inorganic scintillating materials and scintillation detectors

Review

Inorganic scintillating materials and scintillation detectors

By Takayuki YANAGIDA*1,†

(Communicated by Toshimitsu YAMAZAKI, M.J.A.)

Abstract: Scintillation materials and detectors that are used in many applications, such asmedical imaging, security, oil-logging, high energy physics and non-destructive inspection, arereviewed. The fundamental physics understood today is explained, and common scintillators andscintillation detectors are introduced. The properties explained here are light yield, energy non-proportionality, emission wavelength, energy resolution, decay time, effective atomic number andtiming resolution. For further understanding, the emission mechanisms of scintillator materials arealso introduced. Furthermore, unresolved problems in scintillation phenomenon are considered, andmy recent interpretations are discussed. These topics include positive hysteresis, the co-doping ofnon-luminescent ions, the introduction of an aimed impurity phase, the excitation density effect andthe complementary relationship between scintillators and storage phosphors.

Keywords: scintillator, scintillation detector, ionizing radiation, TSL, OSL, RPL

Introduction

Classification and principles of scintillator.Ionizing radiations have been used for many indus-trial and scientific purposes since their discoveriesmore than one hundred years ago. Ionizing radiationsare invisible to the naked eye, and some of them havea high penetration power against dense matters. Inorder to use such ionizing radiations, special tools arenecessary to detect and visualize. Such tools are oftencalled radiation detectors; there are two types of solidmaterials that are most commonly used for radiationdetectors. One is semiconductors, and the other isluminescent materials known as scintillators andstorage phosphors.1) The former absorbs the energyof ionizing radiation and converts to a large numberof carriers, while the latter converts it to a largenumber of photons; such photons are detected byphotodetectors. The number of these carriers orphotons is proportional to the quantity or energy ofthe incident ionizing radiation; thus, we can measureany ionizing radiation. From the view point of thedetector types, two kinds of detection methodologies,counting-type and integration-type, are known. Inthe counting-type detectors, because each radiation

signal is processed event-by-event, a fast time-response is essentially important. In contrast, theintegrated-type detectors detect multiple events overtypically a few ms so fast time-response is notrequired. Semiconductors and scintillators can beapplied to the both types of detectors, while storagephosphors can be applied only to the integrated typedetectors with a very long integration time (e.g., afew weeks to months). These storage phosphors aremainly used for personnel protection dosimetry, andin this paper I call them dosimeter materials.Figure 1 summarizes a classification of radiationdetector materials and detector types.

In this paper, I focus on that scintillators andscintillation detectors. Scintillators are one of theluminescent materials that have a function to absorbionizing radiations and to emit low-energy photons.2)

First, the absorption of ionizing radiation energy byscintillating materials occurs. The interaction proc-esses depend on the species of ionizing radiation andthe elements of the scintillators. When high-energyphotons, including X- and .-rays, are absorbed bythe scintillator, three interaction processes occur,which are called photoelectronic absorption, Comp-ton scattering and pair creation. Photoelectricabsorption is generally used to analyze the radiation,and in this process, one primary electron is generatedper event. This primary electron can generate manyexcited secondary electrons via Coulomb scattering,

*1 Nara Institute of Science and Technology, Nara, Japan.† Correspondence should be addressed: T. Yanagida, Nara

Institute of Science and Technology, 8916-5 Takayama, Ikoma,Nara 630-0192, Japan (e-mail: [email protected]).

Proc. Jpn. Acad., Ser. B 94 (2018)No. 2] 75

doi: 10.2183/pjab.94.007©2018 The Japan Academy

http://dx.doi.org/10.2183/pjab.94.007

and these secondary electrons dissipate their kineticenergy by interactions with lattice or other freeelectrons. Finally, these secondary electrons recom-bine with holes and emit scintillation photons. In thecase of charged particles, such as ,-rays, an energeticcharged particle creates many secondary electrons viasome interactions such as Coulomb scattering, andthese secondary electrons act in the same ways asin the case of X- and .-rays. Although the basicphenomena are similar, a difference can be observedin the excitation density. Figure 2 shows schematicdrawings of an ionizing photon and a charged particleinteracting with various forms of matter. As sche-matically shown, the density of excited secondaryelectrons is high under charged-particle irradiation,and this high excitation density sometimes causesdifferent physical phenomena from the case of photonexcitation. Such a difference due to the excitationdensity is called the linear energy transfer (LET)effect.3)

Figure 3 represents a schematic drawing ofenergy transportation processes of scintillators com-pared with those of storage phosphors. Generally, the

energy-transfer processes in scintillators are under-stood to consist of three processes. The first is calledthe conversion process, and at the end of this process,many energetic (excited) secondary electrons aregenerated. The next is called the transfer process,and in this case, the secondary electrons dissipatetheir kinetic energy via interactions with the latticeand other electrons. Here, some of the electrons aretrapped at localized centers (e.g., lattice defects), andsome others can make it to emission centers. The finaltype is called the luminescence process, in whichscintillation photons are emitted. The luminescenceprocesses are mainly two: one is involving therecombination of electrons and holes; the otherinvolve the excitation of luminescent ions by inter-actions with the energetic secondary electrons.Therefore, in scintillator materials, the secondaryelectrons are desired to reach the luminescencecenters as much as possible. The spatial scale of thedispersion of secondary electrons is around 100 nm.4)

Within a volume of 100 nm3, there are approximately109 atoms, and the number of secondary electrons isaround 105 per incident radiation quantum of 1MeV.The main luminescence process is a direct recombi-nation of electrons and holes, which is often the casein semiconductor-based (e.g., ZnO) scintillators.In contrast, the excitation of luminescent ions isdominant in activator-type scintillators, such as Ce-doped materials.

Concerning the processes in dosimeter materials,the first two processes are essentially the same, butthe main difference is that most of the generatedsecondary electrons are captured at trapping centers.These trapped electrons are meta-stable, and can bere-excited by external stimulations, and then emitphotons. Thermally stimulated luminescence (TSL)5)

and optically-stimulated luminescence (OSL)6) areobserved under thermal and optical stimulations,respectively. Taking into account energy conserva-

Semiconductors(Si, Ge, CdTe, TlBr, …)

Luminescent materials(NaI, CsI, Bi4Ge3O12,…)

Scintillators

Storage phosphors(TSL, OSL, RPL)

Integrated-type detectors

Counting-type detectors

Fig. 1. Classification of solid-state radiation detectors and their detector types.

Charged particle

Incident particle

Secondary electrons

X- or γ-rayPrimary electron

Secondary electrons

Fig. 2. Schematic drawings of the energy trajectories of chargedparticle (top) and high-energy photon (bottom) irradiations.

T. YANAGIDA [Vol. 94,76

tion, the scintillation and storage luminescenceshould be complementarily related to each other. Inother words, bright scintillator materials are dark instorage luminescence, and vice versa. Such a relation-ship has recently been confirmed experimentally,7),8)

which is one of important approaches to understandthe luminescence properties induced by ionizingradiations.

Scintillation detector properties. Thescintillation properties required for practical detec-tors depend on the application. The most commonapplications of scintillation detectors are medicalimaging (PET, X-ray CT, SPECT),9) security,10) oil-logging,11) non-destructive studies of cultural ob-jects,12) astro-physics13) and particle-physics.14) Thetypical construction of a scintillation detector isdepicted in Fig. 4. A scintillator is coupled with aphotodetector, such as a photomultiplier tube(PMT) and a Si-photodiode (Si-PD) which have afunction to convert a scintillation photon to electronsvia a photoelectric conversion. The electronic outputsignals from the photodetector are typically fed into a

preamplifier, shaping amplifier, multichannel analyz-er, and computer in pulse-height analysis. Therequired scintillator properties depend on the appli-cation, and typical properties to be considered are thelight yield, emission wavelength, energy resolution,scintillation decay time, afterglow and high cross-section with the radiation type of interest.

The light yield is one of the most importantproperties of scintillators as well as other phosphors.Ones with high light yields enable us to detectradiations of low energy (or intensity) with a highsignal-to-noise ratio (S/N). Although there are nosolid theories to predict bright scintillators, onephenomenological model is used to explain theobserved light yield.15)–18) The model is formulatedas LYsc F E/(OEg) # S # Q, where LYsc is the scin-tillation light yield, E the deposited energy of ionizingradiation, O the constant parameter, Eg the bandgap energy, S the energy migration efficiency fromthe host to emission centers and Q the quantumefficiency, which is equal to the photoluminescence(PL) quantum efficiency. In this formula, OEg is the

photonparticle

VB

CB

excitation

e-

e-

h h hh

-

+

-

+

Trap levels luminescence

VB

CB

e-

e-

e-

e-

h h hh

-

+

-

+Trap levels luminescence

excitation Optical or thermal stimulation

Scintillator

Dosimeter materials

e- e-

Fig. 3. Energy transport processes of scintillator and dosimeter materials.

e

Single Radiation (MeV)

ee

Scintillator

Electrical signal

Photodetector (PMT, PD)Photons (eV) Electrons

Ionizing radiation

Fig. 4. Sketch of a typical scintillation detector.

