- Email: [email protected]
Europium concentration effects on the scintillation properties of Cs4SrI6:Eu and Cs4CaI6:Eu single crystals for use in gamma spectroscopy
Journal of Luminescence 216 (2019) 116740
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Europium concentration effects on the scintillation properties of Cs4SrI6:Eu and Cs4CaI6:Eu single crystals for use in gamma spectroscopy
T
Daniel Rutstroma,b,∗, Luis Standa, Merry Koschana, Charles L. Melchera,b,c, Mariya Zhuravlevaa,b a
Scintillation Materials Research Center, University of Tennessee, Knoxville, USA Department of Materials Science and Engineering, University of Tennessee, Knoxville, USA c Department of Nuclear Engineering, University of Tennessee, Knoxville, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Scintillator Alkali metal halides Rare earth elements Bridgman technique Growth from melt Single crystal
Recently discovered scintillators Cs4SrI6:Eu and Cs4CaI6:Eu have shown promising properties for gamma-ray detection in homeland security applications, with < 4% energy resolution (at 662 keV) and light yields above 50,000 ph/MeV. However, a thorough investigation of the effects of europium concentration has not yet been conducted. In this work, Eu2+ concentration was varied from 0.5 mol% to 9 mol%, and Ø7 mm single crystals were grown from the melt via the vertical Bridgman method. Scintillation performance was evaluated as a function of Eu2+ concentration to determine the optimal amount. The observed trend was improved energy resolution and higher light yield with increasing Eu2+ concentration up to 7 mol%. The best energy resolution achieved was 3.2% for Cs4SrI6:Eu 7% and 3.6% for Cs4CaI6:Eu 7%. Their respective light yields were 71,000 ph/ MeV and 69,000 ph/MeV.
1. Introduction
critical aspect of nuclear security and accurate identification of radiological isotopes of potentially harmful materials. A new family of zero-dimensional perovskites with the formula A4BX6 (A+ = Cs; B2+ = Ca, Eu, Sr, or Yb; and X– = Br or I) have recently been shown to be efficient scintillators for gamma-ray detection in nuclear security applications [5–8]. Cs4SrI6, Cs4CaI6, Cs4YbI6, Cs4EuI6, and Cs4EuBr6 have demonstrated the ability to be grown from the melt and all adopt the K4CdCl6 structure type, crystallizing in the 3 c [5–8]. The most promising trigonal crystal system with space group R‾ of these are Eu2+-activated Cs4SrI6 and Cs4CaI6, which are reported to have energy resolution as low as 3.3% (at 662 keV) and light yields of ~70,000 ph/MeV. With scintillation properties approaching those of commercially available scintillators such as LaBr3:Ce3+ and SrI2:Eu2+ (energy resolutions < 3% and light yields from 70,000 to 90,000 ph/ MeV), further development of Cs4SrI6 and Cs4CaI6 is of interest [9,10]. Work has already been done toward the development of these compounds by investigating effects of monovalent cation substitution, Cs4-x(K or Rb)xSrI6:Eu and Cs4-x(K or Rb)xCaI6:Eu for example, where it was found that energy resolution slightly improved for Cs3.5Rb0.5SrI6:Eu compared to Cs4SrI6:Eu [8]. Additional ways in which Cs4SrI6 and Cs4CaI6 may further be developed include optimizing parameters for growth of large diameter (> 1 inch) crystals, compositional engineering via mixed divalent cation compositions (ex. – Cs4Sr1-
Currently the ideal scintillator that meets all the necessary criteria for a given application does not exist, therefore, pursuing novel materials with improved detection capabilities continues to be a worthwhile endeavor. Nuclear security is one specific application that could benefit significantly from further improvements to scintillator technology. The basic principle behind the scintillation process is that incident ionizing radiation (gamma-rays, X-rays, etc.) gets converted into visible light that can be collected by a photodetector and further processed as an electrical signal [1–3]. The signal pulse amplitudes correspond to the energy deposited by the incident photon, which can then be utilized in the form of a pulse height spectrum (or energy spectrum) to accurately identify different radioisotopes. The degree of accuracy depends on a variety of factors, but it is ultimately limited by the energy resolution (ER) of the scintillator. Energy resolution is essentially a measure of the detector's ability to discriminate between similar energies of radiation. Poor resolution results in multiple adjacent photopeaks in the energy spectrum appearing as one broad peak, as is the case with a detector such as NaI (~7% energy resolution at 662 keV) [4]. On the other hand, a detector with good resolution will have narrow energy peaks that are possible to distinguish even when in close proximity to one another. This ability to resolve complex features of the energy spectrum is a
∗
Corresponding author. Scintillation Materials Research Center, University of Tennessee, Knoxville, USA. E-mail addresses: [email protected] (D. Rutstrom), [email protected] (L. Stand), [email protected] (M. Koschan), [email protected] (C.L. Melcher), [email protected] (M. Zhuravleva). https://doi.org/10.1016/j.jlumin.2019.116740 Received 28 May 2019; Received in revised form 26 August 2019; Accepted 10 September 2019 Available online 11 September 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
Journal of Luminescence 216 (2019) 116740
D. Rutstrom, et al. xCaxI6:Eu), searching for alternative activators that may be better suited to each matrix, or optimizing the Eu2+ activator concentration. This work focuses solely on the latter – studying the effects of Eu2+ concentration on scintillation properties such as energy resolution, light yield, and non-proportionality. Some materials can intrinsically scintillate, however, many of the widely used scintillators require a luminescent activator in the form of a dopant. The activator provides the recombination centers for electronhole pairs created from the incident radiation, and this recombination is what leads to the emission of a photon of visible light [2,3]. In the past 15 years, Eu2+ has become one of the leading candidate activators for high performance scintillators due to its efficient 5d → 4f electronic transition and emission wavelengths that fall within the spectral sensitivity range of a standard bialkali photomultiplier tube (PMT) [1,2]. Some recent developments toward Eu2+-doped scintillators include SrI2:Eu, BaBrI:Eu, CsBa2I5:Eu, KSr2I5:Eu, KCaI3:Eu, and LiI:Eu [10–16]. In addition to being a known efficient activator, Eu2+ was chosen for the initial development of Cs4SrI6 and Cs4CaI6 due to its compatible valence state with Sr2+ and Ca2+, as well as its similar ionic radius to Sr2+ (rEu = 1.17 Å and rSr = 1.18 Å, for a coordination number of 6) [17]. The influence of Eu2+ concentration on scintillation properties has been investigated before in other host matrices, such as SrI2:Eu, KSr2I5:Eu, and KCaI3:Eu [12–14,18]. Dopant concentration optimization was also carried out in Ref. [8], but this only included Cs3KCaI6:Eu rather than Cs4SrI6:Eu and Cs4CaI6:Eu, which are addressed in this paper. A typical consequence of increasing Eu2+ concentration is an increased decay time. This is in part due to the increased probability of self-absorption (overlap of excitation and emission spectra) that occurs with higher Eu2+ concentration [13]. The primary decay component for Eu2+ is generally in the 1 μs–3 μs range, but depending on concentration can be within ~600 ns–1000 ns [12–14,18]. Starting from low concentrations of about 0.5% Eu2+, light yield often increases and energy resolution improves as the concentration is increased; however, at a certain point self-absorption effects overtake the benefits of adding more Eu2+ activator sites, causing light yield and energy resolution to degrade. Similar effects also occur when increasing the size of a scintillator crystal with a fixed Eu2+ concentration and are also attributed to self-absorption. This work only addresses the concentration effects in Ø7 mm crystals.
