A Review of Recent Improvements in Novel Liquid Scintillator Materials

Abstract

Liquid scintillator detectors have great advantages in the field of radionuclide detection because of their low detection limit, high sensitivity, and diverse functions. However, the material properties of liquid scintillators directly determine their detection effectiveness, which leads to their poor vertex resolution and particle identification. In this work, we introduce the improvement methods, choices, and properties of different novel liquid scintillator materials in recent years. This article is expected to provide references for the development and research of liquid scintillator materials in various application fields.

1. Introduction

Scintillator detectors are widely used in military, medical, and environmental fields [1]. The scintillator material, which determines the performance of scintillator detectors, is the core component of the detectors. Scintillators can be divided into two categories based on their chemical properties: inorganic scintillators and organic scintillators [2,3]. Compared with inorganic scintillators, organic scintillators have higher processing performance and controllability [4]. Organic scintillators are mainly aromatic hydrocarbons with a benzene ring structure, and they include organic crystal scintillators and liquid scintillators [5]. As the standard materials of organic crystal scintillators, organic crystals have relatively high fluorescence efficiency [6,7]. However, with the property of being difficult to grow, organic crystals are brittle and prone to damage, signifying their poor processability [8]. Since liquid scintillator materials can be detected in the 4π direction, they are mainly used in the field of low–activity monitoring [9]. The liquid scintillators usually consist of three parts: an organic solvent, a first scintillator, and a second scintillator. All of the scintillators have benzene rings. The mechanism of light emission of liquid scintillators is more complicated [10]. First of all, charged particles (electrons, ions, and π mesons) deposit most of the energy in organic solvents. Then, the organic solvents transfer the energy to the first scintillator through non–radiative energy transfer, and the energy is released in the form of light through the first scintillator. After that, the second scintillator moves the wavelength of the released light to an appropriate range, and the light is finally input. This process is mainly the process of interaction between a radionuclide and a liquid scintillator, generating ionization, excitation, attenuation, and fluorescence. The core process of luminescence is carried out by absorbing the particle energy of the π–π electronic structure in the liquid scintillator. During the deposition process, the excitation energy is transferred to the first scintillator, and fluorescence is emitted. Therefore, liquid scintillators have high flexibility in the selection, matching, and ratio of materials [11,12,13].

The liquid scintillator’s detection of particles can be traced back to 1911 to be used to detect alpha particles [14]. In 1947, after Broser and Kallmann detected a fast electron with a large–size, high–efficiency organic scintillator, the method of scintillation counting began to attract public attention [15]. Its main features are high detection efficiency and short resolution time. With the development of technology and the expansion of application fields, the performance of scintillator detectors and their devices have been continuously improved, and new scintillators have emerged. Because they are made of organic substances, liquid scintillators also have some shortcomings when they are applied, such as a low flash point, strong volatility, flammability, and an explosion hazard. Meanwhile, the organic substances in the formula of liquid scintillators are ecologically harmful to the environment, which also causes great harm. In general, a good scintillator should have the characteristics of high detection efficiency, high energy resolution, high light output, fast decay rate, and low afterglow. Scintillator detectors have a wide range of applications in medical, environmental, and physical sciences [16,17,18,19].

In recent years (after 2010), with the emergence of more solvents and surfactants, the research work on liquid scintillators has gradually developed towards high luminescence performance, high flash point, and low toxicity [18,20]. The main trends are to improve the light yield of liquid scintillators by doping chemicals and to increase the flash point and toxicity by replacing oil–based solvents with water–based solvents [21,22].

In this review, we will summarize the preparation method of liquid scintillators including the development of liquid scintillators over the past few years.

2. The Composition of Liquid Scintillators

2.1. The Solvent

Liquid scintillators mainly include solvents and scintillators, and the density is generally about 1 g/cm³. Among them, the solvent is the main component of the liquid scintillator, accounting for more than 99% of the entire liquid scintillator, and acts as a carrier and energy transfer for the solution, so the physical and chemical properties of the solvent have a decisive influence on the performance of the liquid scintillator.

A good solvent should have a good high flash point, high refractive index, optical transparency, long–term stability, and environmental friendliness. The high–energy particle irradiating the scintillator mainly deposits energy in the solvent and excites the electrons in the benzene ring structure of the solvent. The excitation energy is usually quickly transferred to the first scintillator through double dipole action. The first solvent is mainly responsible for the liquid carrier and energy conversion function. Given the first solvent’s low detection efficiency, adding the second solvent can greatly improve the detection efficiency.

Feng Zongyou reported that adding naphthalene to a dioxane solvent with lower energy conversion efficiency increased the relative detection efficiency from 40% to 80% [23]. This is mainly because the energy of the scintillator is transferred from the excimer molecules of the inferior solvent to the naphthalene before it is triggered; then, it is transferred to the scintillator molecules, which reduces the energy quenching of the inferior solvent and improves the energy conversion efficiency. In addition, the sensitivity of naphthalene to many quenchers is very low.

The commonly used liquid scintillator solvents mainly include toluene, xylene (p–Xylene, PX), Pseudocumene, LAB, Di–isopropylnaphthalene, and 1–phenyl–1–xylylethane, as shown in Figure 1 [15].

In the liquid scintillator, the solvent occupies more than 90% of the mass. Therefore, the purity and optical properties of the scintillator solvent greatly affect the sensitivity of the detector. The performance of the liquid scintillator increases as the purity of the solvent increases.

For large liquid scintillation detectors of tens to hundreds of tons, the solvent used is analytical–grade, while for liquid scintillation counters with small–volume solvents, the scintillation solvent used should be chromatographically pure. In the scintillation solvent, impurities have an important influence on the production of luminous efficiency [24]. Notably, the influence of solvent impurities on the scintillator mainly comes from three aspects: (1) the influence of impurities on the excited state lifetime of the scintillation solute; (2) the characteristic absorption of the light of special wavelengths by impurities; and (3) some special chemical reactions that impurities may catalyse and the resulting unpredictable results. Therefore, a high–purity solvent can greatly improve the performance of a liquid scintillator. In addition, the dissolved oxygen in the solution is the quencher, and its presence greatly reduces the performance of the scintillator.