Inorganic scintillating materials and scintillation detectorsNo. 2] 77

energy required to generate a single electron-holepair, and which is called the C-value in radiationdetector physics. Recently, a convenient empiricalformula is used; in that formula, O is fixed to 2.5 basedon experimental studies of numerous scintillators.18)

Sometimes a misunderstanding occurs whendistinguishing scintillation and PL. Although thereis no concrete or universal definition of scintillation,one important aspect is whether the incidentexcitation energy can generate multiple carriers ornot. In PL, we generally observe the excitation andrelaxation of one electron per process. In thisdefinition, an excitation energy of 90.1 keV shouldbe a rough border between scintillation and PL.Actually, some people call the VUV-excited lumines-cence (e.g., synchrotron experiments) scintillation,but in most cases these are not scintillations,according to this definition, since the VUV photonenergy is on an order of 10–20 eV, and it can exciteonly one electron for most of insulator materials.

In addition to the definition of scintillation(border between scintillation and PL), anotherargument is sometimes regarded as evaluationmethodologies to determine the light yield. Emissionspectrum excited by ionizing radiation is calledradioluminescence, and some groups use the area ofthe radioluminescence to evaluate the scintillationlight yield. Unfortunately, it is an incorrect method-ology in most cases. In the radioluminescencespectrum, the signal intensity depends not only onthe scintillation light yield, but also on the stoppingpower of the material. Simply speaking, the intensityof the radioluminescence is roughly proportional tothe product of the scintillation light yield and thestopping power. If we compare two materials withthe same absolute light yields, but with differentstopping powers, the heavier material will show ahigher radioluminescence intensity, although thescintillation light yields are the same. For this reason,except for some special cases, pulse-height spec-troscopy must be conducted to determine thescintillation light yield. Special cases are, for exam-ple, to compare those materials with practically closestopping power (chemical composition). Actually,the light-yield values of common scintillators areevaluated by pulse height analysis, and with thepresent technologies available to determine thescintillation light yield of slow materials that have ascintillation decay time longer than 100 µs is difficult.

Another important aspect is energy nonpropor-tionality of the scintillation light yield. In scintilla-tors, the number of emitted photons per unit energy

differs according to the excitation energy, and such arelation is called the energy nonproportional responseof a scintillator, or simply nonproportionality.Figure 5 demonstrates the nonproportionality ofEu-doped SrI2.19) In this graph, ratios of the numberof scintillation photons to the corresponding incidentenergy is plotted relative to that measured with662 keV .-ray excitation, which is defined as 1.Although the vertical axis of the nonproportionalitygraph does not represent any physical value, it can beunderstood as being something like (not exactly) anenergy conversion efficiency from ionizing radiationto scintillation photons. As shown in Fig. 5, Eu-doped SrI2 has a peak at around 100 keV, and it iswell-known that almost all scintillators have similarfeatures. To the best of my knowledge, there is noconfirmed theories to explain this phenomenon, butfor practical applications we must select a scintillatorsuited for the target energy. Otherwise, energycorrections of the obtained data become very difficult.

The emission wavelength must match withthe sensitive wavelength of photodetectors. Typicalphotodetectors are PMT and Si-PD. The formertypically has a spectral sensitivity from 300 to600 nm, while the latter is sensitive to photons withwavelengths longer than 500 nm. Figure 6 representsthe quantum efficiencies of PMT, Si-PD and somegas-type photodetectors for vacuum ultra-violet(VUV) detections. Although the quantum efficiencyof PMT is lower than PD, PMT allows for a muchhigher multiplication gain of up to 106. TMAE(tetrakis dimethylamine ethylene) and TEA (tetraethyl amine) are gases, which are used in gas-PMT.20)

0.9

1

10 100 1000Energy (keV)

Rel

ativ

e lig

ht y

ield

(a. u

.)

Fig. 5. Relative light yield of an Eu-doped SrI2 crystal scintilla-tor plotted against .-ray energy. The light yield is defined as 1 at662 keV.


VUV photodetectors are currently undergoing theR&D phase, and especially the stability and repro-ducibility are problems. The signal intensity ofscintillation detectors depends on the product ofthe scintillation light yield and the quantum effi-ciency of the photodetectors; thus, the commonpractical scintillators are ultraviolet (UV) and visible(VIS) photon-emitting materials since the PMT andPD devices are sensitive from UV to near-infrared(NIR) wavelengths.

Energy resolution ("E) is defined as "E F

FWHM/Chpeak, where FWHM and Chpeak representthe full-width at half-maximum (FWHM) and thepeak channel of the photoabsorption peak in pulse-height spectrum, respectively. In a first-order ap-proximation, the energy resolution depends on thePoisson statistics of the electrons from the photo-detectors, which is essentially proportional to thelight yield. If the energy resolution is sufficiently high,we can distinguish the radiation species with veryclose emission energies in practical applications asscintillation counters. For example, the .-ray energiesof 134Cs and 137Cs are 604 and 662 keV, respectively,and a sufficiently high energy resolution is required todistinguish these two peaks in pulse-height spectrum.Although there are no solid explanations available,empirical observations show that the energy resolu-tion is 3–4% at 662 keV, which is the highest of all; itdoes not seem to improve even if the scintillation lightyield increases. In the case of Pr-doped Lu3Al5O12,it has a light yield of 20,000 ph/MeV and the energyresolution of 3–4% at 662 keV.22) The energy reso-lutions of Ce-doped LaBr323) which has a light yieldof 60,000 ph/MeV and Eu-doped SrI2,19) which shows80,000 ph/MeV, are also 3–4% at 662 keV. Therefore,

if the scintillation light yield is sufficiently high, theenergy resolution is no longer governed by Poissonstatistics. The difference between Poisson statisticsand the actual energy resolution is called the Fanofactor. In order to understand the physics governingthe energy resolution, investigations on the Fanofactor must be made. Before the 21st century, sucha discussion was not considered, since the energyresolution was at most 7–10% at 662 keV, and therecent development of novel high-energy resolutionscintillators has opened up this new discussion. Up tonow, the highest energy resolution has been achievedby co-doped LaBr3; the resolution was around 2% at662 keV.24) If the energy resolution becomes 1% at662 keV, it offers a comparable resolution of CdTe-based semiconductor detectors, and many new appli-cations of scintillation detectors would be established.

The scintillation decay time is one of theimportant properties for scintillation detectors,and is directly related to the timing resolution ofradiation detectors. The decay time is fundamentallygoverned by the speed of transfer of free electronsand holes from an ionization track to the emissioncenter and the lifetime of the luminescence stateof the activator. In PL, the luminescence decaytime can be analytically deduced as being � ¼1� / n

�3 ðn2þ23 Þ2 Pf jhf j�jiij2, where !, =, n and 6

represent the decay rate of an excited state, decaytime, refractive index and emission wavelength,respectively. The matrix element connecting theinitial state, |ii, with the final state, hf |, via thedipole operator, 7, will only be of appreciable size fortransitions between states of different parity.25)

Although other processes of energy migration fromthe host lattice to emission centers can be involved,the scintillation decay time is approximately propor-tional to 63. Previously, the relation between thescintillation decay time and refractive index was well-studied experimentally.26) Following this study, weinvestigated the relation between the scintillationdecay time and the emission wavelength experimen-tally; the decay time was proportional to 962.2.27)

Figure 7 represents the relationship between thedecay time, =, and the emission wavelength, 6, inPL and the scintillation. In this work, it was provedthat the relationship in PL decay and the emissionwavelength could also be applied to the scintillationphenomenon.

In the R&D of scintillator materials, the chemi-cal formula must be considered. For example, if theaim is to detect high-energy photons, such as X-and .-rays, dense materials composed of high Zeff

0

20

40

60

80

100

200 400 600 800 1000

Si-PD

PMT (UBA)

TMAE

TEA

Wavelength (nm)

Qua

ntum

eff

icie

ncy

(%)

Fig. 6. Quantum efficiency (%) plotted against the wavelength(nm). The data is taken from.21)


(effective atomic number) are preferred. Althoughthere are no concrete expressions, the effective atomicnumber for photoelectric absorption can be expressedas Zeff F (’wiZi

4)1/4 where wi and Zi are the fractionof the total mass associated with the i-th element andthe atomic number of the i-th element, respectively.For dosimeter applications, sometimes the power ofZi may take 2.94. Therefore, empirically, powers of3–4 are used; the value depends on the application.In addition, if the purpose is to detect neutrons,materials composed of light elements are used, and6Li or 10B should be included in the matrix, sincethese elements have a high cross-section to thermalneutrons. In the case of charged-particle detectors,materials with intermediate Zeff are preferred becausethere are many background photons and neutrons inthe measurement environment of charged particles.

