12	Lanthanum Halide and Cerium Bromide Scintillators Oliver J. Roberts

CONTENTS

12.1  Motivation: The Need for New Scintillators

12.2  Characteristics of Lanthanum Halide Scintillators

12.2.1  Intrinsic Activity

12.2.2  Nonproportionality and Energy Resolution

12.2.3  Timing Resolution

12.2.4  Novel Codoped Lanthanum Halide Scintillators

12.2.5  PMT Selection and Other Readout Devices

12.2.6  Proton and Neutron Activation

12.3  Characteristics of Cerium Tri-Bromide (CeBr₃) Scintillators

12.3.1  Proton Activation and Intrinsic Activity of CeBr₃ Scintillators

12.3.2  Timing Properties of CeBr₃ Scintillators

12.4  Summary

References

12.1  MOTIVATION: THE NEED FOR NEW SCINTILLATORS

In developing a new scintillator crystal, a variety of important characteristics need to be addressed, such as the following:

•  A high stopping power, related to the scintillator’s density (ρ) and atomic number (Z).

•  A high light output; that is, it should be able to readily convert the energy from incident radiation into light.

•  Minimal afterglow; that is, the scintillator should be transparent to its own emitting wavelength, ensuring good light collection.

•  Good linearity; that is, the conversion of quanta to light should be proportional and uniform throughout the crystal over a wide range of energies.

•  Fast response; that is, induced luminescence should decay quickly.

•  The material should be robust, low in cost, and available in a range of sizes that make it practical to use for various applications.

Currently, these requirements cannot be met by a single type of scintillator, and therefore interest still exists in developing a scintillator that will match all of the aforementioned criteria. In the 65 years since the discovery of NaI(Tl) for use as a scintillator material by Hofstadter [1], research and development of scintillator materials has led to the discovery of Bi₄Ge₃O₁₂ (BGO) [2], CsF [3], and BaF₂ [4]. High-efficiency bismuth-germanate or BGO crystals have been employed in fields such as high-energy physics [5] and are still commonly used as anti-Compton shields [6] around high-purity germanium detectors (HPGe) in nuclear spectroscopy. CsF and BaF₂ crystals allowed the first timing studies to be realized [7,8,9], which had previously been done only with plastic detectors.

The last two decades have seen significant progress in the development of scintillator crystals, driven largely by technological advances and a greater interest in applications. In industry and research, candidates such as lanthanum halide and CeBr₃ crystals have emerged as new inorganic crystals with attractive scintillator properties.

12.2  CHARACTERISTICS OF LANTHANUM HALIDE SCINTILLATORS

Inorganic scintillators are the most popular scintillators used in the field of nuclear spectroscopy due to their high light outputs and stopping powers. The scintillation mechanism in inorganic scintillators is due to the electron band structure in the crystals [10]. These types of scintillator crystals are principally alkali-halide crystals containing a small “activator” impurity, which plays an important role in the scintillation mechanism. The activator creates special sites in the crystal lattice, creating energy states within the forbidden gap through which an excited electron can de-excite back to the valence band. The photon generated from this de-excitation is responsible for the visible scintillation light. The de-excitation sites or luminescence centers determine the emission spectrum of the scintillator.

A nuclear or charged particle traversing through the detector medium will form a large number of electron–hole pairs as the electrons move from the valence to the conduction band. Positive holes are ionized by the activator site, which acts as an impurity trap. The electron will continue to pass through the medium until it encounters an ionized activator site, where it will create a neutral configuration with a set of excited energy states. All of these excited energy states are formed at once due to the short travel time of the electron. The time characteristics of the scintillation light emitted by the crystal are associated with the subsequent decay from these excited states [11].

In lanthanum halide crystals, the luminescence is due to the Ce³⁺ 5d→4f transition, which is shown in Figure 12.1a. “Lattice relaxation” in the crystals causes a minimization in energy, resulting in the excited states populating the band-gap region of the crystal. The difference between the energy absorbed initially (prerelaxation) and emitted (postrelaxation) is called the Stokes shift, which is on the order of 0.55 eV in LaBr₃(Ce) [12,13].

Other competing processes exist, such as when the transition from an excited state (created by an electron and impurity trap) to the ground state is forbidden. In this case, an additional amount of energy in the form of thermal excitations allows a transition to a higher-lying state, from which de-excitation is allowed, to the ground state. This results in a slow component to the resulting scintillation light called phosphorescence, a problem in scintillators as it can be a notable source of “afterglow.” Another process involves transitions between some of the excited states in the gap region, which emit no radiation as a result of an electron becoming captured by an impurity trap.

FIGURE 12.1 (a) A schematic representation of the energy levels of the 4f and 5d configurations of Ce³⁺, responsible for the luminescence in lanthanum halide scintillators. (b) A diagram showing the typical electronic band structure found in scintillators. Variations in the size of the forbidden band-gap region result in different levels of conductivity and are ultimately responsible for each material’s characteristic properties.

An electron–hole pair that travels after the excitation of an electron from the valence to the conduction band is known as an exciton. This delocalized configuration of a hole and electron moves freely within the material from the exciton band until it comes into contact with an activator site, which acts as a generator of electron levels in the forbidden energy-gap region (a schematic diagram of this process is shown in Figure 12.1b). The de-excitation from these states to the ground configuration ultimately results in the creation of scintillation light.

