Novel gamma-ray and thermal-neutron scintillators: Search for high-light-yield and fast-response materials

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Novel γ-ray and thermal-neutron

scintillators

Search for high-light-yield and fast-response materials

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Novel γ-ray and thermal-neutron

scintillators

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the section Radiation Detection and Matter of the department of Radiation, Radionuclides and Reactors, faculty of Applied Sciences,

Delft University of Technology, Mekelweg 15, 2629JB Delft

This research was financially supported by The Dutch Technology Foundation (STW)

Cover: Photograph of LaBr3: Ce3+ commercial crystals from Saint Gobain and

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Novel γ-ray and thermal-neutron

scintillators

Search for high-light-yield and fast-response materials

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft

op gezag van de Rector Magnificus prof.dr.ir.J.T.Fokkema voorzitter van het College voor Promoties,

in het openbaar te verdedigen op maandag 28 januari 2008 om 12.30 uur

door

Muhammad Danang BIROWOSUTO

doctorandus in de natuur- en sterrenkunde. geboren te Jakarta, Indonesie.

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Promotor: Prof.dr.ir. C.W.E. van Eijk Toegevoegd promotor: dr. P. Dorenbos

Samenstelling promotiecommissie:

Rector Magnificus, voorzitter

Prof.dr.ir. C.W.E. van Eijk (Technische Universiteit Delft, promotor) dr. P. Dorenbos (Technische Universiteit Delft, toegevoegd promotor) Prof. dr. A. Meijerink (Universiteit van Utrecht)

Prof. dr. P. A. Rodnyi (St. Petersburg State Polytechnical University) Prof. dr. ir. T. H. J. J. van der Hagen (Technische Universiteit Delft) Prof. dr. L. D. A. Siebbeles (Technische Universiteit Delft)

dr. H. T. Hintzen (Technische Universiteit Eindhoven) Copyright c° 2007 by M. D. Birowosuto and IOS Press

ISBN: 978-1-58603-816-8

Keywords: scintillator, γ-ray, thermal-neutron, cerium, halide, elpasolite, ternary

Published and distributed by IOS Press under the imprint Delft University Press

Publisher IOS Press Nieuwe Hemweg 6b 1013 BG Amsterdam The Netherlands Telephone: +31-20-688 3355 Fax: +31-20-687 0019 E-mail: info@iospress.nl www.iospress.nl www.dupress.nl LEGAL NOTICE

The publisher is not responsible for the use which be made of the following infor-mation

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For my parents,

An ultimate scintillator, the final frontier

These are the voyages of a science cadet of RD&M. His four-year mission: to explore new materials,

to seek out the fast-response and the high-light-yield scintillator, to boldly search where no man has searched before.

(Modified from the opening voiceover in Star Trek)

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The similitude of His light is a niche in which there is a lamp. The lamp is in a Glass, the Glass, like a glistening star, kindled from a blessed olive tree, neither of the east nor of the west, whose oil well night glows though no fire has touched it: light upon light.

God guides to His light whom He wills, and God strikes parables for human beings,

and God knows all things. (Quran 24:35)

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8 Thermal quenching of Ce3+ _emission in PrX3 (X = Cl, Br) 95 8.1 Introduction . . . 95 8.2 Crystal lattice of PrX3 (X = Cl, Br) . . . 96 8.3 Results . . . 97 8.3.1 Luminescence Characteristics . . . 97 8.3.2 Temperature dependence . . . 100 8.4 Discussion . . . 103 8.4.1 Radiative lifetimes of Ce3+ _{in PrCl} 3: Ce3+ and PrBr3: Ce3+ 104 8.4.2 Quenching mechanisms of Ce3+ _{emission . . . 105}

8.4.3 Energy level schemes in LaBr3: Ce3+ and PrBr3: Ce3+ . . 106

8.4.4 A model for the thermal luminescence quenching . . . 107

8.5 Conclusion . . . 109 9 Ce3+ _{doped Cs} 2NaREBr6 (RE=La,Y,Lu) 113 9.1 Introduction . . . 113 9.2 Materials . . . 114 9.3 Results . . . 114

9.3.2 Gamma spectroscopy . . . 116

9.3.3 Scintillation and intrinsic Ce3+ _{emission decay times . . . 116}

9.3.4 Spectroscopy . . . 118

9.3.5 Temperature Dependence of Anomalous Emission . . . 121

9.4 Discussion . . . 124 9.4.1 Host properties . . . 124 9.4.2 Ce3+ _{spectroscopy . . . 124} 9.4.3 Scintillation mechanism . . . 126 9.4.4 Anomalous Emission . . . 127 9.5 Conclusion . . . 132

10 Rb2LiYBr6: Ce3+ and other Li-based thermal-neutron scintilla-tors 135 10.1 Introduction . . . 135

10.2 Ce3+ _{activated Rb} 2LiYBr6: Ce3+ . . . 136

10.2.1 Materials . . . 136

10.2.2 Results and Discussion . . . 136 ix

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10.3.1 Thermal-neutron detection efficiencies of new scintillators . 141

10.3.2 Results and Discussion . . . 142

11 Ce3+ _{doped ternary cesium halides} ₁₄₉ 11.1 Introduction . . . 149

11.2 Sample preparation and gamma interaction properties . . . 149

11.3 Results and discussion . . . 150

11.3.2 Luminescence characteristics . . . 153

11.3.3 Scintillation light yield and energy resolution . . . 157

11.3.4 Scintillation decay curves . . . 160

12 Emission, light yield and time response of Ce3+ _{doped halides 165} 12.1 Introduction . . . 165

12.2 5d→4f Ce3+ _{emission wavelength . . . 166}

12.3 Light yield . . . 168

12.3.1 Influence of the structure, the type of lanthanide and the type of anion to the fundamental limit . . . 169

12.3.2 Influence of the structure, the type of lanthanide and the type of anion to light yield losses . . . 171

12.4 Time response . . . 178

12.4.1 Radiative lifetime of Ce3+ _{. . . 178}

12.4.2 Influence of self absorption on the scintillation decay curve 179 12.4.3 Influence of the anion on the scintillation decay curve . . . 182

12.5 Discussion and Conclusion . . . 184

Summary and Conclusion 189

Samenvatting en Conclusie 195

Acknowledgments 201

Curriculum Vitae 203

List of Publications 205

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Introduction

In 1895, R¨ontgen investigated light emission generated by electrical current in a highly-evacuated glass tube. He discovered the glow of a barium platino-cyanide screen, placed next to his operating tube. R¨ontgen speculated that a new kind of rays originating from the cathode-ray tube might be responsible. Later, he termed the new rays as X-rays which is the mathematical designation for something unknown [1].

Not so long after this discovery, Becquerel found that uranium-covered plates inside his drawer were able to blacken photographic plates wrapped by black paper and aluminium foil. This meant that uranium emits radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity, the spontaneous emission of radiation by a material [2]. Later, Becquerel demon-strated that the radiation emitted by uranium shared certain characteristics with X-rays but, unlike X-rays, part of this radiation could be deflected by a magnetic field and therefore must consist of charged particles. Later, Rutherford proved that the new rays discovered by Becquerel were actually composed out of three types of radiation; alpha, beta and γ-rays. These types of radiation are different in their ”hardness” or ability to penetrate material. Alpha particles penetrate only a small thickness of material or a few centimeters of air. Beta particles were found to penetrate a hundred times thicker layer than alpha particles. γ-rays penetrate even several feet of concrete. Beside these types of radiation, many others have been discovered since. One of them is the neutron discovered by Chadwick in 1932.