The timing resolution is similar, but different,from the scintillation decay time. The timingresolution is important for high counting-rate appli-cations, such as PET. On the side of scintillatormaterials, the timing resolution mainly depends onthe number of scintillation photons of the fastcomponent. This fast component can be effectivelydetected and converted to an electrical signal as longas the spectrum of incident photons matches thespectral sensitivity of the detector. In addition, theresponse time of the photodetector also affects thetiming resolution. Typically, a scintillator has amulti-component decay time, and the presence of aslow component degrades the timing resolution. Itmust be noted that the timing resolution is not aphysical property of scintillators, but a detectorproperty determined by the combination of a

scintillator, photodetector and readout electronics;thus, it is impossible to control the timing resolutiononly by tuning the scintillator properties. This is adifferent point of scintillation properties from othersmentioned above.

Recent R&D progress of scintillators

Brief history. The discovery of ionizingradiation was in the late 19th century; at that time,pioneers such as Roentgen and Edison discoveredthat Ba[Pt(CN)4] emitted visible photons under theirradiation of X-rays.28) Obviously, these visiblephotons were scintillation photons. By a severaldecades after the discovery of X-rays, other species ofionizing radiation had been discovered continuously,and together with the invention of photodetectors,many kinds of scintillators had been developed. Atthe beginning of the R&D history of the scintillator,alkali halide materials (e.g., NaI29) and CsI30)) andnatural phosphors (e.g., CaF2

31) and CdWO432)) were

targeted to be investigated. As known, CaF2 andCdWO4 are called fluorite and scheelite in mineral-ogy, respectively. In the era from the 1940s to 1960s,some of the scintillators well-known today weredeveloped, such as Tl-doped NaI,29) Tl-doped CsI30)

and Eu-doped CaF2.31) After the 1980s, some newmaterials that did not exist in the natural world weredeveloped, and some standard materials today, suchas Bi4Ge3O12 (BGO),33) were found. Compared withNaI, CsI and CaF2, the effective atomic number ofBGO is very high, so BGO was used in the earlytypes of PET. Following the invention of BGO inthis era, notable R&D was performed, which wasCe-doping in insulating host materials, and the Ce-doping became a standard approach to develop newscintillators until now. Ce-doped Lu2SiO5 (LSO)34)

and its Y-admixture LYSO35) are now standardmaterials. Actually, the most recent types of PETare equipped with Ce-doped LYSO instead of BGO.Recently, Ce-doped Gd3(Al,Ga)5O12 (GAGG)36) wasdeveloped by the author and collaborators, and isnow widely used with Si-based photodetectors.Except for Ce-doped garnet scintillators, the emissionwavelength of commercial scintillators is 300–400 nm; these Ce-doped garnet scintillators arethe only type applicable for PD-based radiationdetectors. Figure 8 summarizes the R&D history ofcommon scintillators. Among these scintillators, theauthor contributed to develop GYAG,37) GAGG,36)

Eu:SrI238) and LiCaAlF6.39),40) The recent trend is todevelop host materials of complex compositions, andthe combination of an insulator host and a doped

1

10

100

100 200 300 400 500 600

ScintillationPL

Emission wavelength (nm)

Dec

ay ti

me

(ns)

Fig. 7. Relationship between the emission wavelength (nm) andthe decay time (ns) in PL and scintillation.


emission center has become a standard approach toachieve higher performance.

Emission mechanisms. In the history ofscintillators and scintillation detectors, many kindsof scintillators have been developed. The emissionmechanisms are sometimes considered so as tooverview the scintillator materials, since importantscintillation properties (e.g., emission wavelengthand decay time) strongly depend on the emissionmechanisms. Also these properties greatly affectscintillation detector properties. Figure 9 shows aclassification of the emission mechanisms involved inscintillators. It should be noted that there are noconfirmed classification in this field of study, and thisfigure is a classifications I use.

There are two kinds of luminescence in scintilla-tors: intrinsic and extrinsic. The intrinsic lumines-cence represents free-exciton luminescence, self-trapped exciton (STE) luminescence, Auger freeluminescence and self-activation luminescence. Thefree exciton luminescence is observed in wide band-gap semiconductor materials having direct transi-tions; common examples are ZnO41)–43) and GaN.44)

Scintillation due to free exciton is characterized by

fast scintillation decay with a sharp emission peak inthe spectrum. Recently, I found a newly developedsemiconductor material, Ga2O3, which also showsfast and very intense scintillation upon .-rayirradiation,45) although it is under discussion whetherthe emission is like that of a direct transitionsemiconductor or not. Among the semiconductorscintillators developed so far, Ga2O3 has beenconfirmed to show the highest scintillation lightyield, and we continue to study this scintillator.46),47)

STE can be observed in wide-band-gap insulatormaterials, such as undoped alkali halides. The mostcommon materials are BaF2,48) SrF2,49) CaF2

50) andtheir mixed compounds.51),52) Common properties ofSTE are a broad emission peak in the emissionspectrum, a large Stokes shift, a relatively high lightyield in scintillation, a relatively fast decay time fromseveral hundred ns to few µs and a large temperaturedependence of the scintillation light yield.

Auger free luminescence is observed in somematerials having a special condition, which is a largerband gap energy than the energy between the coreand the valence bands.53) Auger free luminescenceundergoes a very fast luminescence decay with a

1940 1960 1980 2000 2010

NaICsITl:CsICdWO4

Eu:LiILi-glassEu:CaF2

Na:CsI

BGOCe:GSOCe:YAGCe:YAP

BaF2

Ce:LuAGCe:LuAPCe:LSOPbWO4CeF3

Ce:LaCl3Ce:LaBr3

Pr:LuAGCe:GYAG Eu:SrI2

Ce:GAGGLiCaAlF6Elpasolite

199019701950

Imitation of natural phosphors Development of artificial materials

Fig. 8. R&D History of common scintillators. Arrows indicate modifications of materials.

Luminescence with activators (Extrinsic luminescence)

Luminescence without activators (Intrinsic luminescence)I Free exciton luminescenceII Self-trapped exciton (STE) luminescenceIII Auger free luminescenceIV Self-activation

V 1s ↔ 2p transition (e.g., F-center)VI ns2 ↔ nsnp transition

(Ga+, In+, Tl+, Ge2+, Sn2+, Pb2+, Sb3+, Bi3+, …)VII 3d ↔ 3d, 4d ↔ 4f transition (transitional metals)VIII 4f ↔ 4f, 5f ↔ 5f transition (rare earth, Actinoid)IX 4f ↔ 5d transition (Ce3+, Eu2+, …)X charge transfer luminescence (Yb3+, VO4

3-, …)

Fig. 9. Classification of the emission mechanisms of scintillators.


short emission wavelength; the common materials areBaF2,48) BaMgF4,54) CsF55) and Cs2ZnCl4.56) As inthese examples, Auger free luminescence is observedin halide materials, and a search for other compounds(oxides and nitrides) is an interesting research topic.Conventionally, it was considered that they have notemperature dependence concerning the scintillationlight yield;57) but we recently revealed that Augerfree luminescence materials also shows a temperaturedependence.58)

Self-activation type scintillators are composed bythe host, and in this context this type can be classifiedas belonging to the intrinsic luminescence typematerials. The difference from other intrinsic-typematerials is that the host material consists ofluminescent ions as a main constitutional element.Common scintillators of this type are BGO,33) CeF3

59)

and CeBr3.60) Scintillations from BGO are caused byns2 transitions of Bi3D, and those of the latter twomaterials are due to the 5d-4f transitions of Ce3D. Theemission properties are governed by the luminescentions of the host, and interestingly, this class ofmaterials do not suffer from concentration quenching.

We now consider luminescence with activators.Materials containing F-centers show scintillation.The F-center is a defect-based luminescence center,and one electron is captured at an anion vacancy.Common materials with an F-center are simple oxidematerials, such as MgO61) and Al2O3.62) In additionto F-centers, there are some other defect-basedluminescence centers known as, for example, FD-and F!-centers. Although defects are not exactlyactivators, defect-based luminescence is sometimescategorized as being extrinsic emission.

Luminescence due to the ns2 $ nsnp transitionis one of the common electron transitions involved inmany scintillators, such as Tl-doped NaI, Tl-dopedCsI and BGO. The spectral feature of the ns2 $ nsnptransition is characterized by broad emission band,and the emission wavelength is from near UV to VIS.Because of the decay time is from several hundred nsto several µs, it is acceptable for pulse-height-basedradiation detectors. In this type of luminescence,in addition to TlD and Bi3D, InD 63) and Sn2D 64) werediscovered to show promising scintillation propertiesin recent years.

