136
`
`7 . Cathode-Ray Phosphors
`
`red
`
`Fig. 7.3. Principle of projection television. The images of three smaller cathode-ray tubes (blue,
`green and red) are projected to a large screen
`
`There are many other types of cathode-ray tubes which find application: oscillo(cid:173)
`scope tubes, tubes with very short decay times, radar tubes, tubes with high-resolution
`screens, and so on. For these tubes and the phosphors they require, we refer to a recent
`review paper [ 1].
`
`7.2 Preparation of Cathode-Ray Phosphors
`
`Much of what has been said in Sect. 6.3 on lamp phosphors, can be repeated here.
`Since fast electrons excite the host lattice and create mobile electrons and holes,
`the restrictions to the impurity level are usually even more severe. This is certainly
`the case for the frequently applied sulfides. In principle zinc sulfides are made by
`dissolving ZnO in sulfuric acid yielding an aqueous solution of ZnS04 • Subsequently
`H 2 S is bubbled through the solution, converting ZnS04 into insoluble ZnS. The shape,
`size and crystal quality of this precipitate depends on the reaction conditions (pH,
`temperature, concentration).
`The raw material is then fired with a flux and the activator, and the product is
`sieved. Finally the flux is removed and the product milled. Suitable processes may
`be applied to obtain particles of the described size ('""--' S ,urn).
`The red-emitting Y 2 0 2 S: Eu 3+ is made from a mixture of the oxides and elemental
`sulfur which is heated in a flux consisting of sodium carbonate and alkaline phosphate.
`The fired product is washed with diluted HCI in order to remove Na2 S. Figure 7.4
`shows a scanning electron microscope photograph of an oxysulphide phosphor.
`Several methods are in use to fabricate the luminescent screen in the cathode(cid:173)
`ray tube. For conventional applications the screen thickness is 15-30 ,urn, i.e. 2-4
`particles deep, resulting in a weight of 3-7 mg/cm 2
`• The setting method has been
`used since the beginning of tube manufacturing and is still in use for monochrome
`
`Vizio EX1018 Page 0147
`
`

`

`7 .3. Cathode-Ray Phosphors
`
`137
`
`Fig. 7.4. Scanning electron microscope photograph of particles of the Y 202S: Eu phosphor.
`(reproduced with permission from Ref. [I))
`
`cathode-ray tubes. In order to produce color tubes, predominantly a slurry method is
`used. Further a dusting and an electrophoretic method can be used. For more details
`on screening, see Ref. [ 1].
`
`7.3 Cathode-Ray Phosphors
`
`7 .3.1 Some General Remarks
`
`The host lattices which yield the highest radiant efficiency (Sect. 4.3) for cathode-ray
`excitation are undoubtedly ZnS and its derivatives. For the blue-emitting ZnS: Ag
`values higher than 20% have been reported. We note that this fits our discussion
`in Sect. 4.4, where the maximum efficiency for host lattice excitation was found
`to occur for small values of the band gap Eg and the vibrational frequency VLo
`(Eqs (4.5)-(4.7)). For ZnS Eg = 3.8 eV and vLo = 350 cm- 1 • These values satisfy our
`requirements. This can be illustrated by Y 2 0 3 with Eg = 5.6 eV and vLo = 550 cm- 1
`,
`yielding a maximum efficiency of only 8%. Although the latter value is for red
`emission, this value is much lower than for ZnS.
`It is in principle very simple to change the emission wavelength of the efficient
`blue cathode-ray phosphor ZnS : Ag+, viz. by replacing part of the zinc by cadmium.
`
`Vizio EX1018 Page 0148
`
`

