Flexible Cs₃Cu₂I₅ Nanocrystal Thin-Film Scintillators for Efficient α-Particle Detection

Abstract

Thin-film detection technology plays a significant role in particle physics, X-ray imaging and radiation monitoring. In this paper, the detection capability of a Cs₃Cu₂I₅ thin-film scintillator toward α particles is investigated. The flexible thin-film scintillator is fabricated by a facile and cost-effective in situ strategy, exhibiting excellent scintillation properties. Upon α-particle excitation, the light yield of the Cs₃Cu₂I₅ thin-film is 2400 photons/MeV, which greatly benefits its application for single-particle events detection. Moreover, it shows linear energy response within the range of 4.7–5.5 MeV and moderate decay time of 667 ns. We further explored the cryogenic scintillation performance of Cs₃Cu₂I₅@PMMA film. As the temperature decreases from 300 K to 50 K, its light yield gradually increases to 1.3 fold of its original value, while its decay time remains almost unchanged. This scintillator film also shows excellent low-temperature stability and flexible operational stability. This work demonstrates the great potential of the Cs₃Cu₂I₅@PMMA film for the practical utilization in α-particle detection application.

1. Introduction

Scintillators, which can convert the high-energy rays/particles into ultraviolet/visible photons, play a key role in detector systems for high-energy physics, security surveillance and non-destructive testing [1,2,3]. In past decades, single-crystal scintillators (such as LYSO, CsI:Tl and NaI:Tl) are the mainstream choices in spectroscopic detection application due to their effective radiation absorption capacity [4,5,6]. However, in some specific situations when rare single-particle radiation events are generated accompanied by excessive γ-ray background interference, thin-film scintillators are considered to be irreplaceable [7,8,9]. For example, in tokamaks, precise detection of high-energy ion beams, which is essential for diagnosing plasma states and energy distributions, is severely challenged by strong γ-ray background interference [10]. Thin-film scintillators mitigate γ-ray interference by reducing their sensitive layer thickness, enabling accurate charged particle measurements in high-background radiation environments. Currently, various scintillator materials have been fabricated into thin-film scintillators to address such challenges. The plastic thin-film scintillator stands out for its excellent stability and low fabrication cost [11,12]. However, it is restricted by the issues of low light yield and poor energy resolution. The inorganic thin-film scintillator features high light yield and excellent energy resolution, and its inherent hygroscopicity and complicated fabrication procedures impede its further application [13,14]. Driven by the higher detection requirements, it is quite urgent to develop the novel thin-film scintillator with better performance.

Lead-free halide perovskite and their related derivatives, which replace lead with low-toxic or non-toxic elements, have rapidly developed to be one class of promising scintillators for X-ray imaging technology [15,16,17]. Among various lead-free halide perovskites, Cs₃Cu₂I₅ was considered as the most competitive candidate thin-film scintillator owing to its high light yield, strong stopping power, environmental stability and simple preparation process [18,19,20]. So far, intensive studies have been devoted to the development of high-performance Cs₃Cu₂I₅ scintillation film [21,22,23,24]. However, current reports predominantly focus on its X-ray imaging performance, and there are few reports on its detection capability for α particles [25,26,27,28,29]. This motivated us to conduct in-depth research on its α-particle detection performance. Herein, the facile in situ fabrication method was adopted to prepare the Cs₃Cu₂I₅@PMMA film. The scintillation performance of the thin-film for α-particle detection was reported, demonstrating a high light yield, linear energy response and moderate response time. Furthermore, its cryogenic scintillation performance towards α particles was first investigated, and the temperature-dependent light yield and decay time were determined.

2. Materials and Methods

2.1. Materials

Cuprous iodide (CuI, 99.9%), cesium iodide (CsI, 99.9%), dimethylformamide (DMF, 99.8%), dimethylsulfoxide (DMSO, ≥99.5%) and poly (methyl methacrylate) (PMMA, MW ~ 35,000) were purchased from Aladdin (Riverside, CA, USA). All raw chemicals were directly used without other purification.

