Structure and scintillation of Eu2+-activated BaBrCl and solid solutions in the BaCl2–BaBr2 system

Journal of Luminescence 138 (2013) 143–149

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Structure and scintillation of Eu2 þ -activated BaBrCl and solid solutions in the BaCl2–BaBr2 system Gautam Gundiah n, Zewu Yan, Gregory Bizarri, Stephen E. Derenzo, Edith D. Bourret-Courchesne Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

a r t i c l e i n f o

abstract

Article history: Received 15 November 2012 Received in revised form 10 January 2013 Accepted 16 January 2013 Available online 4 February 2013

The structure and scintillation properties of BaBrCl:xEu2 þ (x ¼ 0–0.12) and Eu2 þ activated solid solutions in the BaCl2–BaBr2 system are reported. Samples were synthesized in the form of 1–5 mm crystals by melting the reactants in sealed quartz tubes followed by slow cooling. The solid solutions form an orthorhombic PbCl2-type crystal structure with an ordered arrangement of the anions. Upon optical and X-ray excitation, the samples show an intense emission centered between 407 and 412 nm. The samples exhibit a fast decay characteristic of Eu2 þ , with the primary decay component between 550 and 700 ns, depending on the Eu concentration. The luminosity for the solid solutions is estimated to be similar to the binary halide end members. & 2013 Elsevier B.V. All rights reserved.

Keywords: Scintillator Gamma-ray detector Solid solutions BaBrCl Europium Barium bromide chloride

1. Introduction In the last few years, there has been a large increase in efforts to discover new scintillators for radiation detection applications. These efforts led to the discovery and improvement of a number of scintillators [1–4], all exhibiting very good performance. Single crystals of BaBrCl:Eu2 þ (5%) were reported to show a light yield of 52,000 photons/MeV and an energy resolution of 3.55% [5]. The compound had previously been studied as an X-ray storage phosphor activated with 0.5% of Eu2þ [6]. In this paper, we describe the optimization of the Eu concentration, details of the structure, melting behavior and hygroscopic nature of BaBrCl. We further extend the work by studying Eu2þ activated solid solutions in the BaCl2–BaBr2 system. Solid solutions offer the possibility to tailor properties by incrementally varying the composition. We have previously shown that solid solutions such as BaBrI can exhibit better scintillation light yield than the binary halides that form them [2]. Hodorowicz et al. reported that BaCl2 and BaBr2 are fully miscible over the whole range of compositions and that the resulting solid solutions crystallize in an orthorhombic, anion ordered PbCl2 structure-type with the lattice parameters varying depending on the Cl/Br ratio [7]. This offers the possibility to change the Cl/Br ratio and study the impact on the scintillation properties. We present the results from samples with a general composition BaBrxCl2 x (0rxr2) activated with 5% Eu2þ , the concentration determined to be optimal for BaBrCl. We have also

n

Corresponding author. Tel.: þ1 510 486 5651; fax: þ1 510 486 4768. E-mail address: [email protected] (G. Gundiah).

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.01.017

explored the effects of the Cl/Br ratios on the structure, melting characteristics as well as scintillation properties. 2. Experimental procedure 2.1. Synthesis High-purity reactants in the form of beads were obtained from Sigma-Aldrich and used without additional purification. The reactants and products were handled in an Ar-filled glovebox maintained below 0.1 ppm of O2 and H2O. Samples were obtained by heating the reactants BaCl2, BaBr2 and EuCl2/EuBr2 in sealed quartz tubes to 960 1C, a temperature that is above the melting temperature of each of the reactants. The quartz ampoules were positioned off the vertical and held at this temperature for 1 h to achieve good mixing. This followed by cooling the tubes to ambient temperature at the rate of 0.1 1C/min. The slow cooling allowed small crystals (1– 5 mm-crystals) to form. The concentration of Eu in the starting reactants mixture was varied from 0 to 20 mol%. The percentages of Eu dopant used in the text are nominal compositions that were added to the melt. First, we synthesized samples of composition BaBrCl, then we varied the proportion of Br and Cl to produce samples with the general composition BaBr1 xClx (0oxo1). 2.2. Characterization 2.2.1. Structure, melting point and hygroscopic nature The high-throughput facility described in Ref. [8] was used for characterization of the samples. For luminescence measurements,