`

`138
`
`7. Cathode-Ray Phosphors
`
`i\.s a consequence the band gap decreases, so that the emission wavelength shifts
`to the red. Actually Zn0 _6xCd0 _32 S: Ag+ is a green-, and Zn0 _13 Cd0 _x7 S: Ag+ a red(cid:173)
`emitting cathode-ray phosphor. The emission color is not determined by the nature
`of the luminescent center, but by the value of the band gap. Figure 7.2 shows the
`chromaticity diagram with three phosphors ftom the (Zn,Cd)S : Ag+ family.
`However, the (Zn,Cd)S system has several disadvantages. In the first place, the use
`of cadmium has become inacceptable for environmental reasons. The red phosphor
`on this basis has still another large disadvantage, viz. in order to obtain red emission
`the larger part of the broad band emission of this phosphor is situated in the near
`infrared. The maximum of the emission band is close to 680 nm. This implies that the
`lumen equivalent of this phosphor is low (25%). For Y 20 2 S: Eu3+ with line emission
`this is 55% [ 1].
`Long ago it was predicted that color television with a satisfying brightness would
`only be possible with a phosphor which emits in the red by line emission around
`610 nm [2]. Now we know that only the Eu 3+ ion is able to satisfy this requirement.
`In fact the introduction of Eu3+ -activated phosphors in the color-television tube was a
`breakthrough: not just the red, but the total brightness increased strongly. It was also
`the introduction of rare-earth phosphors leading to many other improvements (e.g. in
`luminescent lamps) and the end of the domination of the sulfides.
`
`7 .3.2 Phosphors for Black-and-White Television [3]
`
`In a sense this is a historical paragraph. The color preferred for black-and-white televi(cid:173)
`sion is bluish-white. This can be realized by many combinations of two phosphors as
`prescribed by the chromaticity diagram. The best is ZnS: Ag+ and (yellow-emitting)
`Zn0 _5 Cd0 _5 S: Ag+ or Zn0 _9 Cd0 _ 1 S: Cu,AL Single-component white phosphors have also
`been found, but none has found practical application.
`
`7 .3.3 Phosphors for Color Television
`
`We will discuss the possible materials for the blue- , the green- , and the red-emitting
`phosphor in turn. For the blue the phosphor ZnS: Ag+ has been continuously in use.
`As mentioned above, its radiant efficiency is very high and close to the theoretical
`limit. In Fig. 7.5 its emission spectrum is given.
`The emission of ZnS: Ag+ is of the donor-acceptor pair type (see Sect. 3.3.9).
`Silver is an acceptor in ZnS. The donor is shallow and can be aluminium or chlorine
`(on zinc or sulfur sites, respectively). The energy level scheme is given in Fig. 7.6.
`For the green the sulphide ZnS: Cu,Cl (or AI) is used. This has also an emission
`of the donor-acceptor pair type, but the copper acceptor levels are located at higher
`energy above the valence band than those of silver. This yields for copper an emission
`band with a maximum at 530 nm. For practical reasons it is profitable to shift the
`
`Vizio EX1018 Page 0149
`
`

`

`7 .3 .3. Phosphors for Color Television
`
`139
`
`400
`
`600nm
`
`Fig. 7.5. Emission spectrum of the blue-emitting cathode-ray phosphor ZnS : Ag
`
`CB --------------------------------
`
`VB - -- --------- - - - -- - - - - -- -- --
`
`Fig. 7.6. Ene rgy level scheme for the donor-acceptor pair emi ssion of ZnS: Ag. The valence
`and conduction bands of ZnS are indicated by VB and CB, respectively. D is the shallow donor
`(aluminium or chlorine), A is the acceptor (silver)
`
`emission maxtmum to a slightly longer wavelength. This can be done by replacing
`part of the zinc (~ 7%) by cadmium, or by adding a deeper acceptor such as gold.
`The emission color depends critically on the defect chemistry of the system [4].
`For the red-emitting phosphor originally (Zn,Cd)S: Ag and Zn3 (P04h: Mn2+ have
`been used. After the prediction that the red emission should consist of a narrow
`emission around 610 nm [2], it took another 10 years before Levine and Pal ilia [5]
`proposed YV04 : Eu3+ as the red phosphor for color television tubes. This interesting
`material has already been discussed before (Sects. 5.3.2 and 6.4.3). A few years
`later it was replaced by Y 20 2 S: Eu3+ which gave increased brightness [6]. This host
`lattice will reappear in the chapter on X-ray phosphors. Many years later Kano et al.
`proposed Y 2 (W04 h: Eu3+ because of its high lumen equivalent [7].
`The great succes of the Eu3+ luminescence in this aspect is illustrated in Fig. 7.7 .
`This shows the eye sensitivity curve which drops sharply in the red spectral area, and
`the emission spectra of a Eu3+ -activated phosphor and red-emitting (Zn,Cd)S: Ag. It
`is clear that the main part of the emission of the sulfide lies outside the eye sensitivity
`
`Vizio EX1018 Page 0150
`
`