The inorganic halide salts LaCl₃ and LaBr₃ were first synthesized in 2001 by the Universities of Delft and Bern [14,15,16] and are now made commercially by Saint-Gobain under the product names BrilLanCe® 350 and BrilLanCe® 380, respectively [17]. The crystals are hexagonal in nature (similar in shape to the uranium chloride compound UCl₃), with a P63/m space group [18,19]. LaBr₃ has a density of 5.07 g/cm³ and a melting point of 783°C, and LaCl₃ has a density of 3.64 g/cm³ and melts at 859°C [18]. The low melting points of both compounds allow the crystals to be grown via the Bridgman and Czochralski methods [20], using ultra-pure dry forms of the compounds’ white powder.

The Bridgman crystal-growth technique requires these crystals to be sealed in quartz ampoules. Both LaBr₃ and LaCl₃ are rare-earth compounds and thus are easily contaminated by moisture and oxygen in the atmosphere. In order to prevent this, the crystal powders are loaded into an ampoule in a nitrogen-purged glove box, where the oxygen and moisture are purged [18]. The ampoule is then lowered into a vertical Bridgman furnace, in which independently controlled heater zones maintain a steady temperature gradient along the entire length of the furnace to facilitate large crystal growth. Failure to maintain an optimal, uniform temperature gradient along the furnace results in the build-up of internal stresses inside the crystal due to non-uniform thermal expansion, which is likely to cause fracturing. This is why lanthanum halide crystals are difficult to grow in large ingots [21]; the largest crystals currently available at the time of writing are Ø3.5” × 8” (89 × 203 mm²) [22]. The build-up of stress during the crystal-making process also makes the initially cylindrical ingots hard to cut, as they are likely to crack and split when cut against their natural cleaving planes. Despite this, different shapes and sizes such as cubes or tapered geometries are readily available. After the crystals are grown, they are subsequently stored under very dry environmental conditions in order to prevent contamination. More information about how these crystals are grown can be found in [18,23,24].

The scintillation properties of both LaBr₃ and LaCl₃ crystals change with different concentrations of Ce³⁺, as shown in Table 12.1. Using x-ray-excited luminescence, the variation of the emitting wavelength at various temperatures for different concentrations of Ce³⁺ doping in LaBr₃(Ce) crystals can be studied. Pure LaBr₃ crystals were found to have two maxima in their emission light of approximately 340 and 430 nm at 100 K due to self-trapping exciton luminescence [25]. This type of luminescence is responsible for similar bands in other bromides [26,27,28,29] and pure LaCl₃ crystals [28,30]. As the temperature is increased to room temperature, however, the maximum near 340 nm disappears and the other maximum at 430 nm shifts to shorter wavelengths, resulting in an x-ray-excited optical luminescence spectrum being dominated by Ce³⁺ luminescence between 325 and 425 nm. Additional luminescence in LaBr₃(Ce:10%) crystals has been reported near 275 nm and is likely to be due to a defect or impurity related to Ce³⁺ [25].

12.2.1  INTRINSIC ACTIVITY

Lanthanum halide crystals exhibit large amounts of intrinsic activity that limit the use of the crystals in low-counting experiments. These spurious lines arise due to the naturally abundant (0.09%) radioisotope ¹³⁸La and the α-decay from ²²⁷Ac into its daughter nuclides. The former manifests itself in two ways: 66.4% of the time, ¹³⁸La undergoes electron capture resulting in the population of the 2⁺ level in ¹³⁸Ba, which decays by emitting a 1436 keV γ-ray, summed with the 32 and 5 keV coincident x-rays; 33.6% of the time, ¹³⁸La β⁻ decays into ¹³⁸Ce, resulting in a β-continuum with an end point of 255 keV and a peak at 789 keV due to the population and subsequent decay of the 2⁺ level in ¹³⁸Ce. This 789 keV γ-ray line in the resulting energy spectrum is smeared to higher energies as it is in coincidence with the β-electrons. All of these features are seen clearly below ~1.7 MeV in Figure 12.2, which was acquired over a period of 12 hours.

Above ~1.7 MeV, a large number of broad peaks are present up to ~3 MeV due to the α-decay of ²²⁷Ac, which is shown in Figure 12.2. These a energies from the decay of ²²⁷Ac actually range from 4 to 8 MeV, meaning that their light is quenched by ~35% [44,45]. A list of the α energies, along with their intensities and calibrated energies as detected in a LaBr₃(Ce) scintillator, is shown in Table 12.2. Discrimination between γ and α radiation in both LaBr₃(Ce) and LaCl₃(Ce) scintillators using pulse-shape analysis techniques has been investigated and has been found to be on the order of a maximum of 10% [46].

FIGURE 12.2 (a) A γ-ray spectrum showing the intrinsic activity of a single Ø1.5” × 2” LaBr₃(Ce) detector. (b) An energy spectrum showing the intrinsic activity of a single #x00D8;1.5” × 2” LaBr₃(Ce) detector due to ²²⁷Ac contamination. A schematic diagram of the decay path from ²²⁷Ac is also shown. The data in both panels was acquired using “singles-mode” triggering over a period of 12 hours, in a lead castle.