In scintillators, transitional metal ions aresometimes included to obtain a high luminescentefficiency when a fast decay time is not required. Thespectrum of 3d-3d and 4d-4d (d-d) transitions has abroad luminescence feature, and the decay time istypically on the ms order. Among the transitional

metal elements, Mn2D 65) and Cr3D 66) are sometimesselected for scintillation detectors due to a goodspectral matching with Si-PD.

Transitions of the 4f-4f and 5f-5f levels are calledf-f transitions, and some scintillators use the 4f-4ftransitions of rare-earth elements. The most commonscintillator based on the 4f-4f transition is Pr3D-doped Gd2O2S (GOS),67) which is equipped inmedical X-ray CTs. The Pr-doped GOS shows avery high scintillation intensity with a decay time ofmedium range (several µs). The luminescence due tothe 4f-4f transitions of Pr3D appear in yellow-green,which is well-matched with the spectral sensitivityof Si-PD. For practical uses, Ce3D is co-doped tosuppress the afterglow level. Recently, Eu3D-doped(Lu,Gd)2O3

68) has attracted much attention for X-ray CT. The emission intensity is higher than othermaterials, and the emission wavelength is in red,which is suited to Si-PD readout. In addition to thesematerials, we have investigated doping with otherrare-earth ions in order to develop near infraredemitting scintillators. For this purpose, Nd3D 69) hasbeen selected as the emission center.

Emission due to 5d-4f transitions of trivalentand divalent rare-earth ions are very important forscintillation detectors because they show intenseand fast emissions by the spin- and parity-allowedtransitions. The most common emission center isCe3D, and examples of commercial scintillators areCe-doped LSO,34) LYSO,35) GAGG,36) YAG,70)

YAP,71) LaCl3,72) LaBr373) and Cs2LiYCl6.74) Exceptfor garnet materials, including GAGG, most scintil-lators have emission wavelengths of 300–400 nm with30–60 ns decay times; these properties are suitablefor PMT readout. In addition to Ce3D, Pr3D can alsoshow luminescence due to the 5d-4f transition insome host materials. The appearance of the 5d-4ftransition depends on the relative positions betweenthe lowest 5d and 1S0 levels. Common Pr-dopedscintillators are Pr-doped LuAG75) and YAP,76)

which show light yields of 10,000–20,000 ph/MeVwith a 20 ns decay time. Compared with Ce-dopedscintillators, the emission of Pr-doped materialsappears in a shorter wavelength range. Some othertrivalent rare-earth ions show 5d-4f transitions onlyin hosts having a wide-band gap energy, and theemission wavelength is in VUV. A common exampleis Nd-doped LaF3, which has an emission wavelengthat 175 nm with a few-ns decay time77) due to the 5d-4ftransition of Nd3D. Divalent Eu can also show a veryhigh scintillation light yield with typically a 1 µsdecay time; example scintillators are Eu2D-doped


SrI2,38) CaF2,31) LiI78) and LiCaAlF6.40) Recently,luminescence due to the 5d-4f transitions of Sm2D

ions was also reported,79)–81) and investigations arein progress. Sm2D shows emission in the near-infraredrange, and has attracted much attention for usewith Si-PD type photodetectors. Recent trends inthe development of scintillators involve those rare-earth-doped materials possessing 5d-4f transitions;Fig. 10 shows typical scintillation spectra of suchscintillators. Nd3D, Er3D and Tm3D show emissions inthe VUV range, Pr3D in the near UV range, Ce3D andEu2D in the VIS range, and Sm2D in the near-infraredrange. It should be noticed that the emission wave-length by 5d-4f transitions strongly depends on thecrystal field, which is specific to each host lattice; thepresented data are only examples.

Charged transfer luminescence can be observedin some atomic groups, such as Yb3D-doped materialsas well as VO4

3! and WO42!. Examples of the former

type is Yb-doped Lu2O383) and Yb-doped garnet

materials,84) while examples of the latter are YVO485)

and CaWO4.86)

On one hand, the charge-transfer luminescenceof Yb3D is known to have a very fast decay with alow light yield due to thermal quenching at roomtemperature. On the other hand, the charge transferluminescence of the VO4

3! and WO42! atomic groups

are slow (typically several µs), but they have a largelight yield of 10,000–20,000 ph/MeV. Among thematerials with charge-transfer luminescence, CdWO4

has been used with Si-PD87) for X-ray detectors forsecurity applications.

Recent R&D of scintillators for X- and .-raydetectors. Among many applications of ionizingradiation, high-energy photons, such as X- and .-rays, are the most commonly utilized, since they canbe generated relatively easier than other species ofionizing radiation. Especially, these high-energyphotons have a high penetration power to mattersincluding the human body; thus, a non-destructivestudy of inner objects is a main application. In X- and.-ray detectors, scintillators are required to have ahigh density (; in g/cm3) and Zeff. Interactions of X-and .-rays with matter mainly include three types:photoelectric absorption (sometimes called photo-absorption), Compton scattering and pair creation,as well as their interaction probabilities depend on9;Zeff

4, ;Zeff and ;Zeff2, respectively. The events of

photoelectric absorption are often utilized in prac-tical detectors (pulse-counting type), since it repre-sents information concerning the incident X- or .-rayenergy. Recently, Compton scattering is also usedto determine the position of X- or .-ray sources(Compton camera detector); this concept works onlyin the pulse-counting mode.

Figure 11 shows the typical detector configura-tions of X- and .-ray detectors. For X-ray detectors,scintillators are typically coupled with Si-PDs, andeach pixel is separated by Pb or W collimator tomaximize the imaging resolution. Because most X-ray detectors are used for medical imaging andsecurity inspection systems, a position sensitivity is ofprimal requirement. To observe a clear image, thedetection of scattered X-rays should be avoided asmuch as possible, and collimators are typically usedfor this purpose. In conventional detectors, thecollimators on the detection plane are a dead area,but recently R&D has attempted to use them as anactive area. One of the methodologies uses a so-called functional collimator.88) In this concept, thecollimator material acts as both a collimator anda scintillator, and shows scintillation. In order todistinguish the signals detected in the primaryscintillators and collimator parts, a pulse-shapediscrimination technique can be applied.

In pulse-counting-type detectors, the most con-ventional detectors consist of a large scintillatorcoupled with PMT, and are mainly used for .-rays.Although there are no confirmed definitions toseparate between X-rays and .-rays, a typicalunderstanding is that .-rays are high-energy photonsemitted via transitions between energy levels inradioisotopes (radioactive decay), while X-rays aregenerated by interactions with the accelerated

0

0.5

1

1.5

160 240 320 400 480 560 640 720 800

Ce:GAGGPr:LuAGEu:SrI2Sm:FCZNd:LuF3Tm:LuF3Er:LuF3

Wavelength (nm)

Ce3+Sm2+Pr3+

Eu2+

Nd3+

Er, Tm3+

Fig. 10. Scintillation spectra of Nd3D-, Er3D- and Tm3D-dopedLuF3,82) Ce-doped GAGG,36) Pr-doped LuAG,75) Eu-dopedSrI238) and Sm-doped FCZ glass-ceramics.79)


electrons and anode materials or by bending trajec-tories of accelerated particles. Thus, the .-rayspectrum appears to be a sharp line structure dueto the well-defined energy levels while X-rays consistof a broad spectrum structure with the endpoint ofenergy defined by the highest energy of acceleratedelectrons. Radiation detectors consisting of onescintillator with one PMT are probably the simplestconfiguration, and are used for survey meters andwell-logging detectors. In most particle detectors,such as ,-ray and neutron detectors, the sameconfiguration is used on a pulse-counting mode.Unlike the case of X-ray detectors, most .-raydetectors do not use collimators, since it is difficultto collimate high-energy photons, and also the useof most .-ray detectors are not to obtain images, butto count the number of .-rays in the pulse-heightspectrum. Among detectors for nuclear imaging,SPECT uses collimators, since the target .-rayenergies are not high, and PET use the coincidencetechnique for 511 keV annihilation .-rays.