Fig. 1. (a) Three crystals being grown simultaneously in a semi-transparent furnace and (b) an example of the quartz boat in which the crystals were contained during growth.
quartz ampoules using the vertical Bridgman technique. A detailed schematic of this growth method can be seen in Chapter 2 of ref. [20]. The ampoules all contained a self-seeding capillary region to initiate nucleation and act as a grain selector. Crystals were translated at a rate of 0.8 mm/h and cooled at 7 °C/hr. The majority were grown using a semi-transparent gold-coated Bridgman furnace with two heating zones. The benefit of the transparent furnace was the ability to monitor the growth in real time and adjust growth parameters (pull rate, temperature settings, etc.) as necessary. Crystals grown in this furnace were done in sets of three, with the exception of Cs4SrI6:Eu 9% and Cs4CaI6:Eu 9%. This was accomplished by use of a quartz boat, in which three separate ampoules were placed at once. This can be seen in Fig. 1. During growth the entire boat was translated through the furnace, and the result was simultaneous growth of three crystals in one growth run (usually the same host compound with varying Eu2+ concentrations). The remaining crystals were grown using a standard three-zone Bridgman furnace (zero transparency). All furnaces were equipped with a 1″ to 2” thick insulating diaphragm to enhance the thermal gradient. Thermal profiles of the furnaces were measured prior to growth. Zone temperatures were adjusted until the temperature near the top of the diaphragm was at the compounds’ melting points and thermal gradients of at least 25 °C/cm were achieved. Temperatures of the hot zones ranged from 640 °C to 675 °C and for the cold zone ranged from 380 °C to 400 °C. The melting temperatures for Cs4SrI6 and Cs4CaI6 are 533 °C and 555 °C, respectively [5].
2. Experimental 2.1. Synthesis and crystal growth
2.2. Characterization Anhydrous beads of CsI, SrI2, CaI2, and EuI2 all with > 99.99% purity were used as starting raw materials. To avoid degradation due to their extreme sensitivity to air and moisture, all raw materials were handled inside of a nitrogen-filled glovebox with < 0.1 ppm H2O and O2. The mass of each individual component required to obtain the target compounds was calculated under the assumption that the dopant substitutes for the divalent cation (Sr2+ or Ca2+) in both Cs4SrI6 and Cs4CaI6; for example, Cs4Sr1-xEuxI6 and Cs4Ca1-xEuxI6. The samples that were evaluated contained nominal molar concentrations of 0.5%, 1%, 3%, 5%, 7%, and 9% europium (or 0.005 < x < 0.09). Using the calculated stoichiometric ratios, raw materials were mechanically mixed and loaded into quartz ampoules. The ampoules were then evacuated to a pressure between 10−6 and 10−7 Torr, heated at 250 °C for up to 12 h, cooled to room temperature, and sealed under vacuum with an H2–O2 torch. Materials were melt-synthesized prior to crystal growth by heating above the melting temperatures of the individual components for > 12 h and cooling to room temperature over several hours. This was carried out using the M.A.D. (multiple ampoule directions) mixing technique to ensure thorough mixing and incorporation of the luminescent activator into the host matrix [19]. Crystals were grown from the melt in Ø7 mm (inner diameter)
Smaller crystals, < 5 × 5 × 5 mm3, were cut from the as-grown boules and polished for characterization. Due to the hygroscopic nature of these crystals, luminescence and scintillation measurements were conducted with the samples submerged in mineral oil to prevent degradation. Steady-state photoluminescence (PL) spectra were measured in a reflection geometry using a Horiba Jobin Yvon Fluorolog 3 Spectrofluorometer equipped with a Xe lamp and dual scanning monochromators. Emission spectra were collected using an excitation wavelength of 280 nm, and excitation spectra were collected monitoring emission at 460 nm and 462 nm depending on the host matrix. The PL decay time was measured using a Horiba Jobin Yvon NanoLed with an excitation wavelength of 370 nm, emission monitored at 460–462 nm, and a pulse width of 1 ns. Radioluminescence (RL) was measured under excitation by a Cu target X-ray tube operated at 35 kV and 0.1 mA using a transmission geometry. The resulting emission spectra were collected from 200 nm to 800 nm with a 150 mm focal length monochromator. Energy resolution measurements were conducted using crystals with sizes of approximately 3 × 3 × 3 mm3. The crystals were placed into a quartz housing filled with mineral oil, which was then optically coupled 2
Journal of Luminescence 216 (2019) 116740
D. Rutstrom, et al.
to a Hamamatsu R6231 PMT. The housing and PMT were covered with reflective Teflon sheets and a Spectralon solid diffuse reflector dome to maximize light collection. Pulse height spectra were collected with the PMT operated at 1 kV using a signal processing chain consisting of a Canberra 2005 preamplifier, an Ortec 672 amplifier set to 10 μs shaping time, and a Tukan 8 K multi-channel analyzer. The 662 keV photopeak was fit with a Gaussian function, and percent energy resolution was calculated by dividing the full width at half maximum (FWHM) by centroid channel number. For instances where energy resolution was good enough to resolve the iodine escape peak (~28 keV below the 662 keV energy peak), two Gaussian functions were fit. A137Cs gamma source was used as the excitation source. Light yield was measured using a similar setup as with energy resolution, with the primary difference being the use of a Hamamatsu R2059 PMT operated at 1.5 kV instead of the R6231. Absolute light yield was calculated in terms of number of photons per MeV using the method described by Dorenbos et al., which accounts for the position of the single photoelectron (SPE) centroid and the quantum efficiency of the PMT [21]. Typical error in light yield due to slight sample-tosample variation (different samples of the same composition) is around ± 2000 ph/MeV to ± 5000 ph/MeV. In the current study, error was not significant enough to change the overall light yield trends. Non-proportionality was measured using the same PMT, voltage, and signal processing chain as with energy resolution. Excitation sources and energies of interest included 137Cs (32 keV and 662 keV), 22 Na (511 keV), 133Ba (356 keV), 57Co (122 keV), and 241Am (59.6 keV). Light yield at each energy was then normalized to the light yield at 662 keV. The method described in Ref. [22] was used to measure scintillation decay time. 137Cs was used as the excitation source.
Fig. 2. (a) Simultaneously-grown crystals of Cs4SrI6:Eu with 7%, 3%, and 1% Eu (top to bottom) under room lighting and (b) excited by UV light. (c) Cs4SrI6 and Cs4CaI6 crystals doped with 9% Eu before removal and (d) after removal from ampoules and polishing.
Cs4SrI6:Eu and Cs4CaI6:Eu is centered around 460 nm–462 nm. For RL, peak emission wavelengths occur around 466 nm. These emissions are characteristic of the 5d → 4f electronic transition of Eu2+. The slight shift to a higher wavelength with RL compared to PL is a result of the measurement geometries. The transmission geometry, which was used for RL measurements, leads to an increased probability for self-absorption thus shifting the emission to higher wavelengths [18]. A low intensity RL emission band from about 500 nm to 600 nm was also observed in the low Eu2+ concentration samples. It is most apparent for Cs4CaI6:Eu 0.5%, seen in Fig. 3b. This has been attributed to the intrinsic self-trapped exciton (STE) emission of undoped Cs4SrI6 and Cs4CaI6 [8]. This band becomes suppressed at higher Eu2+ concentrations as the 5d → 4f transition begins to dominate. The position of PL excitation and emission bands also does not vary significantly with different amounts of Eu2+. However, the relative intensities of the excitation bands do show a slight dependence on Eu2+ concentration. For low concentrations of Eu2+, such as 0.5 mol%, there is a dominant excitation band near 260 nm with a drop in intensity from 300 nm to 330 nm followed by additional lower intensity bands from 330 nm to 445 nm. At higher concentrations, the excitation spectra are more continuous across all wavelengths from 260 nm to 445 nm. This effect was also observed in Refs. [8,18] for 0.5% Eu2+-doped Cs4SrI6 and Cs4CaI6 and KCaI3:Eu 0.2%, respectively. Additionally, a larger degree of self-absorption (overlapping of excitation and emission spectra) can be observed as Eu2+ concentration increases (Fig. 3). This is a well-known effect of Eu2+ that is intensified with either increasing concentration or increasing crystal size [12,13,18].