`

`l40
`
`7. Cathode- Ray Phosphors
`
`550
`
`600
`
`650
`A. ---j~
`
`700
`
`750nm
`
`Fig. 7.7. (a) Emission spectrum of red-emitting (Zn,Cd)S: Ag. (b) emission spectrum of a
`red-emitting Eu3+ phosphor. (y): eye-sensitivity curve
`
`Table 7.1. Luminescent properties of red-emitting cathode-ray phosphors (after Ref. [I]).
`
`Phosphor
`
`Zn3(P04h: Mn2+
`(Zn,Cd)S: Ag
`YV04 : Eu3+
`Y2 0 3 : Eu3+
`Y2 0 2 S: Eu3+
`Y 2 (W04)) : Eu3+
`61 I nm radiation
`
`Radiant
`efficiency (%)
`6.7
`16
`7
`8.7
`13
`4.3
`
`L*
`
`47
`25
`62
`70
`55
`81
`100
`
`Relative
`brightness
`39
`51
`55
`88
`100
`46
`
`* Lumen equivalent relative to monochromatic light of 611 nm wavelength.
`
`curve, so that the lumen equivalent is low. Table 7.1 gives a survey of the red-emitting
`cathode-ray phosphors with their properties.
`7 F 4 emission of Eu3+ at about 700 nm is as weak as
`It is important that the 5 D 0 -
`possible, since this will decrease the lumen equivalent. The high value of the lumen
`equivalents of Y 2 0 3 : Eu3+ and Y 2 (W04)3: Eu3+ are actually due to the low 5 D 0 -
`7 F4
`intensity. Further, the 5 D 1 emission of Eu3+ should be quenched by cross relaxation
`as in the lamp phosphor (see Sect. 6.4.1.4). This makes a Eu3+ concentration of some
`3% necessary.
`
`Vizio EX1018 Page 0151
`
`

`

`7 .3.3. Phosphors for Color Television
`
`141
`
`: 3.10- 2 Tb
`InB0 3
`Y3AI 5 0 12 : 5.10- 2 Tb
`7.10-2 Tb
`Y2Si 05
`
`La 0 Br
`
`Gd202 S
`ZnS
`
`: 1 0 -l Tb
`: 10-2 Tb
`: 2.1o-4 cu
`
`10-4
`
`10-3
`D (J/ em 2 ) - - - ?o -
`
`10-2
`
`Fig. 7.8. Non-linearity of green PTY phosphors
`
`7 .3.4 Phosphors for Projection Television
`
`As argued in Sect. 7. 1 one of the problems with cathode-ray phosphors in projection
`television is the saturation of their light output under high excitation density. Related
`to this is the temperature increase of the phosphor under these circumstances: the
`screen temperature can rise to 1 OO"C [8].
`The nonlinear behavior of the light output was originally ascribed to ground
`state depletion of the activator. In sulfides, where the activator concentration is low
`('"'"' 0.01 %), this is certainly important. Therefore, attention shifted to the oxidic phos(cid:173)
`phors where the activator concentrations are much higher('"'"' 1 %). However, excited
`state absorption and Auger processes (Sect. 4.6) will also result in saturation effects.
`Detailed analysis of interactions between excited activator ions are available [9, 10].
`In Fig. 7.8 the light output of a number of green-emitting phosphors is plotted
`versus the density of cathode-ray excitation. In Fig. 7 .9 their temperature dependence
`is given. Although the sulfide has the highest radiant efficiency at low excitation
`densities, viz. 20%, it shows a pronounced saturation. Note also the drop in light
`output upon increasing the temperature.
`the exception of
`The Tb3+ -activated materials perform much better with
`Gd20 2S: Tb3+. The bad performance of the latter seems to be due to its bad temper(cid:173)
`ature dependence.
`Serious candidates for application as a green phosphor in PTV tubes are
`Y 2Si05 : Tb3+, Y 3Al5 012: Tb3+ and Y3(Al,Ga)sOt2 : Tb3+. The phosphor InB03 : Tb3+
`is excellent from several points of view, but its decay time is very long, viz. 7.5 ms
`
`Vizio EX1018 Page 0152
`
`

