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Phosphors for Luminescent Image Plates
JOURNAL OF X-RAY SCIENCE AND TECHNOLOGY ARTICLE NO.
6, 48–62 (1996)
0003
Phosphors for Luminescent Image Plates A. M. GURVICH,* C. HALL,† I. A. KAMENSKIKH,‡ I. H. MUNRO,† V. V. MIKHAILIN,‡ AND J. S. WORGAN† *Moscow Research Institute for Roentgenology and Radiology, 117837 Moscow, Russia; †EPSRC, DRAL, Daresbury Laboratory, Warrington, Cheshire WA4 4AD, United Kingdom; and ‡Moscow State University, 117234 Moscow, Russia Received June 25, 1993; revised May 10, 1995 This is a review of the properties of some photostimulable phosphors for luminescent image plates as applied to digital radiography. In particular, the properties of BaFBr:Eu and other barium fluorohalides that are useful for this application are considered. The main emphasis of the review is on the effect of the preparative conditions and the origin of the photostimulated luminescence and its features under VUV excitation. q 1996 Academic Press, Inc.
I. INTRODUCTION
Phosphors that can store energy and release it as light or photostimulated luminescence (PSL) when subsequently illuminated with IR or visible light have been known for a long time. They have been applied extensively in dosimetry and in the visualisation of IR (1–3). Computers and lasers have enabled the range of possible applications of photostimulable phosphors to be extended, leading to the creation of luminescent digital radiography (LDR) (4). The distinguishing feature of this technique is the use of an image screen covered by a photostimulable phosphor instead of a photographic (radiographic) film or by its combination with an intensifying screen. Processing of the image is achieved by scanning the screen with a focused helium–neon laser beam. The photostimulated light flashes thus produced are converted by a photomultiplier into a sequence of electric pulses which, after digitization, are recorded on an optical disc or a magnetic tape. After computer processing, the information is again converted into analogue signals which modulate the intensity of an electron beam or a laser scanning over a photofilm. In the most recent Fuji systems, the same laser is used for scanning over both the image plate and the photo film. Among the advantages of the LDR technique (5) compared to conventional film radiography is its larger dynamic range (6, 7). This results in a wide range of contrast variation (1) and makes the choice of the exposure less crucial. The idea of LDR has been around for many years (see e.g. (1)) but its practical realization has had to wait until phosphors meeting all the requirements could be identified. II. THE SELECTION OF PHOSPHORS FOR IMAGE PLATES
Phosphors for image plates are expected to possess the following properties (5, 9): (i) a high energy yield of PSL at room temperature to minimize the noise contribution 48
0895-3996/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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due to photon intensity fluctuations; (ii) the emission and excitation bands must be well separated; (iii) the time decay of light flashes stimulated by the laser should be fast enough to provide an acceptable read-out time and to minimize the overlap of signals from different areas of the image; (iv) long term storage of the absorbed energy (minimal fading) is important; (v) the PSL intensity should be linear with the irradiation dose since this requirement will determine the achievable dynamic range; (vi) the absorption of ionizing radiation must be sufficiently high to reduce the contribution of quantum noise and to minimize the doses required for high quality images. A large number of phosphors which exhibit PSL have been studied. They include the sulfides of zinc and alkaline earth metals, e.g., SrS:Ce,Sm and SrS:Eu,Sm (1, 15); halides of alkali metals (8, 11), especially RbBr:T1 (7, 9); oxides (ThO2:Er) and oxysulfides (La2O2S:Eu,Sm) (10); fluorohalides of alkaline-earth metals (4, 12); several sulfates, borates, and silicates (9, 11, 13); galloborates, gallosilicates, gallophosphates, and gallogermanates of alkaline earth metals, especially Ba5SiO4Br:Eu (9, 14). Barium fluorohalides (BaFHal:Eu) proved to be the most efficient from the viewpoint of LDR and their PSL properties (4, 12) have been widely studied. The most favorable combination of properties is shown by BaFBr9Eu (10, 15–17) which is now used in commercial image plates (15). Its PSL energy yield amounts to 5% (17); the emission (lmax Å 390 nm) and excitation (lmax Å 590–600 nm) bands are well separated; the PSL decay time is approximately t Å 0.8 ns6; its fading characteristics are good, less than 25% is lost in 8 h (15); the presence of barium in the composition of the phosphor ensures a high absorption cross section for x rays (18). Attempts to further improve the properties of BsFBr:Eu have been made, for example, by the partial substitution of Br by Cl or I (11). III. THE STRUCTURE AND PRINCIPAL PROPERTIES OF BARIUM FLUOROHALIDES
BaFCl was the first material to attract the attention of physicists as a subject for the study of color centers (19 – 22) and as an efficient (when doped by Eu) phosphor for x-ray intensifying screens (23, 34). Also, the high efficiency of photoand cathodoluminescence of MeFHal:Eu2/-phosphors (Me|Sr, Ba; Hal|Cl, Br) have been noted (25). Fluorohalides of barium and strontium have a layered tetragonal structure of the PbCl-type (26, 27) which can be projected as a series of layers F2 MeHalMe . . . , situated along the [001] directors (Fig. 1). The fluorine symbol, F2, means that the number of ions in the F0 layer is twice that in the Hal or Me layers. It can be seen, q 0 from Fig. 1, that the distance between neighbouring F ions is 2 times less than the distance between Br0 or Ba/ ions in the appropriate layers. There are two types of anions in the barium fluorohalides and hence there exist two different types of color (F) centers. These are, for example, a fluorine vacancy with a trapped electron (VF) or a chlorine vacancy (VCl) in BaFCl or vacancies of fluorine (VF) and bromine (VBr) in BaFBr. A subscript associated with the vacancy (V) indicates the lattice site it occupies; an interstitial site is shown as (i). Absorption bands related to these F centers have been observed experimentally. According to EPR data (20) and to theoretical calculations (21) it is the long wavelength band which is related to the VF centers, which are frequently indicated as F(F0). Another
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FIG. 1. Ions in the crystal of BaFBr. The symbols F0, Ba2/, Br0 stand for the layers of F0, Ba2/, and Br0 ions, respectively.
band attributable to VBr centers falls within the same region (28). It has been shown that additive coloring results from the creation of F0 centers only of the VF type (29). This can be accounted for by the penetration of surplus Ba into the bulk of the crystal, along the dislocations which appear at the edge of two adjacent F2 and Ba layers and terminate in the bulk of the crystal. Their creation at the layer edges is less probable because of unfavorable geometry. A study of the luminescent properties of the BaCl2 –BaF2 system (12) revealed that an excess of BaF2 created a solid solution in BaFCl. This means that Cl0 ions are partially substituted by F0 ions with the creation of anti site defects, FCl (17). Such • • defects are expected to exist also in BaFBr (30). Their association with VCl (VBr ) leads to the creation of F-centers that are somewhat different from the isolated ones (17) where the superscript dot represents an effective position charge relative to the lattice and a dash represents negative charge. Barium fluorohalides show luminescence which is not related to the impurities. As in the case of other halide phosphors (31) it is attributed to the recombination of electrons with Vk centers (31–34) (i.e., molecular centers, Hal20, created by the capture of a hole by two adjacent halide ions). Since there are two types of halide anions in the crystal, at least the same number of different types of Vk centers can exist there, e.g., F20 and Cl20 in BaFCl. F20 is associated with the short wavelength emission band which is observed only at low temperatures in
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FIG. 2. Emission (curve 1) and stimulation (curves 2, 3) spectra of BaFBr:1004Eu phosphor prepared by heating to 8007C without (2) and with (3) additional annealing at 5507C, measured at 207C. The phosphor mix was initially prepared with a concentration of 1004 g-atom/mol of Eu. The abscissa represents the wavelengths of emitted (lem) and stimulating (lst) light. The ordinate represents the intensity of x-ray luminescence (IXL) and PSL (IPSL). The curves are normalized at the maxima. In the case of BaFBr:Eu, the PSL spectrum and the x-ray luminescence spectrum are essentially identical.
