New halide scintillators for
γ-ray detection
New halide scintillators for gamma ray detection
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op 26 november 2013 om 15:00 uur door Mikhail Sergeevich ALEKHIN
Master of Science in Physics, Lomonosov Moscow State University
Samenstelling promotiecommissie: Rector Magnificus, voorzitter
Prof. dr. P. Dorenbos Technische Universiteit Delft, promotor Prof. dr. E. H. Brück Technische Universiteit Delft
Prof. dr. A. Meijerink Universiteit Utrecht Prof. dr. C. R. Ronda Universiteit Utrecht
Prof. dr. L. D. A. Siebbeles Technische Universiteit Delft Prof. dr. C. W. E. van Eijk Technische Universiteit Delft Dr. K. W. Krämer Universität Bern, Zwitserland
Prof. dr. H. T. Wolterbeek Technische Universiteit Delft, reservelid
This research was financially supported by the Dutch Technology Foundation STW (project 07644 “The ultimate scintillator”).
Cover design: Dmitry Kileynikov
Printed by: Proefschriftmaken.nl | | Uitgeverij BOXPress
List of acronyms and abbreviations iv
Chapter 1 Introduction 1
Chapter 2 Scintillation materials 3
2.1 Scintillation mechanisms 3
2.1.1 Interaction of γ-rays with matter 3
2.1.2 Interaction of electrons with matter, thermalization and transport 4
2.1.3 Emission 5 2.1.4 Self-absorption of emission 6 2.2 Scintillator requirements 6 2.2.1 Light yield 7 2.2.2 Non-proportionality 7 2.2.3 Energy resolution 8 2.2.4 Decay time 9 2.2.5 Other requirements 9
2.3 Scintillation materials overview 10
2.4 Applications 11
2.4.1 High energy physics 11
2.4.2 Medical diagnostics 12
2.4.3 Space explorations 13
2.4.4 Scientific investigations in nuclear physics 13
2.5 Selection of the studied compounds 14
Chapter 3 Experimental Techniques 17
3.1 X-ray excited luminescence and thermoluminescence 17
3.2 VUV excitation and emission spectroscopy 18
3.3 Diffuse reflectance 19
3.4 Pulse-height measurements 19
3.4.1 Light yield 19
3.4.2 Non-proportionality of energy response 21
4.2 Crystal growth 26
4.3 Results and discussion 27
4.3.1 Proportionality improvement of LaBr3:Ce and CeBr3 27
4.3.2 Light yield and energy resolution of LaBr3:Ce,Sr 29
4.3.3 Scintillation properties of LaBr3:Ce co-doped with Li, Na, Mg,
Ca, Sr, and Ba 34
4.4 Summary and conclusion 40
Chapter 5 Optical properties and defect structure of Sr2+ co-doped
LaBr3:5%Ce scintillation crystals 43
5.1 Introduction 43 5.2 Crystal growth 44 5.3 Results 45 5.3.1 Optical properties 45 5.3.2 Time response 49 5.4 Discussion 52
5.4.1 Three different Ce3+ sites 52
5.4.2 Re-absorption of Ce3+ emission and its decay time 54
5.4.3 Point defects in LaBr3:Ce,Sr 55
5.5 Conclusion 58
Chapter 6 Scintillation properties of and self-absorption in SrI2:Eu2+ 61
6.1 Introduction 61
6.2 Crystal growth 62
6.3 Results 62
6.3.1 Excitation and emission spectra 62
6.3.2 Photoelectron yield 70
6.3.3 Decay time measurements 74
6.4 Discussion 76
6.5 Conclusion 81
Chapter 7 Self-absorption in SrI2:2%Eu2+ between 78 K and 600 K 83
7.1 Introduction 83
7.2 Experimental methods 84
7.3 Results and discussions 85
Chapter 8 Non-proportional response and energy resolution of pure
SrI2 and SrI2:5%Eu scintillators 91
8.1 Introduction 91
8.2 Experiment 92
8.3 Results 93
8.3.1 Scintillation non-proportionality to X-rays 94 8.3.2 Energy resolution of X-ray total absorption peaks 95 8.3.3 Scintillation non-proportionality to K-shell photoelectrons 98
8.4 Discussion 100
8.5 Conclusion 103
Chapter 9 Optical and scintillation properties of CsBa2I5:Eu2+ 105
9.1 Introduction 105
9.2 Crystal growth 106
9.3 Results 106
9.3.1 Excitation and emission spectra 106
9.3.2 Light yield and energy resolution 109
9.3.3 Decay time 113
9.4 Discussion 114
9.4.1 Optical properties 114
9.4.2 Light yield and energy resolution 116
9.4.3 Decay time and probability of self-absorption 117
9.5 Summary and conclusion 119
Summary 121
Samenvatting 125
Acknowledgements 129
Curriculum Vitae 131
List of acronyms and abbreviations
a Probability of Self-absorption η Quantum EfficiencyAPD Avalanche Photo Diode
Bq Becquerel
CB Conduction Band CT Computed Tomography
e Electron
FWHM Full Width at Half Maximum
h Hole
HPGe High Purity Germanium Detector LHC Large Hadron Collider
LY Light Yield
PMT phe
N
Number of photoelectrons produced in photo multiplier tube nPR Non-proportional Responseph Photon
phe Photoelectron
PMT Photo Multiplier Tube R Energy Resolution SiPM Silicon Photo Multiplier
SLYNCI Scintillator Light Yield Non-proportionality Characterization Instrument SPECT Single-Photon Emission Computed Tomography
SSL Steady-State X-ray Excited Luminescence STE Self Trapped Exciton
TE Trapped Exciton
ToF PET Time-of-Flight Positron Emission Tomography UV Ultraviolet
VUV Vacuum Ultraviolet
VB Valence Band
Introduction
A scintillator is a material that converts the energy of an ionizing particle into a flash of light. Scintillation materials are used for the detection of ionizing particles and measurement of their types and energies. The position of an interaction between an ionizing particle and a scintillator and the moment of this interaction can also be determined.
Fig. 1. Schematic of the scintillation mechanism.
A typical inorganic scintillator is an insulator with 4-12 eV gap between the top of the valence band (VB) and the bottom of the conduction band (CB), the so called forbidden gap, see Fig. 1. In the VB, all electrons (e) are bound to atoms. When excited into the CB, electrons leave holes (h) in the VB. Both electrons in the CB and holes in the VB can move freely within the crystal. The general idea of a scintillation material is to find an effective way to recombine all the excited electrons in the CB with the holes in the VB via a photon emission. For this, one requires efficient e-h transport to efficient luminescence centers.
In 1895, Wilhelm Röntgen discovered X-ray radiation by observing a faint glow from barium platinocyanide. Visual scintillation counting was introduced by Crookes and Regener in 1908 [1]. The photomultiplier was applied to scintillation counting by Curran and Baker in 1944 [1, 2]. An intensive search for scintillation materials started after the discovery of NaI:Tl by Robert Hofstadter in late 1940s [3]. Tens of compounds have been discovered and successfully applied since then.
Valence band Forbidden gap Conduction band
γ
phonon h h h h h h h h h h e e e e e e e e e e photonToday, the search for new scintillation materials is a separate scientific branch with numerous institutes and industries involved. Scintillator research is driven by the ever-increasing requirements for more advanced applications. Among them are medical diagnostics, high energy physics, space exploration, oil well logging, and different scientific investigations. There are various types of scintillation materials: inorganic single crystals, ceramic, plastic, liquid, and organic scintillators. Single crystal inorganic scintillators are the most efficient and wide-spread ones, and are the subject of this thesis.
