Eu2+ Scintillating Crystals

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Toward high energy resolution in CsSrI3:Eu2+ scintillating crystals: effects of off-stoichiometry and Eu2+ concentration Yuntao Wu, Sasmit S. Gokhale, Adam C. Lindsey, Mariya Zhuravleva, Luis Stand, Jesse Ashby Johnson II, Matthew Loyd, Merry Koschan, and Charles L. Melcher Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01375 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 27, 2016

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Crystal Growth & Design

Toward high energy resolution in CsSrI3:Eu2+ scintillating crystals: effects of off-stoichiometry and Eu2+ concentration Yuntao Wu,*,†,‡ Sasmit S. Gokhale,†,‡ Adam C. Lindsey,†,‡ Mariya Zhuravleva,†,‡ Luis Stand,†,‡ Jesse Ashby Johnson II,†,‡ Matthew Loyd,†,‡ Merry Koschan,† and Charles L. Melcher†,‡ †

Scintillation Materials Research Center and ‡Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA

ABSTRACT: CsSrI3:Eu2+ has very promising scintillation properties for gamma-ray spectroscopy applications, but it has proven difficult to grow high quality single crystals in large sizes. This paper reports a composition-engineering strategy, in this case a combination of off-stoichiometric melts and Eu2+ concentration optimization, to obtain large-size CsSrI3:Eu2+ crystals with excellent energy resolution. Crystals of a series of off-stoichiometric compositions, Cs(1+x)(Sr,Eu)(1-x)I(3-x) (x=0, 0.05, 0.06, and 0.1), were grown by the Bridgman method. The Cs1.06Sr0.94I2.94:Eu2+ single crystal has the highest optical transmittance between 450 and 800 nm. Cs1.06Sr0.94I2.94 single crystals doped with 0.5, 1, 3, 5, and 7 mol% Eu2+ ions were also grown by the Bridgman method. The effects of Eu2+ concentration on the phase purity, optical and scintillation properties were studied. X-ray diffraction patterns confirmed the phase purity of all samples with the exception of a hydrate phase formed during measurement. Increasing Eu2+ concentration leads to longer decay components due to the effect of self-absorption. An unexpected relationship was found between Eu2+ concentration and the appearance of two photopeaks in a pulse height spectrum acquired under a single gamma-ray energy of 662 keV irradiation. The origins of this phenomenon are proposed from experimental insights. The optimal composition we developed achieved an excellent energy resolution of 3.4% for ∅22 mm× 2 mm, 3.9% for ∅22 mm× 15 mm and 4.1% for ∅22 mm× 19 mm at 662 keV. The results of this paper lead to

a better understanding of the effects of composition-engineering in optimization of nonstoichiometric scintillator compounds. SECTION: Crystal Growth, Optical Materials, Scintillation, Off-stoichiometry, Eu2+ concentration. The first method is called vacuum-pumping or melt aging 17-19. The processes of this method are that the metal halide raw materials are held above their melting point under dynamic vacuum for a number of hours, during which any excess of the halogen element is removed by volatilization from the nonstoichiometric melt surface and subsequently captured in a cold trap. However, the resulting crystals presented an anomalous “two full energy peaks” phenomenon under a single 137Cs gamma-ray source irradiation.20 A second approach of intentionally creating an off-stoichiometric melt by introducing an excess of the more volatile melt constituent was also employed by considering the different partial pressures of components during crystal growth. The vapour partial pressure for CsI at the melting point (Tm) of CsSrI3 is over five orders of magnitude higher than that of SrI2.20 It means that CsI is a more volatile component during crystal growth than SrI2, and the greater material loss can result in a compositional deficiency. The strategy is to add excess CsI to compensate for its loss during crystal growth. However, adding excess CsI, namely Cs(1+x)(Sr,Eu)I(3+x), proved to exert a negative impact on the scintillation properties, such as worse energy resolution and inducing a second full energy peak.20 Until now poor attention was devoted to nonstoichiometric scintillator compounds, especially on the role of composition in determining the optical quality and defect state in the crystals and finally their scintillator performances.21,22 In this work, by using a totally different stoichiometric composition, namely Cs(1+x)(Sr,Eu)(1-x)I(3-x), we achieved the best energy resolution for large-size CsSrI3:Eu2+ scintillator ever reported, which is close to that of state-of-art halide scintillators. In most Eu2+ doped iodides, the intense self-absorption associated with the small Stokes shift of the Eu2+ center can negatively impact the performance of large-size crystals. A socalled light trapping resulting from the self-absorption effect can cause an increase in scintillation decay time and the dete-