For .-ray detectors, a Compton camera hasrecently been developed and used in practicalapplications. The root of the Compton camera washigh-energy astrophysics in the 1990s,89) since theCompton scattering was a dominant event in MeV.-rays, which were a main target of high-energyastrophysics. Generally, a Compton camera uses twodetection parts. One is a scatterer where Comptonscattering takes place, and the other is an absorberwhere photoelectric absorption is utilized. In thesetwo detector parts, radiation energies are deposited,and the sum of these energies equals to the incident .-ray energy. From the formula of Compton scattering,if we know the incident radiation energy (specie), wecan find the angle of the Compton scattering (3, in

Fig. 11), and we know that the radiation source issomewhere on the circle drawn in Fig. 11 (right). Bysuperposing circles in many events, the position ofthe radiation source can be determined eventually. Inthe Compton camera, not only scintillators, but alsosemiconductor detectors, are used. Figure 12 showsa .-ray detector that I constructed. It consisted ofCe-doped GSO90) and BGO scintillators. The scintil-lation decay times of GSO (960 ns) and BGO(9300 ns) were different, and the pulse-shape dis-crimination was used to distinguish. As shown in thefigure, the size of the detector was several tens of cmin length to absorb .-rays efficiently; 16 detectorunits were arrayed. This .-ray detector was actuallyonboard the Suzaku satellite,91) and observed high-energy .-rays from stars and galaxies in the universe.

For such X- and .-ray detectors, the requiredproperties on scintillation materials are a high Zeff

(>50) and a high light yield in both integration andpulse-counting type detectors. In the pulse-counting

scintillatorphotodetector

collimator

RI

Integration type(Mainly for X-rays)

RI

Pulse counting type(Mainly for γ-rays)

Pulse counting type(Compton camera)

RI

Compton scattering

Photoelectric absorptionPhotoelectric absorption

Photoelectric absorption

Fig. 11. Typical detector configurations of the integration and pulse counting type.

Fig. 12. Picture of the .-ray detector onboard the Suzakusatellite.


type detectors, a high energy resolution and a fastdecay time are particularly required, while lowafterglow is required in integration-type detectors.As mentioned above, in my opinion, the first-generation scintillators were simple chemical com-pounds, such as NaI and CsI and natural phosphormaterials, including CdWO4 and CaF2, developedand used since the 1940s; second-generation scintilla-tor was BGO, available since the 1970s. After the1990s, third-generation scintillators, which are typi-cally a combination of host and emission center type(e.g., Ce-doped LSO and LYSO), had started to beused. Thus, most state-of-the-art scintillators devel-oped today are those doped with an emission center.

The trends of the third-generation scintillatorshave three streams: rare-earth silicate, rare-earthgarnet and some halide materials. The root of rare-earth silicate was Ce-doped Gd2SiO5 (GSO),90) whichwas found by Hitachi Chemical, Japan. Inspired bythis invention, Lu-substituted scintillators, such asLSO and LYSO, were developed in western countries,and these scintillators are now standards for detec-tors of PET. Unfortunately, the patent of GSOmissed to include Lu-substituted materials, and theindustrial market is now dominated by westerncountries, which have patents on LSO and LYSO.Thus, Japan is the origin of one of the threemainstreams in the scintillator field today. Thescintillation properties of these Ce-doped silicatesare characterized by a high light yield (10,000–30,000 ph/MeV) and a fast scintillation decay (30–60 ns) with blue emission appearing at around430 nm. These properties are suited for detectorsin PET. Since Lu-based silicate scintillators havesuperior light yield (20,000–30,000 ph/MeV) thanGSO (10,000 ph/MeV), most PET use LSO or LYSO.In order to improve the scintillation properties, Cacan be added as a co-dopant,92),93) which are nowused in practical detectors. Following silicate materi-als with a composition of RE2SiO5, where RE denotesrare-earth elements, other materials with differentchemical compositions of RE2Si2O7 have also beeninvestigated, and are called pysosilicate. Among thepyrosilicate scintillators, Ce-doped Gd2Si7O7 (GPS)was developed by groups at Hokkaido University andHitachi Chemical, and it was reported to show ahigher light yield than the LSO series and a fastdecay of several ten ns.94),95) Following this work, Laadmixed GPS (LGPS) was found,96) which exhibitedcomparable scintillation properties as Ce-dopedGPS97),98) but are easier to be fabricated in singlecrystal. Unlike the case of RE2SiO5 materials, a Lu-

based composition (Lu2Si2O7) (LPS)99) has worsescintillation properties than GPS in terms of the lightyield and energy resolution. In addition to RE2SiO5

and RE2Si2O7, other chemical compositions, such asRE9.33(SiO4)6O2 and (AE2RE8)(SiO4)6O2 (AE repre-sents alkali earth elements) are also possible candi-dates of scintillators,100) and are called apatitescintillators. Apatite scintillators have started to beinvestigated recently,100)–102) and at present muchless information concerning to scintillation propertiesof such apatite scintillators is available compared toother silicate compounds. Apatite scintillators areinteresting, since the type of (AE2RE8)(SiO4)6O2,especially, has two sites (AE and RE) to accommo-date rare-earth dopant ions. I expect future studieswill discover novel silicate scintillators.

Garnet materials of scintillators typically havechemical compositions of RE3(Al,Ga)5O12, and someimpurity ions are doped as emission centers. The rootof the garnet materials for phosphor applications wasY3Al5O12 (YAG), which was developed in the 1960sfor laser applications.103) In the scintillator field,the scintillation properties of a Ce-doped YAG wereinvestigated in the 1990s,104),105) and it was thenreported to exhibit intense and fast emission due tothe 5d-4f transitions of Ce3D. The Ce-doped YAG hasbeen mainly used for charged-particle detectors, andit was attempted to substitute Lu for the Y (Ce-doped LuAG) in order to achieve a higher detectionefficiency for high-energy photons.106) Instead of Lu,Gd-substitution was also considered, and at first,Ce-doped (Gd,Y)3Al5O12 (GYAG) was developed.107)

Further studies succeeded to fully replace by Gd, andthe Ce-doped Gd3(Al,Ga)5O12 (GAGG) was foundto show distinct scintillation light yield (946,000ph/MeV), and became a commercial product.36)

A few years ago, we achieved 70,000 ph/MeV bydeveloping a Ce-doped GAGG ceramic, which hadthe highest light yield among the oxide scintilla-tors.108) We have continued to study Ce-doped garnetmaterials, and very recently, we discovered that Ce-doped Tb3Al5O12 (TAG) also shows very promisingscintillation properties: a high light yield of 57,000ph/MeV with a decay time of 38 ns.109),110) Before thisstudy, Ce-doped TAG was only considered in a thin-film form,111) and it was impossible to evaluate thescintillation light yield under .-ray irradiation.Therefore, this discovery opens a new possibility tobe considered as a host of garnet scintillators. Animportant property of Ce-doped garnet scintillatorsis the emission wavelength as well as the high lightyield and fast decay. The peak emission wavelength


typically appears at 500–550 nm, which matches thespectral sensitivity of Si-based photodetectors. Atpresent, Ce-doped garnet scintillators are the onlychoice to be used with Si-PD, avalanche PD (APD)and Geiger-mode APD.

In the 2000s, it was also discovered that Pr3D-doped LuAG also showed a 5d-4f transition ofPr3D.112) Prior to this study in the 1990s, Pr-dopedYAG was known to have a relatively high lightyield,75),113) but further studies were not conducted.The emission wavelength of Pr3D in garnet materialsis near UV, so it is difficult to use with conventionalphotodetectors, such as PD, and PMT is difficult.This is the reason why Pr-doped garnet scintillatorswere not accepted in practical applications in spiteof their relatively high light yield (10,000–20,000 ph/MeV), fast decay (920 ns) and high energy resolution(4.3% at 662 keV).114) For practical applications,Pr-doped garnet scintillators, photodetectors, whichhave a high quantum efficiency in the UV must bedeveloped.

The last trend of R&D is a recent focus on halidescintillators. As mentioned above, the history ofscintillators and scintillation detectors started withhalide scintillators, such as NaI, CsI and CaF2, andNaI had become one of the standard scintillators. Inorder to develop a “next” NaI, continuous efforts havebeen made, since halide scintillators have a greatpotential to display a high light yield due to therelatively smaller band gap than that of commonmaterials of oxide, fluoride and nitride. This trendalso started from early in the 2000s at DelftUniversity. Ce-doped LaCl372) and LaBr373) werediscovered, which have shown a very high light yield(960,000 ph/MeV) with a very high energy resolu-tion (3–4% at 662 keV). Before the discoveries ofthese materials, the highest light yield of scintillatorsfor .-ray detectors were at most 40,000–50,000 ph/MeV by NaI and CsI, and this discovery was quitesensational. Following these pioneering investiga-tions, numerous researchers started to investigatemany halide materials as potential scintillators,and some new materials were found. Among thesenewly developed scintillators, Eu-doped SrI219),38),115)

became a commercial product for .-ray detectors.Actually, Eu-doped SrI2 was invented in the mid-20th century,116) but no one paid any continuousattention on this material, possibly due to itsfeatureless properties. However, owing to the recentprogress of crystal growth technology, the crystalquality had improved, and the scintillation propertieswere dramatically enhanced (e.g., scintillation light

yield of up to 120,000 ph/MeV, energy resolution of3% at 662 keV and a decay time of around 1µs).Since the main patent was lost in effect, manyindustrial suppliers of scintillators started to developthis material to productize, and the purpose wasobviously the “next” NaI. At present, as matter offact, it is not realistic for LaBr3 and SrI2 to becomethe next NaI because of its very high hygroscopicnature. These scintillators must be capsulated toavoid moisture, and the freedom of the detectordesign is strongly limited by the shape of thescintillators. Typically, products of LaBr3 and SrI2are cut to 1–2 inches in diameter and 1–2 inchesin height; other sizes are not available for mostresearchers who develop scintillation detectors.