all fluorohalides as well as BaF2. The luminescence from undoped BaFCl and BaFBr at room temperature is attributed to the recombination of electrons with Cl20 or Br20 centers, respectively. They are formed by Hal0 ions belonging to the adjacent layers and are located at their minimum distance. Due to the reduced electron affinity of Br (compared to that of Cl), centers of the Br20 type dominate in mixed crystals BaFClxBr10x (32). Barium and strontium fluorohalides are easily doped by two-valent rare-earth ions such as Eu2/ (2, 23, 34, 35) and Sm2/ (36, 37) since such doping does not require charge compensation. The emission spectrum of Eu2/ at room temperature is an intense band peaking at 385 nm for BaFCl and at 390 nm for BaFBr and is quite favorable for LDR applications as it falls into the maximum sensitivity range for conventional multi-alkali photocathodes and at the same time is shifted relative to the excitation bands (37). The situation for BaFBr:Eu is presented in Fig. 2 (38). One of the most complicated problems which limits the usefulness of the BaFCl:Euphosphor as an intensifying screen is its weak but very long afterglow (24, 35, 39) which is indicative of deep traps. It was the study of this effect, even more pronounced in BaFBr:Eu (23), that led to the discovery that fluorohalides have the ability to store a large amount of light energy to be released later by optical excitation. The charge separation required for light energy storage is provided by the effective capture of holes by the Eu2/ centers and of electrons by anion vacancies. According to recent EPR results (90) the holes are captured not by Eu2/ but by different adjacent traps • of unknown origin. The participation of VF• and VHal vacancies becomes evident from the comparison of the excitation spectra with the absorption due to F centers created
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by the capture of electrons (41). The energy yield of BaFBr:Eu PSL (40) excited by x ray approaches 5%. The rate controlling the process for the duration of the PSL flash is the allowed intracenter transition 4 f 65d r 4 f 7. Under continuous optical excitation the rate of electron release from the traps (t01 em ) is proportional to the maximum PSL intensity (IPSL) which is, in turn, determined by the activation light intensity (Ist) and the initial concentration of electrons trapped by F center (nF). IPSL is given by IPSL Å sIstnF
[1]
where tem is the time required for the PSL intensity to decay to 1/e of its peak value under continuous stimulation and s is the cross section for optical excitation (41). IV. THE EFFECT OF PREPARATIVE CONDITIONS ON THE PROPERTIES OF BaFBr:Eu PHOSPHORS
Three methods are frequently used for barium fluorohalide preparation (18, 23, 27, 43). One method, cited in several papers (see, e.g., 10, 16, 41), is a solid phase reaction within a heated charge of BaF2 and BaBr2 together with europium bromide (10) or fluoride (40). In a second method, the initial stage employs an aqueous suspension of finitely dispersed BaF2 in the BaBr2 solution (23, 44, 45). After evaporation and the addition of EuBr3, the charge is annealed in a reducing atmosphere. The distinguishing feature of the third method is the application of NH4F (17, 39, 44, 45) as a fluorine source which reacts with BaBr2 first in solution and then, following evaporation, in the solid phase. The properties of BsFBr:Eu are quite strongly affected by the preparative conditions (17, 39, 48, 91). Normally the maximum PSL yield is observed at smaller europium concentration than that required for the maximum stationary luminescence yield (39). This optimum europium concentration can be an order of magnitude less for BaFBr:Eu prepared using NH4F, than that produced by the reaction of BaBr2 with BaF2, at 1004 g atom/mol (49). The effect of fluorine from NH4F also favors the insertion of other rare-earth dopants into the lattice as well as the removal of oxygen, which leads to an increase in phosphorescence and reduction in PSL yield. The oxygen source is associated with water of hydration which readily reacts with barium halides. The drop • in PSL yield seems to occur through the creation of defects of O1BrVBr type (17). 2/ Though the doping of BsFBr by Eu is efficient when the charge is annealed with NH4F and such admixtures as, e.g., KBr (39) in air, a slightly reducing atmosphere, either nitrogen with an admixture of hydrogen (10, 42) or CO (17, 24, 50), produces better results. In this case O1Hal centers can be formed. The admixture to the charge has a substantial effect on the stimulation spectrum (43) and its concentration (along with the annealing temperature) on the grain size. In choosing the method of admixture removal it should be noted that the hygroscopic nature of the compounds MeFHal and their tendency to hydrolysis will increase with the size of cation (Me2/) and anion (Hal0) radii (27) so that they are more pronounced for BaFBr than for BaFCl. An important role is played by the process of charge annealing (38) and the ratio of the components in the charge. This ratio determines the probability of the solid
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phase reaction (27, 49), the stimulation spectrum, and fading (10). The effect of cooling rate and annealing conditions on the properties of the BaFBr:Eu phosphor is of particular interest (37, 51, 52). It appears that annealing at 450 to 6507C shifts the maximum of the stimulation spectrum from the yellow-green region (520 to 540 nm) to the orange-red region (595 to 600 nm) (Fig. 2). Meanwhile, the intensity of stationary x-ray luminescence decreased, room temperature phosphorescence is greatly reduced, and the thermally stimulated luminescence light sum becomes at least an order of magnitude less. High temperature glow peaks are reduced by a greater amount than low temperature ones. The PSL yield however can sometimes increase. The stimulation spectrum measured during the process of the successive annealing of peaks of the glow curve revealed that there is no direct relationship between the characteristics of thermally and optically stimulated luminescence (38). This is confirmed by the fact that optical stimulation scarcely affects thermoluminescence or phosphorescence (47, 49). A study of the infrared emission of F-centers (53) has confirmed the existence of two types of electron traps in BaFBr:Eu: those participating and those not participating in PSL. V. LATTICE DEFECTS IN BaFBr:Eu AND THE PSL MECHANISM
With the assumption that both Frenkel and Schottky thermal disordering of the • BNaFBr lattice can occur, as well as antisite disordering, the point defects V11 Ba, VF, • 1 1 VBr, Fi , Bri , FBr, and EuBa must be considered in the BaFBr:Eu phosphor (17, 31, 53, 54). Computer simulation of the defects in these crystals (55) permits the enthalpy and entropy of BaFBr lattice thermal disordering (31) to be evaluated. A thermodynamic analysis based on these data (54) has shown that among effectively charged • point defects relative to the normal lattice are VBr and F1i. These defects dominate at the phosphor preparation and annealing temperatures. At 5507C their concentration is an order of magnitude smaller than that at 8007C. Equilibrium in this case can be represented by the approximate equation: • [F1i ] É [VBr ].
[2]
A comparison of the stimulation spectrum with the spectra for EPR signal reduction • under stimulation (5) reveals however that the vacancies VBr are responsible only for the long wavelength part of the stimulation spectrum with a maximum at 595 to 600 nm. The short wavelength region which dominates in the spectra from non-annealed phosphors (Fig. 2) is due to VF• . This apparent inconsistency between the calculations and the experiments becomes less significant if we take into account the annealing effect on the thermo- and photostimulated luminescence of BaFBr:Eu (39, 54) which was discussed earlier. The annealing (as well as cooling in the case of ZnS:Eu-electrophosphor (30, 56) may lead to the segregation of point defects at the regions of dislocations and grain boundaries. It has already been mentioned in Section 3 that dislocations are more likely to be found at the edges of two adjacent F2 and Ba layers. Such dislocations can generate the vacancies VF• as well as F11 Ba, which will remain in the vicinity of the extended • defect. Annealing of the phosphor results in the diffusion of the VBr vacancies which
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are dominant in the bulk of the crystal, of interstitial ions F1i , and impurity defects EuBa to the regions of dislocations and grain boundaries. This process is accompanied by the interaction between F1i and VF• , F1i / VF• Å FF,
[3]
• ]/[VF• ]. If we now assume which leads to an increase of the concentration ratio [VBr that PSL processes take place mainly in the regions of linear and surface defects, while thermally stimulated luminescence is controlled by the processes in the normal lattice, then all the data concerning annealing become understandable. This includes (i) the PSL increase in the long wavelength part of the stimulation spectrum, attributed • to VBr ; (ii) the absence of correlation between thermally stimulated luminescence, which is suppressed by annealing, and PSL, which may even increase after annealing; and (iii) the reduction of the afterglow as well as of the stationary luminescence yield. In addition, some results indicative of the tunneling mechanism associated with PSL also can be understood on this basis. This description also provides an explanation for the small effect of the temperature on the PSL decay time following stimulation by short light pulses (16, 97) (the same behavior is observed for phosphors of the K1:T1 type (8)) and also the absence of the photoconductivity under stimulation in the wavelength region related to VF centers (57). It also follows that the segregation of point defects in regions of linear and surface defects results in the roughly adjacent location of emission centers (EcBa) and anion vacancies, increasing the probability of electron tunneling between them (39). PSL related to VBr centers is accompanied by photoconductivity whose excitation spectrum is similar to the stimulation excitation spectrum. This should be regarded as an indication of the coexistence of two mechanisms for PSL stimulation: a tunneling mechanism and one associated with the transfer of electrons through the conduction band (39). Recent results on the temperature dependence of PSL for BaFBr:Eu single crystals (90, 92) confirm this assumption. It has been demonstrated that in powder samples the contribution of tunneling to the PSL is more pronounced than in single crystals (92). This may be accounted for by the segregation of point defects to regions of dislocations and subboundaries. At the same time, in thermally stimulated luminescence and room temperature phosphorescence processes the conduction band is always involved and the processes are mainly linked to the defects in the normal lattice. Stimulation spectra measured after the successive excitation of the glow peaks showed that annealing in this manner increases the PSL yield and the contribution of the short wavelength region of the stimulation spectrum (39). This can be accounted for by the redistribution of the electrons between the thermally stimulated luminescence traps and those accounting for the SPL. An identical explanation has been given to the phenomenon of the partial reduction of the PSL image after the complete optical read-out of the phosphors (49). The kinetics of this process are described by the equation
q
IPSL Å k t,
[4]
where t is the time after optical stimulation has ceased. This result [4] is indicative
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of the association of a diffusion mechanism with electron redistribution. It should be noted that the proposed PSL model, based on the radiative creation of F centers as a result of F–H pair creation by the decay of the impurity exciton (16, 34), does not fully account for all the experimental results. For example, effects of cooling and annealing on the stimulation spectrum and on the PSL yield in particular remain to be explained. Closer inspection of the stimulation spectrum reveals that it has a complex structure which cannot be reduced to two bands (17, 39). This could be a consequence of the fact that the F centers which determine the shape of the spectrum can differ not only in terms of anion origin (VF, VBr) but also in terms of the nearest neighbors, and in particular, due to their association with antisite defects of the FBr type (17, 58) or with the activator (92). The splitting of the excited p-state of F centers is also expected to manifest itself (20, 59). M-centers, or pairs of anion vacancies with a trapped electron, can also be formed (92). A comparison of the excitation spectrum for the infrared emission from color centers with the stimulation spectrum suggests (92) that the 445 and 680 nm bands are associated with nonphotostimulable FA and MA centers located in the vicinity of the activator. The study of BaFCl phosphors doped by Eu2/ or Sm2/ revealed a considerable change in their properties upon the inclusion of excess BaF2 (36, 37, 43), e.g., the enhancement of phosphorescence. There are indications that excess BaF2 affects the properties of BaFBr:Eu as well, by increasing the contribution due to VBr-related bands in the stimulation spectrum (10). This behavior can be linked to intercrystal reactions of the following types: • FBr Å VBr / F*i • F
[5]
• Br
FBr / V Å FF / V .
[6]
Equation [5] can be seen to represent Frenkel anti-site disordering (49). It seems to • be especially important because the main lattice defects VBr and F*i participate in it. We have therefore reached the conclusion that the preparative conditions must be rigorously controlled and in particular those which effect the BaF2 concentration. Great care should be taken when annealing the BaFBr:Eu phosphor (49, 60). VI. THE STATIONARY LUMINESCENCE, PHOSPHORESCENCE, AND PSL EXCITATION SPECTRA OF BARIUM FLUOROHALIDES
The spectra of stationary luminescence, phosphorescence, and the PSL excitation spectra for barium and strontium fluorohalides have been studied across a wide energy range in a series of papers (12, 34, 36–38, 60–65). They are of interest both from a scientific and a technological point of view. The long wavelength threshold of the phosphorescence excitation spectrum was used to determine the energy gap (Eg) of BaFcl (12) and it has been found to be 8.5 eV (38, 41, 66). For E õ Eg the yield of stationary luminescence depends on the emission centers themselves, resulting in differences between phosphor materials undoped or doped by Eu2/ or Sm2/ (36, 38). The yield is also affected, at least in the case of Eu-doped phosphors, by the oxidizing and reducing properties of the annealing atmosphere (36).