3. Results and discussion 3.1. Crystal growth Growth of Ø7 mm Cs4SrI6:Eu and Cs4CaI6:Eu resulted in transparent crack-free crystals in most cases. A few of the Ca-containing compounds had one or two minor cracks, however, this was only observed for the crystals that were grown simultaneously. In contrast, the simultaneously grown Sr-containing crystals did not show any cracking. Fig. 2 illustrates the good crystal quality obtained for the Sr-containing compound with Eu2+ concentrations ranging from 1% to 7%. Due to the slight mismatch in ionic radii between Ca2+ and Eu2+ (1.00 Å and 1.17 Å, respectively, for a coordination number of 6) there was initial uncertainty as to whether high concentrations of Eu2+ could be incorporated into the Cs4CaI6 matrix [17]. Despite this concern, excellent quality crystals of both Cs4CaI6:Eu 9% and Cs4SrI6:Eu 9% were grown. This is shown in Fig. 2d. The lack of a significant last-tofreeze region in either crystal suggests successful incorporation of the dopant. Additionally, powder X-ray diffraction data for these samples was consistent with what was observed in Ref. [5] and confirms the same phase was obtained. The fact that Cs4SrI6, Cs4CaI6, and Cs4EuI6 all share the same crystal structure and space group (trigonal, space group R-3c) likely contributes to why both compounds are capable of incorporating such high amounts of Eu2+ [5,6]. 3.2. Characterization
3.2.2. Scintillation performance and decay time Examples of the fits used to calculate energy resolution are shown in Fig. 4 for Cs4SrI6:Eu and Cs4CaI6:Eu doped with 0.5%, 3%, and 7% Eu2+. As discussed in Section 2.2, peaks were fit with two Gaussian functions when energy resolution was sufficient for resolving the iodine escape peak. Fig. 5 shows light yield and energy resolution of Cs4SrI6:Eu and Cs4CaI6:Eu as a function of dopant concentration from 0.5% to 9% Eu2+. The best energy resolution was obtained with Cs4SrI6:Eu 7%, achieving 3.2% at 662 keV. It can be seen that the energy resolution and light yield of Cs4SrI6:Eu and Cs4CaI6:Eu improve as Eu2+ concentration
3.2.1. Photoluminescence and radioluminescence The luminescent properties of Eu2+-doped Cs4SrI6:Eu and Cs4CaI6:Eu have previously been reported in Refs. [5,8]. Fig. 3 shows RL emission and PL emission/excitation spectra dependence on concentration for 0.5%, 3%, and 7% Eu2+. The additional compounds grown (1%, 5%, and 9%) had similar features in their excitation and emission spectra. Emission wavelength is weakly dependent on Eu2+ concentration, with a negligible < 2 nm shift toward longer wavelengths occurring as concentration is increased. PL emission of both 3
Journal of Luminescence 216 (2019) 116740
D. Rutstrom, et al.
Fig. 3. Radioluminescence (RL) and photoluminescence (PL) emission/excitation of (a) Cs4SrI6:Eu and (b) Cs4CaI6:Eu with 0.5%, 3%, and 7% Eu2+. The vertical line at ~460 nm is included to allow for an easier visual comparison between peak centroid positions.
energy resolution to 3.9% (~41% total improvement). The poor resolution at low Eu2+ concentrations for Cs4SrI6:Eu may be a result of its extreme non-linearity combined with a low light yield, whereas Cs4CaI6:Eu has a relatively flat response at low Eu2+ concentrations (Fig. 6). The scintillation response as a function of energy, or non-proportionality, for Cs4SrI6:Eu and Cs4CaI6:Eu is shown in Fig. 6. Only two different Eu2+ concentrations are plotted for each compound for ease of interpretation. The least proportional response for the Cs4SrI6:Eu occurs with 0.5% Eu, with a maximum deviation from ideal at 32 keV (90.8% relative light yield normalized to 662 keV). The trend observed with this compound was improved proportionality as Eu2+ concentration was increased up to 5%, which deviates by < 1% at 32 keV (100.9% relative light yield) and has maximum deviation at 122 keV (102% relative light yield). The relative light yield at low energies continues to rise with increasing Eu2+ concentration beyond 5%, approaching 104.2% at 32 keV, 104.5% at 59.5 keV, and 103.4% at 122 keV for Cs4SrI6:Eu 9%. Contrast to the Sr-based compound, the most proportional response for Cs4CaI6:Eu was obtained with 0.5% Eu2+, which has a maximum deviation at 122 keV (102.2% relative light yield). The trend in Cs4CaI6:Eu was similar to Cs4SrI6:Eu, where relative light yields at lower energies increased with increasing Eu2+ concentration. The only
is increased from 0.5% to 7% in both compounds. Raising the concentration from 7% to 9% in Cs4SrI6:Eu results in an improved light yield (71,000 ph/MeV to 78,000 ph/MeV), however, energy resolution remains at 3.2% for both. The benefits from this light yield increase are relatively marginal when there is not also an improvement in energy resolution, therefore it is still reasonable to consider 7% Eu the optimal amount for use in nuclear security applications, in which energy resolution is the more performance-limiting property. In contrast, energy resolution of Cs4CaI6:Eu worsens from 3.6% to 3.9% despite a minimal change in light yield from 69,000 ph/MeV to 68,000 ph/MeV when increasing Eu2+ concentration from 7% to 9%. This nearly equivalent light yield for the 9% Eu2+-doped sample is also accompanied by improved non-proportionality (discussed below) compared to 7% Eu2+ suggesting that this degradation of energy resolution could be related to crystal inhomogeneity or an increased degree of self-absorption. Cs4CaI6:Eu has superior energy resolution over Cs4SrI6:Eu at low Eu2+ concentrations, an effect that was also observed in Ref. [8]. This is apparent from Fig. 5b. Increasing from 0.5% to 3% Eu2+ in the Cacontaining matrix lead to an improvement from 4.8% to 4.3% (~10.4% improvement) energy resolution at 662 keV. A more significant change occurred when increasing from 0.5% to 3% Eu2+ in the Sr-containing matrix. Whereas 0.5% and 1% Eu2+ in Cs4SrI6:Eu have energy resolutions > 6%, increasing the concentration to 3% Eu2+ improves the
Fig. 4. Pulse height spectra of 0.5%, 3%, and 7% Eu2+-doped (a) Cs4SrI6 and (b) Cs4CaI6 measured with a137Cs source. Sample dimensions are approximately 3 × 3 × 3 mm3. 4
Journal of Luminescence 216 (2019) 116740
D. Rutstrom, et al.
Fig. 5. (a) Absolute light yield and (b) energy resolution at 662 keV as a function of Eu2+ concentration for approximately 3 × 3 × 3 mm3 crystals.
Table 1 Summary of scintillation and photoluminescence decay times for Cs4SrI6:Eu and Cs4CaI6:Eu with different Eu2+ concentrations.
Cs4SrI6
Cs4CaI6
Fig. 6. Non-proportionality of Cs4SrI6:Eu and Cs4CaI6:Eu showing Eu2+ concentrations with the best and worst response for each compound. Non-proportionality of a NaI crystal was also measured and is shown as a reference.
exception to this trend was with Cs4CaI6:Eu 9%, which had an improved response compared to 7% Eu. For Cs4CaI6:Eu 7%, the maximum deviation occurs at 59.6 keV with a relative light yield of 108.7%, whereas Cs4CaI6:Eu 9% has 105.3% relative light yield at 59.6 keV (also its maximum deviation). Scintillation and PL decay constants for Cs4SrI6:Eu are plotted in Fig. 7a. A summary of decay time values is provided in Table 1. The
Scintillation Decay
Photoluminescence Decay
Eu2+ Conc.
Primary Component
Secondary Component
Primary Component
Secondary Component
(mol %)
(μs)
(μs)
(μs)
(μs)
0 0.5 1 3 5 7 9 0 0.5 1 3 5 7 9
– 1.73 1.69 1.74 1.77 1.88 2.11 – 1.63 1.52 1.65 1.78 1.90 1.74
– 12.2 (23%) 14.0 (17%) 16.5 (12%) 12.6 (6%) 0.948 (15%) 1.47 (18%) – 4.35 (25%) 3.90 (16%) 0.491 (3%) 0.399 (2%) 0.605 (5%) 0.410 (3%)
1.00 1.12 1.27 1.34 1.54 (88%) 1.73 (81%) 1.97 (98%) 0.965 1.14 1.19 1.31 1.54 (89%) 1.62 (85%) 1.67 (90%)
– – – – 0.460 0.483 0.537 – – – – 0.507 0.469 0.463
(77%) (83%) (88%) (94%) (85%) (82%) (75%) (84%) (97%) (98%) (95%) (97%)
(12%) (19%) (2%)
(11%) (15%) (10%)
general trend in both compounds was a longer decay time with increasing Eu2+ concentration. As mentioned in the introduction, this is a common effect of Eu2+ that is attributed to an increased degree of selfabsorption at higher concentrations [13,18]. For PL decay this ranged from 1.12 μs (for 0.5% Eu) to 1.97 μs (for 9% Eu) for the primary component. PL decay profiles for Cs4SrI6:Eu were fit using a single-
Fig. 7. Primary components of scintillation and PL decay as a function of Eu2+ concentration for (a) Cs4SrI6:Eu and (b) Cs4CaI6:Eu. PL decay of the undoped crystals are included for each, however, scintillation decay could not be measured due to the undoped samples being insufficiently bright. 5
