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Nuclear Instruments and Methods in Physics Research A 546 (2005) 180–187 www.elsevier.com/locate/nima

Phosphors and in radiation imaging detectors

Glenn C. TyrrellÃ

Applied Scintillation Technologies Ltd., 8 Roydonbury Industrial Estate, Harlow, Essex CM19 5BZ, UK

Available online 18 April 2005

Abstract

A review is presented of recent development of and scintillators for X-ray and thermal radiation imaging detectors. Data is presented on an improved range of X-ray intensifying screens constructed from Gd2O2S:Tb, which shows a shift in the expected trade-off line for resolution versus output. A high uniformity CsI:Tl fibre optic sensor is also presented. In addition, a new high 6LiF formulation of screen is described, as well as recent radiation imaging detector projects using 6Li scintillators. r 2005 Elsevier B.V. All rights reserved.

PACS: 29.40; 87.59; 85.60

Keywords: Phosphors; ; X-ray imaging; Neutron imaging

1. Introduction significant contribution to the quality of life in the developed world. Although impressive advances in electronic Most of the seminal papers on design and direct detection materials continue to have focused on single- materials due to drive progress in development of radiation ima- their more predictable and reproducible proper- ging detectors, the role of luminescent materials is ties. However, there have been many significant still of continued importance in this field. In papers on luminescence focused on cathode-ray [1] addition to satisfying the state-of-the-art demands and -stimulated [2] phosphors due to of the high-energy physics community, it is the their high commercial importance for display and mainstream usage of phosphors and scintillators applications. across industrial sectors such as medical imaging, It is easy to overlook the early pioneers of the security (threat detection), and non-destructive mechanisms of scintillation and their studies of evaluation of engineering materials that provides a alkali halides [3] and alkaline earth sulphide [4] luminescence. It is with the evolution of other technologies, such as solid-state imaging arrays, ÃTel.: +44 1279 641234; fax: +44 1279 413679. that their work is now being used as a basis for E-mail address: [email protected]. imaging detectors for X-rays, ultraviolet and

0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.03.103 ARTICLE IN PRESS

G.C. Tyrrell / Nuclear Instruments and Methods in Physics Research A 546 (2005) 180–187 181 infrared radiation, as well as energetic particles The major volume of commercially available such as , alpha and beta particles. The intensifying screen product is matched to specific interested reader can find excellent historical photographic film formulations. Products of this perspectives [5], reviews [6], and textbooks [7,8], type may be recognized by trade names, e.g. in the public domain which cover the broad range Lanex. Companies that supply into screen-film of and scintillators, and their applica- markets will often describe screens as ‘fine’, tions. ‘regular’ or ‘fast’ depending upon the film speed In this paper we define phosphors as lumines- with which they are paired. These screens are cent materials that are typically between 2–30 mm usually limited to a maximum size of 1400 Â 1700. in particle size, and for high-resolution applica- However, for most radiation imaging detectors tions require a particle size of o10 mm. They that rely on either lens-coupled or direct coupling consist of a host inorganic matrix with a lumines- to a solid-state array detector, such as a charge cent centre at an appropriate concentra- coupled device (CCD) or CMOS pixel sensor, an tion–typically between 0.01 and 5%. Scintillators understanding of the material properties of the are usually defined as the compound in single- phosphor to a different level of complexity is crystal morphology. usually assumed in order to provide differentiating This paper reviews some past and present performance over standard formulations. activities across two relevant areas of luminescence The basic problems encountered in choosing the in radiation imaging and detection, namely X-rays correct scintillator for a particular application will and neutrons, and looks at the perspectives for rely on understanding fundamental parameters, improvements in radiation imaging detectors. such as

(a) X-ray absorption of the phosphor. 2. Phosphors and scintillators for X-ray imaging (b) Gain of optical for each absorbed X- detectors ray quantum. (c) Matching of the optical photons generated in 2.1. Phosphors (Gd2O2S:Tb) the scintillator with the spectral response of the detector. The pioneering work in this area was done by (d) Reduction of optical in the lumines- Levy and West on the use of sulphide cent layer. phosphors for fluoroscopic X-ray imaging [9]. (e) Temporal characteristics of the generated Since then, many improved phosphor systems optical photons. have been developed, and these can be referred to in the earlier references. For this review we will Phosphors and scintillators suitable for this refer primarily to oxysulphide, application require a high atomic number (Z) Gd2O2S:Tb (P43), phosphor [10]. This is an ideal and high quantum efficiency for conversion of model phosphor system because the phosphor X-ray energy into optical photons that can be morphology is well known and controlled, lumi- detected by devices such as charge-coupled detec- nescent efficiency is high (15%), and the emitted tors (CCD) or photomultipliers. light (54 nm) matches well to the spectral response The X-ray absorption of a phosphor is deter- of detection devices. mined by the atomic number of the material. As Phosphor screens can be produced by a variety phosphors are generally spherical particulates of of different methods including sedimentation, dimensions described previously, they have a spraying, screen printing and doctor blade which packing that is significantly less than the tend to be highly skilled techniques. The binder single crystal density. Therefore, the concept of formulations are complex. In the design of a coating weight of phosphor per unit area (mg/cm2) luminescent screen, there is usually a trade-off is used. Typically, screens of 20–50 mg/cm2 will be between light output and image resolution. used for high-resolution applications, such as ARTICLE IN PRESS

