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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 8: 1214 - 1228

DOI: https://doi.org/10.7536/PC230109

Review

Rare Earth Based Neutron and Gamma Composite Shielding Materials

  • Yidong Lu 1, 2 ,
  • Zhipeng Huo , 1, * ,
  • Guoqiang Zhong 1 ,
  • Hong Zhang 1 ,
  • Liqun Hu 1
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  • 1 Hefei Institutes of Physical Science, Chinese Academy of Sciences,Hefei 230031, China
  • 2 University of Science and Technology of China, Hefei 230026, China
*Corresponding author e-mail :

Received date: 2023-01-28

  Revised date: 2023-06-10

  Online published: 2023-07-10

Supported by

Comprehensive Research Facility for Fusion Technology Program of China(2018-000052-73-01-001228)

Institute of Energy, Hefei Comprehensive National Science Center(21KZL401)

Institute of Energy, Hefei Comprehensive National Science Center(21KHH105)

Institute of Energy, Hefei Comprehensive National Science Center(21KZS205)

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Abstract

With the development of aerospace, nuclear technology and the wide use of nuclear energy, the requirement for the performance of nuclear radiation shielding materials have gradually increased. Since the high energy and strong penetrating ability of neutrons and gamma rays produced by nuclear reactions, they are of great harm to human beings and the environment. Therefore, the research on neutron and gamma radiation shielding materials has become a hot research topic of radiation protection. Rare earth elements have been gradually attracted considerable academic attention, and applied to research and development of neutron and gamma radiation shielding materials owing to their high neutron absorption cross section and high atomic numbers. This paper briefly introduces the application of rare earth materials in radiation shielding materials, and introduces the interaction mechanisms of rare earth elements with neutrons and gamma rays. According to the different types of substrate materials, the rare earth based neutron and gamma composite shielding materials can be divided into three categories: rare earth metal based, rare earth polymer based and rare earth glass based materials. The research progress of these three kinds of rare earth based radiation shielding materials is introduced respectively, and the possible problems and prospects of rare earth materials for neutron and gamma shielding radiation are analyzed.

Contents

1 Introduction

2 Interaction of neutron and gamma with rare earth elements

2.1 Interaction of neutron with rare earth elements

2.2 Interaction ofγ-ray with rare earth elements

3 Research progress of rare earth composite shielding materials

3.1 Rare earth metal based composite shielding materials

3.2 Rare earth polymer based composite shielding materials

3.3 Rare earth glass based composite shielding materials

4 Conclusion and outlook

Key words： rare earth; radiation shielding; neutron; gamma

Cite this article

Yidong Lu , Zhipeng Huo , Guoqiang Zhong , Hong Zhang , Liqun Hu . Rare Earth Based Neutron and Gamma Composite Shielding Materials[J]. Progress in Chemistry, 2023 , 35(8) : 1214 -1228 . DOI: 10.7536/PC230109

1 Introduction

Because of the non-renewable nature of fossil energy, people urgently need sustainable clean energy. Under this background, people have increased the development and utilization of nuclear energy[1]. The development and utilization of nuclear energy can alleviate the energy crisis to a certain extent, but the high-energy neutrons and gamma rays produced in nuclear reactions have strong energy, which can penetrate the human body and cause various health problems such as nerve damage, reproductive damage, heart disease, leukemia, cancer and so on[2,3]. The storage of spent fuel in nuclear reactors, the collimation shield for nuclear measurement in new generation fusion devices, and the radiation protection of nuclear medicine all need to shield the influence of neutrons and gamma rays, so the research and development of neutron and gamma radiation shielding materials are particularly necessary[4⇓~6][7,8].
The research on neutron and gamma radiation shielding materials can be traced back to the nuclear fission reactor Chicago II in the United States in 1943. The reactor was re-installed with concrete radiation protection devices on the basis of Chicago I reactor to ensure the safe operation of the equipment. After decades of development, the research on neutron and gamma radiation shielding materials has made great progress, and the research focus has also changed from neutron or gamma single functional shielding materials to neutron and gamma composite functional shielding materials[9,10]. Moreover, research is being carried out in various sub-fields, from shielding materials for high-energy neutrons and gamma radiation to flexible shielding materials for low-energy neutrons and gamma radiation[11][12⇓~14].
In recent years, it is worth noting that the application of rare earth elements in the field of neutron and gamma radiation shielding materials has developed rapidly because of their high thermal neutron absorption cross-section and atomic number. For example, rare earth elements and metal alloys form composite materials, which can shield neutrons and gamma rays at the same time and have high stability, and can be used for spent fuel storage. The composite material formed by the rare earth element and the polymer matrix can be used as a lightweight lead-free radiation shielding material for occasions where the mass of the shielding material is limited; The rare earth element doped glass can enhance the radiation shielding performance of the glass composite material, so that the glass composite material can be used in service places such as monitoring equipment of nuclear power plants, radiology departments of hospitals and the like. Rare earth composite shielding materials can be divided into metal matrix, glass matrix, polymer matrix and so on according to the type of matrix material[15,16][17][18]. China's rare earth reserves and output rank first in the world, so it is of great significance to make full use of China's abundant rare earth resources and improve the research and development level of rare earth materials.
In this paper, the interaction of rare earth elements with neutrons and gamma rays is introduced firstly, and then the research progress of rare earth composite shielding materials is introduced by classification.Finally, the existing problems of rare earth composite materials are sorted out and the development trend of rare Earth composite materials is summarized.

2 Interaction of neutron and gamma with rare earth elements

2.1 Interaction between neutron and rare earth elements

The shielding absorption of neutrons is mainly divided into two processes: fast neutron moderation and thermal neutron absorption[19]. Fast neutron moderation refers to the elastic and inelastic scattering of high-energy neutrons with substances with high hydrogen content, which reduces the energy of neutrons and turns them into thermal neutrons[20]. Thermal neutron absorption refers to the process in which a neutron undergoes a capture reaction with a substance with a high thermal neutron absorption cross section and is finally absorbed[10,21].
When the energy of fast neutrons is high (> 5 MeV), they are easy to scatter inelastically with atoms of medium mass number (100 ~ 150), and the nucleus gains energy and emits gamma rays to return to the ground state. The mass number of rare earth elements is between 89 and 175, which is easy to scatter inelastically with fast neutrons with higher energy, resulting in the loss of neutron energy and gradually becoming fast neutrons with energy of about 1 MeV. When the neutron energy is reduced to about 1 MeV, it is easy to collide with light nuclear elements (such as hydrogen, carbon, oxygen, etc.), which rapidly reduces the neutron energy to the thermal neutron energy level (0.025 eV), and the thermal neutron is easy to capture with substances with large thermal neutron absorption cross section.
Fig. 1 shows the variation of the neutron total cross section of several common rare earth elements and boron elements in the incident neutron energy range of 10-5~107eV. The neutron total cross section of rare earth elements is much higher than that of B elements. Table 1 lists the total thermal neutron cross sections of several elements obtained from Fig. 1. It can be seen from the table that the total thermal neutron cross sections of Gd, Eu, Sm and other elements in rare earth elements are much higher than those of B elements. Although Gd has a high thermal neutron total cross section, it will emit high-energy secondary gamma rays when the thermal neutron capture reaction occurs, which limits its application in neutron shielding materials[22,23].
图1 Gd、Sm、Eu、B的中子总截面

