1. Introduction

Gamma rays play an important role in various fields such as radiation detection, medical imaging, environmental monitoring, and nuclear material detection. Gamma-ray detection technology has evolved over time and is now widely applied in various areas, such as radiation monitoring systems and nuclear material detection equipment. Gamma-ray detectors are manufactured in a variety of sizes and forms, from large detectors to small ones, enabling efficient use even in high-radiation environments or areas that are difficult to access. The development of this technology has contributed to improving the accuracy and sensitivity of gamma-ray detection, and ongoing research is continuously working toward the development of more efficient and sensitive gamma-ray detectors.

Traditional scintillators are divided into organic and inorganic scintillators, each with its own advantages and limitations. Organic scintillators are easy to manufacture and flexible, but due to their low atomic number and low material density, they have limitations in gamma-ray nuclide identification [1-2]. Inorganic scintillators offer high light-emitting efficiency but face size limitations when creating large detectors, and there are technical constraints to overcome these limitations [3-6]. Recently, quantum dot (QD)-based scintillators have gained attention as a promising technology not only for gamma-ray detection but also for PSD (neutron and gamma-ray discrimination) research, thanks to their unique optical properties and high emission efficiency. Quantum dots can precisely control the emission wavelength and efficiency through size adjustment, and since they are composed of materials with high atomic numbers and high material densities, they are highly advantageous for gamma-ray nuclide identification [1, 7-8]. Moreover, with PSD technology, it is possible to distinguish neutrons from gamma rays, thereby increasing the accuracy and efficiency of radiation detection systems [7-8].

High atomic number materials have gained significant attention in the development of quantum dots for radiation detection due to their enhanced ability to interact with gamma rays [9-10]. These materials, when used as the core or in the shell structure of quantum dots, offer superior gammaray absorption capabilities. High atomic number elements such as lead (Pb) [11], bismuth (Bi) [12], or Gadolinium (Gd) [13] can be incorporated into quantum dots to enhance the overall radiation detection performance. These materials improve the efficiency of gamma-ray interaction by increasing the atomic cross-section for gamma-ray absorption, which results in higher radiation sensitivity and detection efficiency [9]. Additionally, high atomic number quantum dots can also contribute to improved energy resolution and reduced background noise, making them particularly useful for precise radiation measurements and discriminating between different radiation types [9, 14-16].

Conventional quantum dots are composed of a single material, with unique bandgaps and emission properties determined by their size and the material’s characteristics [16- 17]. However, these conventional quantum dots can suffer from surface defects or non-radiative recombination, which may reduce emission efficiency and optical stability [18]. In contrast, Core/Shell quantum dots are composed of a core and a shell made of different materials, where the shell protects the core and reduces surface defects, significantly enhancing emission efficiency [18-19]. The shell structure also efficiently facilitates electron-hole recombination, improving emission characteristics and minimizing non-radiative recombination. As a result, Core/Shell quantum dots offer higher emission efficiency, optical stability, and energy transfer efficiency, enabling superior performance in radiation detection compared to conventional quantum dots [9].

Organic scintillators are generally composed of organic compounds that absorb light and emit it. The emission characteristics of organic scintillators work by emitting light generated from the energy released during the recombination of electron-hole pairs (excitons). Organic scintillators are known for their fast response times and high efficiency, but their low atomic number and low material density limit their performance in gamma-ray detection [1]. On the other hand, quantum dots exhibit the Quantum Confinement Effect, where the smaller the particle size, the larger the bandgap, and this allows for precise control over the emission efficiency and spectrum [1, 9]. Quantum confinement forces electrons and holes to move within a confined space inside the quantum dot, causing the energy levels to become discrete. As a result, the emission characteristics are closely linked to the size of the particle. Smaller quantum dots emit shorter wavelengths of light, while larger quantum dots emit longer wavelengths [16-17]. By utilizing these characteristics, it is possible to enhance luminescence efficiency and control emission wavelengths, providing superior performance in the field of gamma-ray detection.

This study synthesized CdS/CdZnS/ZnS multi-shell quantum dots, performed characterization, and fabricated scintillators by adding the directly synthesized QDs to evaluate their performance. A 20–30% improvement in emission efficiency was achieved compared to traditional scintillators. By using multi-shell quantum dots, energy transfer efficiency was maximized, non-radiative recombination was reduced, and optical properties were optimized. The results of this study demonstrate that quantum dot-based scintillators can offer superior emission efficiency, high sensitivity, and efficient gamma-ray detection performance compared to traditional scintillators. Moreover, quantum dot-based scintillators may be a promising alternative not only for gamma-ray detection but also for PSD (neutron and gamma-ray discrimination) applications.