Thesis outline: chapter 2 of this thesis reviews all the steps of scintillation
mechanisms, from interaction of γ-rays and high energy electrons with matter to emission from luminescence centers and its self-absorption. Several well-known compounds, applications, and application requirements are discussed. The choice of the studied materials is motivated. Chapter 3 explains the principles of the experimental techniques used. Chapters 4-9 are devoted to the studied compounds. In chapter 4, a new outstanding LaBr3:5%Ce,Sr scintillator is presented. Its energy resolution, γ-ray
and electron response proportionality are considerably improved as compared to standard LaBr3:5%Ce. The effects of Li+, Na+, Mg2+, Ca2+, Sr2+, and Ba2+ co-dopants
on light yield, energy resolution, proportionality, decay time, and charge carrier trap creation of LaBr3:Ce are also discussed. In chapter 5, attention is paid to the optical
properties and lattice point defects of LaBr3:Ce,Sr. Three different Ce3+ sites are
revealed and their possible origins are discussed. Chapter 6 deals with scintillation properties and self-absorption in SrI2:Eu. Temperature, sample size and Eu
concentration appear to strongly affect the emission spectrum, decay time and light yield of SrI2:Eu. A model of self-absorption is introduced to explain this behavior. In
chapter 7, the scintillation properties and probability of self-absorption in SrI2:2%Eu
are compared to those of SrI2:5%Eu. Studies of non-proportionality of pure SrI2 and
SrI2:5%Eu at temperatures between 80 K and 600 K gain attention in Chapter 8.
Chapter 9 is devoted to the optical and scintillation properties of CsBa2I5:Eu, which
appears to have similar amount of self-absorption as SrI2:Eu. Finally, the thesis is
summarized.
References:
[1] J. B. Birks, The theory and practice of scintillation counting, Macmillan, New York, 1964. [2] S. C. Curran and W. R. Baker, Rev Sci Instrum 19 (1948) 116.
Scintillation materials
2.1 Scintillation mechanisms
2.1.1 Interaction of γ-rays with matter
Among a number of possible interaction mechanisms of γ-rays with matter, only three major types are important for radiation detection and measurement: photoelectric absorption, Compton scattering, and pair production [1].
In photoelectric absorption, an incident γ-ray interacts with an atom transferring all of its energy to one of the electrons of the atom, and a so-called photoelectron is created. Mainly, this interaction occurs with the most tightly bound electron of the atom, usually a K- or L-shell electron, whose binding energy is still lower than that of the γ-ray. The photoelectron is ejected from the atom with a kinetic energy equal to the difference of the incident γ-ray energy and the binding energy to the atom. The electron vacancy is then filled with one of the outer-shell electrons followed by electronic rearrangement and characteristic X-ray emission. The probability of photoelectric absorption per atom is proportional to:
3.5 n
Z
Eγ
τ
∝ (1)where Z is the atomic number, Eγ is the γ-ray energy, and n is a parameter varying
between 4 and 5. To compare the photoelectric absorption probabilities in compounds consisting of different elements, an effective atomic number was introduced [2]:
4 4
eff i i
i
Z =
∑
w Z (2)where wi is the fraction by weight of an element i with an atomic number Z. Such a
compound can be regarded as containing only one element with an atomic number Zeff.
In Compton scattering, an incident γ-ray transfers part of its energy to a weakly bound electron of an atom. The γ-ray is scattered through an angle θ from its original direction. The transferred energy is maximal when θ= π. At smaller θ, the amount of
transferred energy decreases. The probability of Compton scattering increases with increasing Z or decreasing Eγ.
Electron-positron pair production is possible when the energy of an incident γ-ray exceeds twice the rest mass energy of an electron mec2 =1.02 MeV. γ-ray excess energy
is then shared as a kinetic energy of the produced particles which move in opposite directions. Eventually, the positron annihilates with another electron emitting two 511 keV photons. The probability of pair production per atom is:
2 2 0 2 ln E Z m c γ
ρ
∝ (3)Inorganic scintillators generally contain at least one heavy element with an atomic number Z between 40 and 80. For such an element, photoelectric absorption is dominant for γ-rays with energies below 200-500 keV, Compton scattering is dominant for γ-ray energies between 200-500 keV and 5-7 MeV, and pair production becomes dominant for γ-ray energies above 5-7 MeV [3].
2.1.2 Interaction of electrons with matter, thermalization and transport
Each of the processes described above results in the creation of energetic electron(s), so-called primary electron(s). They pass through the material losing kinetic energy by collisions with other electrons and producing ionization track(s) of electron-hole pairs. The path length of a 1 MeV electron in a moderate density compound is approximately 1 mm [1]. The linear ionization density along the track dE/dx is inversely proportional to the energy of the primary electron: -dE/dx~1/E, i. e. the ionization density increases along the track. At the end of the track, where the energy of the primary electron drops to few keV, the density is the highest.
The next step is the thermalization of free electrons and holes to the bottom of the CB and to the top of the VB respectively. The charge carriers then form defects or can be trapped by impurities followed by radiative recombination. Another possibility is a non-radiative e-h quenching. The efficiencies of these processes depend on the density of the ionization track. This relationship is not linear, but generally the non-radiative quenching rate increases and the luminous efficiency decreases at the densest parts of the track.
Various examples of defects including formation of F-centers, H-centers, Vk
-centers, trapped excitons, and various types of energy transfer processes to the luminescence centers in alkali-(earth) halides are discussed in Refs. [4, 5]. Examples of
non-radiative energy transfer between the luminescence centers can be found in Ref. [6]. Charge carriers can be trapped by impurity atoms or lattice defects containing energy levels within the forbidden gap of the scintillator. This usually results in significant delay of the scintillation process and appearance of afterglow [7].
2.1.3 Emission
Host luminescence of undoped scintillators, while usually quite efficient at low temperatures, normally quenches at room temperature. To avoid this, scintillators are often doped with specific impurities. Past years, a significant effort was put into investigation of lanthanide impurities among which Ce3+, Pr3+ and Eu2+ are most well studied. They have an efficient and relatively fast dipole allowed 4fn-15d → 4fn (5d → 4f) electronic transition, where n=1, 2, 7 for Ce3+, Pr3+ and Eu2+ respectively. For efficient luminescence, the 5d level must be within the forbidden gap of the scintillator. Moreover, the distance between 5d level and the bottom of the CB must exceed 0.5 eV to avoid temperature induced ionization of the 5d electron into the CB [8].
The energy of the 5d → 4f emission depends on three factors: covalency, crystal field splitting of the 5d levels, and the Stokes shift. In crystals, the 4f electron shells of the lanthanides are shielded from the crystal field by filled 5s and 5p shells. The energies of the 4f levels strongly depend on the type of the lanthanide, but not on the type of the host compound. The 5d shell is not shielded by other filled shells from the crystal field. The energies of the 5d levels are almost invariable with the type of the lanthanide, but strongly depend on the type of the host compound. The 5d configuration is split into maximum 5 levels due to the crystal field interaction, which depends on the size and shape of the first anion polyhedron surrounding lanthanide [9]. The 5d level is also lowered due to the nephelauxetic effect for an amount known as the centroid shift. The latter is defined as the difference between the average 4f-5d1…5
transition energy for Ce3+ in a compound and that for the free Ce3+ ion in vacuum. The information on the centroid shift of the Ce3+ 5d levels in numerous compounds is compiled in Ref. [10]. The Stokes shift is the difference between the maxima positions of the lowest energy excitation and the highest energy emission bands. The origin of the Stokes shift is the lattice relaxation following the 4f-5d excitation and emission processes.
The energies of the 4f and 5d levels of the lanthanides are easy to predict using the Dorenbos models [11-14]. For example, if one knows the positions of the 4f and 5d
levels of one lanthanide in a compound, the positions of the 4f and 5d levels of all other lanthanides in the same compound can be derived.