INTRODUCTION Inorganic scintillators are playing important roles in the detection and spectroscopy of gamma and x-rays as well as neutrons and charged particles. In recent years, the search for advanced scintillators with excellent discrimination ability for radioactive isotopes has intensified due to the growing demand for advanced nuclear detection systems for homeland security and nuclear non-proliferation applications. This ability directly relies on the energy resolution of the scintillator selected for the detector. Recent research has resulted in the discovery of several Eu2+ doped halide scintillators with an energy resolution of ≤3% at 662 keV, such as SrI2:Eu2+,1 CsBa2I5:Eu2+,2 KSr2I5:Eu2+,3 KCaI3:Eu2+,4 and KCa0.8Sr0.2I3:Eu2+ 5-7. Several compounds belonging to the ABX3:Eu2+ (A=Cs, B=Ca, Sr, X=Cl, Br, I) material family have been developed during the last five years because of their attractive scintillation properties.8-11 Our group has reported previously promising performance of CsSrI3:Eu2+ single crystals such as a light yield of ∼65,000 photons/MeV and an energy resolution of 5.9% at 662 keV for a small sample (∼5 mm× 5 mm× 2 mm).8 Because of the attractive performance, many fundamental research efforts have been carried out regarding hygroscopicity improvement,12 electronic structure calculation,13 and investigation of optical propert14 of CsSrI3:Eu2+. Nevertheless, there have been few reports on the growth of large-size single crystals of this composition. The development stagnation of this scintillator can be attributed to the difficulty in high quality crystal growth. As one of the ABX3 material compounds, large CsCaI3:Eu2+ crystals have poor energy resolution, i.e. an energy resolution of 11% at 662 keV for a size of 4,560 mm3 (∅22 mm× 12 mm) 15 or 16.3% at 662 keV for a size of 50-300 mm3 16 . This was ascribed to volumetric non-uniformity.15,16 Recently, we employed two different methods in an attempt to improve the quality of CsSrI3:Eu2+ single crystals.

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rioration of light yield and energy resolution.23,24 It was proved that a low-concentration doping strategy can benefit the Eu2+ doped halide scintillators in maintaining their performance in large sizes.8,25 Thus, the effects of Eu2+ doping concentration on the scintillation properties of off-stoichiometric Cs(1+x)(Sr,Eu)(1-x)I(3-x) deserve to be examined. In this work, we developed a composition-engineering strategy to achieve excellent scintillation properties for largesize CsSrI3:Eu2+ crystals. The content is organized as follows: First, off-stoichiometric crystals with different CsI excess amounts were grown by the Bridgman method, and the optical transmittance of the crystals was evaluated; second, based on the best excess CsI amount we found, five off-stoichiometric CsSrI3 crystals doped with 0.5, 1, 3, 5, and 7 mol% Eu2+ ions were also grown by the Bridgman method. The effects of Eu2+ concentration on the phase purity, optical and scintillation properties were studied. An unexpected relationship between features of the full energy photopeak observed in pulse-height spectra full energy peak and Eu2+ concentration was observed. A physical model was proposed to explain this phenomenon based on the results of scintillation decay time and thermoluminesence measurements. The energy resolutions of the best sample with different sizes, such as ∅22 mm× 2 mm and ∅22 mm× 15 mm were characterized at 122 and 662 keV; finally, a reproducibility experiment of the composition-engineering strategy on crystal quality and performance was performed.

