaks due to very weak. One can clearly see 356 kev peaks separated from near peaks. As can be observed in the figure, the non-proportionality of the BaCl 2 compounds without Eu 2+ activators (BaCl 2 and BaCl 2 :La(11 at%)) gradually rolled-off with decreasing photon energy. On the other hand, the non-proportionality of BaCl 2 :Eu 2+ (0.5 at%) was less variable than those of the compounds without Eu 2+, whereas the non-proportionality of BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) exhibited a halide hump that is unique to halide compounds 36, 37, 38. The differences in the shapes of the non-proportionality curves for BaCl 2 without and with Eu 2+ activators began to occur near the region for 133 Ba (273, 303, 356 kev) -ray energies. Namely, the non-proportionality of the BaCl 2 compounds without Eu 2+ activators rapidly rolled off around 133 Ba irradiation energy, whereas that of the BaCl 2 compounds with Eu 2+ doping exhibited a typical halide hump or moderately rolled off around 133 Ba energy region. Additionally, similar behaviors were observed in the curves obtained with 0.5 and 10 s shaping times. These results indicate that the emission mechanism affected the non-proportionality curves. According to previous reports 14, 15, the non-proportionality curves for undoped SrI 2 and NaI were different from those for the 11

12 corresponding Eu 2+ doped compounds 14, and the curve for BaF 2 also changed following Tm 3+ doping 15. These results support our interpretation. As mentioned in Introduction part, the origins of the emission bands in BaCl 2 compounds without Eu 2+ activators are mainly attributed to STE (intrinsic emission). Thus, these compounds are classified as excitonic scintillators 12, 13. The non-proportionality curves for the two BaCl 2 compounds without Eu 2+ activators evaluated in this study declined from 662 kev and were nearly consistent with the predicted curves corresponding to f x = 1 12, 13. The luminescence band around 400 nm for BaCl 2 compounds without Eu 2+ activators is regarded as an extrinsic emission. Therefore, the curves of the compounds without Eu 2+ activators contain the effect of extrinsic origin. On the other hand, the BaCl 2 compounds with Eu 2+ activators are categorized as non-excitonic scintillators. However, judging from the non-proportionality curves of these two, they should be categorized as intermediate scintillators. This is not arbitrary because the emission bands in the spectra of both compounds with Eu 2+ are assigned as mainly extrinsic (Eu 2+ ) and unidentified. The shapes of the non-proportionality curves for BaCl 2 :Eu 2+ (0.5 at%) and BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) indicated that 0 < f x < 1 12, 13 for these compounds. The curve for BaCl 2 :Eu 2+ (0.5 at%) declined moderately and corresponded to f x ~ 1, whereas that for BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) exhibited a tiny hump between 200 and 500 kev and corresponded to f x ~ 0. The presence of the hump in one curve and absence in the other is due to the differences in the rates of radiative and non-radiative recombination and the fraction concentration f x within the track, both of which were caused by La substitution. Importantly, the shapes of the non-proportionality curves for both BaCl 2 :Eu 2+ (0.5 at%) and BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) were also consistent with those of the predicted curves corresponding to 0 < f x < 1 12, 13. On the basis of all these results, therefore, the non-proportionality results for the four specimens investigated in the present study were determined to be in agreement with the predicted curves 12, 13. To quantify the deviation from the reference (662 kev from 137 Cs), the degree of photon non-proportionality ( photon-np ) was evaluated using the standard deviation such as following equation 39, 40, 41 : N i 1 662keV LY i photon NP, (3) N LY where N is the number of mono-energetic photons from each radiation source and LY i /LY 662keV is the non-proportionality plot for a given energy excitation. As photon-np reaches zero, non-proportionality is improved. The estimated photon-np values for the four compounds evaluated in this study are presented in Table V. The photon-np values for the Ba compounds with Eu 2+ activators were better than those for the compounds without Eu 2+ activators. Additionally, 12

