Low Energy Beta Emitter Measurement: a Review

chemosensors

Review Low Energy Beta Emitter Measurement: A Review

Hara Kang 1,2, Sujung Min 1,2, Bumkyung Seo 2, Changhyun Roh 2,3,* , Sangbum Hong 2,* and Jae Hak Cheong 1,* 1 Department of Nuclear Engineering, Kyung-Hee University, Yongin-si 17104, Gyeonggi-do, Korea; [email protected] (H.K.); [email protected] (S.M.) 2 Decommissioning Technology Research Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea; [email protected] 3 Quantum Energy Chemical Engineering, University of Science and Technology (UST), 217 Gajeong-ro, Daejeon 34113, Korea * Correspondence: [email protected] (C.R.); [email protected] (S.H.); [email protected] (J.H.C.)

Received: 22 September 2020; Accepted: 25 October 2020; Published: 28 October 2020

Abstract: The detection and monitoring systems of low energy beta particles are of important concern in nuclear facilities and decommissioning sites. Generally, low-energy beta-rays have been measured in systems such as liquid scintillation counters and gas proportional counters but time is required for pretreatment and sampling, and ultimately it is difficult to obtain a representation of the observables. The risk of external exposure for low energy beta-ray emitting radioisotopes has not been significantly considered due to the low transmittance of the isotopes, whereas radiation protection against internal exposure is necessary because it can cause radiation hazard to into the body through ingestion and inhalation. In this review, research to produce various types of detectors and to measure low-energy beta-rays by using or manufacturing plastic scintillators such as commercial plastic and optic fiber is discussed. Furthermore, the state-of-the-art beta particle detectors using plastic scintillators and other types of beta-ray counters were elucidated with regard to characteristics of low energy beta-ray emitting radioisotopes. Recent rapid advances in organic matter and nanotechnology have brought attention to scintillators combining plastics and nanomaterials for all types of radiation detection. Herein, we provide an in-depth review on low energy beta emitter measurement.

Keywords: sensor; nanomaterial; plastic scintillator; low energy beta-ray emitter; decommissioning; radiation measurement; radiation detection

1. Introduction Globally, the characterization of residual radioactivity is of important concern to treat radioactive waste generated during the operation and decommissioning in nuclear facilities. The characterization is essential to estimate the radiological hazard and support decision making at decommissioning sites [1,2]. A final status survey has been carried out to meet the regulation release via lower radioisotope concentrations to the derived concentration guideline level (DCGL) [3]. Mainly, low energy beta-ray emitting radioisotopes such as 3H, 14C, and 63Ni are usually detected by a liquid scintillation counter (LSC). However, this approach requires a long time for pretreatment and waste generated after the analysis is harmful to the environment [4–8]. Additionally, the specific and large size equipment of the LSC makes it unsuitable for measuring beta-ray emitting isotopes on site due to its complex systems [9]. Furthermore, the LSC is suitable to measure beta-rays due to their low atomic number and density because the elemental composition of plastics is H and C. Additionally, plastic scintillators are easy to manufacture in desired size and can be inexpensively fabricated in a large size [10]. Above all, characteristics of the different additives in the plastic scintillator grant special properties such as high Z material loading for gamma-ray measurement or boron loading for thermal neutron detection [11].

Chemosensors 2020, 8, 106; doi:10.3390/chemosensors8040106 www.mdpi.com/journal/chemosensors Chemosensors 2020, 8, 106 2 of 42

Due to these characteristics, plastic detectors have been used to measure β-rays [12]. The plastic scintillator converts radiation into scintillation [13], and then a spectrum analysis of incident radiation is performed by converting scintillation into current through a photosensor and amplifying it. Since the physical/chemical properties vary depending on the type of scintillator, it is important to select an appropriate scintillator upon target radiation [14]. Thus, the characteristics of low energy beta-ray emitting isotopes and trends regarding in-situ detectors to measure low-energy beta-ray emitting isotopes were investigated. This article provides an overview of the characteristics of low energy beta-rays and the characteristics of radioisotopes occurring at the decommissioning site. In addition, we elucidated commercial plastics and detectors to carry out an analysis of the technical requirements and detector structure for an in-situ beta-ray measurement by analyzing technology trends for a low energy beta-ray measurement. Thus, recent commercial detectors for measurement of low energy beta-ray emitting isotopes were investigated. Furthermore, the technical status of the measurement of radiation levels for monitoring or measuring low energy β-emitting radionuclides at home and abroad was investigated. Ultimately, this paper could provide basic data for the development of technologies for measurement of low energy β-emitting isotopes.

2. Low Energy Beta-Emitter Characteristics Radioisotopes generated at decommissioning sites such as 60Co and 137Cs are easy to measure without requiring chemical separation from other isotopes. In the meantime, beta-ray emitters that cannot penetrate a thick medium should be completely separated from other isotopes for measurement [15,16]. Low energy β emitters mainly originate from neutron activation due to their low atomic mass. Radiation protection against internal exposure via intake, inhalation, and ingestion is essential [17,18], while risk arising from external exposure is very slight because of its low energy and short range [19,20]. There are numerous radioisotopes at the decommissioning site. Tritium and carbon-14 are the main isotopes that emit low energy β-ray, with respective maximum energy of 18.6 keV and 156 keV [21]. Owing to their low energy, they are not treated as radioisotopes that require protection from external exposure. Nevertheless, these radionuclides interact in some environmental mechanisms and turn into various forms influencing the human body. Additionally, low energy of the beta-emitters leads to some difficulties in detection and consequently conventional detectors such as the ionization chamber or the Geiger–Muller counter are incapable of measuring low energy beta emitters. Table1 shows representative beta-emission isotopes, and also presents the characteristics and human effects of low-energy beta-emitters at the decommissioning site.

Table 1. Characteristics of beta-ray emitters.

Nuclide Half-Life Principal Radiation 3H 12.3 y 0.0186 MeV 14C 5.73 103 y 0.156 MeV × 36Cl 3.01 105 y 0.709 MeV × 63Ni 100 y 0.659 MeV 90Sr/90Y 29.1 y 0.546 MeV + 2.28 MeV 94Nb 2 104 y 0.47 MeV × 99Tc 2.13 105 y 0.293 MeV × 129I 1.6 107 y 0.15 MeV × 241Pu 14.1 y 0.21 MeV Chemosensors 2020, 8, 106 3 of 42

2.1. 3H Although tritium has the same chemical behavior as that of hydrogen, this isotope emits radiation via beta decay, unlike hydrogen or deuterium [22]. Tritium has two neutrons and one proton and releases beta-rays that are converted into a stable isotope (Equation (1)).

3 3 H He + 1e− + ve + 18.6 keV (1) 1 → 2 − In general, the main contribution to the accumulation of 3H in concrete is the neutron radiation reaction of 6Li(n, α)3H[23]. Additionally, neutron capture of 2H(n, γ)3H and 3He(n, p)3H in the reactor evaporator is mainly detected in nuclear facilities. Tritium travels only 6 mm in air, and it cannot penetrate the dead layer of skin [18,24]. It also penetrates only about several micrometers in graphite [25] and less than 1 pm in water [26]. Nevertheless, as a result of having the same chemical properties as hydrogen, tritium tends to replace stable hydrogen in the human body in a gaseous state or the form of tritiated water. Tritium gases are rarely dissolved in the human body, and objects can be exposed to and adsorb the vapor of tritium. Only 0.004% or less of tritium gas is absorbed once inhaled, but 98–99% of it is absorbed by the human body when it is breathed for 4–5 min in air saturated by evaporating tritiated water [27].

2.2. 14C 14C is mainly produced by the neutron capture reaction of 14N(n, p)14C, 13C(n, α)14C, and 17O(n, α)14C at the reactor core during the operation of a nuclear reactor [21]. Among others, the reaction 14N(n, p)14C at the concrete shield [28] around the reactor core is the main contribution of the 14C production due to the high neutron cross-section [23,29]. In addition, 14C, which is highly volatile, exists mainly in the form of carbonate and is highly mobile in groundwater and in the form of 14 17 CO2 [30]. Stable isotopes N and O are very common in building materials, and they are mainly detected in radioactive metals from reactors and can be detected in all materials examined for neutrons.

2.3. 36Cl 36Cl is a radioisotope with a 301,000 year half-life that decays to the stable state of 36Ar emitting maximum energy of 709 keV (98.1%) or electron capture (1.9%). 36Cl is created via neutron activation of rocks on the ground. Additionally, neutron activation of the stable state radioisotope in nuclear fuel, graphite, coolant, steel, and ion-exchangers produces 36Cl via 35Cl(n, γ)36Cl [31,32].

2.4. 63Ni 63Ni is the most abundant radioisotope in nuclear facilities at decommissioning, from graphite, pipes, and concrete to ion-exchangers [33]. 63Ni is created by the reaction of stable Ni and Cu of 62Ni(n, r)63Ni, 63Cu(n, p)63Ni, and decays to 63Cu after emission of β-ray energy of 66.95 keV. Ni has high resistance to water and air and is used for metal protection coating alloys for corrosion resistance metal. Therefore, 62Ni produces 63Ni via neutron activation in structural iron and steel of nuclear reactors and internal components and 63Ni is released through corrosion of the surface and circulating coolant of metals such as stainless steel or Inconel.

2.5. 90Sr 90Sr is mainly produced by nuclear ﬁssion and emits 90Y after high-energy beta-decay. It is found in radioactive waste such as ion exchange resin, ﬁlter sludge, and at the bottom of the evaporator. In addition, 90Sr is highly soluble and thus easily transported through precipitation and groundwater. 90Sr constitutes a long-term biological hazard as it accumulates in bone tissues and can lead to cancer via ingestion [31]. Chemosensors 2020, 8, 106 4 of 42

Chemosensors 2020, 8, x 4 of 42 2.6. 94Nb 2.6.94 94NbNb is generated by nuclear fission or neutron activation of a stable state of 235U and 239Pu at 4 the nuclear94Nb reactor.is generated STable by 9nuclearNb exists fission in largeor neutron quantities activation in reactor of a stable vessel state material of 235U and and fuel239Pu cladding at the 94 componentsnuclear reactor. with largeSTable amounts 94Nb exists of Inconelin large [ 34quantities]. Nb isin mainlyreactor generatedvessel material by neutron and fuel capture cladding and 94 decaycomponents to Mo withwith 472large keV amounts of β-ray. of Inconel [34]. 94Nb is mainly generated by neutron capture and decay to 94Mo with 472 keV of β-ray. 2.7. 99Tc 2.7. 99Tc 99Tc is not a naturally occurring radioisotope but it is generated by nuclear fission of uranium and plutonium.99Tc is99 notTc isa mainlynaturally found occurring at the radioisotope bottom of the but evaporator it is generated or radioactive by nuclear wastefission suchof uranium as in the 99 99 filterand and plutonium. sludge [ 35Tc]. Theis mainly main found form ofat theTc bottom is [TcO of4] −the, which evaporator is highly or radioactive mobile in waste groundwater such as in and hasthe a longfilter half-life, and sludge which [35]. can The pose main long-term form of 99 radiologicalTc is [TcO4]− risks., which is highly mobile in groundwater and has a long half-life, which can pose long-term radiological risks. 2.8. 129I 2.8. 129I 129I is mainly produced by uranium nuclear fission in the reactor [36] and is not a naturally 129 occurringI nuclide.is mainly129 producedI is found by in uranium radioactive nuclear waste fission such asin ionthe exchangereactor [36] resin, and enriched is not a wastenaturally fluid 129 of coolant,occurring filter nuclide. sludge, I is and found cartridge in radioactive filters. Whenwaste such inhaled as ion and exchange ingested, resin, most enriched is dissolved waste in fluid body of coolant, filter sludge, and cartridge filters. When inhaled and ingested, most is dissolved in body fluids and deposited in the thyroid gland. fluids and deposited in the thyroid gland. 2.9. 241Pu 2.9. 241Pu In the case of 241Pu, it is produced in the reactor through neutron absorption by the β decay of In the case of 241Pu, it is produced in the reactor through neutron absorption by the β decay of 241Am of trans-uranium elements [37]. In general, plutonium waste is classified as high-level waste, 241Am of trans-uranium elements [37]. In general, plutonium waste is classified as high-level waste, butbut various various wastes wastes generated generated during during reactor reactor operationsoperations are also present in in low-level low-level waste. waste. 3. Beta Ray 3. Beta Ray 60 152 γ Gamma-rayGamma-ray emitters emitters such such as asCo 60 andCo andEu 152 canEu becan easily be easily detected detected when analyzingwhen analyzing the spectrum, the γ butspectrum, in the case but of inβ theemitters, case of it β is emitters, difficult it to is distinguish difficult to radioisotopesdistinguish radioisotopes due to poor due energy to poor resolution energy in theresolution spectrum in [30 the]. Thisspectrum is particularly [30]. This noticeableis particularly in the noticeable low-energy in theβ low-energyarea, and there β area, is also anda there problem is thatalso energy a problem measurement that energy is di measurementfficult because is thedifficult noise because generated the innoise low-energy generated areas in low-energy overlaps with areas the radioisotopesoverlaps with [32 the]. Figure radioisotopes1a,b show [32] the. Figure track of 1a,bγ and showβ-ray the throughtrack of aγ medium.and β-ray As through shown a in medium. Figure1 a, gamma-raysAs shown losein Figure energy 1a, for gamma-rays a short period lose ofenergy time for due a toshort the photoelectricperiod of time e ffdueect, to Compton the photoelectric scattering, andeffect, electron Compton pair production scattering, and [38, 39elec]. tron pair production [38,39].

FigureFigure 1. 1.Radiation Radiation interaction interaction in in scintillator scintillator ((aa)) gamma-raygamma-ray interaction interaction and and (b (b) )beta-ray beta-ray interaction. interaction.

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The interaction within the medium of the γ-ray can be classiﬁed according to the energies of the incident photons, which are shown in Table2.

Table 2. Gamma interaction with the energy region.

Energy Region Interaction Phenomena E < 30 keV Photoelectron Eﬀect and Auger Electrons 30 keV < E < 2 MeV Compton Scattering E > 2 MeV Pair Production

Meanwhile, β particle continuously loses energy in the medium (shown in Figure1b) [ 40,41]. As a result, the eﬀective penetration range of the electron is short because the total range through which particles have moved and the electrons travel in a straight line do not match [42,43]. Therefore, low-energy β isotopes such as 3H and 14C are analyzed by methods such as the separation of radioisotopes by a liquid scintillation counter (LSC) [44,45] or beta-ray induced X-ray spectrometry (BIXS) [46] due to their short range, making it diﬃcult to transfer energy to detectors. The number of neutrons in a nucleus is excessive and it tends to emit electrons (β particles) [42]. This is called β-decay, the neutron converts into a proton and β particle (electron), and nuclear conversion occurs with electron emission [30]. On the other hand, when the number of protons in a nucleus is excessive, the proton is converted into a neutron and electron. Nucleus conversion then occurs, releasing positrons (β+ particles), which is called β+ decay. When the particles are emitted from radioactive decay, the sum of the energy should be constant, and thus an anti-neutrino is released with β− particle emission, and a neutrino is released with a β+ particle (Table2)[ 47]. Due to this mechanism, the emitted energy from beta decay distributed between the energy of β particles and the energy of the neutrinos produces a continuous energy spectrum. β decay leads the nucleus to enter an excited state, and excess energy released by emitting one or more photons. Additionally, excess energy emitted via internal conversion transfers the surplus energy that subtracted binding energy to the orbital electron, to produce an internal conversion electron.

4. Interaction with Matter Electrons besieged by the Coulomb electric ﬁeld that interacts with all of the particles passing through it [48,49]. For most of the interactions, only a minuscule fraction of the kinetic energy of an incident particle can be transferred [29]. These interactions are similar to an electron losing its energy by friction. This process is commonly referred to as CSDA, or continuous slowing-down approximation [32]. In principle, there are three interaction processes.

Hard collisions: Inelastic scattering with atomic electrons generates excitation or ionization of • electrons, and delta-rays (secondary electrons) are originated. The probability of this interaction is proportional to the atomic number, Z. Interactions by Coulomb force with an external nucleus ﬁeld: Inelastic scattering with nuclei results • in photons means Bremsstrahlung. The probability of this interaction process is proportional to Z2. Soft collisions: Elastic scattering, in which electrons lose a small amount of energy, is necessary • to satisfy the conservation of momentum with a collision. The probability of this interaction is proportional to Z2.

Beta particles have the same mass and charge as the electrons, which diﬀer from their origin. Beta particles are emitted from the nucleus during radioactive decay, while electrons are produced or exist outside of the nucleus of an atom. Additionally, beta particles are generated by pair production that contains negatively charged particles (negatrons) and positively charged particles (positrons) at the same time. Each particle is released in a diﬀerent direction at an angle of 180◦ in pair production by the conversion of gamma radiation in the vicinity of a nucleus. Chemosensors 2020, 8, 106 6 of 42

There are two mechanisms by which beta particles interact with matter, ionization and electron orbital excitation [50], dissipating their kinetic energy. Plus, there is a third mechanism via which beta particles interact with matter, their radioactive energy is dissipated via Bremsstrahlung production, which distinguishes the beta particle compared to other radiation. Thus, a β particle has stopping power, described by Equation (2), which is given by the sum of the collisional and radioactive contributions [51]. ! ! ! dE dE dE = + (2) − dx tot − dx col − dx rad The expected value of the energy loss rate per unit path length, x, is called the stopping power (dT/dx)Y,T,Z by the type of charged particle, Y, in the medium of atomic number, Z, and the kinetic energy of the charged particle, T. Dividing the stopping power by the density (ρ) of the absorbing medium gives the mass stopping power (dT/ρdx)[48]. Stopping power can be divided into collision stopping power and radiative stopping power. Collision stopping power is the energy loss rate resulting from the sum of soft collisions and hard collisions, commonly referred to as collision interactions. Unless otherwise speciﬁed, radiation stopping power is assumed to originate solely from photons (bremsstrahlung). The energy consumed by radiative collision is generated from the track of a charged particle, and it leads to ionization and excitation nearby the track of the particle. The mass collision stopping power may be expressed as follows in Equation (3): ! ! ! dT dT dT = s + h (3) ρdx −ρdx c ρdx c where c is the collision interaction, s is the soft collision, and h is the hard collision.