`

`142
`
`7 . Cathode-Ray Phosphors
`
`t
`
`~
`
`cr
`
`a
`b
`c
`d
`e
`
`InB03
`Y3 Al 5 0 12 : Tb
`: Tb
`Y2 Si05
`LaOBr
`: Tb
`Gd202S : Tb
`ZnS
`: Cu
`
`b
`c
`
`d
`
`e
`
`0
`
`100
`
`200
`T ( °C) ----------
`
`300
`
`Fig. 7.9. Thermal quenching of the green emission of some cathode-ray phosphors
`
`103
`
`: 5.10-2 Eu
`Y203
`Y 2 0 2 s : 5. 1 o-2 Eu
`
`/
`
`/
`
`---
`
`/
`
`/
`
`/
`
`/
`
`/
`
`/
`
`/
`
`/
`
`/
`
`/
`
`/
`
`/
`
`/
`
`10 2
`
`t
`
`_J
`
`10 1
`
`/
`/.
`/;
`
`10°
`
`Q
`
`10-5
`
`10- 4
`
`10-3
`
`10-2
`
`10- 1
`
`D {J/cm 2 ) ---
`
`Fig. 7.10. Non-linearity of the red cathode-ray phosphors. Figs. 7 .8-7. 10 are reproduced with
`permission from Ref. [ 14]
`
`[8]. This is due to the calcite structure of InB03 which places Th3+ on a site with
`inversion symmetry which forbids the forced electric-dipole transitions (Sect. 2.3.3) .
`An important criterion in the final phosphor selection is their degradation behavior in
`the tubes under high-density excitation [II].
`The red phosphor in PTV tubes is usually Y 2 0 3 : Eu 3+, because Y 2 0 2 S : Eu3+
`shows saturation (see Fig. 7 .I 0). The properties of Y 2 0 3 : Eu3+ were discussed in
`Sect. 6.4. I .4.
`An ideal blue phosphor for PTV tubes has not yet been found. Materials acti (cid:173)
`vated with Eu 2+ suffer from severe degradation. The Ce3+ emission in (La,Y)OBr
`and (La,Gd)OBr is attractive, but the host lattice is unfavorable for screen making
`
`Vizio EX1018 Page 0153
`
`

`

`7 .3.3. Phosphors for Color Tele vision
`
`143
`
`[12]. The Tm3+ ion can give blue emission, but cross-relaxation limits the activator
`concentration, so that saturation occurs. In spite of its saturation at high excitation
`densities, the old ZnS : Ag+ has not yet been surpassed under these conditions. As a
`consequence the screen brightness is limited by the blue-emitting phosphor. Extensive
`research by many laboratories has not changed this situation.
`
`7.3.5 Other Cathode-Ray Phosphors
`
`In the literature many other cathode-ray phosphors are known for several purposes.
`Two of these we will mention here, partly because they are generally known as
`luminescent materials, partly because their properties are also interesting from a fun(cid:173)
`damental point of view. They are Zn2 Si04 : Mn2+, also known by the mineral name
`willemite, and the family of Ce3+ -activated phosphors. Still more cathode-ray phos(cid:173)
`phors can be found in Ref. [1 ].
`The green-emitting phosphor Zn2 Si04 : Mn2+ was also mentioned in the chapter
`on lamp phosphors. This might suggest that an efficient phosphor can find application
`everywhere. However, this is by no means true. The halophosphates, for example, are
`highly efficient under ultraviolet excitation (Chapter 6), but not under cathode rays.
`The reason for this is that efficient excitation of the activator itself is no guarantee
`for efficient excitation via the host lattice. However, if an activator can be efficiently
`excited via the host lattice, it can also be efficiently excited directly. In addition,
`when ultraviolet irradiation excites the host lattice, the efficiency of this excitation
`and cathode-ray excitation will run parallel. An illustration of the latter is formed by
`the sulfides and YV0 4 : Eu3+; an illustration of the former is Y 2 0 3 : Eu3+ (compare
`the discussion in Sect. 2.1 ).
`Willemite is used as a cathode-ray phosphor in terminal displays and oscilloscope
`tubes. The decay time is very long, viz. 25 ms [I]. This is mainly due to the spin- and
`parity- forbidden nature of the 4T 1 -+ 6 A 1 emission transition in the 3d5 configuration
`of the Mn 2+ ion (Sect. 3.3.4.c), but there is also a contribution of afterglow. An even
`longer persistance is observed for samples to which As has been added. As a result
`of the As addition, electron traps are formed which trap the electrons for a certain
`time, so that the emission is delayed (Sect. 3.4).
`Cathode-ray phosphors with such a long persistence are suitable to avoid or min(cid:173)
`imize flicker in the display. This is especially of importance when high-definition
`figures need to be displayed. For application in television tubes (moving pictures) or
`high-frequency oscilloscopes, such a long persistence is of course fatal.
`The Ce3+ -activated cathode-ray phosphors are used in applications where a very
`short decay time is a requirement [13]. Since the emission is a completely allowed
`transition (5d-+ 4f, Sect. 3.3.3.a), the decay time of Ce3+ varies between about 15
`and 70 ns, depending on the emission wavelength. One application is in the beam(cid:173)
`index tubes which generate color images by means of one electron gun [I, 13]. This
`system could, however, never compete with the above-mentioned shadow-mask tube.
`The beam-index phosphor indicates the location of the electron beam. Therefore, the
`emission should have a very short decay time and, in order not to disturbe the image,
`should be situated in the ultraviolet. A good choice is Y 2Sh07 : Ce3+ with a radiant
`
`Vizio EX1018 Page 0154
`
`