Along with the discovery of the types of ionizing radiation, radiation detec-tion is also becoming a growing field of research. One of the main factors of radiation detection is the fundamental mechanism of interaction of the radiation and the detector. For X-rays and γ-rays, there is a large number of possible interaction mechanisms. Of those three play an important role in radiation mea-surement: photoelectric absorption, Compton scattering and pair production. These processes, which have a well-defined probability lead to the partial or com-plete transfer of the energy of a γ-ray photon to electrons. They result in abrupt changes in the γ-ray photon history, i. e. the photon either disappears entirely or

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is scattered through a significant angle. The high energy primary electrons cre-ated through these processes loose their energy by collisions with other electrons in the materials. These collisions generate secondary electrons and holes. The avalanche of secondaries continues until the electrons are not able to further ion-ize. The excess energy of these electrons and holes is then lost by thermalization. For the detection of the thermalized electrons and holes, there are two meth-ods. In the first method, the charge carriers are separated by an electric field and collected at electrodes. The transported charge is a measure of the energy of the absorbed radiation. This method is mainly applied in gas and semicon-ductor based detectors. These detectors are most suitable for X-ray, low-energy

γ-ray and charged particle detection. For high energy γ-rays, another detection

principle is introduced, explained as follows. The electrons and holes, created after radiation absorption, recombine or transfer their energy to a luminescence center. The emitted light is then detected by a photomultiplier tube (PMT), a photodiode or any other photosensor. This photosensor converts the light into an electric pulse. The charge of the electric signal is a measure of the absorbed energy. The material emitting the flash of light is called scintillator. This word is derived from scintilla, which is latin for spark.

Scintillators have been around since the discovery of ionizing radiation. As mentioned in the first paragraph, the scintillator was actually introduced by R¨ontgen when he detected the X-ray radiation. However, this scintillator ap-peared to be rather inefficient for X-ray registration. Pupin then introduced calcium tungstate powder, which was used coupled together with a photographic film [3]. This phosphor material has been employed for visual counting of X-rays for more than 75 years.

Figure 1: A scintillation detector with the main constituent parts.

The construction of a scintillation detector was made possible by the devel-opment of the PMT as shown on the right in Fig. 1. It resulted in the discovery

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Introduction 3

of activated and pure alkali halide scintillators. NaI: Tl and CsI: Tl, introduced by Hofstadter five decades ago, provide efficient γ-ray detection [4]. These crys-tals show a large scintillation light output. Many other efficient scintillators have been discovered since. Today, these materials play a prominent role in detec-tion devices in many fields of fundamental research, such as nuclear and high energy physics. Their properties have also lead to applications in a large variety of domains, including medical imaging, geophysical exploration and homeland security.

There are huge demands of scintillators in medical imaging. The required properties of these scintillators vary widely. Positron Emission Tomography (PET) requires high density and atomic number, fast decay and high light output, whereas the most important parameters for X-ray Computed Tomography (CT) scintillators are efficiency and low afterglow. Therefore, there is an ongoing search for new scintillator materials which fulfill most, if not all of these requirements.

Recently, LaBr3: Ce3+ was discovered and it shows the highest light yield of

70,000 photons/MeV and the best energy resolution of 2.9% ever reported for an inorganic scintillator [5]. The performance of this crystal in a detector module for Time-of-Flight (TOF) PET proved to be excellent [6]. Despite this discovery, there is still room for improvement of scintillator materials, whereby high density and atomic number, fast response and high light yield are among the criteria. The aim of higher-light-output scintillators may possibly be achieved by smaller band gap materials [7]. In theory, they provide more ionization per unit energy and potentially more scintillation photons can be emitted.

The investigations in this thesis are therefore subjected to the scintillation and the luminescence properties of some small-band-gap materials particularly bromide and iodide compounds. Some compounds were found to have high light yields whereas others were not. Additionally, knowledge about the scintillation mechanisms and the level positions of lanthanide in the band gap of the host material is necessary. It may help to understand why some materials show a low light yield and it may give a better predictability of the new scintillator material properties.

This thesis is summarized as follows. The scintillation process, which includes the formation of electrons and holes and the scintillation mechanism is treated theoretically in Chapter 1. Chapter 2 reviews the scintillator requirements for several applications and the properties of the scintillation materials known today. The second section of Chapter 2 is devoted to the aspects of the improvement of the scintillation properties. The choice of the studied compounds is also motivated in this section. Chapter 3 discusses the experimental methods that were used for investigations. A few remarks are made on data analysis.

Scintillation properties of LuI3: Ce3+, the highest light yield scintillator ever

reported, are presented in Chapter 4. LuI3: Ce3+ also shows an excellent energy

resolution. For application, this crystal is better than LaBr3: Ce3+ with respect

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the quenching mechanism of LuI3: Ce3+ are discussed in Chapter 5. Chapter 6

deals with the scintillation and the luminescence properties of GdI3: Ce3+. The

structure of the material is closely related to that of LuI3: Ce3+. However, the

scintillation properties are less attractive. Chapter 7 is devoted to the scintillation properties of Ce3+ _{doped PrCl}

3: Ce3+ and PrBr3: Ce3+. PrBr3: Ce3+ has a

faster response than LaBr3: Ce3+ and therefore it is important for fast counting

application. Nevertheless, the light yield of PrBr3: Ce3+ is lower compared to

that of LaBr3: Ce3+. Quenching of Ce3+ emission due to metal-to-metal charge

transfer of Ce3+_{and Pr}3+_{is then proposed in Chapter 8. Chapter 9, 10 and 11 deal}

with the scintillation and luminescence properties of Ce3+ _{doped complex}

rare-earth bromides and iodides. Anomalous Ce3+ _{emission in elpasolites is presented}

in Chapter 9. Rb2LiYBr6: Ce3+ as a new thermal neutron scintillator is discussed

in Chapter 10. Cesium rare-earth bromides and iodides doped with Ce3+ _are

presented in Chapter 11. In Chapter 12, an elaborate study is performed of the results obtained on all investigated halide scintillators. Finally, concluding remarks are presented.

Bibliography

[1] W. C. R¨ontgen Sitz. Ber. Phys. Med. Ges. Wuerzb. 9 (1895) 132; W. C. R¨ontgen Science 3 (1896) 227

[2] H. Becquerel M´emoires de l’Acad´emie des Sciences 46 (1903) 1 [3] T. A. Edison Nature 53 (1896) 470

[4] R. Hofstadter Phys. Rev. 74 (1948) 100; R. Hofstadter Nucleonics 6 (1950) 79

[5] E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. W. Kr¨amer and H. U. G¨udel Appl. Phys. Lett. 79 (2001) 1573

[6] A. Kuhn, S. Surti, J. S. Karp, G. Muehllehner, F. M. Newcomer and R. VanBerg IEEE Trans. Nucl. Sci. 53 (2006) 1090

[7] K. W. Kr¨amer, P. Dorenbos, H. U. G¨udel and C. W. E. van Eijk Journal of Materials Chemistry 16 (2006) 2773

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Chapter

1

Scintillation

The detection of ionizing radiation by the scintillation light produced in certain materials was inextricably bound up with the discovery of X-rays and other types of ionizing radiation. Although this detection is the oldest technique on record, the scintillation process remains one of the most useful methods available for the detection and spectroscopy of a wide assortment of radiations. Therefore the research and the development of inorganic scintillators are of interest.

One of the main goals of the scintillator research is to disclose the exact way a scintillator converts radiation into light. If this were perfectly understood, it would not be any problem to predict which materials would be perfect scintilla-tors. In this chapter, a general introduction to scintillation is given. The process that leads to the formation of electrons and holes and, in particular, the scintil-lation mechanism are discussed.

1.1 Scintillation process

The scintillation process starts when an inorganic crystal is subjected to radiation. This leads to changes in both the intrinsic lattice ions as well as in the extrinsic impurities. For the interaction of a charged particle with matter, the charged particle transfers its energy to electrons in the lattice by means of the Coulomb force resulting in the decrease of the particle velocity. This process continues until the particle has lost its energy, i. e. it stops. For the interaction of electromagnetic radiation with matter, a quantum is completely or partially absorbed. In both cases, an energetic electron is produced. This electron will further ionize and excite atoms. Secondary electron and holes will result which in turn will form more energetic electrons and holes. Eventually, they will transfer their energy to luminescent centers.