2.1.4 Self-absorption of emission
When the Stokes shift of a luminescence center is relatively small, its excitation and emission spectra overlap, and the emitted light can be absorbed by other luminescence centers of the same type, a process known as self-absorption. Self-absorption can strongly affect the scintillation properties of the material. High energy photons are re-absorbed, then re-emitted, and after numerous repetitions that results in a red-shift of the emission spectra. Each time the photon is re-absorbed, it requires an additional time to be re-emitted again, which eventually increases the time necessary for the photon to leave the scintillator. If quantum efficiency of the emission is not 100%, a certain amount of light is lost during each re-absorption event, which eventually affects the light yield and energy resolution of the scintillator.
The probability of self-absorption can be defined as a probability that an emitted photon is re-absorbed by another luminescence center before leaving the crystal. Several models were proposed to study the effects of self-absorption in gases, liquids, and solids. Sakai et al. [15] studied the effect of self-absorption using deconvolution by a sum of each absorption-emission process contribution. Visser et al. [16] derived the probability of self-absorption using variations in the decay time constant of Ce3+ emission. Wojtowicz et al. [17] proposed to calculate the probability of self-absorption measuring the ratio of the short-wavelength to long-wavelength Ce3+ emission. Ahn et al. [18] calculated the probability of self-absorption using variations in the quantum yield and luminescence spectrum.
2.2 Scintillator requirements
There are numerous important requirements for the ideal performance of a scintillation material:
1. High light yield
2. Proportional scintillator response 3. Good energy resolution
4. Short rise and decay time
6. Low afterglow
7. Transparency to its own light
8. Emission spectrum should match photo-detector sensitivity 9. Refractive index close to that of photo-detector window material 10. Low internal radioactivity
11. Temperature stability of the scintillation properties 12. Non-hygroscopic material
13. Good radiation hardness 14. Good mechanical ruggedness
15. Ability to grow certain sized crystals 16. Low cost of the material
None of the existing materials fit all of those requirements. Moreover, it is not necessary since the importance of each requirement depends on a particular application.
2.2.1 Light yield (requirement 1)
The light yield is defined as an amount of photons produced by a scintillator per unit of absorbed energy (ph/MeV). The maximum obtainable photon yield Yph of a
scintillator is limited by the size of its forbidden gap Eg (eV): 6 10 ph g SQ Y E β = × (4)
where S is the electron-hole transfer efficiency to the luminescence center, Q is the luminescence quantum efficiency, and β is a parameter ranging between 2 and 3 for wide band gap compounds [19]. Considering S=Q=1 the maximum obtainable light yield for 2.8-3.8 eV forbidden gap compounds is 110,000-140,000 ph/MeV [20]. Generally, the forbidden gap decreases in a sequence: fluorides → oxides → chlorides → bromides → iodides → sulfides.
2.2.2 Non-proportionality (requirement 2)
The non-proportionality at an X-ray or γ-ray energy (Ex) is defined as the ratio
between the number of photoelectrons per MeV observed at Ex and that observed at
662 keV. The non-proportionality of scintillator light yield with γ-ray or primary electron energy is related to the density of the ionization track. This density increases with decreasing primary electron energy. Therefore, the scintillator responds with
different light yield to electrons with different energies, the so-called non-proportionality of electron response. Since the total γ-ray response is a sum of primary electron(s) responses, this results in non-proportionality of the γ-ray response. The responses of all the known scintillators appear non-proportional [21-23]. There are several current approaches to model and explain the non-proportionality [24].
2.2.3 Energy resolution (requirement 3)
The energy resolution is defined as the ratio of the Full Width at Half Maximum intensity (FWHM) to the peak position of the photopeak in a pulse-height spectrum, see also section 3.4.1. The energy resolution is determined by several contributions. It can be written as
2 2 2 2 2
M nPR inh tr
R = R +R +R +R (5)
where RM is the contribution from the photomultiplier tube (PMT) gain and photon
detection Poisson statistics, RnPR is the contribution from the non-proportional response
of the scintillator, Rinh is the contribution from crystal inhomogenieties, and Rtr is the
contribution from the transfer of the scintillation photons from the crystal to the PMT. The latter two can be minimized by means of proper crystal growth and packaging. The first two impose fundamental limits on energy resolution improvement.
RM is given by 1 var( ) 2.35 M PMT phe M R N + = (6)
where var(M) is the fractional variance in the PMT gain, PMT phe
N is the amount of photoelectrons produced in the PMT. Improvement of RM is thus limited by the
PMT phe N
or the light yield of the scintillator.
RnPR originates from the stochastic nature of the ionization track(s) creation. For the
same incident γ-ray, the amount and energies of the primary electrons vary from interaction event to interaction event, varying the total γ-ray response and broadening the full absorption peak. The more proportional the scintillator response, the smaller the expected RnPR.
2.2.4 Decay time (requirement 4)
The time response of a scintillator depends both on the speed of energy transfer from the host to the luminescence centers and on the lifetime of the emitting state of the luminescence center. The core-to-valence luminescence of barium, cesium, and rubidium fluorides has the shortest decay times, which are in a sub-nanosecond range [25]. However that type of luminescence is not very efficient as compared to that of the scintillators based on Ce3+, Pr3+, or Eu2+ 5d→4f radiative transitions. The radiative lifetime τ of the 5d excited state of a lanthanide is defined by [9]:
2 2 2 3
1
n n
(
2)
f
µ
i
τ
λ
+
Γ = ∝
(7)where λ is the emission wavelength, n is the refractive index of the host material,
2
f µ i is the matrix element connecting the initial and the final states via the electric dipole operator µ. The lifetime of Ce3+ 5d excited state cannot be shorter than 15-17 ns, for Pr3+ the limit is 7-9 ns, and for Eu2+ it is 350-400 ns [20].
2.2.5 Other requirements
γ-ray detection applications require dense scintillators with high Zeff. The densest
materials contain the small F-, O2-, or N3- as an anion, and the heavy Cs+, Ba2+, La3+, Gd3+, Lu3+, Hf4+, Ta5+, W6+, Pb2+, Bi3+, Th4+, or U6+ as a cation [26]. Many scintillators have afterglow which limits their applications. No general mechanism is known to eliminate or reduce afterglow. In certain materials, co-doping with various elements might help [27, 28]. Requirements 7, 8, and 9 are among general mandatory requirements implying an efficient transport of the emitted light to the photo-detector and an efficient conversion of this light into an electric signal. To avoid internal radioactivity (requirement 10) a scintillator should not contain elements with a high concentration of naturally radioactive isotopes, like 87Rb, 40K, 176Lu. Highly radioactive Th and U are obviously not suitable for scintillation applications. The temperature behavior of the scintillation properties (requirement 11) is different in each material. It can be affected by self-absorption of the scintillation light, charge carrier trapping, electron-hole transfer efficiency to the luminescence centers, and luminescence quantum efficiency. Hygroscopicity of a scintillator can be a serious barrier for its application. It can be overcome by scintillator packaging, which however complicates its production and use. Requirements 13-16 are beyond the scope of this thesis.
2.3 Scintillation materials overview
Table I compiles main characteristics of a number of prominent and applied scintillators. A broader list of materials can be found in Ref. [29].