2.2. The Solute

The fluorescence quantum efficiency of the solvent in the liquid scintillator is very low, and the luminescence time is very long. If a liquid scintillator only contains solvent, then a lot of electronic excitation energy will be released in a non–radiative manner, then there will be very little scintillation light available for detection, and the energy resolution of the scintillator will be poor. In previous researches, the emission spectra of common solvents are generally concentrated, and photomultiplier tubes have a very low flash point in this band, resulting in poor measurement accuracy. And because the solvent concentration is very high, the self–absorption of the scintillation light will be very serious. Therefore, it is necessary to add a solute to greatly increase the light yield of the liquid scintillator.

The solute of a liquid scintillator has a short fluorescence lifetime, a high fluorescence quantum yield, and high solubility. Generally, it is dissolved in the scintillator solvent (aromatic liquid) to make an organic liquid scintillator. In liquid scintillators, because the Stokes shift between the solvent absorption spectrum and the fluorescence spectrum is very small (usually only 10–20 nm), the solvent absorption–re–emission effect is very strong. Adding a scintillator can not only effectively increase the Stokes shift (~60 nm) to suppress absorption and re–emission, but also greatly improve the detection efficiency [25]. Considering the fluorescence characteristics and effects of the solute, the scintillation solute includes two types: the first scintillator and the second scintillator (wavelength conversion agent).

The concentration of the first scintillator is generally 1 g/L–5 g/L. Due to the low concentration, the self–absorption is small. The quantum efficiency of luminescent materials is generally relatively high (over 70%), and the common first–level scintillation solutes (primary scintillators) are 2,5–Diphenyloxazoze, 2–(4–biphenyl)–5–(4–tert–butylphenyl)–1,3,4–oxadiazole, and p–Terophenyl. The ultraviolet absorption wavelength is in the ultraviolet region (~300 nm), and the emission spectrum wavelength is about 365 nm.

Since the fluorescence spectrum of the first scintillator has not yet reached the sensitive wavelength (>400 nm) of the response range of the photomultiplier tube [26], the second scintillator can absorb the short–wavelength photons generated by the first scintillator and generate long–wavelength fluorescent photons that match the photomultiplier tube, thereby improving the quantum efficiency of the photomultiplier tube. The choice of wavelength shifting agent must also ensure that its absorption spectrum falls within the emission spectrum of the first scintillator. The concentration of the second luminescent substance is generally 10 mg/L–50 mg/L.

Since in the liquid scintillator, the concentration of the first scintillator is much higher than that of the second scintillator, and the emission spectrum of the first scintillator cannot be completely absorbed by the second scintillator, the emission spectrum is composed of the first scintillator and the second scintillator. Although the concentration of the second scintillator is very low, its emission spectrum accounts for 40% of the total emission spectrum [27].

The spectral characteristic of the solute is that the spectral response range of the photomultiplier tube is consistent with the fluorescence spectrum range of the liquid scintillator. Generally, the solute emission spectrum and its own absorption spectrum rarely overlap. Burks once proposed to use the spectral matching factor m to describe the spectral characteristics [28]. The definition of the figure of merit F of the scintillator solution is as follows (Equation (1)):

F = mfqs

(1)

where m is the spectral matching factor, f is the energy transfer efficiency of the solute and solvent, q is the fluorescence quantum yield of the solute, and s is the energy transfer factor of the solvent. It can be seen from the parameter m that F is related to the properties of the photomultiplier tube.

3. The Luminescence Mechanism of Liquid Scintillators

In particle physics, luminescent materials can be used to effectively detect charged particles. When the charged particles pass through luminescent material, the particles lose energy mainly through electromagnetic action. The electric field generated in the luminescent material ionizes or excites the solvent molecules to a medium level, and part of the energy is released in the form of photons during the de–excitation process. Luminescent materials used in detectors have the advantages of a fast response time, good pulse shape performance, and a linear relationship between energy deposition and luminous efficiency.

When the fluorescent substance absorbs energy (radiation energy, heat energy, photons, chemical reactions, etc.), the electrons transition from the ground state to the excited state. Because the molecules in the excited state are unstable, the molecules in the excited state are quickly de–excited from the excited state to the ground state when they generate ultraviolet light and visible light (about 10⁻⁹ ns). The energy difference between the two energy levels is the energy of the fluorescent photon.

This section mainly describes the electronic structure of molecules, the luminescence principle of liquid scintillators, the types of molecular luminescence, and the relationship between fluorescence and molecular structure.

3.1. The Electronic Structure of Molecules

Due to the different shapes of atomic orbitals, their overlapping methods are also different. According to different overlapping methods, covalent bonds can be divided into σ bonds and π bonds. As shown in Figure 2, σ bonds are formed by overlapping atomic orbitals with the same sign in the direction of the internuclear link, that is, the “head–to–head” method, including s–s track, s–p_x track, and p_x–p_x track. The π bond is formed when two atomic orbitals are perpendicular to the nucleus and the lines are parallel to each other, that is, “side by side”.

There are three main structures of molecules. The first is the sp hybrid orbital. For example, the electronic structure of BeCl₂ and Be atoms is 1s²2s². Figure 3 shows that when a covalent bond is formed, a 2s electron is excited to an empty 2p orbital. At the same time, an s orbital and two p orbitals j combine to form two new hybrid orbitals, and each new hybrid orbital contains 0.5s and 0.5p components. The hybrid orbitals are on the same straight line and form 180° with each other.

In aromatic organic compounds, the p orbitals in the C atom interact to form a π bond, and the stability of the π bond is very weak. When the excited π orbital absorbs energy and then transitions from the excited state to the ground state, fluorescent photons are generated.