2.2. Synthesis Methods of Cs₃Cu₂I₅@PMMA Scintillation Film

First, 0.234 g CsI, 0.114 g CuI and 1.4 g PMMA were dissolved in 3 mL N,N-dimethylformamide (DMF) to prepare the precursor solution. The proportion of the Cs₃Cu₂I₅ embedded in the PMMA was 20%. After magnetic stirring for one hour at 120 °C, the solution completed the reaction and became transparent. The prepared precursors with a certain viscosity were dropped onto the glass substrate and formed a uniform film via a facile spin-coating process. The pre-formed film was annealed at 90 °C to facilitate the crystallization of Cs₃Cu₂I₅ NCs. After solvent evaporation, a uniform and transparent Cs₃Cu₂I₅@PMMA film with a size of 5 cm × 5 cm × 40 μm can be acquired and torn off the glass substrate.

2.3. Methods for Evaluation

The XRD pattern was collected on an X-ray diffractometer meter (RIGAKU, Smartlab 9 kW, Beijing, China) with Cu Kα radiation (λ = 1.5406 Å). The XPS measurement was performed on an Escalab 250Xi (Thermo Fisher Scientific, Shanghai, China) with Al Kα as the X-ray source. The PL spectrum was obtained on a spectrometer (Edinburgh, FLS1000, Scotland, UK) under the 302 nm excitation. The UV–vis absorption spectrum was measured by using UV–Vis–NIR spectrophotometer (PerkinElmer, LAMBDA1050+, Springfield, IL, USA). The TRPL was performed on a spectrometer (Edinburgh, FLS1000) with a 300 nm pulsed diode laser (150 fs, 1 kHz). The temperature-dependent pulse height spectrum consists of a PMT (Hamamatsu CR173-Q1, Beijing, China), preamplifier (ORTEC 113, Beijing, China), spectroscopy amplifier (ORTEC 672) and multichannel analyzer (ORTEC ASPEC-927MCB). The temperature-dependent decay time measurement consists of a PMT (Enterprises Limited 9850, Sweetwater, TX, USA), oscilloscope (Tektronix TDS7104, Hong Kong, China) and cryocooler (Lihan, TC4188, Shenzhen, China).

2.4. Calculation

The range of the α particle was calculated by using Geant4. The Cs₃Cu₂I₅@PMMA thin-film model was established, featuring dimensions of 5 cm × 5 cm × 40 μm. The density of the film was 1.84 g/cm³. Stopping power: The calculation of -dE/dx was performed by the ComputeDEDXPerVolume() function directly through physical models. The film was divided into 5000 zones along its thickness. The range was obtained by counting the distribution of energy deposition within these zones. QGSP_BIC physical list was used and one million particles were emitted perpendicular to the surface of the Cs₃Cu₂I₅@PMMA film from a point source.

3. Results and Discussion

In our experiment, an in situ method was adopted to fabricate the Cs₃Cu₂I₅@PMMA scintillation film [30]. As shown in Figure 1a, the powder X-ray diffraction (XRD) pattern of Cs₃Cu₂I₅@PMMA matches well with the Cs₃Cu₂I₅ simulated data, which proved the crystallographic structure of the as-synthesized sample [31,32]. In addition, the X-ray photoelectron spectroscopy (XPS) analysis was implemented to ascertain the chemical composition and the valence state of constituent elements. The wide-sweep XPS spectrum (Figure 1b) confirmed the presence of the Cs, Cu and I peaks. Moreover, the high-resolution XPS spectra were employed to study their valence states as seen in Figure 1c,d. The peaks centered at 724.8 eV and 738.8 eV are the characteristic signals of Cs 3d, which proves the existence of Cs⁺. The Cu 2p doublet can be well fitted with Gaussian–Lorentzian mixed function. Two main peaks located at 932.4 eV and 952.1 eV are the characteristic peaks of Cu⁺ 2p_3/2 and Cu⁺ 2p_1/2. Another two peaks were resolved at 947.3 eV and 930.2 eV. The peak located at 947.3 eV can be ascribed to the satellite peak of Cu²⁺, indicating partial oxidation of trace Cu⁺ cations to Cu²⁺ [33]. One peak at 930.2 eV can be attributed to the existence a tetrahedral site of Cu⁺ in the sample [34]. Two peaks for I 3d at 619.8 eV and 631.1 eV were corresponding to I⁻. The above results can demonstrate the successful synthesis of Cs₃Cu₂I₅ @PMMA film by the in situ method [35].

