Large-Diameter Bulk Crystal Growth and Scintillation Characterization of Thallium-Based Ternary Halide Crystals for Detection and Imaging

Abstract

Scintillators are important for a wide range of applications in detection and imaging. In this paper, the growth and performance results of advanced large-diameter thallium-based ternary halide crystals are presented. Intrinsic crystals of TlMgCl₃, TlCaCl₃, and other small-diameter TlCaBr₃, and TlCa(Cl,Br)₃, as well as europium-doped TlCa₂Br₅, TlCa(Cl,Br)₃, and TlSr₂I₅ are melt-grown by the Bridgman method. These compounds have a high effective atomic number (Z_eff) and physical densities due to thallium. The best crystal quality and energy resolution (FWHM) at 662 keV are observed for TlMgCl₃, TlCaCl₃, and TlSr₂I₅:Eu at 3.8%, 4.6%, and 3.5%, respectively. The primary decay constants for these compounds are in the range of 0.45 to 0.63 μs. These ternary Tl-halide compounds have proportional or linear response (±0.05%) to γ-rays above 40 keV.

1. Introduction

Advancement in high-energy physics depends largely on the advancement in materials research. The need for new low-cost, bright, high-performing bulk scintillators for high-energy particle detectors requires more research in scintillation materials with properties suitable for the application [1]. Some of the most studied radiation detector materials currently available in the market are inorganic binary compound scintillators [2]. Ternary compounds that are based on these binary compounds are usually formed by adding metal cations. Initial examples of these ternary compounds are listed in an online table [2], and many more ternary compounds have been published in recent years. These recent advances in materials study for radiation sensors, particularly the latest discovery of high-density thallium-based inorganic scintillation crystals, are spurred by a necessity to improve isotope identification capability, for example, for homeland security applications. These new materials also possess many of the required properties for detection materials in high-energy physics applications and research, as well as other fields such as medical imaging and oil well logging. The continuous search for improved scintillation materials for better radiation detection is important, since an ideal scintillator for such applications has yet to be discovered.

Recently, high-detection-efficiency Tl-based scintillation crystals have attracted a good amount of attention from worldwide scintillator researchers. These compounds, for example Ce-doped Tl₂LaCl₅ (TLC) [3,4,5,6], as well as intrinsic (i.e., not doped) TlMgCl₃ (TMC) and TlCaI₃ (TCI) [7,8], have been investigated and very promising initial results have been published. These new compounds are of high densities (>5 g/cm³), are bright (light yields between 31,000 and 76,000 ph/MeV for 662 keV photons), have fast decay times (36 ns (89%) for TLC; 46 ns (9%) for TMC; 62 ns (13%) for TCI), and have moderate melting points (between 500 and 700 °C). Other compounds that have been investigated in the past ten years include Tl₂HfCl₆ [9,10,11], Tl₂ZrCl₆ [9,11,12], Tl₂LaBr₅:Ce [5,13], and Tl₂CeCl₅ [14]. As seen in these published results, Tl-based scintillators, such as the ones previously mentioned, have promising properties desirable for high-energy physics as well as homeland security applications [3,4,5,6,7,8,9,10,11,12,13,14,15,16].

Following the results of studied compounds, Xtallized Intelligence, Inc. (XI, Inc.) has grown several large-diameter (≥14 mm) single bulk crystals of intrinsic and doped thallium-based ternary compound scintillators for high energy physics and other applications.

Table 1. List of ternary Tl-halide compounds grown and characterized.

Compound
TlMgCl3	∅16 mm	∅1-inch
TlCaCl3	∅16 mm	∅1-inch
TlCaBr3	∅14 mm
TlCa2Br5	∅16 mm
TlCa2Br5:Eu	∅16 mm
TlCa(Cl,Br)3	∅16 mm
TlCa(Cl,Br)3:Eu	∅16 mm
TlSr2I5:Eu	∅16 mm	∅1-inch

shows a list of compounds grown and characterized by XI, Inc. For high-energy physics purposes, physical dimensions of a detection material should accommodate at least 20 times the radiation length of the material. Therefore, to obtain amenable crystals, the large-diameter growth of crack-free, single crystals of high quality and performance must be accomplished. In the following sections, the growth and crystal analysis of intrinsic crystals of TlMgCl₃, TlCaCl₃, TlCaBr₃, and TlCa(Cl,Br)₃, as well as europium-doped TlCa₂Br₅, TlCa(Cl,Br)₃, and TlSr₂I₅ are presented.