The second is the sp² hybrid orbital and Figure 4 shows the formation of the sp². For example, in BF3, the electronic structure of the B atom is 1s²2s¹2p¹. When the bond is formed, a 2s electron is excited to an empty 2p orbital. At the same time, an s orbital and two p orbitals combine to form three new hybrid orbitals, and each new hybrid orbital contains components of 0.33s and 0.66p. The three hybrid orbitals are in the same plane and form an angle of 120° with each other.

The third type is sp3 hybridization. For example, the electronic structure of a CH₄, C atom is 1s²2s¹2p³; that is, there are two unpaired p electrons. When the C atom is excited, an electron in the outer layer of the atom is excited from the 2s orbit to an empty orbit of 2p, and the s orbital combines with three p orbitals to form four new orbitals. Each new orbital contains 0.25s and 0.75p components. According to theoretical calculations, the angles of the four hybrid orbitals are all 110°. Each hybrid orbital has a certain extension direction in space. When forming a bond, the electron cloud must overlap along this specific direction to obtain the maximum overlap and form a stable covalent bond. Therefore, this σ bond is very stable and cannot emit fluorescent photons.

There are six extranuclear electrons in a carbon atom. The state of existence and energy of the carbon atom depends on the movement state and energy level of these electrons. When the energy of the carbon atom is in the ground state, there are two 1s electrons in the inner layer and two 2s electrons and two 2p electrons in the outer layer, and the four outer electrons are called the valence electrons of the carbon atom. The ground state of the carbon atom can be expressed as 1s²2s²2p². The interaction between atoms is the process of the superposition and reorientation of the valence electron motion states between atoms as far as possible to ensure the maximum overlap between the orbitals, thereby forming a more stable and lower energy covalent bond.

As shown in Figure 5, three types of carbon atoms are sp hybridization, sp² hybridization, and sp³ hybridization, because sp and sp² hybridization are the basis for the formation of π electrons and the fluorescence of organic molecules. When sp² hybridization occurs, two 2p orbitals in the carbon atom and one 2s orbital form three sp² hybrid orbitals; a C–C bond is formed by two carbon atoms through the sp² hybrid orbital; and four C–H bonds are formed by the rest. Each of the two sp² hybrid orbitals is composed of the 1s orbital of the hydrogen atom, so that the two carbon atoms and the four chlorine atoms are on the same plane, which can effectively reduce the electron repulsion effect. The π bond is formed by overlapping the remaining 2p orbitals of two carbon atoms in the direction parallel to the bond axis.

3.2. The Luminescence Principle of Liquid Scintillators

The interaction between the radionuclide and the liquid scintillator detector will cause the liquid scintillator to ionize, excite, de–excite, and emit fluorescence. Liquid scintillator detectors use this property to detect radionuclides. A liquid scintillator is mainly composed of a scintillator, light guide, photomultiplier tube, voltage divider, and signal output. The working process of liquid scintillator detectors mainly goes through the following stages [29]:

(1)

When incident particles enter the liquid scintillator medium, they will lose part or all of their energy to excite or ionize atoms or molecules in the scintillator.

(2)

Excited atoms and molecules produce fluorescent photons during the de–excitation process.

(3)

Fluorescent photons can be reflected by the emission layer and then transmitted to the photocathode of the photomultiplier tube through the optical fibre. Due to the influence of the photoelectric effect, the photocathode absorbs photons and emits photoelectrons.

(4)

The photoelectrons multiply between the dynodes of the photomultiplier tube and finally reach the anode to form a voltage or current pulse.

In organic liquid scintillators, both solvents and solutes are aromatic hydrocarbon compounds with a benzene ring and heterocyclic structure characteristics. In the ring structure, each atom combines with neighbouring atoms through a limited σ electron. At the same time, there are non–limiting π–electron covalent bonds. Liquid scintillator photons mainly come from the excitation of π electrons.

The multiple states of electronic excited states in an atom are represented by 2S + 1; S represents the algebraic sum of the electron spin angular momentum quantum numbers, and S is equal to 0 or 1. The two electrons occupied in the same orbital in a molecule must have opposite spin pairing; that is, all electrons in the molecule are spin paired. When the spin direction of the electron changes during the transition, the molecule has two electrons with unpaired spins, that is, S = 1, and the molecule is in the triplet state (T_n, when n = 0, it is in the ground state, and when n > 1, it is in the excited state); when the electron spin direction does not change during the transition after the molecule absorbs energy, that is, S = 0, the molecule is in a singlet state (S_n, n = 0 is the ground state, and n>1 is the excited state).

The properties of the excited molecules are unstable, and they are usually de–excited to the ground state by means of non–radiative transitions and radiation transitions. The non–radiative transition decay process includes relaxation (VR), intersystem crossing (ISC), and internal conversion (IC). Vibration relaxation is the process in which molecules transfer the remaining vibrational energy to the medium and decay to the lowest vibrational energy level of the same electronic excited state. Intersystem crossing refers to the non–radiative transition between two electronic states of different multiple states (e.g., S₁ to T₀, T₁ to S₀). Internal transformation refers to the radiation–free transition process between two electronic states in the same multiple state (e.g., S₁ to S₀, T₁ to T₀).

The decay process of the radiation transition produces photons, that is, fluorescence or phosphorescence: Fluorescence originates from Sn by allowing rotational transitions, and photons of a few ns are mainly generated by fast components. On the contrary, the rotational radiation transition is very few between the triplet state and the singlet state, and the corresponding emission of light exceeding the millisecond level is called phosphorescence.

The internal conversion rate of S_n between excited singlets is usually very fast (about 10¹¹–10¹³ s⁻¹), and the lifetime of an excited singlet S_n is usually very short (about 10⁻¹¹–10⁻¹³ s⁻¹). Usually, the molecule has undergone a non–radiative transition and decays to the S₁ state before the radiation transition occurs. Therefore, fluorescence is usually the radiation transition of the lowest vibrational energy level of the S₁ state.