144

G. Gundiah et al. / Journal of Luminescence 138 (2013) 143–149

samples of crystalline particles of size ranging from 0.5 to 2 mm were packed in gas tight quartz ampoules with an inner diameter of 4 mm and length 15 mm. It was shown previously [9] that particle sizes in that range can be used to estimate the light output of the compound. Phase identification was performed by powder X-ray diffraction (XRD) with a Bruker Nonius FR591 rotating anode X-ray generator equipped with a Cu target at a 50 kV and 60 mA electron beam. This was followed by a precise structure determination carried out by single crystal XRD on a Bruker SMART-CCD platform diffractometer at the University of California, Santa Barbara. The crystal was mounted on a glass fiber and was transferred to the diffractometer. The SMART [10] program was used to determine the unit cell parameters and data collection (15 s/frame, 0.31/frame for a sphere of diffraction data). The data were collected at 150 K using an Oxford nitrogen gas cryostream system. The raw frame data were processed using the SAINT [11] program. The empirical absorption correction was applied based on psi-scan. Subsequent calculations were carried out using the SHELXTL [12] program. The crystal structure visualization software Diamond was used to view the crystal structure. The melting temperature was measured by differential thermal analysis (DTA)/differential scanning calorimetry (DSC) using a Netzsch STA 409PC thermal analyzer. The measurements were performed with samples ground to powder contained in alumina crucibles in contact with Pt thermocouples Data were acquired with a heating and cooling rate of 10 K/min under flowing Ar gas. The hygroscopicity of BaBrCl:Eu was compared quantitatively to the related halide scintillators BaBrI:Eu and LaBr3:Ce. Samples were finely ground and sieved to a uniform size (below 40 mm) in an Ar filled glovebox. The powders were then exposed to moisture under ambient conditions (temperature 20 1C and relative humidity 55%) and the increase in mass, assumed to be entirely due to reaction with moist air, noted at regular time intervals for several hours.

4.66 g/cc (Fig. 1). All atoms occupy the fourfold special position (4c, site symmetry m) and lie on the mirror planes, perpendicular to the b-axis at y¼ 70.25. The structure contains a single cation site and two types of anion positions. As shown in Fig. 1(b), each Ba cation is surrounded by 9 anions in a tricapped trigonal prismatic arrangement. The anions are not equidistant from the Ba but located at two different positions. The smaller chloride ion occupying the position at distances between 3.07 and 3.11 A˚ and ˚ This is the larger bromide ions located between 3.35 and 3.50 A. contrary to observations by Hodorowicz et. al. [7], who reported that the bromide ions occupy the site nearer to Ba and chloride occupy the site that is further away. Our results agree with the ordering observed in the isomorphous mixed halide compounds

2.2.2. Scintillation properties X-ray excited luminescence was obtained using the rotating anode described above as excitation source and a SpectraPro2150i spectrometer (Acton Research Corp., Acton, MA) coupled to a PIXIS:100B charge-coupled detector (Princeton Instruments, Inc., Trenton, NJ) to record the spectral response. The time response of the X-ray excited luminescence was measured using a custom-made pulsed X-ray system consisting of an ultrafast laser (200 fs pulses at 165 kHz), a light-excited X-ray tube, a Hamamatsu R3809U-50 microchannel PMT and an Ortec 9308 ps time analyzer. The impulse response of the system is 100 ps FWHM [13]. The luminosities were estimated from the pulsed X-ray decay measurements by summing all detected photons. Relative luminosity of each sample was obtained by comparison to a CsBa2I5:Eu single crystal with known luminosity (100,000 photons/MeV), measured under identical conditions. Error bars correspond to statistical variations obtained in the measurements. Photoluminescence excitation and emission spectra were obtained using an in-house-built optical spectrometer. A Xebroad-spectrum lamp was the excitation source used to excite the samples between 200 and 600 nm. The emission was measured from 250 to 1000 nm using the same set-up that was described for the X-ray excited luminescence. 3. Results and discussion 3.1. BaBrCl:xEu2 þ 3.1.1. Structure and physical properties BaBrCl crystallizes in an orthorhombic, three-dimensional PbCl2 structure-type with a Pnma space group and density of