2. Materials and Methods

Each of the Tl-based ternary compounds was grown by stoichiometric mixing of the respective binary compounds and growing them by the vertical Bridgman technique [17]. Table 1 shows the ternary compounds that have been grown by XI, Inc. (Chapmansboro, TN, USA). Starting materials of the highest available purity (up to 99.995%) were purchased from US-based chemical vendors APL Engineered Materials, Millipore Sigma, and Thermo Fisher Scientific. For each targeted ternary compound, stoichiometric amounts of the binary compounds and dopants were purified, if necessary, and loaded into a freshly cleaned and baked quartz ampoule sized 16 mm and 1 inch in inner diameter. Material loading was conducted inside an inert glove box with an argon atmosphere. Using a turbo vacuum pump, the loaded ampoule was dehydrated, if necessary, and then sealed in a high vacuum. The ampoule was placed in a two-zone vertical furnace for growth. The top zone of the furnace was set a few degrees above the melting temperature of the compound to ensure a complete melt. The bottom zone was set a few degrees below the crystallization temperature for the compound. The growth was conducted at a rate of 25–35 mm/day. After the crystallization was completed, the ampoule was cooled down 75–150 °C/day to room temperature. The crystal was then retrieved from the growth ampoule and samples were extracted, lapped, and polished for analysis.

For sample characterization, each polished sample was hermetically encapsulated or packaged. The packaged crystal was then coupled to a Hamamatsu R6231-100 photomultiplier tube (PMT) with BC-630 silicone optical coupling compound. The PMT was biased with a Hamamatsu C9525 High Voltage Power Supply and the PMT anode signals were processed through a chain of equipment (a Canberra 2005 preamplifier, a Canberra 2022 spectroscopy amplifier, and an Amptek MCA8000D multichannel analyzer (MCA)) for gamma-ray spectroscopy.

After background subtraction, the 662 keV full energy peak from ¹³⁷Cs was analyzed to determine the energy resolution of each scintillator and relative light yield with respect to NaI:Tl. Similarly, after background subtraction, full energy peak and x-ray peak positions and full-width half maxima (FWHMs) from other gamma ray sources, such as ²⁴¹Am (60 keV), ¹³³Ba (31 keV, 53 keV, 81 keV, 276 keV, 303 keV, 356 keV, and 384 keV), ²²Na (511 keV and 1275 keV), ⁶⁰Co (1173 keV and 1333 keV), and ¹⁵²Eu (40 keV, 122 keV, 245 keV, 344 keV, 779 keV, 964 keV, 1112 keV, and 1408 keV) were also used to determine relative light yield (i.e., non-proportionality) and energy resolution data, respectively. Energy uncertainties were obtained from the 1999 edition of Table of Isotopes [18] and were treated as one standard deviation [19]. Peak position uncertainties were calculated from their respective FWHMs. Error bars on non-proportionality data were calculated by using the division error propagation formula on energy and peak position uncertainties.

Decay time was determined by recording the PMT anode signals using a CAEN DT5720 digitizer. The signals were analyzed offline by first normalizing each signal pulse then adding the normalized pulses to get an average decay curve or temporal data. The averaged data were fitted with an exponential decay function expressed in Equation (1) as follows:

$$y=y_0+{\sum }_{i=1}^N{A}_i{e}^{\left(-{\displaystyle \frac{x-x_0}{t_i}}\right)},$$

(1)

where x₀ and y₀ are the x-axis and y-axis offsets, t_i is the ith time constant, and $A_i={\displaystyle \frac{f_i}{t_i}}$, where f_i is the fraction of t_i. The ExpDecayn (n = 1, 2, or 3) fitting procedure in Origin^® 2019 software was used for this analysis. The associated standard errors (i.e., standard deviations [20]) on the decay constants from this analysis were noted as uncertainties.

X-ray excited optical luminescence spectra were collected from polished samples of TlMgCl₃, TlCaBr₃, and TlCa₂Br₅:Eu. The spectrum collection was carried out using an x-ray tube source that provided <30 keV x-rays and the emission was collected by a fiberoptic-coupled Ocean Optics spectrometer, employing a silicon CCD readout.

3. Results

To start this material investigation and in order to keep both research cost and material waste low, small-diameter (14-to-16 mm) crystal growth runs were carried out initially. After measurements of optical and physical properties, the most promising compounds that were also feasible for larger-diameter crystal growth were grown in 1-inch-diameter sizes.