Fig.1 The neutron total cross-section of Gd, Eu, Sm, B

表1 图1中几种元素的热中子总截面

Table 1 Thermal neutron total cross-section of elements obtained from Fig.1

Element Thermal neutron total cross-section (barn)
B 765
Gd 48869
Sm 5690
Eu 4553

2.2 Interaction of gamma rays with rare earth elements

The interaction between gamma rays and rare earth elements mainly includes photoelectric effect, Compton effect and electron pair effect. The cross section σ represents the probability of interaction between incident photons and target atoms per unit area.Because of the relative independence of photoelectric effect, Compton effect and electron pair effect, the total reaction cross section of gamma ray interaction with rare earth elements is σγ[24,25].
σ γ = σ p h   + σ c + σ p
Where σγstands for the total reaction cross section, σph for the photoelectric effect reaction cross section, σc for the Compton reaction cross section and σp for the electron pair reaction cross sections.
Photoelectric effect refers to the interaction between the incident photon in the gamma ray and the electron outside the rare earth nucleus. All the energy of the incident photon is transferred to the electron. The electron breaks away from the binding of the nucleus and becomes a photoelectron. Vacancies are produced in the electron shell, which makes the atom in an excited state. There are two ways of deexcitation of excited atoms: (1) the outer electron of an atom jumps to the inner layer to emit X-rays; (2) The atomic excitation energy is transferred to the outer electron, which escapes from the nuclear binding and is called Auger electron[26]. Fig. 3A is a schematic diagram of the photoelectric effect. It can be seen from Fig. 2 that when the gamma photon energy is low (< 0.1 MeV), the photoelectric effect dominates. The photoelectric cross section σph is shown in formula (2)[27].
图2 三种相互作用随能量和原子序数的变化关系[28]

Fig.2 The relationship of three interactions with energy and atomic number[28]

图3 (a)光电效应示意图;(b)康普顿效应示意图;(c)电子对效应示意图[29]

Fig.3 The schematic of (a) photoelectric, (b) compton scattering and (c) electron pair effect[29]

σ p h ~ Z 4 ( h ν ) 3
Compton effect means that the gamma photon interacts with the electron outside the nucleus of the rare earth atom. The electron outside the nucleus (usually the outermost electron) gets part of the energy and escapes from the atom to form a recoil electron. The incident photon loses part of the energy and becomes a scattered photon and changes its trajectory, as shown in Figure 3B[26]. It can be seen from fig. 2 that when the gamma photon energy is between 0.5 and 4 MeV,
The Compton effect dominates the interaction between photons and matter. The relationship between the Compton cross-section σc and the atomic number Z is shown in Equation (3)[27].
σ c ~   Z
The electron pair effect means that when the gamma photon energy is higher than the rest mass of two electrons (Eγ>1.02 MeV), the incident photon passes near the rare-earth element nucleus, and under the action of the nuclear Coulomb force, the incident photon will be converted into a positron and a negative electron, as shown in Fig. 3C[26]. As shown in fig. 2, when the gamma photon energy is higher than 4 MeV, the electron pair effect dominates the interaction between the gamma photon and the rare earth element. The electron pair effect cross section σp is related to the atomic number Z as shown in equation (4)[27].
σ p ~ Z 2
To sum up, the interaction cross section between gamma rays and matter is mainly related to the atomic number Z, so elements with larger atomic number Z (such as Pb, W, Bi, etc.) Are usually used for gamma radiation shielding. The atomic number of rare earth elements is between 57 and 71, and the larger atomic number can produce a better shielding effect on photons.

3 Research Progress of Rare Earth Based Composite Shielding Material

3.1 Rare earth metal matrix composite shielding material

At present, the metal matrix used for radiation shielding materials mainly includes aluminum matrix, stainless steel matrix, boron steel matrix, iron matrix and so on. Metal matrix materials have the advantages of high hardness, good corrosion resistance and heat resistance. In order to make metal matrix materials have better radiation shielding performance and mechanical properties, researchers add a variety of reinforcing fillers in the metal matrix to increase the radiation shielding ability and mechanical properties of the metal matrix. At present, B4C, BN, WC, W, lead-containing compounds and so on are widely used as reinforcing fillers[30,31][32][33][34⇓~36][37]. Reinforcing fillers such as B4C and BN increase the neutron shielding performance of materials by increasing the thermal neutron absorption cross-section of the system. WC and lead-containing compounds mainly enhance the shielding ability of gamma rays by adding substances with higher atomic number or density to the system[34,38]. Among the rare earth elements, Gd has a large thermal neutron absorption cross section, and the thermal neutron absorption cross section of 155Gd is 15.8 times that of 10B. Gd is considered to be a new neutron shielding filler which can replace B4C because of its excellent thermal neutron absorption properties[39,40]. On the basis of B4C/Al composite, a new type of composite plate material, Gd/B4C/Al, was successfully prepared by doping Gd by vacuum hot rolling method in Jiang's group[41]. The test results show that Gd can improve the mechanical properties of the composite. Compared with the B4C/Al composite, the tensile strength, yield strength and elongation at break of the Gd/B4C/Al composite after hot rolling are increased by 46%, 67.6% and 35%, respectively[42]. The hot rolling process enables the Gd/B4C/Al composite to have better strength and toughness. Radiation shielding materials should consider not only the radiation shielding performance of materials, but also the mechanical properties of materials. Fig. 4 compares the relationship between the macroscopic neutron absorption cross section of different shielding materials and the product of strength and ductility (the product of tensile strength and elongation at break, which represents the strength and toughness level of metal materials). By comparing with the B4C/6061Al composite prepared by Chen, Zhang, Li, Park, etc., it is found that the Gd/B4C/Al composite has better comprehensive performance and can be used as a spent fuel storage material with excellent performance[42,43][44][45][46].
图4 屏蔽材料宏观中子吸收截面与强塑积的关系[41⇓⇓⇓⇓~46]

Fig.4 Relationship between macroscopic neutron absorption cross section and the product of tensile strength and elongation of shielding materials[41⇓⇓⇓⇓~46]. Copyright 2019, Elsevier

Cong et al. Prepared the Gd2O3/W/Al composite, in which Gd2O3 and W were uniformly distributed in the aluminum matrix, and the solid solutions such as Gd3Al5O12, Al3Gd, and Al12W generated in the reaction were uniformly distributed in the composite[47]. The mechanical test results show that both the holding time and the sintering temperature have an effect on the hardness of the composite, and the increase of the holding time and the sintering temperature will increase the proportion of solid solutions such as Gd3Al5O12, Al3Gd and Al12W in the composite, and reduce the pores between the composites, thus making the composite more compact. Under the irradiation of 137Cs(0.662 MeV) gamma radiation source, the gamma radiation shielding rate of 25 wt%Gd2O3/25 wt%W/Al composite is much higher than that of 30 wt%B4C/Al composite with the same thickness.
Due to the low solubility of Gd element in the alloy, the concentration of Gd element in the formed solid solution is limited, and most of them exist as brittle Gd-containing metal intermediate compounds[48,49]. In order to solve this problem, Huang Qunying Research Group of Institute of Nuclear Energy Safety and Technology, Chinese Academy of Sciences, used nanoscale Gd2O3 as filler and 316L alloy (alloy composition is shown in Table 2) as matrix, and successfully prepared Gd-316L stainless steel composite alloy with Gd3+ mass fraction of 7.87 wt%[50]. The results show that 0. 2 mm of 7. 87 wt% Gd-316L alloy can shield 90% of the thermal neutrons, and 33 mm of 316L alloy is needed to achieve the same effect.
表2 316L合金成分表[50]