Table I. Overview of inorganic scintillators for X-ray and γ-ray detection and their characteristics: density (ρ), effective atomic number Zeff, light yield, energy resolution (R), principal decay (τ), and
emission wavelength (λ). Scintillator ρ (g/cm3) Zeff Light yield (ph/MeV) R (% at 662 keV) τ (ns) λ (nm) Ref. BaF2 4.9 52.7 1,500 10,000 10 0.8 600 220 310 [30, 31] CeF3 6.2 53.3 4,000 20* 30 330 [32, 33] CdWO4 7.9 64.2 20,000 6.8 ~104 500 [34, 35] PbWO4 8.3 75.6 ~200 30-40 ~10 ~450 [36, 37] Bi4Ge3O12 (BGO) 7.1 75.2 9,000 9 300 480 [35, 38]
Gd2SiO5:Ce (GSO) 6.7 59.4 8,000 7.8 60, 600 430 [36, 39]
Lu2SiO5:Ce (LSO) 7.4 66.4 26,000 7.9 40 420 [36, 40]
LuAlO3:Ce (LuAP) 8.3 64.9 12,000 15 18 365 [36, 41]
Lu2Si2O7:Ce (LPS) 6.2 64.4 26,000 9.5 38 385 [42] Lu1.8Y0.2SiO5:Ce (LYSO) 7.1 65.0 34,000 7.5 40 420 [43] Gd2O2S:Pr,Ce,F 7.3 61.1 35,000 - 4x103 510 [35] Y1.34Gd0.60O3:Eu, Pr0.06 5.9 51.6 42,000 - 106 610 [35] LaCl3:Ce 3.9 49.5 49,000 3.1 26 340 [44] LaBr3:Ce 5.3 46.9 65,000 2.7 15 360 [45] CeBr3 5.2 47.6 52,000 3.6 17 370 [46] NaI:Tl 3.7 50.8 41,000 5.6 230 415 [35, 47] CsI:Tl 4.5 54.1 56,000 5.5** 800 540 [47, 48] *at 511 keV
** measured with an APD
Invented in the late 1940s, NaI:Tl and CsI:Tl had at that time an unequalled combination of scintillation properties such as relatively high light yield, good energy
resolution, and fairly short decay time. Due to those properties and low production cost, they are still the most widely applied scintillators. For 60 years NaI:Tl remained a figure-of-merit for newly discovered materials.
BaF2 has the shortest decay time of 0.8 ns, which is due to the core-to-valence
charge transfer transition from the upper F- valence band to the upper Ba2+ valence band [25]. Unfortunately it yields only 1,500 ph/MeV.
CdWO4, PbWO4, and BGO are among the densest scintillators available with the
highest Zeff. Despite low light yield and poor energy resolution of PbWO4 and BGO, or
long decay time of CdWO4, these scintillators are applied where high density and Zeff
are among the most important requirements. Lu and Gd based oxides also found their application due to quite high density, short decay time, reasonable light yield and energy resolution.
Discovered in 2001, LaBr3:Ce possess a unique combination of very high light
yield, excellent energy resolution, and very short decay time. LaBr3:Ce is capable to
replace NaI:Tl in most of its applications. However, high production costs impose the most serious limitations for large scale LaBr3:Ce applications.
2.4 Applications
2.4.1 High energy physics (HEP)
High energy physics experiments require the densest materials available to efficiently stop and measure the energy of GeV-TeV particles. Since the amount of deposited energy in scintillator is extremely high, a low light yield of 150 ph/MeV can already be sufficient. The time response of the scintillator should be faster than the bunch collision rate. The crystals must also be grown at large volumes and at low cost and they should possess sufficient radiation hardness.
Among the commercially available scintillators, BGO and PbWO4 meet such
requirements the best. In the past, 11,400 22 cm long BGO crystals weighing more than 10 metric tons were used as scintillators in L3 detector at Large Electron–Positron Collider (LEP) in CERN [49]. Currently, Compact Muon Solenoid (CMS) detector at Large Hadron Collider (LHC) in CERN contains ~70,000 23 cm long PbWO4 crystals
weighing over 91 metric tons [50]. Besides possessing the highest density and Zeff,
PbWO4 has a decay time of 10 ns which allow its efficient operation at 25 ns
2.4.2 Medical diagnostics
A decade ago medical diagnostics required 175 metric tons of scintillation materials annually [51]. Medical diagnostics includes computed tomography (CT), single-photon emission computed tomography (SPECT), and (time-of-flight) positron emission tomography (ToF PET).
In CT, the patient is irradiated with ~150 keV X-ray flux. The X-rays passing through the patient are detected in the integration mode with a set of individual scintillators coupled to the photodiodes. The whole system rotates around the patient and a cross-sectional two-dimensional (2D) image of one slice of the patient is obtained. A three-dimensional (3D) image is then reconstructed from a series of stacked 2D-images. CT requires a scintillator with low afterglow (<1% at 3ms), high chemical, temperature, and radiation damage stability, high density (>6 g/cm3), and high light yield (>15,000 ph/MeV) [51]. The commercially accepted materials for CT are CdWO4, (Y,Gd)2O3:Eu,Pr, and Gd2O2S:Pr,Ce,F.
In SPECT, a biologically active compound with a typically 140-245 keV emitting radioactive isotope attached (radionuclide) is delivered into the body of a patient, transported, and bound to a place of interest, e.g. cancer tissue. The emitted γ-rays are detected by 2D-position sensitive detectors consisting of collimators, scintillation material, and PMTs. The whole assembly rotates around the patient, a set of planar projections is obtained, and a 3D-image of the radionuclide distribution is reconstructed. SPECT requires scintillators with high light yield, relatively high density (>3.5 g/cm3), and short decay time (<1 μs) [51]. The materials currently used for SPECT imaging are Ø60cm x 9mm plates of NaI:Tl and CsI:Tl due to their low cost, high light yield, and sufficiently short decay time.
While SPECT uses γ-emitting isotopes, PET systems use positron-emitting isotopes. The emitted positron annihilates with an electron in the patient’s tissue, two 511 keV γ-rays are emitted in opposite directions and detected by two individual scintillator detectors. The line connecting these detectors is used for the reconstruction of the position of interaction. In ToF PET, the position of interaction on this line is determined by measuring the time difference between the detection of two 511 keV γ-rays. ToF PET requires scintillators with high density and Zeff to efficiently detect 511
keV γ-rays, and high light yield in combination with short decay time to precisely determine the moment of interaction of a γ-ray with the scintillator. In the past, BGO was the most commonly used material in PET. In ToF PET, BGO was replaced by much brighter and faster LSO [52] and LYSO [53]. Other potentially suitable
scintillators for ToF PET are GSO, LuAP, and LPS [35]. LaBr3:Ce is also considered
to be a promising candidate for ToF PET [54].
2.4.3 Space explorations
Gamma-ray spectroscopy is an efficient tool to determine the elemental composition of a planet’s surface up to several tens of centimeters in depth. High density and Zeff are required for efficient detection of γ-rays. Good energy resolution is
needed to distinguish individual peaks in the γ-ray spectrum. Internal radioactivity must be sufficiently low not to obscure environmental γ-ray detection events. The importance of scintillator radiation hardness depends on the detector operation time and its proximity to the sun.
A γ-ray spectrometer with a Nal(Tl) scintillator was included in the Apollo 15 and 16 spacecrafts in 1971 and 1972 to determine the lunar-surface composition and to measure the cosmic γ-ray flux [55]. The Lunar Prospector with a Ø7.1 x 7.6 cm BGO crystal onboard was launched in 1998 [56]. The current MESSENGER mission to Mercury uses a CsI(Tl) scintillator embedded into a cup-shaped BGO crystal [57]. Use of high-purity germanium (HPGe) instead of scintillation materials is another possibility for γ-ray detection. For example, it was implemented on the γ-ray spectrometer in the Mars Odyssey spacecraft launched in 2001 [58]. A HPGe γ-ray spectrometer, however, requires a sophisticated and high power consuming cooling system. The recently discovered LaBr3:Ce scintillator appeared to be a rival for HPGe.