The counting rates are approximately 1–2 and 0.1 counts/s/cm³ for contamination from ¹³⁸La and ²²⁷Ac, respectively [108]. In larger, more recently manufactured ingots, the contamination from ²²⁷Ac has been reduced significantly in comparison with older crystals due to the purification of raw materials used in the crystal growing process [45,47].

12.2.2  NONPROPORTIONALITY AND ENERGY RESOLUTION

One of the main purposes of a radiation detector is to measure its response to a monoenergetic source of radiation. The resulting distribution of the detected radiation is known as the response function of the detector at that incident energy. A detector with perfect resolution has a δ-like response function. A realistic resolution has a larger variation of incoming pulses for the same incident energy, resulting in a peak with a greater width. The resolution of a detector for a particular energy can be described as being the width of the distribution at a level that is half the maximum (FWHM) of the peak divided by the centroid of the peak (the energy of the detected radiation). One of the dominant sources for fluctuations in the response function arises from statistical noise, which occurs due to a discrete number of charge carriers generated within the detector by a quantum of radiation [48]. The number of discrete carriers fluctuates from event to event, regardless of whether the incident energy is similar, and it can be estimated by describing the formation of each carrier as a Poisson process. Assuming this, the standard deviation of the number of charge carriers, N, is √N.

The detector response function can usually be described by the Gaussian function:

$G(x)=\frac{A}{\sigma \sqrt{2\pi }}\text{exp}\left[-\frac{{(x-x_{\text{o}})}^2}{2{\sigma }^2}\right],$

(12.1)

where:

σ = width parameter

x_o = peak centroid

A = area of the function

The width parameter σ defines the FWHM of the Gaussian function through the relationship FWHM = 2.35σ, which can be used to determine the Poisson limited resolution as

$R_{\text{Poisson limit}}=\frac{2.35}{\sqrt{N}}.$

(12.2)

This limiting resolution is what gives each detector type its characteristic energy response. If the number of charge carriers (N) increases, then the resolution improves due to a decrease in the limiting resolution. For a R_{Poisson limit} of 1%, the average number of charge carriers ~55,000. This means that, in order to have the best energy resolution available (and thus the smallest limiting resolution possible), one needs to have as many charge carriers as possible. Semiconductor detectors such as CdZnTe and HPGe detectors are popular as these detectors generate a large number of charge carriers. However, discrepancies in the measured values of limiting resolutions have revealed that the total number of charge carriers cannot be completely described by Poisson statistics alone [48]. In an attempt to describe this discrepancy between the statistical fluctuation in the number of charge carriers and the number of charge carriers derived from Poisson statistics, the Fano Factor is introduced. Incorporating this factor of the observed and predicted variance in the number of charge carriers into Equation 12.2, the complete equation describing the nature of the limiting resolution of a detector is now

$R_{\text{Poisson limit}}=2.35\sqrt{\frac{F}{N}}.$

(12.3)

The energy resolution of a Ø1” × 1” LaBr₃(Ce:0.5%) detector is 2.9% FWHM for a γ-ray photon energy of 662 keV, although this number varies depending on the concentration of the Ce³⁺ dopand, as shown in Table 12.1.

Many characteristics of a scintillator can influence the resulting energy resolution, such as the variation of the light output of the scintillator with respect to the incident energy (the photopeak position is proportional to the light output). This nonproportionality or non-linear dependence of the light output due to the variation in the absorbed amount of ionization energy, and thus the emitted number of photons per megaelectronvolts at different energies, generally occurs at energies in the x-ray region rather than at energies above 100 keV. The effect nonproportionality has on the resulting energy resolution [49,50,51,52] can be described such that

${\left[\frac{\Delta E}{E}\right]}^2=R^2=R_{nPr}^2+R_{inh}^2+R_p^2+R_M^2.$

(12.4)

where:

RnPr	=	effect the nonproportionality of the scintillator has on the energy resolution
Rinh	=	inhomogeneous defects in the scintillator (which can cause fluctuations in the light output)
RP	=	transfer resolution
RM	=	contribution to the energy resolution from the photosensor (i.e., PMT) and Poisson statistics of detected photoelectrons [53,54,55]

The latter effect on the resolution can be derived from the equation

$R_M=2.35\sqrt{(1+v(M))/N_{phe}^{PMT}},$

(12.5)

where:

v(M)	=	variance from the electron multiplication process in the PMT (typically 1+v(M) = 1.25)
$N_{phe}^{PMT}$	=	photoelectron yield from the PMT [53]

For LaBr₃(Ce:5%), this photoelectron yield is ~21 keV. Inhomogeneities are responsible for an observed deterioration in the recorded energy resolution with increasing crystal size. In a pure, bare homogeneous crystal we assume the terms R_inh and R_p are negligible (<1% contribution to the overall resolution), leaving only the nonproportionality and photosensor terms R_nPr and R_M, respectively. Therefore, in order to obtain the best energy resolution possible, the nonproportionality of the crystal must be as low as possible, such that the energy response is mainly reliant on the gain and other characteristics of the PMT.