Quantum dots were synthesized using the Hot-Injection method, successfully producing CdS/CdZnS/ZnS multishell quantum dots, which were then compared with existing commercial scintillators through radiation detection experiments to evaluate their efficiency. The experimental results showed that quantum dot-based scintillators exhibited 20–30% higher efficiency than traditional scintillators, which was attributed to the enhanced energy transfer and emission characteristics.

This study suggests that quantum dot-based scintillators can serve as a promising alternative not only for gamma-ray detection but also for PSD technology to distinguish neutrons and gamma rays, making significant contributions to the development of high-efficiency radiation detectors. It also demonstrates that multi-shell quantum dots play a crucial role in improving the emission efficiency of scintillators, establishing their potential for future applications in various fields such as medical imaging, nuclear material detection, and environmental radiation monitoring. Furthermore, by incorporating high-Z materials into quantum dots, the gamma-ray detection capability is further enhanced, providing an efficient and reliable solution for advanced radiation detection systems. If further study on quantum dot optimization is conducted in the future, gamma-ray nuclide identification could be achieved, leading to the development of even more advanced systems.

2. Materials and Methods

2.1 Quantum dot Synthesis

In this study, we directly synthesized CdS/CdZnS/ZnS multi-shell quantum dots. The selection of these materials was based on the unique electronic and optical properties of Cd, Zn, and S, which enable efficient light emission, a broad spectral range, and superior electrical and optical performance. These characteristics make them widely utilized in the production of quantum dots. The reasons for utilizing the CdS/CdZnS/ZnS combination can be explained as follows: CdS is used as the core material due to its narrow bandgap and high optical density. These properties make CdS a popular choice for various scintillation applications, where efficient light emission is crucial. CdZnS is employed as the intermediate shell. This shell acts as a lattice buffer that adjusts the bandgap difference between the CdS core and the ZnS outer shell. By reducing the lattice mismatch between these two layers, the CdZnS shell enhances the structural stability of the quantum dots and minimizes nonradiative losses. ZnS is utilized as the outer shell because of its wide bandgap. This material provides excellent chemical stability and mechanical strength, which significantly improve the durability of the quantum dots. Conventional quantum dots and core/shell quantum dots are structurally different, which results in differences in optical and physical properties. Conventional quantum dots are composed of a single material, while core/shell quantum dots have additional characteristics, with a core surrounded by a shell.

Firstly, structurally, conventional quantum dots are made from a single semiconductor material and have unique bandgaps and luminescent properties based on their size and material properties [20-23]. On the other hand, core/shell quantum dots have a core surrounded by a shell, with the core and shell materials having different bandgaps. The core mainly contributes to the luminescence, while the shell serves to protect the core and reduce defects [21]. Secondly, in terms of photoluminescence efficiency, conventional quantum dots are more likely to undergo non-radiative recombination due to surface defects, which can lower the photoluminescence efficiency. However, for core/shell quantum dots, the shell surrounding the core reduces surface defects, and this increases the likelihood that the electron-hole recombination in the core will emit light, resulting in higher photoluminescence efficiency [23]. Thirdly, in terms of stability and durability, conventional quantum dots are sensitive to external factors, especially oxidation, heat, and moisture, which can degrade their optical performance over time [24-25]. In contrast, core/shell quantum dots offer higher environmental stability because the shell protects the core, preventing chemical changes like oxidation and shielding the quantum dot from external factors [24-26]. This makes core/shell quantum dots particularly advantageous in applications requiring humidity and thermal stability. Fourthly, in terms of controlling photoluminescence wavelength and color stability, conventional quantum dots change their emission wavelength according to their size. However, since they are made from a single material, the ability to fine-tune the emission wavelength is limited [27]. In contrast, core/shell quantum dots can precisely control the bandgap through the combination of the core and shell, allowing for more accurate tuning of the emission wavelength. Moreover, the shell improves the purity of the emitted color, leading to better color stability [21, 27]. Finally, in terms of energy transfer efficiency and lifespan, conventional quantum dots emit light through the recombination of electron-hole pairs in the core. However, due to the many defects present, the charge separation efficiency is low, and the lifespan is limited. For core/shell quantum dots, the shell aids in charge separation, which increases both the lifespan and energy transfer efficiency. The shell reduces non-radiative energy loss, and because charge separation and recombination are controlled, the lifespan is extended [28]. In conclusion, core/shell quantum dots outperform conventional quantum dots in terms of efficiency, stability, and lifespan, making them ideal for various applications. Additionally, multi-shell quantum dots have several advantages over single-shell quantum dots [29]. For example, multi-shell quantum dots use multiple shell layers to protect the core, improve the recombination efficiency of electron-hole pairs, reduce defects between the core and outer layers, and minimize non-radiative recombination. This increases the amount of light emitted without energy loss, thereby enhancing photoluminescence efficiency (PLQY) [29-30]. Therefore, multi-shell quantum dots, with their superior luminescence efficiency, durability, color stability, and flexibility, were employed in this study.