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Optical property measurements Optical transmissoin spectra were measured with a Varian Cary 5000 UV-VIS-NIR spectrophotometer in the 300–800 nm range. Photoluminescence emission and excitation spectra were measured with a HORIBA Jobin Yvon Fluorolog-3 spectrofluorometer. The excitation light went through an excitation monochromator with a 1 nm bandpass to ensure monochromaticity. Similarly, the emission monochromator was set at 1 nm bandpass to select emission light of a specific wavelength. In the case of emission and excitation spectra, a 450W continuous xenon lamp was used as the excitation source. Scintillation property measurements Scintillation decay times were acquired with an Agilent DSO6104A digital oscilloscope in single shot mode under 137 Cs source irradiation. Absolute light yield measurements were recorded by using a pulse processing chain consisting of a Hamamatsu R2059 photomultiplier tube (PMT) operated at -1500 Vbias, an Ortec 672 Amp, a Canberra model 2005 pre-Amp and a Tukan 8k multi-channel analyzer. Each sample was directly coupled to the PMT using mineral oil, and a dome-shaped Spectralon reflector with a 50 mm radius was used to maximize the collection of light. The photoelectron yields were estimated by using the single photoelectron peak method. Measurements on the samples were made with 10 µs shaping time for current pulse integration. Each sample was measured under irradiation with a 15 µCi 137Cs source. The reproducibility of the LY measurements is ±5%. The energy resolution was measured by using a 2-inch diameter high quantum efficiency Hamamatsu R6231-100 PMT. This PMT was operated at -1000 Vbias. The 137Cs (662 keV) and 57Co (122 keV) γ-ray sources were used to irradiate the crystals. The energy resolution (E.R.) was calculated as the full width at half maximum (FWHM) divided by the photopeak centroid. An X-ray tube operated at 35 kV and 0.1 mA was used as the excitation source for X-ray excited radioluminescence (RL) measurements.

EXPERIMENTAL METHODS Crystal growth We used the vertical Bridgman technique to grow three sets of off-stoichiometric CsSrI3:Eu2+ single crystals: (i) offstoichiometric ratio optimization (1+x)CsI:(1x)(Sr0.93Eu0.07)I2 (x=0, 0.05, 0.06, and 0.1); (ii) Eu2+ concentration optimization - (1.06)CsI:(0.94)(Sr1-xEux)I2 (x=0.05, 0.1, 0.3, 0.5 and 0.7); (iii) a reproducibility test of offstoichiometric melt growth of a 22 mm diameter 7 mol% Eu2+ doped CsSrI3:Eu2+ single crystal. All the compounds were produced by varying the mixtures of CsI, SrI2 and EuI2 as received from the chemical supplier. The crystal growth conditions for all three sets were the same. High-purity anhydrous CsI, SrI2 and EuI2 beads (5N) from APL Engineered Materials Inc. were used. These CsI, SrI2 and EuI2 beads were loaded and mixed in the quartz ampoules according to the specific composition. The loaded ampoule was evacuated to 10-6 mbar and heated to 250°C and kept at this temperature for 12-20 h for growth of the ∅22 mm crystal in order to remove residual water and oxygen impurities. After baking, the ampoule was sealed and transferred to a Bridgman growth furnace. We used a temperature gradient of ∼25 °C/cm and a translation rate of 0.5 mm/h. The grown crystals were cooled to room temperature (RT) at a speed of 7 °C/h. We then cut and polished samples from the crystal boules for measurements.

Thermoluminescence measurements For each thermoluminescence (TL) measurement, a ∼5 mm cube sample was mounted on a cold finger of the cryostat. The pressure was reduced to 20 mTorr and the sample was then heated to 600 K in order to ensure that all traps were empty in the temperature range of interest. The samples were cooled to 5 K and irradiated by an X-ray generator (X-ray Model; CMX003) at 35 kV and 0.1 mA for 3 min. Subsequently, the sample was again heated to 600 K at a rate of 9 K/min; noise due to thermionic emissions precluded the acquisition of good quality data above this temperature. A Hamamatsu H3177 PMT optically coupled to the cryostat’s light transport interface was used to measure the spectrally unresolved emission from sample. The PMT current signal was transformed into a voltage signal using standard NIM electronics. A National Instruments 6002-E data acquisition card was then used to digitize this voltage signal. Software developed in-house was used to correlate the sample temperature with the signal intensity.

X-ray diffraction measurements A Bruker D2 Phaser with an X-ray source operated at 30 kV and 10 mA using a copper target which produced K-alpha emission lines detected by a 1-dimensional LYNXEYE detector was used to confirm the phase purity. Powder samples were measured through a protective Kapton amorphous polymer domed sample stage that protects the sample from decomposition in ambient air.