13 the calculated photon-np values for all the compounds were less than 0.1, which is better than those for most commercial scintillators, including CsI:Tl, NaI:Tl, BGO, GSO:Ce, and LSO:Ce 39, 41. Furthermore, it should be noted that the photon-np values for the Eu 2+ -doped compounds examined with a 0.5 s shaping time are equivalent to that for YAP:Ce ( , , 41 ), which is famous for being a proportional scintillator, although they are inferior to the values for BaBrI:8%Eu 2+ (0.024), CsBa 2 I 5 :4%Eu 2+ (0.0096), and LaBr 3 (0.013) 41. The energy resolution plots with 0.5 and 10 s shaping times are displayed in Figs. 8 and 9. The solid lines are fitting curves and guides for the eye. Fitting was performed using a linear function when plotting on a log-log scale. Energy resolution curves can be theoretically introduced as a function of the energy 33 : K R, (4) E where R is the energy resolution, K is the constant of proportionality, and E is the irradiated energy. On the basis of eq. (4), when using linear function fitting on a log-log scale, the slope must be theoretically 0.5. The energy resolution theoretically consists of several components as follows 16, 42, 43 : R 2 R 2 sc 2 t R R 2 st, (5) where R is the estimated energy resolution, R sc is the intrinsic energy resolution of the scintillators, R t is the transfer contribution, and R st is the statistical contribution of the PMT, i.e., the contribution for Poisson statics. R st can be described as follows: R st , (6) N phe where is the variance of the electron multiplier gain, which is equal to , 43, 44 for modern PMTs, and N phe is the number of photoelectrons generated in the PMT. For an ideal scintillator and current PMT, R sc and R t are negligible 43. Therefore, R can be ideally described as only R st, which is equal to eq. (4). In this case, the slope should be 0.5 when plotting on a log-log scale. However, the actual value for R sc does not become zero and is considerably affected by certain factors such as the inhomogeneity (R inh ) of the samples and the non-proportional contribution (R NP ) as follows 16, 43, 45 : R 2 sc 2 inh 2 NP R R. (7) R inh represents the inhomogeneity of the samples due to segregation of the defects and activators which is related to their distribution, the non-uniform transmission of the scintillator, and the variation of localized scintillation light 16, 42, 43. The slopes of the energy resolution plots for the BaCl 2 compounds without Eu 2+ doping were evaluated to be in the range from 0.52 to 0.58 using linear fitting curves on log-log 13

14 scale, which are close to the expected value. The slopes of BaCl 2 :Eu 2+ (0.5 at%) were estimated to be 0.41 and 0.45, which are also close to 0.5. However, the slopes of energy resolution plots for BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) were significantly scattered from the theoretical slope and found to range from 0.30 to This large discrepancy from expected value could be caused by the inhomogeneity of Eu 2+ distribution and/or non-proportional contribution. Considering the slopes of BaCl 2 :Eu 2+ (0.5 at%) and BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%), La doping might cause the inhomogeneity of Eu 2+ distribution. Furthermore, these figures do not show the clear differences of energy resolution plots derived from different emission origins. Nonetheless, the effect of the emission origin to the energy resolution should be investigated further. IV. Conclusions Crack-free La 3+ -substituted BaCl 2 single crystal rods were grown by the micro-pulling-down technique for the first time. Scintillation properties for these La 3+ -substituted BaCl 2 single crystals were reported together with non-substituted crystals with cracks. Scintillation light yields following 137 Cs excitation were estimated to be , , 13,400 18,300, and 15,000 19,900 ph/mev for BaCl 2, BaCl 2 :La 3+ (11 at%), BaCl 2 :Eu 2+ (0.5 at%), and BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%), respectively. Additionally, the energy resolutions under 137 Cs excitation were evaluated to be approximately 18% for the non- and La-substituted BaCl 2 compounds without Eu 2+ activators and 8.1% 12.4% for the non- and La-substituted BaCl 2 compounds with Eu 2+ activators. Non-proportionality curves and energy resolution plots for the four compounds were also reported for the first time. The non-proportionality curves derived from mainly intrinsic (STEs) and extrinsic (Eu 2+ activators) were consistent with the predicted curves for each of the compounds. Importantly, the non-proportional curves from different emission origins exhibited different curve shapes although the host material was the same. These results experimentally demonstrated that the emission mechanism influenced scintillator non-proportionality. However, the influence of the emission mechanism to energy resolution could not be confirmed. Acknowledgments This work was supported by the U.S. Department of Energy/NNSA/NA22 and carried out at Lawrence Berkeley National Laboratory under Contract NO. AC02-05CH The author is grateful to Edith D. Bourret-Courchesne, Gregory Bizarri, Martín Gascón, Christopher Ramsey, Stephan Hanrahan, Ivan Khodyuk, Kathleen Brennan, and Woon-Seng Choong for their help. 14