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FIG. 3. Normal (steady state) luminescence excitation spectra for: (a) non-doped BaFCl; (b) BaFCl:1002Eu; (c) BaFCl:1002Sm; (d) non-doped BaFBr; (e) BaFBr:1002Eu; (f) BaFBr:1002Sm. In each case, the number in front of the dopant element defines its concentration in g atom/mol in the charge used to prepare the phosphor. The ordinate represents the quantum yield relative to sodium salicylate.
Thus, the excitation spectrum of BaFCl:Eu (2 1 1002), prepared in a CO atmosphere, has an intense broad impurity band extending from 3.3 to 5.4 eV with evidence of slight splitting into two bands related to the 5d(2eg) and 5d(2t2g) states of the Eu2/ ion. Annealing in air between 6507 and 7007 suppresses the Eu2/ band although the intensity of stationary x-ray luminescence may increase weakly (24). A small peak at 318 nm on the low energy side of the excitation band is preserved, while on the high energy edge features appear related to the 2t2g-state which we attribute to electronic transitions from the 4 f 7 level to the spin-orbit split levels of 4 f 6(8F7)5d(2t2g). Annealing in CO suppresses these features because of the Eu reduction at the phosphor surface, where Eu2/ ions can achieve a different nearest neighbor arrangement (36). The features in the excitation region beyond the Eu intercenter transition and up to Eg are attributed to Eu2/ ionization and G-excitons at 8.4 eV for BaFCl and 7.4 and 7.9 eV for BaFBr (37, 62, 66), related to the 4p-states of the bromine ion. The stationary luminescence yield for E ú Eg is essentially unaffected by impurities and the excitation spectra of BaFCl and BaFBr are largely similar in this range (Fig. 3). Their most remarkable feature is a two-peak maximum between 16 and 20 eV for BaFCl and between 15 and 19.5 eV for BaFBr. These features can be attributed to
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FIG. 4. Excitation spectra of (a) steady state luminescence and (b) PSL for BaFBr:1004Eu phosphor, annealed at 5507C.
the combined effect of a plasmon and core exciton created by the excitation of electrons from the 5p-shell of Ba2/. An increase in the absorption coefficient linked to the creation of the exciton results in the increase of surface losses which then appear as a narrow dip at around 18.1 to 18.3 eV. The effect of photon multiplication produces a tendency to a constant energy yield rather than an increase of h, the quantum yield (37, 38). Between 20 and 30 eV, h already approaches values characteristic for x-ray excitation, giving 11–12% for BaFCl:1002Eu and 16–18% for BaFBr:Eu (38, 39). Annealing BaFBr:Eu at 5507 results in a reduction in the stationary luminescence yield by a factor of two for E ú 11 eV. At about 10 eV an intense band appears (Fig. 4a) which is weak in nonannealed phosphors. It can be attributed to the creation of clusters of FBr, since 10 eV is the onset energy for the creation of excitons associated with the 2p-state of F0 ions (34). The PSL excitation spectra for BaFBr:Eu have been measured independently by two groups under somewhat difference conditions (34, 48, 60–62). For E ú 11 eV the spectrum mainly reproduces the features of the stationary luminescence excitation (see Fig. 4) while the principle differences are noted for E õ 11 eV. Thus the maximum at 10 eV which is observed in the stationary luminescence excitation spectra of annealed phosphors does not appear in their PSL excitation spectra. A characteristic feature of PSL excitation spectra is a sharp peak at 7 eV. According to the two different PSL mechanisms presented above, it can be attributed either to an electron transition from Eu2/ to an adjacent anion vacancy or to the creation of F–H pairs by
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exciton decay (34, 48). It is important that the onset of phosphorescence excitation is linked to larger energies of about 8 eV (48). VII. EXPERIMENTAL MEASUREMENTS OF PHOSPHOR TIME PROFILES AND EFFICIENCIES
An experimental setup was made in the detector development laboratory at Daresbury to measure the time profiles and efficiencies of phosphors and scintillators. Initially the apparatus was to be used to measure the PSL from phosphors used in commercial image plates. It was designed to allow x rays and the stimulating light to illuminate the sample simultaneously, whilst being viewed by a photomultiplier tube to measure the PSL. The sample stage is comprised of a light-tight aluminum cube with three orthogonal ports. The x-ray illumination enters through one port via a 0.5-mm beryllium window. The second port has a photomultiplier and filter assembly which allows the luminescence to be measured at specific wavelength ranges. The third port allows the stimulating radiation to fall on the sample. In the initial experiments this was the light from a He–Ne laser. The sample holder is a tetragonal block with one surface inclined at 457 to all ports. The inside of the vessel is treated with anti-reflectance paint to reduce indirect illumination of the sample by scattered laser light. The vessel was installed in an x-ray safety hutch with a fixed anode x-ray generator. The beam from the x-ray head was accurately aligned with the beryllium window in the sample vessel. Two targets were used in the tests, copper which produces 8 keV photons and molybdenum which produces 17.3 keV. The image plate samples were the BaFBr:Eu type laid down on the substrate in a variety of ways. A 35-mW He– Ne laser was used to produce the stimulating radiation which was appropriate for this phosphor. The laser was mounted on the top of the x-ray hutch on an optical bench with a series of mirrors and an opto-acoustical modulator which was used to switch the light. The mirrors allowed a long optical path from the modulator head to the sample to gain the most benefit from the ability of the modulator to deflect the laser beam through small angles. The beam was reflected at right angles down into the sample vessel in the hutch through a radiation-safe and light-tight tube. The output of the phosphor was measured using the photomultiplier tube, an amplifier, and a digital storage oscilloscope (DSO). The modulator switching speed is 30 ns for both deflection and intensity modulation. The extinction ratio for the modulation between fully on and fully off is 30 dB. This was inadequate for some of the experiments, so the deflection modulation was used to switch the beam out of the path. This provided a high extinction ratio, but with a slower leading edge to the laser switching function. During the time profile measurements a pinhole mask was placed over the sample. This limited the illuminated area to the central part of the laser intensity profile, ensuring a uniform illumination and improving the speed of the on/off switch of the stimulating radiation when the deflection modulation was used. A high-speed x-ray shutter was mounted on the x-ray head to provide accurate dose control on the phosphors. For the time-profile measurements the phosphor was beached before insertion into the sample vessel. A fixed dose of x-ray radiation was given and then the time profile
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of the PSL was captured on the DSO as the modulator switched the laser light. For the efficiency measurements, a standard sample of commercial phosphor was used to compare the test samples. The best samples produced ‘‘in house’’ were of the same efficiency as the standard. The PSL rise time of 800 ns as measured by von Seggern and others (93) was confirmed by our measurements. VIII. APPLICATIONS OF LUMINESCENT IMAGE PLATES
The wide dynamic range, low noise, and good spatial resolution of luminescent image plates have opened up new opportunities for x-ray diagnostics, x-ray crystallography, and x-ray imaging, and in X-ray diffraction and scattering studies, particularly those making use of synchrotron radiation. There are already reports on their application to the investigation of lungs, breasts, and other organs (67–71). The radiation background has also been measured although it should be noted that the barium salts which are used as phosphor matrices normally contain a small amount of radium. Wide spread application of the technique in medicine is restricted to some extent, however, by technical and financial problems (57, 75). A series of papers (76–82) have been dedicated to the application of image plates for diffraction pattern measurements of biological specimens using synchrotron radiation. High resolution patterns can be obtained within approximately one-tenth of the exposure time required using conventional methods (76, 79, 80). Image plates have been suggested for use in conjunction with transmission electron microscopy (83), for the detection of all types of electromagnetic radiation extending from the VUV (38, 84) and up to the S-ray bremsstruhling region for electron accelerators (52) and also for particles (84). It can also be exploited as a beam position monitor and to obtain data on the dose distribution in the irradiated body for radiation therapy. Image plates may have applications in astrophysical research, including x-ray astronomy, which would gain substantially from their wide dynamic range. The many and various applications of image plates have promoted a study of those parameters which define the quality of images and of the threshold flux density required for the detected radiation (76, 85–87). As expected, the decisive role is played by the efficiency of the initial energy conversion process, noise, frequency-contact characteristics, dynamic range, and other kinetic parameters (88). Of course, image plates and their associated readout systems have a number of specific limitations (5, 85). These include scattering of the stimulating radiation (9), signal fading (11, 91), and fluctuations of light level (luminescence noise) (85), which are determined by the properties of the phosphors in use. This final factor makes a substantial contribution to the total noise at low levels of PSL and when there are large light losses on the way from the screen to the photomultiplier. Nevertheless, the preparation and exploitation of high-efficiency phosphors for image plates is helping to make an important input in many areas of x-ray research. REFERENCES 1. V. V. ANTONOV-ROMANOVSKI, V. L. LEVSHIN, Z. L. MIRGENSHTERN, An SSR 54, 19 (1946) [in Russian].