The processes behind nonproportionality can be explained in the following way. At the beginning of the scintillation process, the efficiency of transporting the charge to the luminescence centers is dependent on the ionization density, which is generated in the scintillator crystal by the interaction of incident radiation. This ionization density increases with the low-energy electrons subsequently generated after an interaction with an x-ray or γ-ray, resulting in an increase of competition with the scintillation process, which lowers the charge transport efficiency to the luminescence centers. This scintillation yield is consequently no longer proportional to the amount of ionizing x-ray or γ-ray radiation, thereby degrading the energy resolution [48,56].

The response of the nonproportional scintillation light in both LaBr₃(Ce) and LaCl₃(Ce) scintillators has been measured in various studies [51,57,58,59,60] down to a photon energy of 5 keV and an electron energy of 3 keV. Figure 12.3 shows the light yield as a function of γ- and x-ray energies relative to the yield at 662 keV. The nonproportionality of the scintillator response of a Ø25 × 31 mm² LaBr₃(Ce) crystal is compared with that of a NaI(Tl) crystal with similar dimensions. For energies above 1 MeV, the determination of the light yield (particularly for LaBr₃(Ce)), might depend on the non-linearity of the PMT. For energies above 100 keV, the LaBr₃(Ce) detector seems to be fairly proportional, with a similar degree of proportionality to the NaI(Tl) detector above ~300 keV [61]. At lower energies, the light production in LaBr₃(Ce) is much lower with decreasing energy, but is still seen to have a higher degree of light proportionality than NaI(Tl).

FIGURE 12.3 A comparison of the nonproportionality curves measured for both LaBr₃(Ce) and NaI(Tl) scintillators, each with crystal dimensions of Ø25 × 31 mm². (From Syntfeld, A., et al., IEEE Trans. Nucl. Sci., 53, 3938. © (2006) IEEE. With permission.)

Nonproportionality has also been examined for various Ce³⁺ concentrations in LaBr₃ scintillators over a temperature range of 80–450 K [62]. In this study, it was found that for Ce³⁺ concentrations of 5% and 30%, the best proportionality and energy resolution were recorded at 80 K. For LaBr₃(Ce:0.2%), the lowest energy resolution and nonproportionality were obtained at room temperature [62].

12.2.3  TIMING RESOLUTION

The time resolution (pulse-resolving time) of a detector is another important criterion that influences the final choice of scintillator crystal. An ideal timing signal is one whose rising edge is almost vertical, as this gives a more precise moment in time (time pickoff). The criterion for a detector with good timing resolution is how quickly a timing signal is generated and decays, as the duration of the total time signal ultimately dictates the pileup rate. The insensitivity of a detector caused by a long decay time contributes to the dead time and limits the counting rate at which it can be operated.

The timing resolution can be influenced by the size, material, and outer surface condition of the scintillator, as well as by the properties of the light guide, photosensor, and subsequent electronics. Larger crystals will generally have longer timing responses due to the distance the light has to travel through the scintillator. The light guide in the case of lanthanum halides is particularly important, as the crystals are hermetically sealed due to their hygroscopic nature. The effect of a fluctuating gain and the spread in transit time of the accompanying photosensor on the resulting time resolution will be discussed in further detail in Section 12.2.5. In the case of lanthanum halide scintillators, the timing resolution of the detector also depends on the Ce³⁺ doping concentration, as shown in Figure 12.4.

Typically, the brighter and faster the light pulse generated by the scintillation mechanism in a crystal, the better its ability to resolve the arrival time of two light signals at the end of the scintillator. The uncertainty in the arrival-time difference for these two photons is known as the coincidence resolving time (CRT). In medical applications such as positron emission tomography (PET), the detection of two coincident 511 keV photons caused by electron-positron annihilation forms the basis of this method by using direct coincidences. However, the CRTs of current scintillators used in medical applications, such as LaBr₃(Ce), are so good that the emission point of these annihilation photons can be constrained to a small segment along the line of coincidence using time-of-flight (ToF) methods. The CRTs of such detectors are directly responsible for mapping the emission point, where a quicker or smaller CRT between two detectors results in a more accurate location of the initial emission point of the annihilation photons. Typically, the time resolutions in ToF measurements vary between 0.1 and 1 ns for high-quality scintillators. Small LaBr₃(Ce) crystals have been tested with silicon photomultipliers (SiPMs) and avalanche photodiodes (APDs) in such medical applications and have been found to give an exceptionally good CRT of ~100 ps, which corresponds to a ToF position resolution of 15 mm [64]. Other studies of CRTs involving small lanthanum halide detectors up to an inch in size can be found in [65,66]. The CRTs of larger LaBr₃(Ce) scintillators can be found in Table 12.3.

FIGURE 12.4 The effect of changing the dopant concentration on the timing resolution of a LaBr₃(Ce) scintillator. The timing signal shows the prompt detection of two 511 keV annihilation photons in coincidence. (From Glodo, J., et al., IEEE Trans. Nucl. Sci., 52, 5. © (2005) IEEE. With permission.)