Table 2 Chemical composition of 316L alloy[50]

Element Fe Cr Ni Mo Si O C
wt% 69.63 16.7 11.28 1.7 0.5 0.17 0.02
When Gd reacts with neutrons, it will emit high-energy secondary gamma rays, which limits the use of Gd. Zhang et al. Used a core-shell structure formed by Gd2O3@W(Gd2O3 and W, with Gd2O3 as the core and W as the shell) powder and aluminum powder as raw materials to prepare Gd2O3@W/Al composite using plasma sintering technology[51]. The Gd element in the core-shell structure of the Gd2O3@W has a capture reaction with a neutron to generate a secondary gamma ray, and meanwhile, the tungsten shell at the outer layer of the core-shell structure directly absorbs the secondary gamma ray. The Gd2O3@W/Al can absorb 99% of thermal neutrons when its thickness is 3 mm, which is far more than the neutron shielding rate of 10 wt%B4C/Al and 20 wt%B4C/Al.
The rare earth metal matrix composite shielding materials are mainly aluminum matrix and stainless steel alloy matrix, as shown in Table 3. The neutron shielding performance of the composite can be improved by increasing the content of Gd2O3 (1 wt% – 30 wt%)[52]. When the content of Gd2O3 is excessive, the disadvantages of low solubility and clusters of Gd2O3 in the composite system begin to highlight, which leads to the decrease of ductility and toughness of the material[52,53]. Therefore, optimizing the proportion of composite materials and balancing the radiation shielding performance and mechanical properties of materials are the key research contents.
表3 典型稀土金属基复合屏蔽材料及其性能参数a)

Table 3 Typical metal-based rare earth composite shielding materials and their performance parametersa)

Chemical Composition Mechanical Property Shielding Field Shielding Performance ref
10 wt% Gd2O3/6061Al σb: 240; Neutron Sn: 99.64 (THK: 10, En: 0.03232) 52
(1 vol% Gd + 15 vol% B4C)/6061 Al σb: 380 ± 11;
σs: 310 ± 8; E: 105 ± 2;
δ: 5.0 ± 0.3		 Neutron Sn: 99.9 (THK: 3, En: 0.025); 41
41
41
20 wt% Gd-Fe Hν: 588.8 ± 37.9 Neutron Sn: 99 (THK: 0.15, En: 0.0253) 49
2.5 wt% Gd2O3/316L Hν: 270; σs: 247;
σuts: 345; δ: 5.5		 Neutron Sn: 99 (THK: 3, En: 0.025) 65
65
25 wt% Gd2O3/25 wt% W/Al Hν: 140; σbc: 316 Neutron Σt: 125 (En: 0.0253); Sn: 99.9 (THK: 2.5, En: 0.1) 47
47
7.87 wt% Gd2O3/316L alloy / Neutron Sn: 90 (THK: 0.2, En: 0.0253) 50
Gd2O3@W/Al σs: 310 Neutron Sn : 99 (THK: 3, En: 0.0253) 51

a) σb: Tensile stress (MPa); Sn: neutron shielding rate (%); THK: thickness of sample (mm); Hν: vickers hardness (GPa) ; En: incident neutron energy (eV); σs: yield strength (MPa); E: elastic Modulus (GPa); δ: elongation (%);σbc: compressive strength (MPa); Σt: total cross sections (cm-1); σuts: ultimate tensile strength (MPa)

3.2 Rare earth polymer matrix composite shielding material

Polymer-based shielding materials contain a large number of light nuclear elements such as H, C and O, which have a significant moderating effect on neutrons, but the shielding effect of light nuclear elements on thermal neutron capture and gamma rays is limited. In order to make up for the deficiency, researchers have developed a variety of polymer-based composite shielding materials, such as B4C, BN, PbWO4, Gd2O3, Sm2O3 and other polymer-based shielding materials[54,55][56⇓~58][59,60][61,62][63]. The addition of high-Z rare earth elements makes the composite material not only have strong neutron capture and absorption ability, but also have gamma radiation shielding ability, which has a good application prospect.
As an important polymer material, polyethylene has the advantages of easy processing, non-toxic, light weight and so on. Because of its high hydrogen content, it has become the most widely used neutron shielding matrix material. 。 İrrimet al. Used an arc discharge device to synthesize UUNK composite functional shielding materials with high density polyethylene (HHDPE, nano UUNK and hexagonal boron nitride (h-BBN as raw materials[64]. Compared with HDPE, the tensile strength and elastic modulus of the new composite functional shielding material are increased by 55% and 13%, respectively. Neutron-gamma radiation shielding tests were then conducted on the composite, and the results showed that the neutron and gamma radiation shielding properties of the h-BN/Gd2O3/HDPE composite were increased by 280% and 52%, respectively, compared with those of the HDPE material.
The interfacial compatibility between high molecular weight polyethylene and inorganic rare earth materials is poor, which will cause the phenomenon of inorganic filler clustering and uneven distribution. In order to alleviate this problem, researchers have tried to use coupling agents to modify the surface of inorganic fillers to increase their dispersion in the polymer matrix. The research team of Huo Zhipeng, Institute of Plasma Physics, Chinese Academy of Sciences, has designed a high-performance lead-free surface modified gadolinium oxide/boron carbide/high-density polyethylene composite shielding scheme[66]. The surface of Gd2O3 is modified by coupling agent, which improves the interface compatibility and dispersivity of the filler in the matrix, and makes the radiation particles interact more fully with the functional components in the material and attenuate rapidly. The composite material adopts a Gd-H-B system to moderate and absorb neutrons, utilizes the interaction characteristics of light nuclei and heavy nuclei with neutrons and the high thermal neutron absorption cross section characteristics of Gd and B,High-energy incident neutrons collide inelastically with Gd, collide elastically with H, C and O until they become thermal neutrons, and are finally absorbed by Gd and B, in which Gd, as a heavy nuclear element, also has the function of absorbing gamma rays. When the thickness of the optimized composition 10 wt%Gd2O3/20 wt%B4C/70 wt%HDPE composite plate is 9. 1 cm, the neutron shielding rate of the plate for 252Cf neutron source can reach 90%, and when the thickness is 13. 7 cm, the gamma shielding rate of the plate for 137Cs gamma source can also reach 70%. At the same time, the mechanical strength and heat resistance of the composite board are obviously improved, and the composite board has high practical value as a lightweight neutron and gamma composite shielding material. Toyen et al. Used KBE903 silane coupling agent to modify the surface of Sm2O3 powder, and prepared a Sm2O3/ polyethylene composite shielding material by hot pressing[67]. Scanning electron microscopy (SEM) analysis showed that the modified Sm2O3 powder had better dispersion in the polyethylene matrix, and compared with the unmodified Sm2O3, the tensile strength of the modified Sm2O3/ polyethylene reached 24.The simulation calculation results of PHITS software (a program for analyzing neutron and gamma ray transport problems based on Monte Carlo algorithm) are shown in Fig. 5, from which it can be seen that the addition of Sm2O3 improves the neutron and gamma radiation shielding ability of the composite shielding material. When the content of Sm2O3 is 13 wt%, it can reach the level of commercial 5 wt% borated polyethylene radiation shielding.
图5 Sm2O3聚乙烯复合屏蔽材料的热中子、伽马线性衰减系数[67]