RESULTS AND DISCUSSION Off-stoichiometric ratio optimization - (1+x)CsI:(1-x)(Sr,Eu)2


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Crystal Growth & Design Five Cs1.06Sr0.94I3 crystals doped with different Eu2+ concentrations were grown by the Bridgman method to evaluate the Eu2+ concentration effects. The as-grown crystals are shown in Figure 2(a). All the crystals were colorless and inclusion-free. The opaque polycrystalline layer at the last-tofreeze region should be associated with the rejection of part of the excess CsI. The slabs used for measurements were all cut from the cylinder region that is next to the cone (Figure 2b). The flat face surfaces of each sample were polished inside a glove box with an ultra-dry atmosphere (H2O and O2 levels < 0.1 ppm).

Polished crystal slabs and naturally cracked pieces of stoichiometric and off-stoichiometric CsSrI3:Eu2+ are shown in Figures 1(a)-(d). It is clear that the sample with 6 mol% CsI excess is more transparent than the others. The stoichiometric sample and the sample with 5 mol% CsI excess have less transparency, and the sample with 10 mol% CsI excess has an issue of cracking. To quantitatively evaluate the optical quality, the collimated optical transmission spectra of 2 mm thick slabs were measured. The scattered light is not measured in this geometry, only unscattered light reaches the detector. The spectra shown in Figure 1(e) indicate that the sample with 6 mol% CsI excess has a 40% higher transmittance than that of both stoichiometric and 5 mol% CsI excess samples, and over 50% higher than that of the sample with 10 mol% CsI excess. Thus, it is believed that the usage of 6 mol% excess CsI is favorable for the optical transparency of CsSrI3:Eu2+. The improvement should relate to the suppression of the formation of inclusions as scattering centers. For CsSrI3, there is a subtle endothermic peak at 283 °C in the differential scanning calorimeter data,20 which is possibly associated with the solid-solid (S-S) phase transition, similar to CsSrBr3.21 Because of the existence of the endothermic peak in all off-stoichiometric samples, it is believed that the CsI excess has negligible influence on the S-S phase transition. For perovskite family, it was reported that one mode of twinning is possible via a S-S transition from a high symmetry at high temperature to the low symmetry at room temperature.26 The lamellar twinning is observed in all samples of CsSrI3:Eu2+ investigated by using polarized light microscope, similar to that of CsCaI3:Eu2+.15 It could possibly contribute to the decrease of transmittance of all samples between 800 nm and the absorption edge at 440 nm.

Figure 2. (a) As-grown Cs1.06Sr0.94I3:Eu2+ single crystals doped with different Eu2+ concentration. (b) ∅22 mm × 10 mm Cs1.06Sr0.94I3:Eu2+ slabs. The samples shown from left to right are 0.5, 1, 3, 5, and 7 mol% Eu2+ doped, respectively.

Powder

XRD

patterns

of

off-stoichiometric

Cs1.06Sr0.94I2.94:Eu2+ samples obtained by grinding the grown

crystals are shown in Figure 3 as well as the reported pattern for CsSrI3 single crystal as a reference.14 Since the XRD measurements could not be performed without the total exclusion of air, some reflections corresponding to the in situ formation of hydrates of the respective iodide due to the hygroscopic nature of CsSrI3:Eu2+ are shown in the XRD patterns as marked. Similar phenomena were also observed in Ref. 14 and our previous experiments for CsSrI3:Eu2+ 20. Except for that, all reflection peaks are in good agreement with the reference pattern. It indicates that the compositions of all synthesized compounds are a single phase of stoichiometric CsSrI3.

Figure 1. Crystal pictures of CsSrI3:Eu2+ with stoichiometric and off-stoichiometric melts: (a) stoichiometric, ∅15 mm × 8 mm; (b) 5 mol% CsI excess, ∅22 mm × 19 mm; (c) 6 mol% CsI excess, ∅22 mm × 15 mm, and (d) 10 mol% CsI excess, naturally cracked pieces. (e) Optical transmission spectra of 2 mm thick plates that were cut from the samples shown in (a-d).

Figure 3. Measured powder X-ray diffraction patterns of Cs1.06Sr0.94I2.94:Eu2+ samples doped with different Eu2+ concentrations. The single crystal data derived from Ref. 27 is plotted for comparisons.