As described above, conventional halide scintil-lators are very bright, but hygroscopic. Recently,some groups including the author’s groups havestarted to search for non-hygroscopic halide scintilla-tors. Within these few years, Cs2HfCl6117),118) wasintroduced as a promising alternative. This materialwas discovered by two independent studies by anAmerican group and our group a coincidence by.Cs2HfCl6 is non-hygroscopic, and it includes Hf as ahost constitution, so the cross-section to high energyphotons is very high. The light yield was evaluated tobe around 30,000 ph/MeV under .-ray exposure witha few µs decay time. With the knowledge of a similarcompound, Cs2ZrCl6, the emission origin was blamedfor the charge-transfer luminescence of the [HfCl6]2!

anion complex. Motivated by this work, we havefurther developed non-hygroscopic halide scintilla-tors, including Tl- and In-doped CsCl,119) TlCdCl3,120)

Tl- and Ce-doped Cs2HfCl6,121) Rb2HfCl6,122) Ce-doped CsCaCl3123) and TlMgCl3.124) Among thesenewly developed scintillators, especially TlMgCl3shows the most promising properties of a high lightyield (46,000 ph/MeV), a high energy resolution (5%at 662 keV) and a relatively fast decay (60 ns).

Recent R&D of scintillators for neutrondetectors. Scintillators for neutron detectors havebeen attracting much attention recently due to adecreasing supply of 3He gas, which has been afundamental of conventional neutron detectors overthe past decades.125),126) As an alternative to 3He gas,many efforts have been made to develop scintillatorscontaining 6Li and 10B, which have high interactionprobabilities against thermal neutrons. Before the3He problem, the applications of scintillators forneutron detectors were not many, and this is a newsubject of scintillator research. For this purpose,many scintillators were developed, and 6Li-enriched


elpasolite and LiCaAlF6 scintillators have reached tothe commercial stage. The former one, elpasolite, hasa chemical composition of Cs2AMRE(Cl,Br)6, whereAM means alkali metal elements, and Li is chosen todetect neutrons. The root of the elpasolite was also atDelft University,127) but the R&D for practicalapplications was conducted in the U.S. Up to now,many elpasolite materials have been developed, andCe-doped Cs2LiYCl6 (CLYC) demonstrated the bestperformance as a neutron detector.128) AlthoughCLYC and other elpasolite scintillators have a high,/O ratio and distinct functions of a pulse-shapediscrimination of .-rays and neutrons, a large degreeof hygroscopicity is a disadvantage for practicalapplications, as for LaBr3 and SrI2. LiCaAlF6 hasbeen developed by the author’s groups for neutrondetectors.39),40) The advantage of LiCaAlF6 is a non-hygroscopicity and a high scintillation light yieldwhen Eu2D is doped as an emission center. Pulse-shape discrimination is possible when Ce3D is doped.At present, the productization of 6Li-containingscintillators for neutron detectors has converged onthese two scintillators.

Compared with 6Li, 10B has a higher cross-section against thermal neutrons, but has lower Q-value. Thus, it is difficult to develop a 10B-basedbright scintillator for neutron detectors. Up to now,the brightest 10B-based bulk crystalline scintillatorhas been developed by us, which is Ce-dopedCaB2O4, exhibiting 2,200 ph/n under neutron exci-tation.129) The light yield must be improved forpractical detector applications.

Recent R&D of scintillators for chargedparticle detectors. Charged-particle detectors aremainly used for detecting ,-rays, which have a highinteraction probability with most materials. Inscintillation detectors for ,-rays, Ag-doped ZnS hasbeen used historically.1) ZnS is the most famous of allscintillators because a ZnS screen was utilized in thefamous Rutherford experiments employed to provethe existence of the atomic nucleus. Ag:ZnS is onlyavailable as a polycrystalline powder (visibly white),and applications are limited to thin screens madeby, for example, spraying. Therefore, in spite of ahigh light yield of 90,000 photons/MeV, the energyresolution of Ag:ZnS is not so good, except for someideal detector geometries. Recently, scintillators for,-ray detection have become highly demanded allover the world. In nuclear processing plants, uraniumand plutonium are manufactured, that elements emit,-rays, which should be monitored.130) They cancontaminate in the human body and cause serious

damage by internal exposure. Since the environmentrequired to monitor ,-rays often comprise back-ground .-rays; scintillators for ,-ray detections mustbe insensitive to .-rays. In addition, a high energyresolution is desired to distinguish from variousradioisotopes.131) Therefore, new scintillators for ,-ray detectors should require a high energy resolution.To achieve this resolution, the scintillator materialsshould have high transparence. Moreover, to avoiddetecting background .-rays and neutrons, scintilla-tors must have an intermediate Zeff, and thus thecandidates are elements with Z F 10–30. In orderto develop novel scintillators for ,-ray detectors, wehave focused on ZnO thin-film scintillators. Since thepenetration depth of ,-rays is not so deep (around10–20 µm), film scintillators are ideal. In addition,Zn has a low cross-section to .-rays and neutrons.Up to now, we have developed some ZnO filmscintillators,132) and have demonstrated ,-ray realtime imaging by using ZnO.133)

Unresolved problems. In scintillator-relatedfields, there are many unresolved problems in spiteof a large number of practical applications. Here,I select some unresolved topics and discuss our mostrecent understandings of these phenomena alongwith some experimental results.

Positive hysteresis. In scintillation phenom-ena, there are many unresolved problems. One of theimportant problems involved in both basic scienceand detector applications is positive hysteresis. Posi-tive hysteresis is defined as an enhancement of thescintillation light yield observed after a large amountof radiation exposure. This phenomenon is sometimescalled by other names: radiation drift and brightburn. To my understanding, positive hysteresis isnot a common phenomenon, and observations werereported only in selected materials, including Pr-doped Lu2Si2O7 (LPS),134) Tl-doped CsI,135) Ce-doped and Ce/Zr co-doped Gd2SiO5 (GSO),136) Ce-doped Gd3(Al,Ga)5O12 (Ce:GAGG),137) Tb-dopedSiO2 glass138) and Ce/Nd co-doped YPO4.139) Onone hand, from a physics point of view, the originof positive hysteresis is currently under discussion,and at least it has been considered that carriertraps138),139) and the generation of new excitationbands above the band gap energy136),137) are impor-tant. On the other hand, from the viewpoint ofpractical applications, positive hysteresis needs to besuppressed, or corrected, by the readout electronics orsoftware, since it can cause some ghosts in a radiationimage. In addition, it should be noted that twodifferent measurements were previously considered


to determine the scintillation light yield involvedin this problem: by the pulse-height spectrum136),137)

and radioluminescence.134),138),139) In order to discussthe light yield of the scintillator, any pulse-heightanalysis should be conducted as mentioned in theintroduction, though some groups still continue touse the radioluminescence intensity.

In our most recent study,140) we found that Pr-doped LPS did not show any positive hysteresis(confirmed by the pulse height analyses), and wethought that the positive hysteresis of the pioneeringreport in Pr-doped LPS may have been be affected byradiation-induced afterglow (their study was basedon a radioluminescence analysis). Therefore, in myopinion, the materials confirmed to show positivehysteresis up to now are CsI, GSO and GAGG.Figure 13 shows schematic drawings of the energytransportation processes under the normal and thepositive hysteresis conditions. In the normal con-dition, many secondary electrons are generated byionizing radiation exposure, and some secondaryelectrons (energies) are transported to the emissioncenter, while other carriers (energies) are transferredto trapping or non-radiative centers. In the positivehysteresis condition, some of the secondary electrons(energies) are captured at newly created energylevels, and then efficiently transferred to the emissioncenter directly. In the examples of CsI, GSO andGAGG, the light yield enhancement was at most20%. Possible origin of the newly created energylevels would be anion vacancies, since such a vacancytypically appears upon large-dose exposure. If theorigin of the positive hysteresis is understood, thenwe would be able to control the light-yield enhance-

ment phenomenon. For example, if the anion vacancyis the origin of positive hysteresis, we should considerincluding stable anion vacancies by, for example,synthesizing in a strong reducing environment orexposing to a large dose to obtain bright scintillatorproducts.