Fig. 1. (a) Schematic of the crystal structure of BaBrCl with (b) the environment around a single Ba/Eu atom. Atoms in black represent Ba/Eu, blue represent Cl and light blue represent Br atoms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

G. Gundiah et al. / Journal of Luminescence 138 (2013) 143–149

sensitivity to moisture would be beneficial in reducing the related defects during crystal growth that adversely affect the scintillation characteristics.

PbBrCl [14], PbBrI [14], BaBrI [15]. We attempted to refine the structure to verify the ordering of the anions as they surround the Eu site and their position may impact the scintillation properties. Attempts to vary the occupancy of the halide anions between the two sites were unsuccessful, indicating that the structure is an anion ordered PbCl2 structure. Based on size and charge con˚ is expected to siderations, the activator Eu (ionic radius 1.30 A) ˚ in the lattice [16]. The phase replace Ba (ionic radius 1.47 A) purity of every sample was confirmed by powder XRD. For samples containing higher Eu dopant concentrations, we were able to see a distinct shift in the peak positions in the powder XRD pattern due to a decrease in the unit cell volume. EuBrCl is reported to have a similar structure as BaBrCl [17]. This and the fact that we were able to obtain pure samples containing up to 20% Eu without the presence of additional impurity phases, indicates that Eu is highly soluble in BaBrCl. DTA was performed at a heating rate of 10 1C/min from room temperature up to 1050 1C and a cooling rate of 10 1C/min. The results are shown in Fig. 2. A melting temperature of 886 1C was obtained as the intersection of the tangent drawn at the point of greatest slope of the leading edge of the endothermic peak with the x-axis. The melt solidifies at the same temperature as evidenced by the sharp, exothermic peak in the cooling curve. Additional peaks due to phase transformations or formation of impurity products were absent. A mass loss of  0.5% was observed while keeping the melt 50 1C above the melting temperature for 30 min, thus indicating that the melt has a low vapor pressure and shows a very small amount of decomposition on melting. The small step in the thermogravimetric curve at 480 1C is an instrumental artifact in the measurement. The samples are mildly sensitive to moisture and a white coating appears on the surface of the samples after exposure to air for several days under ambient conditions. In order to have a quantitative estimate of the hygroscopic nature, finely ground samples of BaBrCl, BaBrI and LaBr3 were sieved below 40 mm in a glovebox, exposed to moisture under ambient conditions and their weight increase was monitored over time. Due to the high surface area, the powders react with moisture faster than larger crystals. As can be seen from Fig. 3, BaBrCl is much less hygroscopic than the other scintillators. Although halide scintillator crystals can be packaged to prevent reaction with moisture, lower