Eu2+ concentration effects in Cs1.06Sr0.94I3:Eu2+



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doped samples. Specifically, for 5 mo% Eu2+ doped samples, a single full energy peak can be achieved when using 3 µs shaping time, but for 0.5 mol% Eu2+ doped sample, the second full energy peak still can be identified when using only 0.5 µs shaping time. The “two full energy peaks” phenomenon is also found in melt-aged CsSrI3:Eu2+ and off-stoichiometric Cs(1+x)(Sr,Eu)I(3+x) crystals, and the cause is ascribed to the existence of a slow scintillation event in a time scale between 15 and 50 µs.20

The pulse height spectra of ∅22 mm × 10 mm Cs1.06Sr0.94I2.94:Eu2+ samples doped with different Eu2+ concentrations irradiated under a 137Cs gamma-ray source and recorded at different shaping times are plotted in Figure 4. When using 10 µs shaping time, two full energy peaks can be identified in the pulse height spectra of samples doped with ≤5 mol% Eu2+ whereas the 7 mol% Eu2+ doped sample exhibits a single peak. Also, the second full energy peak tends to disappear when the shaping time is continuously decreased, and this phenomenon becomes more pronounced in more highly Eu2+

Figure 4. Pulse height spectra of ∅22 mm × 10 mm Cs1.06Sr0.94I2.94:Eu2+ single crystals irradiated under a 137Cs gamma-ray source recording at different shaping time: (a) 0.5 mol% Eu2+, (b) 1 mol% Eu2+, (c) 3 mol% Eu2+, (d) 5 mol% Eu2+, and (e) 7 mol% Eu2+.

slow event becomes negligible with increasing Eu2+ concentration because its constant is much larger than the shaping time used and its fraction significantly decreases.

The scintillation decay profiles of the Cs1.06Sr0.94I2.94:Eu2+ samples doped with different Eu2+ concentrations acquired with a digital oscilloscope are shown in Figure 5(a). All the scintillation decay profiles could be well fitted by a twoexponential equation except a single-exponential equation for the 7 mol% Eu2+ doped sample. The decay constants and the corresponding fractions are plotted in Figure 5(b) and (c), respectively. We found that when the Eu2+ concentration increases from 0.5% to 5%, both fast and slow decay components increase. Specifically, the fast component increases from 2 µs to 3.5 µs, but the slow component has a significant increase from 7.2 µs to 40 µs. The 7% doped sample has a single decay component of 3.7 µs. As observed in Figure 5(c), the fraction of slow decay component reduces as the increase of Eu2+ concentration. The results confirm that the formation of a second full energy peak in the pulse height spectra is correlated to the presence of the slow event. The influence of this

Figure 5. (a) Scintillation decay profiles of ∅22 mm × 10 mm Cs1.06Sr0.94I3:Eu2+ single crystals doped with different Eu2+ concentration using a digital oscilloscope. (b) The fast and slow component constants, and (c) the fractions of fast and slow components as a function of Eu2+ concentration.


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The prolonged decay time with increasing Eu2+ concentration in metal iodides, such as SrI2:Eu2+,5 KCaI3:Eu2+,28 and KCa0.8Sr0.2I3:Eu2+,23 is associated with a light trapping effect, the consequence of self-absorption of Eu2+ centers. To compare the self-absorption effect in Cs1.06Sr0.94I3:Eu2+ doped with different Eu2+ concentrations, the values of the Stoke shift were evaluated by measuring the PL and PLE spectra. The spectra are shown Figure 6(a-e). When the Eu2+ concentration increases from 0.5 to 7 mol%, the emission peak position remains almost the same at about 460 nm, but the excitation bands gradually shift to the longer wavelength. When further increasing the Eu2+ concentration to 7 mol%, in addition to the redshift of the excitation band, the Eu2+ 5d-4f emission peak also has a redshift to 469 nm. The trend of Stokes shift as a function of Eu2+ concentration plotted in Figure 6(f) shows that it gradually decreases from 0.22 eV for 0.5 mol% Eu2+ to 0.075 eV for 7 mol% Eu2+. In such a case, highly doped samples should have a more intense self-absorption. The x-ray excited RL spectra measured in a reflection mode (Figure 7) confirm the self-absorption effect, namely the redshift of the Eu2+ 5d→4f emission. Hence, it is reasonable to believe that the prolonged fast and slow components are associated with the light trapping effect. The schematic diagrams of scintillation processes of fast and slow decay components in offstoichiometric Cs1.06Sr0.94I2.94 doped with low and high Eu2+ concentration are presented in Figure 8. For the low Eu2+ concentration, the fast decay time is a prompt sequential electronhole capture at a Eu2+ center, and the slow decay component is a delayed recombination because electrons being captured by the shallow traps before reaching the Eu2+ centers. However, with further increasing Eu2+ concentration, due to the light trapping effect, both fast and slow decay components are prolonged. Another possible explanation of formation of relatively deep electron traps with longer detrapping time needs to be considered. Hence, it is necessary to investigate the electron traps in the low and high Eu2+ doped samples by the TL technique.