LaBr3:Ce excellent energy resolution and high γ-ray detection efficiency in
combination with good radiation hardness [59] qualified it for the BepiColombo mission to Mercury [60] which is scheduled to fly in 2014. A 1.2 Bq/cm3 intrinsic activity of LaBr3:Ce limits its application where high sensitivity is required. An
alternative scintillator CeBr3 with much lower intrinsic activity was recently proposed
as a replacement for LaBr3:Ce in such applications [61].
2.4.4 Scientific investigations in nuclear physics
Scintillators are also used to study the radioactive decays of their composing elements. Examples are the search for nuclear processes forbidden by the Pauli exclusion principle in 23Na and 127I using NaI:Tl [62], investigation of 2β decay of 116Cd using CdWO4 [63], and 2β decay of 84Sr using SrI2:Eu [64]. The better the
is obtained. An excellent energy resolution of LaBr3:Ce allowed one to accurately
study the shape of β continuum emitted in β-decay of 138La, which revealed a significant deviation from the standard nuclear theory [65].
2.5 Selection of the studied compounds
LaBr3:Ce has a unique combination of high light yield, excellent energy resolution
and short decay time, which makes this scintillator suitable for many applications. I. Khodyuk showed in his dissertation work [22] that the energy resolution of LaBr3:Ce is
limited by the non-proportional scintillation response as function of gamma ray energy. In order to further improve resolution one therefore should make the scintillator more proportional. The amount of non-proportionality in LaBr3:Ce was found to depend on
temperature and Ce concentration.
In this thesis work, we decided to study the effect of co-dopants on the non-proportional response of LaBr3:Ce. In parallel to this, a new line of co-dopants research
was also introduced for commercial LaBr3:Ce scintillators by Saint-Gobain company.
At first, we observed a considerable improvement of energy resolution and proportionality of LaBr3:5%Ce by Sr2+ co-doping. This motivated us to grow
LaBr3:5%Ce co-doped with various other alkali-(earth) metals Li+, Na+,Mg2+, Ca2+,
and Ba2+, to verify whether these co-dopants also improve the scintillation properties of LaBr3:5%Ce. The results of these studies are presented in chapters 4 and 5 of this
thesis.
Recently discovered SrI2:Eu [66] and CsBa2I5:Eu [67] are very promising
scintillators. Their light yield and energy resolution are comparable to those of LaBr3:Ce, and their internal radioactivity is much lower. This makes SrI2:Eu and
CsBa2I5:Eu potentially suitable for applications where high sensitivity of gamma ray
detection is required. However, information on scintillation properties of these materials is scarce, and their scintillation mechanisms are barely understood. This motivated us to study them, and the results of these studies are presented in chapters 6-9 of this thesis.
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Experimental techniques
3.1 X-ray excited luminescence and thermoluminescence
Fig. 1. Experimental setup for X-ray excited luminescence and thermoluminescence measurements.
Fig. 1 shows the setup used for X-ray excited luminescence and thermoluminescence measurements. The luminescence spectra were recorded in reflection mode using an X-ray tube with a Cu anode operated typically at 60 kV and 25 mA. The samples under study were pressed by a needle spring to the bottom of the sample holder inside a Janis VPF-800 cryostat. The emission of the samples was focused via a quartz window and a lens on the entrance slit of an ARC VM504 monochromator (blazed at 300 nm, 1200 grooves/mm), dispersed, and recorded with a Hamamatsu R943-02 PMT. Both the cryostat and the monochromator were operated under vacuum. The emission spectra were corrected for the monochromator transmission and the quantum efficiency of the PMT. X-ray excited luminescence measurements were performed
between 80 K and 600 K using a Janis VPF-800 cryostat operated with a LakeShore 331 temperature controller. Due to the high moisture sensitivity of the studied samples the cryostat was baked out to remove all water from the system prior to sample mounting. The PMT was outside the cryostat and remained always at room temperature.
For thermoluminescence measurements, ~1 mm thick crystals were pressed by a needle spring to the bottom of the sample holder inside the cryostat. The crystals were cooled down to 78 K and then irradiated with X-rays during 20 minutes leading to a steady state X-ray excited luminescence (SSL). After switching off the X-rays, the crystals were heated with a rate of 0.1 K/s using a LakeShore 331 temperature controller. The thermoluminescence emission was monitored at 380 nm for LaBr3:Ce and at 430 nm for CsBa2I5:Eu, and measured
with a Hamamatsu R943-02 PMT.
3.2 VUV excitation and emission spectroscopy
Fig. 2. Experimental setup for UV-vis-IR spectroscopy.
Fig. 2 shows the setup used for optically excited luminescence measurements. They were performed using a Hamamatsu L1835 Deuterium lamp with a 120-350 nm excitation range (1) in combination with an ARC VM502 monochromator (2) and an FL-1039 Horiba 450W Xenon lamp with a 200-900 nm excitation range (3) in combination with a Gemini-180 Horiba double-grating monochromator (4). The emission from the sample was dispersed with an Acton SP2300 Princeton Instruments monochromator (5) and detected by a Hamamatsu C9100-13 CCD Camera sensitive to 200-2000 nm photons (6). The crystals under study were pressed by a needle spring to
the bottom of the sample holder inside a Janis VPF-800 cryostat. Excitation spectra were corrected for the background counts of the CCD, the lamp spectrum, and the monochromator transmission. Emission spectra were not corrected for the transmission of the monochromator and the quantum efficiency of the CCD.
3.3 Diffuse reflectance
Diffuse reflectance spectra were recorded using a Bruker Vertex 80 V Fourier transform spectrometer. The crystals were ground into powders which were then sealed into a sample holder with a quartz window on the top. With different combinations of light sources (tungsten lamp and Globar), beamsplitters (CaF2 and KBr) and detectors
(GaP, Si, InGaAs, and HgCdTe), the diffuse reflectance was measured between 0.1 and 2 eV [1].
3.4 Pulse-height measurements
Pulse-height measurements were performed to study the light yield and energy resolution of the scintillator, and non-proportionality of the scintillator response.
3.4.1 Light yield PMT preamplifier ADC Spectroscopic Amplifier Pulse-height spectrum Alum. case scintillator
Fig. 3. Schematic of the experimental setup used for recording pulse-height spectra.
γ-ray excited pulse-height spectra at room temperature were recorded with a standard bialkali Hamamatsu R1791 photo multiplier tube (PMT) or a super bialkali Hamamatsu R6231-100 PMT connected to a Cremat CR-112
pre-amplifier and an Ortec 672 spectroscopic pre-amplifier with 0.5-10 µs shaping time, see Fig. 3. The bare crystals were mounted on the window of the PMT and covered with several 0.1 mm thick Teflon layers. Due to hygroscopic nature of the studied crystals, all pulse-height measurements were performed inside an M-Braun UNILAB dry box with a moisture level of less than 1 part per million. The light yield expressed in photoelectrons per MeV of absorbed γ-ray energy (phe/MeV) was determined without an optical coupling between the scintillator and the PMT-window. The yield was obtained from the ratio between the peak position of the 662 keV photopeak and the position of the mean value of the single photoelectron peak in pulse-height spectra [2]. Single photoelectron spectra were recorded with a Hamamatsu R1791 PMT connected to a Cremat CR-110 pre-amplifier. The absolute light yield expressed in photons per MeV (ph/MeV) was determined by correcting for the quantum efficiency and reflectivity of the PMT as outlined in [2]. The energy resolution was defined as the Full Width at Half Maximum intensity (FWHM) over the peak position of the photopeak in a pulse-height spectrum. Pulse-height measurements as a function of temperature were performed using a Janis VPF-800 Cryostat operated with a LakeShore 331 Temperature Controller [3].