Timing resolutions are also crucial in nuclear structure physics, as the electromagnetic reduced matrix element B(λL) can be extracted by measuring the half-lives of excited nuclear states. The transition probability T_FI(λL) of a state decaying from a state with initial spin J to a final state with spin J_i is given by the formula [72]

$T_{fi}(\lambda L)=\frac{8\pi (L+1)}{\hslash L{((2L+1)!!)}^2}\times {\left[\frac{E_{\gamma }}{\hslash c}\right]}^{2L+1}\times B(\lambda L:J_i\to J_f).$

(12.6)

The half-lives of excited states can vary considerably over many orders of magnitude and thus need to be determined using various techniques [73,74]. One of these techniques, the centroid shift method [75, 76, 77, 78], compares the moments of the prompt– and delayed-response functions using the delayed-coincidence method [79]. Until recently, fast coincidences between discrete nuclear states were measured with BaF₂ detectors, which have a fast decay time component of 0.6–0.8 ns [38]. However despite having such a fast decay time, the detector suffers from weak light output (1800 photon/MeV [38]) and poor energy resolution of about 9% FWHM for a 662 keV γ-ray [39,40]. The use of superior, readily available scintillators such as LaBr₃(Ce) has allowed for the construction of large arrays of fast-timing scintillators [68,80]. It is envisaged that such arrays will measure the half-lives of excited states using the delayed-coincidence method down to tens of picoseconds [67,81,82,83,84,85,86,87,88]. In order to measure the half-lives in this sub-nanosecond regime effectively, it is necessary to have a good understanding of the prompt response function, the width of which depends on the sum of the CRTs between pairs of fast-timing LaBr₃(Ce) detectors [81]. The CRTs of various LaBr₃(Ce) detectors of different shapes and sizes are shown in Table 12.3.

12.2.4  NOVEL CODOPED LANTHANUM HALIDE SCINTILLATORS

In an effort to improve the currently acclaimed LaBr₃(Ce) crystals, ionic co-doping of LaBr₃(Ce) was investigated [34]. Small (Ø60 × 80 mm²) LaBr₃(Ce: 5%) crystals were co-doped with Sr²⁺ and Ba²⁺, resulting in an improvement of the light output of ~25% and a better energy resolution over a dynamic range of 122–2615 keV, due to the increase in the light output combined with improved energy proportionality. Despite this, co-doping LaBr₃(Ce:5%) scintillators with 0.5% of Sr and 0.17% of Ba degrades the initial decay time of the LaBr₃(Ce:5%) scintillator by 3–4 ns. Deconvolving the time profiles using an impulse-response function, however, results in the extraction of “true” decay times of 18.2 and 19.1 ns for the Sr and Ba co-doped LaBr₃(Ce) scintillators, respectively [34]. Similar studies using other co-dopands such as Li+, Na+ Mg²⁺, and Ca²⁺ with LaBr₃(Ce) scintillators can be found in [89,90].

12.2.5  PMT SELECTION AND OTHER READOUT DEVICES

Selecting a PMT is crucial in order to maintain the anticipated performance of the scintillator by adhering to its intrinsic properties. In order to do this, the PMT has to be compatible with the scintillator in various ways:

•  It needs to be matched in light; that is, the peak emission wavelength of the crystal needs to match the highest quantum efficiency offered by the PMT at a similar wavelength.

•  It needs to be properly gain-matched in order to maintain linearity in the response.

•  The grease used to couple the scintillator and PMT window must ensure maximum light transmission.

It is important to match the peak emission wavelength (λ_max) of the PMT with the crystal to ensure that all or most of the light at the emitted wavelength is registered. As we know from Section 12.2, the peak emission wavelength of LaBr₃(Ce) and LaCl₃(Ce) scintillation light occurs principally between 350 and 420 nm, although changes in the concentration of Ce³⁺ changes the light output by producing variations in the scintillation process. In order to reduce the loss of the collected scintillation light, a PMT needs to be sensitive to blue ultraviolet (UV) light (with a range of 13–15 μA/lmF), and have a high quantum efficiency (such as a Bi-alkali PMT).

The PMT used with a very high-light-yielding crystal (like LaBr₃(Ce) or LaCl₃(Ce)), needs to be chosen carefully as space charge effects and saturation can occur, resulting in the degradation of the signal. This can be solved by reducing the gain on the PMT or the number of PMT stages used in the cascade. However, although reducing the bias voltage lowers the gain and current of the PMT by slowing down the electrons during the multiplication process, non-linearity in the generated energy spectrum is likely to occur. Taking the signal from one of the dynode stages (reducing the number of PMT stages used during the multiplication process) is seen to be the most favorable option in avoiding this; however, a greater amount of variance in the signal is produced due to the higher voltages between each dynode stage. Changing the impedance in the voltage divider on the stage used to take the signal is another alternative in trying to improve the energy resolution of a detector [68].

For timing applications, it is important that the selected PMT has a high number of photoelectrons generated by the photocathode and a low spread in gain due to the electron multiplier, and that it limits the transit time jitter associated with the cascading electrons travelling to the first dynode from the photocathode. In some PMTs, a screening grid is placed inside the last dynode, resulting in an improvement in the charge collection due to the reduced ToF of the electrons between the last dynode and the anode [70]. In doing so, a “parasitic” component is induced in the resulting anode signal, as the travelling electrons in the last stage of the PMT increase the triggering point in the fast discriminator due to the charge from this shifted component. This is found to improve the rise time of the anode pulse as the triggering point is much higher in comparison to the main component (generated by the collection of electrons travelling from the last dynode), where the properties of the scintillation detector require only a small fraction of the anode-pulse height in order to achieve the best time resolution.