100.4

300

400

500

145

3.1.2. Scintillation properties Fig. 4 shows the room temperature X-ray excited emission spectra for BaBrCl doped with different amount of Eu. The undoped sample shows a broad emission from the host between 325 and 600 nm. The emission shows a maximum at 406 nm and appears to be a superposition of several different peaks. A broad emission with maxima between 420 and 430 nm has been reported for the related undoped halides BaCl2 and BaBr2 upon X-ray excitation [21]. This has been attributed to F-center and VK center recombination. We attribute the emission with maximum at 406 nm to either the F–VK pair recombination or contamination with traces of Eu2 þ . Upon addition of 0.25% Eu, the intensity increases and the spectrum is dominated by an emission band with a maximum at 407 nm. The emission spectra are similar to those reported earlier for BaBrCl:Eu powders [6] and is attributed to the 4f65d1-4f7 transition of Eu2 þ . Thus, the absorbed energy is efficiently transferred to the dopant Eu ions. Further addition of Eu results in an increase in the intensity of emission with a maximum obtained for 5% Eu. Increasing the Eu concentration results in a red-shift of the emission maximum to 407.5 nm, 408 nm, 410.5 nm and 412.5 nm for samples containing 1%, 2%, 5% and 8% Eu, respectively. The minor shift in the emission maximum could be due to reduction of the unit cell volume that occurs with increase of the Eu concentration (calculated to be 4.5 A˚ 3 for 10% Eu doping). The latter results in an increase in the crystal field splitting of the 5d energy levels leading to a red-shift of the 4f65d1-4f7 emission. The luminosities as a function of Eu2 þ concentration were estimated from the pulsed X-ray decay measurements by summing all detected photons (Fig. 5). Relative luminosity of each sample was obtained by comparison to a CsBa2I5:Eu single crystal with known luminosity, measured under identical conditions. Error bars correspond to statistical variations in the measurements, obtained by repeated measurements. BaBrCl containing 4– 8% Eu dopant shows the highest light output. Room temperature X-ray excited luminescence decay curves, normalized to 1 at zero time, are presented in Fig. 6. The results are tabulated in Table 1. The decay curves of all samples are

600

700

800

900

0.4

100.2

Cooling 100.0

0.0

DTA (µV/mg)

Mass (%)

0.2

Heating

-0.2

99.8

300

400

500 600 700 Temperature (°C)

800

900

Fig. 2. DTA curves for BaBrCl recorded during the heating and cooling cycles. The curve in dashed line indicates the mass loss for the sample.

146

G. Gundiah et al. / Journal of Luminescence 138 (2013) 143–149

0

200

400

600

800

1000

1200

150 mg powder (< 40 microns) 160

160

Mass gain (%)

LaBr3:Ce 140

140

BaBrI:Eu 120

120

BaBrCl:Eu

100

100 0

200

400

600 800 Time (minutes)

1000

1200

Fig. 3. Hygroscopic nature of BaBrCl:Eu as compared to BaBrI:Eu and LaBr3:Ce measured under identical conditions.

Evergy (eV) 3.5

3

2.5

0% Eu 0.25% Eu 1% Eu 2% Eu 5% Eu 8% Eu

Intensity (arb. units)

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

2

Relative luminosity

4

0.0

0

2

4 6 8 Eu concentration (%)

10

12

0.0

Fig. 5. Estimated luminosities for BaBrCl containing different Eu concentrations.

300

400

500 Wavelength (nm)

600

Fig. 4. Room temperature X-ray excited luminescence spectra for BaBrCl containing different Eu concentrations.

multi-exponential and can be fitted with one or two decay components in addition to a slow constant fraction of more than 15 ms. The undoped sample shows a slow decay with 11% of the light decaying in 8240 ns and the remaining light decaying slower than 15 ms. Upon addition of 0.25% Eu, we obtain 2 decay times of 456 ns and 3080 ns accounting for 14% and 4% of the total light, in addition to the slow component. Further increase of Eu results in t1 and t2 becoming faster. No rise time was observed for any of the compositions. BaBrCl:Eu2 þ (5%) shows the fastest decay with 265 ns and 551 ns decay fractions accounting for 17% and 58% of the total emission respectively.