Fig. 4. Pulse-height spectrum of a 137Cs source measured with a LaBr3:5%Ce,Sr scintillator.
Fig. 4 shows an example of a 137Cs pulse-height spectrum measured with a LaBr3:5%Ce,Sr scintillator [4]. (1) is a peak created due to total absorption of
the 662 keV energy, the so-called full absorption peak or photopeak. (2) is the peak due to escape of La Kα X-rays. (3) is the Compton edge of the Compton
interacts with the material via Compton effect and the scattered photon escapes the crystal. (4) is the backscatter peak. It arises when the γ-ray photon interacts with the surrounding materials via the Compton effect and then scattered into the crystal. (5) is the Ba Kα X-ray peak from the 137Cs source.
3.4.2 Non-proportionality of energy response
The non-proportional response (nPR) at an X-ray or γ-ray energy (Ex) is
defined as the ratio between the number of photoelectrons per MeV observed at Ex and that observed at 662 keV. nPR was studied with a standard set of
radioactive sources (241Am, 137Cs, 133Ba, 60Co, 22Na) plus an Amersham variable energy X-ray source. The last contains a 241Am source that emits 59.5 keV γ-rays producing characteristic Kα and Kβ X-rays from Cu, Rb, Mo, Ag,
Ba, and Tb targets.
Fig. 5. Schematic of the X-1 beam line experimental facility at HASYLAB taken from http://hasylab.desy.de.
In addition, nPR was studied at the X-1 beam line at the Hamburger Synhrotronstrahlungslabor (HASYLAB) synchrotron radiation facility. A schematic of this setup is shown in Fig. 5. A highly monochromatic pencil X-ray beam of 9 to 100 keV energy was used as an excitation source. The energy resolution of the beam varied from 1 eV at 9 keV to 20 eV at 100 keV. A tunable double Bragg reflection monochromator with Si[511] and Si[311] was used to select the energy. The size of the beam spot was set by a pair of 50x50 μm2 slits positioned in front of the sample. The
beam intensity was reduced with an ionization chamber to avoid pulse pile-ups. The sample was protected from the background irradiation with a lead shielding [5].
Both bare and encapsulated samples were used in the experiments. The bare samples were fixed at the bottom of a parabolic-like stainless steel reflector covered with Al foil. The reflector was mounted on a cold finger of a Janis VPF-800 Cryostat operated with a LakeShore 331 Temperature Controller. The reflector faced a Hamamatsu R6231-100 PMT, which was placed outside the cryostat and kept at room temperature during all experiments. The encapsulated samples were directly mounted on the window of the PMT and covered with several Teflon tape layers. The PMT was connected to a Cremat CR-112 pre-amplifier, an Ortec 672 spectroscopic amplifier with 10 µs shaping time, and an Amptek 8000A multichannel analyzer (MCA).
3.5 Scintillation time profiles
Homemade preamplifier Spectroscopy amplifier ORTEC 572 SCA Canberra 1431 Gate Generator LeCroy 222 ADC AD114 ORTEC TAC ORTEC 567 QUAD CFD ORTEC 935 QUAD CFD ORTEC 935 Dynode Anode Anode PC XP2020Q start PMT Cold finger Reflector Sample Windows Filter Sample chamber XP2020Q stop PMT Mode 1 Mode 2
Fig. 6. Schematic of the setup for scintillation rise and decay time measurements.
Scintillation time profiles were recorded by the conventional delayed coincidence single photon counting method developed by Bollinger and Thomas [6] with a setup described in Ref. [3]. A 137Cs γ-ray source was used for the excitation. The studied
crystals were fixed at the bottom of a parabolic-like stainless steel reflector covered with Al foil. Smaller than 5mm thick crystals were pressed by a needle spring to the bottom of the sample holder. Larger samples were additionally embedded into Al foil for better thermal contact. The reflector was mounted on a cold finger of a Janis VPF-800 Cryostat facing a Philips XP2020Q PMT. The PMT was placed outside the cryostat and kept at room temperature during all experiments. 15-20% of the scintillation light from the crystal was collected by the photocathode of the start PMT.
The electronics of the setup had two modes, see Fig. 6. The first one was used for fast LaBr3:Ce scintillator with 15-20 ns decay time. The anode pulse from the start
PMT passed to an Ortec 935 Constant Fraction Discriminator (CFD). If the amplitude of this pulse was high enough to pass the threshold, a ‘start’ Nuclear Instruments Pulse (NIM) pulse was generated by the CFD. A hole in the cold finger allowed a small number of photons to reach the photocathode of the stop PMT. Optical filters in front of the stop PMT were used, firstly, to reduce the amount of events when a photon reaches the stop PMT to 2%, secondly, to select a specific emission range. The signal from the stop PMT was converted into a ‘stop’ NIM pulse by another Ortec 935 CFD. The time difference between the ‘start’ and the ‘stop’ pulses were registered by an Ortec 567 Time-to-Analog Converter (TAC), digitized by an AD114 16k Analog-to-Digital Converter (ADC), and stored by a PC. The number of photons detected in a time interval (t, t+Δt), where Δt is the resolution of the TAC, was then plotted as a function of time t.
The second mode was used for the slower SrI2:Eu and CsBa2I5:Eu crystals with
0.5-8 μs decay time. The anode pulse from the start PMT is two orders of magnitude longer as compared to LaBr3:Ce. That reduces the amplitude of the scintillator response to the
level of single-electron response. To reduce the noise level and to avoid the jittering of the start pulse, an additional electronic filter that blocked all the false signals from the start PMT was used. The dynode signal from the start PMT was integrated via a home-made preamplifier and a spectroscopy amplifier ORTEC 572. If the amplitude of the integrated signal was sufficiently high, a Canberra 1431 Single Channel Analyzer (SCA) triggered LeCroy 222 Gate Generator which then allowed the ADC to digitize the signal from the TAC.
For optically excited luminescence decay time measurements, the ‘start’ PMT was replaced with a PicoQuant Sepia diode laser in combination with an HP Agilent 8116A pulse generator.
References:
[1] E. Rogers, P. Dorenbos, J. T. M. de Haas, et al., J Phys Condens Mat 24 (2012) 275502. [2] J. T. M. de Haas and P. Dorenbos, IEEE Trans Nucl Sci 55 (2008) 1086.
[3] G. Bizarri, J. T. M. de Haas, P. Dorenbos, et al., Phys Stat Sol (a) 203 (2006) R41.
[4] M. S. Alekhin, J. T. M. de Haas, I. V. Khodyuk, et al., Appl Phys Lett 102 (2013) 161915. [5] I. V. Khodyuk, P. A. Rodnyi, and P. Dorenbos, J Appl Phys 107 (2010) 113513.