The scintillation process can be divided into three consecutive stages [1, 2, 3]. These stages, shown in Fig. 1.1, are

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(a) the conversion process in which the energy of the incoming radiation or par-ticles is converted into a large number of electron-hole pairs. This process consists of the interaction of radiation with matter, multiplication, relax-ation and thermalizrelax-ation of the resulting electrons and holes,

(b) the formation of defects and the transfer process in which the energy of an electron-hole pair is transferred to the luminescent ion involved,

(c) the emission process in which the luminescent ion returns radiatively from an excited state to the ground state.

Figure 1.1: A sketch of the scintillator process in a wide band-gap single crystal. The process is divided into three consecutive stages of (a) conversion, (b) transport and (c) luminescence, which are described in the text.

There are various theoretical models to describe the conversion process [4, 5]. These models were also used to predict the efficiency and the energy loss in inorganic scintillators. Some models of the mechanisms by which the energy is transferred by the electrons and holes to the luminescent center, were presented by Dorenbos [6]. They concern the prompt energy transfer of electrons and holes from the ionization track to the luminescent centers and competing mechanisms involving the role of mobile defects. These models provide deeper insight than the old models based on the assumption of energy transfer dependence on energy carrier density and electron stopping power [7, 8]. For the last stage in the scintillation process, the radiative decay rate (or the lifetime) is the characteristic of the emission process [9]. Different models were developed to formulate the radiative decay rate [10, 11].

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1.2. Conversion process 7

1.2 Conversion process

1.2.1 Interaction of radiation with matter

The initial step in the conversion process is the absorption of energy upon the interaction of radiation with matter. The linear absorption coefficient µ in cm−1

is used to define this absorption according to

I I0

= e−µx (1.1)

where I0 and I are the numbers of initial photons and transmitted photons after

being absorbed by a material with a thickness of x cm, respectively.

The nature of the interaction is different for various types of radiation. Since we will deal with electromagnetic and thermal-neutron radiations, both types of interactions will be discussed.

Interaction of electromagnetic radiation

Although a large number of possible interaction mechanisms is known for electro-magnetic radiation in matter, only three types play an important role at relatively low energies (< 10 MeV) [12]. These are photoelectric absorption, Compton scat-tering and pair production. All these processes lead to the partial or complete absorption of the radiation quantum.

In the photoelectric absorption process, the full photon energy is transferred to a bound electron of the atoms and this electron, which is mostly a K- or L-shell electron, is ejected. For a photon with an energy E, the cross section κ per atom having atomic number Z is proportional to:

κ ∼ Zn

E3.5 (1.2)

where n varies between 4 and 5. Eq. 1.2 shows that absorption of photons is most efficient for high-Z materials and low-energy photons. The absorption probability increases as the photon energy decreases until the energy is not sufficient to excite

K-shell electrons. At that point, a discontinuity in the curve of the interaction

probabilities versus energy, an absorption edge, is observed. This also happens at lower energy for L- and M-shell electrons.

Compton scattering is a dominant mechanism for photon energies larger than approximately 100 keV. In Compton scattering, the incoming photon scatters off a weakly bound electron that is initially at rest. The electron gains energy and the scattered photon has an energy less than that of the incoming photon. The cross section per atom for Compton scattering is proportional to

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If the photon energy exceeds twice the rest-mass energy of an electron of 1.02 MeV, the process of pair production is possible. The probability of this interaction is very low until the photon energy approaches several MeV and therefore pair production is predominantly confined to high-energy photons. The cross section per atom, τ , is proportional to

τ ∼ Z2_ln 2E

m0c2

(1.4) where m0c2 is the rest-mass energy of an electron of 0.511 MeV.

The relation between the linear γ-absorption coefficient µγ and the cross

sec-tion per atom σ is given by

µγ = ρ

NA

A σ ≈ ρ NA

2Zσ (1.5)

where ρ is the scintillator density, NA the Avogadro’s number and A the atomic

mass. The last term in Eq. 1.5 is an approximation using A ∼ 2Z for the band of stability. The linear γ-absorption coefficients for photoelectric absorption, Compton scattering and pair production are therefore proportional to ρZ3−4_{, ρZ}0

and ρZ, respectively. The total linear attenuation coefficient for electromagnetic radiation is determined by the sum of the linear attenuation coefficients of the three interaction mechanisms mentioned above.

Considering the linear attenuation coefficients of all processes, a high ρ and high Z are important for efficient γ-ray detection. Of course we should realize that the importance of the different interaction mechanisms for the primary in-teraction process varies strongly with energy. Photoelectric absorption dominates at energies below 100 keV. If we consider Z ≈ 80, the linear attenuation coeffi-cient for Compton scattering equals that of photoelectric absorption at 500 keV. Compton scattering is most important at higher energies and pair production can only occur at energies above 1022 keV.

Variation in the absorption cross sections for different compounds containing different atoms is not simply described by atomic number of the ith _{element Z}

i in

the compound. Therefore, the atomic effective number Zef f has been introduced.

Zef f is defined as [13]

Zef f = 4 sX

i

wiZi4 (1.6)

where wiis the fraction by weight of element i with an atomic number Zi. Eq. 1.6

is showing the Z4 _{dependence of photoelectric absorption. For energies between}

100 keV and 1 MeV, the total linear attenuation coefficient contains contribu-tions of both photoelectric absorption and Compton scattering. As the latter is generally followed by photoelectric effect, Eq. 1.6 is important for this energy region as well.

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1.2. Conversion process 9

Interaction of thermal neutrons

Like photons, neutrons are uncharged. However, unlike photons, they do not interact with the electromagnetic field 1_{. Hence neutron can travel appreciable}

distances in matter without interacting. Free charge carriers can only be created either by direct collisions with nuclei which are subsequently displaced or by utilization of nuclear reactions. In case of thermal neutrons with an energy of 0.025 eV, there is not sufficient energy available to accomplish displacement by collisions so only the second option remains. The nuclear reactions of importance in this case are those with nuclei with a high neutron capture cross section in order to limit the neutron-absorbing volume of a detector.

The radiative capture reaction or (n,γ) reaction is the most probable and plays an important part in the shielding of neutrons. This capture reaction can be useful in the indirect detection of neutrons using activation foils but it is not widely applied in active neutron detectors because the secondary product takes the form of γ-rays, which are difficult to detect. Instead, reactions such as (n,p) and (n,α) are more attractive since the reaction products are charged particles.

6_{Li and} 10_{B are materials with reaction products of heavy charged particles.}

Cross sections σn in both materials are inversely proportional with the square

root of the neutron energy √En. The linear absorption coefficient µn in cm−1 is

obtained from formula

µn = ρ

NAw

A σ (1.7)

This equation is similar with Eq. 1.5. In addition, the weight fraction of the neutron-sensitive nucleus w is introduced. Details about the thermal neutron scintillators are discussed in Chapter 2.

1.2.2 Multiplication, relaxation and thermalization

electron-hole pairs

After absorption of ionizing radiation in an ionic crystal, the energy is present in the form of high energetic primary electrons and holes in a material with normally empty conduction and valence bands, respectively. The hot electrons collide with other electron in the lattice, generating secondary electrons and holes. This avalanche of secondaries continues until the electrons and holes are not able to further ionize. The excess energy of these charge carriers is then lost by thermalization and optical phonon excitation. The time scale of this process is estimated to be in the order of 1 ps. Various models have been developed for the description of the energy dissipation process [4]. These models consider only a single mechanism without taking into account the whole variety of processes. The 1_{A neutron has a magnetic dipole. However, this is not relevant for a thermal-neutron} scintillator application

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models are simple phenomenological [14, 15], ”crazy carpentry” [16], plasmon [17] and polaron [18].