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17 Figure captions Fig. 1. Powder X-ray diffraction patterns for BaCl 2, BaCl 2 :La 3+ (11 at%), BaCl 2 :Eu 2+ (0.5 at%), and BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%). The hump from 10 to 25 degrees is due to the plastic tape used to seal the powders. The BaCl 2 peaks were identified as PDF Fig. 2. Radioluminescence spectra as a function of the photon energy for (a) BaCl 2 without Eu 2+ activators and (b) BaCl 2 with Eu 2+ activators under X-ray excitation. The BaCl 2 samples without Eu 2+ activators include BaCl 2 and BaCl 2 :La 3+ (11 at%). The BaCl 2 samples with Eu 2+ activators include BaCl 2 :Eu 2+ (0.5 at%) and BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%). All the spectra were decomposed by multiple fittings using a Gaussian function. The red and green dotted curves are the individual fitting curves, and the red and green solid curves are the merged curves. The corresponding rods grown using the micro-pulling-down technique are also shown. Scale in inset pictures is in mm. Fig. 3. Photoluminescence and excitation spectra of (black solid lines) BaCl 2 :Eu 2+ (0.5 at%) and (red dotted lines) BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) single crystals. Measurements were performed at room temperature. Fig. 4. Spectrally unsolved scintillation decay profiles for the (a) BaCl 2 samples without Eu 2+ activators and (b) BaCl 2 samples with Eu 2+ activators under X-ray excitation. The red solid lines show the fitting curves. Measurements were performed at room temperature. Fig. 5. Pulse height spectra under 137 Cs excitation for the (a, b) BaCl 2 samples without Eu 2+ activators and (c, d) BaCl 2 samples with Eu 2+ activators. The spectra shown in (a, c) and (b, d) were obtained using 0.5 and 10 s shaping times, respectively. Fittings were performed using the multiple Gaussian function considering the characteristic X-ray escape peaks and multiple Compton scattering (background). The sky blue and green dotted curves are the individual fitting curves, and the red solid curves are the merged curves. Measurements were performed at room temperature. All figures are displayed using the same gain. Fig. 6. Non-proportionality of BaCl 2, BaCl 2 :La 3+ (11 at%), BaCl 2 :Eu 2+ (0.5 at%), and BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) with (a) 0.5 s and (b) 10 s shaping times. The lines are guides for the eye and were drawn without considering the characteristic X-ray absorptions of Ba, La, and Eu. Therefore, the dips corresponding to K -escape and K -escape for Ba, La, and Eu are not shown. The pulse height measurements under various radiation sources excitation were performed at room temperature in the glove box. Fig. 7. Pulse height spectra under 133 Ba excitation for the BaCl 2 samples with Eu 2+ activators obtained using (a) 0.5 s and (b) 10 s shaping times. Fittings were performed using the multiple Gaussian functions considering multiple Compton scattering (background). The blue and green dotted curves are the individual fitting curves, and the red solid curves are the merged curves. Measurements were performed at room temperature. All pulse height spectra under 137 Cs 17

18 (shown in Fig. 5) and 133 Ba excitations were measured using the same gain. Fig. 8. Energy resolution plots for BaCl 2, BaCl 2 :La 3+ (11 at%), BaCl 2 :Eu 2+ (0.5 at%), and BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) as a function of irradiated energy. Fittings were performed using linear functions when plotting on a log-log scale. The numbers in parentheses on the right side of the sample names are the slopes obtained from the fittings. Error bars were evaluated by fitting. 18