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2. V. L. LEVSHIN, N. V. MITROFANOVA, YU. P. TIMOFEEV, S. A. FRIDMAN, AND V. V. STCHAENKO. Proc. Phys. Inst. AN SSSR 59, 64 (1972) [in Russian]. 3. S. W. S. MCKEEVER, ‘‘Thermoluminescence of Solids,’’ Cambridge Univ. Press, Cambridge, 1985. 4. M. SONADA, M. TAKANO, J. MIYAHARA, AND H. KATO, Radiology 148, 833 (1983). 5. A. M. GURVICH, M. G. MYAGKOVA, AND YU. RU¨DIGER, Med. Technol. 27(3), (1990) [in Russian]. 6. K. TAKAHASHI, J. MIYAHARA, AND Y. SHIBAHARA, J. Electrochem. Soc. 132, 1492 (1985). 7. H. VON SEGGERN, T. VOIGT, AND K. SCHWARZMICHEL, Siemens Forsch:u.Entwickl.-Ber. 17(3), 125 (1988). 8. G. I. VLASOV, R. A. KALNYNSH, L. E. NAGLI, V. P. OB’EDKOV, I. K. PLYAVIN, AND A. K. TALE, Avtometriya 66(1), (1980) [in Russian]. 9. A. MEIJERINK, ‘‘Luminescence of New X-Ray Storage Phosphors,’’ Drukkerij Elinkwijk B V, Utrecht, 1990. 10. K. TAKAHASHI AND T. NAKAMURA. U.S.A. Patent No. 4 535 237, 13.1283/13.08.85. 11. J. DEGENHARDT, FRG Patent No. (OS) DE 3 347 207 A1, 27.12.83/11.07.85. 12. A. M. GURVICH, M. A. IL’INA, V. P. KAVTOROVA, L. I. KONSTANTINOVA, N. I. LEONOVA, M. G. MYAGKOVA, AND T. I. SAVIKHINA, J. Prikl. Sektr. 28, 765 (1983) [in Russian]. 13. A. MEIJERING, G. BLASSE, AND M. GLASBEEK, J. Phys.: Condens. Mater. 2, 6303 (1990). 14. A. MEIJERING AND G. BLASSE, Mater. Chem. & Phys. 21, 261 (1989). 15. J. MIYAHARA, in ‘‘Computed Radiographie’’ (Y. Tateno, T. Linuma, and M. Takano, Eds.), p. 7, Springer-Verlag, Berlin, 1987. 16. H. VON SEGGERN, T. VOIGT, W. KNU¨PFER, AND G. LANGE, J. Appl. Phys. 64, 1405 (1988). 17. A. M. GURVIC, W. P. KAVTOROVA, M. G. MYAGKOVA, J. RUDIGER, AND V. D. CERNOVSKI, ‘‘Proceedings, 2nd International Mtg. on Luminescence, ‘35 Years of Luminescence,’ Greifswald, 1989,’’ p. 14, Ernts-Moritz-Arndt-Universitat Greifswald. 18. A. M. GURVICH, ‘‘Ro¨ntgenleuchtstoffe und Ro¨ntgenlumineszenzbildwandler,’’ Akademische Verlagsgesellschaft Geest & Portig K.-G., Leipzig, 1988. 19. E. NICKLAUS AND F. FISCHER, Phys. Status Solidi B 52, 453 (1972). 20. M. YUSTE, L. TAUREL, M. RAHMANI, AND D. LEMOYNE, J. Phys. Chem. Solids 37, 961 (1976). 21. S. LEFRANT AND A. H. HARKER, Solid State Comm. 19, 853 (1976). 22. K. SOMAIAH, P. VEERESHAM, K. L. N. PRASAD, AND V. HARI BABU, Physica Status Solidi A 56, 737 (1979). 23. A. L. N. STEVELS AND F. PINGAULT, Philips Res. Rep. 30, 277 (1975). 24. A. M. GURVISH, R. V. KATOMINA, AND N. P. SOSTCHIN, Izv. AN SSR, Ser. Phys. 41, 1372 (1977) [in Russian]. 25. J. L. SOMMERDIJK, J. M. P. VERGSTEGEN, AND A. BRIL, J. Luminesc. 8, 502 (1974). 26. M. SAUVAGE, Acta Crystallogr. B 30, 2786 (1974). 27. H. P. BECK, J. Solid State Chem. 17, 275 (1976). 28. D. LEMOYNE, J. DURAN, M. JUSTE, AND M. BRILLARDON, J. Phys. C: Solid State Phys. 8, 1455 (1975). 29. F. A. KRO¨GER, ‘‘The Chemistry of Imperfect Crystals,’’ North-Holland, Amsterdam, 1964. 30. A. M. GURVICH, ‘‘Introduction to Physical Chemistry of Crystal Phosphors,’’ Vysshaya Shkola, Moscow, 1972; 2nd ed., 1982 [in Russian]. 31. R. C. BAETZOLD, Phys. Rev. B 36, 9182 (1987). 32. K. A. KALDER, ‘‘Electronic Excitation in AIIB VII,’’ Ph.D. thesis, Tartu, 1972. 33. M. K. CRAWFORD, L. H. BRIXNER, AND K. SOMAIAH, J. Appl. Phys. 66(8), 3758 (1989). 34. H. H. RUTER, H. VON SEGGERN, R. REININGER, AND V. SAILE, Phys. Rev. Lett. 65, 2438 (1990). 35. J. L. SOMMERDIJK, J. M. P. VERSTEGEN, AND A. BRIL, J. Luminesc. 8, 502 (1974). 36. A. M. GURVICH, V. B. GUTAN, M. A. IL’INA, V. P. KAVTOROVA, R. V. KATOMINA, N. G. MYAGKOVA, AND T. I. SAVIKHINA, Opt. i Spektrosk. 52, 289 (1982) [in Russian].
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37. A. M. GURVICH, M. A. IL’INA, V. V. MIKHAILIN, M. G. MYAGKOVA, B. V. RYBAKOV, AND M. A. TEREKHIN, J. Prikl. Spektr. 49, 246 (1988) [in Russian]. 38. A. M. GURVICH, V. V. MIKHAILIN, M. G. MYAGKOVA, AND M. A. TEREKHIN, Vest. Moskov. Univ. Ser. 3 Phys. Astron. 31, 44 (1990) [in Russian]. 39. A. M. GURVICH, N. G. MYAGKOVA, RU. RU¨DIGER, AND V. P. KAVTOROVA, J. Prikl. Spectr. 53, 56 (1990) [in Russian]. 40. L. H. BRIXNER, Mater. Chem. Phys. 16, 253 (1987). 41. K. TAKAHASHI, K. KOHDA, J. MUYAHARA, Y. KANEMITSU, K. AMITANI, AND S. SHIONOYA, J. Luminesc. 31/32, 266 (1984). 42. D. M. DE LEEUW, T. KOVATS, AND S. P. HERKO, J. Electrochem. Soc. 134, 491 (1987). 43. Research Disclosure No. 165, 17 (1978). 44. A. N. CAMPBELL AND A. J. R. CAMPBELL, Trans. Faraday Soc. 35, 241 (1939). 45. A. L. N. STEVELS, A. D. M. SCHRAME-DE PAUW, AND F. PINGAULT, USA Patent No. 4 157 981, 3.04.78/ 12.06.79. 46. M-Z. SU, X-L. XU, S-K. RUAN, AND M-L. GONG, J. Less-Common Metals 93, 361 (1983). 47. A. M. GURVICH, S. I. GOLOVKOVA, M. A. IL’INA, M. G. MYAGKOVA, L. A. BENDERSKI, E. A. BLYAKHMAN, AND O. A. SLEPYSHEVA, ‘‘Luminescent Detectors and Converters of Ionising Radiation,’’ p. 72, Nauka, Novosibirsk, 1985 [in Russian]. 48. A. N. BELSKY, A. M. GURVICH, I. A. KAMENSKIKH, V. V. MIKHAILIN, M. G. MYAGKOVA, AND M. A. TEREKHIN, Nucl. Instrum. Methods (A) 308, 190 (1991). 49. A. M. GURVICH, M. G. MYAGKOVA, J. RUDIGER, AND V. P. KAVTOROVA, ‘‘Abstracts of International Symposium on Luminescent Detectors and Converters, Riga 1991,’’ in press. 50. B. YE, J-H. NIN, AND M-Z. SU, J. Luminesc. 40/41, 323 (1988). 51. A. M. GURVICH, V. P. KAVTOROVA, M. G. MYAGKOVA, J. RU¨DIGER, AND V. D. CHERNOVSKY, ‘‘International Symposium, ‘Luminescent X-Ray Screens’, Ab. Moscow, 1989,’’ p. 29. 52. A. M. GURVICH, R. S. MIL’STEJN, M. G. MYAGKOVA, S. G. GOLOVKOVA, J. RU¨DIGER, AND V. P. KOVTOROVA, Radiation Protection Dosimetry 34, 265 (1990). 53. R. C. BAETZOLD, J. Phys. Chem. Solids 50, 915 (1989). 54. A. M. GURVICH, ‘‘Abstracts All-Union Luminescence Conference dedicated to the centenary of S. I. Vavilov,’’ p. 20 Lebedev Physics Institute, Moscow, 1991; Izv. AN SSSR, Ser. Phys. 56, 139 (1992) [in Russian]. 55. C. R. CATLOW, M. DIXON, AND W. C. MACKRODT, ‘‘Computed Simulation of Solids,’’ Springer-Verlag, Berlin, 1982. 56. O. N. KAZANKIN, Sb. Trudov Gos. Inst. Prikl. Khim. 53, 12 (1966) [in Russian]. 57. Y. IMABUCHI, CH. UMEMOTO, K. TAKAHASHI, AND S. SHIONOYA, in ‘‘Abstracts, International Conference on Luminescence, Lisbon, 1990,’’ p. 158. 58. A. M. GURVICH, V. P. KAVTOROVA, M. G. MYAGKOVA, J. RU¨DIGER, D. STARICK, AND G. GERZOG, ‘‘Abstracts, International Symposium on Imaging Systems—150 Yrs Photography, Dresden, 1989,’’ p. 179. 59. X-P. SUN AND M-Z. SU, J. Luminesc. 40/41, 171 (1988). 60. A. M. GURVICH, M. G. MYAGKOVA, YU. RU¨DIGER, A. N. BELSKY, I. A. KAMENSKIKH, V. V. MIKHAILIN, AND M. A. TEREKHIN, ‘‘Proc. Second International Conference on Rare Earths, Development & Applications, Beijing, 1991,’’ Vol. 2, p. 730. 61. H. H. RU¨TER, H. VON SEGGERN, R. REININGER, AND V. SAILE, HASYLAB Jahresbericht 149 (1989). 62. H. H. RU¨TER, H. VON SEGGERN, R. REININGER, AND V. SAILE, ‘‘Abstracts, International Conference on Luminescence, Lisbon, 1990,’’ p. 629. 63. V. I. ALEKSEEV, A. N. VASIL’EV, A. M. GURVICH, et al., preprint of P N Lebedev Phys. Inst. AN SSSR 20, (1988) [in Russian]. 64. I. M. TERNOV AND V. V. MIKHAILIN, ‘‘Synchrotron Radiation, Theory, & Experiment,’’ Energoatomizdat, Moscow, 1986 [in Russian].