Journal of Luminescence 216 (2019) 116740
D. Rutstrom, et al.
exponential function for Eu2+ concentrations ≤3% and with a doubleexponential for concentrations from 5% to 9%. The secondary component for 5%, 7%, and 9% Eu accounted for 12%, 19%, and 2% of the total light emitted. This component was faster than the primary decay component and had values of 460 ns, 483 ns, and 537 ns for Eu2+ concentrations of 5%, 7%, and 9%, respectively. All scintillation decay profiles for Cs4SrI6:Eu were fit with a doubleexponential function. The primary component of scintillation decay in Cs4SrI6:Eu accounted for 77%–94% of the total light emitted, and the decay constants ranged from 1.69 μs to 2.11 μs. The secondary component was between 12.2 μs and 16.5 μs for Eu2+ concentrations ≤5% and did not follow a specific trend. The secondary decay component for 7% and 9% Eu was 948 ns and 1.47 μs, respectively. The cause for this drastically shorter secondary component compared to that of the lower Eu2+ concentrations is not yet understood. Fig. 7b shows scintillation and PL decay for Cs4CaI6:Eu, showing a trend similar to what was observed in Cs4SrI6:Eu. As was the case with the Sr-based compound, PL decay profiles for Cs4CaI6:Eu were fit using a single-exponential function for Eu2+ concentrations ≤3% and a double-exponential fit for concentrations from 5% to 9%. The primary PL decay component ranged from 1.14 μs (for 0.5% Eu) to 1.67 μs (for 9% Eu). The secondary component (where applicable) accounted for 10%–15% of the total light emitted. This component became slightly faster with higher Eu2+ concentration, with the following values being obtained for Eu2+ concentrations of 5%, 7%, and 9%: 507 ns, 469 ns, and 463 ns, respectively. Scintillation decay profiles for Cs4CaI6:Eu were all fit using a double-exponential function. The contribution from the primary component was between 75% and 98% (> 95% for concentrations ≥3% Eu), and the decay constants ranged from 1.52 μs to 1.90 μs. The secondary component was 4.35 μs and 3.90 μs for 0.5% and 1% Eu, respectively. A faster component between 399 ns and 605 ns accounting for 2–5% of the total light emitted was observed for concentrations ≥3% Eu. Overall, the scintillation decay time dependence on Eu2+ concentration in Cs4SrI6:Eu and Cs4CaI6:Eu is relatively weak compared to other Eu2+-doped scintillators such as SrI2:Eu, which has shown decay time increase by > 1 μs from 0.5% to 10% Eu [12].
of Energy National Nuclear Security Administration through the Nuclear Science and Security Consortium under Award Number DENA0003180. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or limited, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. One of the authors (DJR) acknowledges the partial support of the Center for Materials Processing, a Tennessee Higher Education Commission (THEC) supported Accomplished Center of Excellence. References [1] M. Nikl, A. Yoshikawa, Recent R&D trends in inorganic single-crystal scintillator materials for radiation detection, Adv. Opt. Mater. 3 (2015) 463–481. [2] G.F. Knoll, Radiation Detection and Measurement, fourth ed., Wiley, 2010. [3] P.A. Rodnyi, Physical Processes in Inorganic Scintillators, CRC Press, 1997. [4] E. Sakai, Recent measurements ON scintillator-photodetector systems, IEEE Trans. Nucl. Sci. 34 (1987) 418–422. [5] L. Stand, et al., Crystal growth and scintillation properties of Eu2+ doped Cs4CaI6 and Cs4SrI6, J. Cryst. Growth 486 (2018) 162–168. [6] Y.T. Wu, et al., Zero-dimensional Cs4EuX6 (X = Br, I) all-inorganic perovskite single crystals for gamma-ray spectroscopy, J. Mater. Chem. C 6 (2018) 6647–6655. [7] Y. Wu, et al., Crystal structure, electronic structure, optical and scintillation properties of self-activated Cs4YbI6, J. Lumin. 201 (2018) 460–465. [8] J.A. Johnson, et al., Discovery of new compounds and scintillators of the A(4)BX(6) family: crystal structure, thermal, optical, and scintillation properties, Cryst. Growth Des. 18 (2018) 5220–5230. [9] E.V.D. Van Loef, P. Dorenbos, C.W.E. Van Eijk, K. Krämer, H.U. Güdel, High-energyresolution scintillator: Ce3+ activated LaBr3, Appl. Phys. Lett. 79 (2001) 1573–1575. [10] N.J. Cherepy, et al., Strontium and barium iodide high light yield scintillators, Appl. Phys. Lett. 92 (2008) 3. [11] G. Bizarri, E.D. Bourret-Courchesne, Z.W. Yan, S.E. Derenzo, Scintillation and optical properties of BaBrI: Eu2+ and CsBa2I5: Eu2+, IEEE Trans. Nucl. Sci. 58 (2011) 3403–3410. [12] J. Glodo, E.V. van Loef, N.J. Cherepy, S.A. Payne, K.S. Shah, Concentration effects in Eu doped SrI2, IEEE Trans. Nucl. Sci. 57 (2010) 1228–1232. [13] M.S. Alekhin, J.T.M. de Haas, K.W. Kramer, P. Dorenbos, Scintillation properties of and self absorption in SrI2:Eu2+, IEEE Trans. Nucl. Sci. 58 (2011) 2519–2527. [14] L. Stand, et al., Exploring growth conditions and Eu2+ concentration effects for KSr2I5:Eu scintillator crystals, J. Cryst. Growth 439 (2016) 93–98. [15] A.C. Lindsey, M. Zhuravleva, L. Stand, Y. Wu, C.L. Melcher, Crystal growth and characterization of europium doped KCaI3, a high light yield scintillator, Opt. Mater. 48 (2015) 1–6. [16] L.A. Boatner, et al., Improved lithium iodide neutron scintillator with Eu2+ activation: the elimination of Suzuki-Phase precipitates, Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip. 854 (2017) 82–88. [17] R.D. Shannon, Revised effective IONIC-radii and systematic studies OF interatomic distances IN halides and chalcogenides, Acta Crystallogr. A 32 (1976) 751–767. [18] A.C. Lindsey, et al., Effects of increasing size and changing europium activator concentration in KCaI3 scintillator crystals, J. Cryst. Growth 449 (2016) 96–103. [19] H. Wei, A. Lindsey, Z. Zhao, M. Koschan, M. Zhuravleva, C.L. Melcher, Tackling single crystal growth challenges for mixed-elpasolite scintillators, Cryst. Growth Des. 16 (2016) 4072–4081. [20] A.C. Lindsey, The Crystal Growth of Cesium Cerium Chloride Scintillator for X-Ray and Gamma-Ray Spectroscopy Applications, Master of Science M.S. Thesis Department of Materials Science and Engineering, University of Tennessee, 2014. [21] J.T.M. de Haas, P. Dorenbos, Advances in yield calibration of scintillators, IEEE Trans. Nucl. Sci. 55 (2008) 1086–1092. [22] L.M. Bollinger, G.E. Thomas, Measurement of the time dependence of scintillation intensity by a delayed‐coincidence method, Rev. Sci. Instrum. 32 (1961) 1044–1050.