5. Scintillation Process Scintillation is a phenomenon that a substance is illuminated by external energy such as radiation, electric field. When radiation is incident to the scintillator, it interacts with particles in the medium to transfer energy to the periphery and excite scintillation molecules. Excited molecules are then immediately stabilized and emit photons with as much light as the difference in the energy level when returning to the ground state [39]. These scintillation characteristics depend on the type of scintillation material, incident radiation, and the nature of secondary charged particles. Beta particles are absorbed in matter and their energy dissipates by colliding with molecules in the scintillator, which transfer the energy via heat, ionization, and excitation. For efficient energy transfer between beta particles and the scintillator, a scintillation cocktail, which converts kinetic energy of beta particles into light energy, is added to the scintillator. As the molecules excited by the scintillator transfer their energy to other molecules or solutions, electrons are excited. After an excited electron emits photon light in the ultraviolet region, it goes to the ground state. One of the key performance measures of the scintillator is the light yield, which is defined below [52].

# o f scintillation photons Light Yield = (4) energy o f particle [MeV]

The intensity of photon light from the scintillator is proportional to the initial energy of the beta particle through linear conversion of the photosensor. Photosensitive devices such as a photomultiplier tube (PMT) or silicon photomultiplier tube (SiPM) amplify the light and convert detected photon light into an electrical form to detect emitted photons eﬃciently. The photomultiplier tube collects the total photon light produced within the scintillator, and the inside face of the PMT is uniformly coated with a photosensitive material that converts photon light energy into electrical energy. The electrons are drawn to the electrodes in the photomultiplier tube by the positive potential of the electrodes to produce more electrons. These secondary electrons are attracted to the following electrodes and repeat Chemosensors 2020, 8, 106 7 of 42 the next diodes that make up the PMT (Figure2). The electrons ampliﬁed at each electrode stage produceChemosensors an 2020 electrical, 8, x pulse proportional to the photons. 7 of 42

Figure 2. Basic composition of the scintillation detector. PMT is very sensitive and generates small pulses even when there is almost no light and produces PMT is very sensitive and generates small pulses even when there is almost no light and noise that appears in the background area of the sample measurement. Additionally, noise is caused produces noise that appears in the background area of the sample measurement. Additionally, noise by external factors such as heat, cosmic rays, and fall out, and it is difficult to distinguish low energy is caused by external factors such as heat, cosmic rays, and fall out, and it is difficult to distinguish β-ray from this noise [53]. Photons incident to the photo-sensor generate photoelectric effects and are low energy β-ray from this noise [53]. Photons incident to the photo-sensor generate photoelectric converted into photoelectrons, which are amplified by electrical signals to a sufficient level for spectral effects and are converted into photoelectrons, which are amplified by electrical signals to a sufficient analysis through preamplifiers and main amplifiers. As the height of the converted current signal level for spectral analysis through preamplifiers and main amplifiers. As the height of the converted pulse is proportional to the energy of the incident radiation, analysis of the incident radiation from the current signal pulse is proportional to the energy of the incident radiation, analysis of the incident pulse-height is possible, and the amount of incident radiation can be derived. radiation from the pulse-height is possible, and the amount of incident radiation can be derived. 6. Characteristics of the Scintillator 6. Characteristics of the Scintillator The scintillator acts by converting radiation into scintillation and converts its light into current and The scintillator acts by converting radiation into scintillation and converts its light into current amplifies it through the light sensor to perform an energy analysis of incident radiation. Scintillators and amplifies it through the light sensor to perform an energy analysis of incident radiation. suitable for radiation detectors afford the following characteristics [27,30–32,40,42,48,51,53]. Scintillators suitable for radiation detectors afford the following characteristics [27,30– 32,40,42,48,51,53].High scintillation efficiency of radiation energy: The luminescence efficiency of the scintillator • is given by the ratio of energy lost by radiation within the scintillator to energy converted to  High scintillation efficiency of radiation energy: scintillation, and the luminescence intensity of the scintillator varies depending on the type of The luminescence efficiency of the scintillator is given by the ratio of energy lost by radiation scintillator and the quality of radiation. Higher scintillation efficiency increases the luminescence within the scintillator to energy converted to scintillation, and the luminescence intensity of the sensitivity of the scintillator due to its high energy absorption and conversion efficiency to photons. scintillator varies depending on the type of scintillator and the quality of radiation. Higher The luminescence efficiency of an organic scintillator is a function of the luminescence intensity of scintillation efficiency increases the luminescence sensitivity of the scintillator due to its high the anthracene to the electrons. A high light output means that when radiation with the same energy absorption and conversion efficiency to photons. The luminescence efficiency of an energy is incident, the number of photons produced is high, which means that the amount of organic scintillator is a function of the luminescence intensity of the anthracene to the electrons. data is high, which has a significant effect on the resolution. The high light output has high A high light output means that when radiation with the same energy is incident, the number of luminescence efficiency because it has excellent linearity proportional to the intensity of light photons produced is high, which means that the amount of data is high, which has a significant emitted by the scintillator and the energy of incident radiation. effect on the resolution. The high light output has high luminescence efficiency because it has High transparency: Higher transparency increases the amount of light that reaches the PMT, • excellent linearity proportional to the intensity of light emitted by the scintillator and the energy which increases the efficiency of light collection. It can be obtained by minimizing the of incident radiation. self-absorption, internal attenuation of the emitted photons, and high transfer efficiency to  High transparency: theHigher photosensor. transparency increases the amount of light that reaches the PMT, which increases the Short decay time of scintillation: The time taken for a luminescence phenomenon to dissipate, • efficiency of light collection. It can be obtained by minimizing the self-absorption, internal calledattenuation the scintillation of the emitted attenuation photons, time, and high classifies transfer luminescence efficiency to characteristics the photosensor. according to the  timeShort remaining decay time in of the scintillation: material as follows: Fluorescence: Light stops as soon as the energy is cutThe o timeff. Phosphorescence: taken for a luminescence Residual lightphenomenon remains even to dissipate, after cutting called off thethe scintillation incident energy. attenuation A short attenuationtime, classifies time luminescence of scintillation characteristics can count high according dose rates to ofthe radiation time remaining because ofin thethe decreasematerial inas thefollows: dead time and because the signal pulses are rapidly produced. In addition, the precision isFluorescence: proportional Light to the stops attenuation as soon as time the energy of the is scintillator, cut off. which improves the possibility of simultaneousPhosphorescence: measurement Residual light applications remains due even to after the short cutting rise off time the of incident the signal energy. pulse. Generally, A short attenuation time of scintillation can count high dose rates of radiation because of the decrease in the dead time and because the signal pulses are rapidly produced. In addition, the precision is proportional to the attenuation time of the scintillator, which improves the possibility of simultaneous measurement applications due to the short rise time of the signal

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there is an attenuation time of several seconds for an inorganic scintillator and several nanoseconds for organic scintillators. The wavelength distribution of the scintillation is suitable for the spectral sensitivity characteristics • of the photomultiplier tube.

When radiation that enters a scintillator is converted into light, it has much smaller energy and longer wavelengths than original radiation. If the wavelength distribution of the scintillation and the spectral sensitivity characteristics of the PMT are suitable, the photons can be efficiently converted into photoelectrons, thus achieving high photoelectron emission efficiency, defined as quantum efficiency. Therefore, the output wavelengths of light emitted from the scintillator should be consistent with the absorption wavelength and refractive index (e.g., the refractive index of glass, 1.5) of the photosensor. Therefore, the scintillator should be fabricated or selected with consideration of its compatibility with the response functions, such as the optimal wavelength area of the photosensor. In addition, there are indicators of comparable performance and influencing factors for determining optimal scintillation, such as radiation integrity, low cost, large-scale production possibility, thermal/mechanical integrity at the environment, and the applicability of pulse shape discrimination techniques. In addition, scintillators for beta-ray measurement have to be thin to detect the incident radiation only for beta-rays and avoid interference from gamma-rays [54].

6.1. Types of Scitillators The types of scintillators can be classiﬁed as follows, depending on their condition and chemical composition.

Depending on the condition: solids, liquids, and gases. • Depending on chemical composition: inorganic and organic scintillator. • Depending on the type of scintillator and the scintillation mechanism, the radiation under the measurement varies, and the characteristics of scintillators according to chemical composition are shown in Table3.

Table 3. Comparison of an organic scintillator and inorganic scintillator.

Organic Scintillator Inorganic Scintillator - short decay time (few ns) - high detection eﬃciency and emission intensity Strength - counts signal rapidly - good energy linearity - relatively inexpensive price - stable crystal structure (mechanical, chemically stable) - bad energy linearity - low atomic number and density - long decay time (hundreds of ns) Weakness - low possibility of photoelectron eﬀect - expensive price - bad energy linearity

6.1.1. Inorganic Scintillator An inorganic scintillator contains about 0.1% of impurities such as Eu, Ce, and Tl to enhance the luminescence and form an energy level at which scintillation can occur [55]. Therefore, it is not an exciton, a weakly coupled electron-electric pair, but an extrinsic crystal resulting from luminescence in impurities. Since a high-density material is added, it is used for measuring γ-rays, which have a long range in the medium. The following two types of inorganic scintillator.

NaI(Tl): It has a high density (3.67 g/cm3) and contains high atomic number composition (I, Z = 53), • and thus oﬀers eﬃcient detection of γ-rays and excellent linearity. Its use has been expanded to large-capacity detectors such as monitoring nuclear power plants, medical care, and security searches due to its relatively low price. However, it is used to measure γ and medium-hard X-rays rather than measuring α, β, and soft X-rays, which have weak permeability, because they have Chemosensors 2020, 8, 106 9 of 42

low mechanical thermal impact and have to be sealed with aluminum to block contact with the air [56]. BGO (Bismuth Germanate, Bi Ge O ): The high atomic number (Z = 83) and high density • 4 3 12 (Bi) (7.3 g/cm3) result in excellent detection eﬃciency and are used for γ-ray and X-ray measurements. In addition, the attenuation time of luminescence is very short, and thus it is used as a detector such as in X-ray CT and PET and has excellent mechanical strength and chemical properties. However, it has a low intensity of luminescence, resulting in lower energy resolution than NaI(Tl) [57,58]. LiI(Eu) is also used for thermal neutron measurements and ZnS(Ag) is used for α-ray measurements.

6.1.2. Organic Scintillator The organic scintillator is an aromatic hydrocarbon compound and has a benzene-ring structure [59]. Organic scintillators are classiﬁed into organic crystals, plastics, liquid scintillators, etc., depending on their densities [60]. Unlike inorganic scintillators, the luminescence is of molecular origin, forming an ion pair (exciton) by electron excitation between energy levels within the composition molecules [55]. The exciton then moves just below the conduction band within the scintillator crystal and falls down to the valence band via capture by the cation. Finally, photons are emitted by the formation of electron-hole pair recombination.

6.2. Liquid Scintillator It is widely used for measurements such as 3H, 14C, etc., due to the high detection eﬃciency in low energy β-rays. Since the samples containing 3H and 14C are measured directly in the liquid scintillator, there is no absorption or attenuation of the β-ray at the incident window. Due to its low atomic number and density, the luminescence eﬃciency of the liquid scintillator is not high.

6.3. Gas Scintillator Luminescence sensitivity is weaker than a solid scintillator because of its small density, but the attenuation time is the shortest. The luminescence eﬃciency is very small for γ-rays, electrons, and neutrons but relatively large for α-rays or ﬁssion fragments.

7. Inorganic Nanomaterials Conventional plastic scintillators are fabricated with an organic solvent containing a scintillation cocktail and then dried it. Therefore, the atomic number and density of the components are low, resulting in low light conversion efficiency in the scintillator. Additionally, they have poor detection efficiency and energy resolution. Adding materials with high atomic numbers and nanomaterials with various properties can increase the light conversion efficiency and density of the scintillator. Plastic scintillators containing nanomaterials of high atomic number and organic/inorganic hybrid scintillators with the addition of a high atomic number inorganic nanomaterial to the conventional plastic are manufactured to compensate for the shortcomings of the organic scintillator (low energy resolution and low measurement efficiency) while having the advantage of the inorganic scintillator. As conventional inorganic nanomaterials such as CdTe and ZnO have poor light conversion efficiency due to poor light absorption because of structural defects or low density, the luminescence rate is increased by adding activators such as Ce3+. Below is a summary of the types and characteristics of inorganic nanomaterials.

7.1. Perovskite (Calcium Titanium Oxide Mineral)

Perovskite, with a chemical formula of ABO3 [61], consists of a material with a high atomic number and high density, and it leads to a high light absorption rate, high charge diffusion coefficient, and excellent charge movement, resulting in high light conversion efficiency in the scintillator [56,62]. Chemosensors 2020, 8, x 10 of 42

Chemosensors 2020, 8, 106 10 of 42 because of its ionic compound, and also is environmentally harmful, addictive, and prone to oxidation due to Pb, its main component. Therefore, to overcome these disadvantages, a perovskite thatHowever, is not it addictive has shortcomings and has including high atmospheric vulnerability safety tomoisture using Sn and instead oxygen of because Pb was of recently its ionic developedcompound, [63]. and also is environmentally harmful, addictive, and prone to oxidation due to Pb, its main component. Therefore, to overcome these disadvantages, a perovskite that is not addictive and has 7.2.high CdTe atmospheric Structure safetyof LaF3 using:Ce/CdTe Sn instead of Pb was recently developed [63].

The CdTe structure of LaF3: Ce/CdTe is a Ce3+ doped fluorescent with Ce3+ as an activator with a 7.2. CdTe Structure of LaF3:Ce/CdTe very fast response speed and emits scintillation in the UV range. The CdTe luminescence of LaF3: 3+ 3+ Ce/CdTeThe CdTeis about structure five times of LaF stronger3: Ce/CdTe than is apure Ce CdTedoped due ﬂuorescent to improved with Ceenergyas antransfer activator and with an absorbenta very fast rate response of light speed and stab andility emits of scintillationstructure defects in the [64]. UV range. The CdTe luminescence of LaF3: Ce/CdTe is about ﬁve times stronger than pure CdTe due to improved energy transfer and an absorbent

7.3.rate CeF of light3/ZnO and stability of structure defects [64].

7.3. CeFZnO3/ ZnOis a semiconductor with a wide bandgap of 60 MeV and has a disadvantage of a low light emission rate due to its high exciton binding energy, and it has fast scintillation speed but relatively ZnO is a semiconductor with a wide bandgap of 60 MeV and has a disadvantage of a low light low density. To improve this, CeF3/ZnO with Ce3+ doping is developed, and energy transfer from emission rate due to its high exciton binding energy, and it has fast scintillation speed but relatively CeF3 in the scintillator to ZnO occurs. Compared with simple ZnO nanoparticles, the luminescence low density. To improve this, CeF /ZnO with Ce3+ doping is developed, and energy transfer from is 30 times higher and the X-ray luminescence3 is four times higher [65]. CeF3 in the scintillator to ZnO occurs. Compared with simple ZnO nanoparticles, the luminescence is 8.30 Beta-Ray times higher Detector and the Technology X-ray luminescence is four times higher [65]. 8. Beta-Ray Detector Technology 8.1. Commercial Plastic Scintillator 8.1. CommercialDepending Plasticon the Scintillatortype of scintillator, the physical and chemical properties such as light output and densityDepending vary, on and the typethus of it scintillator,is important the to physical choose and the chemical appropriate properties scintillator such as according light output to andthe radiationdensity vary, being and measured. thus it is Additionally, important to choosethe longer the appropriatewavelength scintillatorof light emitted according from tothe the scintillator radiation matchesbeing measured. the sensitivity Additionally, of the PMT, the longer there wavelength is less loss of lightlight, emitted and the from wavelength the scintillator of the matchesabsorption the andsensitivity emission of theof the PMT, radiation there is varies less loss from of light, one scinti and thellator wavelength to another. of theTherefore, absorption in this and section, emission the of characteristicsthe radiation variesof commercial from one scintillatorscintillators to mainly another. used Therefore, for the in measurement this section, the of characteristicsβ particles are of compared.commercial scintillators mainly used for the measurement of β particles are compared.

8.1.1.8.1.1. Eljen Eljen Technology Technology AsAs shown shown in in Figure Figure 33,, EJ-260 is a suitable scintillatorscintillator for sensitivity of the light sensor consistent withwith the long wavelength ofof emission.emission. TheThe greengreen fluorescencefluorescence emission emission of of EJ-260 EJ-260 is is su sufficientlyfficiently short short in inwavelength, wavelength, and and the the scintillation scintillation e ffiefficiencyciency is is su sufficientfficient to touse use withwith PMT,PMT, whichwhich is sensitive to blue blue wavelengths.wavelengths. The The characteristics characteristics of of EJ-260 EJ-260 are are shown shown in in Table Table 44..

FigureFigure 3. 3. EJ-260:EJ-260: (a (a) )EJ-260 EJ-260 scintillator, scintillator, ( (bb)) EJ-260 EJ-260 scintillator scintillator emission emission sp spectrumectrum with with wavelength wavelength [66]. [66].

Chemosensors 2020, 8, x 11 of 42

Table 4. Characteristics of the EJ-260 scintillator.