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65. V. V. MIKHAILIN AND M. A. TEREKHIN, Nucl. Instrum. Methods A 282, 607 (1989). 66. E. NICKLAUS, Phys. Status Solidi A 53, 217 (1979). 67. R. D. MU¨LLER, V. JOHN, M. VOSS, E. LOHR, M. HEER, AND S. LEMANN, ‘‘Abstracts, International Symposium, ‘Luminescent X-Ray Screens,’ Moscow, 1989,’’ p. 27. 68. L. R. GOODMAN, C. R. WILSON, AND W. D. FOLEY, Amer. J. Roentgenol. 15, 1241 (1988). 69. B. SCHWERMER, G. WITTE, R. P. SPIELMANN, V. NOCOLAS, AND T. FASSBEND, Fortschr. Rontgenstr. 153, 161 (1990). 70. E. BOIJEN, ‘‘Abstracts, Charite-Symposium ‘CT and NR in Medicine,’ Berlin, 1988,’’ p. 130. 71. B. BERGH AND W. DOHRING, ‘‘Digitale Radiographie, Referate und Vortra¨ge,’’ p. 11, Schnetztor Vortra¨ge Verlag, Konstanz, 1989. 72. K. F. NEUFANG, B. KRUG, K. LORENZ, AND N. SCEINBRICK, Fortschr. Rontgenstr. 152, 501 (1990). 73. R. M. WILENZICK, C. R. B. MERRITT, AND S. BALTER, Med. Phys. 14, 389 (1987). 74. D. GUR, M. DEUTSCH, C. R. FUHRMANN, P. A. CLAYTON, J. C. WAISER, M. S. ROSENTHAL, AND A. G. BUKOVITZ, Med. Phys. 16, 132 (1989). 75. R. SCHITTENHELM, Electromedica 54, 72 (1986). 76. J. MIYAHARA, K. TAKAHASHI, Y. AMEMIYA, N. KAMIYA, AND Y. SATOW, Nucl. Instrum. Methods A 266, 645 (1988). 77. Y. AMEMIYA, N. KAMIYA, AND J. MIYAHARA, Oyo Buturi 55, 978 (1986) [in Japanese]. 78. N. KAMIYA, Y. AMEMIYA, AND J. MIYAHARA, J. Crystall. Soc. Japan 28, 350 (1986). 79. Y. AMEMIYA, T. MATSUSHITA, A. NAKAGAWA, Y. SATOW, J. MIYAHARA, AND H. CHIKAWA, Nucl. Instrum. Methods A 266, 645 (1988). 80. Y. AMEMIYA, K. WAKABAYASHI, H. TANAKA, Y. UENO, AND J. MIYAHARA, Science 237, 164 (1987). 81. R. B. WHITING, J. F. OWEN, AND B. H. RUBIN, Nucl. Instrum. Methods A 266, 628 (1988). 82. D. BILDERBECK, K. MOFFAT, J. OWEN, B. RUBIN, W. SCHILDKAMP, D. SZEBENYI, T. B. SMITH, K. VOLZ, AND B. WHITING, Nucl. Instrum. Methods A 266, 636 (1988). 83. N. MORI, T. OIKAWA, T. KATOH, J. MIYAHARA, AND Y. HARADA, Ultramicroscopy 25, 195 (1988). 84. J. MIYAHARA, J. Soc. Instrum. Control Eng. 26, 657 (1987) [in Japanese]. 85. W. HILLEN, U. SCHIEBEL, AND T. ZAENGEL, Med. Phys. 14, 744 (1987). 86. H. FUKITA, K. UESA, J. MORISHITA, T. FUJIKAWA, A. OTSUKI, AND T. SAI, Med. Phys. 16, 52 (1989). 87. K. KLIENGENBECK AND B. CONRAD, Siemens Forsch. und Entwickl. Ber. 16, 192 (1987). 88. A. M. GURVICH, ‘‘Fundamental Physics of Radiation Control & Diagnostics,’’ Ergoatomizdat, Moscow, 1989 [in Russian]. 89. J. D. NEWALL AND CH. A. KELSEY (Eds.), ‘‘Digital Imaging in Diagnostic Radiology,’’ Churchill Livingstone, New York, 1990. 90. T. HANGLEITER, J. K. KOSCHNICK, J. M. SPAETH, et al., J. Phys. Condens. Matter 2, 6837 (1990). 91. D. STARICK, A. M. GURVICH, M. G. MJAGKOVA, et al., ‘‘Abstracts, 3rd International Mtg. on Luminescence, Trassenheide, 1991,’’ p. 16. 92. M. THOMS, H. VON SEGGERN, AND A. WINNAKER, Phys. Rev. B 44, 9240 (1991). 93. H. VON SEGGERN, Cryst. Lattice Defects 18, 399 (1989). 94. A. M. GURVICH AND V. V. MIKHAILIN, Uspekhi Khim. 61, 1047 (1992) [in Russian].
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Phosphors for Luminescent Image Plates A. M. GURVICH,* C. HALL,† I. A. KAMENSKIKH,‡ I. H. MUNRO,† V. V. MIKHAILIN,‡ AND J. S. WORGAN† *Moscow Research Institute for Roentgenology and Radiology, 117837 Moscow, Russia; †EPSRC, DRAL, Daresbury Laboratory, Warrington, Cheshire WA4 4AD, United Kingdom; and ‡Moscow State University, 117234 Moscow, Russia Received June 25, 1993; revised May 10, 1995 This is a review of the properties of some photostimulable phosphors for luminescent image plates as applied to digital radiography. In particular, the properties of BaFBr:Eu and other barium fluorohalides that are useful for this application are considered. The main emphasis of the review is on the effect of the preparative conditions and the origin of the photostimulated luminescence and its features under VUV excitation. q 1996 Academic Press, Inc.
I. INTRODUCTION
Phosphors that can store energy and release it as light or photostimulated luminescence (PSL) when subsequently illuminated with IR or visible light have been known for a long time. They have been applied extensively in dosimetry and in the visualisation of IR (1–3). Computers and lasers have enabled the range of possible applications of photostimulable phosphors to be extended, leading to the creation of luminescent digital radiography (LDR) (4). The distinguishing feature of this technique is the use of an image screen covered by a photostimulable phosphor instead of a photographic (radiographic) film or by its combination with an intensifying screen. Processing of the image is achieved by scanning the screen with a focused helium–neon laser beam. The photostimulated light flashes thus produced are converted by a photomultiplier into a sequence of electric pulses which, after digitization, are recorded on an optical disc or a magnetic tape. After computer processing, the information is again converted into analogue signals which modulate the intensity of an electron beam or a laser scanning over a photofilm. In the most recent Fuji systems, the same laser is used for scanning over both the image plate and the photo film. Among the advantages of the LDR technique (5) compared to conventional film radiography is its larger dynamic range (6, 7). This results in a wide range of contrast variation (1) and makes the choice of the exposure less crucial. The idea of LDR has been around for many years (see e.g. (1)) but its practical realization has had to wait until phosphors meeting all the requirements could be identified. II. THE SELECTION OF PHOSPHORS FOR IMAGE PLATES
Phosphors for image plates are expected to possess the following properties (5, 9): (i) a high energy yield of PSL at room temperature to minimize the noise contribution 48
0895-3996/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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due to photon intensity fluctuations; (ii) the emission and excitation bands must be well separated; (iii) the time decay of light flashes stimulated by the laser should be fast enough to provide an acceptable read-out time and to minimize the overlap of signals from different areas of the image; (iv) long term storage of the absorbed energy (minimal fading) is important; (v) the PSL intensity should be linear with the irradiation dose since this requirement will determine the achievable dynamic range; (vi) the absorption of ionizing radiation must be sufficiently high to reduce the contribution of quantum noise and to minimize the doses required for high quality images. A large number of phosphors which exhibit PSL have been studied. They include the sulfides of zinc and alkaline earth metals, e.g., SrS:Ce,Sm and SrS:Eu,Sm (1, 15); halides of alkali metals (8, 11), especially RbBr:T1 (7, 9); oxides (ThO2:Er) and oxysulfides (La2O2S:Eu,Sm) (10); fluorohalides of alkaline-earth metals (4, 12); several sulfates, borates, and silicates (9, 11, 13); galloborates, gallosilicates, gallophosphates, and gallogermanates of alkaline earth metals, especially Ba5SiO4Br:Eu (9, 14). Barium fluorohalides (BaFHal:Eu) proved to be the most efficient from the viewpoint of LDR and their PSL properties (4, 12) have been widely studied. The most favorable combination of properties is shown by BaFBr9Eu (10, 15–17) which is now used in commercial image plates (15). Its PSL energy yield amounts to 5% (17); the emission (lmax Å 390 nm) and excitation (lmax Å 590–600 nm) bands are well separated; the PSL decay time is approximately t Å 0.8 ns6; its fading characteristics are good, less than 25% is lost in 8 h (15); the presence of barium in the composition of the phosphor ensures a high absorption cross section for x rays (18). Attempts to further improve the properties of BsFBr:Eu have been made, for example, by the partial substitution of Br by Cl or I (11). III. THE STRUCTURE AND PRINCIPAL PROPERTIES OF BARIUM FLUOROHALIDES
BaFCl was the first material to attract the attention of physicists as a subject for the study of color centers (19 – 22) and as an efficient (when doped by Eu) phosphor for x-ray intensifying screens (23, 34). Also, the high efficiency of photoand cathodoluminescence of MeFHal:Eu2/-phosphors (Me|Sr, Ba; Hal|Cl, Br) have been noted (25). Fluorohalides of barium and strontium have a layered tetragonal structure of the PbCl-type (26, 27) which can be projected as a series of layers F2 MeHalMe . . . , situated along the [001] directors (Fig. 1). The fluorine symbol, F2, means that the number of ions in the F0 layer is twice that in the Hal or Me layers. It can be seen, q 0 from Fig. 1, that the distance between neighbouring F ions is 2 times less than the distance between Br0 or Ba/ ions in the appropriate layers. There are two types of anions in the barium fluorohalides and hence there exist two different types of color (F) centers. These are, for example, a fluorine vacancy with a trapped electron (VF) or a chlorine vacancy (VCl) in BaFCl or vacancies of fluorine (VF) and bromine (VBr) in BaFBr. A subscript associated with the vacancy (V) indicates the lattice site it occupies; an interstitial site is shown as (i). Absorption bands related to these F centers have been observed experimentally. According to EPR data (20) and to theoretical calculations (21) it is the long wavelength band which is related to the VF centers, which are frequently indicated as F(F0). Another
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FIG. 1. Ions in the crystal of BaFBr. The symbols F0, Ba2/, Br0 stand for the layers of F0, Ba2/, and Br0 ions, respectively.