4. Conclusion A study was conducted to determine the optimal Eu2+ concentration in Ø7 mm Cs4SrI6:Eu and Cs4CaI6:Eu single crystal scintillators for gamma spectroscopy applications. The main criteria included energy resolution and scintillation light yield. The best energy resolution was achieved using 7% Eu2+ in both compounds. Cs4SrI6:Eu 7% has a 3.2% energy resolution at 662 keV and 71,000 ph/MeV light yield. Cs4CaI6:Eu 7% has an energy resolution of 3.6% at 662 keV and 69,000 ph/MeV light yield. We have shown that transparent crack-free crystals can be grown with Eu2+ concentrations as high as 9% for Ø7 mm crystals. Future work will investigate concentrations exceeding this value. Further development of these scintillators will also involve growth of larger diameter crystals (> Ø1 inch) and will experiment with compositional engineering as an attempt to improve performance. This may include an in-depth study on crystal inhomogeneity, which was outside the scope of this work. Declarations of interest None. Acknowledgements This material is based on work supported in part by the Department
6
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Europium concentration effects on the scintillation properties of Cs4SrI6:Eu and Cs4CaI6:Eu single crystals for use in gamma spectroscopy
T
Daniel Rutstroma,b,∗, Luis Standa, Merry Koschana, Charles L. Melchera,b,c, Mariya Zhuravlevaa,b a
Scintillation Materials Research Center, University of Tennessee, Knoxville, USA Department of Materials Science and Engineering, University of Tennessee, Knoxville, USA c Department of Nuclear Engineering, University of Tennessee, Knoxville, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Scintillator Alkali metal halides Rare earth elements Bridgman technique Growth from melt Single crystal
Recently discovered scintillators Cs4SrI6:Eu and Cs4CaI6:Eu have shown promising properties for gamma-ray detection in homeland security applications, with < 4% energy resolution (at 662 keV) and light yields above 50,000 ph/MeV. However, a thorough investigation of the effects of europium concentration has not yet been conducted. In this work, Eu2+ concentration was varied from 0.5 mol% to 9 mol%, and Ø7 mm single crystals were grown from the melt via the vertical Bridgman method. Scintillation performance was evaluated as a function of Eu2+ concentration to determine the optimal amount. The observed trend was improved energy resolution and higher light yield with increasing Eu2+ concentration up to 7 mol%. The best energy resolution achieved was 3.2% for Cs4SrI6:Eu 7% and 3.6% for Cs4CaI6:Eu 7%. Their respective light yields were 71,000 ph/ MeV and 69,000 ph/MeV.
1. Introduction
critical aspect of nuclear security and accurate identification of radiological isotopes of potentially harmful materials. A new family of zero-dimensional perovskites with the formula A4BX6 (A+ = Cs; B2+ = Ca, Eu, Sr, or Yb; and X– = Br or I) have recently been shown to be efficient scintillators for gamma-ray detection in nuclear security applications [5–8]. Cs4SrI6, Cs4CaI6, Cs4YbI6, Cs4EuI6, and Cs4EuBr6 have demonstrated the ability to be grown from the melt and all adopt the K4CdCl6 structure type, crystallizing in the 3 c [5–8]. The most promising trigonal crystal system with space group R‾ of these are Eu2+-activated Cs4SrI6 and Cs4CaI6, which are reported to have energy resolution as low as 3.3% (at 662 keV) and light yields of ~70,000 ph/MeV. With scintillation properties approaching those of commercially available scintillators such as LaBr3:Ce3+ and SrI2:Eu2+ (energy resolutions < 3% and light yields from 70,000 to 90,000 ph/ MeV), further development of Cs4SrI6 and Cs4CaI6 is of interest [9,10]. Work has already been done toward the development of these compounds by investigating effects of monovalent cation substitution, Cs4-x(K or Rb)xSrI6:Eu and Cs4-x(K or Rb)xCaI6:Eu for example, where it was found that energy resolution slightly improved for Cs3.5Rb0.5SrI6:Eu compared to Cs4SrI6:Eu [8]. Additional ways in which Cs4SrI6 and Cs4CaI6 may further be developed include optimizing parameters for growth of large diameter (> 1 inch) crystals, compositional engineering via mixed divalent cation compositions (ex. – Cs4Sr1-
Currently the ideal scintillator that meets all the necessary criteria for a given application does not exist, therefore, pursuing novel materials with improved detection capabilities continues to be a worthwhile endeavor. Nuclear security is one specific application that could benefit significantly from further improvements to scintillator technology. The basic principle behind the scintillation process is that incident ionizing radiation (gamma-rays, X-rays, etc.) gets converted into visible light that can be collected by a photodetector and further processed as an electrical signal [1–3]. The signal pulse amplitudes correspond to the energy deposited by the incident photon, which can then be utilized in the form of a pulse height spectrum (or energy spectrum) to accurately identify different radioisotopes. The degree of accuracy depends on a variety of factors, but it is ultimately limited by the energy resolution (ER) of the scintillator. Energy resolution is essentially a measure of the detector's ability to discriminate between similar energies of radiation. Poor resolution results in multiple adjacent photopeaks in the energy spectrum appearing as one broad peak, as is the case with a detector such as NaI (~7% energy resolution at 662 keV) [4]. On the other hand, a detector with good resolution will have narrow energy peaks that are possible to distinguish even when in close proximity to one another. This ability to resolve complex features of the energy spectrum is a
∗
Corresponding author. Scintillation Materials Research Center, University of Tennessee, Knoxville, USA. E-mail addresses: [email protected] (D. Rutstrom), [email protected] (L. Stand), [email protected] (M. Koschan), [email protected] (C.L. Melcher), [email protected] (M. Zhuravleva). https://doi.org/10.1016/j.jlumin.2019.116740 Received 28 May 2019; Received in revised form 26 August 2019; Accepted 10 September 2019 Available online 11 September 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
Journal of Luminescence 216 (2019) 116740
D. Rutstrom, et al. xCaxI6:Eu), searching for alternative activators that may be better suited to each matrix, or optimizing the Eu2+ activator concentration. This work focuses solely on the latter – studying the effects of Eu2+ concentration on scintillation properties such as energy resolution, light yield, and non-proportionality. Some materials can intrinsically scintillate, however, many of the widely used scintillators require a luminescent activator in the form of a dopant. The activator provides the recombination centers for electronhole pairs created from the incident radiation, and this recombination is what leads to the emission of a photon of visible light [2,3]. In the past 15 years, Eu2+ has become one of the leading candidate activators for high performance scintillators due to its efficient 5d → 4f electronic transition and emission wavelengths that fall within the spectral sensitivity range of a standard bialkali photomultiplier tube (PMT) [1,2]. Some recent developments toward Eu2+-doped scintillators include SrI2:Eu, BaBrI:Eu, CsBa2I5:Eu, KSr2I5:Eu, KCaI3:Eu, and LiI:Eu [10–16]. In addition to being a known efficient activator, Eu2+ was chosen for the initial development of Cs4SrI6 and Cs4CaI6 due to its compatible valence state with Sr2+ and Ca2+, as well as its similar ionic radius to Sr2+ (rEu = 1.17 Å and rSr = 1.18 Å, for a coordination number of 6) [17]. The influence of Eu2+ concentration on scintillation properties has been investigated before in other host matrices, such as SrI2:Eu, KSr2I5:Eu, and KCaI3:Eu [12–14,18]. Dopant concentration optimization was also carried out in Ref. [8], but this only included Cs3KCaI6:Eu rather than Cs4SrI6:Eu and Cs4CaI6:Eu, which are addressed in this paper. A typical consequence of increasing Eu2+ concentration is an increased decay time. This is in part due to the increased probability of self-absorption (overlap of excitation and emission spectra) that occurs with higher Eu2+ concentration [13]. The primary decay component for Eu2+ is generally in the 1 μs–3 μs range, but depending on concentration can be within ~600 ns–1000 ns [12–14,18]. Starting from low concentrations of about 0.5% Eu2+, light yield often increases and energy resolution improves as the concentration is increased; however, at a certain point self-absorption effects overtake the benefits of adding more Eu2+ activator sites, causing light yield and energy resolution to degrade. Similar effects also occur when increasing the size of a scintillator crystal with a fixed Eu2+ concentration and are also attributed to self-absorption. This work only addresses the concentration effects in Ø7 mm crystals.
Fig. 1. (a) Three crystals being grown simultaneously in a semi-transparent furnace and (b) an example of the quartz boat in which the crystals were contained during growth.
quartz ampoules using the vertical Bridgman technique. A detailed schematic of this growth method can be seen in Chapter 2 of ref. [20]. The ampoules all contained a self-seeding capillary region to initiate nucleation and act as a grain selector. Crystals were translated at a rate of 0.8 mm/h and cooled at 7 °C/hr. The majority were grown using a semi-transparent gold-coated Bridgman furnace with two heating zones. The benefit of the transparent furnace was the ability to monitor the growth in real time and adjust growth parameters (pull rate, temperature settings, etc.) as necessary. Crystals grown in this furnace were done in sets of three, with the exception of Cs4SrI6:Eu 9% and Cs4CaI6:Eu 9%. This was accomplished by use of a quartz boat, in which three separate ampoules were placed at once. This can be seen in Fig. 1. During growth the entire boat was translated through the furnace, and the result was simultaneous growth of three crystals in one growth run (usually the same host compound with varying Eu2+ concentrations). The remaining crystals were grown using a standard three-zone Bridgman furnace (zero transparency). All furnaces were equipped with a 1″ to 2” thick insulating diaphragm to enhance the thermal gradient. Thermal profiles of the furnaces were measured prior to growth. Zone temperatures were adjusted until the temperature near the top of the diaphragm was at the compounds’ melting points and thermal gradients of at least 25 °C/cm were achieved. Temperatures of the hot zones ranged from 640 °C to 675 °C and for the cold zone ranged from 380 °C to 400 °C. The melting temperatures for Cs4SrI6 and Cs4CaI6 are 533 °C and 555 °C, respectively [5].