Basic Properties Characteristics Light Output (% Anthracene) 60 Chemosensors 2020, 8, 106 11 of 42 Scintillation Efficiency (photons/1 MeV e−) 9200

WavelengthTable of 4. MaximumCharacteristics Emission of the (nm) EJ-260 scintillator. 490

LightBasic Attenuation Properties Length (cm) Characteristics 350

Light OutputRise (% Time Anthracene) (ns) 1.5 60 Scintillation Eﬃciency (photons/1 MeV e−) 9200 Wavelength of MaximumDecay Time Emission (ns) (nm) 9.2 490 Light Attenuation Length (cm) 350 RisePolymer Time (ns) Base Polyvinyltoluene 1.5 Decay Time (ns) 9.2 Refractive Index 1.58 Polymer Base Polyvinyltoluene RefractiveLight IndexOutput 95% of that 1.58 at 20 °C Light Output 95% of that at 20 ◦C

8.1.2. Saint-Gobain 8.1.2. Saint-Gobain BC-400 and BC-404 are suitable for measuring γ, α, and β-rays below 5 MeV. In the case of BC-400 and BC-404 are suitable for measuring γ, α, and β-rays below 5 MeV. In the case of BC-400, BC-400, it is a commonly used scintillator, whereas BC-404 is used for fast counting. BC-408 it is a commonly used scintillator, whereas BC-404 is used for fast counting. BC-408 meanwhile is meanwhile is efficient for X, α, and β-ray measurements below 100 KeV and has high light output. It efficient for X, α, and β-ray measurements below 100 KeV and has high light output. It can be made to can be made to a large extent, suitable for measuring pollution in a wide site. Furthermore, BC-428 is a large extent, suitable for measuring pollution in a wide site. Furthermore, BC-428 is a scintillator a scintillator emitting green fluorescence, similar in efficiency to BC-400, but in the case of light emitting green fluorescence, similar in efficiency to BC-400, but in the case of light output, it has 56% output, it has 56% efficiency in non-alkali photocathode PMT. The emission spectrum and the efficiency in non-alkali photocathode PMT. The emission spectrum and the characteristics of each characteristics of each plastic scintillator are shown in Figure 4 and Table 5, respectably. plastic scintillator are shown in Figure4 and Table5, respectably.

Figure 4. Emission spectrum with wavelength of plastic scintillation (a) BC-400, (b) BC-404, (c) BC-408, Figure 4. Emission spectrum with wavelength of plastic scintillation (a) BC-400, (b) BC-404, and (d) BC-428 [67]. (c) BC-408, and (d) BC-428 [67].

Chemosensors 2020, 8, x 12 of 42

Chemosensors 2020, 8, 106 Table 5. Characteristics of plastic scintillator. 12 of 42

Properties BC-400 BC-404 BC-408 BC-428 CaF2:Eu Light Output (% Anthracene)Table 5. Characteristics 65 of plastic scintillator. 68 64 36 - Wavelength ofProperties Maximum Emission (nm) BC-400 423 BC-404 408 BC-408 425 BC-428 480 CaF2:Eu 435 Light Attenuation Length (cm) 160 140 210 150 - Light Output (% Anthracene) 65 68 64 36 - Wavelength of MaximumRise Time Emission (ns) (nm) 423 0.9 408 0.7 425 0.9 1.6 480 -435 Light AttenuationDecay Time Length (ns) (cm) 160 2.4 140 1.8 210 2.1 12.5 150 940 - RisePolymer Time (ns) Base 0.9 PVT 0.7PVT PVT 0.9 PVT 1.6 - - DecayDensity Time (ns)(g/cc) 2.4 1.023 1.81.023 1.023 2.1 1.032 12.5 3.18 940 Polymer Base PVT PVT PVT PVT - Refractive Index 1.58 1.58 1.58 1.58 1.47 Density (g/cc) 1.023 1.023 1.023 1.032 3.18 Refractive Index 1.58 1.58 1.58 1.58 1.47 CaF2:Eu is used to detect hundreds of keV of γ-rays due to the low atomic number that constitutes the substance and is suitable for detecting β particles due to low back-scattering. In CaF2:Eu is used to detect hundreds of keV of γ-rays due to the low atomic number that constitutes addition, CaF2:Eu is non-hygroscopic and thus radiation in the solution can directly contact the β thecrystals. substance Additionally, and is suitable with resistance for detecting to heatparticles and mechanical due to low impact, back-scattering. it can be made In addition, in various CaF 2forms.:Eu is non-hygroscopic and thus radiation in the solution can directly contact the crystals. Additionally, Figure 5 is a scintillation emission spectrum of CaF2:Eu with a maximum emission value near 435 nm. with resistance to heat and mechanical impact, it can be made in various forms. Figure5 is a scintillation CaF2:Eu penetrates visible light well but absorbs light at 400 nm, which partially overlaps with the emissionscintillation spectrum emission of CaF area,2:Eu causing with a self-absorption maximum emission of the value scintillator. near 435 Thus, nm. CaFfor applications2:Eu penetrates requiring visible lightoptimal well energy but absorbs resolution, light atless 400 than nm, which1 inch partiallyof length overlaps is appropriate with the due scintillation to the self-absorption emission area, causing self-absorption of the scintillator. Thus, for applications requiring optimal energy resolution, of CaF2:Eu. less than 1 inch of length is appropriate due to the self-absorption of CaF2:Eu.

FigureFigure 5.5. Emission spectrum withwith wavelengthwavelength ofof CaFCaF22:Eu [[68].68]. 8.2. Commercial Detector 8.2. Commercial Detector Commercial detectors for β measurement with high user convenience have been manufactured in Commercial detectors for β measurement with high user convenience have been manufactured various countries, including the United States and the Netherlands. in various countries, including the United States and the Netherlands. 8.2.1. United States 8.2.1. United States In 2013, the Savannah River National Laboratory (SRNL) developed a system for rapid analysis In 2013, the Savannah River National Laboratory (SRNL) developed a system for rapid analysis of tritium and other β or α particles at the site. Figure6 shows a portable rapid tritium analysis of tritium and other β or α particles at the site. Figure 6 shows a portable rapid tritium analysis system system (PORTAS) detector produced by SRNL that combines a sample holder, a small PMT, and a (PORTAS) detector produced by SRNL that combines a sample holder, a small PMT, and a multi-channel analyzer into a single package. These packages are surrounded by aluminum cases and multi-channel analyzer into a single package. These packages are surrounded by aluminum cases entirely shielded from outside light. Radioactive isotopes are measured by putting radionuclides in and entirely shielded from outside light. Radioactive isotopes are measured by putting radionuclides a liquid scintillating cocktail container. The container is fixed by a screw cap at the end of the case, in a liquid scintillating cocktail container. The container is fixed by a screw cap at the end of the case, and then by attaching the cap, light is blocked. Ortec fabricated a multi-purpose α/β counter using a and then by attaching the cap, light is blocked. Ortec fabricated a multi-purpose α/β counter using a gas flow proportional detector or a double phosphor scintillator (Figure7). The α/β systems of gas gas flow proportional detector or a double phosphor scintillator (Figure 7). The α/β systems of gas flow proportional counter type produced by Ortec are shown in Table6. flow proportional counter type produced by Ortec are shown in Table 6.

Chemosensors 2020, 8, 106 13 of 42 ChemosensorsChemosensors 2020 2020, ,8 8, ,x x 1313 of of 42 42

FigureFigureFigure 6. 6.6. Portable Portable rapid rapid tritium tritium analysis analysis system system (PoRTAS) (PoRTAS) detector: detector:detector: ( (aa)) PoRTAS PoRTAS detector detectordetector and andand cap capcap and andand (((bbb))) PoRTAS PoRTASPoRTAS systemsystem planeplane [[69]. [69].69].

FigureFigureFigure 7. 7.7. Gas-flow Gas-ﬂowGas-flow type type proportional proportional counter: counter:counter: ( (aa)) IPC-650, IPC-650, ( ((bb))) WPC-1150, WPC-1150, ( ((ccc))) WPC-1150-GFW WPC-1150-GFWWPC-1150-GFW [70,71]. [[70,71].70,71]. Table 6. Compared properties by the Ortec automatic count system. TableTable 6. 6. Compared Compared properties properties by by the the Ortec Ortec automatic automatic count count system. system. Properties IPC-650 (IPC-650-HP) WPC-1050 WPC-1150-GFW-3 PropertiesProperties IPC-650IPC-650 (IPC-650-HP) (IPC-650-HP) WPC-1050 WPC-1050 WPC-1150-GFW-3 WPC-1150-GFW-3 90 90 Sr9090Sr//Sr/YE9090YYﬃ ciency 55% 55% 63% 55%55% 55%55% 63%63% EfficiencyEfficiency Aluminized, Aluminized, Window Windowless 2 2 Aluminized,Aluminized,80 µg/cm Aluminized,Aluminized,80 µg/cm WindowWindow WindowlessWindowless 0.7 cpm22 22 β BKG Performance 0.7 cpm 8080 μ μg/cmg/cm 80801.5 μ μg/cmg/cm cpm ββ BKG BKG (0.90.70.7 cpm cpm cpm Warranted) 0.7 cpm 1.5 cpm 0.7 cpm Gas-Flow Type Gas-Flow1.5 Type cpm Proportional PerformancePerformanceGuard Detector X (0.9(0.9 cpm cpm Warranted) Warranted) GuardGuard Gas-FlowProportionalGas-Flow Type Type Counter Gas-FlowGas-FlowCounter Type Type X Detector X 8.9 cm (3.5. in) dia Gas Flow DetectorDetector 5.7 cm (2.25 inch)ProportionalProportional 5.7 cm (2.25 Counter Counter inch) ProportionalProportional Counter Counter DetectorDetectorDiameter 8.98.9 cm cm (3.5. (3.5.Detector in) in) dia dia Gas Gas 5.75.7 cm cm (2.25 (2.25 inch) inch) 5.7 5.7 cm cm (2.25 (2.25 inch) inch) DiameterDiameterShielding 10.2 cm (4 inch) 10 cm (2.4 inch)FlowFlow 10.2 cm Detector Detector (4 inch) ShieldingShieldingDetector Type 10.2 10.2 Hemispherical cm cm (4 (4 inch) inch) Style 10 10 cm cm Pancake (2.4 (2.4 inch) inch) Style 10.2 10.2 Pancake cm cm (4 (4 Styleinch) inch) DetectorDetector Type Type Hemispherical Hemispherical- Use in Radiochemical Style Style and Pancake Pancake Style Style - UsePancake in Pancake Health Style PhysicsStyle and - Use -- Use UseHealth in in Radiochemical Radiochemical Physics (IPC-650-HP) Environmental-- Use Use in in Health Health Monitoring Physics Physics Notes In Radiochemical - Use in Count for - Use in Air Sampler of Large andand Health Health Physics Physics (IPC- (IPC- and-- Use HealthUse In In Physics andand Environmental Environmental Ultra-Low-Level α/β Capacity Air Filter NotesNotes 650-HP)650-HP) RadiochemicalRadiochemical and and MonitoringMonitoring -- Use Use in in Count Count for for Ultra- Ultra- HealthHealth Physics Physics -- Use Use in in Air Air Sampler Sampler of of Low-LevelLow-Level α α//ββ LargeLarge Capacity Capacity Air Air Filter Filter

Chemosensors 2020, 8, 106 14 of 42 Chemosensors 2020, 8, x 14 of 42

OrtecOrtec used used a a gas gas flow-type ﬂow-type propor proportionaltional counter counter tube tube to to fabricate fabricate an an automatic/manual automatic/manual detection detection systemsystem and and a a single/multiple single/multiple detection detection systems systems and and P-10 P-10 gas gas for for filling ﬁlling gas gas (Figure (Figure 88).). The automatic detectordetector system system is is mainly mainly used used for for relatively relatively short short count count times times to to count count multiple multiple samples. samples. Table Table 6 showsshows Ortec’s Ortec’s automated automated detection detection systems. systems. In In Table Table 7,7, the manual single detector is compared to a longlong count count time time to to measure measure the the small small number number of of samples. samples.

Figure 8. Gas-flow type proportional counter: (a) MPC-900-GFL and (b) MPC-1000-GFL [71]. Figure 8. Gas-flow type proportional counter: (a) MPC-900-GFL and (b) MPC-1000-GFL [71]. Table 7. Compared properties by the Ortec Manual Single Counter. Table 7. Compared properties by the Ortec Manual Single Counter. Properties MPC-900(GFL/GFW) MPC-1000(GFL/GFW) Properties MPC-900(GFL/GFW) MPC-1000(GFL/GFW) 90Sr/90YEfficiency 55% 55% 90Sr/90Y Aluminized, 8055%µg /cm2 Aluminized, 8055%µg/cm 2 EfficiencyWindow (GFW: Windowless) (GFW: Windowless) Aluminized, 80 μg/cm2 Aluminized, 80 μg/cm2 Window 35 cpm 0.7 cpm β BKG Performance (GFW: Windowless) (GFW: Windowless) (55 cpm Warranted) (0.9 cpm Warranted) β BKG 35 cpm 0.7 cpm Cosmic Guard Detector PerformanceGuard Detector (55 cpm X Warranted) (0.9 cpm Warranted) (Large Area Gas Flow Proportional Counter) Cosmic Guard Detector Planchet Diameter 5.1 cm (2 inch) 5.1 cm (2 inch) Guard Detector X (Large Area Gas Flow Shielding - 10 cmProportional (2.4 inch) Thick Counter) PlanchetDetector Type Hemispherical Style Pancake Style 5.1 cm (2 inch) 5.1 cm (2 inch) Diameter - Use in the place where not far - Ultra low-level background radioactive Notes and not require high sensitivity for Shielding - 10counter cm (2.4 inch) Thick Detector Type β-ray Hemispherical measurement Style Pancake Style - Use in the place where not far and not - Ultra low-level background MPC-9604Notes is an α/requireβ multi-detector high sensitivity system for β (MDS)-ray for low-level background radiation radioactive counter measurements and is used for rapid samplemeasurement throughput and high sensitivity (Figure9). Each MPC-9604 contains four separated 2.25-inch diameter pancake type gas-flow proportional counters and aluminum windows.MPC-9604 By connecting is an α up/β tomulti-detector 12 MPC-9604 devicessystem to(MDS) a single for PC, low-level a total of 48background independent radiation channels measurementscould be used. Theand MDSis used system for isra shieldedpid sample with throughput a large gas flowand proportionalhigh sensitivity counter (Figure guard 9). detector Each MPC-9604to block the contains cosmic four rays separated and a 4-inch 2.25-inch lead shield diamet toer remove pancake background type gas-flow noise. proportional It also incorporates counters andamplifiers aluminum of spectroscopy windows. By grade connecting to process up signalsto 12 MPC-9604 and linear devices low voltage to a powersingle supplyPC, a total devices of 48 to independentremove electrical channels interference. could be Theseused. signalsThe MDS are system transmitted is shielded through with the a shieldedlarge gas cable.flow proportional A RFI (radio counterfrequency guard interface) detector guard to block and the metal cosmic enclosure rays and are a applied 4-inch lead to eliminate shield to noise. remove background noise. It also incorporates amplifiers of spectroscopy grade to process signals and linear low voltage power supply devices to remove electrical interference. These signals are transmitted through the shielded cable. A RFI (radio frequency interface) guard and metal enclosure are applied to eliminate noise.

ChemosensorsChemosensors 20202020, ,88, ,x 106 1515 of of 42 42 Chemosensors 2020, 8, x 15 of 42

FigureFigure 9. 9. Gas-flowGas-flow type type proportional proportional counter: counter: (a (a) )MPC-9604 MPC-9604 and and (b (b) )multi-detector multi-detector system system (MDS) (MDS) Figure 9. Gas-flow type proportional counter: (a) MPC-9604 and (b) multi-detector system (MDS) compositioncomposition linked linked with with a a P-10 P-10 gas gas tank tank [72]. [72]. composition linked with a P-10 gas tank [72]. OrtecOrtec has has developed developed a a dual dual phosphor-type phosphor-type counter, counter, ASC-950-DP ASC-950-DP automatic automatic sample sample changer changer and and Ortec has developed a dual phosphor-type counter, ASC-950-DP automatic sample changer and MPC-900-DPMPC-900-DP manual manual single single sample sample changer changer (Figure (Figure 10). 10). It It is is mainly mainly used used for for health health physics physics where where MPC-900-DP manual single sample changer (Figure 10). It is mainly used for health physics where rapidrapid counts counts are requiredrequired suchsuch as as for for smear smear or or air ai filterr filter measurements. measurements. A 1.5A 1.5 kg lightweightkg lightweight probe probe was rapid counts are required such as for smear or air filter measurements. A 1.5 kg lightweight probe wasproduced produced to monitor to monitor low/ low/middle/highmiddle/high energy energyβ particles β particles in a wide-areain a wide-area plane, plane, such such as floors as floors and walls and was produced to monitor low/middle/high energy β particles in a wide-area plane, such as floors and walls(Figure (Figure 11a). The11a). probe The consistsprobe consists of a rectangular of a rectangular large-area large-area window ofwindow 600 cm 2of, making 600 cm it2, sensitivemaking it to walls (Figure 11a). The probe consists of a rectangular large-area window of 600 cm2, making it sensitivethe detection to the of detection low-level of energy-emitting low-level energy-emitting isotopes in a isotopes wide area. in Ita waswide shown area. It to was have shown 19% e ffitociency have sensitive to the detection of low-level energy-emitting isotopes in a wide area. It was shown to have 19%and sensitivityefficiency forand60 sensitivityCo, 26% for for36Cl, 60Co, and 26% 24% for for 9036Cl,Sr/ 90andY. BP19AD 24% for/BP19DD 90Sr/90Y. betaBP19AD/BP19DD probes applied largebeta 19% efficiency and sensitivity for 60Co, 26% for 36Cl, and 24% for 90Sr/90Y. BP19AD/BP19DD beta probesBC-400 applied plastic scintillator large BC-400 probes plas insidetic scintillator the light-alloy probes die-cast inside housing the (Figurelight-alloy 11b). die-cast The reaction housing with probes applied large BC-400 plastic scintillator probes inside the light-alloy die-cast housing (Figurethe low 11b). energy Theβ reaction-ray is good with resultsthe low from energy a low β-ray background. is good results HP-380B from a and low HP-380AB background. are HP-380B portable (Figure 11b). The reaction with the low energy β-ray is good results from a low background. HP-380B andsurvey HP-380AB meters withare portable a high α/ βsurveysensitivity meters including with a lowhigh background α/β sensitivity counts including (Figure 12lowa). background The detector and HP-380AB are portable survey meters with a high α/β sensitivity including low background countscan be (Figure used as 12a). a smart The detector probe, including can be used a memoryas a smart device probe, that including stores a all memory calibration device and that function stores counts (Figure 12a). The detector can be used as a smart probe, including a memory device that stores allparameters. calibration Using and function ZnS(Ag) parameters. for α-ray detection Using ZnS(Ag) and a plastic for α-ray scintillator detection for andβ-ray a plastic detection, scintillator a dual all calibration and function parameters. Using ZnS(Ag) for α-ray detection and a plastic scintillator forphosphor β-ray detection, material fora dual distinguishing phosphor materialα-ray and forβ distinguishing-ray was conjugated. α-ray and In addition, β-ray was a fine conjugated. mesh made In for β-ray detection, a dual phosphor material for distinguishing α-ray and β-ray was conjugated. In addition,of stainless-steel a fine mesh was usedmade to of protect stainless-steel the detector’s was us externaled to protect area from the holesdetector’s inside external of the lightweight area from addition, a fine mesh made of stainless-steel was used to protect the detector’s external area from holesaluminum inside housing.of the lightweight In addition, aluminum a dual phosphor housing. scintillationIn addition, probea dual consisting phosphor ofscintillation DP8A and probe DP8B holes inside of the lightweight aluminum housing. In addition, a dual phosphor scintillation probe consistingis used to of monitor DP8A andα/β DP8Bsurface is contaminationused to monitor such α/β surface as floors contamination and walls. It such has aas high floors sensitivity and walls. to consisting of DP8A and DP8B is used to monitor α/β surface contamination such as floors and walls. Itdistribute has a high radioactive sensitivity contamination to distribute radioactive using a large-area contamination probe ofusing 600 a cm large-area2 (Figure probe 12b). of In 600 Table cm82, It has a high sensitivity to distribute radioactive contamination using a large-area probe of 600 cm2 (Figurethe characteristics 12b). In Table of the8, the probes characteristics from ThermoFisher of the probes are compared.from ThermoFisher are compared. (Figure 12b). In Table 8, the characteristics of the probes from ThermoFisher are compared.