3.1.3. Photoluminescence Fig. 7(a) and (b) shows the room temperature excitation and emission curves and the 2D contour maps for Eu2 þ -activated BaBrCl. The spectra in Fig. 7(a), for BaBrCl containing 0.25%, 5% and 8% Eu, are normalized at the maximum of the emission peak. The emission spectra presented are recorded with an excitation of 350 nm. The emission for the sample containing 0.25% Eu shows a peak maximum at 408 nm. Further addition of Eu results in a very small red shift of only 2 nm with a slight decrease in the width of emission (Fig. 7(b)). The Eu2 þ emission can be excited via a broad band between 250 and 370 nm. For the sample containing 0.25% Eu, the excitation intensity is highest at 275 nm and the less at lower energies. For samples containing higher concentration of Eu, the excitation at lower energies is significantly greater. We observe a slight overlap between the excitation and emission spectra for samples containing higher Eu dopant concentration. There is no indication of strong self-absorption in these samples. This is also confirmed by the observation that there is no

G. Gundiah et al. / Journal of Luminescence 138 (2013) 143–149

4.5

0% Eu 0.25% Eu 1% Eu 2% Eu 5% Eu 8% Eu

0.1

2000 3000 Decay time (ns)

Emission

5% Eu 8% Eu

0.5

0.0 250

Table 1 Decay components with fractions for BaBrCl:xEu2 þ . See Fig. 5 for luminosity values.

300

350 400 Wavelength (nm)

450

500

450

Decay fractions (ns)

t1

t2

t3

456 390 371 265 227

8241 (11%) 3080 (4%) 754 (12%) 663 (23%) 551 (58%) 549 (61%)

Slow Slow Slow Slow Slow Slow

(14%) (36%) (35%) (17%) (11%)

400 (88%) (82%) (53%) (42%) (24%) (28%)

Excitation (nm)

0 0.25 1.00 2.00 5.0 8.0

2.5

4000

Fig. 6. Room temperature X-ray excited decay curves for BaBrCl containing different Eu concentrations.

Eu%

3

0.25% Eu

0.01 1000

3.5

Excitation

Increasing Eu

0

4

1.0 Normalized intensity

Normalized intensity

1

147

350

300

lengthening in the decay lifetimes on increasing the Eu concentration (Table 1).

250 3.2. Solid solutions in the BaCl2–BaBr2 system 3.2.1. Structural properties As Hodorowicz el al. [7] reported earlier, solid solutions in the BaCl2–BaBr2 system form over the entire composition range and crystallizes in an orthorhombic structure with the Pnma space group. The structure of the solid solutions contains a single Ba cation site and two anion sites, similar to BaBrCl. In this case too, contrary to Ref. [7], the chloride ions occupy the site nearer the barium ions. Variation of the lattice parameters, cell volume, density and melting temperatures are shown in Fig. 8. The exact compositions obtained from structure solution and data is also presented in Table 2. As expected, by replacing the smaller chloride ion by a larger bromide ion, we see an increase in the lattice parameters as well as the unit cell volume. Structural data for BaCl2 and BaBr2 are obtained from the literature [18] and shown for comparison. The density (calculated) and cell volume also increases replacing Cl  by Br  in the lattice. Attempts to refine the structure by varying the occupancy of the halide anions between the two sites for different Cl/Br ratios were not successful. Thus, the solid solutions form an ordered PbCl2-type structure over the entire composition range. The melting temperatures for the compositions were obtained from DTA/DSC curves. The melting temperatures of the solid solutions are intermediate to those of BaBr2 and BaCl2 end-members and increases with an increase in the amount of chloride. Similar to BaBrCl, all solid solutions show a mass loss below 1% on heating to 50 1C above the melting temperature, indicating congruent melting with minimal decomposition or volatization.

300

350

400 450 Emission (nm)

500

550

Fig. 7. (a) Room temperature excitation and emission spectra of BaBrCl:xEu2 þ . (b) 2D excitation versus emission contour map for BaBrCl:Eu(5%).