2. Experimental 2.1. Synthesis and crystal growth
2.2. Characterization Anhydrous beads of CsI, SrI2, CaI2, and EuI2 all with > 99.99% purity were used as starting raw materials. To avoid degradation due to their extreme sensitivity to air and moisture, all raw materials were handled inside of a nitrogen-filled glovebox with < 0.1 ppm H2O and O2. The mass of each individual component required to obtain the target compounds was calculated under the assumption that the dopant substitutes for the divalent cation (Sr2+ or Ca2+) in both Cs4SrI6 and Cs4CaI6; for example, Cs4Sr1-xEuxI6 and Cs4Ca1-xEuxI6. The samples that were evaluated contained nominal molar concentrations of 0.5%, 1%, 3%, 5%, 7%, and 9% europium (or 0.005 < x < 0.09). Using the calculated stoichiometric ratios, raw materials were mechanically mixed and loaded into quartz ampoules. The ampoules were then evacuated to a pressure between 10−6 and 10−7 Torr, heated at 250 °C for up to 12 h, cooled to room temperature, and sealed under vacuum with an H2–O2 torch. Materials were melt-synthesized prior to crystal growth by heating above the melting temperatures of the individual components for > 12 h and cooling to room temperature over several hours. This was carried out using the M.A.D. (multiple ampoule directions) mixing technique to ensure thorough mixing and incorporation of the luminescent activator into the host matrix [19]. Crystals were grown from the melt in Ø7 mm (inner diameter)
Smaller crystals, < 5 × 5 × 5 mm3, were cut from the as-grown boules and polished for characterization. Due to the hygroscopic nature of these crystals, luminescence and scintillation measurements were conducted with the samples submerged in mineral oil to prevent degradation. Steady-state photoluminescence (PL) spectra were measured in a reflection geometry using a Horiba Jobin Yvon Fluorolog 3 Spectrofluorometer equipped with a Xe lamp and dual scanning monochromators. Emission spectra were collected using an excitation wavelength of 280 nm, and excitation spectra were collected monitoring emission at 460 nm and 462 nm depending on the host matrix. The PL decay time was measured using a Horiba Jobin Yvon NanoLed with an excitation wavelength of 370 nm, emission monitored at 460–462 nm, and a pulse width of 1 ns. Radioluminescence (RL) was measured under excitation by a Cu target X-ray tube operated at 35 kV and 0.1 mA using a transmission geometry. The resulting emission spectra were collected from 200 nm to 800 nm with a 150 mm focal length monochromator. Energy resolution measurements were conducted using crystals with sizes of approximately 3 × 3 × 3 mm3. The crystals were placed into a quartz housing filled with mineral oil, which was then optically coupled 2
Journal of Luminescence 216 (2019) 116740
D. Rutstrom, et al.
to a Hamamatsu R6231 PMT. The housing and PMT were covered with reflective Teflon sheets and a Spectralon solid diffuse reflector dome to maximize light collection. Pulse height spectra were collected with the PMT operated at 1 kV using a signal processing chain consisting of a Canberra 2005 preamplifier, an Ortec 672 amplifier set to 10 μs shaping time, and a Tukan 8 K multi-channel analyzer. The 662 keV photopeak was fit with a Gaussian function, and percent energy resolution was calculated by dividing the full width at half maximum (FWHM) by centroid channel number. For instances where energy resolution was good enough to resolve the iodine escape peak (~28 keV below the 662 keV energy peak), two Gaussian functions were fit. A137Cs gamma source was used as the excitation source. Light yield was measured using a similar setup as with energy resolution, with the primary difference being the use of a Hamamatsu R2059 PMT operated at 1.5 kV instead of the R6231. Absolute light yield was calculated in terms of number of photons per MeV using the method described by Dorenbos et al., which accounts for the position of the single photoelectron (SPE) centroid and the quantum efficiency of the PMT [21]. Typical error in light yield due to slight sample-tosample variation (different samples of the same composition) is around ± 2000 ph/MeV to ± 5000 ph/MeV. In the current study, error was not significant enough to change the overall light yield trends. Non-proportionality was measured using the same PMT, voltage, and signal processing chain as with energy resolution. Excitation sources and energies of interest included 137Cs (32 keV and 662 keV), 22 Na (511 keV), 133Ba (356 keV), 57Co (122 keV), and 241Am (59.6 keV). Light yield at each energy was then normalized to the light yield at 662 keV. The method described in Ref. [22] was used to measure scintillation decay time. 137Cs was used as the excitation source.
Fig. 2. (a) Simultaneously-grown crystals of Cs4SrI6:Eu with 7%, 3%, and 1% Eu (top to bottom) under room lighting and (b) excited by UV light. (c) Cs4SrI6 and Cs4CaI6 crystals doped with 9% Eu before removal and (d) after removal from ampoules and polishing.
Cs4SrI6:Eu and Cs4CaI6:Eu is centered around 460 nm–462 nm. For RL, peak emission wavelengths occur around 466 nm. These emissions are characteristic of the 5d → 4f electronic transition of Eu2+. The slight shift to a higher wavelength with RL compared to PL is a result of the measurement geometries. The transmission geometry, which was used for RL measurements, leads to an increased probability for self-absorption thus shifting the emission to higher wavelengths [18]. A low intensity RL emission band from about 500 nm to 600 nm was also observed in the low Eu2+ concentration samples. It is most apparent for Cs4CaI6:Eu 0.5%, seen in Fig. 3b. This has been attributed to the intrinsic self-trapped exciton (STE) emission of undoped Cs4SrI6 and Cs4CaI6 [8]. This band becomes suppressed at higher Eu2+ concentrations as the 5d → 4f transition begins to dominate. The position of PL excitation and emission bands also does not vary significantly with different amounts of Eu2+. However, the relative intensities of the excitation bands do show a slight dependence on Eu2+ concentration. For low concentrations of Eu2+, such as 0.5 mol%, there is a dominant excitation band near 260 nm with a drop in intensity from 300 nm to 330 nm followed by additional lower intensity bands from 330 nm to 445 nm. At higher concentrations, the excitation spectra are more continuous across all wavelengths from 260 nm to 445 nm. This effect was also observed in Refs. [8,18] for 0.5% Eu2+-doped Cs4SrI6 and Cs4CaI6 and KCaI3:Eu 0.2%, respectively. Additionally, a larger degree of self-absorption (overlapping of excitation and emission spectra) can be observed as Eu2+ concentration increases (Fig. 3). This is a well-known effect of Eu2+ that is intensified with either increasing concentration or increasing crystal size [12,13,18].