FigureFigure 10.10.Dual Dual phosphor phosphor type counter:type counter: (a) ASC-950-DP (a) ASC-950-DP automatic sampleautomatic changer sample and (bchanger) MPC-900-DP and (Figuremanualb) MPC-900-DP single10. Dual sample manual phosphor changer single [typesample70,71 ].counter: changer ([70,71].a) ASC-950-DP automatic sample changer and (b) MPC-900-DP manual single sample changer [70,71].

Chemosensors 2020, 8, 106 16 of 42 Chemosensors 2020, 8, x 16 of 42

FigureFigure 11. 11. (a()a )BP17A BP17A beta beta probes probes [73] [73 ]and and (b (b) )BP19AD/BP19BDD BP19AD/BP19BDD beta beta probes probes [74]. [74].

FigureFigure 12. 12. (a()a )HP-380AB HP-380AB alpha-beta alpha-beta scintillati scintillationon hand hand probes probes [75] [75 ]and and (b (b) )DP8A/DP8B DP8A/DP8B alpha-beta alpha-beta probesprobes [76]. [76]. Table 8. Compared characteristics of the probes from ThermoFisher. Table 8. Compared characteristics of the probes from ThermoFisher. Model BP17A BP19AD BP19DD HP380B HP380AB DP8A/DP8B Model BP17A BP19AD BP19DD HP380B HP380AB DP8A/DP8B Probe Type Beta Beta Beta Beta Scintillation Alpha-Beta Scintillation Alpha-Beta 2 Beta Alpha-Beta WindowProbe Type Area [cm ] Beta600 Beta 100 100 Beta 100 100Alpha-Beta 600 Weight [kg] 1.5 0.5 0.5Scintillation 0.59 Scintillation 0.59 1.55 s 1 < 3 s 1 < 10 s 1 < 8 s 1 < 10 s 1 < 10 s 1 < 30 BKGWindow Measurement − − − − − − cpm600 < 1800 cpm100< 600 cpm <100480 cpm < 600100 cpm 100< 600 cpm 600 < 1800 Area [cm2] Efficiency (%) Weight [kg] 1.5 0.5 0.5 0.59 0.59 1.55 14 C −1 -−1 14 21−1 28−1 −1 -−1 - BKG s−1 < 3 s−1 < 10 s−1 < 8 s−1 < 10 s−1 < 10 s−1 < 30 36Cl 26 49 48 - - 31 Measurement60Co cpm < 180019 cpm 32 < 600 cpm 34 < 480 cpm - < 600 cpm < - 600 cpm < 1800 12 90Sr/90Y 24 51Efficiency 51 (%) 52 44 36 99 14 - - - - 18 - 14C Tc - 14 21 28 - - 241Am - - - - 36 28 36Cl 26 49 48 - - 31 60Co 19 32 34 - - 12 90AMS-4Sr/90Y is a continuous24 detection 51 system 51 for early warning 52 of worker 44 exposure by 36 airborne emission99Tc such as β-ray- emission particles,- radioactive- iodine,- and noble gas 18 in the air (Figure - 13). Both241 fixedAm and portable- types are - available due - to its light - and rigid composition 36 with 28 a size of 32.5 cm H 27.9 cm W 22.2 cm D and weight of 3.4 kg. Table9 shows the specifications of each × × functionAMS-4 of theis a AMS-4. continuous It provides detection DAC system (Derived for Airearly Concentration)-based warning of worker alertsexposure for radionuclidesby airborne emissionspecified such in 10 as CFR β-ray Part emission 20. An Ar particles,/CO2 gas radioactive proportional iodi detectorne, and can noble monitor gas thein dischargethe air (Figure of effl 13).uent Boththrough fixed an and in-line portable sampling types head are andavailable provides due ato real-time its light γand-ray rigid background composition subtraction with a functionsize of 32.5using cm the H remote× 27.9 cm sampling W × 22.2 function. cm D and weight of 3.4 kg. Table 9 shows the specifications of each function of the AMS-4. It provides DAC (Derived Air Concentration)-based alerts for radionuclides specified in 10 CFR Part 20. An Ar/CO2 gas proportional detector can monitor the discharge of effluent through an in-line sampling head and provides a real-time γ-ray background subtraction function using the remote sampling function.

Chemosensors 2020, 8, 106 17 of 42 ChemosensorsChemosensors 2020 2020, ,8 8, ,x x 1717 of of 42 42

FigureFigure 13. 13. AMS-4 AMS-4AMS-4 beta beta beta air air air monitor monitor monitor [77]. [77]. [77].

Table 9. AMS-4 speciﬁcations. TableTable 9. 9. AMS-4 AMS-4 specifications. specifications. Function Radial Sampling Head Inline Sampling Head Noble Gas Sampling Head FunctionFunction RadialRadial Sampling Sampling Head Head Inline Inline Sampli Samplingng Head Head Noble Noble Gas Gas Sampling Sampling Head Head

ProbeProbeProbe TypeType Type Proportional Proportional Proportional (Ar/CO (Ar/CO (Ar/CO2)22) ) 3 SizeSize (cm (cm3)3) 24.9 24.9 HH × 14.214.2 WW × 18.5 D 24.1 24.1 H H × 23.623.6 W W × 16.316.3 D D 29.5 29.5 H H × 14.214.2 W W × 18.518.5 D D Size (cm ) 24.9 H × 14.2 W ×× 18.5 D 24.1 H ×× 23.6 W × ×16.3 D 29.5 H × 14.2× W × 18.5× D 444πππ Efficiency EfficiencyEﬃciency (%) (%) 6060Co 8.5 5.75 - 60CoCo 8.58.5 5.75 5.75 - - 90 90 9090SrSr/Sr//9090YY 171717 12 12 12 - - - 858585KrKrKr ------6.4 6.4 6.4 133 133133XeXeXe ------4.4 4.4 4.4

HP-210 and HP-360 are designed to detect effluent from radiation workplaces and to prevent HP-210HP-210 and and HP-360 HP-360 are designed toto detectdetect eeffluentffluent from from radiation radiation workplaces workplaces and and to to prevent prevent the the spread of contamination inside and outside the laboratory, designed for daily contamination thespread spread of contaminationof contamination inside inside and outsideand outside the laboratory, the laboratory, designed designed for daily for contamination daily contamination surveys surveys of all surfaces that could be exposed to radiation, such as individuals, tables, floors, and surveysof all surfaces of all surfaces that could that be could exposed be toexposed radiation, to ra suchdiation, as individuals, such as individuals, tables, floors, tables, and floors, equipment and equipment (Figure 14). High sensitivity using thin mica windows protected by etched (stainless-steel equipment(Figure 14 ).(Figure High 14). sensitivity High sensitivity using thin using mica thin windows mica windows protected protected by etched by (stainless-steel etched (stainless-steel screens) screens) to β-ray emission of surface contamination (40 keV) can meet laboratory environmental and screens)to β-ray to emission β-ray emission of surface of surface contamination contamination (40 keV) (40 can keV) meet can laboratorymeet laboratory environmental environmental and safety and safety requirements. For HP-210T, high-density tungsten shielding enables relatively low β-ray safetyrequirements. requirements. For HP-210T, For HP-210T, high-density high-density tungsten tungsten shielding shielding enables relatively enables lowrelativelyβ-ray monitoringlow β-ray monitoring in a γ background, and aluminum-housed HP-210AL allows low energy β-ray monitoringin a γ background, in a γ andbackground, aluminum-housed and aluminum-housed HP-210AL allows HP-210AL low energy allowsβ-ray low monitoring energy β in-ray the monitoring in the low energy background area. Table 10 shows the specifications of the monitoringlow energy backgroundin the low area.energy Table background 10 shows the area. specifications Table 10 ofshows the Geiger–Mueller the specifications detectors of fromthe Geiger–Mueller detectors from ThermoFisher (Waltham, MA, USA). Geiger–MuellerThermoFisher (Waltham, detectors from MA, USA).ThermoFisher (Waltham, MA, USA).

FigureFigure 14. 14. HP-210/HP-360 HP-210/HP-360HP-210/HP-360 series series series pancake pancake pancake Geiger-Mueller Geiger-Mueller detectors: detectors: ( (a (aa)) )HP-210 HP-210 HP-210 [78], [78], [78], ( (b (b)) )HP-360 HP-360 HP-360 [79]. [79]. [79].

ChemosensorsChemosensors 20202020,, 88,, x 106 1818 of of 42 42

Table 10. Specifications of HP-210/HP-360 series pancake Geiger–Mueller detectors. Table 10. Speciﬁcations of HP-210/HP-360 series pancake Geiger–Mueller detectors. Model HP-210 HP-360 SizeModel (cm3) 16.5 L × HP-2108.9 W × 9.7 H 24.8 L HP-360× 6.8 W × 7 H Weight (kg) 16.5 L 0.7–1.98.9 W 9.7 24.8 L 0.236.8 W 7 Size (cm3) × × × × Dead Time (μs) H 50 H Mica Window Size (cm.Weight diameter)/Thickness (kg) (mg/cm2) 0.7–1.94.45/1.4–2.0 0.23 Dead Time (µs) 50 14C 6 Mica Window Size (cm. diameter)/Thickness (mg/cm2) 4.45/1.4–2.0 60Co 16 14 6 with Screen 90Sr/C90Y 32 60Co 16 99Tc 15 with Screen 90Sr/90Y 32 99137Cs 22 β Efficiency (%) Tc 15 13714CsC 6 22 - β Eﬃciency (%) 60Co 16 - 14C 6 - 90 90 without Screen 60Sr/CoY 3216 - 99 without Screen 90Sr/Tc90 Y 1532 - 99137TcCs 2215 - 137Cs 22 - 8.2.2. Netherlands 8.2.2.Scionix Netherlands produced an α/β detector using SIPM (J-60035-4P) as a light sensor. Figure 15 shows a plasticScionix scintillator produced mixed an withα/β detectorZnS(Ag) usingwith an SIPM α/β (J-60035-4P) detector produced as a light by sensor. Scionix Figure with a15 polyester shows a housingplastic scintillator and a double mixed aluminum with ZnS(Ag) miler (mylar, with an0.9α mg/cm/β detector2) incident produced window by Scionixto blockwith out light. a polyester Other characteristicshousing and aof double this commercial aluminum detector miler (mylar,are shown 0.9 in mg Table/cm2 )11. incident window to block out light. Other characteristics of this commercial detector are shown in Table 11.

Figure 15. Silicon photomultiplier tube (SiPM) based alpha-beta detector, Scionix: (a) α/β detector and Figure 15. Silicon photomultiplier tube (SiPM) based alpha-beta detector, Scionix: (a) α/β detector and (b) detector plane [80]. (b) detector plane [80]. Table 11. Characteristics of Scionix alpha-beta detector. Table 11. Characteristics of Scionix alpha-beta detector. Protection 72% Transmission Stainless Steel grid Protection 72% Transmission Stainless Steel grid Power Supply +5–16 V (3 mA) ElectricalPower Connections Supply ERA0S302CLL + 5–16 Flying V (3 LeadsmA) (LEMO) Electrical Connections ERA0S302CLL Flying Leads (LEMO) Performance Performance 150 ns (betas) Pulse Rise Time (0–100%) 150 ns (betas) Pulse Rise Time (0–100%) 530 ns (alphas) 5301.2 nsµs (alphas) (betas) Pulse Fall Time (1/e) 5301.2 μ nss (alphas)(betas) Pulse Fall Time (1/e) Approximately 75 mV/MeV(betas) Gain 530 ns (alphas) ApproximatelyApproximately 500 75 mV mV/MeV(betas)/5.48 MeV (alphas) Gain Approximately 500 mV/5.48 MeV (alphas)

ChemosensorsChemosensors 20202020,, 88,, x 106 1919 of of 42 42

8.2.3. Japan 8.2.3. Japan In 2013, Japan produced a measurement system to analyze 90Sr underwater (Figure 16) based on 90 a combinationIn 2013, Japan of a plastic produced detector a measurement and a gas flow system type to Geiger–Muller analyze Sr underwater counter tube (Figure surrounded 16) based by on a leada combination shield. The of electrons a plastic collected detector andby each a gas detector flow type are Geiger–Muller processed through counter a simultaneous tube surrounded circuit. by Fora lead 90Sr shield. measurement The electrons in groundwater collected byin a each depth detector of 4 m, are a 2processed Bq/L detection through limit a was simultaneous obtained within circuit. 90 10For daysSr [81–83]. measurement Likewise, in groundwater the pico-beta in analyzer a depth requ of 4ires m, a a 2 long Bq/L average detection analysis limit was time obtained of two to within four days10 days per [81bottle–83]. of Likewise, the liquid the sample pico-beta and analyzerrequires requirespretreatment a long such average as fuming analysis nitric time acid of twoor the to strontiumfour days perresin bottle method of the [82]. liquid In addition, sample and due requires to the thick pretreatment lead shielding such as and fuming the nature nitric acidof the or gas the flow-typestrontium resinproportional method [counter,82]. In addition, a Q gas due supply to the thicksystem lead is shieldingessential, andmaking the nature it difficult of the gasto moveflow-type outside. proportional counter, a Q gas supply system is essential, making it difficult to move outside.

Figure 16. Pico-beta analyzer, TEPCO [82]. Figure 16. Pico-beta analyzer, TEPCO [82]. 8.3. Flow Cell Detector 8.3. Flow Cell Detector Tritium inflows to the human body via inhalation or ingestion, and the formed organically bound tritiumTritium (OBT) inflows leads toto thethe detrimenthuman body of DNAvia inhalation [84]. Thus, or theingestion, maximum and tritiumthe formed concentration organically of bounddrinking tritium water (OBT) is legislated leads to in everythe detriment country (EPAof DNA of U.S: [84]. 740 Thus, Bq/L, the EURATOM maximum of tritium EU: 100 concentration Bq/L) [85,86]. ofFurthermore, drinking water detectable is legislated technologies in every withcountry rapidity (EPA andof U.S: precision 740 Bq/L, to preventEURATOM tritium of EU: pollution 100 Bq/L) are [85,86].required. Furthermore, Conventional detectable low energy technologies beta-emitting with rapidity isotopes and are precision measured to prevent by a liquid tritium scintillation pollution arecounter required. (LSC) Conventional due to the short low rangeenergy of beta-emitting the beta particle. isotopes However, are measured a LSC is by not a liquid appropriate scintillation for an counterin-situ measurement, (LSC) due to the and short pretreatment range of forthe sampling beta particle. and longHowever, analysis a LSC time is are not required. appropriate In the for case an in-situof a gas measurement, detector, a complicated and pretreatment system for where sampling the ionizing and long chamber analysis and time gas are tank required. are combined In the withcase ofthe a detectorgas detector, make a automaticcomplicated work system infeasible. where Whilethe ionizing the solid chamber detector and can gas measure tank are beta-rays combined directly, with thethe detector performance make isautomatic degraded work by erosioninfeasible. from While water. the solid To compensate detector can for measure these defects, beta-rays studies directly, on thefabricating performance a flow-cell is degraded type detector by erosion to monitor from andwate preventr. To compensate low beta-ray for e ffltheseuents defects, have been studies carried on fabricatingout at home a andflow-cell abroad. type detector to monitor and prevent low beta-ray effluents have been carried out at home and abroad. 8.3.1. Korea 8.3.1. Korea In UNIST, a study on real-time tritium monitoring at underwater was performed. The tritiated waterIn isUNIST, electrolyzed a study using on real-time a proton tritium exchange monitori membraneng at underwater (PEM) cell, was and performed. is measured The using tritiated a gas waterproportional is electrolyzed counter. using Figure a 17 proton presents exchange the composition membrane of (PEM) the electrolysis cell, and systemis measured with theusing PEM a gas cell. proportional counter. Figure 17 presents the composition of the electrolysis system with the PEM cell.

Chemosensors 2020, 8, 106 20 of 42

Chemosensors 2020, 8, x 20 of 42

Figure 17. Electrolysis system using the proton exchange membrane (PEM) cell [[87].87].

After putting tritium tritium water water into into a a water water container, container, it it is is supplied supplied to to the the PEM PEM cell cell by by connecting connecting a apower power supply supply to tothe the water water pump. pump. In InFigure Figure 18, 18HT, HT gas, gas, H2 Hgas,2 gas, and and a small a small quantity quantity of water of water are areproduced produced and and only only generate generate gases gases that that are are captur captureded by by the the probe, probe, which which comprises comprises of of a plastic scintillator and two PMT (R 878, Hamamatsu) insideinside anan acrylacryl case.case.