The inter–system crossing process and the rate of internal conversion are related to the energy level difference between the lowest vibrational energy levels of the two electronic states, and the conversion rate decreases with the increase in the energy level difference [30]. The energy level difference between the lowest vibrational energy levels of the S₀ and S₁ states is larger than the energy level difference of the other two adjacent excited singlet states, so the internal conversion rate constant from S₁ deexcitation to S₀ is relatively small (about 10⁶–10¹² s⁻¹). In the same way, the transition rate constant between the T₁ transition and the S₀ system is also relatively small (about 10²–10⁵ s⁻¹). In summary, fluorescent photons are mainly produced by the radiation transition from S₁ de–excitation to S₀.

3.3. The Type of Luminescence of Molecules

Molecules mainly have two types of luminescence: excited mode and molecular excited state type. As far as the excitation mode is concerned, the molecular excitation energy comes from the energy or chemical energy released by the organism, and the generated fluorescence is called bioluminescence or chemiluminescence. The luminescence produced when molecules absorb radiant energy is called photoluminescence [31].

According to the type of molecular excited state, the lowest electron excited triplet state T₁ returns to the ground state S₀ through the radiation transition and releases the photon, which is called phosphorescence. The light produced by the lowest electron–excited singlet state S₁ through the radiative transition is called fluorescence.

There are mainly two types of fluorescence: transient fluorescence and delayed fluorescence. Transient fluorescence is formed by the de–excitation of the lowest vibrational energy level of the lowest excited state S₁ to any vibrational energy level of the S₀ ground state. In addition, transient fluorescence will also be generated by the excited dimer formed by the low–excited state molecule S₁ and its ground state molecule S₀, but this situation usually occurs in high–concentration solutions.

Phosphorescence and delayed fluorescence exist in some viscous or planar rigid substances. The fluorescence spectrum waveform of delayed fluorescence is similar to that of transient fluorescence. The only difference is that the lifetime of delayed fluorescence is much longer than that of transient fluorescence. There are mainly the following three types of delayed fluorescence: (1) P–delayed fluorescence; (2) E–delayed fluorescence; and (3) composite fluorescence.

3.4. The Relationship between Fluorescence and Molecular Structure

The luminescence properties of fluorescent materials are closely related to their molecular structure. In order to grasp the relationship between fluorescence and its structure, we first need to grasp the type of molecular transition and then grasp the luminescence process and its mechanism.

After the molecule absorbs energy and is excited to the electronic excited state, it can be de–excited to the electronic ground state in the following series: the formation of fluorescent photons, non–radiative transition processes, and photochemical reactions. The de–excitation method is mainly determined by the maximum rate constant in these three processes. According to literature reports, it is found that strong fluorescent substances generally exhibit the following characteristics: (1) rigid plane structure in the molecule; (2) there is a large conjugated π bond structure in the molecule; (3) the substituents are electron–donating substituents; and (4) lowest singlet excited state.

Conjugated double bonds (π bonds) usually exist in the molecular structure of fluorescent substances, and the fluorescence intensity increases as the conjugated system increases. Therefore, heterocyclic or aromatic ring structures often exist in luminescent materials. The peak position of the emission spectrum is red–shifted with the increase in the conjugated structure, and the fluorescence intensity increases with the increase in the co–rolling structure. The number of conjugated rings is the same, and the emission spectrum wavelength of the nonlinear aromatic ring is smaller than the emission spectrum wavelength of the linear aromatic ring.

4. Solute–Solvent Energy Transfer Efficiency of Liquid Scintillators

4.1. Mechanism of Solute–Solvent Energy Transfer Efficiency

As shown in Figure 6, when the charged particles pass through the liquid scintillator, the scintillator solvent absorbs radiation energy, ionizes or excites the π electrons in the benzene ring structure, and excites the singlet S_ns through the non–radiative transition process (vibration relaxation and internal conversion process). Decaying to S_1s, this process mainly leads to the photon quenching process. The decline from S_1s to S_0s is mainly through the following processes: radiation transition κ_fs, internal conversion process κ_is without radiation transition, energy collision with surrounding solvent molecules κ_tss, and energy collision with solute molecules κ_tsv.

The energy transfer process between the solvent and the solute through the double dipole action produces the double dipole transfer rate equation as follows [32]:

$${\mathrm{K}}_{{\mathrm{S}}_{1\mathrm{s}}^{\mathrm{*}}\to {\mathrm{S}}_{1\mathrm{v}}^{\mathrm{*}}}=\frac{1}{{\tau }_{\mathrm{s}}}{\left(\frac{{\mathrm{R}}_0}{\mathrm{R}}\right)}^6$$

(2)

$${\mathrm{R}}_0^6=\frac{9000\mathrm{l}\mathrm{n}10{\mathrm{x}}^2{\mathsf{\eta }}_{\mathrm{s}}^0}{128{\mathsf{\pi }}^6\mathrm{n}4\mathrm{N}{\mathrm{v}}^4}{\int }_0^{\mathrm{\infty }}{\mathrm{f}}_{\mathrm{s}}\left(\mathrm{v}\right)\mathsf{\epsilon }\left(\mathrm{v}\right)\mathrm{d}\mathrm{v}$$

(3)

where τ_s is the actual average distance of the excitation solvent, the intrinsic lifetime of the excitation solvent and the fluorescence quantum yield τ_s = τ_s⁰*η_s⁰ without double dipole interaction between the solvent, R₀ is the critical distance for double dipole interaction, R is the collision distance between the solvent and the solute, v is the wave number, ε(v) is the molar extinction coefficient, f_s(v) is the distribution function of the peak spectrum, n is the refractive index, and N is Avogadro’s constant. The energy transfer mechanism between the first scintillator and the second scintillator is the same as that of the solvent first scintillator.