Co-doping of non-luminescent ions. Re-cently, the co-doping of ions that have no lumines-cence energy levels has attracted much attention toimprove the scintillation properties. The attemptshave been made to improve an enhancement of thelight yield and to suppress the slow component ofscintillation decay. The most common example isCa2D and Ce3D co-doped LSO. Here, the light yieldand energy resolution have been improved, and asuppression of the slow decay component wasreported.92),93) In LSO, there are two Lu3D sitesthat can accommodate Ce3D ions, which shows fastemission at one site and slow emission at the other.The origin of this improvement in LSO can beunderstood by hypothesizing that the Ce3D ions wereselectively embedded at the former sites by co-doping, so the slow emission was suppressed.92) Inaddition, it was pointed out that individual differenceof the Ce3D/Ce4D ratio was considerably large in LSOsamples,141) and such a ratio may be related to Ca2D

co-doping.Ce- and Eu-doped LiCaAlF6 crystal scintillators

have been developed recently by the author andcollaborators,39),40),142),143) and they have becomecommercial products by Tokuyama Corp. for thermalneutron scintillation detectors. Especially, Eu-dopedLiCaAlF6 is known to have the highest light yield(930,000 ph/n) under neutron irradiation among non-hygroscopic materials. During the R&D processes,we found that the light yield can be enhanced by co-doping with alkali metal elements.144) Alkali metalions including Na, K, Rb and Cs are known to showno luminescence features. We discovered that, whenNa is included to Eu-doped LiCaAlF6, the scintilla-tion light yield was enhanced to 942,000 ph/n.Historically, especially in laser fields, the chargeimbalance of Ce3D and Ca2D in Ce-doped LiCaAlF6

was compensated by an addition of NaD under theassumption that the Ce3D ions would substitute forthe Ca2D sites.145) Therefore, at that time, the sameapproach was considered to be nonsense for Eu2D-doped LiCaAlF6. Based on our experimental results,the effects of a charge balance cannot be applied atleast to Eu-doped LiCaAlF6. In order to clarify theorigin, we observed the excitation spectra of Eu andalkali metal co-doped LiCaAlF6 at the synchrotron

VB

CB

Emission center

Trapping or non-radiative center

Secondary electrons(Carriers)

Normal condition Positive hysteresis

VB

CB

Emission center

Trapping or non-radiative center

Newly created levels

Secondary electrons(Carriers)

ScintillationScintillation

Thermal loss

Fig. 13. Schematic drawings of energy transport processes undera normal condition (left) and a positive hysteresis condition(right).


facility (UVSOR, Japan). Figure 14 shows theexcitation spectra of Eu-doped LiCaAlF6 incorpo-rated with Na, K, Rb and Cs. In addition to the bandgap (9110 nm) and the absorption due to Eu, a newexcitation band appeared at around 80–100 nm inthese materials. Actually, this excitation band wasobserved in undoped LiCaAlF6 at very low temper-atures, and the highlight of this work was theappearance of this band at room temperature. Therole of co-doping with alkali metal elements is toenhance some kinds of defects associated with theLiCaAlF6 host. My idea is that the same interpreta-tions as in Fig. 13 can be applied to understand thisphenomenon. The enhancement of the light yield byco-doping with alkali metal is around 30%, and theratio of the newly created energy level to the bandgap energy is 1.2–1.3. By opening this newly createdpath, the energy transfer efficiency was improvedby 20–30% on average. In the case of Eu-dopedLiCaAlF6, the order of the effectiveness by co-dopingwas Na > Rb > Cs > K, while the best performancewas observed by co-doping with K in the case ofLiSrAlF6.

A similar light yield enhancement by co-dopingwas also observed in Mg2D co-doped Lu3Al5O12.146)

Following this study, other garnet scintillators havebeen studied by mainly the same group, but a similarenhancement effect was not observed. Thus, amonggarnet scintillators, a light yield enhancement can bepossible in Lu3Al5O12 by a coincident combination ofCe3D and Mg2D. The origin of this phenomenon wasconsidered to be the generation of Ce4D by dopingwith Mg2D, and the presence of Ce4D was confirmedby a XANES (X-ray absorption near edge structure)

analysis. Although some groups use this hypothesis,to my understanding, this model has not beenconfirmed yet, since the presented data are onlythe PL spectrum and XANES. In order to approvethe model logically, it is necessary to confirm thefollowing two points in addition to the presence ofCe4D. One is that the main energy-transfer process forCe3D ions is a sequential charge transfer, and notenergy transfer from some relaxed excited states inthe host to the Ce3D ions. The second is that electroncapture by Ce4D ions is faster and more efficientthan hole capture by Ce3D ions. Recently, one of thediscoverer groups denied this hypothesis and re-ported in some conferences and literature.147) In myopinion, some groups use this hypothesis conclusivelywithout experimental evidence of the dynamics,and it is an exaggeration. Further experiments arerequired to understand this phenomenon.

Introduction of aimed impurity phase. Thescintillation light yield can be enhanced by introduc-ing impurity phases. Actually, we have found thisconcept in Ce-doped GAGG ceramics.108) In a GAGGcrystal of the single phase, the scintillation lightyield is known to be 46,000 ph/MeV. In contrast,by including a few % of the perovskite phase, thelight yield was enhanced up to 70,000 ph/MeV.108)

Figure 15 illustrates my understanding of this phe-nomenon. I think the impurity phase works somethinglike a bypass, and it suppresses the energy dissipationduring transfer. Although the light yield could beimproved by introducing the impurity phase, theenergy resolution degraded because the material wasno longer homogeneous on a microscopic scale.

Maybe the same idea is applicable to the positivehysteresis and co-doping with non-luminescent ions,because some “impurities” are included in the lattertwo cases as vacancies and impurity ions, respec-tively. These ideas of interpretation came up from therelation between the defect and scintillation lightyield. In most scientific fields, including semiconduc-tor and laser physics, the creation of defects inmaterials should be avoided as much as possible sincethey degrade the device performance. However, inscintillators, we have proved experimentally that acertain degree of defect concentration is requiredto achieve a high light yield. In an experiment, weprepared Pr-doped Lu3Al5O12 with the same amountof Pr, but different defect concentrations by control-ling the synthesis processes, and evaluated theconcentration of defects by an X-ray rocking curveand scintillation light yield under 137Cs .-raysexposure. When we plotted the light yield as a

1.1x105

1.2x105

60 80 100 120 140 160 180 200

Na:LiCAF

Wavelength (nm)

Inte

nsity

(a.u

.)

KLiCAFRb:LiCAF

Cs:LiCAF

Absorption of Eu

Bandgap

Newly created excitation bands

Fig. 14. Excitation spectra of Eu and alkali metal (Na, K, Rband Cs) co-doped LiCaAlF6 crystalline scintillators.


function of the defect concentrations, deduced fromthe X-ray rocking curve analysis, it showed a convexquadratic function feature with a clear peak.148) Fromthis result, I have noticed that ideal crystals (with nodefects) are not preferred for scintillator uses, andthat the inclusion of some defects or impurities isnecessary. Similar work was done to compare betweenceramic and crystal forms of undoped Y3Al5O12 andLu3Al5O12 scintillators, since the types and numberof defects differ in the ceramic and crystal.149) Tounderstand the phenomenon and to control thescintillation light yield, further studies are required.

Excitation density effect. The excitationdensity (LET) effect can be observed in somescintillators. Frankly speaking, excitation by ionizingphotons (X- and .-rays) has a relatively lowerexcitation density compared with that of chargedparticles (e.g., ,-rays). The excitation density effectis typically observed by measuring the scintillationdecay time profile and the emission spectrum. Themost famous example is BaF2, which shows Augerfree luminescence. The Auger free luminescence isalso called core valence luminescence because theluminescence occurs by the recombination of carrierswithin the core and valence bands. The physicalproperties of Auger free luminescence are character-ized by fast decay, and in the case of BaF2, it isaround 0.8 ns. This Auger free luminescence in BaF2

can be observed in high-energy photon and electronexcitations, but it cannot be seen under ,-rayexcitation.150) The threshold of the excitation densityto make the Auger free luminescence appear wasinvestigated by controlling the excitation density of acathode luminescence technique; the obtained valuewas 2 # 1019 eV/cm3 in BaF2.151) The threshold valueis unique to each material, and the threshold wasaround 1019 eV/cm3. Although there are no con-firmed theories, the quenching of secondary electronswould occur when the excitation density becomeslarge. Some luminescence transitions, especiallyAuger free luminescence, is very sensitive to LET,

and such a quenching due to the interactions ofsecondary electrons is observed.