Figure 18. The flow ﬂow cell detector probe [[87].87].

The aboveabove system system is usedis used to measureto measure the radioactivethe radioactive variation variation of tritium of pertritium unit ofper mass unit according of mass toaccording the current to the applied current to applied the PEM to cell.the PEM It also cell. calculates It also calculates the ratio the of ratio the current of the current change change rate to rate the changeto the change in radioactivity in radioactivity per unit per mass unit inmass the in number the number of electrolyzed of electrolyzed tritium tritium and predicts and predicts the trend the oftrend radioactive of radioactive change change due to due the to change the change in current. in current. As a As result, a resu thelt, current the current was optimizedwas optimized at 7 A,at with7 A, with a detection a detection efficiency efficiency of 31.3% of 31.3%1.3% ± 1.3% and and a minimum a minimum detectable detectable activity activity (MDA) (MDA) calculated calculated at ± 10.3at 10.30.8 ± 0.8 kBq kBq/m/m3 for3 for five five minutes. minutes. Additionally, Additionally, the the detection detection e efficiencyfficiency of ofa a LSCLSC isis 34.4%34.4% ± 0.2%, ± ± and it showed a detection eefficiencyfficiency ofof 0.910.91 ± 0.04 for gas tritium produced by electrolysis. ± 8.3.2. Japan In 2017, the National Institute for Fusion Science (NIFS) developed a monitoring system based In 2017, the National Institute for Fusion Science1 (NIFS) developed a monitoring system based on flowflow cell detectors for real-timereal-time measurement of underwater tritiumtritium concentrations. The flowflow cell detector waswas fabricatedfabricated using using a a granulated granulated CaF CaF2 solid2 solid scintillator. scintillator. Figure Figure 19a 19a shows shows three three types types of flow of flow cell detectors used in the experiment: a single diameter of 3 mm cells, a series of 3 mm cells in diameter, a single diameter of 5 mm cells. The flow cell is made of Teflon PFA tubes (Figure 19a). Figure 19b shows the configuration of the flow cell detectors for tritium, including flow cells, a pair of PMTs, high voltage power, coincidence factor modules, flow cell pumps, and sample bottles. The sample bottle is a model simulating an effluent tank at the radiation facility and is filled with tritiated

Chemosensors 2020, 8, 106 21 of 42 cell detectors used in the experiment: a single diameter of 3 mm cells, a series of 3 mm cells in diameter, a single diameter of 5 mm cells. The flow cell is made of Teflon PFA tubes (Figure 19a). Figure 19b shows the configuration of the flow cell detectors for tritium, including flow cells, a pair of PMTs, high voltageChemosensors power, 2020 coincidence, 8, x factor modules, flow cell pumps, and sample bottles. The sample21 bottle of 42 is a model simulating an effluent tank at the radiation facility and is filled with tritiated water. The flow water. The flow cell pump sends the tritiated water to the flow cell, which is placed between the two cell pump sends the tritiated water to the flow cell, which is placed between the two PMTs for the PMTs for the coincidence system. Samples of various concentrations ae then produced by diluting coincidencethe commercial system. tritiated Samples water of with various distilled concentrations water and measured ae then producedfor 600 and by 10,000 diluting s using the each commercial flow tritiatedcell. As water a result with of the distilled study on water the relationship and measured between for the 600 count and speed 10,000 and s concentration using each flow by passing cell. As a resultsamples of the through study on flow the cells, relationship the 5 mm between diameter the thre counte cell speed series and accurately concentration measured by a passing low tritium samples throughconcentration flow cells, of the 10 5Bq/mL mm diameter while maintaining three cell serieslinearity accurately between measured the count a speed low tritium and the concentration tritium of 10concentration. Bq/mL while maintaining linearity between the count speed and the tritium concentration.

FigureFigure 19. 19.Flow Flow cell cell system: system: ( a(a))the the prototypeprototype flow ﬂow cell, cell, (b (b) composition) composition of the of the detection detection system system [88,89]. [88 ,89].

8.3.3.8.3.3. UK UK In 2019,In 2019, Lancaster Lancaster University University from from thethe UK conducted a astudy study on on the the fabrication fabrication of scintillation of scintillation detectorsdetectors to measureto measure tritium tritium in in groundwater. groundwater. First, CaF CaF2:Eu,2:Eu, a non-hygroscopic a non-hygroscopic inorganic inorganic scintillator, scintillator, was selected and manufactured as a homogeneous scintillator with CaF2:Eu powder using two was selected and manufactured as a homogeneous scintillator with CaF2:Eu powder using two methods: a chemicalmethods: method a chemical and method a granulation and a granulation method. Due me tothod. the Due quantum to the mechanicalquantum mechanical nature of nature the nanosize of the nanosize particles, reducing the radius of the scintillation particles results in structural changes, particles, reducing the radius of the scintillation particles results in structural changes, such as expansion such as expansion of the absorption and emission bands and increase of the forbidden band, which of the absorption and emission bands and increase of the forbidden band, which leads to an increase of leads to an increase of the scintillator luminescence. A study was conducted to fabricate scintillation the scintillator luminescence. A study was conducted to fabricate scintillation particles with a small particles with a small radius via two methods. Chemical approaches, such as a reverse micelle radiusmethod, via two electrodeposition, methods. Chemical and precipitation approaches, methods, such as were a reverse used, and micelle the sizes method, of particles electrodeposition, for each andprocess precipitation are shown methods, in Table were 12. used, and the sizes of particles for each process are shown in Table 12.

TableTable 12. 12.Particle Particle diameterdiameter with with each each chemical chemical method. method.

MethodMethod Particle Particle Diameter Diameter (nm) ReverseReverse Micelle Micelle Method Method 20 20 nm nm ElectrodepositionElectrodeposition 500 500 nm nm Precipitation 10 2 nm (depend on pH) ± ChemicalPrecipitation Coprecipitation 10 ±30–35 2 nm nm,(depend 15–20 on nm pH) Chemical Coprecipitation 30–35 nm, 15–20 nm The other is granulation, a pure mechanical method, which is carried out to produce ﬁne particles. The other is granulation, a pure mechanical method, which is carried out to produce fine There are two methods of granulation, ball milling and use of a mortar and pestle to pulverize a particles. There are two methods of granulation, ball milling and use of a mortar and pestle to monocrystalline scintillator to produce particles of a desired size. Figure 20 shows a comparison of the pulverize a monocrystalline scintillator to produce particles of a desired size. Figure 20 shows a comparison of the scanning electron microscope (SEM) image of CaF2:Eu pulverized with the two methods. The size distribution of particles produced by the mortar and pestle method for the two

Chemosensors 2020, 8, 106 22 of 42

Chemosensors 2020, 8, x 22 of 42 Chemosensorsscanning electron2020, 8, x microscope (SEM) image of CaF2:Eu pulverized with the two methods. The22 of size 42 distribution of particles produced by the mortar and pestle method for the two CaF2:Eu crystals CaF2:Eu crystals showed that the distribution of particle size, 2–11 μm and 2–50 μm, with the centers CaFshowed2:Eu crystals that the showed distribution that ofthe particle distribution size, 2–11 of particleµm and size, 2–50 2–11µm, μ withm and the 2–50 centers μm, ofwith each the particle centers of of each particle of 7 μm and 10 μm, respectively. of7 eachµm and particle 10 µm, of respectively.7 μm and 10 μm, respectively.

Figure 20. SEM images of the CaF2 particle with the granulation method: (a) the ball milling method FigureFigure 20. 20. SEMSEM images images of of the the CaF CaF2 2particleparticle with with the the granulation granulation method: method: (a ()a )the the ball ball milling milling method method andand ( b( b) )the the mortar mortar and and pestle pestle method method [42]. [[42].42].

AsAs producing producing CaF CaFCaF22:Eu2:Eu:Eu powder powderpowder through through variousvarious various methods,methods, methods, chemicalchemical chemical approaches approaches approaches were were were found found found to to beto bebeunsuitable unsuitable unsuitable due due due to to theto the the use use use of of harmfulof harmful harmful chemicals. chemicals. chemicals. Whereas Whereas Whereas the the the mortar mortar mortar and and and pestle pestle pestle method method method was was was found found found to totobe be be suitable suitable suitable due due due to to to its its its simplicity. simplicity. simplicity. Furthermore,Furthermore, Furthermore, thethe the ﬁne-sizedfine-sized fine-sized particles particles of of CaF CaF22:Eu2:Eu:Eu were were successfully successfully manufactured.manufactured. Additionally,Additionally, Additionally, thethe the radiusradius radius ofof of scintillatorscintillator scintillator particlesparticles particles withwith with thethe the highesthighest highest valuevalue value ofof of energyenergy accumulatedaccumulated by by 3 H,3H,H, 210 210210Pb,Pb,Pb, and and and 14 14C14C Ccalculated calculated calculated by by by the the the Geant Geant Geant 4 4 4computational computational computational simulation simulation simulation were were were 3.5 3.5 3.5 μ μµm,m,m, 3030 μ μµm,m, and and 150 150 μ μµm,m, respectively. respectively. The The scintillator scintillator consists consists of of CaF CaF22:Eu:Eu:Eu particles particles particles with with with a a a radius radius radius of of of 3.5 3.5 3.5 μ μµmm depositeddeposited on on on a a aPDMS PDMS PDMS substrate substrate substrate and and and compared compared compared with with with a a single single a single crystal crystal crystal CaF CaF CaF2:Eu2:Eu2 inorganic:Eu inorganic inorganic scintillator. scintillator. scintillator. As As shownshownAs shown in in Table Table in Table 13, 13, 13the the, thesum sum sum counts counts counts of of the ofthe the scintillator scintillator scintillator with with with a a particle aparticle particle radius radius radius of of of3.5 3.5 3.5 μ μmµmm is is is15% 15% 15% higher, higher, higher, increasingincreasing the the the number number number of of of photons photons photons produced produced produced within within within th th theee scintillator, scintillator, leading leading to to greater greater efficiency efficiency eﬃciency to to measuremeasure radioisotopes. radioisotopes.

TableTableTable 13. 13. Scintillator Scintillator efficiency efficiency eﬃciency to to counting counting time. time.

CountingCounting Time Time Time 600 600 ss s 1200 1200 1200 s s s SingleSingle Crystal Crystal Scintillator Scintillator 151,393 151,393 ±406 406 293,481 293,481 ± 568568 Single Crystal Scintillator 151,393± ± 406 293,481 ±± 568 Fabricated Scintillator 173,421 228 339,450 305 FabricatedFabricated Scintillator Scintillator 173,421 173,421± ± ± 228 228 339,450 339,450 ± ±± 305 305

FigureFigure 21 21 represents represents a a prototype prototype flow flow ﬂow cell cell that that was was created created to to detect detect a a short-time short-time concentration concentration spikespike of of radioisotopes radioisotopes spilled spilled to to the the river river (Figure (Figure 21). 2121).).

FigureFigure 21. 21. The TheThe flow flow ﬂow cell cell fabricated fabricated [42]. [42]. [42].

TheThe flow flow cell cell is is made made by by machining machining aluminum aluminum blocks blocks and and a a lid lid made made with with transparent transparent perspex. perspex. TheThe center center of of the the lid lid was was embedded embedded with with SiPM SiPM and and th thee inlet inlet and and outlet outlet of of the the flow flow cell cell were were made made on on thethe side.side. TwoTwo setssets ofof flowflow cellscells werewere fabricatedfabricated foforr thethe experiment,experiment, oneone consistsconsists ofof aa three-layerthree-layer

Chemosensors 2020, 8, 106 23 of 42

The flow cell is made by machining aluminum blocks and a lid made with transparent perspex. ChemosensorsThe center 2020 of, the 8, x lid was embedded with SiPM and the inlet and outlet of the flow cell were23 made of 42 on the side. Two sets of flow cells were fabricated for the experiment, one consists of a three-layer perspexperspex disk, withwith aa diameter diameter of of 4 cm4 cm and and a gap a gap of 8 mmof 8 betweenmm between each layer,each andlayer, the and other the is 12other layers is 12of layers polycarbonate of polycarbonate disks, with disks, a diameter with a ofdiameter 4 cm, a thicknessof 4 cm, a of thickness 1 mm, and of a1 spacingmm, and of a 1 spacing mm. SiPM of 1(Sense mm. SiPM C-series (Sense 60035) C-series was 60035) selected was for selected the light for sensor the light used sensor in the used detector in the with detector a smaller with operatinga smaller operatingvoltage (29.7 voltage V) compared (29.7 V) to PMT.compared The results to PMT. by using The tritiated results water by ofusing 1000 Bqtritiated/mL concentration water of 1000are shown Bq/mL inconcentration Table 14. The are flow shown cell consistingin Table 14. of The 12 layersflow cell has consisting 95% higher of e12ffi layersciency has than 95% a flow higher cell efficiencyconsisting than of three a flow layers. cell consisting of three layers.

TableTable 14. 14. SumSum count count comparison comparison with with the the scintillation scintillation detectors detectors of of different diﬀerent layers. layers.

Scintillator Sum Sum Count Count (6 (6 h) h) 3 Layers Flow Flow Cell Cell 271 271 ± 3131 ± 1212 Layers ofof Flow Flow Cell Cell 11,660 11,660 ± 150150 ±

8.3.4.8.3.4. EU EU InIn 2019, 2019, Southwestern Southwestern European European Instruments Instruments (SEI) (SEI) established established a a real-time real-time low-radiation low-radiation tritium tritium monitoringmonitoring system system to to measure measure low-level low-level tritium tritium within within a a river river near near nuclear nuclear power power plants. plants. To To this this end,end, two two types types of of flow flow cell cell prototype prototype detectors detectors we werere fabricated fabricated to to obtain obtain an an optimized optimized flow flow cell cell detector.detector. Geant Geant 4 4simulation simulation results results showed showed that that most most (99.7%) (99.7%) ββ particlesparticles reaching reaching the the fiber fiber optic optic surfacesurface were were emitted emitted at at a a distance distance of of less less than than 5 5 μµm,m, and and only only radioactive radioactive decay decay from from very very thin thin layers layers ofof water water near near the the fiber-optic fiber-optic surface surface was was detected. detected. Therefore, Therefore, cladding cladding was was excluded excluded to to maximize maximize the the exposedexposed detection detection area duedue toto the the low low energy energy of of tritium tritium (average (average 6 keV). 6 keV). Additionally, Additionally, lead lead bricks bricks were wereused used to remove to remove cosmic cosmic and natural and natural radioactivity radioactivity to reach to tritium reach tritium levels below levels 100 below Bq/L. 100 In Bq/L. addition, In addition,the maximum the maximum tritium concentration tritium concentration of drinking of drinking water was water presented was presented by EURATOM. by EURATOM. To this end,To thisa cosmic end, a veto cosmic detector veto detector consisting consisting of two layers of two of la plasticyers of scintillator plastic scintillator (Epic crystal) (Epic wascrystal) used, was with used, the withentire the detection entire detection being positioned being positioned within a Pb within shield ofa severalPb shield centimeters of several of thickness.centimeters Figure of thickness. 22 shows Figurea prototype 22 shows detector a prototype of the IFIC detector version, of the consisting IFIC vers ofion, 64 opticalconsisting fiber ofBCF-12 64 optical (Saint. fiber Gobain BCF-12 crystals) (Saint. Gobain25 cm in crystals) length and25 cm 1 mmin length in diameter. and 1 mm in diameter.

Figure 22. Tritium-1, IFIC proto-type detector: (a) bundle of the optical ﬁbers, (b) PTFT container [90]. Figure 22. Tritium-1, IFIC proto-type detector: (a) bundle of the optical fibers, (b) PTFT container [90].

The fiber optic was connected to two PTFE containers and measured by two SiPMs configured in the coincidence mode to eliminate noise from the light detector. TRITIUM-1 IFICs were used to assess the stability of optical fibers over time and showed a stable response for nine months.

Chemosensors 2020, 8, 106 24 of 42

The fiber optic was connected to two PTFE containers and measured by two SiPMs configured in the coincidence mode to eliminate noise from the light detector. TRITIUM-1 IFICs were used to Chemosensors 2020, 8, x 24 of 42 assessChemosensors the stability 2020, 8 of, x optical fibers over time and showed a stable response for nine months.24 Figure of 42 23 showsFigure an 23 Aveiro shows version an Aveiro of aversion prototype of a prototype detector with detector a larger with detectiona larger detection surface surface consisting consisting of a larger Figure 23 shows an Aveiro version of a prototype detector with a larger detection surface consisting numberof a larger of fibers number (400) of than fibers other (400) prototypes. than other pr Theototypes. optical The fibers optical were fibers positioned were positioned between thebetween two PMTs of a larger number of fibers (400) than other prototypes. The optical fibers were positioned between (Hamamatsuthe two PMTs R2154-02) (Hamamatsu for the R2154-02) coincidence for the mode coincidence to increase mode theto increase sensitivity the sensitivity of tritium of detection tritium by the two PMTs (Hamamatsu R2154-02) for the coincidence mode to increase the sensitivity of tritium eliminatingdetection PMTby eliminating noise. Figure PMT 24 noise. shows Figure a Tritium-2 24 shows prototype a Tritium-2 module, prototype which module, is the which final detectoris the to detection by eliminating PMT noise. Figure 24 shows a Tritium-2 prototype module, which is the final detector to be installed in the Arrocampo dam at the Alamaz nuclear power plant in Spain. It be installedfinal detector in the to Arrocampo be installed dam in the at Arrocampo the Alamaz dam nuclear at the power Alamaz plant nuclear in Spain. power It plant contains in Spain. 500 optical It contains 500 optical fibers with a length of 25 cm and a diameter of 1 mm, arranged in 4 × 4 SiPM fiberscontains witha 500 length optical of fibers 25 cm with and a a length diameter of 25 of cm 1 mm,and a arrangeddiameter of in 1 4 mm,4 SiPMarranged (Hamamatsu) in 4 × 4 SiPM for a (Hamamatsu) for a coincidence system. It is also positioned in a parallel× container made of Teflon coincidence(Hamamatsu) system. for Ita iscoincidence also positioned system. in It a is parallel also positioned container in made a parallel of Teflon container walls, made and theof Teflon reflection walls, and the reflection of light is optimized. of lightwalls, is optimized.and the reflection of light is optimized.