3.2.2. Scintillation properties BaBrCl containing 5% Eu2 þ dopant showed high luminosity and the fastest decay. This dopant concentration was used to study the variation in scintillation properties for the solid solutions. Fig. 9 shows X-ray luminescence curves for compositions with varying Cl/Br ratios recorded at room temperature where the peak values have been normalized to 1. The band observed for each composition is attributed to the characteristic 4f65d1-4f7 transition of Eu2 þ . The d–f transitions are influenced by the environment around the Eu2 þ ion. The Eu2 þ -activated binary halides, BaCl2 and BaBr2, show an emission with maximum at 405 and 409 nm, respectively. The emission is narrow with a full width at half maximum (FWHM) of  25 nm. For the mixed halides, we observe an inhomogeneous broadening of the emission band with the FWHM increasing to 35–40 nm. The chloriderich mixed halides emit at a slightly higher energy than the bromide-rich samples. However, the magnitude of change in emission wavelength for the mixed halides is small due to minor changes in local environment on varying the Cl/Br ratio. Shown in Fig. 10 are the room temperature decay curves for the BaBr2  xClx:Eu2 þ (5%) compositions normalized to 1 at zero time. The decay curves are multi-exponential and can be fitted

G. Gundiah et al. / Journal of Luminescence 138 (2013) 143–149

with two decay components in addition to a slow constant fraction (above 15 ms). The components with their fractions are shown in Table 3. All the compositions show a first decay component between 200 and 300 ns accounting for 10–20% of the decaying light. The other components are slower for the solid solutions compared to those of the 2 binaries. For the binary halides BaCl2 and BaBr2, the majority of light decays at 700 ns. The major decay fraction for the mixed halides, on the other hand, is between 550 and 600 ns. The slowest of all compositions is the BaBrxCl2  x (x¼1):Eu2 þ (5%). Shown in Fig. 11 are the luminosities measured for compositions containing 5% Eu with varying Cl/Br ratios. The values were obtained by comparison to a CsBa2I5:Eu single crystal of known

Melting temp. (degC)

BaCl2

BaBrCl

BaBr2

950 900

luminosity with similar grain size/granularity measured under identical conditions. Error bars correspond to variations obtained in the measurements. Also shown in the figure (blue squares) is the maximum luminosity that can be expected for some of the compositions. The limit on scintillation yield is given by the

375

400

425

450

475 BaCl2

1.0

BaBr 0.67Cl1.33 Normalized intensity

148

BaBrCl BaBr1.33Cl0.67 BaBr2 0.5

850

Density (g/cc)

5.0

0.0

4.5

375

400

450

475

Fig. 9. X-ray excited luminescence spectra for BaBr2  xClx:Eu2 þ (5%).

400 1

375

BaCl 2

350

BaBr 0.67Cl1.33

10

BaBrCl BaBr 1.33Cl0.67

c

9

8

a

5.0 4.5

Normalized intensity

Cell parameters (A°)

Cell volume (Ao3)

4.0

425 Wavelength (nm)

0.01

b

0.0

0.5

BaBr 2 0.1

1.0 x in BaBr xCl2-x

1.5

2.0 0

Fig. 8. Variation of the lattice parameters, cell volume and density for BaBrxCl2  x (0r xr 2).

1000

2000 Time (ns)

3000

4000

Fig. 10. X-ray excited decay curves for BaBr2  xClx:Eu2 þ (5%).

Table 2 Structural properties for BaCl2–BaBr2 system. Composition

BaCl2 BaBr0.5Cl1.5 BaBr0.67Cl1.33 BaBrCl BaBr1.09Cl0.91 BaBr1.31Cl0.69 BaBr1.51Cl0.49 BaBr2

Melting point (1C)

952 908 903 887 883 877 866 850

Density (g/cc)

3.94 4.2935 4.37579 4.5675 4.60089 4.66621 4.75753 4.85

˚ Cell parameters (A) a

b

c

7.865 7.9571 8.0286 8.1059 8.1265 8.1703 8.1754 8.276

4.731 4.7453 4.7638 4.7864 4.7981 4.8363 4.8571 4.956

9.421 9.4331 9.442 9.4711 9.4956 9.6031 9.6774 9.919

˚ 3 Cell volume (A)