Figure 7 is a schematic diagram of the energy transfer kinetics process of the solvent and the solute in the liquid scintillator. In the liquid scintillator, assuming that the excited solvent S is generated at the rate of R_S, the rate of the excited solvent transfer to the scintillator through the double dipole action is C_v; it generates self–quenching at the rate of C_q and generates the internal solvent at the rate of r_f. The conversion effect and the rate of r_f emit photons. That is, energy is lost at the rate of r_f. The rate equation of the solvent is as follows:

$$\frac{\mathrm{d}{\mathrm{C}}_{{\mathrm{S}}^{\mathrm{*}}}}{\mathrm{d}\mathrm{t}}={\mathrm{R}}_{{\mathrm{S}}^{\mathrm{*}}}-{\mathrm{r}}_{\mathrm{t}}{\mathrm{C}}_{\mathrm{v}}-{\mathrm{r}}_{\mathrm{q}}{\mathrm{C}}_{{\mathrm{S}}^{\mathrm{*}}}{\mathrm{C}}_{\mathrm{Q}}-{\mathrm{r}}_{\mathrm{f}}{\mathrm{C}}_{{\mathrm{S}}^{\mathrm{*}}}$$

(4)

For the solute, in the low concentration range, the reaction rate increases with the increase in the solute concentration. When the concentration increases to a certain value, self–absorption (V) will occur. The rate equation of the solute is as follows:

$$\frac{\mathrm{d}{\mathrm{C}}_{{\mathrm{v}}^{\mathrm{*}}}}{\mathrm{d}\mathrm{t}}={{\mathrm{r}}_{\mathrm{t}}{\mathrm{C}}_{\mathrm{v}}\mathrm{C}}_{{\mathrm{S}}^{\mathrm{*}}}-{\mathrm{r}}_{\mathrm{e}}{\mathrm{C}}_{{\mathrm{v}}^{\mathrm{*}}}-{\mathrm{r}}_{\mathrm{s}\mathrm{q}}{\mathrm{C}}_{{\mathrm{v}}^{\mathrm{*}}}{\mathrm{C}}_{\mathrm{v}}$$

(5)

4.2. Influence Factors of Solute–Solvent Energy Transfer

The energy transfer mechanism diagram and the energy transfer dynamic process show that the liquid scintillator has the following supply and demand relationships in the energy transfer process: particle–solvent, solvent–first scintillator, first scintillator–wavelength conversion agent. The quenching effect is an important factor affecting the relationship between energy supply and demand [33]. Fluorescence quenching refers to a physical and chemical process between fluorescent molecules and solvents, solutes, or quencher molecules, resulting in a decrease in fluorescence intensity. Fluorescence quenching mainly includes phase quenching, ionization quenching, chemical quenching, etc.

4.2.1. Phase Quenching

The quenching before fluorescence is called phase quenching, and it usually occurs before the fluorescent photon. Since the liquid scintillator is mainly distributed in the container in a homogeneous form, when the incident particles transfer the radiant energy to the solvent, they may absorb other substances (dust and small water droplets in the air), causing the energy of incident particles to decrease, reducing the number of fluorescent photons produced.

4.2.2. Concentration Quenching

In the low concentration range, the luminous efficiency of the liquid scintillator increases as the concentration of the scintillator increases. However, when the concentration exceeds a certain value, the luminous efficiency will decrease as the concentration increases. This phenomenon is called the concentration quenching effect. Concentration quenching includes two types, including the self–absorption of photons. Since the absorption spectrum of the fluorescent substance overlaps with the fluorescence spectrum to a certain extent, the short–wavelength photons generated by the fluorescent substance that overlap with the absorption spectrum will be absorbed by itself.

The self−absorption will increase with the increase in the concentration; when the concentration reaches a certain level, the excited−state molecules of the solute and the ground−state molecules form a dimer. Non−luminescent dimers cause fluorescence quenching effects. In the liquid scintillator, the energy transfer between the solvent and the scintillator substance is a double dipole non−radiative transition. Due to the short time, it is almost difficult to form dimers between solvent molecules. The dimer can only exist in the scintillator; the concentration of the first scintillator is a few g/L, and the concentration of the second scintillator is tens of mg/L. Therefore, the concentration quenching effect in the liquid scintillator is mainly produced by the first scintillator.

4.2.3. Chemical Quenching

Chemical quenching usually occurs when photons are generated. In the process of solvent–solute energy transfer, some impurity substances form a competitive relationship with scintillator molecules and form a supply–demand relationship with excited solvent molecules. Some impurities also form complexes with scintillator molecules, thereby effectively reducing the scintillator molecules. Concentration results in a decrease in the yield of fluorescent light; this phenomenon is called chemical quenching. In addition, oxygen in the air is a strong quencher, so oxygen must be removed during the preparation and testing process, and nitrogen is often passed through the solution to remove dissolved oxygen in the solution [34].

5. The Research on Improving the Liquid Scintillators

The improvement of the performance of liquid scintillators can usually be studied from the three perspectives of luminous performance, particle identification performance, and neutrino detection performance. We will summarize the improvement method of liquid scintillators in the following section.

5.1. Improving Luminous Performance of Liquid Scintillators

The relatively low light yield of liquid scintillators compared to solid scintillators results in the limited capability of liquid scintillators for dosimetry and radiographic imaging. Doucet et al. studied the ability of chemical substances to change the emission wavelength of liquid scintillators earlier. As shown in Figure 8a, they used the Hamamatsu R1635P with a bialkali photocathode (PMT) to research parameters such as the light yield, diameter, fibre position and the reflecting surface of different doped substances, thus summarising the key points for improving the light output of liquid scintillators [35]. Wang et al. proved that the doping of quantum dots to liquid scintillators could improve their luminescent properties by changing the parameters of ZnS quantum dot distribution and particle size in liquid scintillators [36]. From Figure 8b, one can see that the doping of quantum could enhance the α value compared to pure polyvinyl alcohol.