19 Table captions Table I. Actual La 3+ concentrations at the Ba sites determined using energy-dispersive X-ray spectroscopy and nominal La concentrations. No signal from Eu 2+ was detected because the Eu 2+ concentration was less than the detection limit of the EDX and/or was buried in the noise. Therefore, the La 3+ concentrations at the Ba sites were estimated without the Eu 2+ concentration. The nominal chemical compositions were used as sample notations in this study. Table II. Results of multi-gaussian component deconvolution for the investigated samples. Table III. Spectrally unsolved scintillation rise and decay times under X-ray excitation. The error was estimated to be 5%. Table IV. Light yields (LY) and energy resolutions (ER) at room temperature obtained with 0.5 and 10 s shaping times. Errors were estimated to be 5% for LY and by fitting for ER. The light yield of the NaI:Tl(0.2 at%) reference was regarded to be 25,000 ph/mev. The centroid photo-peak channels of the reference were 7500 ch and 8900 ch for 0.5 s and 10 s shaping times, respectively. The LYs were calculated using integral quantum efficiency (QE) derived from radio-luminescence spectra and the PMT QE curve. The QE for R PMT disclosed by Hamamatsu Photonics range from 1.8 ev (690 nm) to 4.6 ev (270 nm). Therefore, the light yields reported in this study were estimated within this range. The excitation source was 137 Cs 662 kev -rays. Table V. Degree of photon non-proportionality ( photon-np) for the investigated samples. 19

20 composition Nominal La concentration at the Ba site (at%) Actual La concentration at the Ba site (at%) BaCl 2 :La 3+ (11at%) BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) Residual solid in the crucible after NA 17 BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) growth Table I 20

21 Composition Components Energy (ev) FWHM (ev) Contribution to the total emission (%) BaCl BaCl 2 :La 3+ (11 at%) BaCl 2 :Eu 2+ (0.5 at%) BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) Table II 21

22 composition Rise (ns) 1st (ns) 2nd (ns) 3rd (ns) 4th (ns) BaCl ± ± 0.05 (43%) 25.7 ± 1.3 (8%) 84.6 ± 4 (33%) 1430 ± 72 (11%) BaCl 2 :La 3+ (11 at%) 0.94 ± 0.05 (42%) 25.6 ± 1.3 (8%) 78.5 ± 4 (29%) 1426 ± 71 (12%) BaCl 2 :Eu 2+ (0.5 at%) 0.63 ± 0.03 (0.2%) 407 ± 20 (51%) 860 ± 43 (40%) BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) 0.64 ± 0.03 (0.2%) 475 ± 24 (64%) 1372 ± 69 (27%) Table III 22

23 composition LY (Ph/MeV) ER (%) LY (Ph/MeV) ER (%) shaping time 0.5 s shaping time 0.5 s shaping time 10 s shaping time 10 s BaCl ± ± ± ± 0.2 BaCl 2 :La 3+ (11 at%) 730 ± ± ± ± 0.3 BaCl 2 :Eu 2+ (0.5 at%) 13,400 ± ± ,300 ± ± 0.4 BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) 15,000 ± ± ,900 ± ± 0.4 Table IV 23

24 composition Degree of photon-np, shaping time 0.5 s Degree of photon-np, shaping time 10 s BaCl BaCl 2 :La 3+ (11 at%) BaCl 2 :Eu 2+ (0.5 at%) BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) Table V 24

25 Intensity [a.u.] BaCl 2 :La 3+ (7.7 at%):eu 2+ (0.5 at%) BaCl 2 :Eu 2+ (0.5 at%) BaCl 2 :La 3+ (11 at%) BaCl 2 :BaCl 2 (H 2 O) and/or BaCl 2 (H 2 O) θ [degree] Fig. 1.

26 Normalized intensity [a.u.] Normalized intensity [a.u.] 1.0 (a) without Eu 2+ activator BaCl 2 BaCl 2 :La 3+ (11 at%) Energy [ev] (b) with Eu 2+ activator BaCl 2 :Eu 2+ (0.5 at%) 0.6 BaCl2:La3+(7.7 at%):eu2+(0.5 at%) Energy [ev] Fig. 2.