From these models, a rule of thumb was proposed. The energy needed to create an electron-hole pair after absorption is about two to three times the band gap energy for materials having a relatively small valence bandwidth compared to the energy of the forbidden band gap.

1.3 Formation of defects and transfer process

1.3.1 Formation of defects

Interaction of radiation with materials with mainly ionic bonding produces Frenkel defects [19]. This is a defect consisting of a vacancy and an interstitial which arise when an atom is plucked out of a normal lattice site and forced into an interstitial position. Consequently, electrons or holes can be trapped at a vacancy. Three classes of defects are discussed.

Figure 1.2: Configuration of self-trapped carriers in an ionic crystal. Shaded parts illustrate hole centers that are trapped on a halide ion and bounded a neighboring halide ion. Trapped electron orbits are illustrated by dotted lines.

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1.3. Formation of defects and transfer process 11

Electron center

For an electron in the conduction band of a perfect ionic crystal, it may be energetically favorable to move into a spatially localized level. This slow moving electron, interacting with lattice ions through long-range forces will permanently be surrounded by a region of lattice polarization and deformation caused by the moving electron. A quasiparticle composed of an electron plus its accompanying polarization field is called polaron. Such an entity happens to be very mobile and is generally not considered as a defect.

One or more electrons may also be bound to a negative ion vacancy, depending on the charge of the missing ion in the crystal. This electron center is called an

F -center as an abbreviation from German word Farbzentrum (color center). The F -center has a series of energy levels. It can absorb light and jump to excited

states. When it falls back, it emits energy in the form of electromagnetic waves. This process is responsible for the color of a crystal. Paramagnetic resonance absorption due to the presence of F -centers was first observed for irradiated LiF crystals [20]. Its paramagnetic nature is interesting for electron paramagnetic resonance (EPR) studies. A schematic of an F-center is shown in Fig. 1.2. Some defects, which can be regarded as F -center with an adjacent impurity or another defect, are discussed elsewhere [21].

Hole center

A hole trapped in ionic crystals is not a defect in the usual sense: there are no atoms missing, no extra atoms, no any impurities either. There is simply a hole in the valence band, which causes substantial local distortion of the host. This distortion renders the hole relatively immobile. The trapped hole moves only at higher temperatures by a diffusion like hopping from one lattice position to the next. Some theoretical attempts to understand the formation of self trapped hole in alkali halides were developed by Nettel [22] and Das et al. [23]. Jette et

al. showed quantitatively that the self-trapping of hole occurs if the energy to

localize a hole, which equals half the width of the valence band, is much less than the energy obtained from polarization and binding with neighboring ions [24].

When a self-trapped hole is localized on a halide ion X−_{, it forms an X}0

atom-like defect. After polarization and relaxation of the lattice, it binds a neighboring halide ion, forming a pseudomolecule X−

2 on the site of two X−ions in the lattice.

Such a center is called Vk-center.

Another hole center, the H -center is formed when an interstitial halide ion is bound to a lattice ion by a hole. The lattice configuration can be regarded as a singly ionized halide molecule X−

2 occupying an X− anion site. Therefore the

H -center resembles the Vk-center in a crystalline environment but the net charge

of the H -center with respect to the lattice is zero whereas the Vk-center has a

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Excitons

So far, the point defects are consisted of missing ions (vacancies), excess ions (interstitial) or the wrong kind of ions (substitutional impurities). Another class of defects, that differs from its colleagues, is formed by excitons. Excitons are charge carriers of electronic excitation energy in ionic crystals and they can be regarded as a correlated electron-hole pair. When an electron delocalizes in an orbit around a Vk-center (Vk+ e−), a self-trapped exciton (STE) is formed. This

is called an on-center STE.

However, this exciton can relax further. The Vkcenter transforms into a H

-center and the electron is localized at the anion site which is now vacant, creating an F -center. This F-H pair is called an off-center STE. The STEs as (Vk+ e−)

and F-H pair are shown in Fig. 1.2.

STE luminescence is observed after recombination of a free or self-trapped electron and a self-trapped hole takes place. Two luminescence bands are ob-served. These are the σ- and π-polarized bands. The σ-luminescence bands are generally found in the ultraviolet part of the spectrum, typically 1 or 2 eV higher in energy than the π-luminescence bands. The σ-luminescence bands are observed only at liquid helium temperatures. They were found in only about half of the alkali halide whereas the π-luminescence bands were found in every alkali halide. The σ-polarized transition has a short lifetime of a few nanoseconds since it is a dipole- and spin-allowed transition from a singlet STE state of overall Σu

symmetry in the free-molecule approximation of the STE to the1_Σ

g ground state

[25]. The π-polarized transition has lifetimes ranging from 90 ns in NaI up to 5 ms in KCl [26]. It is a transition from a pure triplet3_{Π excited state to the singlet} 1_Σ

g ground state of the STE. This strictly forbidden transition in alkali halides

is relaxed by the halogen spin-orbit interaction of the Vk core of the STE [27].

Kabler and Patterson also found a basic trend of shorter π-polarized transition lifetimes in crystals with heavier halogen constituents, thereby confirming the role of spin-orbit interaction in determining the lifetimes of π-luminescence [26].

1.3.2 Energy transport

There are many mechanisms of scintillation light loss. Free electrons and holes may recombine non-radiatively already inside the ionization track or defect sites can trap them.

The scope of this thesis is Ce3+ _{activated halide scintillators and we therefore}

discuss the scintillation mechanisms in Ce3+ _{doped halide scintillators. This was}

previously discussed by Dorenbos [28]. Three different energy transfer processes are defined.

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Prompt transfer of free electron and hole

This type of transfer is the preferred scintillation mechanism. Defects have no role in this transfer. After the thermalization process, a free hole and electron in the valence band and conduction band, respectively, are sequentially captured within 1 ns by Ce3+ _{ion leading to 4f→5d excitation. This mechanism is followed}

by 5d→4f emission.

Ce3+_{+ h → Ce}4+ _Ce4+_{+ e → Ce}3+∗ _Ce3+∗_{→ Ce}3+_{+ hv} _(1.8)

In principle, a Ce3+ _{center can also trap an electron. Such a case is not common}

since the Ce2+ _{center is usually unstable. This is because the energy of the Ce}3+_4f

ground state with Ce3+ _{4f lattice configuration is lower than that of the Ce}2+_4f5d

ground state with Ce2+ _{4f5d lattice configuration. Ab initio calculation of Ce}2+

energy levels in Ce3+ _{doped alkaline-earth halides was performed by Visser et al.}

They concluded that the capture of an electron by Ce2+ _{is unfavorable for these}

compounds [29].

Delayed transfer by binary electron-hole diffusion

This transfer involves the role of self-trapped holes (VK or H-center). At low

tem-peratures, the self trapped hole is immobile; its migration to Ce3+ _becomes

possi-ble at higher temperatures [30]. The migration of VK-centers has been studied by

Keller and Murray [31]. These works established that the migration consists of a series of hopping motions in which one of two halogen ions remains the common partner in the old and new VK-center. This motion can be explained

satisfacto-rily by applying the small polaron transport theory developed by Yamashita and Kurosawa [32, 33]. A VK-center jumps to overcome a potential barrier with a

thermal activation Ea.

The hopping probability of a VK-center from one lattice position to an adjacent

one, W, in the limit of high temperature T, ~ω/kT → 0, can be put in the following form W = W0exp(−Ea/kT ) (1.9) where W0 = 2π ~ |J|2 ~ω r ~ω/kT 4πS (1.10) Ea = S~ω/4 (1.11)

~ω is some average energy of the phonons involved in the hopping, J is the elec-tronic transfer energy between the two sites, k is Boltzmann’s constant and S is

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the Huang-Rhys factor, see Ref. [21]. The hopping probability W is proportional to the diffusion coefficient of the hopping motion D [31].