Winslow et al. (2012) characterized the light yield of liquid scintillation doped with quantum dots and conducted neutrino experiments. As shown in Figure 9a, they used a quantum–dot–doped scintillator instead of the plain liquid scintillator in KamLAND’s kiloton and Large Area Picosecond Photo Detectors (LAPPDs) to search for neutrinoless double beta decay. The characterization results showed that quantum dots would reduce the total light output of the scintillator, but it would not exceed the level of the double β decay experiment. In the next step, they further studied the characteristics of liquid scintillators doped with quantum dots. The results showed that larger sample size makes it possible to study the attenuation length, which was a crucial measurement for large detectors [37]. Bignell et al. (2013) grafted a third fluorophore component on the surface of the silver–core silicon shell nanoparticles, as shown in Figure 9b. By appropriately selecting the plasmon energy and scintillation fluorescence, the luminescence of the liquid scintillator was improved [38].

Aberle et al. (2013) studied the optical properties of three quantum dot–doped liquid scintillators for the precipitation and fluorescence yield of separated quantum dots in solution. The results showed that the luminescence performance of the liquid scintillator doped with three quantum dots was greatly improved [41]. Xia et al. (2014) studied the effect of temperature on the light yield of an LAB liquid scintillator and mesitylene liquid scintillator. Figure 10 shows the ADC distribution shift of the measurement at different temperatures. These results indicated that when the temperature dropped from 26 °C to −40 °C, the light output of each liquid scintillator was increased by 23%, thereby correcting the temperature response of the photomultiplier tube. Lowering the operating temperature of the detector to approximately 4 °C could increase its photoelectron yield by 13% [42].

To improve organic solvent liquid scintillators, He et al. optimized liquid scintillators in terms of solvent selection. PX (p–xylene) and 2,5–diphenyloxazol and p–bis(o–methylatyryl)–benzene were selected as organic solvents and fluorescents, respectively, for the liquid scintillators. The optimized liquid scintillators showed excellent properties such as high reflectivity, good transparency in the wavelength range of the fluorescence emission spectrum, and short decay time [43]. Zheng et al. (2014) used a mixed solvent of trimethylene and linear alkylbenzene as the liquid scintillator solvent to prepare a liquid scintillator with high light output, large decay length, fast fluorescence decay time, and low chemical activity. The optimal ratio was studied, and it was found that the overall performance of the liquid scintillator was the best under the mixed solvent of 20% PC and 80% LAB. In 2017, according to this conclusion, Zheng selected a combination of 20% volume of pseudocumene (PC) and 80% volume of linear alkylbenzene (LAB) as a mixed solvent with the main fluorescent agents of 2,5–diphenyloxazole (PPO) and wavelength transfer agents of 4–bis(2–methylstyryl) benzene (bis–MSB) to obtain the best light emission prescription and prepare a new liquid scintillator based on mixed solvents, which achieved good overall performance.

Cho et al. prepared novel liquid scintillators by the surface hybridization of colloidal metal halide–encapsulated CsPbA3 (A: Cl, Br, I) nanocrystals (NCs) with an organic molecule (2,5–diphenyl oxazole). Compared with other novel liquid scintillators, the liquid scintillator based on CsPbBr3 hybrid NCs shows a very high competitive emission quantum yield, which dramatically improves the disadvantage of liquid scintillators with relatively low light yield [44]. As shown in Figure 11a, Ye et al. (2015) researched the influence of PPO (fluorine) and bis–MSB (wavelength shifter) on the light output of a liquid scintillator and obtained the concentration of bis–MSB when the concentration of PPO was higher than 4 g/L. It could also be concluded that the influence of the two substances on the luminescence of the liquid scintillator was not obvious when above 8 mg/L [45]. Buck et al. (2015) used 2,5–diphenyloxazole (PPO) and 4–bis–(2–methylstyryl) benzene (bis–MSB) as the primary and secondary wavelength transfer agents for liquid scintillation, and a new measurement of the quantum yield of various aromatic molecules is presented in Figure 11b [46].

Delage et al. (2016) developed a liquid scintillator based on cQD and studied the response of the scintillator under X–ray radiation. For the beam energy of 180 kVp, it was found that most of the signal came from cQD, not the solvent. Even when using an ultra–low concentration (μM), cQD could emit enough light, which fully proved the improvement effect of quantum dots on the luminous performance of the liquid scintillator [47]. Tam et al. (2018) studied the doping and luminescence characteristics of liquid scintillators by using ZnS quantum dots. They also dispersed 2,5–diphenyloxazole (PPO) and cadmium sulphide (CdS) quantum dots (QD) emitting at 418 nm coated with oleic acid in a matrix medium of polyvinyl toluene (PVT), and they confirmed by photomultiplier device probing that the mixtures exhibited a 50% increase in light yield at larger wavelengths [48]. Graham et al. (2019) also performed similar work. They prepared a liquid scintillator doped with lead–based perovskite nanocrystal and applied it to the neutrino experiment, proving that this method was feasible to improve the luminous efficiency of a liquid scintillator, as shown in Figure 12 [49].

In general, the luminescence performance of the liquid scintillator is related to its energy conversion efficiency of the liquid scintillator after absorbing the radiation, which also directly determines the final count when it is detected. In recent years, researchers have done a lot of work to change the luminous efficiency and luminous wavelength of the liquid scintillator. Reasonably selecting the ratio and modification method can better enhance the light emitting performance of the liquid scintillator, which is a promising way to improve the measurement count in various measurement environments.

5.2. Improving the Particle Identification Performance of Liquid Scintillators

In the past few years, liquid scintillation technology for particle detection applications has made major technological breakthroughs. Liquid organic scintillators have been used for rapid neutron detection due to their rapid response and ability for pulse shape discrimination (PSD) to distinguish the interaction between neutrons and gamma rays. In addition to the above characteristics, the deuterated liquid organic scintillator also has a structure in its pulse height spectrum due to the different characteristics of n–d and n–p scattering.