Figure 23. TRITIUM-1, AVEIRO detector prototype [90]. FigureFigure 23. 23.TRITIUM-1, TRITIUM-1, AVEIROAVEIRO detector prototype prototype [90]. [90 ].

Figure 24. TRITIUM-2 detector: (a) TRITIUM-2 probe prototype and (b) detection composition with FigureFigure 24. TRITIUM-224. TRITIUM-2 detector: detector: (a ()a) TRITIUM-2 TRITIUM-2 probeprobe prototype prototype and and (b ()b detection) detection composition composition with with the TRITIUM-2 module [91]. the TRITIUM-2the TRITIUM-2 module module [91 [91].]. 8.4. Beta-Ray Detector Understudying 8.4.8.4. Beta-Ray Beta-Ray Detector Detector Understudying Understudying Studies other than the flow cell on the measurement for low energy beta ray emitting StudiesStudies other other than than the ﬂow the cellflow on cell the measurementon the measurement for low energyfor low beta energy ray emittingbeta ray radioisotopes emitting radioisotopes have been carried out. This section described the detector fabrication to measure beta haveradioisotopes been carried have out. been This carried section out. described This section the described detector fabricationthe detector tofabrication measure to beta measure ray emitting beta ray emitting radioisotopes in radioactivity contamination and radioactive waste at radioisotopesray emitting in radioactivity radioisotopes contamination in radioactivity and radioactive contamination waste atand decommissioning radioactive waste sites. at decommissioning sites. decommissioning sites.

Chemosensors 2020 8 Chemosensors 2020, 8, , x, 106 2525 of of 42 42 Chemosensors 2020, 8, x 25 of 42 8.4.1. Myong-Ji University 8.4.1. Myong-Ji University 90 Myong-Ji University from Korea studied 90Sr, a major artificial radioisotope generated via Myong-Ji University from Korea studied 90Sr, a major artificial radioisotope generated via nuclearMyong-Ji fission among University the radioisotopes from Korea outpouredstudied Sr,to thea majorocean fromartificial the Fukushimaradioisotope Daiichi generated nuclear via nuclear fission among the radioisotopes outpoured to the ocean from the Fukushima Daiichi nuclear powernuclear generator, fission among for real-time the radioisotopes monitoring. outpoured Therefore, to physical the ocean and from structural the Fukushima detection Daiichiefficiency nuclear and power generator, for real-time monitoring. Therefore, physical and structural detection efficiency power generator, for real-time monitoring.90 Therefore, physical and structural detection efficiency and a water/dustproof detector for Sr 90measurement underwater was designed and produced. and a water/dustproof detector for90 Sr measurement underwater was designed and produced. Additionally,a water/dustproof the photon detector transfer for efficiencySr measurement of PMT underwaterwith detector was thickness designed is calculatedand produced. and Additionally, the photon transfer efficiency of PMT with detector thickness is calculated and optimized Additionally, the photon transfer efficiency of PMT with detector 90thickness90 is calculated and optimized using LightTools software. Detection characteristics90 90 of Sr/ Y in an underwater using LightTools software. Detection characteristics of Sr/ Y in an underwater90 90 environment were environmentoptimized using were LightToolsevaluated bysoftware. developing Detection a PM Tcharacteristics and SiPM photosensor of Sr/ Y basedin an scintillationunderwater evaluated by developing a PMT and SiPM photosensor based scintillation detector. Figure 25 represents detector.environment Figure were 25 represents evaluated a prototypeby developing of the asc intillationPMT and detectorSiPM photosensor applied to a basedlarge-area scintillation probe. a prototype of the scintillation detector applied to a large-area probe. In light of the maximum energy Indetector. light of theFigure maximum 25 represents energy a of prototype the beta ray of theand sc physicalintillation characteristics detector applied of the to scintillator, a large-area a 1probe. mm ofIn thelight beta of the ray maximum and physical energy characteristics of the beta of ray the and scintillator, physical a 1characteristics mm thick CaF of2:Eu the inorganic scintillator, scintillator a 1 mm thick CaF2:Eu inorganic scintillator and a 6 mm × 6 mm of the small size SiPM photosensor were and a 6 mm 6 mm of the small size SiPM photosensor were exploited and a Teflon reflector was used exploitedthick CaF and2:Eu× a inorganicTeflon reflector scintillator was used and asa 6the mm reflector. × 6 mm The of voltagethe small gain size of SiPM SiPM photosensoris as high as thatwere as the reflector. The voltage gain of SiPM is as high as that of PMT, but the detection area is relatively ofexploited PMT, but and the a Teflondetection reflector area iswas relatively used as thesmal reflector.l and that The decreases voltage gain the ofdetection SiPM is efficiency.as high as thatTo small and that decreases the detection efficiency. To compensate for this shortcoming, a study on compensateof PMT, but for the this detection shortcoming, area ais study relatively on fabricating small and a thatlarge-scale decreases probe the was detection carried efficiency.out. To fabricatingcompensate a for large-scale this shortcoming, probe was a carriedstudy on out. fabricating a large-scale probe was carried out.

Figure 25. SiPM-CaF2:Eu prototype detector [92]. FigureFigure 25.25. SiPM-CaF2:Eu:Eu prototype prototype detector [92]. [92].

Based on results of the SiPM linked CaF2:Eu scintillation detector, an in-situ detector was Based on results of the SiPM linked CaF2:Eu scintillation detector, an in-situ detector was produced producedBased as shownon results in Figure of the 26. SiPM PMT linked B51B03 CaF (ADIT2:Eu Inc.,scintillation Sweetwater, detector, IN, USA) an in-situexhibited detector a circular was asproduced shown inas Figureshown 26in. Figure PMT B51B03 26. PMT (ADIT B51B03 Inc., (ADIT Sweetwater, Inc., Sweetwater, IN, USA) IN, exhibited USA) exhibited a circular a CaF circular2:Eu CaFscintillator2:Eu scintillator with the with same the area same of photoreception area of photoreception of PMT with of PMT a diameter with a ofdiameter 50.8 nm of and 50.8 thickness nm and of thicknessCaF2:Eu ofscintillator 1 mm. with the same area of photoreception of PMT with a diameter of 50.8 nm and 1thickness mm. of 1 mm.

FigureFigure 26. 26. In-situIn-situ prototype prototype detector detector [92]. [92]. Figure 26. In-situ prototype detector [92]. InIn additiion, additiion, minimum minimum detectable detectable concentration waswas 330330 Bq Bq/L/L for for 10 10 min, min, but but it it did did not not retain retain the In additiion, minimum detectable concentration was 330 Bq/L for 10 min, but it did not retain thedomestic domestic emission emission standard standard of 20of 20 Bq Bq/L,/L, suggesting suggesting the the possibility possibility of of real-time real-time monitoring monitoring for for90 Sr.90Sr. the domestic emission standard of 20 Bq/L, suggesting the possibility of real-time monitoring for 90Sr.

Chemosensors 2020, 8, 106 26 of 42

8.4.2.Chemosensors KAERI 2020, 8, x 26 of 42 In 2017, KAERI conducted a study to in-situ measure radiological contamination from beta rays 8.4.2.90 KAERI 238 such as Sr and U. Plastic scintillators were fabricated adding nanomaterials such as Gd2O3, CdS, and CdTe.In 2017, Detection KAERI effi conductedciency with a study scintillator to in-situ thickness measure was radiological evaluated contamination through MCNP from (Montebeta rays Carlo N-Particle)such as 90 simulationSr and 238U. toPlastic measure scintillators the high were energy fabricated beta adding ray of nanomaterials90Sr (maximum such energy as Gd2 ofO3, 2.3 CdS, MeV), showingand CdTe. good Detection detection eefficiencyfficiency with above scintillator 3 mm thickness. thickness In was the caseevaluated of a low through energy MCNP beta ray(Monte of 204Tl Carlo N-Particle) simulation to measure the high energy beta ray of 90Sr (maximum energy of 2.3 (maximum energy of 763 keV), the detection efficiency was higher with 1 mm thickness. From the MeV), showing good detection efficiency above 3 mm thickness. In the case of a low energy beta ray results of a MCNP computational simulation for 545.9 keV low energy beta ray emission by 90Sr of 204Tl (maximum energy of 763 keV), the detection efficiency was higher with 1 mm thickness. (standard activity 18.0 kBq), plastic scintillator thickness above 6 mm was found to be appropriate. From the results of a MCNP computational simulation for 545.9 keV low energy beta ray emission by Considering90Sr (standard that beta-rayactivity 18.0 contamination kBq), plastic at thescintillator decommissioning thickness above site was6 mm 0.0629 was Bq found/g, much to be lower thanappropriate. the radioactivity Considering of 18.0 that kBq beta-ray at the experimentalcontamination source,at the decommissioning the minimum thickness site was 0.0629 to fully Bq/g, absorb beta-raysmuch lower was calculatedthan the radioactivity to be 4 mm, of 18.0 even kBq consideringat the experimental the shielding source, the of minimum other radiation thickness emitted to fromfully the absorb site. The beta-rays fabricated was calculated plastic scintillator to be 4 mm, is aeven mixture considering of organic the shielding scintillator of other into epoxy,radiation using 2,5-Diphenyloxazoleemitted from the site. (PPO) The fabricated as the first plastic material scintillator and 1,4-bis(5-phenyloxazol-2-yl) is a mixture of organic scintillator benzene into epoxy, (POPOP) as theusing second. 2,5-Diphenyloxazole PPOs absorb UV(PPO) generated as the first by ma betaterial rays and and 1,4-bis(5-phenyloxazol-2-yl) emit 320 nm of visible lightbenzene (visible light–violet,(POPOP) VL–V),as the second. while POPOPPPOs absorb acts asUV a wavegenerated shift by that beta absorbs rays and VL–V emit and 320 emits nm of 420 visible nm oflight visible light(visible (visible light–violet, light–blue, VL–V), VL–B) while of a POPOP long wavelength. acts as a wave As shift a result, that absorbs plastic VL–V produced and emits by mixing 420 nm PPO 0.2 wtof visible % and light POPOP (visible 0.01 light–blue, wt % had VL–B) the highestof a long ratiowavelength. of scintillation As a result, at plastic 380–446 produced nm (VL–V by mixing to VL–B) PPO 0.2 wt % and POPOP 0.01 wt % had the highest ratio of scintillation at 380–446 nm (VL–V to and the highest emission intensity at a 420 nm wavelength. VL–B) and the highest emission intensity at a 420 nm wavelength. For PS-0201 and commercial plastic, BC-400 (Saint. Gobain) in terms of emission intensity and For PS-0201 and commercial plastic, BC-400 (Saint. Gobain) in terms of emission intensity and transmissiontransmission ratio ratio with with the the light, light, thethe emissionemission intensity intensity at at a awavelength wavelength of of420 420 nm nm showed showed that that PS-0201PS-0201 was was 20 20 times times weaker weaker than than BC-400 BC-400 (Figure(Figure 27).27). Comparing Comparing the the transmittance transmittance of scintillators, of scintillators, thethe transmittance transmittance of of PS-0201 PS-0201 atat 420420 nm was was 83.0%, 83.0%, which which was was 3.7% 3.7% higher higher than thanthe value the value(79.3%) (79.3%) of of BC-400.BC-400. Additionally,Additionally, a a comparison comparison of of radiation radiation absorbance absorbance for for90Sr90 (18.0Sr (18.0 kBq, kBq, 545.9 545.9 keV keVβ-ray)β-ray) showedshowed 96% 96% incident incidentβ-ray β-ray absorbance absorbance inin PS-0201,PS-0201, while while commercial commercial scintillators scintillators had had a transmittance a transmittance of lessof less than than 2% 2% and and 98% 98% absorbance, absorbance, respectively. respectively.

Figure 27. BC-400 and fabricated plastic scintillators: (a) emission intensity with wavelength and Figure 27. BC-400 and fabricated plastic scintillators: (a) emission intensity with wavelength and (b) ransmittance(b) transmittance with with wavelength wavelength [93 [93,94].,94].

Chemosensors 2020, 8, 106 27 of 42

Chemosensors 2020, 8, x 27 of 42 A study on improving the detection efficiency of the fabricated plastic scintillator was then carried outA by study mixing on each improving nanomaterial the detection of Gd2O3 ,efficiency CdS, and CdTeof the with fabricated the premixture, plastic scintillator and the transmittance was then carriedand relative out by effi mixingciency ofeach each nanomaterial scintillator were of Gd compared2O3, CdS, (Figure and CdTe 28). Comparingwith the premixture, the transmittance and the at transmittance420 nm wavelengths and relative (Figure efficiency 28a), CdTe of each and Gdscin2Otillator3 had awere transmittance compared of (Figure about 80%,28). Comparing while CdS had the a transmittancelow transmittance at 420 of nm about wavelengths 20%. When (Figure calculating 28a), CdTe the relative and Gd effi2Ociency3 had a of transmittance each plastic scintillator, of about 80%,it was while found CdS that had the a plastic low transmittance scintillator with of about 0.1 wt 20%. % of When CdTe addedcalculating was the the highest relative (Figure efficiency 28b). of each plastic scintillator, it was found that the plastic scintillator with 0.1 wt % of CdTe added was the highest (Figure 28b).

1.2

(a) (b) FigureFigure 28. 28. ComparedCompared plastic plastic scintillat scintillatorsors with with nanomaterial: nanomaterial: (a ()a )transmittance transmittance with with wavelength wavelength and and (b(b) )relative relative efficiency eﬃciency of of scinti scintillatorsllators with with nanomaterial nanomaterial [95]. [95].

8.4.3.8.4.3. UNIST UNIST InIn 2019, 2019, UNIST UNIST assembled assembled a aplastic plastic detector detector to to measure measure radiological radiological contamination contamination by by long-lived long-lived beta-raybeta-ray emitting emitting radioisotope radioisotope at at the the decommissioni decommissioningng site site (Figure (Figure 29). 29). Not Not only only beta-rays beta-rays but but also also variousvarious radioisotopes radioisotopes areare jumbled jumbled at at the the nuclear nuclear facility facility and decommissioningand decommissioning sites, asites, scanning a scanning detector detectorthat could that immediately could immediately distinguish distinguish beta-rays beta-r and gamma-raysays and gamma-rays was developed. was developed. The radiation The radiation sensitivity sensitivitydue to scintillator due to thickness scintillator was thenthickness compared was through then compared an experiment through and a computationalan experiment simulation and a computationalusing MCNP. simulation using MCNP.

FigureFigure 29. 29. AA plastic plastic detector detector composed composed with with two two scintillators: scintillators: (a ()a )conceptual conceptual diagram diagram and and (b (b) )plastic plastic detectordetector bundle bundle [40]. [40]. Plastic scintillators with diﬀerent thicknesses of 1 mm and 10 mm and a diameter of 50 mm were Plastic scintillators with different thicknesses of 1 mm and 10 mm and a diameter of 50 mm were used to assemble the detector bundle and PMT was attached using an optical cement, as shown in used to assemble the detector bundle and PMT was attached using an optical cement, as shown in Figure 30. For two types of contamination, homogenous and surface contamination, 90Sr and 60Co Figure 30. For two types of contamination, homogenous and surface contamination, 90Sr and 60Co sources were measured for 600 s at a point 100 mm from the detector. The measured contaminated sources were measured for 600 s at a point 100 mm from the detector. The measured contaminated areas were deﬁned as an area of 40 cm 40 cm and a depth of 50 cm for homogeneous contamination, areas were defined as an area of 40 cm ×× 40 cm and a depth of 50 cm for homogeneous contamination, and an area of 40 cm × 40 cm and depth of 5 cm for surface contamination. The detection efficiency of each source by two different thicknesses of the plastic scintillator is shown in Table 15.

Chemosensors 2020, 8, 106 28 of 42 and an area of 40 cm 40 cm and depth of 5 cm for surface contamination. The detection eﬃciency of × Chemosensorseach source 2020 by, 8 two, x diﬀerent thicknesses of the plastic scintillator is shown in Table 15. 28 of 42

FigureFigure 30. 30. Multi-sensitiveMulti-sensitive (MS) (MS) and and tritium tritium (TR) (TR) screens screens [96–99]. [96–99]. Table 15. Radiation detection eﬃciency with source and scintillator thickness. Table 15. Radiation detection efficiency with source and scintillator thickness. Scintillator Thickness 1 mm 10 mm Scintillator Thickness 1 mm 10 mm 90 Sr 90Sr 1.5%1.5% 1.5% 1.5% 60Co 0.067% 0.21% 60Co 0.067% 0.21%

TheThe experiment showed showed that two scintillators for β-ray-ray emitting emitting isotopes isotopes had had similar similar detection efficiency,efficiency, but for γ-ray emitters, the the detection detection efficiency efficiency was was three three times times higher higher in in 10 10 mm mm thick thick scintillators.scintillators. Simulating Simulating a a detector detector in in soil soil by by MCNP MCNP with with a a plastic plastic scintillator scintillator 12 12 mm mm in in diameter diameter and and 2020 mm in thickness to measure a 90SrSr source source resulted resulted in in an an effective effective distance distance of of 19 19 mm, mm, an an effective effective volume ofof 37.337.3 cm cm3,3 and, and the the detection detection effi ciencyefficiency of 4.2%. of 4.2% Based. Based on the on results the above,results the above, MDA the satisfying MDA satisfyingrelease criteria release of 90 criteriaSr were of calculated 90Sr were in 335calculated s for the in concentration 335 s for the of 1.0concentration Bq/g, and 33,100 of 1.0 s forBq/g, 0.1 Bqand/g. 33,100In addition, s for 0.1 the Bq/g. surface In addition, was scanned the surface to detect was scan radioactivened to detect hotspots radioactive at site surfacehotspots contamination, at site surface contamination,and the MDC result and ofthe 34 MDC dpm /100result cm of2 over 34 dpm/100 two-minute cm2 measurement over two-minute time wasmeasurement lower than time the MDC was lowerof the than Geiger–Muller the MDC of and the theGeiger–Muller gas proportional and the modulus gas proportional (550 dpm /modulus100 cm2 and(550 170dpm/100 dpm/ 100cm2 cmand2, 170respectively). dpm/100 cm However,2, respectively). for the above However, results for it wasthe assumedabove results that soil it contaminationwas assumed bythat90 Srsoil is contaminationhomogeneous atby the 90Sr contaminated is homogeneous site, at and the this contaminated may result in site, a decrease and this of may detection result einffi aciency decrease as soil of detectioncharacteristics efficiency may diasff soiler from characteristics actual measurements. may differ from actual measurements.