The method used in this study is the high-temperature injection method. When synthesizing quantum dots with the high-temperature injection method, a Group 6 precursor is put in a reactor containing a Group 2 precursor to create supersaturation conditions in which quantum dot particles can be generated. Under this condition, the nucleus of the quantum dot is generated, after which the quantum dot grows as the precursors not involved in the nucleation step react with the surface of the nucleus.

When the monomer concentration is high, small particles grow quickly and relatively large particles grow slowly due to the difference in their surface areas. Afterwards, when the concentration of the precursor in the container is reduced to below the critical concentration, the smaller particles melt and the larger particles grow, thereby increasing the size distribution of the quantum dots. Therefore, if the chemical reaction is terminated by lowering the temperature of the reactor before coarsening of the particles occurs, quantum dots having a uniform distribution can be synthesized. When synthesizing quantum dots, conditions such as the reaction temperature and concentration and the type of ligand bound to the crystal determine the nucleation concentration, the quantum dot growth rate, and the shape of the quantum dot. Depending on the crystal of the quantum dot, the binding energy of the organic ligand bound to the crystal surface changes. By utilizing this energy difference to promote the growth of quantum dot crystals in a specific crystal direction, it is possible to synthesize various types of nanostructures, such as rods and stars, as opposed to spherical structures.

The materials used to synthesize the quantum dots in this study are as follows: cadmium oxide (≥99.99% trace metal basis, Sigma Aldrich, USA), zinc acetate (99.99% trace metal basis, Sigma Aldrich, USA), oleic acid (technical grade 90%, Sigma Aldrich, USA), 1-octadecene, (technical grade 90%, Sigma Aldrich, USA), trioctylphosphine (technical grade 90%, Sigma Aldrich, USA), 1-dodecanethiol (≥98%, Sigma Aldrich, USA), sulfur powder (98%, Duksan General Science, Korea), ethyl alcohol (Anhydrous 99.9%, Duksan General Science, Korea), acetone (99.5%, Duksan General Science, Korea), and toluene (99.5%, Duksan General Science, Korea).

First, 7 ml of OA (oleic acid), 0.128 g of CdO, and 1.83 g of Zn(Ac)₂ were added to a 250 ml three-neck roundbottomed flask, after which moisture and oxygen were removed at 120°C for one hour to create a vacuum state. Subsequently, an amount of 53 ml of ODE (1-octadecene) was added, with nitrogen then added to the flask to change the atmosphere. The temperature was raised to 300℃ in the nitrogen atmosphere, and when it reached 300℃, 6 ml of precursor 1 was quickly injected and reacted for 7 minutes to form a CdS core. Here, precursor 1 is a substance obtained by dissolving 0.11 g of sulfur powder in 6 ml of ODE.

Afterwards, 0.83 ml of DDT (dodecanethiol) was slowly injected to form an intermediate layer shell and reacted for 30 minutes. Next, 4 ml of precursor 2 was rapidly injected and reacted for 20 minutes to form CdS/CdZnS. Here, precursor 2 is a substance obtained by dissolving 0.192 g of sulfur powder in 4 ml of TOP (n-trioctylphosphine). After purifying the synthesized quantum dots, they were dispersed in ODE.

In this study, an additional procedure was performed to form a thick multi-shell structure. In this case, 3.5 ml of OA, 0.064 g of CdO, 0.917 g of Zn(Ac)₂, and 26.5 ml of ODE were placed in a 250 ml three-neck round-bottomed flask and moisture and oxygen were removed at 120°C for one hour to create a vacuum. Then, purified quantum dots (CdS/ CdZnS) were put into the flask and a nitrogen atmosphere was created inside the flask. After raising the temperature to 300°C in the nitrogen atmosphere, upon reaching that temperature, 2 ml of the third precursor was slowly injected and reacted for 20 minutes. After cooling to room temperature, the synthesized quantum dots were purified and then dispersed in ODE to obtain the quantum dot sample used in this study.

During the quantum dot purification process, the synthesized quantum dots, ethyl alcohol, and toluene were mixed at a ratio of 1:4:4 and centrifuged at 4,500 rpm. Then, an amount of 100 ml of acetone was added to the precipitated quantum dots, followed by centrifugation. This process was repeated twice, and the precipitated quantum dots were dispersed in ODE or toluene to obtain the quantum dot sample. Fig. 1 shows the quantum dot manufacturing method.

Fig. 1

Manufacturing process of multi-shell structured quantum dots (QDs).

2.2 Evaluation of Optical Properties

For the evaluation of the optical properties of the synthesized quantum dots, an absorption/transmission analysis, XRD, Raman spectrograph, a microstructural analysis, fluorescence spectroscopy, and a decay time analysis were conducted. The optical characterization items performed in this study are as follows in the Table 1.