Monnier et al. and Ascarelly et al. calculated the activation energy for Vk

diffusion Ea for alkali halides and they showed that Ea decreases in the series

Cl→Br→I [34, 35]. Van Loef therefore proposed that the diffusion rate of the

Vk-center towards Ce3+ is likely to increase in the series Cl→Br→I [36].

When the VK-center reaches Ce3+, it can be trapped by Ce3+. The following

reaction takes place:

Ce3+_{+ V}

k → Ce4+ Ce4++ e → Ce3+∗ Ce3+∗→ Ce3++ hv (1.12)

A reaction involving the capture of an electron and then the diffusion of Vk

center to Ce2+ _{is not common. This was previously discussed for the capture of}

an electron by Ce3+ _{center in prompt transfer.}

The scintillation decay of this binary-hole electron recombination can be ex-plained using the bimolecular law if the electrons and holes are distributed ran-domly and the diffusion of electrons is random [36]. This law is commonly used in photostimulation and glow curves [37]. The bimolecular decay would be of the form [38]

I(T ) ∼ (t + t0)−2 (1.13)

where t0 is a characteristic time constant that will vary with temperature. This

time constant is determined not only by the lifetime of the 5d state of Ce3+ _but

also by the transfer speed of Vk-center to Ce3+ and electrons to Ce4+ or Ce3+- Vk.

Delayed energy transfer from STE to Ce3+

Yet another type of energy transfer involves the role of excitons. Instead of being trapped by Ce3+_{, a V}

k-center can trap an electron to form an STE. The STE

is a mobile defect and it may transfer its energy to Ce3+ _{whenever it is in the}

neighborhood of Ce3+ _{leading to delayed Ce}3+ _{emission. In some compounds,}

the STE emission is quenched at low temperature, and when the lifetime of the STE is shorter than the time needed to transfer energy, this can be an important scintillation loss factor.

Two types of energy transfers, radiative transfer and STE diffusion, are pro-posed for Ce3+ _{doped halide scintillators [6]. An overlap between STE emission}

and Ce3+ _{absorption is necessary for radiative transfer. For some compounds, the}

scintillation decay time associated with the radiative transfer is the characteristic lifetime of the π-luminescence of STE. This is due to the much longer lifetime of

π-luminescence of STE compared to the intrinsic lifetime of the 5d state of Ce3+_,

see Ref. [26].

STE diffusion was previously discussed in Ref. [36]. The STE moves through the lattice by a hopping like motion [39, 40, 41]. This is a thermally activated

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process and therefore raising the temperature increases the mobility of STE.

Ce3+_{+ ST E → Ce}3+∗ _Ce3+∗_{→ Ce}3+_{+ hv} _(1.14)

This thermally activated energy transfer can be described mathematically by the following rate equations:

dN_Ce(t,x)3+∗ dt = −ΓCe3+∗N (t,x) Ce3+∗+ Γ (T,x) t NST E(t,x) (1.15) dN_{ST E}(t,x) dt = −ΓST EN (t,x) ST E − Γ (T,x) t NST E(t,x) − Γ(T )q NST E(t,x) (1.16) where N_Ce(t,x)3+∗ and N (t,x)

ST E are the respective number of excited Ce3+ ions and

STE as function of time t and Ce3+ _{concentration x in the crystal. Γ}

Ce3+∗ and ΓST E are the radiative decay rate of Ce3+ and STE, respectively. The STE

non-radiative quenching rate as function of temperature T is defined as Γ(T )q . Γ(T,x)t

is the energy transfer rate from STE to Ce3+ _{ion as function of temperature}

and Ce3+ _{concentration. Both transfer rates are assumed to follow an Arrhenius}

behavior. Γ(T ) q = vqe −Eq kT Γ(T,x) t = vt(x)e −Et kT (1.17)

where k is the Boltzmann constant. In this formula, vq and Eq are the frequency

factor and the activation energy for thermal quenching, respectively, whereas v(x)_t and Et are corresponding terms for the energy transfer.

The STE energy transfer in the high temperature limit (kT >> Et) can be

regarded as a diffusion-limited reaction [40]. The rate of energy transfer from the STE to Ce3+ _{in an isotropic medium Γ}(T,x)

t is therefore given by [42, 43, 44]

Γ(T,x)_t ≈ v_t(x)= 4πDRx (1.18)

where D, R, and x are the diffusion constant of the STE, the radial distance from a Ce3+ _{center in which the STE transfers its energy, and the Ce}3+ _{concentration,}

respectively.

From Eq. 1.15 and 1.16, N_{ST E}(t,x) and N_Ce(t,x)3+∗ are given by

N_{ST E}(t,x) = N_{ST E}(0,x)e−(ΓST E+Γ(T,x)t +Γ (T ) q )t _(1.19) and N_Ce(t,x)3+∗ = N (0,x) Ce3+∗e−ΓCe3+∗t + ... ... Γ (T,x) t NST E(0,x) ΓST E + Γ(T,x)t + Γ(T )q − ΓCe3+∗ × (e−ΓCe3+∗t− e−(ΓST E+Γ(T,x)t +Γ (T ) q )t₎ _(1.20)

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where N_Ce(0,x)3+∗ and N (0,x)

ST E are the respective number of excited Ce3+ ions and STE

as function of Ce3+ _{concentration at time t = 0, respectively. The first term in}

Eq. 1.20 is attributed to direct sequential electron-hole capture by Ce3+ _whereas

the second term is due to the energy transfer from STE to Ce3+_.

In general at room temperature, the decay rate of Ce3+ _{is more than an order}

of magnitude higher than that of an STE. Then, if we consider that there is no non-radiative quenching (Γ(T )q = 0), Eq. 1.19 and 1.20 essentially depend on the

energy transfer rate and the radiative decay rate of STE. Under these conditions, we may observe three cases of energy transfer from STE to Ce3+_{. The first case is}

when the STE is located close to the Ce3+ _{site. The decay time is then governed}

by the lifetime of STE and the scintillation decay curve is exponential. The second case is the migration of a relatively distant STE to Ce3+ _{ion. The decay}

time is governed by the migration process and the scintillation decay curve is exponential. However, the decay time of STE is much faster than the migration time. The third case is the migration from a relatively distant STE to Ce3+ _ion

in a disordered medium. Tunneling, diffusion, or percolation through a lattice with a distribution of site-to-site tunneling probabilities or energy barriers, and with a random distribution of defects often result in power-law-like diffusion rate [45]. The decay curve then follows a power law of the form intensity [46]

I(t) ∝ t−s _(1.21)

where s is fractional exponent with 0 < s < 2.

In alkali halides, the calculated thermal activation Et for the migration of

the STE is smaller than that for the Vk-center [42]. The diffusion constant of the

STE is also larger than that of the Vk-center [40, 41]. The STE diffusion therefore

proceeds at higher rate than the Vk-center diffusion. This has been attributed to

the significant difference in the polarization and the lattice relaxation around a

Vk-center and an STE due to the additional electron of the latter. The STE is

electrically neutral; the Vk-center has a net positive charge.

1.4 Emission process

The final stage of the scintillation process is the emission of the luminescent center. After being transferred to luminescence center, the electron and hole recombine and emit quanta with a certain probability. In order to obtain a fast luminescence decay, one should consider luminescence centers with dipole allowed transitions. An example of a dipole allowed transition is the 4fn−1_5d→4fn

transition of some rare earth ions.

The rare earth Ce3+ _{ion has been intensively used as an activator for}

scintil-lators. The Ce3+ _{ion has one electron in the 4f ground state configuration. This}

electron is strongly correlated and localized and its energy is hardly affected by the crystalline environment since it is shielded by 5p and 5s electrons. Having

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1.4. Emission process 17

only one 4f electron, there are only two terms arising from this electron configura-tion: the2_F

5/2 ground term and the2F7/2 spin-orbit split excited term at roughly

2200 cm−1_{. This characteristic doublet can be seen in the 5d→4f emission spectra}

with wavelengths ranging from the near-infrared to the near-ultraviolet [47]. An electron excited into the 5d state of Ce3+ _{efficiently returns radiatively to}

the 4f ground state of Ce3+_{. The characteristic lifetime of 5d→4f Ce}3+ _transition

of 15-60 ns is fast and hence a great interest for the scintillator performance. Multiphonon-relaxation from the 5d to 4f ground state is unlikely to occur since the energy difference from the lowest 5d state and the highest 4f state of Ce3+ _is

large. However, recombination may occur nonradiatively.