Optical grease needs to be chosen to allow for maximum light transmission between the PMT window and the crystal (or light guide in the case of hygroscopic LaBr₃(Ce) and LaCl₃(Ce) scintillators). The coupling grease needs to have a similar refractive index to the crystal or light guide and the PMT window, and it needs to be transparent. Air bubbles need to be eliminated during the coupling process, as these will affect the performance of the detector.

In some applications, the use of other readout devices can have advantages over conventional photosensors such as PMTs. One such field is medical physics, where LaBr₃(Ce) and LaCl₃(Ce) scintillators are coupled to SiPMs, solid-state sensors that are insensitive to magnetic fields, unlike PMTs. Additionally, SiPMs are transparent to 511 keV γ-rays and are very compact detectors. Such a detector system enables novel designs of compact arrays that are high resolution and allow for depth-of-interaction correction [91]. Moreover, SiPMs are compatible with magnetic resonance imaging (MRI) devices, allowing the possibility of hybrid (PET and MRI) devices to be explored [92,93]. Other applications of SiPMs and LaBr₃(Ce) scintillators, such as Compton cameras, can be found in [94].

12.2.6  PROTON AND NEUTRON ACTIVATION

In applications that involve high proton and neutron fluxes (such as space science), the durability of the detector is challenged. In such hostile environments, radiation damage can compromise the performance of the detector and thus needs to be tested.

Exposing LaBr₃(Ce) and LaCl₃(Ce) scintillators to proton fluxes of 10^8–12 protons/cm² results in an activation spectrum due to the proton capture of the constituent elements of the crystal. Although the proton inelastic reaction cross section for the lighter elements (Br and Cl) is less than the ^138,139La+p cross section, Br and Cl are more abundant. Consequently, the resulting activation spectrum for a LaBr₃(Ce) scintillator is dominated by γ-ray transitions from ⁷⁷Br and ⁷⁹Kr [95]. Similarly, the activation spectrum of a LaCl₃(Ce) scintillator is dominated by ³³Cl and ³²Cl. Other nuclides that feature in the activation spectrum include ¹⁴⁰Cs and ¹³⁹Ce from the ^138,139La+p reaction channel. Measurements also report that the proton light yield is 44% less than the light yield of γ-rays, due to the higher ionization density that is generated, which quenches the light [45].

Neutron activation of lanthanum halide crystals also occurs due to the large (n, γ) cross sections of stable ¹³⁹La (9.0 b) and the two stable isotopes in bromine: ⁷⁹Br and ⁸¹Br (11.0 and 2.4 b, respectively) [96,97]. Neutron inelastic scattering of thermal neutrons off these isotopes results in the population of excited levels in ¹⁴⁰La and ^80,82Br, which subsequently undergo β⁻-decay and electron capture. Similar results in the case of thermal neutron activation have been found [98].

Pulse shapes of thermal and fast neutron signals have also been compared with those of signals from γ-ray detections in LaBr₃(Ce) scintillators using “software CFD,” a method in which the values for the slow and fast components are analyzed in a similar manner to a nuclear instrumentation module (NIM) discriminator module. The fast and slow signals are integrated and the values for the fast and slow components deduced via the zero crossover method. Figure 12.5 shows the sum of the fast and slow components from γ-ray and neutron-anode pulses plotted against the slow component only, showing little or no quenching of the light from the signals of neutrons detected in LaBr₃(Ce) scintillators. However, other investigations report that there is modest discrimination between neutrons and γ-rays, which are detected in LaCl₃(Ce) scintillators [99]. Further coincidence data with a beamline would aid in verifying these results.

FIGURE 12.5 (See color insert) A spectrum showing a comparison between acquired neutron pulses (which also include coincident γ-rays) from an AmBe source (black) and pulses acquired from a ⁶⁰Co source (red). No discrimination between both sets of pulse data is possible.

12.3  CHARACTERISTICS OF CERIUM TRI-BROMIDE (CeBr₃) SCINTILLATORS

Cerium tri-bromide (CeBr₃) crystals were initially developed by Radiation Monitoring Devices in 2004 [33] and offer a cheaper alternative to expensive LaBr₃(Ce) and LaCl₃(Ce) scintillators. They are currently marketed by Scionix Holland B.V. and can now be grown up to Ø3” × 4” in size [56]. CeBr₃ (like lanthanum halide scintillators) is grown via the Bridgman and Czochralski techniques [20] as it melts at 722°C and has a UCl₃ lattice structure. Growing the crystals using these melt-based techniques is advantageous as it allows for the growth of larger ingots [100]. The crystal has an asymmetrical hexagonal structure and a tendency to crack due to the build-up of internal stresses from non-uniform thermal expansion inside the crystal, similar to lanthanum halide scintillators. However, compared to the lanthanum in LaBr₃(Ce) and LaCl₃(Ce) scintillators, the ionic radius of cerium is smaller (122 vs. 120 pm, respectively) [101] and is responsible for CeBr₃ having a larger “effective” atomic number. Consequently, the density of CeBr₃ is 0.11 g/cm³ larger than LaBr₃(Ce:5%) at 5.18 g/cm³ [56]. Due to the hygroscopic nature of the crystal, it has to be prepared using nonaqueous slurries of Al₂O₃ grit and mineral oil [37]. They are also encapsulated in aluminum containers, with a quartz window for transmitting the light from the scintillator to a photosensor device.