The equipment used to perform each characteristic evaluation is as follows. First, the absorption/transmission analysis was performed using an optical spectrometer (FLAME, Ocean Insight, USA); this system consists of a spectrometer, a light source (DH-2000), two optical fibers and a sample holder. The DH-2000 light source consists of deuterium and tungsten halogen, and the light source produces stable output from 215 to 2,000 nm.

In addition, a deep-UV light source having a wavelength range of 190 to 1,700 nm can be used. The two optical fibers have a core diameter of 400 μm and a wavelength range of 300 to 1,100 nm. The absorption/transmission analysis spectrum was obtained by averaging 10 measurements at random.

XRD equipment (SmartLab, RIGAKU, Japan) was used to analyze the peaks diffracted by irradiating X-rays on the sample. The XRD was equipped with a Cu anode operating at 9 kW and generating Cu Kα radiation. A 2D detector (Hypix-3000) and a high speed 1D detector (D/Tex Ultra 250) were used to take 2θ measurements from 10° to 80°.

Raman equipment (RAMANtouch, Nanophoton, Japan) was used to analyze the energy change of the molecules, with an excitation wavelength of 532 nm. The detector used a highly sensitive TE-cooled CCD (1,650 pixels). The weakest point of a Raman microscope used to be the extraordinary long measurement time. However, the equipment (RAMANtouch) used in this study can scan by emitting laser beams along the most appropriate paths without any preliminary information on the samples, obtaining images at a speed 5 to 10 times faster than a conventional scanning Raman microscope.

A quantum dot microstructural analysis was conducted using a transmission electron microscope (Tecnai G2 F30 S-TWIN, Philips/FEI, USA). The TEM used for the analysis is a device capable of analyzing the microstructure of a material by irradiating an accelerated electron beam onto the sample with a thickness of 100 nm or less and utilizing the diffraction pattern obtained from the transmitted and diffracted electrons. This device can analyze the components of a specific site through the installed EDS detector. Therefore, in this experiment, the particle size and distribution were confirmed through TEM, and the component analysis was performed through EDS.

In addition, a time-resolved fluorescence spectrophotometer (Fluorolog3, HORIBA, Japan) was used for fluorescence spectroscopy and fluorescence decay time analyses of the liquid scintillator. Fluorescence and phosphorescence can be measured in the UV-Vis-NIR region with high sensitivity for a material, and the time-correlated single-photon counting (TCSPC) method using pulsed nanoLED and a pulsed laser diode as a light source is used to measure the decay time of the fluorescence and phosphorescence of a material. The photoluminescence lifetime of a fluorescent material is affected by both the radiative and nonradiative transition processes of excited electrons. The radiative transition is determined by the molecular structure of the phosphor, and the nonradiative transition is sensitively changed by the phosphor-fluorescence interaction, the phosphorsolvent interaction, and the energy transfer.

2.3 Fabrication of Solid/Liquid Scintillator

A solid scintillator (SS) and a liquid scintillator (LS) were fabricated using the quantum dots synthesized in Section 2.1. POPOP (Sigma Aldrich, Germany), widely used as a secondary solute, absorbs in the wavelength range of 260 to 340 nm and emits in the wavelength range of 330 to 450 nm. In general, POPOP shows multiple emission peaks and has a wide emission range. However, the scintillator synthesized with quantum dots shows a single emission peak with a narrow emission region of 420 to 430 nm compared to a commercial scintillator. Therefore, the photon sensitivity of the photomultiplier tube (PMT, ET-9266KB, ET Enterprises, UK) used is improved because the wavelength of the maximum response range of the PMT used in this study is 420 to 430 nm. In other words, the photoluminescence quantum efficiency is improved. POPOP also has disadvantages in that it has poor solubility in solvents and poor transparency.

Fig. 2 shows the manufacturing process used to fabricate the solid scintillator and the liquid scintillator. For the solid scintillator, a mixed solution of primary and secondary solutes was placed in a glass vial with a diameter of 50 mm, as described above, with the amount of solution added being 95 ml to create a thickness of 50 mm. The amount of solution was determined by taking into account the amount that would be lost due to cutting. The mixed solutions were polymerized at 80 to 120 degrees for approximately 3 days after removing microbubbles in a vacuum oven. First, slow heating was performed to 100 degrees, after which the temperature was stabilized for about 12 hours and slow heating was then performed again up to 120 degrees, remaining at that temperature for 3 hours. Afterwards, slow cooling was performed to 80 degrees, the temperature was stabilized for approximately 12 to 20 hours, with rapid cooling to room temperature then conducted. After polymerization, the surface was cut to ensure transparency. Fig. 3 shows the polymerization temperature change during the manufacturing of the solid scintillator. The manufactured scintillator was wrapped with Teflon as a reflector and then additionally wrapped with black tape to block light.