Fig.5 Thermal neutron and gamma linear attenuation coefficients of Sm2O3 polyethylene composite shielding materials[67]

Epoxy resin is widely used in the field of neutron and gamma shielding materials because of its chemical stability, good heat resistance, easy processing and excellent mechanical properties. Li Ran of Beijing University of Aeronautics and Astronautics used Er2O3 as reinforcing filler to prepare fiber reinforced epoxy resin composite by hot pressing method, marked as BE[68]. Subsequent gamma shielding tests on fiber-reinforced epoxy BE using 133Ba and 137Cs gamma radiation sources (photon energy of 0.356 and 0.662 MeV, respectively) showed that the BE composite has good gamma radiation shielding properties. In order to solve the problem of interfacial incompatibility between inorganic fillers and epoxy resin, researchers used coupling agents to modify inorganic fillers. Wang et al. Used Sm2O3 and AFG-90H epoxy resin as raw materials, and silane coupling agent (APTES) was used to modify the surface of Sm2O3 by ultrasonic-assisted method to prepare Sm2O3-APTES/AFG-90H composite, which enhanced the interfacial compatibility between Sm2O3 and epoxy resin matrix, improved the distribution uniformity of Sm2O3 filler in the matrix, and avoided large-scale cluster effect[69]. The thermogravimetric analysis results show that the T5% (the temperature at which the mass loss of the material is 5%, which is used to evaluate the thermal stability of the composite, the same below) of the 10 wt%Sm2O3-APTES/AFG-90H composite is about 309. 6 ℃. When the mass fraction of Sm2O3 filler increases from 10 wt% to 30 wt%, the T5% of the material increases. Neutron and secondary gamma radiation shielding test of Sm2O3-APTES/AFG-90H composite shows that the neutron radiation shielding rate of 2 mm thick 30 wt%Sm2O3-APTES/AFG-90H composite under thermal neutron source irradiation is 98.86%, and the test results of secondary gamma rays show that the amount of secondary gamma rays produced by all Sm2O3-APTES/AFG-90H composites is less than 10-14Gy. Compared with 5 wt%B4C and 3 wt% Gd inorganic filler doped epoxy resin matrix composite, it is found that 30 wt%Sm2O3-APTES/AFG-90H material has the best neutron shielding performance and can be used as an advanced neutron shielding material for nuclear facilities[70][71].
In order to better understand the influence of filler particle size on the shielding performance of epoxy resin matrix composites, Li et al. First modified the surface of Gd2O3 fillers with different sizes by KH550 silane coupling agent, then blended the modified Gd2O3 fillers with epoxy resin materials, and finally obtained Gd2O3/ epoxy resin composites[72]. Compared with micron Gd2O3/ epoxy materials, epoxy matrix composites doped with nano-sized Gd2O3 fillers have the advantages of low stiffness loss and high flexural strength. The results of gamma radiation shielding properties show that the mass attenuation coefficient of epoxy matrix composites filled with nano-sized Gd2O3 is better than that filled with micro-sized Gd2O3, and the mass attenuation coefficient of nano-sized and micro-sized Gd2O3 gradually tends to be the same with the increase of gamma photon energy and Gd2O3 content.
Polyethylene and epoxy resin are two common polymer-based radiation shielding matrix materials.Due to the low melting temperature of polyethylene and epoxy resin materials, their application in the field of neutron and gamma radiation shielding in high temperature environment is limited, so researchers began to turn their attention to polyimide (PI) materials with high temperature resistance[73,74][75,76]. Polyimide materials have high melting temperature (Tm) and thermal decomposition temperature (Tm:300~400℃,T5%:450~600℃), and its molecular structure contains a large number of light nuclear elements, which has a significant moderating effect on high-energy neutrons[77,78][79,80]. Wang et al. Successfully prepared a new type of carbon fiber reinforced Sm2O3/ polyimide composite with Sm2O3, polyimide (TY005-1) and carbon fiber as raw materials by hot pressing method[81]. The research team then studied the mechanical properties and neutron and gamma radiation shielding properties of the composite. The results show that the tensile strength and elastic modulus of the composites increase with the increase of Sm2O3/ carbon fiber layers in the composites, and the tensile strength of the continuous carbon fiber reinforced Sm2O3/PI composites can reach more than 200 MPa, which is close to the mechanical properties of aluminum alloy. The researchers used 241Am-Be neutron source, 137Cs and 60Co photon source to test the neutron and gamma shielding performance of carbon fiber reinforced Sm2O3/PI composite. The test results show that low-energy neutrons can be effectively absorbed by the material, and the neutron absorption cross section of the composite decreases with the increase of neutron energy, and the neutron shielding ability of the composite gradually weakens. When the thickness of carbon fiber reinforced Sm2O3/PI composite is 5 cm, the gamma ray shielding rate is 42. 4%, 34.4% and 32. 2% under the irradiation of 137Cs (characteristic gamma ray energy is 0. 662 MeV) and 60Co (gamma ray energy is 1. 17 and 1. 33 MeV) gamma radiation sources, respectively. Baykara et al. Successfully synthesized functional polyimide composite h-BN/Gd2O3/PI by using polyimide, h-BN and Gd2O3 as raw materials in a twin-screw extruder, and studied the mechanical properties, neutron and gamma radiation shielding properties of the material[82]. The mechanical test results of the composites show that the tensile strength of the functional polyimide composites is increased by about 2. 6% with 3 wt% h-BN doping. Then the effects of the thickness and the content of h-BN and Gd2O3 on the radiation shielding properties of the functional polyimide composite were studied, and the mass attenuation coefficients of the composite for neutron and gamma radiation were calculated. The results show that the composite with 11 wt%h-BN/3 wt%Gd2O3/PI has the best shielding properties for neutron and gamma radiation.When the thickness is 3. 0 cm, the radiation shielding rate for 239Pu-Be neutron source is close to 90%, and when the thickness is 6 cm, the radiation shielding rate for gamma ray is about 50%, which is much higher than that of pure polyimide material.
With the rapid development of aerospace, nuclear energy and other fields, high requirements for the heat resistance and flexibility of radiation shielding materials have been put forward. In order to solve these problems, Castley's research group has developed a kind of neutron composite shielding material based on room temperature vulcanized silicone rubber elastomer matrix.Three kinds of nano-powder fillers, B4C, Gd2O3 and Sm2O3, were added to the elastomer material respectively, and the thermogravimetric results showed that the mass loss of the elastomer composite at 300 ℃ was 1.6%, indicating that the elastomer material had good heat resistance[83]. By comparing the effects of the three fillers on the heat resistance of elastomer composites, it was found that the heat resistance of elastomer decreased gradually with the increase of the proportion of B4C filler, while the other two additives had no significant effect on the heat stability of elastomer composites. Subsequently, the research group carried out neutron and secondary gamma ray radiation shielding tests on elastomer composites, and the test results showed that with the increase of Gd2O3 and Sm2O3 content, the neutron shielding performance of composites increased, and the secondary gamma ray also increased.The amount of secondary gamma rays can be reduced by increasing the amount of B4C in the composite, and the final results show that the elastomer composite containing 10 wt%Gd2O3/2 wt%B4C has better neutron shielding properties and produces the least secondary gamma rays.
With the development of nuclear science, radiation medicine and nuclear medicine, nuclear facility constructors and radiologists are at greater risk of neutron and gamma ray radiation, and the demand for flexible shielding materials is increasing. Chen Wei et al. Of Nanjing University of Aeronautics and Astronautics successfully synthesized two kinds of flexible shielding materials, WO3/Gd2O3/RTV and WO3/Gd2O3/CTS/RTV, using nano Gd2O3, nano WO3, multi-dimensional carbon nanotubes (MWCNTS) and room temperature vulcanized silicone rubber (RTV) as raw materials, as shown in Fig. 6[13]. Subsequently, radiation shielding tests were carried out using gamma sources with different energies of 241Am(0.0596 MeV), 137Cs(0.662 MeV), 60Co(1.173 and 1.332 MeV), and the results showed that the WO3/Gd2O3/RTV and WO3/Gd2O3/CTS/RTV composites had better gamma radiation shielding properties.
图6 柔性辐射屏蔽材料(a)WO3/Gd2O3/RTV和(b)WO3/Gd2O3/CTS/RTV[13]