One possibility is thermal quenching due to the ionization processes of the 5d electron into the conduction band. Upon delocalization, the electron non-radiatively returns to the 4f ground state of Ce3+_{. This explains why La}

2O3:

Ce3+_{, Lu}

2O3: Ce3+, and LaAlO3: Ce3+ do not show Ce3+ emission [48].

Re-cently, the absence of Ce3+ _{emission in LaI}

3: Ce3+ at room temperature is also

attributed to this thermal quenching [49]. Other possible quenching mechanisms have been discussed by Blasse et al. [50]. These include large Stokes Shift, electron transfer and F¨orster-Dexter energy transfer. The knowledge of the lumi-nescence characteristics and the positions of the Ce3+ _{5d levels inside the band}

gap gives insight about these luminescence losses.

Bibliography

[1] C. W. E. van Eijk Nucl. Tracks Radiat. Meas. 21 (1993) 5

[2] A. Lempicki, A. J. Wojtowicz and E. Berman Nucl. Instrum. Meth. in Phys. Res. A333 (1993) 304

[3] G. Blasse Chem. Mater. 6 (1994) 1465

[4] P. A. Rodnyi, P. Dorenbos and C. W. E. van Eijk Phys. Stat. Sol. B 187 (1995) 15 [5] A. N. Vasil’ev ”Elementary Processes in Scintillation and Their Interconnection in

Scin-tillation Process” Proc. Int. Conf. on Inorganic Scintillators and their Applications Sep-tember 19-23 2005 Crimea Ukraine p. 1

[6] P. Dorenbos Phys. Stat. Sol. A 202 (2005) 195

[7] R. B. Murray and A. Meyer Phys. Rev. 122 (1961) 815 [8] M. Luntz Phys. Rev. B4 (1971) 2857

[9] A. Einstein Z. Phys. 18 (1917) 121

[10] M. E. Crenshaw and C. M. Bowden Phys. Rev. Lett. 85 (2000) 1851 [11] P. R. Berman and P. W. Milonni Phys. Rev. Lett. 92 (2004) 053601 [12] R. D. Evans The Atomic Nucleus Krieger New York 1982

[13] C. W. E. van Eijk ”New Scintillators, New Light Sensors, New Applications” Proc. Int. Conf. on Inorganic Scintillators and their Applications September 22-25 1997 Shanghai China p. 3

[14] W. Shockley Sol. St. Elec. 2 (1961) 35 [15] C. A. Klein J. Appl. Phys. 39 (1968) 2029 [16] W. van Roosbroeck Phys. Rev. 139 (1965) A1702

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[17] A. Rothwarf J. Appl. Phys. 44 (1973) 752

[18] D. J. Robbins J. Electrochem. Soc. 127 (1980) 2694

[19] G. H. Kinchin and R. S. Pease Repts. Prog. Phys. 18 (1955) 1 [20] C. A. Hutchinson Jr. Phys. Rev. 75 (1949) 1769

[21] A. M. Stoneham Theory of Defects in Solids Oxford University Press New York 1975 [22] S. J. Nettel Phys. Rev. 121 (1961) 425

[23] T. P. Das, A. N. Jette and R. S. Knox Phys. Rev. 134 (1964) A1079 [24] A. N. Jette, T. L. Gilbert and T. P. Das Phys. Rev. 184 (1969) 884 [25] R. B. Murray and F. J. Keller Phys. Rev. A137 (1965) 942

[26] M. N. Kabler and D. A. Patterson Phys. Rev. Lett. 19 (1967) 652

[27] P. E. Cade, A. M. Stoneham and P. W. Tasker Phys. Rev. B30 (1984) 4621 [28] P. Dorenbos Phys. Stat. Sol. A 202 (2005) 195

[29] R Visser, J Andriessen, P Dorenbos and C W E van Eijk J. Phys.: Condens. Matter 5 (1993) 5887

[30] T. G. Castner and W. K¨anzig J. Phys. Chem. Solids 3 (1957) 178

[31] F. J. Keller and R. B. Murray Phys. Rev. Lett. 15 (1965) 198; F. J. Keller and R. B. Murray Phys. Rev. B 150 (1966) 670

[32] K. S. Song J. Phys. Chem. Solids 31 (1970) 1389

[33] J. Yamashita and T. Kurosawa J. Phys. Chem. Solids 5 (1958) 34

[34] R. Monnier, K. S. Song and A. M. Stoneham J. Phys. C.: Sol. St. Phys. 10 (1977) 4441 [35] G. Ascarelli and R. H. Stulen Phys. Rev. B 11 (1975) 4045

[36] E. V. D. van Loef Halide Scintillators Ph.D. Dissertation Delft University of Technology Delft University Press The Netherlands 2003

[37] J. T. Randall and M. H. F. Wilkins Nature 143 (1939) 978

[38] H. W. Leverenz An Introduction to Luminescence in Solids in: Structure and Matter Series Wiley London 1950 p. 270

[39] E. A. Vasil’chenko, N. E. Lushchik and Ch. B. Lushchik Sov. Phys.-Solid State 12 (1970) 167

[40] K. Tanimura and N. Itoh J. Phys. Chem. Solids 10 (1981) 901 [41] L. F. Chen and K. S. Song J. Phys.: Condens. Matter 2 (1990) 3507 [42] K. S. Song J. Phys. (Paris) C 34-9 (1973) 495

[43] T. R. Waite Phys. Rev. 107 (1957) 463 [44] R. M. Noyes Prog. React. Kinet. 1 (1961) 129

[45] D. G. Thomas, J. J. Hopfield and W. M. Augustyniak Phys. Rev. 104A (1965) 202; P. J. Dean Progr. Solid State Chem. 8 (1973) 1; M. Kastner J. Non-Cryst. Solids 35-36 (1980) 807; R. A. Street Advan. Phys. 30 (1981) 593

[46] D. J. Huntley J. Phys.: Condens. Matter 18 (2006) 1359; L. A. Dissado Chem. Phys. Lett. 124(3) (1986) 206; A. K. Jonscher and A. de Polignac J. Phys. C.: Sol. St. Phys. 17 (1984) 6493

[47] K. W. Kr¨amer, P. Dorenbos, H. U. G¨udel and C. W. E. van Eijk Journal of Materials Chemistry 16 (2006) 2773

[48] W. M. Yen, M. Raukas, S. A. Basun, W. van Schaik and U. Happek J. Lumin. 69 (1996) 287; Y. Shen, D. B. Gatch, U. R. Rod´roguez Mendoza, G.Cunningham, R. S. Meltzer, W. M. Yen and K. L. Bray Phys. Rev. B 65 (2002) 212103

[49] A. Bessiere, P. Dorenbos, C. W. E. van Eijk, K. W. Kr¨amer and H. U. G¨udel Nucl. Instrum. Meth. in Phys. Res. A537 22 (2005)

[50] G. Blasse, W. Schipper and J. J. Hamelink Inorg. Chim. Acta 189 (1991) 77; G. Blasse and N. Sabbatini Mater. Chem. Phys. 16 (1987) 237

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Chapter

2

Scintillation materials

2.1 Introduction

The search for the new scintillators, driven to a large degree by the rapidly grow-ing needs of old and new applications, has become increasgrow-ingly successful in re-cent years. The applied scintillators do not always meet the requirements of the detection systems. The main challenge of the scintillator research is to find a scintillator material that has the correct combination of properties to match the needs of a given application rather than to find a scintillator with a single out-standing characteristic [1]. This chapter reviews the direction in the search for the new scintillation materials which leads to the selection of the investigated materials. First the applications and the drivers of the scintillator development are discussed. The currently known scintillators are presented in the next sec-tion. We then formulate the fundamental limits for the properties of scintillators. Finally, the selection of the scintillation materials in this thesis is motivated.