Fig. 2

Manufacturing process of Solid and Liquid scintillator.

Fig. 3

Polymerization temperature change for manufacturing solid scintillator.

The liquid scintillator used transparent macro cuvettes with an inner diameter of 47 mm, an outer diameter of 50 mm, a thickness of 50 mm, and a volume of 86 ml. The inside of the cuvettes was filled with a toluene-based scintillation solution to prevent air bubbles. After filling the cuvettes with the scintillation solution and sealing them, they were additionally taped with Teflon and black tape to prevent light loss and interference by miscellaneous sources of light.

2.4 Gamma & Neutron Detection Performance Evaluation

To evaluate the measurement performance capabilities of the quantum-dot-based solid scintillator and liquid scintillator, a detection system was constructed using electronic devices, in this case a PMT (ET-9266KB, ET Enterprises, UK) and an MCA (DT5730SB, CAEN, Italy). In order to remove the afterglow of the PMT generated during the scintillator replacement process, it was placed in a dark room for about 6 to 12 hours after being connected to the scintillator. The radiation sources used in this experiment are ¹³⁷Cs (for gamma detection) and ²⁵²Cf (for gamma/neutron discrimination test). With regard to the half-life, the radioactivity of ¹³⁷Cs is 331.4 kBq and the radioactivity of ²⁵²Cf is 0.7798 MBq. For the measurement test, the source was placed at a distance of 150 mm, and data was acquired for 600 seconds.

Fig. 4 presents an experimental photograph taken during the gamma and neutron identification performance test. Fig. 4(a) shows an experiment using a ¹³⁷Cs nuclide (gamma) source, while Fig. 4(b) shows a PSD experiment using a ²⁵²Cf nuclide (neutron) source. In the PSD experiment, the commercial liquid scintillator (BC501A, Saint- Gobain) with a 3-inch size and the scintillator developed in this study were placed parallel to each other. In case of neutron detection test, it was evaluated whether the neutron peak and the gamma-ray peak are well distinguished (figure of merit, FOM), and the FOM calculation formula used here is as follows. When the interval between the neutron peak and the gamma peak is defined as Δ, the ratio of Δ to the sum of the half widths of the neutron peak and gamma peak is the FOM value [31-33]. The larger the FOM, the better the peak discrimination performance.

Fig. 4

Equipment configuration for gamma measurement (a) and gamma/neutron identification performance experiments (b).

3. Results and Discussion

3.1 Evaluation Results of the Optical Properties of Quantum Dots

First, absorbance is a method of measuring the amount of light that a sample absorbs in a certain narrow wavelength range. The visible and ultraviolet absorption spectra of a material change depending on the chemical structure of the material. When light passes through a certain material, the ratio between the intensity of transmitted light (I) and the intensity of incident light (I₀) is called transmittance (T), and the common logarithm of the reciprocal of transmittance is called absorbance (A). The formulas for transmittance and absorbance are shown below. The absorbance measurement result is shown in Fig. 5(a). In Fig. 5(a), the black line is the absorption spectrum, and the red line is the transmission spectrum.

Fig. 5

Result of evaluation of optical properties of quantum dots (a) Absorption and transmittance, (b) Decay time at 316 nm excitation wavelength.

The second optical characteristic is the decay time (fluorescence extinction time) of quantum dots, and the result is shown in Fig. 5(b). For the decay time, the time-resolved fluorescence analysis equipment used to analyze the photoluminescence characteristics was used again. The decay time was analyzed for each sample at an excitation wavelength of 316 nm; this was calculated through Equation (4) based on raw data. Equation (4) is an equation for the multiexponential fitting of the spectrum over time, where A_n is the amplitude of nth component and τ_n is the lifetime of the nth component.

As the third optical characteristic, the emission wavelength of the quantum dots was analyzed by means of steady-state fluorescence measurements. As a result of analyzing the emission spectrum with an excitation wavelength of 316 nm, the average emission wavelength was found to be approximately 434 nm (Fig. 6). A wavelength close to 434 nm is suitable for the response of the PMT used in this study, and it has a quantum efficiency of about 25% or more. Additionally, the quantum dots exhibit a larger Stokes shift compared to POPOP, which is used as a secondary solute, reducing the impact of self-absorption. It has been demonstrated that the quantum dots have excellent emission color purity due to their narrow emission peak characteristics.

Fig. 6

Absorbance/Photoluminescence graphs of the primary and secondary scintillators and quantum dots (The dashed line represents absorption, and the solid line indicates emission).