Reference

350.55 356.18 361.12 367.46 370.25 379.46 384.28 406.84

[18] This This This This This This [18]

work work work work work work

G. Gundiah et al. / Journal of Luminescence 138 (2013) 143–149

4. Conclusions

Table 3 Decay components with fractions for BaBr2  xClx:Eu2 þ (5%). Composition

Decay fractions (ns)

t1 BaCl2 BaBr0.67Cl1.33 BaBrCl BaBr1.31Cl0.69 BaBr2

1.0

239 257 285 310 214

t2 (14%) (18%) (14%) (12%) (19%)

BaBr2

704 562 546 590 702

149

t3 (84%) (72%) (56%) (61%) (76%)

Slow Slow Slow Slow Slow

BaBrCl

(2%) (10%) (30%) (27%) (5%)

We have presented a detailed investigation of the structure, melting characteristics and scintillation properties of Eu2 þ -activated BaBrCl and BaCl2–BaBr2 solid solutions. All compositions were confirmed to form an ordered PbCl2 structure-type with an ordered arrangement of the anions. We show that the optimum Eu concentration is in the range 4–8%. For BaBrCl containing 5% Eu, we measured a luminosity of 59,500 photons/MeV on small mm-sized crystals. For the mixed halide solid solutions with different Cl/Br ratios, there is a marginal increase in the light output as compared to the end members, BaCl2 and BaBr2.

BaCl2 Acknowledgments

Relative luminosity

0.8

This work has been supported by the US Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded Contract IAA HSHQDC-07-X-00170 and carried out at the Lawrence Berkeley National Laboratory under Contract no. DE-AC02-05CH11231. This support does not constitute an express or implied endorsement on the part of the Government. We thank Dr. Guang Wu for crystal structure determinations, Drs. Andrew Canning, Marvin J. Weber, Eric Samulon for useful discussions regarding the structure/scintillation properties and Mr. S.M. Hanrahan for measurements.

0.6

0.4

0.2

References

0.0

0.0

0.5

1.0 x in BaBr 2-xCl x

1.5

2.0

Fig. 11. Black dots show the luminosities estimated for BaBr2  xClx:Eu2 þ (5%) along with the error bars. The red circles represent the maximum expected luminosity for the compositions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

following equation [19]: Max: Luminosity ¼

106 SQ bEg

where Eg is the bandgap, b is a constant related to the absorption of energy with a value between 2 and 3 [20], S is the transfer efficiency and Q is the quantum efficiency of the Eu2 þ luminescence. We note that our measurements show a luminosity of 53,000 ph/MeV for BaCl2:Eu (5%) and 55,000 ph/MeV for BaBr2:Eu (5%). BaCl2 and BaBr2 activated with 0.1% Eu2 þ have been reported to have a light output of 19,000 and 15,000 ph/MeV respectively [21], well below the theoretical maximum values of 63,000 and 74,000 ph/MeV, respectively, theoretical values calculated from Ref. [19] and assuming a value of b ¼2. The light output for BaCl2:Eu (5%) is very close to its theoretical luminosity and the value recently reported by Yan et al. [22]. The luminosity for BaBr2:Eu (5%) is about 25% below its theoretical limit. The higher values obtained in our study are due to the optimized dopant concentration. BaBrCl:Eu (5%) has been grown as a single crystal [5]. The light output of the single crystal was measured at 52,000 ph/MeV. This value is very close to the value we estimated in this work (59,500 ph/MeV) using small crystalline pieces. Contrary to what was observed for BaBrI [2], there seems to be only a marginal increase in light output for BaBrCl and mixed halides containing different Cl/Br ratios compared to BaCl2 and BaBr2.