8.4.4.8.4.4. CEA CEA InIn 2017,2017, French French Alternative Alternative Energies Energies and Atomicand At Energyomic Energy Commission Commission (CEA) from (CEA) France from developed France developeda digital auto a digital radiography auto radiography (DA) technique (DA) to characterizetechnique to radioactive characterize waste radioactive through localizationwaste through and localizationquantiﬁcation and of residualquantification radioactive of residual contamination. radioactive Auto radiographycontamination. was Auto originally radiography developed was for originallybiological developed research but for found biological non-destructive research but andfound sensitive non-destructive in order of andα, sensitiveβ, γ-rays, in which order usedof α, β as, γequipment-rays, which provides used as radiological equipment images provides of samples.radiologic Theal images screens of used samples. in the digitalThe screens auto radiographyused in the digitalare shown auto in radiography Figure 30. are shown in Figure 30. Radiation-sensitiveRadiation-sensitive detection detection was was assembled assembled with with two two screens, screens, a a phosphor phosphor crystal crystal screen screen and and a a backingbacking screen. screen. TR TR (tritium) (tritium) screens screens did did not not have have protective protective layers layers and and were sensitive to β-rays-rays of of tritiumtritium (average (average energy energy of of about about 6 6 keV), keV), whereas whereas th thee MS (multisensitive) screen screen was was coated coated with with a a 3+ protective layer.layer. RadiationRadiation energy energy was was exposed exposed to to a layer a layer of 633of 633 nm nm to produce to produce Eu Euin3+ an in unstable an unstable state, 2+ state,which which emitted emitted 390 nm photons,390 nm photons, returned toreturned its initial to Eu its state,initial and Eu was2+ state, collected and bywas a photomultipliercollected by a 2+ photomultipliertube. Trapped electrons tube. Trapped in Br traps elec oftrons BaFBr:Eu in Br trapscrystal of BaFBr:Eu were excited2+ crystal by a 633were nm excited laser, thenby a released633 nm 2+ 3+ laser,and re-excited then released electrons and re-excited of the Eu electronsvalence band,of the transitionedEu2+ valence inband, a meta-stable transitioned Eu in banda meta-stable with the Eurelease3+ band of with 390 nmthe release energy. of After 390 nm scanning energy. theAfter DA sca screen,nning the it canDA bescreen, exposed it can to be strong exposed white to strong light, whiteremoving light, radiological removing radiological information information within minutes within and minutes reusing and it. The reusing time toit. initializeThe time radiologicalto initialize radiologicalinformation dependsinformation on the depends energy on of thethe isotope,energy theof the exposure isotope, time, the andexposure the intensity time, and of the the white intensity light ofsource. the white Methods light and source. order of Methods radiation and measurement order of usingradiation digital measurement auto radiography using are digital as follows. auto radiography are as follows. (1) Obtain radiological information based on the ratio of radiation, exposure time, etc., using MS/TR screens where a MS screen and TR screen are stacked. (2) Scan the screen with a 663 nm laser scanner and then collect the information in digital light units for each compartment.

Chemosensors 2020, 8, 106 29 of 42

(1) Obtain radiological information based on the ratio of radiation, exposure time, etc., using MS/TR screens where a MS screen and TR screen are stacked. Chemosensors(2) Scan the2020 screen, 8, x with a 663 nm laser scanner and then collect the information in digital light29 units of 42 for each compartment. (3)(3) UseUse OptiQuant OptiQuant software software to to quantify quantify the the digi digitaltal light light unit unit and and map map the two-dimensional radiationradiation traces. traces.

TheseThese screens screens can can be be placed placed and and used used multiple multiple times times at at different diﬀerent locations locations to to obtain obtain a a two-dimensionaltwo-dimensional radiological radiological mapping. mapping. In addition, In addition, a study a study on identifying on identifying particle particle types and types energy and ofenergy radioisotopes of radioisotopes at the decommissioning at the decommissioning site was ca siterried was out. carried The radiation out. The intensity radiation was intensity compared was bycompared placing a by source placing at the a source top of atthe the stacked top of screens, the stacked and collecting screens, and the collectingdigital light the unit digital (DLU) light signals unit detected(DLU) signals from detectedeach screen from to each calculate screen the to calculate ratio of thethe ratio DLU. of theBy comparing DLU. By comparing the response the response of two subsequentlyof two subsequently stacked stacked screens screens to determine to determine the thesignal signal reduction reduction ratio, ratio, radioisotopes radioisotopes were were then then deduceddeduced byby determiningdetermining the the last last screen screen on whichon which the signal the signal can be can detected be detected according according to the maximum to the maximumenergy of the energy radioisotopes. of the radioisotopes. Figure 31 shows Figure the 31 ratioshows of the signals ratio of ofβ signals-ray emitting of β-ray isotopes emitting to radiationisotopes toexposure radiation time exposure between time successive between screens. successive screens.

FigureFigure 31. 31. SignalSignal discrimination discrimination ratio ratio with with radiatio radiationn exposure time of the radioisotope ((aa)) 1414C, ( b)) 3636Cl,Cl, ((cc)) 9090Sr/Sr90/90YY discrimination discrimination ratio ratio [97]. [97].

3 14 RadiologicalRadiological informationinformation was was collected collected on oneon one screen screen for H, for a second3H, a second screen for screenC, andfor 2014C, screens and 90 90 20for screensSr/ Y.for DA 90Sr/ technology90Y. DA technology provides highprovides spatial high resolution spatial resolution to monitor to radioactive monitor radioactive images directly images so directlythat contamination so that contamination of radioactive of radioactive waste can be waste observed can be in aobserved two-dimensional in a two-dimensional map. This allowed map. usThis to allowedidentify us the to hot identify spots andthe hot homogeneity spots and homogeneit of certain wastes,y of certain thereby wastes, improving thereby eﬃ improvingciency and efficiency ensuring andsample ensuring representativeness. sample representativeness. Thus, the DA Thus, radiography the DA radiography technique can technique observe can surfaces observe or wastes surfaces at orthe wastes contaminated at the contaminated site as a complementary site as a measurecomplementary if the conventional measure if detectionthe conventional technique detection (camera, techniqueprobe, etc.) (camera, does not probe, detect radioactiveetc.) does not contamination, detect radioactive or if it iscontamination, diﬃcult to determine or if it theis difficult presence to of determinean isotope. the However, presence these of an technologies isotope. However, can only th identifyese technologies contamination can only of identify one radioisotope, contamination not a ofmixed one radioisotope, radioactive environment. not a mixed radioactive environment.

8.4.5.8.4.5. Canada Canada InIn 2018,2018, McMaster McMaster University University from from Hamilton, Hamilt Canadaon, Canada demonstrated demonstrated Thick Gas Thick Electron Gas MultiplierElectron β Multiplier(THGEM) (THGEM) technology technology to produce to a proportionalproduce a proportional counter for counter measuring for measuring low-energy low-energy-rays (Figure β-rays 32). (FigureFor tritium 32). measurementsFor tritium measurements through the conventional through the gas conventional proportional gas counter, proportional the amount counter, of tritium the amountmeasured of tritium is severely measured overestimated is severely because overesti othermated isotopes because commonly other isotopes exist commonly in the sampled exist in gases. the sampledFor GEM gases. detectors For developedGEM detectors in 2005, developed spatial ionizationin 2005, spatial cluster ionization size information cluster size could information be used to coulddistinguish be used tritium to distinguish from other tritium sources. from Therefore, other so aurces. recent Therefore, study was a conducted recent study to produce was conducted a thick gas to produceelectron multipliera thick gas (THGEM), electron multiplier which is similar(THGEM), in structure which is to similar GEM butin structure is ﬁve to 20to GEM times but larger is five in size. to Figure 32a represents a THGEM based β-ray detector to produce a 42 mm 42 mm thick detector. 20 times larger in size. Figure 32a represents a THGEM based β-ray× detector to produce a 42 mm × 42 mm thick detector.

Chemosensors 2020, 8, 106 30 of 42 Chemosensors 2020, 8, x 30 of 42

FigureFigure 32. 32. (a()a )THGEM THGEM detector detector composition composition and and (b (b) )aluminum aluminum chamber chamber [100]. [100].

InIn addition, addition, the the THGEM THGEM detector detector consists consists of of aluminum aluminum vacuum vacuum chambers chambers (Figure (Figure 32b), 32b), copper copper collimators,collimators, collection collection anodes, anodes, an andd voltage voltage dividers. dividers. With With the thedistance distance between between the source the source and the and detectorthe detector set at set 10 at mm, 10 mm, the theexperiment experiment was was cond conducteducted with with THGEM THGEM detectors detectors in in low-pressure low-pressure TE-propaneTE-propane and and P-10 P-10 gases. gases. TheThe principle principle of of measurement measurement of of the the proportional proportional counter counter is is as as follows. follows. When When radiation radiation particles particles passpass through through cylinders, cylinders, gases gases are are ionized ionized by by accelerated accelerated ion ion pairs pairs via via an an electric electric field field in in the the chamber. chamber. IfIf a a low low voltage voltage is is applied applied to to a a proportional proportional counter, counter, only only αα particlesparticles can can be be measured, measured, but but if if the the appliedapplied voltage voltage increases increases above above the the thresh thresholdold voltage, voltage, separation separation detection detection of of αα particlesparticles and and ββ particlesparticles is is possible, possible, distinguishing distinguishing particles particles by by distinguishing distinguishing the the pulse pulse height height due due to to the the difference difference inin specific-ionization. specific-ionization. Compared Compared to complex to complex GEMs, GEMs, THGEM THGEM detectors detectors are easy are to easy fabricate to fabricate and have and robusthave robustfeatures features because because of their ofsimple their design simple and design application and application of commercial of commercial PCB design PCB software. design Theysoftware. can also They compensate can also compensate for the shortcomings for the shortcomings of the existing of the gas existing proportional gas proportional counter with counter low detectionwith low detectionefficiency e ffidueciency to the due limited to the limited surface surface area. area.They They are arealso also appropriate appropriate for for monitoring monitoring contaminationcontamination of of beta beta ray ray emitting emitting isotopes isotopes in in nuclear nuclear facilities facilities as as costs costs per per unit unit area area are are economical. economical.

9.9. Scintillators Scintillators TheThe Sahani Sahani group group from from the the Defence Defence Laboratory Laboratory (DRDO) (DRDO) in in India India showed showed a a ZnO:Ga ZnO:Ga nanorod nanorod scintillatorscintillator for αα-ray-ray detection detection (Figure (Figure 33 ).33). A scintillator A scintillator of a one-dimensionalof a one-dimensional thin layer thin was layer produced, was produced,due to the due properties to the properties of α particles, of α particles, which have which high have mass high and mass charge and charge and short-range and short-range in matter. in matter.Gallium Gallium served served as a dopant as a dopant in zinc oxidein zinc matrix oxide bymatrix the hydrothermal by the hydrothermal method thatmethod forms that a seedforms on a a seedﬂorine-doped on a florine-doped tin oxide (FTP) tin glassoxidesubstrate, (FTP) glass followed substrate, by low-temperature followed by low-temperature solution growth. solution The ZnO growth.seed layer The is ZnO deposited seed layer by spin-coating is deposited a gelby thatspin-coa containsting a zinc gel acetatethat contains and is preheatedzinc acetate to and form is a preheatednanoseed. to This form process a nanoseed. is repeated This process until obtaining is repeated the until desired obtaining seed layer. the desired The seeded seed substratelayer. The is seededthen kept substrate in a solution is then containingkept in a solution a gallium containi and zincng a precursorgallium and for zinc the Gaprecursor doped seedfor the layer Ga growthdoped seedand heatedlayer growth at 450 ◦andC. Finally, heated a at semitransparent 450 °C. Finally, ZnO:Ga a semitransparent/glass scintillator ZnO:Ga/glass is produced scintillator (Figure 33 isa), and the diameter of the nanorods was about 150 10 nm. The optical band gap was 3.22 eV, slightly produced (Figure 33a), and the diameter of the na±norods was about 150 ± 10 nm. The optical band gapless was than 3.22 the valueeV, slightly (3.37 eV) less of than pristine the ZnO value nanorods (3.37 eV) due of topristine Ga defect ZnO states nanorods that lower due theto absorptionGa defect statesedge. that Additionally, lower the absorption the peak ofedge. the Additionally photoluminescence, the peak (PL) of the spectrum photoluminescence is 393 nm. The (PL)α spectrumradiation isdetector 393 nm. constituting The α radiation the developed detector scintillator constituting coupled the with developed a photomultiplier scintillator using coupled silicon greasewith a is photomultiplierfabricated. They using were wrappedsilicon grease using is black fabricated. tape to blockThey externalwere wrapped light. Figure using 33 blackb shows tape a schematicto block externalof the fabricated light. Figure detector. 33b shows a schematic of the fabricated detector.

Chemosensors 2020, 8, 106x 31 of 42 Chemosensors 2020, 8, x 31 of 42

Figure 33. (a) Semitransparent nature of grown nanorods (ZnO:Ga/glass) and (b) pulse height spectra FigureFigure 33. 33. (a(a) )Semitransparent Semitransparent nature nature of of grown grown nanorods nanorods (ZnO:Ga/glass) (ZnO:Ga/glass) and and ( (bb)) pulse pulse height height spectra spectra for different α sources [101]. forfor different diﬀerent αα sourcessources [101]. [101].

FigureFigure 34a 34a34a presents presents pulse pulse height height spectra spectra spectra for for for thr thr threeeeee different different diﬀerent sources sources sources and and counts counts counts of of of the the the experiment; experiment; experiment; thethe distance distance between between the the sources sources to toto the the detector detector was waswas 5 5 mm mm and and the the counting counting time time was was 300 300 s. s. It It It resulted resulted resulted inin aa nearlynearly linearlinear net net count count rate rate vers versusversusus the the activity activity spectra spectra (Figure (Figure 34b).3434b).b).

Figure 34. ((a)) Pulse height spectrumspectrum forfor didifferentﬀerent α αsources sources and and ( b(b)) net net count count rate rate versus versus activity activity for forα Figure 34. (a) Pulse height spectrum for different α sources and (b) net count rate versus activity for αsources sources [101 [101].]. α sources [101]. Herein, we present recent commercial plastic scintillators and detectors regarding fabricated ﬂow Herein,Herein, wewe presentpresent recentrecent commercialcommercial plasticplastic scscintillatorsintillators andand detectorsdetectors regardingregarding fabricatedfabricated cells and other detectors to measure low energy β-rays. Table𝛽 16 shows the commercial scintillators flowflow cellscells andand otherother detectorsdetectors toto measuremeasure lowlow energyenergy 𝛽 -rays.-rays. TableTable 1616 showsshows thethe commercialcommercial produced by Sain. Gobain, Eljen technology, and Epic crystal. scintillatorsscintillators producedproduced byby Sain.Sain. Gobain,Gobain, EljenEljen technology,technology, andand EpicEpic crystal.crystal.

Table 16. Commercial scintillators. Table 16. Commercial scintillators. Item Saint. Gobain Eljen TechnologyEljen Epic Crystal ItemItem Saint. Saint. Gobain Gobain Eljen Technology Epic CrystalCrystal Scintillator BC-400 BC-404 BC-408 BC-428 Technology EJ-260 Plastic Scintillator Scintillator BC-400 BC-404 BC-408 BC-428 EJ-260 Plastic Scintillator LightScintillator Output, BC-400 BC-404 BC-408 BC-428 EJ-260 Plastic Scintillator Light Output, 65 68 64 36 60 50–60 % Anthracene 65 68 64 36 60 50–60 Light% Output, Anthracene % Anthracene 65 68 64 36 60 50–60 Emission Wavelength (nm) 423 408 425 480 490 415 EmissionEmission Wavelength Wavelength (nm) (nm) 423 423 408 408 425 425 480 480 490 490 415 415 BaseBase Material Material Polyvinyltolue Polyvinyltoluenn PolyvinyltoluenPolyvinyltoluen PolysterenePolysterene Base Material 3 Polyvinyltoluen Polyvinyltoluen Polysterene DensityDensity (g/cm (g/cm3) ) 1.0321.032 1.023 1.023 1.051.05 Density (g/cm3) 1.032 1.023 1.05

InIn commercialcommercial detectors,detectors, UnitedUnited States,States, NetherlandNetherland Netherland,,, andand JapanJapan fabricatedfabricated user-friendlyuser-friendly user-friendly betabeta rayray measurementmeasurement systems.systems. SRNLSRNL SRNL fromfrom USUS fabricatedfabricated anan inin in-situ-situ-situ liquidliquid scintillationscintillation detectordetector thatthat hashas highhigh detectiondetection efficiencyefficiency eﬃciency toto supplement supplement shortcomingsshortcomings ofof thethe in-situin-situ solidsolid scintillator, scintillator, long long measurement measurement time,time, and and complicated complicated complicated design. design. Additionally, Additionally, Additionally, Ortec Ortec assembled assembled a a gas gas flow flow ﬂow type type and and dual dual dual phosphor phosphor phosphor type type detectordetector to to measure measure alpha alpha and and beta beta rays. rays. Table Table 17 17 pr presentsesents the the dual dual phosphor phosphor type type and and gas-flow gas-flow type type proportionalproportional counter.counter.