Fig.6 Flexible radiation shielding materials (a) WO3/Gd2O3/RTV, (b) WO3/Gd2O3/CTS/RTV[13]

Table 4 shows several typical rare earth polymer matrix composite shielding materials. The results show that the thermal stability, mechanical properties and neutron and gamma radiation shielding properties of the composites can be improved by filling rare earth fillers into the polymer matrix materials. The low thermal stability of polymer materials limits the use of polymer matrix in high temperature environment, so how to further effectively improve the thermal stability of composites remains to be further studied.
表4 典型的稀土聚合物基复合屏蔽材料及其性能参数a)

Table 4 Typical polymer-based rare earth shielding materials and their performance parameters a)

Chemical composition Mechanical property Shielding field Shielding performance ref
11 wt% h-BN/3 wt%
Gd2O3/HDPE		 σb : 33 ; σk: 330 Neutron Σ: 0.3804 ± 0.0112; μ/ρ: 0.1123 ± 0.035;
Sn: 67% (THK: 6, En: 4.5 MeV)		 64
25 wt% Sm2O3/UHMWPE σb: 24.9±0.6 ; σk: 156.8 ± 20.9; HS: 68 ± 1 γ-ray HVL = 6.7214 (Eγ: 0.712 MeV); μ/ρ: 0.0845 (Eγ: 0.712 MeV) 67
Carbon-fiber/Sm2O3/PI σb: 200; E: 35 γ-ray Sn: 42.4% (THK: 5, Eγ: 0.662 MeV) 81
9.7 wt% nano-Gd2O3/Epoxy σf: 140; σm : 4.2 γ-ray μ/ρ: 0.0826 (356 MeV) 72
11 wt% h-BN/3 wt%
Gd2O3/PI		 σb : 73 ± 1; δ: 13 Neutron and γ-ray Σ: 0.4052; Sn: 90% (THK: 3, En: 4.5 MeV) 82
15 wt% Sm2O3-APTES/
AFG-90H		 σb: 28.645 ; σk : 5300 ; δ: 6.8;
HS: 83		 Neutron Sn: 78%
(THK: 0.2, En: 0.0253 eV)		 69
10 wt% Gd2O3/ 20 wt%
B4C/70 wt% HDPE		 σk: 1297.9; σb: 19.6; δ: 7.9 Neutron and γ-ray Sn: 90% (THK: 9.1, En: 2.45 MeV)
Sγ: 70% (THK: 13.7, Eγ: 0.661 MeV)		 66
14 vol% Er2O3/Epoxy — γ-ray μ/ρ: 0.073 (Eγ: 0.662 MeV) 68

a) σb: Tensile stress (MPa); σk: tensile Modulus (MPa); δ: elongation at break (%); σf: flexural strength (MPa); σm: flexural modulus (GPa); HVL: half-value layer (cm); Σ: neutron absorption cross-section (cm-1) ; Sn: neutron shielding rate; Sγ: gamma shielding rate; THK: thickness of sample (cm); Eγ: incident gamma photon energy ; En: incident neutron energy ; μ/ρ: mass attenuation coefficient (cm2/g); E: elastic modulus (GPa); HS: shore hardness (HA)

3.3 Rare earth glass matrix composite shielding material

Glass-based materials have the advantages of high refractive index, high transmittance, easy processing and manufacturing, and low cost. Doping various heavy metal elements and rare earth elements in the glass matrix can effectively improve the radiation shielding ability of the glass and meet the needs of some special places. Materials with high transmittance and high neutron and gamma shielding performance shall be used for observation windows and other components in nuclear medicine radiology departments, nuclear research laboratories and radiation medical centers[84][85,86]. Borate glass, tellurite glass, phosphate glass and so on are the common substrate materials of rare earth glass-based composite shielding materials. The results show that the doping of rare earth elements (such as gadolinium, europium, samarium, cerium, neodymium, etc.) Has a significant impact on the structure, optical properties and radiation shielding properties of glasses[87⇓~89].
Borate glass has become the most widely used glass matrix material because of its low thermal expansion coefficient, low melting temperature and low glass transition temperature. The increase of the proportion of rare earth elements can improve the gamma shielding ability of borate glass, and also change the transmittance of composite glass materials. Alatawi et al. Used the melt-quenching technique to prepare Al(0.1-x)Bi1.8B0.6O3Y2x(x=0,0.02,0.04,0.06,0.08 mol%) glass, as shown in Fig. 7, from which it can be seen that the transmittance of the composite glass material gradually decreases with the increase of Y2O3 content[90].
图7 Al(0.1-x)Bi1.8B0.6O3Y2x玻璃样品[90]