An important aspect of the excitation densityeffect for practical detector applications is a discrim-ination of ionizing radiations. The most commontechnique is pulse shape discrimination, which usesthe difference of the scintillation decay profile (pulseshape) under high-energy photon and charged-particle excitations. A common example is 6Li-basedscintillators for thermal neutron detectors. By usingthe difference of the pulse shapes under .-rays and ,-rays, caused by the 6Li(n,,)3H reaction, the neutronand .-ray induced events can be discriminated.Imagine some typical situations where neutrondetectors are used. One of the common situationsis around a nuclear reactor. The status of a nuclearreaction can be monitored by measuring neutronsgenerated by nuclear reactions, and in this situation,a large number of background .-rays are alsodetected by scintillation detectors. In such a sit-uation, the pulse-height discrimination is used inorder to separate the signals by .-rays and neutronsand to monitor the status of the nuclear reactorprecisely. Common scintillators possessing the func-tionalities of pulse discrimination are LiCaAlF6

152)

and elpasolite153) materials. Pulse-shape discrimina-tion may be possible by employing some organicmaterials,154) but the radiation tolerance is a problemin organic scintillators. Concerning future prospects,if the excitation density effect is fully understood, thepulse-shape discrimination technique can be appliedfor other ionizing radiations in practical applications.

Complementary relationship. As mentionedin the introduction, most of the basic physics ofscintillation is still unclear in spite of a large numberof practical applications. Especially, the importantproperties to be understood are the scintillationlight yield and the dynamics (rise and decay ofscintillation). I have particular interest in the formerphenomenon. In other words, I am eager to know whysome materials emit bright scintillation and some

Luminescence center Luminescence

center

Main phase Main phase

Impurity phase

Fig. 15. Schematic drawings of secondary electrons transported in a single-phase material (left) and a mixture of the main and impurityphases (right).


others do not. In my opinion, one of the key points isto comprehensively consider both scintillation anddosimetric properties, since these processes sharesome common aspects as shown in Fig. 3.

Historically, scintillators and dosimeter materi-als have been investigated and developed by differentcommunities due to the differences of detectiontechniques and measurement time scales, while theirpurpose are the same: to detect ionizing radiation byluminescent materials. However, if we assume energyconservation, the scintillation and OSL/TSL proper-ties can be connected, and they should be comple-mentarily related. Considering the total energy of thescintillation output per incident radiation energy, animmediate conclusion is that only a small fraction ofthe radiation energy is used for luminescence. Here, asimple question may arise: ‘why some scintillators arebright and the others are not?’ In other words, wherehas the remaining energy gone? I would answer thatsome portion (maybe not all) of the remaining energyis stored in the material, and that it can be releasedby stimulation (OSL or TSL). Also, it is possible thatanother amount remaining energy is converted tothermal energy, which is used in the calorimeter forradiation detection. Therefore, we consider that thescintillation and OSL/TSL properties can be com-plementarily related, and understanding of thisrelation will be a great benefit for the futuredevelopment of scintillator/dosimeter and our under-standing of the scintillation light yield. Actually, weproposed a hypothesis that the luminescence inten-sities of scintillation and dosimeter are complemen-tarily related, assuming the energy conservation law,and we tested the hypothesis experimentally. Ifmaterials are not bright in scintillation, they shouldeffectively store absorbed energy and should showstrong luminescence as dosimeters (TSL or OSL),and vice versa. Recently, this hypothesis was con-firmed experimentally in Ce-doped CaF2, Eu-dopedLiCaAlF6 and Pr-doped Lu3Al5O12 crystals andceramics.155),156) Figure 16 demonstrates a relation-ship between the scintillation light yield and OSLintensity of 0.1–20% Ce-doped CaF2. With increasingthe concentration of Ce, the scintillation light yielddecreased while the OSL intensity increased. Thedegradation of the light yield of highly Ce-dopedCaF2 may be considered to be due to concentrationquenching. However, it is not the present case inwhich most of the energy was stored in those heavilydoped materials. Such a systematic study based onenergy conservation is a conventional approach inhigh-energy physics, and I would like to find the key

for understanding the scintillation light yield bysystematic investigations with many different mate-rials, particularly by focusing on the scintillation andstorage luminescence properties together.

Summary

The basic properties of scintillators have beenexplained especially for applications in scintillationdetectors, and a number of scintillators used inpractical applications today introduced. In addition,state-of-the-art scintillators and scintillation detec-tors currently in the R&D phase were reviewed.Finally, some unresolved problems were pointed out,and my interpretations based on experimental resultsexplained.

Acknowledgement

This work was supported by a Grant in Aid forYoung Scientists (B)-21760705 and (A)-23686135, aGrant in Aid for Young Scientists ChallengingExploratory Research-23656584, Grant-in-Aid forScientific Research (A)-26249147 and (A) 17H01375from the Ministry of Education, Culture, Sports,Science and Technology of Japan (MEXT) andpartially by JST Development of advanced measure-ment and analysis systems (Sentan) and Adaptableand Seamless Technology Transfer Program throughTarget-driven R&D (A-step). Partial assistance fromthe Yazaki Memorial Foundation for Science andTechnology, Hitachi Metals Materials Science Foun-dation, Shimadzu Sci. Foundation, JFE 21st CenturyFoundation, The Mazda Foundation, Kato Founda-tion for Promotion of Science, and Nippon SheetGlass Foundation for Materials Science and Engi-neering, Tokuyama Science foundation, The Murata

1000

1x104

1 10 100OSL intensity (quantum yield, %)

LY (p

h/5.

5 M

eV-a

lpha

)

Fig. 16. Relationship between the scintillation light yield under5.5MeV ,-ray excitation and the OSL intensity after 1Gy X-rayexposure in 0.1–20% Ce doped CaF2 crystals. The data are takenfrom past experiments.156)


Science Foundation, Kansai Research Foundation fortechnology promotion, The Kazuchika Okura Memo-rial Foundation, Inamori Foundation, Suzuki Foun-dation, Iketani Science and Technology Foundation,The Taisei Foundation, SEI Group CSR Foundationand The Asahi Glass Foundation, are also gratefullyacknowledged. The Cooperative Research Project ofResearch Institute of Electronics, Shizuoka Univer-sity is also acknowledged.

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118) Saeki, K., Fujimoto, Y., Koshimizu, M., Yanagida,T. and Asai, K. (2016) Comparative study ofscintillation properties of Cs2HfCl6 and Cs2ZrCl6.Appl. Phys. Express 9, 042602.

119) Sakai, T., Koshimizu, M., Fujimoto, Y., Nakauchi,D., Yanagida, T. and Asai, K. (2017) Lumines-cence and scintillation properties of Tl- andIn-doped CsCl crystals. Jpn. J. Appl. Phys. 56,062601.

120) Fujimoto, Y., Saeki, K., Yanagida, T., Koshimizu,M. and Asai, K. (2017) Luminescence andscintillation properties of TlCdCl3 crystal. Radiat.Meas. 106, 151–154. doi: 10.1016/j.radmeas.2017.03.034.

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124) Fujimoto, Y., Koshimizu, M., Yanagida, T., Okada,G., Saeki, K. and Asai, K. (2016) Thalliummagnesium chloride: a high light yield, largeeffective atomic number, non-hygroscopic crystal-line scintillator for X-ray and gamma-ray detec-tion. Jpn. J. Appl. Phys. 55, 090301.

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137) Yanagida, T., Fujimoto, Y., Koshimizu, M.,Watanabe, K., Sato, H., Yagi, H. et al. (2014)Positive hysteresis of Ce-doped GAGG scintilla-tor. Opt. Mater. 36, 2016–2019.

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(Received Aug. 30, 2017; accepted Oct. 27, 2017)

Profile

Takayuki Yanagida was born in Japan in 1978. He received a B.S. degree from theFaculty of Science in the University of Tokyo, Japan, in 2002, as well as M.S. and Ph.D.degrees in Physics from the University of Tokyo in 2004 and 2007, respectively. Heworked in Tohoku University as a Research Assistant professor from 2007 to 2010, inInstitute of Multidisciplinary Research for Advanced Materials, as Associate Professorfrom 2011 to Aug. 2012, in New Industry Creation Hatchery Center. He also worked inFrontier Research Academy for Young Researchers at Kyushu Institute of Technology,as Associate Professor from Sept. 2012 to Mar. 2015. In 2015, he joined the GraduateSchool of Materials Science, Nara Institute of Science and Technology as Professor.During his career, he received a total of 17 awards. In 2014, he received the YoungScientist Award from the Ministry of Education, Culture, Sports, Science and Technology. In 2017, he received theJSPS Prize from the Japan Society for the Promotion of Science.

He is on the Editorial Board of the Journal of Materials Science: Materials in Electronics. His researchinterests include the synthesis and evaluation of scintillation and dosimeter materials, and to develop radiationdetector devices by using these materials.


inorganic scintillating materials and scintillation detectors

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