Chemosensors 2020, 8, 106 32 of 42 detector to measure alpha and beta rays. Table 17 presents the dual phosphor type and gas-ﬂow type proportional counter.

Table 17. Dual phosphor type and gas-ﬂow type proportional counter.

Dual Phosphor Item WPC-1150-GFW-3 MPC-1000-GFL MPC-9604 Type Counter 90Sr/90YEfficiency 45% 63% 55% 55% Aluminized, Aluminized, Aluminized, Aluminized, Window 80 µg/cm2 80 µg/cm2 80 µg/cm2 100 µg/cm2 β BKG 40 cpm (65 cpm 0.7 cpm (0.9 cpm 0.4–0.7 cpm (0.9 cpm 1.5 cpm Performance Warranted) Warranted) Warranted) Cosmic Guard Detector (Large-Area Gas-Flow type Guard Detector - Gas Flow Cosmic Guard Detectors Proportional Counter Proportional Counter) Detector Diameter 5.1 cm (2 inc) 8.9 cm (3.5 inch) 5.1 cm (2 inch) 81.3 cm 41 cm 41 cm × × 4 inch 4π Virgin Lead Shielding - 10.2 cm (4 inch) 10 cm (2.4 inch) thick Shield ZnS + Plastic Dual Detector Type Pancake Style Pancake Style Gas Flow Type Phosphore Health physic Spectroscopy grade Health physic • Extremely • • and environment • amplifier field requiring rapid low-level Note monitoring RFI guard count such as smear, background • Large capacity air Linear low voltage air filter count • radioacivity counter • sampler supplier

The dual phosphor type counter has better portability than others because it operates without external devices. However, the detection efficiency for 90Sr/90Y sources was 45%, which was lower than that of gas-flow type counters (WPC-1150-GFW-3: 63%, MPC-1000 (GFL/GFW): 55%, MPC-9604: 55%). As the diameter of the detector was increased, the background count rate decreased for extreme low-level α/β ray measurement in the proportional counter. On the other hand, detectors with high background count rates (dual phosphor type and WPC-1150-GFW-3) are suitable for health physics and environmental monitoring, which require fast count rates. In the US, systems that typically use 2 inches of detector have a β background factor of 1–2 cm for low level radioactive monitoring systems. MPC-9604, a multiple detection system for low-level background radioactivity measurements, is suitable for analysis that requires high sensitivity and throughput. MPC-9604 has a low background count rate of 0.4–0.7, diminished external radioactivity, electrical interference, and background noise by applying a virgin lead shield, cosmic guard detector, spectroscopy grade amplifier, RFI guard, and shielding cable. ThermoFisher Scientific Co. Ltd. (Waltham, MA, USA) produced a lightweight probe weighing 0.5–1.55 kg to monitor low/medium/high energy β-rays (Table 18). It has high sensitivity to low-energy emitting radioisotopes due to its large-area window. In BP19DD probes, a plastic scintillator (BC-400) in a light-alloy housing is exploited for higher measurement efficiency in low/medium/high energy than for other counters. For HP-210, a thin mica window protected by etched stainless-steel screens improved the sensitivity to β rays. Chemosensors 2020, 8, 106 33 of 42

Table 18. ThermoFisher counter.

Model BP19DD AMS-4 HP-210 counter type Plastic probe Proportional Pancake GM Size (cm3) 100 cm2 24.9 H 14.2 W 18.5 D 16.5 L 8.9 W 9.7 H × × × × Weight (kg) 0.5 3.4 0.7–1.9 14C 21 - 6 36Cl 48 - - 60 Eﬃciency (%) Co 34 8.5 16 90Sr/90Y 51 17 32 99Tc - - 15 137Cs - - 22

Scionix from the Netherlands produced a SiPM-based detector that could detect α/β-rays by fabricating nanomaterials with incorporation of a plastic compound. Table 19 presents a comparison of the flow cell detectors from different countries. In Japan, a measurement system composed of plastic detectors and Geiger–Muller counters for an underwater 90Sr analysis presented a low detection limit of 2 Bq/L. UNIST, Japan, United Kingdom, EU, and others have conducted studies on the fabrication of flow cells to monitor tritium on-site in real-time. Table 19 represents a LSC system with flow cells produced in each country. By connecting two PMTs with the flow cell for the coincidence mode, the noise from PMT was eliminated to increase the sensitivity of tritium detection. The Aveiro detector from EU was placed inside a lead shield and cosmic-rays were removed using a veto detector and a 4 4 SiPM (Hamamatsu) was arranged for a coincidence system. Additionally, parallel containers × with inner Teflon walls were used to optimize the reflection of light, and all of these were placed inside the lead shield. Lancaster University from the UK carried out a study to maximize energy deposition using a CaF2:Eu inorganic scintillator fabricated by granulation. They found that tritium, 14C, and 210Pb of each isotope had different radius particles that maximize the deposition efficiency: 3.5 µm, 30 µm, and 150 µm, respectively. They then compared the count of the tritium source with a CaF2:Eu single crystal scintillator, and found that scintillators fabricated with particle size maximizing energy deposition efficiency had a 15% higher count. Additionally, a scintillator was fabricated by spraying 3.5 µm radius CaF2:Eu particles on a PMSD substrate and flow cells were produced to determine the detection efficiency with the number of scintillator layers. The experiments showed that flow cells with 12 layers had 95% higher efficiency than those with three layers, and they confirmed improved detection efficiency by increasing the surface area consisting of the flow cell with multiple layers. UNIST and EU used commercial plastic scintillators while Japan and United Kingdom used granulated CaF2 to constitute the flow cell. UNIST, EU, and the UK fabricated flow cells with multiple plastic plates or optical fibers to broaden the surface area where water and scintillators contact, and Japan formed three layers of detector. The results showed that UNIST and EU had lower minimum detectable activity (MDA) of 0.01 Bq/mL and 0.1 Bq/mL, respectively, compared to Japan (10 Bq/mL). Chemosensors 2020, 8, 106 34 of 42

Table 19. Flow cell detector.

LSC UNIST Lancaster NIFS EU Liquid CaF :Eu + PMSD Granulated Plastic Scintillator Scintillator Plastic Scintillator 2 Scintillator Substrate CaF2 (Epic crystal) PMT (R878, PMT (R2154-02, SiPM (Sense Photosensor - Hamamatsu) PMT ( 2) Hamamatsu) C-series 60035) × ( 2) ( 2) × × Thin Plate (Φ 4 cm, Flow Cell (Φ 5 mm) (Φ 1 mm, Length - Thin Plate ( 13) 1 mm Thick, 1 mm Composition × Cell ( 3) 25 cm) 500 fiber Gap) ( 12) × × × Flow Cell - Acryl Case Aluminum Case Teflon Tube PTFE Case Tube/Case Material Anti-Coincidence - O × × Minimum 0.077 Bq/L 0.01 Bq/mL - 10 Bq/mL Below 0.1 Bq/mL Detectable Activity Detection 34.4% 0.2% 31.3% 1.3%, - - - Efficiency ± ± Gas Total count • • measurement after comparison with Cosmic veto Note - - • tritium electrolysis with scintillator detector using PEM particle radius

Beyond research on flow cells, various studies have been carried out to measure low energy β-ray emitting isotopes, and the characteristics of the detectors and counters from each country are 90 compared in Table 20. Myong-Ji University assembled a CaF2:Eu detector to monitor Sr in the marine environment. To this end, they produced two prototypes of plastic detectors using SiPM and PMT as a photosensor, respectively. While the detector with SiPM should have a probe with large-area to compensate for low light detection efficiency, the PMT coupled detector had a small diameter of 50.8 mm and possibility of monitoring 90Sr was confirmed. KAERI fabricated plastic scintillators by mixing PPO and POPOP and compared them with BC-400. The fabricated plastic scintillator showed 3.7% better light transmittance than BC-400, while its luminous intensity was 20 times weaker. Additionally, the relative efficiency of CdTe, Gd2O3, and CdS nanomaterials was compared, and CdTe had the highest efficiency among them. UNIST assembled a bundle with different thicknesses of two plastic scintillators and measured a standard source buried in soil. Additionally, the detection efficiency of detectors in homogeneous soil was obtained through MCNP simulations and MDA was derived. Furthermore, CEA developed a reusable auto radiography screen for in situ measurements of low energy β-rays in decommissioning sites. Detection of radioisotopes and radiological mapping of isotopes can be achieved by stacking several screens, but only one isotope can be detected. Chemosensors 2020, 8, 106 35 of 42

Table 20. Scintillation detector and proportional counter.

Myong-Ji McMaster KAERI UNIST EU University University Digital Auto Gas Proportional Detector Type Plastic Detector - Plastic Detector Radiography Detector Plastic (epoxy, Scintillator CaF2:Eu Hardener + PPO, Epic crystal BaFBr:Eu - POPOP) Scintillator Emission 435 420 415 390 - Wavelength (nm) Φ 50.8 mm, Φ 50 mm, Scintillator Φ 50 mm, 4 mm Phosphor 42 mm 42 mm, 1 mm Thick 1 mm/10 mm Thick × Composition Thick Disc Screen 0.4 mm Thick Disc Disc PMT (ADIT Inc, Photosensor - PMT - - B51B03) Reflector Teflon Reflector ---- MDA 330 Bq/L- 34 dpm/100 cm2 -- 20 times lower • emission intensity Uncomplicated Detector bundle • compared with • design and composited with Single BC-400 • fabrication Note - the two different radioisotope 3.7% higher Robust system • thicknesses of detection • transmittance Inexpensive plastic scintillator • compared with fabrication expense BC-400

Monitoring systems in the US typically use 2 inches of detector and a β background factor of 1–2 cm. MPC-9604 from Ortec is a multi-detection system for measuring low-level background radioactivity, with application of a virgin lead shield, cosmic guard detector, spectroscopy grade amplifier, RFI guard, shielding cable, etc., resulting in a low background count rate of 0.4–0.7 by reducing external radioactivity, electrical interference, and background noise. Additionally, Aveiro detectors produced in the EU used veto detectors to remove spacecraft and arranged a 4 4 SiPM × (Hamamatsu) for a coincidence system. Additionally, the interior has a Teflon wall to optimize the light reflection in the parallel form of the container, all of which are placed inside a lead shield

10. Conclusions Characterization of the residual radioactivity of radioactive waste generated for the operation and decommissioning in nuclear facilities is an important concern in nuclear power plants. The initial characterization gives information to conduct radiation protection by understanding the requirements for the safety of workers. The final characterization is critical for the release of radioactive waste generated from the decommissioning of nuclear facilities under the regulations of each country. Therefore, precise characterization is vital for managing radioactive waste generated at the decommissioning site. For γ-ray emitting isotopes such as 60Co and 137Cs, it is easy to measure and analyze radioactivity, and chemical separation from other radioisotopes is hardly required. On the other hand, for β-ray emission it is difficult to distinguish isotopes through a spectrum analysis because β-rays cannot penetrate a thick medium and the energy resolution of the spectrum is poor. These characteristics are conspicuous in low energy β-ray emitting isotopes such as 3H, 14C, and 63Ni, and it is difficult to distinguish them from the noise that occurs in low energy areas. Low energy beta-ray emitters have low penetration, and thus the risk of external exposure is negligible while inflow in the human body via intake or inhalation is critical and thus people should be protected against radiation exposure. Therefore, commercial plastic scintillators and optical fibers have been exploited Chemosensors 2020, 8, 106 36 of 42 to assemble flow cell or various types of detectors and to conduct research using these detectors for radiation measurement. Detection of low energy β-rays has been mainly carried out through the liquid scintillation counter, as low energy is difficult to transfer to the detector due to its short-range in the medium. However, this method takes a lot of time for pretreatment and sampling and also is not ecofriendly. Furthermore, large equipment and complex systems make it unsuitable for use directly at the site. In the case of commercial detectors, various companies have produced portable detectors and low-level radioactive monitoring systems for user convenience. The SRNL group developed portable liquid scintillator detectors to compensate for the shortcomings of long measurement times and complex design. Ortec’s MPC-9604 was shown to have a low background count rate of 0.4–0.7 by reducing external radioactivity, electrical interference, and background noise by applying a virgin lead shield, cosmic guard detector, spectroscopy grade amplifier, RFI guard, shielding cable, etc. ThermoFisher Scientific produced a large-area window to assemble a probe with high sensitivity to low-energy radioisotopes. Subsequently, studies on low energy β ray measurement using plastic scintillators were carried out. From flow cells, various studies have been carried out to measure low-energy β-emitting isotopes. Studies have been conducted in many countries, including South Korea, Japan, the UK, and the EU, to produce flow cells for monitoring radioisotopes released underwater, including tritium in particular. By connecting the two PMTs with the flow cell and forming a coincidence system, noise was eliminated from the PMT to increase the sensitivity of radioisotope detection. In the case of the Aveiro detector of the EU, the detector is placed inside a lead shield and the cosmic-rays are removed using a veto detector. Additionally, the inside of the container has Teflon walls, which optimize the reflection of light. In Lancaster, UK, a CaF2:Eu inorganic scintillator was fabricated by granulation to maximize energy deposition efficiency. It has been shown that the detection efficiency varies with the radius of scintillation particles. A flow cell was then composed with a scintillator made by spraying 3.5 um radius CaF2:Eu particles upon PMSD substrates. In the case of UNIST, EU, and the UK, flow cells with 13 thin plastic plates and 500 optical fibers were assembled to make the surface area where water and scintillators contact while Japan formed a three cell detector. From flow cells, various types of detectors are made with plastic, which has the advantage of reducing the time, cost, etc., required for pretreatment to analyze samples and wasteless, convenient composition that can be applied to the site. Myong-Ji University intended to monitor 90Sr underwater by producing CaF2:Eu detectors. To this end, two prototypes of plastic detectors were produced using SiPM and PMT. Although a large-scale detector was required to compensate for the low light detection efficiency when SiPM was used, a small detector with a diameter of 50.8 mm was produced to confirm the possibility of field monitoring of 90Sr with PMT. KAERI compared relative efficiency by adding CdTe, Gd2O3, and CdS nanomaterials to plastic scintillators, and found that CdTe was the most efficient. CEA in France developed a reusable auto radiography screen for in-situ detection of low-energy β-rays. In the case of detectors with plastic scintillators, the system is simple and mechanically robust, making it more suitable for use in field measurement compared to other types of detectors. It is also easy to process and can be manufactured in desired sizes and shapes and on a large scale as it is cheaper than other scintillators such as NaI and BGO. It was also found to be suitable for detecting low-energy β-emitting isotopes due to their low atomic number and density. However, the development of beta-ray measuring detectors combined with nanomaterials is still a significant issue. The following are major challenges for beta-ray measurement: first, low light conversion efficiency due to short atypical and self-absorbing; second, difficulty in distinguishing noise from low energy, and various approaches combining nanomaterials will lead us to seize upstairs. Low energy β emitters originate mostly from neutron activation due to their low atomic mass. This includes 3H, 14C, 63Ni, 94Nb, 99Tc, 129I, and 241Pu. Among these, 3H, 14C, and 36Cl can be detected in nuclear reactors, concrete barriers, rebars, and aluminum alloys. Tritium easily changes into HTO by reacting with the surrounding water. Additionally, when tritiated water evaporates, about 98% of it in saturated air is absorbed into the body 14 by inhalation. For C, it has high mobility in groundwater and is volatile in a gaseous state (CO2). Chemosensors 2020, 8, 106 37 of 42

Thus, the risk for external exposure of low energy β-rays is less because the penetration distance within tissue is very short [74], but radiation protection against inflow into the body is essential. However, the measurement of low energy β-rays has poor energy resolution, and noise from low-energy areas makes it difficult to distinguish radioisotopes and energy. Furthermore, low energy makes it difficult to transfer energy to the detector, and thus an isotope analysis is performed using methods such as a liquid scintillation counter or scaling factor. Unlike γ-ray emitting isotopes, beta-rays are difficult to measure because of their low penetration and short range, and furthermore pretreatment is essential because it should be measured after being separated from other radioisotopes. However, this method requires a lot of manpower, cost, and time to produce samples, and produces waste after sample measurement. Therefore, this paper investigated the status of the technology on the beta-ray measurement, which has characteristics of being waste-free and reduced time and cost required for pretreatment and sampling, and is also applicable for in-situ measurement. Owing to a simple system and mechanical durability plastic scintillators are applicable to in-situ measurement. Next, the commercial detectors and status of prototype detectors that have been fabricated to monitor low energy β-ray emitting isotopes were investigated. For commercial detectors, portable detectors and low-level monitoring systems were produced for user convenience in United States, the Netherlands, and elsewhere. In the case of scintillator fabrication, it has been shown that the efficiency varies with the radius of scintillation particles. Additionally, for the measurement of low energy β-ray emitting isotopes, flow cells were produced to monitor those isotopes released underwater. Studies have been carried out to increase the detection efficiency of the flow cell and to lower MDA by using an organic/inorganic scintillator. The count rate of the flow cell in the form of multiple layers was found to be higher than that of the flow cell in a single scintillator layer. This may be used as basic data according to the development of technology on the measurement of low energy β-ray. In this paper, we provided an outlook on the progress in the development of radiological approaches for the determination of beta rays in low energy beta particle measurements in the environments. Thus, low energy beta particle measurements are of significant concern in nuclear facilities including decommissioning sites at nuclear power plants. The main achievements are briefly summarized in this study. The characterization and the localization of surface and matrix contamination are of primary convern at the decommissioning site in a nuclear facility. In this regard, digital authoradiography measurements capable of giving a real time image of the contaminated obervables and providing an accurate mapping of contamination at the nuclear facility will address a major challenge to investigate residual contamination on site and to proceed toward proper remediation of a contaminated facility and decommissioning site. Furthermore, nanotechnology-based advances in low energy beta particle detection and monitoring have brought attention to the production of scintillators including nanomaterials.

Author Contributions: Conceptualization, original draft preparation, review and editing: H.K., S.M., and B.S.; supervision, C.R., S.H., and J.H.C. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: This work was supported by a grant from the National Research Foundation of Korea (NRF), funded by the Korean government, Ministry of Science ICT (No. 2017M2A8A5015143). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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