Fig.7 Al(0.1-x)Bi1.8B0.6O3Y2x glass samples[90]. Copyright 2020, Elsevier

The valence state of Ce ion is adjustable with the increase and decrease of the number of extranuclear electrons, and the gamma radiation shielding performance of the glass composite material can be improved by capturing carriers such as holes, electrons and the like generated by the reaction of gamma rays and glass. Gomaa et al. Prepared a cerium-containing borate glass by doping sodium borate glass with a mass ratio of 0 to 10 wt% of CeO2, and studied the optical properties, thermal stability, and gamma radiation shielding properties of the borate glass[91]. The optical test results show that the transmittance of borate glass in the visible region (390 ~ 770 nm) increases with the increase of CeO2 content. With the increase of the mass fraction of CeO2, the energy band gap of borate glass increases, the absorption edge of the material moves to the ultraviolet direction, and the transmittance of borate material in the visible region increases. TG results show that the glass transition temperature of borate glasses decreases with the increase of CeO2 content. The gamma ray shielding ability of borate glass matrix composite containing 10 wt%CeO2 is optimal in the photon energy range of 0. 08 ~ 1.33 MeV. Kavaz et al. Improved the neutron and gamma shielding properties of barium bismuth borate glass by doping CeO2 on the basis of barium bismuth borate glass[92]. The results show that the mass attenuation coefficient of Ba-Bi borate composite glass decreases with the increase of the mass fraction of CeO2 under the irradiation of low energy gamma photons (< 0.662 MeV) and high energy gamma photons (8 ~ 20 MeV). When the content of CeO2 increases from 1 wt% to 10 wt%, the effective fast neutron removal cross section of barium bismuth borate glass increases.
Eu element has a high neutron absorption cross section in the thermal neutron energy range and a long half-life with neutron capture reaction[93]. At the same time, Eu element also has a high atomic number and a good shielding effect on gamma rays, so it is used to prepare glass-based composite shielding materials. Kilic et al. Successfully prepared a zinc borate (ZB) glass doped with Eu3+, which was marked as ZBEu[94]. The results show that the physical and gamma radiation shielding properties of ZB glass are affected by the doped Eu3+. When the mole fraction of Eu2O3 increases from 0 to 3 mol%, the density of ZBEu glass increases linearly from 3.6716 g·cm-3 to 3.972 g·cm-3,Eu2O3. The gamma radiation shielding ability of ZB glass gradually increases with the increase of Eu2O3 content. Then the effective neutron removal cross sections of ZBEu glasses with different mole fractions were compared, and the results show that the ZBS glass doped with 3 mol%Eu2O3 has the highest effective neutron removal cross section. Saudi et al. Have shown that with the increase of Eu3+ concentration, the BO4 in borate glass will be transformed into BO3, which can improve the structural stability of borate glass[95]. Researchers conducted gamma radiation shielding experiments on doped borate glass under 0.662, 1.173 and 1.33 MeV photon irradiation by using two gamma radioactive sources, 137Cs and 60Co, and studied the influence of Eu2O3 on the mass attenuation coefficient, half-value layer and other parameters of borate glass. The results show that the Eu2O3 filler can effectively enhance the gamma radiation shielding performance of borate glass. Then the fast neutron effective removal cross section of the material was calculated, and the results show that the fast neutron effective removal cross section of the borate glass material doped with 14 mol%Eu2O3 is 1. 25 times of that of the undoped borate glass.
Mariselvam et al. Prepared a new type of gamma radiation shielding glass by doping Yb3+ into barium fluoborate glass (BBFB)[96]. The results show that the density of barium fluoborate glass increases and the structure of the material changes with the increase of the concentration of Yb3+. When the mass fraction of Yb3+ is 2 wt%, the density of barium fluoborate glass is 3.94 g·cm-3, and the length of B — B bond decreases with the increase of the mass fraction of Yb3+. The gamma radiation shielding results show that the gamma shielding ability of barium fluoborate glass increases with the increase of Yb3+ content. Gd is a substance with a high thermal neutron absorption cross section of 46000 barn, while W, as a high-Z heavy metal element, has a good effect on gamma ray radiation shielding. Based on the above characteristics of materials, Kaewnuam et al. Successfully prepared WO3-Gd2O3-B2O3 ternary glass system by melt-quenching method[97]. The 137Cs gamma radiation source and XCOM software were used to test and theoretically simulate the radiation shielding performance of WO3-Gd2O3-B2O3 glass under the condition of 0.225 ~ 0.662 MeV gamma photon energy, and the results are shown in Fig. 8 a and B. It can be seen from Figure 8 a that the error between the test data of the 137Cs emitter and the simulation data of the XCOM software is small; With the increase of gamma photon energy, the mass attenuation coefficient (μm), effective atomic number (Zeff), effective electron density (Neff) of 17.5 mol%Gd2O3/ tungsten-borate glass decrease gradually. Fig. 8C shows the half-value layer of WBG(WO3-Gd2O3-B2O3) glass and commercial standard shielding materials such as ordinary concrete, ferromagnetic concrete, RS-253 commercial glass, barite concrete and RS-360 commercial glass under 0.662 MeV photon energy irradiation. It is found that the half-value layer of WBG glass is much smaller than these standard shielding materials, indicating that it has good gamma shielding performance.
图8 (a)Gd 17.5玻璃对不同能量伽马射线屏蔽参数的理论值与实验值;(b)WGB玻璃在0.662 MeV伽马光子能量辐照条件下的屏蔽参数;(c)在0.662 MeV伽马光子能量辐照条件下的WGB玻璃和标准屏蔽材料的半值层[97]

Fig.8 (a) Theoretical and experimental values of shielding parameters of Gd 17.5 glass at different γ-ray energy; (b) shielding parameters of WGB glass at 0.662 MeV photon energy; (c) half value layer of WGB glass and standard shielding materials at 0.662 MeV photon energy[97]. Copyright 2022, Elsevier

Compared with borate glass, tellurite glass can be formed at a lower temperature (close to the temperature limit of known elements to form glass), and has many advantages such as high transmittance, high refractive index, low thermal expansion coefficient and high chemical durability, so it is widely used as the matrix material of doped glass[98,99]. Sm3+ doped glasses have attracted the attention of researchers due to their excellent mechanical properties, corrosion resistance, and high thermal neutron absorption cross section[100]. Kavaz group doped Sm2O3 into tellurium zinc salt glass to obtain a new type of tellurium zinc salt composite glass (60TeO2-(40-x)ZnO-xSm2O3:x=0,0.1,0.2,0.3,0.4,0.5 mol%)[101]. Then the mass attenuation coefficient, half-value layer, mean free path, effective atomic number and other parameters of tellurite composite glass were studied. The results show that the neutron shielding performance of tellurite composite glass reaches the maximum when the content of Sm2O3 reaches 0. 4 mol%, and the neutron shielding performance of tellurite composite glass decreases with the increase of Sm2O3 content when the content of Sm2O3 exceeds 0. 4 mol%. Kozlovskiy et al. Prepared a new type of TeO2-(1-x)ZnO-xSm2O3 composite glass, and studied the gamma radiation shielding performance of this tellurium zinc salt composite glass by using three kinds of gamma radiation sources, including 57Co(0.136 MeV), 137Cs(0.661 MeV) and 22Na(1.274 MeV). The results show that when the mole fraction of Sm2O3 is 80 mol%, the composite glass has the best shielding effect for three kinds of gamma rays[102]. Ilik et al. Doped CeO2 into lithium-tellurium borate glass and synthesized 50TeO2-30B2O3-(20-x)Li2O-xCeO2(x=0~20 mol%), labeled as TBLC0-TBLC20, as shown in Fig. 9 a, and studied the optical and radiation shielding properties of TBLC glass. With the increase of the mole fraction of CeO2, the transmittance of TBLC material in the visible region gradually decreased, as shown in Fig. 9 B[103]. The absorption edge of TBLC glass is red-shifted with the addition of CeO2. The absorption edge of TBLC0 glass is about 350 nm, and the absorption edge of TBLC20 glass is about 750 nm. The red-shift of the absorption edge causes the decrease of the transmittance of TBLC glass. The mass attenuation coefficient and half-value layer of TBLC glass (X = 0 ~ 20 mol%) irradiated with 0.015 ~ 15 MeV photon energy were simulated by FLUKA and Phy-X/PSD software (two photon shielding transport calculation software), and the results showed that TBLC20 had the best gamma radiation shielding ability.
图9 (a)TBLC玻璃样品和(b)TBLC玻璃的透射光谱[103]