3. Results and discussion 3.1. Crystal growth Growth of Ø7 mm Cs4SrI6:Eu and Cs4CaI6:Eu resulted in transparent crack-free crystals in most cases. A few of the Ca-containing compounds had one or two minor cracks, however, this was only observed for the crystals that were grown simultaneously. In contrast, the simultaneously grown Sr-containing crystals did not show any cracking. Fig. 2 illustrates the good crystal quality obtained for the Sr-containing compound with Eu2+ concentrations ranging from 1% to 7%. Due to the slight mismatch in ionic radii between Ca2+ and Eu2+ (1.00 Å and 1.17 Å, respectively, for a coordination number of 6) there was initial uncertainty as to whether high concentrations of Eu2+ could be incorporated into the Cs4CaI6 matrix [17]. Despite this concern, excellent quality crystals of both Cs4CaI6:Eu 9% and Cs4SrI6:Eu 9% were grown. This is shown in Fig. 2d. The lack of a significant last-tofreeze region in either crystal suggests successful incorporation of the dopant. Additionally, powder X-ray diffraction data for these samples was consistent with what was observed in Ref. [5] and confirms the same phase was obtained. The fact that Cs4SrI6, Cs4CaI6, and Cs4EuI6 all share the same crystal structure and space group (trigonal, space group R-3c) likely contributes to why both compounds are capable of incorporating such high amounts of Eu2+ [5,6]. 3.2. Characterization
3.2.2. Scintillation performance and decay time Examples of the fits used to calculate energy resolution are shown in Fig. 4 for Cs4SrI6:Eu and Cs4CaI6:Eu doped with 0.5%, 3%, and 7% Eu2+. As discussed in Section 2.2, peaks were fit with two Gaussian functions when energy resolution was sufficient for resolving the iodine escape peak. Fig. 5 shows light yield and energy resolution of Cs4SrI6:Eu and Cs4CaI6:Eu as a function of dopant concentration from 0.5% to 9% Eu2+. The best energy resolution was obtained with Cs4SrI6:Eu 7%, achieving 3.2% at 662 keV. It can be seen that the energy resolution and light yield of Cs4SrI6:Eu and Cs4CaI6:Eu improve as Eu2+ concentration
3.2.1. Photoluminescence and radioluminescence The luminescent properties of Eu2+-doped Cs4SrI6:Eu and Cs4CaI6:Eu have previously been reported in Refs. [5,8]. Fig. 3 shows RL emission and PL emission/excitation spectra dependence on concentration for 0.5%, 3%, and 7% Eu2+. The additional compounds grown (1%, 5%, and 9%) had similar features in their excitation and emission spectra. Emission wavelength is weakly dependent on Eu2+ concentration, with a negligible < 2 nm shift toward longer wavelengths occurring as concentration is increased. PL emission of both 3
Journal of Luminescence 216 (2019) 116740
D. Rutstrom, et al.
Fig. 3. Radioluminescence (RL) and photoluminescence (PL) emission/excitation of (a) Cs4SrI6:Eu and (b) Cs4CaI6:Eu with 0.5%, 3%, and 7% Eu2+. The vertical line at ~460 nm is included to allow for an easier visual comparison between peak centroid positions.
energy resolution to 3.9% (~41% total improvement). The poor resolution at low Eu2+ concentrations for Cs4SrI6:Eu may be a result of its extreme non-linearity combined with a low light yield, whereas Cs4CaI6:Eu has a relatively flat response at low Eu2+ concentrations (Fig. 6). The scintillation response as a function of energy, or non-proportionality, for Cs4SrI6:Eu and Cs4CaI6:Eu is shown in Fig. 6. Only two different Eu2+ concentrations are plotted for each compound for ease of interpretation. The least proportional response for the Cs4SrI6:Eu occurs with 0.5% Eu, with a maximum deviation from ideal at 32 keV (90.8% relative light yield normalized to 662 keV). The trend observed with this compound was improved proportionality as Eu2+ concentration was increased up to 5%, which deviates by < 1% at 32 keV (100.9% relative light yield) and has maximum deviation at 122 keV (102% relative light yield). The relative light yield at low energies continues to rise with increasing Eu2+ concentration beyond 5%, approaching 104.2% at 32 keV, 104.5% at 59.5 keV, and 103.4% at 122 keV for Cs4SrI6:Eu 9%. Contrast to the Sr-based compound, the most proportional response for Cs4CaI6:Eu was obtained with 0.5% Eu2+, which has a maximum deviation at 122 keV (102.2% relative light yield). The trend in Cs4CaI6:Eu was similar to Cs4SrI6:Eu, where relative light yields at lower energies increased with increasing Eu2+ concentration. The only
is increased from 0.5% to 7% in both compounds. Raising the concentration from 7% to 9% in Cs4SrI6:Eu results in an improved light yield (71,000 ph/MeV to 78,000 ph/MeV), however, energy resolution remains at 3.2% for both. The benefits from this light yield increase are relatively marginal when there is not also an improvement in energy resolution, therefore it is still reasonable to consider 7% Eu the optimal amount for use in nuclear security applications, in which energy resolution is the more performance-limiting property. In contrast, energy resolution of Cs4CaI6:Eu worsens from 3.6% to 3.9% despite a minimal change in light yield from 69,000 ph/MeV to 68,000 ph/MeV when increasing Eu2+ concentration from 7% to 9%. This nearly equivalent light yield for the 9% Eu2+-doped sample is also accompanied by improved non-proportionality (discussed below) compared to 7% Eu2+ suggesting that this degradation of energy resolution could be related to crystal inhomogeneity or an increased degree of self-absorption. Cs4CaI6:Eu has superior energy resolution over Cs4SrI6:Eu at low Eu2+ concentrations, an effect that was also observed in Ref. [8]. This is apparent from Fig. 5b. Increasing from 0.5% to 3% Eu2+ in the Cacontaining matrix lead to an improvement from 4.8% to 4.3% (~10.4% improvement) energy resolution at 662 keV. A more significant change occurred when increasing from 0.5% to 3% Eu2+ in the Sr-containing matrix. Whereas 0.5% and 1% Eu2+ in Cs4SrI6:Eu have energy resolutions > 6%, increasing the concentration to 3% Eu2+ improves the
Fig. 4. Pulse height spectra of 0.5%, 3%, and 7% Eu2+-doped (a) Cs4SrI6 and (b) Cs4CaI6 measured with a137Cs source. Sample dimensions are approximately 3 × 3 × 3 mm3. 4
Journal of Luminescence 216 (2019) 116740
D. Rutstrom, et al.
Fig. 5. (a) Absolute light yield and (b) energy resolution at 662 keV as a function of Eu2+ concentration for approximately 3 × 3 × 3 mm3 crystals.
Table 1 Summary of scintillation and photoluminescence decay times for Cs4SrI6:Eu and Cs4CaI6:Eu with different Eu2+ concentrations.
Cs4SrI6
Cs4CaI6
Fig. 6. Non-proportionality of Cs4SrI6:Eu and Cs4CaI6:Eu showing Eu2+ concentrations with the best and worst response for each compound. Non-proportionality of a NaI crystal was also measured and is shown as a reference.
exception to this trend was with Cs4CaI6:Eu 9%, which had an improved response compared to 7% Eu. For Cs4CaI6:Eu 7%, the maximum deviation occurs at 59.6 keV with a relative light yield of 108.7%, whereas Cs4CaI6:Eu 9% has 105.3% relative light yield at 59.6 keV (also its maximum deviation). Scintillation and PL decay constants for Cs4SrI6:Eu are plotted in Fig. 7a. A summary of decay time values is provided in Table 1. The
Scintillation Decay
Photoluminescence Decay
Eu2+ Conc.
Primary Component
Secondary Component
Primary Component
Secondary Component
(mol %)
(μs)
(μs)
(μs)
(μs)
0 0.5 1 3 5 7 9 0 0.5 1 3 5 7 9
– 1.73 1.69 1.74 1.77 1.88 2.11 – 1.63 1.52 1.65 1.78 1.90 1.74
– 12.2 (23%) 14.0 (17%) 16.5 (12%) 12.6 (6%) 0.948 (15%) 1.47 (18%) – 4.35 (25%) 3.90 (16%) 0.491 (3%) 0.399 (2%) 0.605 (5%) 0.410 (3%)
1.00 1.12 1.27 1.34 1.54 (88%) 1.73 (81%) 1.97 (98%) 0.965 1.14 1.19 1.31 1.54 (89%) 1.62 (85%) 1.67 (90%)
– – – – 0.460 0.483 0.537 – – – – 0.507 0.469 0.463
(77%) (83%) (88%) (94%) (85%) (82%) (75%) (84%) (97%) (98%) (95%) (97%)
(12%) (19%) (2%)
(11%) (15%) (10%)
general trend in both compounds was a longer decay time with increasing Eu2+ concentration. As mentioned in the introduction, this is a common effect of Eu2+ that is attributed to an increased degree of selfabsorption at higher concentrations [13,18]. For PL decay this ranged from 1.12 μs (for 0.5% Eu) to 1.97 μs (for 9% Eu) for the primary component. PL decay profiles for Cs4SrI6:Eu were fit using a single-
Fig. 7. Primary components of scintillation and PL decay as a function of Eu2+ concentration for (a) Cs4SrI6:Eu and (b) Cs4CaI6:Eu. PL decay of the undoped crystals are included for each, however, scintillation decay could not be measured due to the undoped samples being insufficiently bright. 5