3.1. 16 mm Diameter Crystal Growth

3.1.1. Intrinsic TlMgCl₃

The investigation into the growth of TlMgCl₃ was started by growing ∅16 mm crystal boules, then followed by scaling up to several ∅1″ boules. From the ∅16 mm boule, similarly sized (12 × 15 × 15 mm³) single, transparent, crack-free crystal samples of TlMgCl₃ were successfully obtained from three different areas in the crystal boule, which were the tip (first to freeze), mid (dle), and tail (last to freeze), respectively, to check the crystal homogeneity and to measure the performance along the growth direction.

Figure 1a shows ¹³⁷Cs spectra collected by thinner samples retrieved along the growth direction of the 16 mm diameter TlMgCl₃ boule that show a similar detector performance (similar energy resolution ~ 4.6% at FWHM for 662 keV, as well as similar full-energy peak position or relative light yield), indicating that the boule was uniform. Because TlMgCl₃ had no dopant (i.e., intrinsic) and the growth involved a stoichiometric composition, the crystal performance along the boule was expected to be uniform. Uniformity in a detector volume is important especially when an application requires that the entire detector volume be involved in detection. Tl characteristic x-ray escape peaks (low energy peaks adjacent to the full energy peaks) were commonly observed in these spectra collected with thin samples of compounds with a high Z constituent. Figure 1b shows ¹³⁷Cs spectra collected at different amplifier shaping times, with the best energy resolution of 4.4% (FWHM) at 662 keV obtained with a 4 μs shaping time.

The comparison of a ¹³⁷Cs spectrum collected with the TlMgCl₃ sample and that collected with a ∅1″× 1″ (∅ 2.54 cm × 2.54 cm) NaI:Tl produced by Saint Gobain is shown in Figure 1c, showing that TlMgCl₃ has an excellent peak-to-Compton ratio (PCR) of 7.05 even for a small-sized crystal, compared to a PCR of 5.44 for the ∅1″× 1″ NaI:Tl crystal. Figure 1d shows the x-ray emitted optical luminescence spectrum of TlMgCl₃ that shows a broad emission band with a maximum around 436 nm, which is higher than a previously published maximum of 409 nm [7]. Considering the spectral response or quantum efficiency of a PMT with a super bialkali photocathode [21], the emission spectra of both TlMgCl₃ (Figure 1d) and NaI:Tl [22], as well as the published light yield of 38,000 photons/MeV for NaI:Tl [19], the estimated light yield for TlMgCl₃ is 28,000 photons/MeV, which is slightly lower than a previously published number of 30,600 photons/MeV [7]. It is worth noting that there is a difference in the x-ray emission spectrum in Figure 1d and previously published spectrum in [7], which has a maximum of 409 nm. It is also worth noting that the previously published result was based on the analysis of a much thinner and smaller crystal.

Figure 1e shows one of the polished TlMgCl₃ crystal samples from the 16 mm diameter TlMgCl₃ crystal boule. Figure 1f shows the non-proportionality data indicating that the response of TlMgCl₃ to γ-rays is linear (±0.05%) for energy above 30 keV. Decay time was measured and analyzed using Equation (1) with N = 3, resulting in decay constants of 74 ns ± 1 ns (4%), 338 ns ± 4 ns (50%), and 572 ns ± 7 ns (46%) (Figure 1g).

3.1.2. Intrinsic TlCaCl₃

As in the case with TlMgCl₃, the investigation into the growth for TlCaCl₃ was started by growing ∅16 mm crystal boules, and then it was scaled up to several ∅1″ boules. From the 16 mm diameter boule, similarly sized (approximately 3 × 7 × 10 mm³) single, transparent, crack-free crystal platelet-like samples of TlCaCl₃ were successfully obtained from three different areas in the crystal boule (top, middle, and tail, respectively) to check for crystal homogeneity and to measure the performance along the growth direction. Figure 2a shows ¹³⁷Cs spectra collected by the polished TlCaCl₃ samples (inset pictures) that were retrieved along the direction of the growth. The pictures of the crystals were taken after all measurements were completed in a non-inert atmosphere environment; thus, due to the hygroscopicity of the crystals, the samples slightly degraded and turned hazy. The ¹³⁷Cs spectra shows similar energy resolution (approximately 4.6% FWHM at 662 keV) and similar full-energy peak positions, which indicates that the boule was uniform in quality and performance. As in the case with TlMgCl₃, because TlCaCl₃ was also an intrinsic scintillator and the growth involved a stoichiometric composition, the crystal performance along the boule was also expected to be uniform. ¹³⁷Cs spectra collected with a slightly large sized TlCaCl₃ crystal (9 × 10 × 10 mm³) at different amplifier shaping times resulted in a best energy resolution of 5.2% (FWHM) at 2 μs shaping time (Figure 2b).