Fig.9 (a) TBLC glass samples, (b) transmittance spectra of TBLC glass[103]. Copyright 2022, Elsevier

There have been many reports on the application of rare earth element doped glasses such as Sm, Eu and Gd in radiation shielding materials, but there are few reports on the application of rare earth element doped glasses such as La and Er in radiation shielding materials. Rammah et al. Used Phy-X/PSD software (a photon shielding calculation and analysis software) to calculate the mass attenuation coefficient and linear attenuation coefficient of TeO2-B2O3-ZnO-La2O3(TBZL) composite glass materials under different gamma photon energies, and the results showed that the gamma shielding performance of composite glass materials increased with the increase of La2O3 mole fraction[104]. The radiation shielding performance of composite glass with different La2O3 content was compared with that of ordinary concrete and RS-253 commercial glass under the condition of gamma photons with energy ranging from 0. 015 MeV to 15 MeV. It is found that in the low energy photon region (< 0.5 MeV, photoelectric effect is dominant) and high energy region (> 5 MeV, electron pair effect is dominant), the mass attenuation coefficient, linear attenuation coefficient and half-value layer of La2O3-TeO2-B2O3-ZnO composite glass are much higher than those of ordinary concrete and RS-253 commercial glass. The mass attenuation coefficient, linear attenuation coefficient and half-value layer of La2O3-TeO2-B2O3-ZnO composite glass, concrete and RS-253 commercial glass tend to be the same at gamma photon energies of 0.5 ~ 5 MeV (dominated by Compton effect). Subsequently, the researchers studied the effective fast neutron removal cross section of several materials, and the results showed that when the mole fraction of La2O3 exceeded 3 mol%, the effective fast neutron removal cross section of La2O3-TeO2-B2O3-ZnO quaternary glass system was much higher than that of ordinary concrete and RS-253 commercial glass.
Phosphate glass has the advantages of low melting temperature and easy forming, but the disadvantages of poor chemical stability and strong hygroscopicity of phosphate materials limit the use of phosphate glass[105,106]. The low chemical stability and strong hygroscopicity of phosphate materials are due to the presence of an oxygen atom at the end of its molecule. Studies have shown that the bridge oxygen bond P — O — P will be destroyed by doping alkali and alkaline earth ions into phosphate glasses.The terminal oxygen bond P — O — M is formed, and the original phosphate glass structure is destroyed to form a new P-O-M bond with high hydration resistance, and the accumulated charges around the P-O-M bond can significantly enhance the chemical stability of phosphate-based glasses[107,108]. Luo Qing et al. Of Southwest University of Science and Technology used Gd2O3, BaCO3, and NH4H2PO4 as raw materials to synthesize two phosphate composite glasses, xGd2O3-(40-x)BaO-P2O5(0<x<14 wt%) and xGd2O3-(50-x)BaO-P2O5(0<x<7 wt%), labeled as sample 1 and sample 2, respectively, by melt-quenching method[109]. The effect of Gd2O3 content on the transmittance of phosphate composite glass was studied, and the results are shown in Fig. 10. With the increase of Gd2O3 content, the transmittance of sample 1 increases; The transmittance of sample 2 increases first and then decreases, and the transmittance of glass reaches the highest when the content of Gd2O3 is 6 wt%. Subsequently, the gamma shielding test of phosphate composite glass with a size of Φ35 mm × 4 mm was carried out by using 60Co gamma radiation source under the irradiation conditions of photon energy of 1.173 and 1.332 MeV, respectively (dominated by Compton scattering).By comparing the mass attenuation coefficient, half-value layer, mean free path, effective atomic number and other parameters of composite glass with different formulations, it is found that the gamma shielding performance of Gd2O3-BaO-P2O5 ternary system glass increases with the increase of Gd2O3 content, and by comparing two phosphate glass samples, it is found that the gamma shielding performance of composite glass with 7 wt%Gd2O3-43 wt%BaO-50 wt%P2O5 is the highest.
图10 (a)样品1的透射光谱图和(b)样品2的透射光谱图[109]

Fig.10 (a) Transmission spectrum of sample 1, (b) transmission spectrum of sample 2[109]

Suresh et al. Prepared a variety of rare earth (Dy2O3, Pr2O3, Nd2O3) doped aluminoborophosphate (ABP) composite glass systems by melt-quenching method[110]. The mass attenuation coefficients of aluminoborate composite glasses doped with different rare earth elements under 10-3~105MeV photon energy irradiation were calculated by Phy-X/PSD software. The half-value layer, mean free path, effective atomic number and effective electron density were calculated from the mass attenuation coefficients. The results show that the gamma shielding properties of Dy2O3/ABP composite glass, Nd2O3/ABP composite glass and Pr2O3/ABP composite glass decrease in turn under the same photon energy irradiation. In order to better evaluate the gamma ray shielding ability of aluminoborate composite glass system, the G-P fitting method was used to fit the exposure accumulation factor (EBF) and energy absorption accumulation factor (EABF) curves, which revealed the relationship between the transmission depth of photons in composite glass and the incident photon energy.The results show that the EBF value of ABP composite glass doped with Dy element is the lowest, which indicates that the Dy2O3/ABP composite glass material has the best gamma radiation shielding performance, which is consistent with the simulation results of Phy-X/PSD software.
Table 5 lists some typical rare earth glass-based composite shielding materials and their performance parameters. Due to the high transmittance of glass, it plays a significant role in nuclear safety monitoring, nuclear medical protection and other aspects. The combination of rare earth oxides and glass matrix can effectively improve the gamma radiation shielding ability of glass, but also affect the structure and optical properties of glass. How to further improve the radiation shielding performance of rare earth glass-based composite shielding materials while ensuring the transmittance is an important direction to be further studied.
表5 典型稀土基玻璃屏蔽材料及其性能参数a)

Table 5 Typical glass-based rare earth shielding materials and their performance parameters a)