Journal of Luminescence 216 (2019) 116740
D. Rutstrom, et al.
exponential function for Eu2+ concentrations ≤3% and with a doubleexponential for concentrations from 5% to 9%. The secondary component for 5%, 7%, and 9% Eu accounted for 12%, 19%, and 2% of the total light emitted. This component was faster than the primary decay component and had values of 460 ns, 483 ns, and 537 ns for Eu2+ concentrations of 5%, 7%, and 9%, respectively. All scintillation decay profiles for Cs4SrI6:Eu were fit with a doubleexponential function. The primary component of scintillation decay in Cs4SrI6:Eu accounted for 77%–94% of the total light emitted, and the decay constants ranged from 1.69 μs to 2.11 μs. The secondary component was between 12.2 μs and 16.5 μs for Eu2+ concentrations ≤5% and did not follow a specific trend. The secondary decay component for 7% and 9% Eu was 948 ns and 1.47 μs, respectively. The cause for this drastically shorter secondary component compared to that of the lower Eu2+ concentrations is not yet understood. Fig. 7b shows scintillation and PL decay for Cs4CaI6:Eu, showing a trend similar to what was observed in Cs4SrI6:Eu. As was the case with the Sr-based compound, PL decay profiles for Cs4CaI6:Eu were fit using a single-exponential function for Eu2+ concentrations ≤3% and a double-exponential fit for concentrations from 5% to 9%. The primary PL decay component ranged from 1.14 μs (for 0.5% Eu) to 1.67 μs (for 9% Eu). The secondary component (where applicable) accounted for 10%–15% of the total light emitted. This component became slightly faster with higher Eu2+ concentration, with the following values being obtained for Eu2+ concentrations of 5%, 7%, and 9%: 507 ns, 469 ns, and 463 ns, respectively. Scintillation decay profiles for Cs4CaI6:Eu were all fit using a double-exponential function. The contribution from the primary component was between 75% and 98% (> 95% for concentrations ≥3% Eu), and the decay constants ranged from 1.52 μs to 1.90 μs. The secondary component was 4.35 μs and 3.90 μs for 0.5% and 1% Eu, respectively. A faster component between 399 ns and 605 ns accounting for 2–5% of the total light emitted was observed for concentrations ≥3% Eu. Overall, the scintillation decay time dependence on Eu2+ concentration in Cs4SrI6:Eu and Cs4CaI6:Eu is relatively weak compared to other Eu2+-doped scintillators such as SrI2:Eu, which has shown decay time increase by > 1 μs from 0.5% to 10% Eu [12].
of Energy National Nuclear Security Administration through the Nuclear Science and Security Consortium under Award Number DENA0003180. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or limited, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. One of the authors (DJR) acknowledges the partial support of the Center for Materials Processing, a Tennessee Higher Education Commission (THEC) supported Accomplished Center of Excellence. References [1] M. Nikl, A. Yoshikawa, Recent R&D trends in inorganic single-crystal scintillator materials for radiation detection, Adv. Opt. Mater. 3 (2015) 463–481. [2] G.F. Knoll, Radiation Detection and Measurement, fourth ed., Wiley, 2010. [3] P.A. Rodnyi, Physical Processes in Inorganic Scintillators, CRC Press, 1997. [4] E. Sakai, Recent measurements ON scintillator-photodetector systems, IEEE Trans. Nucl. Sci. 34 (1987) 418–422. [5] L. Stand, et al., Crystal growth and scintillation properties of Eu2+ doped Cs4CaI6 and Cs4SrI6, J. Cryst. Growth 486 (2018) 162–168. [6] Y.T. Wu, et al., Zero-dimensional Cs4EuX6 (X = Br, I) all-inorganic perovskite single crystals for gamma-ray spectroscopy, J. Mater. Chem. C 6 (2018) 6647–6655. [7] Y. Wu, et al., Crystal structure, electronic structure, optical and scintillation properties of self-activated Cs4YbI6, J. Lumin. 201 (2018) 460–465. [8] J.A. Johnson, et al., Discovery of new compounds and scintillators of the A(4)BX(6) family: crystal structure, thermal, optical, and scintillation properties, Cryst. Growth Des. 18 (2018) 5220–5230. [9] E.V.D. Van Loef, P. Dorenbos, C.W.E. Van Eijk, K. Krämer, H.U. Güdel, High-energyresolution scintillator: Ce3+ activated LaBr3, Appl. Phys. Lett. 79 (2001) 1573–1575. [10] N.J. Cherepy, et al., Strontium and barium iodide high light yield scintillators, Appl. Phys. Lett. 92 (2008) 3. [11] G. Bizarri, E.D. Bourret-Courchesne, Z.W. Yan, S.E. Derenzo, Scintillation and optical properties of BaBrI: Eu2+ and CsBa2I5: Eu2+, IEEE Trans. Nucl. Sci. 58 (2011) 3403–3410. [12] J. Glodo, E.V. van Loef, N.J. Cherepy, S.A. Payne, K.S. Shah, Concentration effects in Eu doped SrI2, IEEE Trans. Nucl. Sci. 57 (2010) 1228–1232. [13] M.S. Alekhin, J.T.M. de Haas, K.W. Kramer, P. Dorenbos, Scintillation properties of and self absorption in SrI2:Eu2+, IEEE Trans. Nucl. Sci. 58 (2011) 2519–2527. [14] L. Stand, et al., Exploring growth conditions and Eu2+ concentration effects for KSr2I5:Eu scintillator crystals, J. Cryst. Growth 439 (2016) 93–98. [15] A.C. Lindsey, M. Zhuravleva, L. Stand, Y. Wu, C.L. Melcher, Crystal growth and characterization of europium doped KCaI3, a high light yield scintillator, Opt. Mater. 48 (2015) 1–6. [16] L.A. Boatner, et al., Improved lithium iodide neutron scintillator with Eu2+ activation: the elimination of Suzuki-Phase precipitates, Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip. 854 (2017) 82–88. [17] R.D. Shannon, Revised effective IONIC-radii and systematic studies OF interatomic distances IN halides and chalcogenides, Acta Crystallogr. A 32 (1976) 751–767. [18] A.C. Lindsey, et al., Effects of increasing size and changing europium activator concentration in KCaI3 scintillator crystals, J. Cryst. Growth 449 (2016) 96–103. [19] H. Wei, A. Lindsey, Z. Zhao, M. Koschan, M. Zhuravleva, C.L. Melcher, Tackling single crystal growth challenges for mixed-elpasolite scintillators, Cryst. Growth Des. 16 (2016) 4072–4081. [20] A.C. Lindsey, The Crystal Growth of Cesium Cerium Chloride Scintillator for X-Ray and Gamma-Ray Spectroscopy Applications, Master of Science M.S. Thesis Department of Materials Science and Engineering, University of Tennessee, 2014. [21] J.T.M. de Haas, P. Dorenbos, Advances in yield calibration of scintillators, IEEE Trans. Nucl. Sci. 55 (2008) 1086–1092. [22] L.M. Bollinger, G.E. Thomas, Measurement of the time dependence of scintillation intensity by a delayed‐coincidence method, Rev. Sci. Instrum. 32 (1961) 1044–1050.
4. Conclusion A study was conducted to determine the optimal Eu2+ concentration in Ø7 mm Cs4SrI6:Eu and Cs4CaI6:Eu single crystal scintillators for gamma spectroscopy applications. The main criteria included energy resolution and scintillation light yield. The best energy resolution was achieved using 7% Eu2+ in both compounds. Cs4SrI6:Eu 7% has a 3.2% energy resolution at 662 keV and 71,000 ph/MeV light yield. Cs4CaI6:Eu 7% has an energy resolution of 3.6% at 662 keV and 69,000 ph/MeV light yield. We have shown that transparent crack-free crystals can be grown with Eu2+ concentrations as high as 9% for Ø7 mm crystals. Future work will investigate concentrations exceeding this value. Further development of these scintillators will also involve growth of larger diameter crystals (> Ø1 inch) and will experiment with compositional engineering as an attempt to improve performance. This may include an in-depth study on crystal inhomogeneity, which was outside the scope of this work. Declarations of interest None. Acknowledgements This material is based on work supported in part by the Department
6