2.2 Applications and drivers of scintillator

de-velopment

2.2.1 High energy physics

Presently, the largest high energy physics project is the Large Hadron Collider (LHC) at CERN. In this experiment, colliding proton beams produce many par-ticles, among which electrons with energies ≥ 100 GeV. Electron and photon energies from this experiment are measured in detectors, called calorimeters, in which showers of secondary particles are produced. One of the calorimeters is a homogeneous calorimeter. This calorimeter of scintillating crystals provides an excellent energy resolution over a wide energy range, with high detection efficiency [2].

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The major requirements of these scintillating crystals are high density, fast decay and good radiation hardness. The cost is also an important issue taking into consideration the very large volume of several cubic meters of such detectors.

2.2.2 Medical imaging

Inorganic scintillators are employed in most of the current medical diagnostic imaging modalities using X-rays or γ-rays, viz. X-ray computed tomography (CT) and radionuclide imaging [3, 1].

In CT, a patient is consecutively irradiated from a large number of directions by an X-ray fan beam. Attenuation profiles are registered and from them, cross-sectional images of the patient are reconstructed. In X-ray CT, X-ray energies are in the range up to ∼ 140 keV (80-140 kVp). Inorganic scintillators are there-fore the perfect detection medium. The scintillator requirements for X-ray CT are low afterglow, high stability, high density, effective atomic number, emission wavelength well matched with photodiode readout and high luminous efficiency [4].

Radionuclide medical procedures are diagnostic services that use isotopes to image organs and study their function. A biologically active compound is in-troduced into the body either by injection or inhalation. This compound then accumulates in the patient and the pattern of its subsequent radioactive emissions is used to estimate the distribution of the radioisotope and hence of the tracer compound. In single photon emission computed tomography (SPECT), the ra-dioisotopes are γ-ray emitters with emissions in the range 60-511 keV and the

γ-radiation emitted by the isotopes is used for simple projections. From

projec-tions in many direcprojec-tions, an image is reconstructed. The scintillator requirements for SPECT are high light yield, good energy resolution, high density, fast decay time and a proportional energy response [4].

In positron emission tomography (PET), the simultaneous position-sensitive detection of two collinearly emitted positron-annihilation quanta of 511 keV is used. From many events, an image is reconstructed. PET requires scintillators with high density, high effective atomic number Z, fast decay time, low cost and high light yield.

2.2.3 Neutron physics

The need for thermal neutron detectors is rapidly expanding primarily due to the increasing use of spallation neutrons for fundamental investigations of the structure of matter [5]. In April 2006, the accelerator of the Spallation Neutron Source (SNS), Oak Ridge, USA, was commissioned and the Japanese Neutron Spallation Source (JNSS), Tokai, Japan, will become operational in 2007 [6, 7]. For optimal use of these sources, the development of advanced neutron detectors is very important. Furthermore, detection of radioactive and fissile materials by

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2.3. Known scintillators and their properties 21

neutron detection is becoming increasingly important for inspection and security systems. Also for these applications, detector development is of interest.

There have been growing efforts in research and development of inorganic scin-tillators as neutron detectors. Good thermal neutron scinscin-tillators should have a high neutron detection efficiency, a low γ sensitivity or an efficient discrimination, a high light yield, and a fast response. The materials should be also transparent and easy to produce [8].

Materials containing 6_{Li are well known for the thermal neutron scintillators}

[9]. 6_{Li isotopes capture thermal neutrons and convert them into ionizing particles}

according to the reaction

6

3Li +10n →31 H + α (2.1)

The two charged particles produced in the reaction have a total kinetic energy of 4.8 MeV and scintillation light is produced along their ionization tracks. The large kinetic energy offers an advantage whenever pulse-height discrimination of neutrons against γ-background is a requirement [9].

Some of the inorganic thermal neutron scintillators show core valence (CV) luminescence, which is useful for γ-background and neutron discrimination. In these scintillators, CV luminescence is only produced by γ-ray excitation but not by neutron irradiation. Thus, the γ-ray events can be very effectively vetoed based on pulse shape. This method results in a clean neutron signal.

2.3 Known scintillators and their properties

Table 2.1 presents the main characteristics for a number of scintillators for X-ray and γ-X-ray detection, of which some have found practical application. In this table, the density, effective atomic number, light yield, energy resolution, decay time and emission wavelength are compiled.

It is apparent that each scintillator approaches the theoretical limit of prop-erties or what is generally perceived as a practical limit based on current experi-ence. CsF and BaF2 have the fastest response among all inorganic scintillators.

This led to studies of the applicability of these scintillators for Time of Flight (TOF) PET [10, 11]. However, it did not result in commercial systems. CeF3

and LuF3:Ce3+ have been considered for application in high energy physics. The

oxide scintillators is the group with the highest density and the highest effective atomic number among all scintillators. PbWO4 was chosen for the

electromag-netic calorimeter of the high energy physics Compact Muon Solenoid (CMS) experiment at LHC at CERN [19]. This crystal reduces the detector volume of the calorimeter by factor of two compared to CeF3. The oxide group is most used

for medical imaging applications. LuAlO3:Ce3+ (LuAP), Bi4Ge3O12 (BGO),

Gd2SiO5:Ce3+ (GSO) and Lu2SiO5:Ce3+ (LSO) are PET scintillators whereas

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Table 2.1: Overview of some scintillators for X-ray and γ-ray detections. The abbreviations in the heading denote the density (ρ), the effective atomic number (Zef f), the emission wavelength (λ) and the main scintillation decay time (τ ).

Scintillator ρ Zef f Light yield Resolution λ τ Ref.

(g/cm3₎ _{(photons/MeV)} _{(%, at 662keV)} _(nm) _(ns) CsF 4.6 53.2 1,900 19 390 2-4 [10] BaF2 4.9 52.7 1,430 10 175 0.8 [11] 9,500 220-300 630 CeF3 6.2 53.3 4,500 - 330 28 [12] LuF3:Ce3+ 8.3 66.2 8,000 - 310 28 [13] LuAlO3:Ce3+ 8.3 64.9 11,400 23 365 17 [14] Gd2SiO5:Ce3+ 6.7 59.4 10,000 14 430 300 [15] Lu2SiO5:Ce3+ 7.4 66.4 30,000 10 420 40 [16] Lu2Si2O7:Ce3+ 6.2 64.0 26,000 9.5 378 38 [17] CdWO4 7.9 64.2 19,700 6.5 495 104 [18] PbWO4 8.3 75.6 140 - ∼475 ∼10 [19] Bi4Ge3O12 7.1 75.2 8,200 27 505 300 [20] LaCl3:Ce3+ 3.9 49.5 49,000 3.3 330 25 [21] CeCl3 3.9 50.4 28,000 - 360 25 [22] K2LaCl5:Ce3+ 2.9 44.1 30,000 5.1 344 1,000 [23] RbGd2Cl7:Ce3+ 3.7 53.9 40,000 5.0 370 90 [24] LaBr3:Ce3+ 5.1 46.9 67,000 2.8 358 15 [25] GdBr3:Ce3+ 4.6 52.4 44,000 - 350 20 [26] K2LaBr5:Ce3+ 3.9 42.8 40,000 4.9 359 100 [27] RbGd2Br7:Ce3+ 4.8 50.6 56,000 3.8 420 43 [24] NaI:Tl+ _3.7 _50.8 _43,000 _6.7 ₄₁₅ ₂₃₀ _{[28, 29]} CsI:Tl+ _4.5 _54.0 _66,000 _6.6 ₅₆₀ _1,000 _{[28, 30]} K2LaI5:Ce3+ 4.4 52.4 57,000 4.2 401 24 [27] Gd2O2S:Pr3+,Ce3+ 7.3 61.1 40,000 - 511 3,000 [31] ZnS:Ag+ _4.1 _27.4 _73,000 _- ₄₅₀ ₁₀5 _[32] CdS:Te2+ _4.8 _48.0 _17,000 ₁₄ ₆₄₀ _270-3,000 _[33]

Of the presented scintillators, NaI:Tl+ _{and CsI:Tl}+ _{dominate a wide}

applica-tion area because of their high light yield, reasonably fast decay time and the low cost to grow these materials. However, the recently discovered LaBr3: Ce3+ is

predicted to take over this domination. Scintillation properties of LaBr3: Ce3+are

superior to those of NaI:Tl+ _{and CsI:Tl}+_{, see Table 2.1. Noteworthily, LaBr} 3:

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2.3. Known scintillators and their properties 23

This material is also considered for application in Time-of-Flight (TOF) PET [34].