band attributable to VBr centers falls within the same region (28). It has been shown that additive coloring results from the creation of F0 centers only of the VF type (29). This can be accounted for by the penetration of surplus Ba into the bulk of the crystal, along the dislocations which appear at the edge of two adjacent F2 and Ba layers and terminate in the bulk of the crystal. Their creation at the layer edges is less probable because of unfavorable geometry. A study of the luminescent properties of the BaCl2 –BaF2 system (12) revealed that an excess of BaF2 created a solid solution in BaFCl. This means that Cl0 ions are partially substituted by F0 ions with the creation of anti site defects, FCl (17). Such • • defects are expected to exist also in BaFBr (30). Their association with VCl (VBr ) leads to the creation of F-centers that are somewhat different from the isolated ones (17) where the superscript dot represents an effective position charge relative to the lattice and a dash represents negative charge. Barium fluorohalides show luminescence which is not related to the impurities. As in the case of other halide phosphors (31) it is attributed to the recombination of electrons with Vk centers (31–34) (i.e., molecular centers, Hal20, created by the capture of a hole by two adjacent halide ions). Since there are two types of halide anions in the crystal, at least the same number of different types of Vk centers can exist there, e.g., F20 and Cl20 in BaFCl. F20 is associated with the short wavelength emission band which is observed only at low temperatures in
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FIG. 2. Emission (curve 1) and stimulation (curves 2, 3) spectra of BaFBr:1004Eu phosphor prepared by heating to 8007C without (2) and with (3) additional annealing at 5507C, measured at 207C. The phosphor mix was initially prepared with a concentration of 1004 g-atom/mol of Eu. The abscissa represents the wavelengths of emitted (lem) and stimulating (lst) light. The ordinate represents the intensity of x-ray luminescence (IXL) and PSL (IPSL). The curves are normalized at the maxima. In the case of BaFBr:Eu, the PSL spectrum and the x-ray luminescence spectrum are essentially identical.
all fluorohalides as well as BaF2. The luminescence from undoped BaFCl and BaFBr at room temperature is attributed to the recombination of electrons with Cl20 or Br20 centers, respectively. They are formed by Hal0 ions belonging to the adjacent layers and are located at their minimum distance. Due to the reduced electron affinity of Br (compared to that of Cl), centers of the Br20 type dominate in mixed crystals BaFClxBr10x (32). Barium and strontium fluorohalides are easily doped by two-valent rare-earth ions such as Eu2/ (2, 23, 34, 35) and Sm2/ (36, 37) since such doping does not require charge compensation. The emission spectrum of Eu2/ at room temperature is an intense band peaking at 385 nm for BaFCl and at 390 nm for BaFBr and is quite favorable for LDR applications as it falls into the maximum sensitivity range for conventional multi-alkali photocathodes and at the same time is shifted relative to the excitation bands (37). The situation for BaFBr:Eu is presented in Fig. 2 (38). One of the most complicated problems which limits the usefulness of the BaFCl:Euphosphor as an intensifying screen is its weak but very long afterglow (24, 35, 39) which is indicative of deep traps. It was the study of this effect, even more pronounced in BaFBr:Eu (23), that led to the discovery that fluorohalides have the ability to store a large amount of light energy to be released later by optical excitation. The charge separation required for light energy storage is provided by the effective capture of holes by the Eu2/ centers and of electrons by anion vacancies. According to recent EPR results (90) the holes are captured not by Eu2/ but by different adjacent traps • of unknown origin. The participation of VF• and VHal vacancies becomes evident from the comparison of the excitation spectra with the absorption due to F centers created
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by the capture of electrons (41). The energy yield of BaFBr:Eu PSL (40) excited by x ray approaches 5%. The rate controlling the process for the duration of the PSL flash is the allowed intracenter transition 4 f 65d r 4 f 7. Under continuous optical excitation the rate of electron release from the traps (t01 em ) is proportional to the maximum PSL intensity (IPSL) which is, in turn, determined by the activation light intensity (Ist) and the initial concentration of electrons trapped by F center (nF). IPSL is given by IPSL Å sIstnF
[1]
where tem is the time required for the PSL intensity to decay to 1/e of its peak value under continuous stimulation and s is the cross section for optical excitation (41). IV. THE EFFECT OF PREPARATIVE CONDITIONS ON THE PROPERTIES OF BaFBr:Eu PHOSPHORS
Three methods are frequently used for barium fluorohalide preparation (18, 23, 27, 43). One method, cited in several papers (see, e.g., 10, 16, 41), is a solid phase reaction within a heated charge of BaF2 and BaBr2 together with europium bromide (10) or fluoride (40). In a second method, the initial stage employs an aqueous suspension of finitely dispersed BaF2 in the BaBr2 solution (23, 44, 45). After evaporation and the addition of EuBr3, the charge is annealed in a reducing atmosphere. The distinguishing feature of the third method is the application of NH4F (17, 39, 44, 45) as a fluorine source which reacts with BaBr2 first in solution and then, following evaporation, in the solid phase. The properties of BsFBr:Eu are quite strongly affected by the preparative conditions (17, 39, 48, 91). Normally the maximum PSL yield is observed at smaller europium concentration than that required for the maximum stationary luminescence yield (39). This optimum europium concentration can be an order of magnitude less for BaFBr:Eu prepared using NH4F, than that produced by the reaction of BaBr2 with BaF2, at 1004 g atom/mol (49). The effect of fluorine from NH4F also favors the insertion of other rare-earth dopants into the lattice as well as the removal of oxygen, which leads to an increase in phosphorescence and reduction in PSL yield. The oxygen source is associated with water of hydration which readily reacts with barium halides. The drop • in PSL yield seems to occur through the creation of defects of O1BrVBr type (17). 2/ Though the doping of BsFBr by Eu is efficient when the charge is annealed with NH4F and such admixtures as, e.g., KBr (39) in air, a slightly reducing atmosphere, either nitrogen with an admixture of hydrogen (10, 42) or CO (17, 24, 50), produces better results. In this case O1Hal centers can be formed. The admixture to the charge has a substantial effect on the stimulation spectrum (43) and its concentration (along with the annealing temperature) on the grain size. In choosing the method of admixture removal it should be noted that the hygroscopic nature of the compounds MeFHal and their tendency to hydrolysis will increase with the size of cation (Me2/) and anion (Hal0) radii (27) so that they are more pronounced for BaFBr than for BaFCl. An important role is played by the process of charge annealing (38) and the ratio of the components in the charge. This ratio determines the probability of the solid
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phase reaction (27, 49), the stimulation spectrum, and fading (10). The effect of cooling rate and annealing conditions on the properties of the BaFBr:Eu phosphor is of particular interest (37, 51, 52). It appears that annealing at 450 to 6507C shifts the maximum of the stimulation spectrum from the yellow-green region (520 to 540 nm) to the orange-red region (595 to 600 nm) (Fig. 2). Meanwhile, the intensity of stationary x-ray luminescence decreased, room temperature phosphorescence is greatly reduced, and the thermally stimulated luminescence light sum becomes at least an order of magnitude less. High temperature glow peaks are reduced by a greater amount than low temperature ones. The PSL yield however can sometimes increase. The stimulation spectrum measured during the process of the successive annealing of peaks of the glow curve revealed that there is no direct relationship between the characteristics of thermally and optically stimulated luminescence (38). This is confirmed by the fact that optical stimulation scarcely affects thermoluminescence or phosphorescence (47, 49). A study of the infrared emission of F-centers (53) has confirmed the existence of two types of electron traps in BaFBr:Eu: those participating and those not participating in PSL. V. LATTICE DEFECTS IN BaFBr:Eu AND THE PSL MECHANISM
With the assumption that both Frenkel and Schottky thermal disordering of the • BNaFBr lattice can occur, as well as antisite disordering, the point defects V11 Ba, VF, • 1 1 VBr, Fi , Bri , FBr, and EuBa must be considered in the BaFBr:Eu phosphor (17, 31, 53, 54). Computer simulation of the defects in these crystals (55) permits the enthalpy and entropy of BaFBr lattice thermal disordering (31) to be evaluated. A thermodynamic analysis based on these data (54) has shown that among effectively charged • point defects relative to the normal lattice are VBr and F1i. These defects dominate at the phosphor preparation and annealing temperatures. At 5507C their concentration is an order of magnitude smaller than that at 8007C. Equilibrium in this case can be represented by the approximate equation: • [F1i ] É [VBr ].