182 G.C. Tyrrell / Nuclear Instruments and Methods in Physics Research A 546 (2005) 180–187 digital dental X-radiography. Whereas lens The result, shown in Fig. 1 (screens 1–5 (D)) is coupled systems which have lower resolution, but achieved by a combination of factors, including are light starved, will require a high light output an improvement in the X-ray phosphors but screen of 50–140 mg/cm2. For high-energy applica- also by subtle changes in the binder formulations tions (4450 keV–25 MeV) very high coating and protective overlayers. It is known from weight screens, such as 200–450 mg/cm2 are avail- intrinsic efficiency measurements [11] that able. A typical example for this type of screen is as processing conditions, particle size and an alignment screen in radiotherapy systems, such content can adjust the efficiency from 9% to 17%. as the Siemens Beamview II system. Some selected papers on the performance A method for benchmarking screens is shown in of Gd2O2S:Tb includes those of Pokric and Fig. 1 that shows a plot of image resolution Allinson [12], and Wang and Cargill [13]. The defined as line pairs per mm at 10% contrast former paper assesses phosphors of particle sizes transfer function (CTF), plotted against light ranging from 2.5 mmto10mm, and reveals that output at fixed X-ray energy. This demonstrates optimised conditions for light output and point the trade-off between light output and image spread function (PSF) for CCD-based X-ray resolution across a wide range of phosphor coating detectors operating in the energy range of weights. A number of well-known commercially 5–25 keV, is 4 mm particles at 10 mg/cm2 coating branded screens are referenced on the graph (J), weight. along with the previous AST screen performance An optically absorbing substrate may be used (&). An improvement to the performance of a for very high resolution, high contrast applications range of different coating weight screens will such as non-destructive testing (NDT) of engineer- displace an imaginary line through this range of ing components. This type of screen design screens. absorbs light emitted towards the back of the This is illustrated by recent results from our screens, and reduces the optical scattering mean group that show an improvement to the base free path. This results in less light, but an position of this light output/image resolution line improved MTF. These formulations are designed that allows significantly more light output using special binders that have the additional (420%) for the same image resolution. This is benefit of being significantly more ‘burn’ resistant particularly important for light starved systems. to high X-ray fluxes.

700

Lanex Fast Back 600 134 mg/cm2 1 1 2

500 2 Kasei DRZ 3 3 Lanex 400 FF/Regular 50 mg/cm2 300 4 Intensity

Min-R 2190 200 5 5 (compared to Lanex Fine (100%))

100 Lanex Fine 34 mg/cm2 0 2 4 6 8 1012141618 Resolution (lp/mm) @ 10% contrast

Fig. 1. Relationship between light output and image resolution for a series of known commercial intensifying screens (&). AST standard screens (J) and AST newly developed screens (1–5). ARTICLE IN PRESS

G.C. Tyrrell / Nuclear Instruments and Methods in Physics Research A 546 (2005) 180–187 183

Modern pixel detectors which are designed for fast readout speeds require phosphors that exhibit fast decay characteristics with minimal afterglow or latent image burn-in. A typical demand for high frame rate (30fps) applications, e.g. fluoroscopy, with a large area, high density of pixels requires fast readout (o1 ms per pixel) unless the pixels are 2 Â 2 binned to reduce the total number of pixels. This requires a faster scintillator than the Tb-doped phosphors (decay time to 10% is 2 ms). A to this problem is to employ a Pr-doped phosphor, e.g. Gd2O2S:Pr. Luminescence of these phosphors has been studied by time-resolved techniques in order to establish an optimised fast decay (o3.5 ms), low afterglow phosphor [14].