The crystal has a high photon-light yield of 68,000 photons/MeV [37,102,103] (although other findings suggest that the absolute light yield of various large encapsulated CeBr₃ crystals varies from 40,000 to 47,000 photons/MeV [56]), with a peak emission wavelength that is reported to be from ~370 nm [37,56] to 390 nm [102], making it compatible with bi-alkali, blue-sensitive PMTs. The luminescence in CeBr₃ is similar to that observed in the lanthanum halide crystals, its doublet structure attributed to the transition from the lowest 5d level to the spin-orbit splitting of the 4f²F_5/2 and ²F_7/2 levels. This doublet structure is seen to be less noticeable in CeBr₃ than LaBr₃(Ce:5%) [102], which might be due to the smaller lattice configuration and site size, shifting the emission of Ce³⁺ by several nanometers [56].

The energy resolution of CeBr₃ has been measured to be between ~3.4% and ~4.4% (FWHM) [37,104,105]. These values for the energy resolution are similar to energy resolutions for smaller CeBr₃ crystal sizes, indicating little loss in the light yield when the size of the crystal is increased. The characterization of these detectors over a dynamic range of γ-ray energies from 81 to 2615 keV, which is determined using ¹³³Ba, ¹³⁷Cs, ⁶⁰Co, and ²³²Th sources, is shown in Figure 12.6a.

The energy dependence of the data in Figure 12.6 can be fitted using the power-law function E^−1/2, similar to that used in [106] for LaBr₃(Ce) crystals. Reference [104] also reports that CeBr₃ is linear over the majority of this energy range, with a linear correlation coefficient of R² = 0.99975, although an exponential term is needed in order to take into account non-linearity above ~1.7 MeV. Other studies report a similar 1/√E dependence for CeBr₃ detectors [56], but add that the nonproportionality component of the overall resolution (defined as R_nPr in Equation 12.4) strongly contributes to the limit of the resulting resolution, R. Therefore, the amount of variance in the electron multiplication process of the PMT cannot be attributed entirely to the degradation in the energy resolution [56,90].

FIGURE 12.6 (See color insert) (a) The measured energy resolutions for γ-ray energies between 81 and 2615 keV. The solid line shows the fit of the power function E^−0.4993. The dashed line shows the anticipated E^1/2 behavior. (b) The residuals for the different fits to the calibration curves described. (c) The relationship between the pulse height and #x03B3;-ray energy using linear (dashed), quadratic (dotted), and “exponential-linear” (solid) fits. (From Billnert, R., Oberstedt, S., Andreotti, E., Hult, M., Marissens, G., and Oberstedt, A., Nucl. Instrum. Methods Phys. Res. A, 647, 94, 2011.)

Section 12.2.2 commented on how the amount of nonproportionality as a function of energy in the light yield can affect the intrinsic performance of a detector. The light yield of a CeBr₃ scintillator is influenced by self-absorption and re-emission processes, which result in the increased likelihood of photon loss within encapsulated crystals, unlike lanthanum halide crystals. Samples of CeBr₃ that are not encapsulated are found to have a higher light yield, but without a corresponding improvement in the energy resolution. The light yield from CeBr₃ scintillators is very proportional over an energy range of 122–1274 keV, with the nonproportionality of the light yield being ~4% [37]. The contribution to the energy resolution of ~4.0% (typically recorded for these scintillators for an incident energy of 662 keV) is dominated by the photoelectron yield (13 phe/keV for CeBr₃ compared to 21 phe/keV reported for LaBr₃(Ce:5%) [56]), and the nonproportionality with energy (R_nPr). The contribution from the non-proportionality term is greater for CeBr₃ than for LaBr₃(Ce:5%) [49,53,56]. At present, a model to assess the energy resolution due to the nonproportionality of the crystal is still being devised.

12.3.1  PROTON ACTIVATION AND INTRINSIC ACTIVITY OF CEBR₃ SCINTILLATORS

The robustness of CeBr₃ scintillators when subjected to high proton fluxes was assessed by replicating energies typically observed from solar events [107], generated by a superconducting cyclotron. The fluxes ranged from 10⁹ to 10¹² protons/cm², where subsequent activation of CeBr₃ was found to be similar to that observed in LaBr₃(Ce). The resulting spectrum was dominated by ⁷⁷Kr, ⁷⁹Kr, and ¹⁴⁰Cs from activated bromine and cerium [56]. This study by Quarati et al. also found that during the irradiation of protons on CeBr₃, little degradation in the energy resolution or the light yield of the detector was observed [56]. This makes CeBr₃ detectors a very promising scintillator to use for future space missions.