In Table 2.2, we give information on commercially available (first three rows) and other recently developed thermal-neutron scintillators. A low γ sensitivity is provided by low mass density materials. From all scintillators in Table 2.2, the lowest density scintillator of 2.5 g/cm3 _{is found for} 6_{Li-glass. The highest}

neutron light yield of 160,000 photons per neutron is recorded for a mixture of

6_{LiF with ZnS:Ag}+_.

Table 2.2: Overview of some traditional and recently developed thermal-neutron scintillators. The data were taken from van Eijk et al. [8]. The abbreviations in the heading denote the mass density (ρ), the absorption length of 1.8 ˚A neutrons (1/µn), the emission wavelength (λ), the main scintillation decay time (τ ) and the ratio of the number of photons per MeV produced by the heavy charged reaction products and the number of photons per MeV produced by electrons (α/β).

Scintillator ρ 1/µn Light yield Peak neutron λ τ α/β

(g/cm3₎ _(mm) _{photons per} _Resolution _(nm) _(ns)

MeV Gamma Neutron (%)

6_Li-glass _2.5 _0.52 _∼4,000 _∼6,000 ₁₃ ₃₉₅ ₇₅ _0.31 6_LiI:Eu2+ _4.1 _0.54 _12,000 _50,000 _3.9 ₄₇₀ _1,400 _0.86 6_LiF/ZnS:Ag+ _2.6 _0.8 _75,000 _160,000 _- ₄₅₀ ₁₀4 _0.45 LiBaF3:Ce3+, K+ 5.3 - 5,000 3,500 - 330 1/34/2,100 0.14 LiBaF3:Ce3+, Rb+ 5.3 - 4,500 3,600 - 330 1/34/2,400 0.17 Cs6 2LiYCl6:Ce3+ 3.3 3.2 18,000 56,000 9 380 ∼1,000 0.66 700 255 3 (CVL) Cs6 2LiYBr6:Ce3+ 4.1 3.7 20,000 73,000 4.6 389 83/1,500 0.76

The α/β ratio presented in column 9 is the ratio of the number of photons per MeV produced by heavy charged particles from the neutron capture reac-tion and the number of photons per MeV produced by electrons. The large

α/β ratios of 0.86, 0.66 and 0.76 are shown for 6_LiI:Eu2+_{, Cs}6

2LiYCl6:Ce3+ and

Cs6

2LiYBr6:Ce3+, respectively. Two of them, 6LiI:Eu2+ and Cs62LiYBr6:Ce3+

even show the best neutron peak resolutions of 3.9 and 4.6 %, respectively. A combination of a large α/β ratio and a good neutron peak resolution offers the possibility of neutron/γ pulse-height discrimination [8].

CV luminescence was observed in LiBaF3:Ce3+, K+, LiBaF3:Ce3+, Rb+ and

Cs6

2LiYCl6:Ce3+ [35, 36]. As discussed in section 2.2.3, this luminescence

mech-anism provides the best neutron/γ discrimination by means of pulse-shape dis-crimination.

(36)

2.4 Fundamental limits and directions in the search

for the new scintillator

2.4.1 Scintillation speed

The response time of scintillation basically depends on the transfer time of free charge carriers from the ionization track to the luminescence center and the life-time of the emitting state of the activator. The transfer life-time is difficult to predict. If the transfer is really fast (less than 1 ns), the fundamental limit is determined by the decay rate Γ of the emitting state which can be written as [37]

Γ = 1 τ ∝ n λ3 em (n 2_{+ 2} 3 ) 2X f | < f |µ|i > |2 (2.2)

where n is the refractive index of the crystal and λem the emission wavelength.

f and i denote the final and initial states, respectively. The matrix element

connecting both states via the dipole operator µ is only of appreciable size when two states are of different parity. Therefore, transitions between 5d and 4f states as in Ce3+_{, Pr}3+_{, Nd}3+ _{and Eu}2+ _{are of interest. Although the response times}

are larger (see below), also transitions between p- and s- configurations as in the so called s2 _{ions Tl}+_{, Pb}2+ _{and Bi}3+ _{are important. These are luminescent ions}

encountered in scintillators since the discovery of the first scintillator.

Transitions starting from a less than half-filled configuration and ending in a more than half-filled ground-state configuration are usually spin forbidden. This is the case for Tl+_{, Pb}2+ _{and Bi}3+ _{leading to relatively long decay times for these}

activator ions. The same applies for the 5d→4f transitions in the lanthanides with more than half filled 4f shell like Er3+_{, Tm}3+ _{and Yb}3+_{. For Eu}2+_{, a slow decay}

time of 390 ns was previously recorded for BaCl2:Eu2+ [38]. The explanation of

the slow decay time of Eu2+ _{can be found in the work of Hoshina [39].}

What remains as candidate ions for fast scintillator development are Ce3+_,

Pr3+ _{and Nd}3+_{. A survey of the literature data reveals that the lifetime of the}

lowest 5d state of Ce3+ _{in inorganic compounds ranges from 17 to 60 ns [37]. The}

5d→4f emission energy of Pr3+ _{is always 1.50 eV higher, and that of Nd}3+ _2.82

eV higher, than that of Ce3+ _{when these three lanthanides are doped in the same}

compound [40]. Using Eq. 2.2, this implies that the decay time of 5d→4f emission in Pr3+ _{is around two times faster than that of Ce}3+ _{and that of Nd}3+ _{is again}

two times faster.

Despite the shorter decay times, most research activities on new scintilla-tors, nowadays, are still devoted to Ce3+ _{activated materials. The wavelength}

of Ce3+ _{5d→4f emission is very suitable for either PMT or silicon photodiode}

readout. The efficiency of luminescence in Ce3+ _{is also close to unity because,}

contrary to Pr3+ _{and Nd}3+_{, non-radiative relaxation from the 5d state to lower}

Novel gamma-ray and thermal-neutron scintillators: Search for high-light-yield and fast-response materials

Novel γ-ray and thermal-neutron

scintillators

Search for high-light-yield and fast-response materials

Novel γ-ray and thermal-neutron

scintillators

Novel γ-ray and thermal-neutron

scintillators

Search for high-light-yield and fast-response materials

Proefschrift

Muhammad Danang BIROWOSUTO

Contents

Introduction

Bibliography

Chapter

1

Scintillation

1.1

Scintillation process

1.2

Conversion process

1.2.1

Interaction of radiation with matter

1.2.2

Multiplication, relaxation and thermalization

electron-hole pairs

1.3

Formation of defects and transfer process

1.3.1

Formation of defects

1.3.2

Energy transport

1.4

Emission process

Bibliography

Chapter

2

Scintillation materials

2.1

Introduction

2.2

Applications and drivers of scintillator

de-velopment

2.2.1

High energy physics

2.2.2

Medical imaging

2.2.3

Neutron physics

2.3

Known scintillators and their properties

2.4

Fundamental limits and directions in the search

for the new scintillator

2.4.1

Scintillation speed