2.2. iodide (CsI:Tl)

This scintillator is well known and characterised in single crystal form, but although available commercially as a columnar thick film Fig. 2. CsI:Tl coated fibre optic faceplates showing (a) a (50–500 mm) from several sources, it is remarkably commercially available dental X-ray imaging plate, and (b) an variable in quality and not well understood or AST plate of similar construction. described. The columnar structure of the as-grown thick film conveys an improved level of signal 150% of a Kodak Lanex Fine X-ray intensifying uniformity and image resolution over conventional screen. phosphor intensifying screens. CsI:Tl has been studied most recently for 2.3. Structured scintillators application in X-ray imaging in intra-oral dental applications. In this scenario, the scintillator is A further enhancement of scintillator design required to cover the major proportion of active towards improved image resolution and confined area in the sensor in order to reproduce the pixellation is achieved by filling of etched hole effectiveness of photographic film while conveying structures with selected scintillators. These materi- the advantages of modern digital imaging. Recent als have been investigated as part of the EU fifth advances in the uniformity of CsI:Tl are reported framework project ‘‘3D-Radiation Imaging Detec- in this paper, where deposition on customised fibre tors—3D-RID’’ [15]. optic faceplates is achieved to the edge of the plate Fig. 3 shows an etched structure filled with uniformity variation of less than 1%. Fig. 2 with CsI:Tl scintillator via a multi-step melt filling shows the comparable uniformity of identical process with (a) an etched silicon structure, and (b) CsI:Tl coatings of approximately 100 mmona a plan view of filled pores. fibre optic faceplate, with (a) a commercially Fig. 4 shows a potential thermal neutron available CsI:Tl screen showing uniformity of +/ scintillator design, which is based on material À2%, and (b) a new AST CsI:Tl screen showing more thoroughly discussed in Section 3. The view o1% variation in uniformity across a 20 Â 30 mm shows a of an etched silicon pore dental X-ray sensor faceplate. These screens have filled with a scintillator of ZnS:Ag with a neutron MTF of 25% at 10 lp/mm with a light output of converter of 6LiF. ARTICLE IN PRESS

184 G.C. Tyrrell / Nuclear Instruments and Methods in Physics Research A 546 (2005) 180–187

Fig. 3. Photomicrographs showing (a) a cross section of a high aspect ratio etched silicon structure (courtesy of Jan Linnros and Xavier Badel at KTH Stockholm), and (b) a plan view of the CsI:Tl filled structure. Fig. 4. A cross sectional view of an etched silicon structure with a ZnS:Ag phosphor and a 6LiF neutron capture material.

3. Phosphors and scintillators for neutron radiation where t is the thickness of the material, s is the imaging detector atomic cross-section in barns, N is the number of 6 3 Li atoms per cm , I0 is the incident neutron flux, I Typical scintillators for detection of neutrons is the transmitted neutron flux. require two processes to occur; (i) capture/reaction This review looks at two different forms of of the neutron and (ii) transfer of energy to a neutron detector, which rely on volume luminescent centre. The process for neutron created by the 6Li-neutron reaction to excite capture in materials described in this paper is by luminescent centres. reaction of a thermal neutron with 6Li atom.

6 3 4 Li þ n ! H þ He þ 4:71 MeV: 3.1. Ce-doped 6Li aluminosilicate scintillation glass The reaction above proceeds with a high reaction cross-section as well as releasing a large Scintillation for neutron detection are a amount of energy. The probability of interaction very well established technology. The - of a thermal neutron with a screen is dependent doped scintillation glasses were first formulated upon the number of 6Li atoms in a fixed volume. by Ginther and Schulman [16], but did not reach The neutron attenuation coefficients of any widespread use until Levy West Labs [17] in the material can be determined using the following UK produced a high-quality commercial manu- relation: facture method which was badged by Bicron and NE Technologies. The glass can now be sourced I ¼ I 0 expðÀNstÞ, directly. ARTICLE IN PRESS

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6Li is used in preference to 10B because of poor scintillator response [20] temperature effects [21] pulse height resolution obtained with the latter and energy transfer processes [22]. [18]. There are at least two important areas for A table of common formulations is shown in reproducible glass performance. The control of the Table 1, and properties in Table 2. The various Ce3+/Ce4+ ratio is critical in order to achieve the grades can be divided into natural, 6Li enriched correct light emission, optical decay and optical and 6Li depleted versions. As these glasses have transmission characteristics. A high cerium con- some sensitivity, they are often used in pairs in centration is found to decrease the amount of order to subtract the gamma sensitivity (as Ce4+ in the scintillation glass [19], but this ratio is exhibited by the 6Li depleted glass). Clearly, use far more profoundly influenced by melting the of an enriched 6Li grade of glass substantially glass in a reducing atmosphere. reduces the thickness of the glass needed to absorb The Ce-doped lithium aluminosilicate glasses a known amount of neutrons and as such reduces have been extensively studied and are still in the gamma sensitivity. widespread use today. Spowart investigated sev- Some examples of usage of 6Li glasses are eral aspects of Levy West glass considering for use in measurement-while-drilling (WMD)