The fourth characteristic is the XRD structure of quantum dots. XRD is an abbreviation of X-ray diffraction, and an XRD analysis analyzes reflected waves from a material using X-rays. X-rays are used owing to their short wavelength because at shorter wavelengths, deeper penetration into the material becomes possible. Fig. 7(a) shows the XRD pattern of the quantum dots synthesized in this study. Based on the 2θ values of 28.5°, 47.4°, 55.1°, and 76.7° in this figure, the observed peaks in the XRD pattern of ZnS are consistent with (111), (200), and (220). Also, based on the 2θ values of 26.76°, 28.36°, 30.35°, 39.38°, 47.25°, 56.03°, and 77.27°, the peaks observed in the XRD pattern of CdS are (100), (002), (101), (102), (110), (200), and (211). This result confirmed that the quantum dots synthesized have a hexagonal structure based on the reference patterns in several earlier works (Ulrich, F., Zachariasen, W.Z. Kristallogr 62 (1925)).

Fig. 7

Results of XRD and Raman (a) XRD (b) Raman spectrum of each sample (Sample with PPO(A), POPOP(B), QD(C), PPO and POPOP(D), PPO and QD(E)).

The fifth characteristic is a Raman spectrograph. During the Raman scattering process, when a photon interacts with a molecule, the molecule is excited and then returns to a stable state, but it may not return to its initial level. At this time, the difference between the incident photon energy and the scattered photon energy is called the Raman shift. In this Raman test, the Raman characteristics of the materials used in manufacturing the liquid scintillator and in the mixed material state were measured. These results are shown in Fig. 7(b). In the peak of the sample ultimately used (Fig. 7(b)(A−E)), the peaks of PPO and POPOP were mainly observed, and it was confirmed that the position of the vibration band of the quantum dot used was mainly located at 200–400 cm⁻¹.

The sixth optical characteristic is the microstructure of the quantum dots. The synthesized quantum dots are Cd-based quantum dots and have a core/shell structure. Although quantum dots have excellent photostability, color purity, and good light efficiency, aggregation, which is a fundamental problem of nanoparticles, is severe. Accordingly, there are many limitations with regard to utilization, and when agglomeration occurs, the photoluminescence intensity is reduced. However, the result of TEM analysis demonstrated that they were evenly spread without agglomeration, as shown in Fig. 8(b), and the particle size was found to be approximately 10 nm. In addition, the results of a component analysis through an EDS analysis are shown in Fig. 9 and Table 2. The synthesized quantum dots were found to be CdS/CdZnS/ZnS through the EDS analysis.

Fig. 8

Synthesized quantum dot (a) and TEM analysis result (×49,000) (b).

Fig. 9

EDS analysis of quantum dots synthesized.

3.2 Evaluation Result of the Optical Properties of the Scintillator

A photoluminescence analysis of the fabricated scintillators was conducted. These results are shown in Fig. 10. Except for the commercial scintillator, all samples were measured using a 1 ml four-window clear quartz cell, and for the commercial scintillator, a 1-mm-thick scintillator was measured. The photoluminescence analysis results were averaged by producing 3 samples: a sample with only PPO (primary solute), a sample with PPO and POPOP (secondary solute), and a sample with PPO and QD (secondary solute). All solution samples were based on toluene. The 1-mm-thick commercial scintillator was also measured 3 times and averaged. As a result, the sample with only the primary solute added showed an emission peak at 350–400 nm, the sample with POPOP added showed an emission peak at about 417 nm, and the sample with QD added showed an emission peak at about 445 nm. The analysis showed that the intensity of the QD-based sample was nearly 10% higher than that of the POPOP sample. This occurred because the energy transfer rate from the primary solute to the QD is high and the solubility is higher than that of POPOP, meaning that the shoulder peak is not visible. In addition, the emission peak of the QD-based sample was narrow, which means that because high-intensity flashes are generated in a specific wavelength range, improved quantum efficiency can be obtained if the response range matches the PMT used. For the POPOP sample, it was confirmed that a shoulder peak existed at 441 nm, and it was analyzed as a broader peak than that of the QD sample. The commercial scintillator also showed an emission peak at 470 nm, but this was at 433 nm and showed a broad emission peak.

Fig. 10

Photoluminescence analysis results of scintillator.

In this study, SS (solid scintillator) and LS (liquid scintillator) were fabricated and their properties were evaluated. Liquid and solid scintillators have distinct physical properties and applications. Liquid scintillators exhibit fast optical responses and high energy transfer efficiency, but their liquid state can lead to long-term stability issues and lower mechanical strength. Additionally, using specific compounds or nanomaterials can maximize the coupling efficiency with optoelectronic devices. In contrast, solid scintillators offer superior durability and stability but may have slower response times and lower efficiency. They are mainly used in applications that require fixed shapes and structures, such as high-energy particle detectors, radiation therapy, and various radiation monitoring equipment. Both liquid and solid scintillators with these characteristics were fabricated and their performance was compared. The photoluminescence characteristics of the solid and liquid scintillators under UV irradiation are shown in Fig. 11. The UV irradiation test involved exposing the samples to 365 nm UV light to directly observe the optical emission. Fig. 11(a) and (c) represent the POPOP (0.01wt%) samples, while (b) and (d) represent the quantum dot (0.01wt%) based samples. When the samples were irradiated with 365 nm UV light, blue light was emitted, confirming that the solute was well dissolved in the solvent.