[2]
A comparison of the stimulation spectrum with the spectra for EPR signal reduction • under stimulation (5) reveals however that the vacancies VBr are responsible only for the long wavelength part of the stimulation spectrum with a maximum at 595 to 600 nm. The short wavelength region which dominates in the spectra from non-annealed phosphors (Fig. 2) is due to VF• . This apparent inconsistency between the calculations and the experiments becomes less significant if we take into account the annealing effect on the thermo- and photostimulated luminescence of BaFBr:Eu (39, 54) which was discussed earlier. The annealing (as well as cooling in the case of ZnS:Eu-electrophosphor (30, 56) may lead to the segregation of point defects at the regions of dislocations and grain boundaries. It has already been mentioned in Section 3 that dislocations are more likely to be found at the edges of two adjacent F2 and Ba layers. Such dislocations can generate the vacancies VF• as well as F11 Ba, which will remain in the vicinity of the extended • defect. Annealing of the phosphor results in the diffusion of the VBr vacancies which
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are dominant in the bulk of the crystal, of interstitial ions F1i , and impurity defects EuBa to the regions of dislocations and grain boundaries. This process is accompanied by the interaction between F1i and VF• , F1i / VF• Å FF,
[3]
• ]/[VF• ]. If we now assume which leads to an increase of the concentration ratio [VBr that PSL processes take place mainly in the regions of linear and surface defects, while thermally stimulated luminescence is controlled by the processes in the normal lattice, then all the data concerning annealing become understandable. This includes (i) the PSL increase in the long wavelength part of the stimulation spectrum, attributed • to VBr ; (ii) the absence of correlation between thermally stimulated luminescence, which is suppressed by annealing, and PSL, which may even increase after annealing; and (iii) the reduction of the afterglow as well as of the stationary luminescence yield. In addition, some results indicative of the tunneling mechanism associated with PSL also can be understood on this basis. This description also provides an explanation for the small effect of the temperature on the PSL decay time following stimulation by short light pulses (16, 97) (the same behavior is observed for phosphors of the K1:T1 type (8)) and also the absence of the photoconductivity under stimulation in the wavelength region related to VF centers (57). It also follows that the segregation of point defects in regions of linear and surface defects results in the roughly adjacent location of emission centers (EcBa) and anion vacancies, increasing the probability of electron tunneling between them (39). PSL related to VBr centers is accompanied by photoconductivity whose excitation spectrum is similar to the stimulation excitation spectrum. This should be regarded as an indication of the coexistence of two mechanisms for PSL stimulation: a tunneling mechanism and one associated with the transfer of electrons through the conduction band (39). Recent results on the temperature dependence of PSL for BaFBr:Eu single crystals (90, 92) confirm this assumption. It has been demonstrated that in powder samples the contribution of tunneling to the PSL is more pronounced than in single crystals (92). This may be accounted for by the segregation of point defects to regions of dislocations and subboundaries. At the same time, in thermally stimulated luminescence and room temperature phosphorescence processes the conduction band is always involved and the processes are mainly linked to the defects in the normal lattice. Stimulation spectra measured after the successive excitation of the glow peaks showed that annealing in this manner increases the PSL yield and the contribution of the short wavelength region of the stimulation spectrum (39). This can be accounted for by the redistribution of the electrons between the thermally stimulated luminescence traps and those accounting for the SPL. An identical explanation has been given to the phenomenon of the partial reduction of the PSL image after the complete optical read-out of the phosphors (49). The kinetics of this process are described by the equation
q
IPSL Å k t,
[4]
where t is the time after optical stimulation has ceased. This result [4] is indicative
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of the association of a diffusion mechanism with electron redistribution. It should be noted that the proposed PSL model, based on the radiative creation of F centers as a result of F–H pair creation by the decay of the impurity exciton (16, 34), does not fully account for all the experimental results. For example, effects of cooling and annealing on the stimulation spectrum and on the PSL yield in particular remain to be explained. Closer inspection of the stimulation spectrum reveals that it has a complex structure which cannot be reduced to two bands (17, 39). This could be a consequence of the fact that the F centers which determine the shape of the spectrum can differ not only in terms of anion origin (VF, VBr) but also in terms of the nearest neighbors, and in particular, due to their association with antisite defects of the FBr type (17, 58) or with the activator (92). The splitting of the excited p-state of F centers is also expected to manifest itself (20, 59). M-centers, or pairs of anion vacancies with a trapped electron, can also be formed (92). A comparison of the excitation spectrum for the infrared emission from color centers with the stimulation spectrum suggests (92) that the 445 and 680 nm bands are associated with nonphotostimulable FA and MA centers located in the vicinity of the activator. The study of BaFCl phosphors doped by Eu2/ or Sm2/ revealed a considerable change in their properties upon the inclusion of excess BaF2 (36, 37, 43), e.g., the enhancement of phosphorescence. There are indications that excess BaF2 affects the properties of BaFBr:Eu as well, by increasing the contribution due to VBr-related bands in the stimulation spectrum (10). This behavior can be linked to intercrystal reactions of the following types: • FBr Å VBr / F*i • F
[5]
• Br
FBr / V Å FF / V .
[6]
Equation [5] can be seen to represent Frenkel anti-site disordering (49). It seems to • be especially important because the main lattice defects VBr and F*i participate in it. We have therefore reached the conclusion that the preparative conditions must be rigorously controlled and in particular those which effect the BaF2 concentration. Great care should be taken when annealing the BaFBr:Eu phosphor (49, 60). VI. THE STATIONARY LUMINESCENCE, PHOSPHORESCENCE, AND PSL EXCITATION SPECTRA OF BARIUM FLUOROHALIDES
The spectra of stationary luminescence, phosphorescence, and the PSL excitation spectra for barium and strontium fluorohalides have been studied across a wide energy range in a series of papers (12, 34, 36–38, 60–65). They are of interest both from a scientific and a technological point of view. The long wavelength threshold of the phosphorescence excitation spectrum was used to determine the energy gap (Eg) of BaFcl (12) and it has been found to be 8.5 eV (38, 41, 66). For E õ Eg the yield of stationary luminescence depends on the emission centers themselves, resulting in differences between phosphor materials undoped or doped by Eu2/ or Sm2/ (36, 38). The yield is also affected, at least in the case of Eu-doped phosphors, by the oxidizing and reducing properties of the annealing atmosphere (36).
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FIG. 3. Normal (steady state) luminescence excitation spectra for: (a) non-doped BaFCl; (b) BaFCl:1002Eu; (c) BaFCl:1002Sm; (d) non-doped BaFBr; (e) BaFBr:1002Eu; (f) BaFBr:1002Sm. In each case, the number in front of the dopant element defines its concentration in g atom/mol in the charge used to prepare the phosphor. The ordinate represents the quantum yield relative to sodium salicylate.
Thus, the excitation spectrum of BaFCl:Eu (2 1 1002), prepared in a CO atmosphere, has an intense broad impurity band extending from 3.3 to 5.4 eV with evidence of slight splitting into two bands related to the 5d(2eg) and 5d(2t2g) states of the Eu2/ ion. Annealing in air between 6507 and 7007 suppresses the Eu2/ band although the intensity of stationary x-ray luminescence may increase weakly (24). A small peak at 318 nm on the low energy side of the excitation band is preserved, while on the high energy edge features appear related to the 2t2g-state which we attribute to electronic transitions from the 4 f 7 level to the spin-orbit split levels of 4 f 6(8F7)5d(2t2g). Annealing in CO suppresses these features because of the Eu reduction at the phosphor surface, where Eu2/ ions can achieve a different nearest neighbor arrangement (36). The features in the excitation region beyond the Eu intercenter transition and up to Eg are attributed to Eu2/ ionization and G-excitons at 8.4 eV for BaFCl and 7.4 and 7.9 eV for BaFBr (37, 62, 66), related to the 4p-states of the bromine ion. The stationary luminescence yield for E ú Eg is essentially unaffected by impurities and the excitation spectra of BaFCl and BaFBr are largely similar in this range (Fig. 3). Their most remarkable feature is a two-peak maximum between 16 and 20 eV for BaFCl and between 15 and 19.5 eV for BaFBr. These features can be attributed to
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FIG. 4. Excitation spectra of (a) steady state luminescence and (b) PSL for BaFBr:1004Eu phosphor, annealed at 5507C.