Figure 7. X-ray excited RL spectra of ∅22 mm × 10 mm Cs1.06Sr0.94I3:Eu2+ single crystals doped with different Eu2+ concentrations.

Figure 8. The schematic diagram of scintillation processes of fast and slow decay components in off-stoichiometric Cs1.06Sr0.94I2.94 doped with low (a) and high (b) Eu2+ concentration.

The TL glow curves of Cs1.06Sr0.94I3:Eu2+ single crystals doped with different Eu2+ concentrations are shown in Figure 9. As shown in Figure 9(a-e), no new TL peaks are formed with increasing Eu2+ concentration. Contrarily, a TL peak at 60 K tends to disappear with increasing Eu2+ concentration, and traps below 200 K are fully suppressed in the 7 mol% Eu2+ doped sample. This result suggests that the prolonged slow decay component from 0.5% to 5% doped sample should be related to a more intense light trapping effect rather than formation of new shallow traps, and the absence of a slow decay component in the 7 mol% Eu2+ doped sample is related to the full suppression of the electron population at shallow traps or the removal of shallow traps. Also, the reduction of the slow decay component fraction with increasing Eu2+ concentration implies that adding more Eu2+ ions could efficiently suppress the electron population at certain shallow traps spatially correlated with Eu2+ centers. Furthermore, to rule out the influence of matrix composition on the defect structure, the TL curves of 7 mol% Eu2+ doped off-stoichiometric CsSrI3 with different CsI/SrI2 ratios, such as CsI/SrI2=1.06/0.94 and 1.05/0.95, are

Figure 6. PL and PL spectra of Cs1.06Sr0.94I3:Eu2+ single crystals doped with different Eu2+ concentration monitoring at their maximum excitation and emission wavelengths: (a) 0.5 mol%, (b) 1 mol%, (c) 3 mol%, (d) 5 mol%, and (e) 7 mol%. (f) The Stokes shift as a function of Eu2+ concentration.



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shown in Fig. 9(f). The results indicate that the significant suppression of shallow traps associated with the TL peaks between 50 and 250 K depends on the high Eu2+ doping level, not the matrix composition. As mentioned above, these nonstoichiometric halide scintillators are developed toward high energy resolution for homeland security and nuclear nonproliferation applications, thus, the afterglow related TL peaks above room temperature are not discussed here. The afterglow level should be expected of high intensity similar to the results reported in Ref. 20.

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single crystal, it is estimated that its light yield is 44,000±2000 photons per MeV. The value is close to that reported for the stoichiometric CsSrI3:Eu2+ with a similar size.20

Figure 10. Pulse height spectra of Cs1.06Sr0.94I2.94:7 mol%Eu2+ single crystals with different sizes irradiated under 137Cs and 57Co gamma-ray sources: (a) ∅22 mm × 2 mm, 137Cs; (b) ∅22 mm × 2 mm, 57Co; (c) ∅22 mm × 15 mm, 137Cs; (d) ∅22 mm × 15 mm, 57 Co. Note: the gain setting of spectrum (a) is two times smaller than that of spectra (b) (c), and (d), but it has negligible influence on energy resolution evaluation. The standard error of energy resolution is ±0.05% for 662 keV and ±0.1% for 122 keV, respectively.

Figure 9. (a-e) TL glow curves of Cs1.06Sr0.94I3:Eu2+ single crystals doped with different Eu2+ concentration. (f) TL glow curves of Cs1.05Sr0.96I2.95:7 mol% Eu2+ and Cs1.06Sr0.94I2.94:7 mol% Eu2+ crystals.