Unlike the ¹³⁸La radioisotope in lanthanum halide detectors, cerium and bromine do not have any long-lived metastable states or radioactive isotopes, and therefore the intrinsic activity in CeBr₃ crystals can be ascribed to the decay of ²²⁷Ac into its daughter nuclides, similar to that seen in lanthanum halide crystals. The presence of this contamination is likely to be due to the fact that the elements Ac, La, and Ce are chemically similar, or may be due to insufficient purification of the raw materials that go into the making of the crystal. However, the source of the contamination is most likely to originate from ²²⁷Ac rather than from raw materials such as ²³⁵U ore, due to the structure of the peaks in both lanthanum halide and CeBr₃ crystals appearing to be very similar in structure [56]. The levels of ²²⁷Ac contamination are largely attributed to the choice of material that is used to grow the crystal, where levels vary from 0.001 cts/s/cm³ to similar levels observed in LaBr₃(Ce) scintillators (0.1 cts/s/cm³). However, the level of ²²⁷Ac can be reduced by growing the CeBr₃ crystals from selected batches, although the long-term availability of batches with low ²²⁷Ac cannot currently be guaranteed [56]. Unlike lanthanum halide crystals (which show different levels of intrinsic activity for various crystal sizes due to the escape probabilities and attenuation lengths of the ¹³⁸La products), the intrinsic activity observed in CeBr₃ scintillators is not very dependent on the crystal size [108]. Light from the a radiation registered in the detector due to the intrinsic ²²⁷Ac contamination is found to be quenched by 26% when compared to γ radiation. The ratio of this α/γ light yield observed with CeBr₃ is thus ~1.33 times lower than that typically observed for LaBr₃(Ce) scintillators [56], reinforcing the fact that the charge-transport efficiency to the luminescence centers in CeBr₃ is strongly influenced by the higher ionization density.

12.3.2  TIMING PROPERTIES OF CEBR₃ SCINTILLATORS

CeBr₃ scintillators have been found to be almost as fast as cerium-doped lanthanum halide scintillators, the short lifetime of the emission of Ce³⁺ luminescence being responsible for the 1/e nature of the decay time constant (τ). For CeBr₃, a decay constant of τ = 17 ns has been obtained by fitting an exponential component over the integrated light output and subtracting the background [37]. Similarly, the rise time of the timing signal was found to be 0.7 ns [37]. On average, the decay constant for CeBr₃ has been reported to be longer than LaBr₃(Ce) due to the self-absorption/re-emission cycle.

Timing resolutions using CeBr₃ scintillators have been measured using various types of PMTs to help establish the best combination of scintillator and photosensor. In a study by Fraile et al. [109], a combination of a CeBr₃ scintillator with a Hamamatsu (R9779) PMT was found to give a timing resolution (FWHM) of 119 ± 2 and 164 ± 2 ps when used with a reference BaF₂ detector at ⁶⁰Co and ²²Na energies, respectively. These values are better than those achieved with a CeBr₃-Photonis (XP20D0) detector combination, which was found in the same study to give timing resolutions (FWHM) of 146 ± 2 and 210 ± 2 ps at ⁶⁰Co and ²²Na energies, respectively [109]. This result is interesting as, traditionally, Photonis tubes such as the XP20D0 model (the properties of which are given alongside the Hamamatsu R9779 model in Table 12.4) have been found to perform very well with LaBr₃(Ce) crystals in previous studies [70]. Despite the non-linearity of the energy spectrum when these PMTs are used with CeBr₃ crystals at an operating voltage above 800 V, the timing resolutions of these scintillators are still 20%–30% worse than results obtained when using a CeBr₃ scintillator coupled to a Hamamatsu R9779 PMT [109].

12.4  SUMMARY

This chapter began by introducing the requirements that need to be met in order to have an ideal scintillator and briefly looked at the history of scintillator development up to the present day. Although not perfect, the lanthanum halide crystals LaBr₃(Ce) and LaCl₃(Ce) have been shown to be popular crystals in the field of radiation detection. The good energy (2.9% FWHM at 662 keV [25,31]) and excellent timing (~110 ps FWHM at ⁶⁰Co energies with a Ø1” × 1” crystal [70]) resolutions of these scintillators allow for many opportunities to be realized in medical and space applications, as well as in scientific research, when matched with a good photosensor. However, these crystals suffer from intrinsic activity due to the decay of the radioisotope ¹³⁸La and a-decay from ²²⁷Ac (with rates of approximately 0.7 and 0.1 counts/s/cm³, respectively [45,47]).

More recently, an alternative to lanthanum halide scintillators, CeBr₃, has been shown to perform almost as well as its sister crystals. Although CeBr₃ does not match the excellent energy resolution offered by LaBr₃(Ce) crystals (3.5%–4.5% FWHM at 662 keV [37,104,105]), the timing resolution is comparable (~120 ps FWHM at ⁶⁰Co energies for a Ø1” × 1” crystal [109]). The most desirable quality of these new scintillators is the absence of a large intrinsic background, making them ideal for low rate/background counting applications.

As technologies and techniques in the field of radiation detection advance, research continues to seek the ideal scintillator detector, with garnet ceramics [42] and other inorganic scintillators (i.e., SrI₂(Eu) [41,42], Cs₂LiYCl₆(Ce) (CLYC) [111,112,113,114,115], Cs₂LiLaCl₆(Ce) (CLLC) [116], and Cs₂LiLaBr₆(Ce) (CLLB) [43,116]) presently leading the way.

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