¹³⁷Cs spectra comparison between the TlCaCl₃ sample and a ∅1″× 1″ (∅ 2.54 cm × 2.54 cm) NaI:Tl is shown in Figure 2c. Using the published emission spectrum of TlCaCl₃ [8], the estimated light yield for TlCaCl₃ is 22,000 photons/MeV. The analysis on the temporal profile in Figure 2d, using Equation (1) and N = 3, shows that the decay constants for TlCaCl₃ are 81 ns ± 2 ns (2%), 409 ± 4 ns (62%), and 966 ns ± 17 ns (36%). Non- proportionality data in Figure 2e show that the response of TlCaCl₃ to γ-rays is linear (±0.05%) for energy above 30 keV.

3.1.3. Intrinsic TlCaBr₃

¹³⁷Cs spectra collected at different amplifier shaping times for a TlCaBr₃ sample are shown in Figure 3a, with a best measured energy resolution of 5.3% (FHWM) at 662 keV collected at a 12 μs shaping time. The comparison of ¹³⁷Cs spectra collected with a TlCaBr₃ sample from the ∅14 × 20 mm boule and a ∅1″ × 1″ NaI:Tl is shown in Figure 3b, showing that even with a smaller diameter, TlCaBr₃ exhibited a larger peak-to-Compton ratio than NaI:Tl. The x-ray excited optical luminescence spectrum of TlCaBr₃ is shown in Figure 3c, showing a broad band with a maximum around 517 nm. Using this emission data, the light yield for TlCaBr₃ is estimated to be 54,000 photons/MeV. ¹⁵²Eu spectrum was also collected with TlCaBr₃ (Figure 3d), from which the relative light yield data were calculated to determine the non-proportionality behavior of TlCaBr₃ (Figure 3e). As in the case of other ternary Tl-based halide crystals described so far, the crystal had a linear response (±0.05%) to γ-rays with energy above 30 keV. The analysis on the temporal profile in Figure 3f, using Equation (1) and N = 3, shows that the decay constants for TlCaBr₃ are 328 ns ± 1 ns (11%), 788 ns ± 13 ns (44%), and 3.28 μs ± 0.06 μs (45%).

3.1.4. Intrinsic and Eu-Doped TlCa₂Br₅

Intrinsic and Eu-doped ∅16 mm crystal boules of TlCa₂Br₅ were grown, processed, and characterized. The undoped crystal did not appear clear or transparent, indicating that there was probably more than one phase in the crystal boule. A dopant (Eu²⁺, with 3% concentration) was utilized in attempt to improve scintillation properties, like light yield, energy resolution, and decay time. Figure 4a shows ¹³⁷Cs spectrum collected with the Eu-doped TlCa₂Br₅ crystal, where an energy resolution of 4.2% (FWHM at 662 keV) was measured. Figure 4b shows the comparison between ¹³⁷Cs spectra collected with TlCa₂Br₅:Eu and NaI:Tl. Figure 4c shows the x-ray excited optical luminescence spectrum of TlCa₂Br₅:Eu, with a maximum peak around 484 nm. With this data, the light yield of TlCa₂Br₅:Eu is estimated to be 42,000 photons/MeV. According to the relative light yield vs. photon energy (non-proportionality) data (Figure 4d), TlCa₂Br₅:Eu has a relatively linear response (±0.05%) for a wide range of photon energy, especially for energies above 40 keV. Doping with europium appears to improve energy resolution; however, crystal quality appears to have been compromised. Temporal data were measured, and with N = 2 in Equation (1), two decay constants were calculated (Figure 4e): 552 ns ± 7 ns (46%) and 1.63 μs ± 0.04 μs (54%).

3.1.5. Intrinsic and Eu-Doped Mixed Halide TlCa(Cl,Br)₃

In an effort to improve the scintillation properties of bromine-based TlCaX₃ compounds, several approaches were considered in this research. As seen previously, undoped TlCaBr₃ was first grown as the base for comparison. Next, Eu-doped TlCa₂Br₅ was grown, which resulted in a better energy resolution, while the crystal quality and light yield suffer. In this section, results from intrinsic and Eu-doped mixed halide TlCa(Cl,Br)₃, with 50:50 Cl/Br mol ratio, are presented.