Figure 11. Pulse height spectra of a ∅22 mm × 10 mm Cs1.06Sr0.94I2.94:7 mol%Eu2+ single crystals irradiated under 137Cs gamma-ray sources acquired with a Hamamatsu R2059 PMT and 10 µs shaping time. The EWQE is calculated to be 22.9%, which is derived from the RL spectrum and quantum efficiency of R2059 PMT shown inset.

The Cs1.06Sr0.94I2.94:7 mol% Eu2+ sample shown in Figure 2(a) was chosen to be cut into two sizes, such as ∅22 mm× 2 mm and ∅22 mm× 15 mm, because of the absence of the “two full energy peaks” phenomenon. The pulse height spectra of these two size samples under 57Co and 137Cs irradiation acquired with a high quantum efficiency R6231-100 PMT are shown in Figure 10. For the ∅22 mm× 2 mm size sample, the energy resolutions at 122 and 662 keV are 6.5% and 3.4%, respectively. When the sample size increases to ∅22 mm× 15 mm, 7.5 times larger, the energy resolutions at 122 and 662 keV still achieve 7.6% and 3.9%, respectively. Based on an emission-weighted quantum efficiency (EWQE) of R2059 of 22.9% for a ∅22 mm ×10 mm Cs1.06Sr0.94I2.94:7 mol%Eu2+

The reproducibility of composition-engineering on crystal quality and performance

To inspect the reproducibility, an off-stoichiometric melt growth of 7 mol% Eu2+ doped CsSrI3 single crystal with a size of ∅22 mm× 50 mm was performed. As observed in Figure 12(a) and (b), the as-grown crystal is transparent with only few cracks on the surface. The cracks originated from the stresses generated due to the different thermal expansions of ampule and crystal during cooling process. Similar phenome-


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Crystal Growth & Design Table 1. The advantages and disadvantages of stoichiometric and offstoichiometric melt growth of CsSrI3:Eu2+ single crystals. Composition Advantages Disadvantages Stoichiometry Weak self-absorption Risk of existence of two 1CsI:1(Sr,Eu)I2 (Eu2+ concentration could full energy peaks; be less than 5 mol%) Worse transparency; Worse energy resolution Off-stoichiometry Single full energy peak; Strong self-absorption； (1+x)CsI: Better transparency; A relative large portion (1-x)(Sr0.93Eu0.07)I2 Better energy resolution of last-to-freeze region (x=0.05-0.07) Off-stoichiometry Existence of two full (1+x)CsI:1(Sr,Eu)I2 energy peaks; A relative large portion of last-to-freeze region

non has been observed for the Bridgman growth of other alkali and alkaline earth iodides.7,28 A ∅22 mm× 19 mm crystal slab was cut from the cylinder region of the ingot, and polished to evaluate the energy resolutions. The pulse height spectra of a ∅22 mm× 19 mm long sample acquired with R6231-100 PMT under 137Cs and 57Co irradiation are shown in Figure 12(c) and (d). The energy resolutions at 122 and 662 keV are 8% and ∼4.1%, respectively. The results are superior to the best reported values for CsSrI3:Eu2+ crystals of smaller size, such as 5.9% at 662 keV for a 5 mm× 5 mm× 2 mm,8 and 5% at 662 keV for ∅15 mm× 8 mm.20 Thus, the growth result and the performance demonstrate the reproducibility of the strategy. The advantages and disadvantages of stoichiometric and off-stoichiometric melt growth of CsSrI3:Eu2+ single crystals are listed in Table 1, including Cs(Sr,Eu)I3, Cs(1+x)(Sr,Eu)I(3+x) and Cs(1+x)(Sr,Eu)(1-x)I(3-x). Apparently, the approach of using off-stoichiometric melts Cs(1+x)(Sr,Eu)I(3+x) reported in Ref. 20 is unfeasible because it will unavoidably induce the formation of two full energy peaks under a single 137Cs gamma-ray source irradiation. Compared to the use of initial stoichiometric melt composition, the large-size crystals grown by using the best off-stoichiometric melt composition Cs(1+x)(Sr,Eu)(1-x)I(3-x) we found has much better transparency. More importantly, by using a combination of offstoichiometric melts Cs(1+x)(Sr,Eu)(1-x)I(3-x) and high Eu2+ concentration, it is for the first time that the large-size CsSrI3:Eu2+ crystals can achieve an excellent energy resolution of 4% with a volume of 7 cm3 and utterly avoid the present of “two full energy peaks” phenomenon. From the crystal yield point of view, one drawback of using off-stoichiometric melt growth is the formation of relative large portion of last-to-freeze region with respect to the whole boule due to the rejection of excess CsI from the melt during crystal growth. A shortcoming of use of high Eu2+ doping level is a strong self-absorption effect. However, it is shown that appropriate use of pulse digitization can efficiently mitigate the negative influence of selfabsorption and further improve the energy resolution of inchsized Eu2+ doped iodide crystals.29 Further improvement of readout system for the optimized CsSrI3:Eu2+ single crystals is in progress.