Lombardi et al. (2013) discriminated measurements of the fluorescence decay time and pulse shape of liquid scintillators manufactured by two methods, namely linear alkylbenzene (LAB) and diisopropylnaphthalene (DIN). The measurement was carried out under the excitation of particles or ultraviolet light of the scintillation solution, which could reveal the characteristics of the fast component and the long tail. The experimental results showed that the PSD properties of LAB and DIN based scintillators did not change much with the increase in PPO content, indicating that the PSD properties are essentially dictated by the solvent features but not by the solute features. Forming the entire scintillation pulse, they are crucial for applications focusing on the increasingly important field of low background detection [49].

Stevanato et al. (2012) used a fast digitizer to compare the neutron–gamma resolution capabilities of the new liquid scintillator EJ–309 with the standard EJ–301 [50]. The abilities of two scintillation detectors to identify weak neutron sources were demonstrated in the background of high gamma rays. In the background of gamma rays corresponding to a dose rate of 100 mSv/h, at a 95% confidence level, the probability of neutron detection was PD 1⁄495%. Bildstein et al. (2013) reported a direct comparison of two small–volume liquid scintillation detectors with identical geometries (diameter 11.43 cm, thickness 2.54 cm) [51]. The first detector was based on NE213 produced by Nuclear Enterprise, and the second was based on EJ–315 produced by Eljen Technologies, which is a deuterated scintillator. The results showed that although the light output of the deuterated detector was lower than that of the conventional non–deuterated detector, it was possible to detect 60 keV neutrons. The PSD function of the deuterated detector matched and exceeded the NE213 detector. When the neutron energy was higher than 2 MeV, the efficiency of the deuterated detector was comparable to the NE213 detector. Below this energy, its relative efficiency was affected by the significantly lower n–d scattering cross–section. For very low neutron energies (200 keV), due to the low noise level of the specific detector, the relative efficiency of the deuterated detector was improved again, which showed the importance of low photomultiplier tube noise for detecting low energy neutrons, as shown in Figure 13a [52]. Bourne et al. (2013) developed a liquid scintillator with particle discrimination capability, a 5.08 × 5.08 cm stilbene crystal grown when exposed to a gamma–ray neutron ratio of 1000 to 1, which operated at a 100 kHz count rate with PSD capability. Its capability of PSD was performed at a threshold of 42 keV (440-keV protons), and for the EJ–309 liquid scintillator, at a threshold of 60 keVee (610-keV protons). The lowest stilbene threshold led to intrinsic neutron efficiency. In high gamma fields, the concentration of bare 252Cf was approximately 14.5%, which was 10% higher than that of the EJ–309 liquid scintillator, and the concentration of 252Cf was 13%. Despite the lower threshold, the γ misclassification rate in stilbene was about 3106, which was nearly five–fifths lower than the value of the equivalent EJ–309 liquid scintillation detector as shown in Figure 13b [53]. As shown in Figure 13c, Becchetti et al. (2016) designed, constructed, and evaluated a three–inch deuterated xylene organic liquid scintillator (C8D10; EJ301D) as a fast neutron detector. This scintillator could provide a good pulse shape distinction between neutrons and gamma rays, which also had good timing characteristics and could provide a spectrum with peaks corresponding to discrete neutron energy levels up to ca. 20 MeV. Unlike benzene–based detectors, the deuterated xylene organic liquid scintillator is less volatile, less toxic, non–carcinogenic, and has a higher flash point. Therefore, it is safer for many applications. In addition, compared with the deuterated benzene scintillator, EJ301D could provide more light output and better PSD. Like the deuterated benzene scintillator, the light response spectrum of the deuterated xylene organic liquid scintillator could be expanded to a useful neutron energy spectrum without time of flight (ToF). A set of these detectors arranged at many angles close to the reaction target was much more effective than a long–distance ToF detector array that must use narrow beams and pulse–selective beams. Therefore, when beam–shaped and pulse–selective beams (time of flight required) are not available, the measurements above can still be performed [54].

In recent years, water–based liquid scintillators (WbLS) exploiting Cherenkov–to–scintillation showed low cost and environmentally friendly characteristics. In addition, WbLS can also improve the discrimination of alpha/beta particles after adopting a discrimination strategy based on the particle–dependent Cherenkov/scintillation light ratio, and may recognise beta/gamma particles to a certain extent [55]. The opaque scintillator was a new concept of scintillators, which reduced the scattering length in the scintillator to the cm level and below, and collected light in opaque scintillator material close to the interaction point of the ionizing radiation using optical fibres [56]. This scintillator could perform detailed particle identification using the topological information. Buck et al. (2019) prepared a new opaque scintillator by adding paraffin wax to a mixture of LAB and PPO, and named it NoWaSH (New opaque Wax Scintillator, Heidelberg) [57]. Compared to standard transparent liquid scintillators, NoWaSH could tune optical properties by temperature adjustment (NoWaSH was a colourless transparent liquid at around 40 °C, and had a milky wax structure below 20 °C), and showed higher spatial resolution, better capabilities for particle identification and reduced constraints on the absorption properties.

In summary, the ability of liquid scintillators to discriminate particles is well used in the detection of alpha, beta, gamma, and n particles, such as the detection of radionuclides in water and neutron measurement, which are easily interfered with by pulses. In the experiment, although the research on particle screening has become more mature in recent years, there are relatively few studies on the material to improve the luminescence performance of liquid scintillators. Moreover, liquid scintillator materials are the fundamental measurement due to the alternative. There are thousands of organic solvents and surfactants, so choosing a suitable ratio to improve the ability of liquid scintillators to distinguish particles from the material level is also a promising research task.