Figure 5a shows ¹³⁷Cs spectra collected with an intrinsic TlCa(Cl,Br)₃ sample at different amplifier shaping times, with a best energy resolution of 5.2% (FWHM) collected at a 12 μs shaping time. Figure 5b shows the temporal profile for intrinsic TlCa(Cl,Br)₃, with decay constants of 378 ns ± 12 ns (18%), 621 ns ± 11 ns (61%), and 2.61 μs (21%). The decay constants are similar in orders of magnitude and fraction sizes to those of TlCa₂Br₅:Eu. The comparison of ¹³⁷Cs spectra collected by TlCa(Cl,Br)₃ and NaI:Tl is shown in Figure 5c. Using the emission spectrum of TlCaBr₃ (Figure 3c), the light yield of TlCa(Cl,Br)₃ is estimated to be 71,000 photons/MeV.

Following the study presented in the previous sections, europium doped TlCa(Cl,Br)₃ (with 3% Eu²⁺ concentration) were grown, with the results shown in Figure 5d,e. The crystals extracted from along the boule were not transparent; however, the crystals had similar performances (energy resolution of 5.5–5.8% (FWHM) at 662 keV, as well as primary decay constant of 0.50–0.58 μs, as the decay constants of TlCa(Cl,Br)₃:Eu were measured at 505 ns ± 3 ns (62%), 1.08 μs ± 0.06 μs (22%), and 4.25 μs ± 0.29 μs (16%) (Figure 5d)), indicating that the boule was grown uniformly. Adding dopant to the mixed halide TlCaX₃ did not result in an observable improvement in the energy resolution nor in the decay constants. Using the emission spectrum of TlCaBr₃ (Figure 3c), the light yield of TlCa(Cl,Br)₃:Eu is estimated to be 63,000 photons/MeV, which is slightly less than the undoped sample (Figure 5c).

3.2. 1-Inch-Diameter Crystal Growth

From the scintillation results of the 16 mm diameter crystal growth, as well as the crystal growth yield results, it was evident that some of the compounds were feasible for a scale-up effort, namely intrinsic TlMgCl₃ and TlCaCl₃ scintillators, due to their reproduceable crystal yield, quality, and performance, as well as the non-existence phase issue. Also feasible for scale up was europium-doped TlSr₂I₅, whose small diameter growth and initial 1-inch-diameter growth were previously reported [8,9,10]. The following sections describe the latest 1-inch-diameter growth results of the aforementioned compounds.

3.2.1. Intrinsic TlMgCl₃

Better purification processes and finely tuned temperature profiles improved the growth of several boules of 1-inch-diameter TlMgCl₃ (Figure 6a). Visible on these boules were dark inclusions, which were residual impurities that were pushed out of the boules and reacted with the walls of the quartz ampoules. From these boules, several ∅1″ × 1″ TlMgCl₃ were cut and processed. Figure 6b shows three as polished ∅1″ × 1″ TlMgCl₃ crystals (top pictures) that were then permanently encapsulated (bottom picture). Although TlMgCl₃ is non hygroscopic, the encapsulation provides protection from accidental mechanical damage.

Figure 7a shows ¹³⁷Cs spectra by one of the ∅1″ × 1″ TlMgCl₃ crystals with different amplifier shaping times, resulting in a best energy resolution of 3.8% (FWHM) at 662 keV for 8 μs. This measurement was slightly better than the results obtained by the smaller crystals (Figure 1), which could be attributed to better processing or shaping of the crystal (right cylinder vs. cuboid) that promotes better light collection, better post-growth crystal processing (i.e., better polishing without cracking or cleaving), and/or better crystal quality (lack of defects or imperfections). Further study on light collection in TlMgCl₃ may be needed. Figure 7b shows the temporal data for the 1-inch TlMgCl₃, with measured decay constants of 71 ns ± 1 ns (4%), 321 ns ± 3 ns (47%), and 548 ns ± 4 ns (49%), which are comparable to the decay constants obtained for the smaller crystals (Figure 1g).