the combined effect of a plasmon and core exciton created by the excitation of electrons from the 5p-shell of Ba2/. An increase in the absorption coefficient linked to the creation of the exciton results in the increase of surface losses which then appear as a narrow dip at around 18.1 to 18.3 eV. The effect of photon multiplication produces a tendency to a constant energy yield rather than an increase of h, the quantum yield (37, 38). Between 20 and 30 eV, h already approaches values characteristic for x-ray excitation, giving 11–12% for BaFCl:1002Eu and 16–18% for BaFBr:Eu (38, 39). Annealing BaFBr:Eu at 5507 results in a reduction in the stationary luminescence yield by a factor of two for E ú 11 eV. At about 10 eV an intense band appears (Fig. 4a) which is weak in nonannealed phosphors. It can be attributed to the creation of clusters of FBr, since 10 eV is the onset energy for the creation of excitons associated with the 2p-state of F0 ions (34). The PSL excitation spectra for BaFBr:Eu have been measured independently by two groups under somewhat difference conditions (34, 48, 60–62). For E ú 11 eV the spectrum mainly reproduces the features of the stationary luminescence excitation (see Fig. 4) while the principle differences are noted for E õ 11 eV. Thus the maximum at 10 eV which is observed in the stationary luminescence excitation spectra of annealed phosphors does not appear in their PSL excitation spectra. A characteristic feature of PSL excitation spectra is a sharp peak at 7 eV. According to the two different PSL mechanisms presented above, it can be attributed either to an electron transition from Eu2/ to an adjacent anion vacancy or to the creation of F–H pairs by
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exciton decay (34, 48). It is important that the onset of phosphorescence excitation is linked to larger energies of about 8 eV (48). VII. EXPERIMENTAL MEASUREMENTS OF PHOSPHOR TIME PROFILES AND EFFICIENCIES
An experimental setup was made in the detector development laboratory at Daresbury to measure the time profiles and efficiencies of phosphors and scintillators. Initially the apparatus was to be used to measure the PSL from phosphors used in commercial image plates. It was designed to allow x rays and the stimulating light to illuminate the sample simultaneously, whilst being viewed by a photomultiplier tube to measure the PSL. The sample stage is comprised of a light-tight aluminum cube with three orthogonal ports. The x-ray illumination enters through one port via a 0.5-mm beryllium window. The second port has a photomultiplier and filter assembly which allows the luminescence to be measured at specific wavelength ranges. The third port allows the stimulating radiation to fall on the sample. In the initial experiments this was the light from a He–Ne laser. The sample holder is a tetragonal block with one surface inclined at 457 to all ports. The inside of the vessel is treated with anti-reflectance paint to reduce indirect illumination of the sample by scattered laser light. The vessel was installed in an x-ray safety hutch with a fixed anode x-ray generator. The beam from the x-ray head was accurately aligned with the beryllium window in the sample vessel. Two targets were used in the tests, copper which produces 8 keV photons and molybdenum which produces 17.3 keV. The image plate samples were the BaFBr:Eu type laid down on the substrate in a variety of ways. A 35-mW He– Ne laser was used to produce the stimulating radiation which was appropriate for this phosphor. The laser was mounted on the top of the x-ray hutch on an optical bench with a series of mirrors and an opto-acoustical modulator which was used to switch the light. The mirrors allowed a long optical path from the modulator head to the sample to gain the most benefit from the ability of the modulator to deflect the laser beam through small angles. The beam was reflected at right angles down into the sample vessel in the hutch through a radiation-safe and light-tight tube. The output of the phosphor was measured using the photomultiplier tube, an amplifier, and a digital storage oscilloscope (DSO). The modulator switching speed is 30 ns for both deflection and intensity modulation. The extinction ratio for the modulation between fully on and fully off is 30 dB. This was inadequate for some of the experiments, so the deflection modulation was used to switch the beam out of the path. This provided a high extinction ratio, but with a slower leading edge to the laser switching function. During the time profile measurements a pinhole mask was placed over the sample. This limited the illuminated area to the central part of the laser intensity profile, ensuring a uniform illumination and improving the speed of the on/off switch of the stimulating radiation when the deflection modulation was used. A high-speed x-ray shutter was mounted on the x-ray head to provide accurate dose control on the phosphors. For the time-profile measurements the phosphor was beached before insertion into the sample vessel. A fixed dose of x-ray radiation was given and then the time profile
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of the PSL was captured on the DSO as the modulator switched the laser light. For the efficiency measurements, a standard sample of commercial phosphor was used to compare the test samples. The best samples produced ‘‘in house’’ were of the same efficiency as the standard. The PSL rise time of 800 ns as measured by von Seggern and others (93) was confirmed by our measurements. VIII. APPLICATIONS OF LUMINESCENT IMAGE PLATES
The wide dynamic range, low noise, and good spatial resolution of luminescent image plates have opened up new opportunities for x-ray diagnostics, x-ray crystallography, and x-ray imaging, and in X-ray diffraction and scattering studies, particularly those making use of synchrotron radiation. There are already reports on their application to the investigation of lungs, breasts, and other organs (67–71). The radiation background has also been measured although it should be noted that the barium salts which are used as phosphor matrices normally contain a small amount of radium. Wide spread application of the technique in medicine is restricted to some extent, however, by technical and financial problems (57, 75). A series of papers (76–82) have been dedicated to the application of image plates for diffraction pattern measurements of biological specimens using synchrotron radiation. High resolution patterns can be obtained within approximately one-tenth of the exposure time required using conventional methods (76, 79, 80). Image plates have been suggested for use in conjunction with transmission electron microscopy (83), for the detection of all types of electromagnetic radiation extending from the VUV (38, 84) and up to the S-ray bremsstruhling region for electron accelerators (52) and also for particles (84). It can also be exploited as a beam position monitor and to obtain data on the dose distribution in the irradiated body for radiation therapy. Image plates may have applications in astrophysical research, including x-ray astronomy, which would gain substantially from their wide dynamic range. The many and various applications of image plates have promoted a study of those parameters which define the quality of images and of the threshold flux density required for the detected radiation (76, 85–87). As expected, the decisive role is played by the efficiency of the initial energy conversion process, noise, frequency-contact characteristics, dynamic range, and other kinetic parameters (88). Of course, image plates and their associated readout systems have a number of specific limitations (5, 85). These include scattering of the stimulating radiation (9), signal fading (11, 91), and fluctuations of light level (luminescence noise) (85), which are determined by the properties of the phosphors in use. This final factor makes a substantial contribution to the total noise at low levels of PSL and when there are large light losses on the way from the screen to the photomultiplier. Nevertheless, the preparation and exploitation of high-efficiency phosphors for image plates is helping to make an important input in many areas of x-ray research. REFERENCES 1. V. V. ANTONOV-ROMANOVSKI, V. L. LEVSHIN, Z. L. MIRGENSHTERN, An SSR 54, 19 (1946) [in Russian].
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2. V. L. LEVSHIN, N. V. MITROFANOVA, YU. P. TIMOFEEV, S. A. FRIDMAN, AND V. V. STCHAENKO. Proc. Phys. Inst. AN SSSR 59, 64 (1972) [in Russian]. 3. S. W. S. MCKEEVER, ‘‘Thermoluminescence of Solids,’’ Cambridge Univ. Press, Cambridge, 1985. 4. M. SONADA, M. TAKANO, J. MIYAHARA, AND H. KATO, Radiology 148, 833 (1983). 5. A. M. GURVICH, M. G. MYAGKOVA, AND YU. RU¨DIGER, Med. Technol. 27(3), (1990) [in Russian]. 6. K. TAKAHASHI, J. MIYAHARA, AND Y. SHIBAHARA, J. Electrochem. Soc. 132, 1492 (1985). 7. H. VON SEGGERN, T. VOIGT, AND K. SCHWARZMICHEL, Siemens Forsch:u.Entwickl.-Ber. 17(3), 125 (1988). 8. G. I. VLASOV, R. A. KALNYNSH, L. E. NAGLI, V. P. OB’EDKOV, I. K. PLYAVIN, AND A. K. TALE, Avtometriya 66(1), (1980) [in Russian]. 9. A. MEIJERINK, ‘‘Luminescence of New X-Ray Storage Phosphors,’’ Drukkerij Elinkwijk B V, Utrecht, 1990. 10. K. TAKAHASHI AND T. NAKAMURA. U.S.A. Patent No. 4 535 237, 13.1283/13.08.85. 11. J. DEGENHARDT, FRG Patent No. (OS) DE 3 347 207 A1, 27.12.83/11.07.85. 12. A. M. GURVICH, M. A. IL’INA, V. P. KAVTOROVA, L. I. KONSTANTINOVA, N. I. LEONOVA, M. G. MYAGKOVA, AND T. I. SAVIKHINA, J. Prikl. Sektr. 28, 765 (1983) [in Russian]. 13. A. MEIJERING, G. BLASSE, AND M. GLASBEEK, J. Phys.: Condens. Mater. 2, 6303 (1990). 14. A. MEIJERING AND G. BLASSE, Mater. Chem. & Phys. 21, 261 (1989). 15. J. MIYAHARA, in ‘‘Computed Radiographie’’ (Y. Tateno, T. Linuma, and M. Takano, Eds.), p. 7, Springer-Verlag, Berlin, 1987. 16. H. VON SEGGERN, T. VOIGT, W. KNU¨PFER, AND G. LANGE, J. Appl. Phys. 64, 1405 (1988). 17. A. M. GURVIC, W. P. KAVTOROVA, M. G. MYAGKOVA, J. RUDIGER, AND V. D. CERNOVSKI, ‘‘Proceedings, 2nd International Mtg. on Luminescence, ‘35 Years of Luminescence,’ Greifswald, 1989,’’ p. 14, Ernts-Moritz-Arndt-Universitat Greifswald. 18. A. M. GURVICH, ‘‘Ro¨ntgenleuchtstoffe und Ro¨ntgenlumineszenzbildwandler,’’ Akademische Verlagsgesellschaft Geest & Portig K.-G., Leipzig, 1988. 19. E. NICKLAUS AND F. FISCHER, Phys. Status Solidi B 52, 453 (1972). 20. M. YUSTE, L. TAUREL, M. RAHMANI, AND D. LEMOYNE, J. Phys. Chem. Solids 37, 961 (1976). 21. S. LEFRANT AND A. H. HARKER, Solid State Comm. 19, 853 (1976). 22. K. SOMAIAH, P. VEERESHAM, K. L. N. PRASAD, AND V. HARI BABU, Physica Status Solidi A 56, 737 (1979). 23. A. L. N. STEVELS AND F. PINGAULT, Philips Res. Rep. 30, 277 (1975). 24. A. M. GURVISH, R. V. KATOMINA, AND N. P. SOSTCHIN, Izv. AN SSR, Ser. Phys. 41, 1372 (1977) [in Russian]. 25. J. L. SOMMERDIJK, J. M. P. VERGSTEGEN, AND A. BRIL, J. Luminesc. 8, 502 (1974). 26. M. SAUVAGE, Acta Crystallogr. B 30, 2786 (1974). 27. H. P. BECK, J. Solid State Chem. 17, 275 (1976). 28. D. LEMOYNE, J. DURAN, M. JUSTE, AND M. BRILLARDON, J. Phys. C: Solid State Phys. 8, 1455 (1975). 29. F. A. KRO¨GER, ‘‘The Chemistry of Imperfect Crystals,’’ North-Holland, Amsterdam, 1964. 30. A. M. GURVICH, ‘‘Introduction to Physical Chemistry of Crystal Phosphors,’’ Vysshaya Shkola, Moscow, 1972; 2nd ed., 1982 [in Russian]. 31. R. C. BAETZOLD, Phys. Rev. B 36, 9182 (1987). 32. K. A. KALDER, ‘‘Electronic Excitation in AIIB VII,’’ Ph.D. thesis, Tartu, 1972. 33. M. K. CRAWFORD, L. H. BRIXNER, AND K. SOMAIAH, J. Appl. Phys. 66(8), 3758 (1989). 34. H. H. RUTER, H. VON SEGGERN, R. REININGER, AND V. SAILE, Phys. Rev. Lett. 65, 2438 (1990). 35. J. L. SOMMERDIJK, J. M. P. VERSTEGEN, AND A. BRIL, J. Luminesc. 8, 502 (1974). 36. A. M. GURVICH, V. B. GUTAN, M. A. IL’INA, V. P. KAVTOROVA, R. V. KATOMINA, N. G. MYAGKOVA, AND T. I. SAVIKHINA, Opt. i Spektrosk. 52, 289 (1982) [in Russian].
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