5.3. Improving Neutrino Detection Performance of Liquid Scintillators

Neutrinos have been a research topic in the international frontier field in recent years. Because neutrinos are not charged, they are difficult to react with other substances. Therefore, large–volume liquid scintillator detectors are generally used to increase the probability of neutrino capture. As many as tens of thousands of tons of liquid scintillators may be required in large equipment, so the low toxicity, safety performance, and environmental protection performance of liquid scintillators are put forward to higher requirements. Grieb et al. (2011) made a major breakthrough in liquid scintillator chemistry and background suppression [58]. A new type of liquid scintillator detector, the “scintillation grid”, could provide extraordinary spatial resolution in a large number of liquid scintillators by segmentation instead of time–of–flight information, so that the time/space coincidence label could be used to properly remove the background. Numerous Monte Carlo studies have determined the feasibility of using the LENS detector (detailed presentations on LENS can be found in [59]) with less than 200 tons of scintillators. Sun et al. (2018) synthesized ionic liquid 1–silicyl–3–benzylimidazolehexafluorophosphate ([SBnIM]PF6) by neutralizing chemical reactions. Subsequently, the [SBnIM]PF6 was applied as a scintillation solvent by solvent–solvent extraction under controlled pH to analyse polycyclic aromatic hydrocarbons (PAHs) in water. The new type of liquid scintillator has good optical properties and performs high–precision analyses of PAHs over a wide range of 0.5 µg·L^–1 to 1000 µg·L^–1 [60]. In Figure 14a, Bass et al. (2013) proposed a liquid scintillator that could be used for neutrino detection pairs, which was produced by mixing a concentrated lithium chloride aqueous solution with 0.40% Li. The optical properties and neutron response capabilities of this scintillator were carried out by tests and the light output spectra of 2.5 MeV, 14.1 MeV, and 252Cf neutrons were measured. The spectrometer made thermal neutron measurements without coincidence. The possible improvement of the performance of the spectrometer was discussed [61]. Enqvist et al. (2013) studied the liquid scintillator that could be used to measure neutrinos on the tandem van der Graff generator of the University of Ohio for the neutron light output response function and detector resolution function. A shapeable EJ–309 was used to project a 7.44 MeV deuterium beam on a 27Al target to produce a continuous spectrum in the energy range from a few hundred keV to over 10 MeV. Exponential fitting was used to determine the light output response function. For the 12.7 × 12.7 and 7.6 × 7.6 cm detectors, the resolution functions were obtained. It turned out that even if the photomultiplier tubes, detector materials, and other detector characteristics were carefully matched, the dependence on the detector size was also important for the light output response function. However, the effect on the resolution function was not the same, as shown in Figure 14b [62]. Chris et al. (2013) proposed a three–by–two–inch measurement neutron response matrix. A cylindrical EJ315 detector was compared with a hydrogen–based liquid EJ309 in other identical components, such as the light output relationship. The measurement used a continuous spectrum neutron source bombarded with 11 MeB by 5.5 MeV deuterons, and the detected neutrons were divided into 100 keV energy groups with an energy range of 0.5 to 15 MeV by time of flight. Matrix conditional analysis showed that EJ315 was superior to EJ309 in terms of the number of independent parameters available in the inversion procedure as shown in Figure 14c [63]. Fischer et al. (2020) investigated the deployment of medium–sized neutrino detectors near the Akkuyu nuclear power plant, Turkey’s first nuclear power plant. A water–based liquid scintillator (WbLS) was applied for the instantaneous monitoring of the Akyouyu nuclear power plant through its anti–neutrino flux. In addition to its physical and technical goals, this will also provide a valuable opportunity for the development of the next generation of particle detectors in the field of neutrino physics for the Turkish nuclear and particle physics community [64].

6. The Improvement of Liquid Scintillator Materials in the Environmental Field

Due to the flammability and high vapour pressure of conventional organic scintillator solvents, increasing the safety and environmental friendliness of liquid scintillator materials is also an important research direction. Yeh et al. (2011) applied water–based solvents instead of organic solvents to prepare a new type of WbLS, which is undoubtedly low–cost and environmentally friendly [65]. In addition, the experiment results showed that 10% WbLS emits scintillation light through the ionization process with a yield slightly lower than that of LAB (120 p.e./MeV at 15% photo–coverage). LAB is one of the most widely used scintillator solvents in recent years, offering excellent compatibility and cost–effectiveness, as well as a flash point well above 100 °C and a vapour pressure of only 0.013. Ding et al. (2012) prepared a new type of liquid scintillator with low toxicity, low corrosion, low volatility, and high flash point by employing LAB as the solvent. It was highly transparent, with an attenuation length of more than 10 m at 430 nm. Meanwhile, it also possessed high stability, and its Gd content and UV–vis absorption spectrum remain basically unchanged after two and a half years of measurements [11]. Polysiloxanes typically have flash points well above 200 °C and very low vapour pressures, so they are completely immune to the generation of harmful vapours. More importantly, these compounds are also chemically inert, offering stability and good material compatibility. Palma et al. (2015) developed a new type of liquid scintillator (1,1,5,5–tetraphenyl–1,3,3,5–tetramethyltrisiloxane (TPTMTS)), based on polysiloxane liquid compounds, which has low toxicity, chemical inertness, extremely low volatility, and low flammability. In addition, TPTMTS has a higher quantum yield of monophenyl emission than other polysiloxanes, higher excited state energy levels relative to PPO, and higher energy transfer efficiency (intensity at 270 nm = 0.362, which was 25% higher than 18–22%–Diphenyl–dimethylsiloxane copolymer) [66].

7. Conclusions

In summary, in recent years, scholars in the international community have adopted methods, such as changing ratios, improving luminescent properties, and reducing toxicity, to develop liquid scintillators for neutrino measurement. Due to the importance of neutrino experiments, the demand for liquid scintillators is particularly high. Therefore, it is vital to improve luminous efficiency and the ability to distinguish neutrons and gamma rays of liquid scintillators over a long period of time. Meanwhile, the promotion and study of liquid scintillators with low toxicity and high flash points, such as water–based liquid scintillators, is likely to become a key research focus, offering wide application prospects.