Improvement of LaBr
3:5%Ce scintillation properties by
Li
+, Na
+, Mg
2+, Ca
2+, Sr
2+, and Ba
2+co-doping
The content of this chapter is based on the following publications:
1. M. S. Alekhin, J. T. M. de Haas, I. V. Khodyuk, K. W. Krämer, P. Menge, V. Ouspenski, and P. Dorenbos, “Improvement of γ-ray energy resolution of LaBr3:Ce3+
scintillation detectors by Sr2+ and Ca2+ co-doping” Appl. Phys. Lett. vol. 102, p. 161915, 2013
2. M. S. Alekhin, D. A. Biner, K. W. Krämer, and P. Dorenbos, “Improvement of LaBr3:5%Ce scintillation properties by Li+, Na+, Mg2+, Ca2+, Sr2+, and Ba2+
co-doping”, J. Appl. Phys., vol. 113, p. 224904, 2013
This chapter reports on the effects of Li+, Na+, Mg2+, Ca2+, Sr2+, and Ba2+ co-doping on the scintillation properties of LaBr3:5%Ce3+. The scintillation responses of the
co-doped LaBr3:5%Ce crystals to the 10-100 keV synchrotron monochromatic X-rays
were studied. Using the K-dip spectroscopy method, the scintillation responses to the 100 eV - 60 keV K-shell photoelectrons were derived. Proportionality of both responses considerably improves with Sr2+ and Ca2+ co-doping. In addition, pulse-height spectra of various gamma and X-ray sources with energies from 8 keV to 1.33 MeV were measured, from which the values of light yield and energy resolution were derived. Sr2+ and Ca2+ co-doped crystals showed excellent energy resolution as compared to standard LaBr3:Ce. The proportionality of the scintillation response to the
8 keV - 1.33 MeV gamma and X-rays also improves with Ca2+, Sr2+, and Ba2+ doping. This confirms the synchrotron experiment results. The effects of the co-dopants on emission spectra, decay time, and temperature stability of the light yield were studied. Multiple thermoluminescence glow peaks, decrease of the light yield at temperatures below 295 K, and additional long scintillation decay components were observed and related to charge carrier traps appearing in LaBr3:Ce3+ with Ca2+, Sr2+,
4.1 Introduction
Discovered in 2001 [1], LaBr3:Ce has a unique combination of high light yield of
70,000 photons/MeV, excellent energy resolution of 2.7% at 662 keV, decent proportional response, and short decay time of 15 ns. Owing to these properties, LaBr3:Ce meets the requirements of numerous applications [2, 3]. For example, the
high light output and the excellent time and energy resolution make LaBr3:Ce a good
detector material for time-of-flight positron emission tomography (ToF PET) [4]. A 100 ps coincidence resolving time (CRT) for 511 keV annihilation photon pairs was achieved with a LaBr3:Ce scintillator in combination with silicon photomultipliers
(SiPM) [5]. The high γ-ray detection efficiency and excellent energy resolution in combination with sufficient radiation tolerance [6] qualified LaBr3:Ce for the
ESA/JAXA BepiColombo mission to Mercury [7]. LaBr3:Ce is a mechanically robust
scintillation material with remarkable light yield at high temperatures, thus a good candidate for online logging in drill heads for oil well prospection [8]. The performance of Ø3.5”x6” LaBr3:Ce detectors is very satisfactory for high energy
γ-ray measurements [9]. LaBr3:Ce has also a potential for the use in scintillation cameras
for X-ray and γ-ray imaging [10].
For the majority of the mentioned applications the energy resolution is a very important parameter, and its further improvement will be very beneficial. In this chapter we will show that by Sr co-doping energy resolution of LaBr3:5%Ce improves
to a record low value of 2% at 662 keV. This improvement is ascribed to a more proportional response of LaBr3:5%Ce,Sr as compared to standard LaBr3:5%Ce. This
chapter also concerns the evaluation of the light yield, energy resolution, non-proportionality, and other scintillation properties of LaBr3:Ce with 6 different
co-dopants: Li+, Na+, Mg2+, Ca2+, Sr2+, and Ba2+. Special attention is paid to an improved energy resolution of Sr co-doped LaBr3:Ce over the 8 keV – 1.33 MeV energy range.
4.2 Crystal growth
LaBr3:5%Ce crystals were independently grown at two institutes. Those from
Saint-Gobain Crystals are referred to as Group A crystals. They were grown using proper source of raw materials of LaBr3, 5 mol% CeBr3, and 0.35-0.75 mol% LiBr or SrBr2
starting materials with a propriety method also used for the commercially available BriLanCe380 standard LaBr3:5%Ce scintillators. 100-200 atomic ppm of Sr were
detected in Sr co-doped LaBr3:Ce crystalline matrix by Inductively Coupled Plasma
(ICP) analysis.
The other crystals, referred to as Group B crystals, were grown at University of Bern. Starting from LaBr3, 5 mol% CeBr3, and 0.5 mol% NaBr, MgBr2, CaBr2, SrBr2,
or BaBr2 crystals were grown by the vertical Bridgman technique from the melt.
Irregularly shaped pieces with sizes ranging from 10 to 100 mm3 were cleaved from the original crystal boules for further studies.
Both groups produced the co-doped crystals together with standard LaBr3:5%Ce for
reference. In this chapter we refer to standard LaBr3:5%Ce as LaBr3:Ce and to
LaBr3:5%Ce co-doped with a further element X as LaBr3:Ce,X. No visible differences
in crystal quality were observed between the standard and the co-doped samples.
4.3 Results and discussion
The first part of this section deals with non-proportionality of Sr co-doped LaBr3:5%Ce and CeBr3. The second part is devoted to the light yield and energy
resolution of Group A LaBr3:Ce,Sr samples, which showed the best performance
among the studied crystals. The third part deals with the effects of all the studied co-dopants on the scintillation properties of LaBr3:Ce.
4.3.1 Proportionality improvement of LaBr3:Ce and CeBr3
Fig. 1. The X-ray response curves for LaBr3:Ce, LaBr3:Ce,Sr, CeBr3, and CeBr3:Sr normalized to
Fig. 1 shows the scintillator response of LaBr3:Ce, LaBr3:Ce,Sr, CeBr3, and
CeBr3:Sr to the 10-100 keV monochromatic synchrotron X-rays. Proportionality of Sr
co-doped LaBr3:Ce significantly improves as compared to a standard commercial
LaBr3:Ce scintillator. The response of LaBr3:Ce,Sr is much closer to the ideal one.
CeBr3 proportionality is poorer than that of standard LaBr3:Ce. Like in LaBr3:Ce,
co-doping with Sr considerably improves the response curve of CeBr3. It is closer to the
ideal response as compared to standard LaBr3:Ce, but not as close as that of
LaBr3:Ce,Sr.
Fig. 2. The K-shell photoelectron response curves for standard LaBr3:5%Ce, and Sr 2+
and Ca2+ co-doped samples normalized to 100% at 662 keV.
Fig. 1 shows a strong drop in the response curves when passing the K-shell and L-shell electron binding energies of the atoms in the scintillator material. This drop is caused by a re-distribution of the available X-ray energy over a set of secondary electrons. A better representation of the scintillator response is obtained by measuring the response as function of electron energy instead of X-ray or γ-ray energy. To obtain such a response down to a very low electron energy of 100 eV, the so-called K-dip spectroscopy method was used [11]. By tuning the X-ray energy just above the K-shell electron binding energy EK of the heaviest element in the scintillator, a K-shell
photo-electron will be ejected. The energy of this photo-electron can be controlled with an accuracy of 10 eV by tuning the X-ray energy. One thus creates an internal electron source tunable from 100 eV to, say, 60 keV. The excess scintillation light produced by that
photoelectron can be derived from the data. With this method, scintillation response curves as a function of K-shell photoelectron energy were obtained as shown in Fig. 2. It reveals that Ca2+ and Sr2+ co-doping also improve the K-shell photoelectron response curve of standard LaBr3:Ce. For example, the point of 10% recombination loss shifts
from 4.1 keV in the standard LaBr3:5%Ce scintillator towards 1.3 keV in the Ca2+ and
Sr2+ co-doped scintillators. Since energy loss -dE/dx of a track creating electron increases with 1/E [12], the ionization density at 1.3 keV is three times higher than at 4.1 keV. Apparently, the scintillator has become a factor of three more tolerant towards high ionization density recombination losses by the co-doping with either Ca2+ or Sr2+.
4.3.2 Light yield and energy resolution of LaBr3:Ce,Sr
Table I. Light yield of Group A LaBr3:Ce and LaBr3:Ce,Sr derived from 137
Cs pulse height spectra measured with various shaping times.