Fig. 11

UV irradiation test result of the manufactured solid and liquid scintillator. The size of the scintillator is 50 mm in diameter and 50 mm in length. (a) SS with added Toluene, PPO and POPOP (0.01wt%), (b) SS with added Toluene, PPO and QDs (0.01wt%), (c) LS with added Toluene, PPO and POPOP (0.01wt%), (d) LS with added Toluene, PPO and QDs (0.01wt%).

3.3 Gamma & Neutron Detection Evaluation Results

Among the materials constituting the liquid scintillator, the primary solute and quantum dots have excellent solubility in toluene. However, because undissolved solute may cause scattering, transparency of the liquid scintillator was secured through mixing for 4 to 5 hours after all substances were added to the toluene. In this section, the results of gamma and neutron measurements and performance evaluation are described using manufactured scintillators.

First, the gamma measurement results were analyzed by post-processing the energy spectrum. For the organic scintillator, because it is composed of a material with a low atomic number, the measurement efficiency for gamma rays is low, and a full-energy peak is not observed in the energy spectrum. Only the effect of Compton scattering is observed. Therefore, by applying the Energy-Weighted Algorithm (EWA), the low-energy noise signal is reduced, and the Compton edge peak is emphasized. Additionally, through the emphasized Compton edge, artificial and natural radionuclides can be distinguished as Compton edges. The EWA was applied using Equation (5). The EWA is calculated by aligning the energy (keV) corresponding to each channel in the energy spectrum and then multiplying by the counts for each channel. C_i refers to the counts value of the i-th channel, and E_i refers to the energy (keV) corresponding to the i-th channel [34-36].

Based on the analysis of the previous papers [14-16], it has been observed that the performance of plastic scintillators was improved by adding QDs above 40wt% or modifying the surface of QDs, and a full-energy peak was observed for gamma sources. However, the full-energy peak in these studies has poor resolution, long measurement times, making peak classification difficult. In such cases, applying the Energy-Weighted Algorithm (EWA) could be useful for clearly separate the full-energy peak. Although the full-energy peak was not observed in this study utilizing QD-based scintillators, a preliminary test was performed through the application of EWA using the Compton edge. By emphasizing the Compton edge, artificial and natural radionuclides can be easily distinguished. In the future, it is expected that precise results can be achieved by applying EWA to the full-energy peak or Compton edge information obtained through the optimization of quantum dot properties. Therefore, EWA was applied in this study.

Fig. 12(a) shows the energy spectrum and the energyweighted spectrum using a solid-type scintillator, and Fig. 12(b) shows the energy spectrum and the energy-weighted spectrum using a liquid-type scintillator. When the EWA is applied, the scattered light or noise signal in the low-energy region is decreased, and the count is increased in the highenergy region. As a result, the region of interest, the Compton edge region, is highlighted as an identifiable peak. Table 3 shows the results of the calculation of the total counts and the relative efficiency based on Fig. 12. With regard to the total counts, this outcome was calculated through the spectrum with the EWA applied, and after designating the area corresponding to 80% of the maximum peak in the normalized spectrum as the Compton edge range, the total count was calculated. The relative efficiency was calculated through Equation (6) based on the calculated total count in each case. The reason why the energy spectrum of the scintillator with added quantum dots shows peaks over a wider channel compared to the commercial scintillator in Fig. 12 can be attributed to several factors. First, the quantum dot-based scintillator, due to its multi-shell structure, may result in a more complex energy transfer process. As a result, the emitted energy can be distributed over a wider range. Furthermore, the quantum dots used in this study are of the multi-shell structure, such as CdS/CdZnS/ZnS, and the core-shell structure can have different luminescent properties at each layer. Since the energy absorption and emission processes differ by layer, the energy is distributed over a broader range.

Fig. 12

Measurement results using ¹³⁷Cs source (Energy spectrum (ES) and Energy weighted spectrum (EWS)), (a) Measurement results of solid type scintillator, (b) Measurement results of liquid type scintillator.

The efficiency was generally calculated only for ¹³⁷Cs sources. As shown in Table 3, the QD-based scintillator showed improved efficiency compared to the POPOP-based scintillator, with an improvement in the approximate range of 34 to 38%. This is thought to stem from the quantum dots, which are composed of a high-atomic-number material such that the reaction rate was improved via incident radiation.