Table 1 Commercial cerium-doped 6Li aluminosilicate scintillation glass formulations

Commercial Type of lithium Weight % designation

SiO2 MgO Al2O3 Ce2O3 Li2O GS1 Natural 55 24 11 4 6 GS2 Enriched 55 24 11 4 6 GS10 Natural 56 4 18 3 18 GS20 Enriched 56 4 18 3 18 KG1 Natural 74 0 0 5 21 KG2 Enriched 74 0 0 5 21

Table 2 Typical properties of cerium-doped 6Li aluminosilicate scintillation glasses

GS1/2/3 GS10/20/30 KG1/2/3

Density (g/ml) 2.66 2.50 2.42

Refractive index n4047 1.58 1.55 1.566 Glass transition Tg (1C) 620 499 464 Softening point Ts (1C) 650 522 490 Strain point Tsr (1C) 350 410 461 Coefficient of linear expansion 7 Â 10À6 9.23 Â 10À6 100 Â 10À6 of maximum emission (nm) 395 395 395 Relative light pulse height per unit energy input 100 85 60 Light output relative to 22–34% 20–30% 20% Decay time (ns) 50–70 50–70 50–70 Resolution on the thermal neutron peak obtained with 13–22% (GS2) 15–28% (GS20) 20–30% (KG2) moderated Po/Be neutrons (depending on geometry of whole system) Peak/trough ratio of above peak (range) for thermal 15:1–40:1 (GS2) 10:1–40:1 (GS20) 10:1–20:1 (KG2) neutrons ARTICLE IN PRESS

186 G.C. Tyrrell / Nuclear Instruments and Methods in Physics Research A 546 (2005) 180–187 processes for oil [23], positional sensitive detection [24], and also for space borne neutron detectors. A recent example was for the and neutron spectrometer (GRNS) for the JHU-APL NASA MESSENGER mission to mercury [25]. Lithium-free scintillation glasses are also described for fast neutron detection [26] and they are also in commercial manufacture. They have advantages over other competitive scintillators in time-of-flight (TOF) applications such as 6LiI:Eu because of the detailed structure in the neutron– reaction. A comprehensive review of other inorganic scintillator development with reference to cerium- doped single has been published by Van Fig. 5. A module from the GEM detector (courtesy of Dr. Eijk [27]. Nigel Rhodes at the ISIS facility at the Rutherford Appleton Laboratories). 3.2. Neutron detection screens (Type ND) gamma radiation is significant and should be in the This type of neutron detection screen is usually a region of 10À6–10À8 for 1 MeV gamma. A detector formulation of 6LiF (for neutron capture) and a module of this type is shown. high-efficiency phosphor such as ZnS:Ag. The All the ISIS detectors have been manufactured resultant screen is opaque to its own emission light with a 4:1 ratio of ZnS:6LiF. Recent work [31] at and so a maximum thickness of approximately Oak Ridge National Laboratories (ORNL) re- 0.5 mm is used so that light can escape from the ports on an optimized 6LiF/ZnS ratio for a shifting screen to the detector (usually a photomultiplier). scintillator prototype for the Powgen 3 powder Nevertheless, for neutron imaging applications diffractometer at the Spallation Neutron Source these screens have a very high light output against (SNS) at ORNL using a 2:1 ZnS:6LiF formulation other neutron detection materials [28]. There are from AST that increases the neutron absorption. further variants available that can be spectrally Further detectors are now available that utilise matched against the red response from typical direct capture of neutrons by 6LiF/silicon pixel CCDs in use. , which bypass the requirement for a These screens are used in positional sensitive scintillator [32]. It is important for these devices detector (PSD) applications. The ISIS pulsed that the conversion layer is sufficiently thin to neutron spallation facility at Rutherford Appleton accommodate the range of the reaction products. Laboratories (RAL) has several beam station Pore filling of three-dimensional silicon diodes detectors that incorporate this material. These using either 6LiF or 6LiF/ZnS is another method include a number of high efficiency and high- that may be used for neutron imaging and resolution position-sensitive detectors and diffract- detection. Fig. 4 in the earlier section on structured ometers, including SANDALS (Small Angle Li- scintillators shows a structure of this type. quids & Amorphous Diffraction), GEM (General Purpose Diffraction), ENGIN-X (Engineering Instrument) and HRPD (High Resolution Powder 4. Summary Diffractometer). A v-shaped construction has been described by Rhodes and Johnson [29] that have This paper has surveyed some of the important been incorporated into the OSIRIS and GEM research that is currently being done on new X-ray powder diffractometers. Fig. 5 shows a module and neutron radiation imaging detector designs from the GEM detector [30]. Low sensitivity to using phosphors and scintillators. ARTICLE IN PRESS

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