[1] E.D. Bourret-Courchesne, G. Bizarri, R. Borade, Z. Yan, S.M. Hanrahan, G. Gundiah, A. Chaudhry, A. Canning, S.E. Derenzo, Nucl. Instrum. Methods Phys. Res. A 612 (2009) 138. [2] E.D. Bourret-Courchesne, G. Bizarri, S.M. Hanrahan, G. Gundiah, Z. Yan, S.E. Derenzo, Nucl. Instrum. Methods Phys. Res. A 613 (2010) 95. [3] N.J. Cherepy, G. Hull, A.D. Drobshoff, S.A. Payne, E. van Loef, C.M. Wilson, K.S. Shah, U.N. Roy, A. Burger, L.A. Boatner, W.-S. Choong, W.W. Moses, Appl. Phys. Lett. 92 (2008) 083508. [4] G. Gundiah, E.D. Bourret-Courchesne, G. Bizarri, S.M. Hanrahan, A. Chaudhry, A. Canning, W.W. Moses, S.E. Derenzo, IEEE Trans. Nucl. Sci. 57 (2010) 1702. [5] E.D. Bourret-Courchesne, G.A. Bizarri, R. Borade, G. Gundiah, E.C. Samulon, Z. Yan, S.E. Derenzo, J. Cryst. Growth 352 (2012) 78. [6] X. Meng, Y. Wang, H. Jin, L. Sun, J. Rare Earths 24 (2006) 503. [7] S.A. Hodorowicz, E.K. Hodorowicz, H.A. Elck, J. Solid State Chem. 48 (1983) 351. [8] S.E. Derenzo, M.S. Boswell, E.D. Bourret-Courchesne, R. Boutchko, T.F. Budinger, A. Canning, S.M. Hanrahan, M. Janecek, Qiyu Peng, Y. PorterChapman, J.D. Powell, C.A. Ramsey, S.E. Taylor, Lin-Wang Wang, M.J. Weber, D.S. Wilson, IEEE Trans. Nucl. Sci. 55 (2008) 1458. [9] M. Janecek, R. Borade, E.D. Bourret-Courchesne, S.E. Derenzo, Nucl. Instrum. Methods Phys. Res. A 659 (2011) 252. [10] SMART Software Users Guide, Bruker Analytical X-ray Systems Inc., Madison, WI, 1999. [11] SAINT Software Users Guide, Bruker Analytical X-ray Systems Inc., Madison, WI, 1999. [12] G.M. Sheldrick, SHELXTL, Bruker Analytical X-ray Systems Inc., Madison, WI, 2001. [13] S.E. Derenzo, W.W. Moses, S.C. Blankespoor, M. Ito, K. Oba, IEEE Trans. Nucl. Sci. 41 (1994) 629. [14] J. Goodyear, S.A.D. Ali, W.J. Duffin, Acta Crystallogr. B 25 (1969) 796. [15] G. Gundiah, S. Hanrahan, F. Hollander, E. Bourret-Courchesne, Acta Crystallogr. E 65 (2009) I76. [16] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B 25 (1969) 925. [17] E.K. Hodorowicz, S.A. Hodorowicz, H.A. Eick, J. Solid State Chem. 49 (1983) 362. [18] E.B. Brackett, T.E. Brackett, R.L. Sass, J. Phys. Chem. 67 (1963) 2132. [19] A. Lempicki, A.J. Wojtowicz, E. Berman, Nucl. Instrum. Methods Phys. Res. A 333 (1993) 304. [20] P. Dorenbos, IEEE Trans. Nucl. Sci. 57 (2010) 1162. [21] J. Selling, M.D. Birowosuto, P. Dorenbos, S. Schweizer, J. Appl. Phys. 101 (2007) 034901. [22] Z. Yan, G. Bizarri, E. Bourret-Courchesne, Nucl. Instrum. Methods Phys. Res. A 698 (2013) 7.