Subsequently, we evaluate the optical properties of Cs₃Cu₂I₅@PMMA film. Figure 2b exhibits the photoluminescence (PL) and absorption spectra of Cs₃Cu₂I₅@PMMA film. The absorption peak and PL emission peak are centered at 325 nm and 442 nm, respectively, resulting in a Stokes shift of 117 nm. Such a large Stokes shift is beneficial for enhancing light output efficiency. The PL spectrum displays a broad emission band with a full width at half maximum (FWHM) of 80 nm, arising from the formation of self-trapped excitons (STE) emissions [10,36]. To investigate the luminescence kinetics, a time-resolved photoluminescence (TRPL) decay curve was measured. As depicted in Figure 2b, the TRPL decay profile of the Cs₃Cu₂I₅@PMMA film exhibits a nearly mono-exponential decay behavior, with a fitted lifetime of 1008 ns. This result is consistent with previous experimental observations [33,34].

We take a step to test the Cs₃Cu₂I₅@PMMA film for α-particle detection, including luminescence efficiency, response time and linear energy response. The range of α particle with 5.5 MeV in Cs₃Cu₂I₅@PMMA film was simulated as seen in Figure 3a. It is clearly shown that the penetration depth is merely 26 μm, which indicates thatα particles can be fully deposited in the prepared Cs₃Cu₂I₅@PMMA film. This effective energy deposition was ascribed to the large stopping power provided by the heavy inorganic component. The response time of Cs₃Cu₂I₅@PMMA film irradiated with a single-particle event is characterized by recording the pulse waveform forα particles from the ²⁴¹Am isotope source, as shown in Figure 3b. The decay time can be accurately fitted by a mono-exponential function, and it is determined to be 667 ns. It is noteworthy that the decay time of the Cs₃Cu₂I₅@PMMA film exhibits a marked acceleration when excited by alpha particles compared to laser excitation. This phenomenon can be attributed to the pronounced exciton–exciton quenching effect, which arises from the high ionization density characteristic of alpha particles [37]. To determine the luminescence efficiency forα particles, the relative light yield was obtained by measuring pulse height spectrum. Given that scintillation light yield is proportional to the channel of the peak, the variation in the light output of the crystal with temperature can be traced by monitoring the change in the peak position. Figure 3c displays the pulse height spectrum of Cs₃Cu₂I₅@PMMA film irradiated by 5.5 MeV α particles from ²⁴¹Am. For comparison, the pulse height spectrum of the LYSO:Ce (33,000 photons/MeV) scintillator under 662 keV γ-rays from ¹³⁷Cs is shown. The channel number of Cs₃Cu₂I₅@PMMA film and LYSO:Ce are 500 ± 3 and 304 ± 2, respectively. The photoelectron peak is observed for LYSO:Ce, whose light yield corresponds to 21,846 photons at 500 channels. Therefore, the peak channel of Cs₃Cu₂I₅@PMMA corresponds to 13,337 photons for 5.5 MeV α particles, yielding a light yield of ~2400 photons/MeV.

Table 1. Performance of α-particle scintillators.

Sample	Emission	Light Yield	Decay Time	Stability	Reference
1,1,4,4-tetraphenyl-1,3-butadiene	410–430 nm	882 ± 210 photons/MeV	11 ± 5 ns/275 ± 10 ns	yes	[9]
Plastic scintillators	400–450 nm	830 photons/MeV	2 ns/10 ns	yes	[11]
Li2CaSiO4	480 nm	21,600 photons/MeV	157 ns	yes	[38]
Cs3Cu2I5@PMMA film	442 nm	2400 photons/MeV	667 ns	yes	This paper

summarizes key characteristics of the Cs₃Cu₂I₅@PMMA film and common α-particle scintillators.