Glass Sample Physical property Shielding field Shielding performance ref
S1 ρ: 4.810 γ-ray μ/ρ: 0.0624 (Eγ: 1), HVL: 2.31 (Eγ: 1) 111
111
S2 ρ: 2.46 ; Hν : 3.37 γ-ray HVL: 2.45 (Eγ: 0.356), μ/ρ: 0.1 (Eγ: 0.356) 91
91
S3 σa : 84.85 ; σb : 64.87 ; σc : 33.1; ρ: 6.260 γ-ray Sγ: 99 (THK: 3, Eγ: 0.284); μ/ρ: 0.0996 (Eγ: 0.662), HVL: 1.18 (Eγ: 0.662) 112
112
112
S4 ρ: 3.77; Vm: 29.468 γ-ray HVL: 6 (Eγ: 10) 94
S5 ρ: 6.259; Vm: 51.6 γ-ray HVL: 3 (Eγ: 3) 113
S6 ρ: 3.26 γ-ray HVL: 6.756 (Eγ: 10) 96
S7 ρ: 5.846 γ-ray HVL: 2.1 (Eγ: 1); μ/ρ: 0.054 (Eγ: 1.173) 101
101
S8 ρ: 2.84 γ-ray HVL: 1.319 (Eγ: 0.15); μ/ρ: 1.633 (Eγ: 0.05); μ: 4.12084 (Eγ: 0.05) 114
114
114
S9 ρ: 6.09; Vm: 51.42 γ-ray μ/ρ: 0.2057 (Eγ: 0.356);
HVL: 0.552 (Eγ: 0.356)		 90
90
S10 ρ: 6.21 Neutron and γ-ray μ/ρ: 0.215 (Eγ: 0.356)
ΣR: 0.13992		 92
92
S11 ρ: 4.57 Neutron and γ-ray μ/ρ: 0.0547 ± 0.00212 (Eγ: 0.662)
HVL: 2.59 ± 0.052 (Eγ: 0.662)
ΣR: 0.032		 95
95
95
S12 ρ: 5.48; Vm: 37.6 γ-ray μ/ρ: 0.090 (Eγ: 0.662)
HVL: 1.412 (Eγ: 0.662)		 97
97
S13 ρ: 3.41; Vm: 45.42 γ-ray μ/ρ: 0.056 (Eγ: 1.173)
HVL: 3.62 (Eγ: 1.173)		 109
109
S14 ρ: 4.1012 ± 0.0001;
Vm: 26.3553 ± 0.006		 γ-ray μ/ρ: 0.08 (Eγ: 0.6)
μ: 0.3281 (Eγ: 0.6)		 103
103

a) σa: Young’s modulus (GPa); σb: Bulk modulus (GPa); σc: Shear modulus (GPa); Hν: Vickers hardness (GPa); HVL: Half value layer (cm); μ/ρ: Mass attenuation coefficient (cm2/g); μ: Linear attenuation coefficients (cm-1); ΣR: Fast neutron effective removal cross sections (cm-1); ρ: Density (g/cm3); Vm: Molar volume (cm3/mol); Sγ: Gamma shielding rate (%); THK: Thickness of sample (cm); Eγ: Incident gamma photon energy (MeV).

Sample: S1: 60 mol%B2O3-15 mol%WO3-25 mol%La2O3; S2: 2 wt%CeO2-Sodium borate glass; S3: 39H3BO3+30PbO+20BaO+10Bi2O3+1Dy2O3; S4: 59.4 mol%ZnO-39.6 mol%B2O3-1 mol% Eu2O3; S5: YBBiOLa2; S6: 69.95 mol%H3BO3-5 mol%Bi2O3-10 mol%BaCO3-7.5 mol%CaF2-7.5 mol%ZnO-0.05 mol%Yb2O3. S7: 0.1 mol%Sm2O3-60 mol%TeO2-39.9 mol%ZnO; S8: 36.4 wt%Y2O3-1.6 wt%Al2O3-61.9 wt%P2O5; S9: Al0.08B0.6Bi1.8O3Y0.04; S10: 2.5 mol%CeO2-20 mol%BaO-17.5 mol%Bi2O3-60 mol%B2O3; S11: 7.5 mol%Eu2O3-47.5 mol%B2O3-10 mol%CaO-10 mol%SiO2-25 mol%Bi2O3; S12: 17.5 mol%Gd2O3-52.5 mol%WO3-30 mol%B2O3; S13: 4 mol%Gd2O3-36 mol%BaO-60 mol%P2O5; S14: 50 mol%TeO2-30 mol%B2O3-19 mol%Li2O-1 mol%CeO2.

4 Conclusion and prospect

In this paper, different types of rare earth based neutron and gamma composite shielding materials are introduced, which can be divided into metal based, polymer based and glass based neutron and gamma composite shielding materials according to the type of matrix. Different types of rare earth based shielding materials have their own advantages and disadvantages. Rare earth metal matrix composites have good ductility, wear resistance and radiation shielding performance, but the solubility of rare earth filler in the metal matrix is small, and excessive rare earth filler doping will reduce the ductility of the metal matrix. Rare earth polymer matrix composites have the advantages of easy processing, light weight, strong plasticity and good radiation shielding performance, but also have the disadvantages of low melting point and poor thermal stability. Researchers doped rare earth oxides into polyimide, epoxy resin, vulcanized silicone rubber and other matrices, which increased the melting temperature and thermal decomposition temperature of rare earth polymer matrix composites, and enhanced the application range of rare earth matrix composites. Rare earth glass matrix composites have the advantages of non-toxicity, good light transmittance, easy forming and processing. The doping of rare earth elements can significantly improve the structural stability, optical properties and radiation shielding properties of glass matrix composites. The rare earth filler doped glass matrix changes the wavelength range of its absorption edge, resulting in the change of the transmittance of the composite material, and the reduction of the transmittance of the composite glass material limits the application range of the rare earth glass-based material. Radiation shielding materials should consider not only the radiation shielding performance of materials, but also the mechanical properties, thermal stability, corrosion resistance, etc. Rare earth composite shielding materials have both good mechanical properties and excellent radiation shielding performance, which is an important development direction of radiation shielding materials in the future.
Although rare earth composite shielding materials have many excellent properties, there are still some problems: (1) Most of the rare earth fillers used for doping reported in the literature are fragments with irregular morphology, which affects the dispersion of the fillers in the composite materials, and when the incident neutrons or gamma rays react with the material, the reaction may be insufficient; (2) Rare earth elements will produce secondary gamma rays after absorbing neutrons, and how to effectively shield secondary gamma rays is an urgent problem to be solved; (3) The interfacial compatibility between the rare earth inorganic filler and the polymer is poor, and the phase separation phenomenon is easy to occur; (4) The solubility of rare earth filler in the metal matrix is small, and excessive rare earth filler will reduce the mechanical properties and ductility of metal materials.
For the above problems of rare earth based composite shielding materials, some feasible solutions are proposed in this paper. For the problem of insufficient interaction between incident neutrons and gamma rays caused by the uneven distribution of rare earth fillers in the composite material,The rare earth filler and the matrix material can be mixed and milled by a high-energy ball mill to increase the dispersion of the filler in the composite material and improve the probability of interaction between the rare earth filler and neutrons and gamma rays, thereby enhancing the radiation shielding performance of the composite material. For the problem that rare earth elements capture neutrons to produce secondary gamma rays, this problem can be solved by blending rare earth materials with high atomic number materials to prepare functional composites. For the problem of interfacial compatibility and phase separation between rare earth filler and polymer, coupling agent can be used to modify the surface of inorganic filler to increase its dispersion in the polymer matrix. For the problem of low solubility of rare earth filler in the metal matrix, the solubility and dispersion of rare earth filler in the metal matrix can be improved by hot rolling, solid solution and reprecipitation.

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