3.2.2. Intrinsic TlCaCl₃

An as-grown bulk ∅1″ TlCaCl₃ single crack-free crystal boule is shown in Figure 8a. The outside of the boule was covered with a thin translucent layer that may have been organics or impurities due to an excess of chloride reacting with the quartz ampoule. Under this translucent layer was a single crack-free crystal, a sample of which was shown in Figure 8b before packaging, a rectangular cuboid with the size of 16 × 16 × 25 mm³, which was subsequently polished and hermetically encapsulated (Figure 8c). The encapsulated crystal was characterized by collecting ¹³⁷Cs and ¹⁵²Eu spectra, as well as collecting the PMT anode signals to determine the temporal profile and determine decay time constants. Figure 8d shows the ¹³⁷Cs spectrum collected by the encapsulated TlCaCl₃ sample with a measured 4.6% (FWHM) energy resolution at 662 keV. Using the ¹³⁷Cs spectra comparison between TlCaCl₃ and NaI:Tl crystals (Figure 8d), the light yield of the 1-inch sample of TlCaCl₃ is estimated to be 19,000 photons/MeV. As also observed in Figure 8d, for a slightly sized crystal (16 × 16 × 25 mm³) than NaI:Tl (∅1″ × 1″), TlCaCl₃ has a much better peak-to-Compton ratio (PCR) than NaI:Tl. Decay constants of 104 ns ± 1 ns (3%), 456 ns ± 2 ns (72%), and 1.28 μs ± 0.02 μs (25%) were measured (Figure 8e). These decay constants were slightly longer than the results of the smaller crystals (Figure 2d); however, they were still in the same order of magnitude. ¹⁵²Eu spectrum was also collected (Figure 8f), from which the relative light yield data were calculated to determine the non-proportionality behavior of TlCaCl₃ (Figure 8g). As in the case of the smaller sized TlCaCl₃ crystals, the crystal had a linear response (±0.05%) to γ-rays with energy above 100 keV.

3.2.3. Eu-Doped TlSr₂I₅

Eu-doped TlSr₂I₅ crystal growth started with ∅16 mm [13], and then scaled up to several ∅1″ crystal boules. The latest 1-inch-diameter TlSr₂I₅:Eu as-grown crystals are shown in Figure 9a. The ¹³⁷Cs spectrum collected with a ∅1″ × 1″ TlSr₂I₅:Eu sample is shown in Figure 9b, with an energy resolution of 3.5% (FWHM) at 662 keV. Tl escape peak was detected around ch. no. 1470; however, due to the large crystal size, it was not as prominently featured in the spectrum as in the case of a thin TlSr₂I₅:Eu sample.

4. Discussion

The results and properties of ternary Tl-halide crystal successfully grown and investigated for imaging and high-energy particle physics applications are shown in

Table 2. Summary of results and properties of analyzed ternary Tl-halide crystals.

∅ 16 mm Crystals
Compound	Zeff	ER	Tprimary (ns)	Light Yield (Photons/MeV)
TlMgCl3	69.7	4.6%	338, 572	28,000
TlCaCl3	68.9	5.2%	409	22,000
TlCaBr3	64.3	5.3%	788, 3.28 μs	54,000
TlCa2Br5:Eu	66.2	4.2%	1.63 μs	42,000
TlCa(Cl,Br)3	64.3	5.2%	621	71,000
TlCa(Cl,Br)3:Eu	66.3	5.5%	505	63,000
		∅ 1-Inch Crystals
Compound	Zeff	ER	Tprimary (ns)	Light Yield (Photons/MeV)
TlMgCl3	69.7	3.8%	321, 548	28,000
TlCaCl3	68.9	4.6%	459	19,000
TlSr2I5:Eu	68.9	3.5%	395	72,000

. These compounds have high Z_eff due to thallium and they are expected to have high physical densities. The peak-to-Compton ratio (PCR) for small-sized crystals, especially TlMgCl₃ and TlCaCl₃, at 662 keV are comparable or better than a ∅1″ × 1″ NaI:Tl, due to better energy resolution and full-energy peak efficiency. Overall, the improvement in crystal growth and performance in 1-inch-diameter vs. 16 mm diameter crystals is due to the effect of material purification in the lab. The best crystal quality and energy resolution (FWHM) at 662 keV were observed for intrinsic TlMgCl₃. Additionally, TlMgCl₃ is not hygroscopic. The primary decay constants for these compounds are in the range of 0.45 to 0.63 μs. All of these compounds have proportional or linear response (±0.05%) to γ-ray above 100 keV. Intrinsic TlMgCl₃ and TlCaCl₃, as well as TlSr₂I₅:Eu, are feasible for large-diameter growth (≥1-inch-diameter) due to uniformity in crystal quality and detector performance along the boule. Currently, a scale-up effort of up to 2-inch-diameter crystal growth is in the process and will be reported in the near future.