Crystal
Light yield at 662 keV (103 ph/MeV)
0.5μs 1μs 3μs 10μs LaBr3:Ce,Sr 68 71 75 78
LaBr3:Ce 72 74 75 76
The light yield of Group A LaBr3:Ce,Sr measured at 662 keV with 10 μs amplifier
shaping time is 78,000 ph/MeV. It is slightly higher than 76,000 ph/MeV of the standard LaBr3:Ce. However, measured with 0.5-3 μs shaping times, the yield of
LaBr3:Ce,Sr is lower as compared to LaBr3:Ce, see Table I. This is due to additional
slow components of the scintillation decay of Sr co-doped LaBr3:Ce. As will be shown
in section 4.3.3, those components have decay time in a range of microseconds.
The energy resolution of Sr co-doped LaBr3:Ce is considerably better as compared
to the standard LaBr3:Ce. Fig. 3 shows that the 662 keV full absorption peak improves
from 2.45% to 2.04%. Note that this record low value was obtained during 24 hours acquisition with ~200,000 counts under the photopeak and 1 μs shaping time. Interestingly, varying the shaping time between 0.5 and 10 μs does not change the energy resolution of LaBr3:Ce,Sr at 662 keV by more than 0.05%.
Fig. 3. Pulse-height spectrum of a 137Cs source measured with a Group A 3x3x1 mm3 LaBr3:Ce,Sr
crystal and a super bialkali R6231-100 PMT. The inset compares LaBr3:Ce and LaBr3:Ce,Sr 662 keV
photo-peaks on an enlarged scale. The peaks are normalized so that the integral numbers of counts below them are equal.
Fig. 4. Pulse-height spectra of a 60Co source measured with Group A LaBr3:Ce and LaBr3:Ce,Sr
crystals and a R6231-100 PMT. The spectra are normalized so that the integral numbers of counts below the 1.33 MeV photo-peaks are equal.
60
Co pulse-height spectra measured with LaBr3:Ce and LaBr3:Ce,Sr are compared
in Fig. 4. The 1.17 MeV (1) and 1.33 MeV (2) full absorption peaks are better resolved in the spectrum of the Sr co-doped sample. One can also clearly distinguish the La-Kα escape peak from the 1.33 MeV peak in the LaBr3:Ce,Sr spectrum.
Fig. 5. Pulse-height spectra of a 241Amsource measured with Group A LaBr3:Ce and LaBr3:Ce,Sr
crystals and a R6231-100 PMT. The spectra are normalized so that the integral numbers of counts below them are equal. The energy scales of both spectra are calibrated at 59.54 keV.
Even more spectacular improvements were observed at energies below 60 keV. Fig. 5 compares the pulse height spectrum of a 241Am source measured with a standard and a Sr co-doped LaBr3:Ce scintillation crystal. Resolution at 59.5 keV improves from
9.4% to 6.5%. Also, the peaks between 10 and 30 keV are much better resolved in the Sr co-doped sample.
Fig. 6 shows the pulse-height spectra from a Tb target of the variable energy X-ray source. 44.2 keV Kα Tb X-rays (1), 50.7 keV Kβ Tb X-rays (2), and 241Am 59.5 keV
γ-rays (3) appear as separate peaks in the LaBr3:Ce,Sr spectrum, while for standard
LaBr3:Ce peaks (2) and (3) are observed only as shoulders. (4) and (5) are La Kα, and
(6) La Kβ escape peaks. They are slightly misaligned in the LaBr3:Ce,Sr spectrum as
compared to the LaBr3:Ce spectrum. This is caused by a more proportional response of
Fig. 6. Pulse-height spectra from a Tb target of the variable energy X-ray source measured with LaBr3:Ce and LaBr3:Ce,Sr crystals and a R6231-100 PMT. The spectra are normalized so that the
integral numbers of counts below them are equal. The energy scales of both spectra are calibrated at 44.2 keV.
Table II compiles the values of the energy resolution, which were determined from the full absorption peaks of pulse-height spectra measured with various radioactive sources. The peaks were fitted with single or multiple Gaussian functions depending on the number of closely-located energy lines. The only exception were the 8.04 keV Kα
and 8.91 keV Kβ Cu target lines, which were fitted with a single Gaussian function. LaBr3:Ce,Sr shows on average 25% smaller values of the energy resolution as
compared to standard LaBr3:Ce at all the studied energies between 8 keV and 1.33
MeV, see Table II. Whereas LaBr3:Ce shows worse performance compared to NaI:Tl
at energies below 100 keV [13], the co-doped LaBr3:Ce,Sr outperforms NaI:Tl.
The energy resolution is determined by several contributions. It can be written as
2 2 2 2 2
M nPR inh tr
R =R +R +R +R (1)
where RM is the contribution from the PMT gain and photon detection Poisson
statistics, RnPR is the contribution from the non-proportional response of the scintillator,
Rinh is the contribution from crystal inhomogenieties, and Rtr is the contribution from
the transfer of the scintillation photons from the crystal to the PMT.
RM is given by 1 var( ) 2.35 M PMT phe M R N + = (2)
where var(M) is the fractional variance in the PMT gain, which is 0.27 for the used PMT [14].
Table II. Energy resolution (R) derived from pulse-height spectra recorded with various γ-ray and X-ray sources with a R6231-100 PMT and 1 μs amplifier shaping time. RM and Rs are the values of the
PMT and Poisson statistics contribution and the scintillator contribution, respectively.
Source Target Energy (keV)
LaBr3:Ce LaBr3:Ce,Sr
R (%) RM (%) Rs (%) R (%) RM (%) Rs (%) Amersham variable energy X-ray source Cu 8.04 35.4 16.4 31.4 26 15.8 20.7 Rb 13.37 25.4 12.2 22.3 17.3 12.1 12.4 Mo 17.44 20.1 10.8 16.9 13.9 10.6 9 Ag 22.1 16.9 9.5 14 11.8 9.4 7.2 Ba 32.06 13.3 7.8 10.7 9.3 7.8 5.1 Tb 44.23 12.5 6.6 10.6 8.1 6.6 4.6 241 Am 59.5 9.4 5.7 7.5 6.5 5.7 3.2 133 Ba 81 7.7 4.8 6 5.7 4.9 2.8 276.4 4.2 2.6 3.3 2.9 2.7 1.2 302.9 3.7 2.5 2.8 2.9 2.5 1.4 356 3.4 2.3 2.5 2.8 2.4 1.4 383.9 3.3 2.2 2.5 2.6 2.3 1.3 22 Na 511 2.8 1.9 2.1 2.4 2 1.4 137 Cs 661.7 2.5 1.7 1.8 2 1.7 1.1 60 Co 1173 1.9 1.2 1.3 1.5 1.3 0.8 1332 1.8 1.2 1.5 1.4 1.2 0.7
Applying Eq. (2) to the number of photoelectrons
N
phePMTproduced in the PMT, wecalculated the values of RM, see Table II. Because of comparable light yield, RM
contributions are roughly equal for both crystals. The other three contributions are often called the scintillator resolution Rs = RnPR2 +Rinh2 +Rtr2 [15]. The values of Rs derived from Eq. (1) and (2) are also compiled in Table II. Rs is on average twice
smaller in LaBr3:Ce,Sr as compared to that of standard LaBr3:Ce. The reason for this
significant improvement is either smaller RnPR, Rinh, or Rtr. Each of them will be further
addressed below.
The non-proportionality contribution to the energy resolution originates from the stochastic nature of the ionization track creation. After interaction with matter, an incident γ-ray creates a high-energy electron due to the photoelectric effect, or several high-energy electrons due to multiple Compton scattering events. These primary