It is well known that the non-proportionality of energy has a significant impact on the light yield and energy resolution. The non-proportionality of α-particle energy was calibrated by using discrete ²⁴¹Am, ²³⁹Pu and ²³⁷Np α isotope source. As shown in Figure 3d, the channel number of peak centroid is proportional to the incident energy within the range of 4.7 MeV–5.5 MeV. This linear correlation proves that the film exhibits a favorable linear response to the energy of heavy charged particles.

Furthermore, we conducted an investigation into the cryogenic scintillation performance of the Cs₃Cu₂I₅@PMMA film in response to α particles across the temperature range from 50 K to 300 K. The temperature-dependent pulse height spectra are shown in Figure 4a. The channel number of detector output signal is proportional to the incident particle energy. As can be observed, the peak exhibits a shift towards higher channels with decreasing temperature, which implies an increase in its light output with the reduction in temperature. At 50 K, the light yield of the Cs₃Cu₂I₅@PMMA film is approximately 1.3 times higher than that at 300 K. The fundamental mechanism stems from the suppression of charge carrier thermal agitation in crystalline materials at cryogenic temperatures, directly reducing intrinsic thermal noise. This reduction in noise baseline enables the detection of previously noise-obscured weak signals, ultimately resulting in an enhancement in light yield [39]. In α-particle spectroscopy detection, energy resolution serves as a critical parameter for assessing a detection system’s ability to distinguish particles with varying energies. It is quantitatively defined as the ratio of the full width at half maximum of the full-energy peak to its centroid position. Figure 4b shows that the energy resolution gradually decreases as the temperature increases. The energy resolution improves as temperature decreases, rising from 28.8% at 300 K to 25% at 50 K. The improved energy resolution of the Cs₃Cu₂I₅@PMMA film at low temperatures stems from its elevated light yield, which curtails the influence of statistical fluctuations in the measurement. To further explore the scintillation kinetics of the Cs₃Cu₂I₅@PMMA film, the temperature-dependent scintillation decay curves were measured as seen in Figure 4c. The intensity of the waveform response gradually increases as the temperature decreases, which is consistent with the test results of the temperature-dependent pulse height spectra. The obtained decay curves are well fitted with the mono-exponential decay time function at all temperatures. As shown in Figure 4d, the fitted decay time shows no significant variation as the temperature decreases, maintaining a consistent value around 0.68 μs. The performance stability of the scintillator is vital for practical applications. We initially conducted an assessment of the low temperature stability of the Cs₃Cu₂I₅@PMMA film. Specifically, the film was subjected to testing in a vacuum environment at a temperature of 50 K for a duration of 10 days. Figure 4e shows that the normalized light yield excited by α particles remains the same, which confirms its excellent low-temperature stability. Moreover, the flexibility of the Cs₃Cu₂I₅@PMMA film is evaluated by using the bending tests. During the bending tests, the film remained unrelaxed, operating at a bending speed of 30 cycles per minute. As shown in Figure 4f, the Cs₃Cu₂I₅@PMMA film maintains more than 90% of its original light yield after 800 bending cycles with a bending radius down to 3 mm, which make the realization of flexible α-particle detector possible. The aforementioned stability tests have confirmed the feasibility of using the Cs₃Cu₂I₅@PMMA film as a low-temperature scintillator for α-particle measurement.

4. Conclusions

To summarize, we have demonstrated Cs₃Cu₂I₅@PMMA film as a novel, efficient and next-generation scintillator for the detection of α particles. The Cs₃Cu₂I₅@PMMA film, synthesized via an in situ method, exhibits favorable characteristics in response to α particles, including a high light yield, a linear energy response and a moderate decay time. These properties render it highly competitive compared with conventional scintillators. Furthermore, the Cs₃Cu₂I₅@PMMA film also exhibits excellent cryogenic scintillation performance and stability. Our finding offers a new option for the thin-film detector and opens a new avenue for the application of perovskite film materials as well.