CONCLUSIONS Crystals of a series off-stoichiometric compositions, Cs(1+x)(Sr,Eu)(1-x)I(3-x) (x=0, 0.05, 0.06, and 0.1), were grown by the Bridgman method. The Cs1.06Sr0.94I2.94:Eu2+ sample with the highest optical transmittance indicates that 6 mol% excess CsI is favorable for optical transparency. Cs1.06Sr0.94I2.94 single crystals doped with 0.5, 1, 3, 5, and 7 mol% Eu2+ ions were grown by the Bridgman method. X-ray diffraction patterns confirm a single phase of CsSrI3 for all samples with the exception of a hydrate phase formed during measurement. Increasing Eu2+ concentration prolongs the fast and slow decay components due to the effect of self-absorption. The Eu2+ concentration dependent “two full energy peaks” phenomenon is related to a slow event. The influence of this slow event becomes negligible with increasing Eu2+ concentration because its constant is much larger than the shaping time used and its fraction significantly decreases. The prolonged slow decay component in the 0.5% to 5% doped samples is caused by a more intense light trapping effect, and the absence of the slow decay component in the 7 mol% Eu2+ doped sample is ascribed to the full suppression of the electron population at shallow traps or the removal of shallow traps. The best composition we found derived from the composition-engineering strategy achieved an energy resolution of 3.4% for ∅22 mm× 2 mm, 3.9% for ∅22 mm× 15 mm and 4.1% for ∅22 mm× 19 mm at 662 keV, which is superior to the best performance reported for CsSrI3:Eu2+ samples of smaller size. More importantly, this composition-engineering approach can be applied to other metal halides confronting the similar problem because of the potential beneficial effect on the crystal quality and energy resolution. AUTHOR INFORMATION Corresponding author ∗E-mail: [email protected], [email protected] (Y.T. Wu). ACKNOWLEDGEMENT This work has been supported by the US Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded grant #2012-DN-077-ARI067-05. This support does not constitute an express or implied endorsement on the part of the Government.

Figure 12. A ∅22 mm× 50 mm crystal grown from the offstoichiometric melt of CsSrI3:7 mol% Eu2+: (a) inside growth ampule, (b) surface polished ingot. Pulse height spectra of a ∅22 mm× 19 mm crystal slab under (c) 137Cs and (d) 57Co irradiation acquired with a high quantum efficiency R6231-100 PMT.

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For Table of Contents Use Only Toward high energy resolution in CsSrI3:Eu2+ scintillating crystals: effects of off-stoichiometry and Eu2+ concentration Yuntao Wu,*,†,‡ Sasmit S. Gokhale,†,‡ Adam C. Lindsey,†,‡ Mariya Zhuravleva,†,‡ Luis Stand,†,‡ Jesse Ashby Johnson II,†,‡ Matthew Loyd,†,‡ Merry Koschan,† and Charles L. Melcher†,‡ † Scintillation Materials Research Center and ‡Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA

Synopsis This paper reports a composition-engineering strategy, in this case a combination of off-stoichiometric melts and Eu2+ concentration optimization, to obtain large-size CsSrI3:Eu2+ crystals with excellent energy resolution for radiation detection applications. The best composition we found derived from the strategy achieved an energy resolution of 4.1% for ∅22 mm× 19 mm at 662 keV, which is superior to the best performance reported for CsSrI3:Eu2+ samples of smaller size. The results lead to a better understanding of the effects of composition engineering in optimization of nonstoichiometric scintillator compounds.



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Figure 1 70x70mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 9 129x208mm (300 x 300 DPI)


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Figure 10 77x59mm (300 x 300 DPI)



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Figure 12 80x54mm (300 x 300 DPI)


Eu2+ Scintillating Crystals

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