Additionally, pulse shape discrimination (PSD) experiments using a liquid scintillator were conducted to discriminate neutrons and gamma nuclides using a neutron source. As mentioned above, the PSD method is a method of distinguishing and measuring incident particles using the ratio of two components. In this experiment, the spectrum of the neutron source (²⁵²Cf) was measured using the prepared liquid scintillator, and the PSD test was performed by evaluating the degree of differentiation between the neutron and gamma ray peaks (figure of merit, FOM).

The CAEN DT5730SB used in this study primarily digitizes analog signals, such as those generated by a PMT, into digital data. The digitized waveform data is stored in the digitizer’s internal memory or transferred to a PC for analysis using CAEN’s software, COMPASS. The DT5730SB with PSD firmware uses the integral of the digitized pulse over a user-defined window (Long gate) as the value characterizing the energy loss of a scintillating particle. To obtain the discrimination parameter, the integral of the fast component (Short gate) is additionally calculated using Equation (7).

CAEN settings (e.g., threshold, time gates, and triggers) were adjusted to optimize neutron-gamma separation. The threshold was set to 150 lsb (CAEN relative units), corresponding to an energy of 180 ± 10 keV, and the short gate and long gate were set to 80 ns and 240 ns, respectively. The PSD results are shown in Fig. 13. EJ-276, as a commercial scintillator, is designed to optimize the ratio of slow and fast components for PSD applications. In contrast, the manufactured scintillators (POPOP, QD) have not yet been optimized for PSD applications. However, since the FOM, an indicator of PSD performance, was evaluated to be at a similar level to EJ-276, it can be concluded that liquid scintillators also hold potential for PSD applications. The FOM values of the commercial scintillators, a 3-inch liquid scintillator (BC501A) and a 2-inch plastic scintillator (EJ276), were evaluated to be 1.3 and 1.22, respectively. When POPOP was added as a secondary solute, the FOM value was evaluated as 1.25. When quantum dots were added instead of POPOP, the FOM value was evaluated as a maximum of 1.15 using Equation (1). Even when the scintillator size was equal to or smaller than that of existing commercial systems, the PSD performance was found to be similar to that of commercial systems. Although the performance of the quantum dot-based scintillator was evaluated to be lower than that of commercial scintillators, it is believed that quantum dots can replace the existing POPOP material through surface modification or content optimization. Furthermore, by optimizing factors such as material composition and signal processing algorithms, the performance of liquid scintillators is expected to improve further in future applications.

Fig. 13

Neutron/gamma identification results for each sample using a neutron source (²⁵²Cf). (the red line is a EJ276 commercial scintillator, the blue line is a POPOP-added liquid scintillator, and the green line is a QD-added liquid scintillator).

4. Conclusion

The photoluminescence efficiency of a scintillator is very important for radiation detection. The efficiency can be improved by controlling the size of the quantum dots, which have discontinuous energy levels for electrons and holes. In this study, solid and liquid scintillators incorporated with a QD-based scintillator were developed and the detection efficiency was compared with that of a commercial scintillator for radioactivity measurements in radiological characterization applications. CdS/CdZnS/ZnS, having a multi-shell structure with a blue wavelength, was successfully synthesized through a hot injection method. The wavelength of the synthesized quantum dots is suitable for the wavelength of the maximum response of a photomultiplier tube (PMT), with quantum efficiency of approximately 25% or more. When compared with the emission characteristics of POPOP, it was confirmed that a single emission peak was observed with a good resolution in quantum dots. In addition, as a result of measuring the gamma detection efficiency of the fabricated solid and liquid scintillators containing quantum dots, the relative efficiency was found to be improved by 34–38% compared to the sample without QDs. This is the effect of the quantum dots, composed of materials with high atomic numbers, and it is necessary to optimize the content of the quantum dots in consideration of the relationship between the light yield and detection efficiency. In neutron detection experiments, the performance of the synthesized quantum dots was slightly lower than that of POPOP. Despite this, the quantum dots showed high potential for neutron/gamma discrimination. The advantage of the developed plastic scintillator is that it can be easily manufactured in various shapes and sizes. It can be manufactured into various sizes with flexible sensors as well for complex structures such as those at nuclear facilities. Therefore, the quantum dots can be improved and replace the existing organic scintillator materials through content optimization, surface modification, and component changes. Also, this study is especially notable in that the organic scintillator incorporated with the newly fabricated QDs can contribute to better gamma and gamma/neutron detection for the operation and decommissioning various nuclear facilities. In the future, we plan to develop a highperformance scintillator with identifying gamma nuclides by observing the full-energy peak through optimization studies on QD materials, surface modification, and material content. This study can be applied to nuclear facility decommissioning, nuclear material inspection, and medical fields.