`

`144
`
`7. Cathode-Ray Phosphors
`
`I 0.5
`
`350
`
`400
`)..
`
`450
`
`500
`
`550nm
`
`Fig. 7.11. Emission spectra of /3- and y-Y2Si 2 07: Ce 3+ under cathode-ray excitation (after
`Ref. [ 13])
`
`efficiency of some 7% and a decay time of 40 ns. The maximum emission wavelength
`is 375 nm. Figure 7.11 shows the emission spectra of the {3 and the y modifications .
`Especially they modification shows clearly the double emission band character which
`is due to the splitting of the 4 f 1 ground configuration into the 2 F 512 and 2 F 712 levels
`(Sect. 3.3.3.a).
`The other application is the flying-spot scanner. In this set up the information
`of slides or film can be transformed into electric signals. The electron beam excites
`a phosphor with short decay time. The luminescence signal scans the object point
`by point and the transmitted radiation is detected with a photomultiplyer. To reduce
`blurring of the signal, the decay time of the phosphor should be of the order of
`magnitude of the time the electron beam scans a picture element ('"""' 50 ns).
`In order to transmit pictures in color, the emission of the phosphor should cover the
`whole visible area. For this purpose a mixture of Y 2 Si05 : Ce3+ and Y 3 Al 5 0 12 : Ce3+
`has been used. The former emits in the blue, the latter in the green and red [ 13).
`The reason that Ce3+ in Y 3 AI 5 0 12 emits at such long wavelengths is due to the
`extended crystal-field splitting of the excited Ce3+ ion (i.e. 5d 1 configuration). This
`was considered in Sect. 3.3.3.a.
`It is interesting to note that Y 3 Al 5 0 12 : Ce3+ was originally developped for the
`flying-spot scanner tube [ 13]. However, today its application lies in the special de
`luxe lamps as discussed in Sect. 6.4.1.7. For the former application its long wave(cid:173)
`length emission with short decay time is essential, for the latter its long wavelength
`absorption and emission. In both cases the low energy position of the lowest crystal(cid:173)
`field component is essential.
`
`Vizio EX1018 Page 0155
`
`

`

`7.4 Outlook
`
`References
`
`145
`
`The field of cathode-ray phosphors is relatively old and has reached a high level of
`maturity. The quality of todays color television is a token of the success of these
`materials. For direct-view television there is not much more to desire as far as the
`luminescent materials are concerned. The combination ZnS: Ag+, ZnS: cu+ ,Al3+,
`and Y 2 0 2 S: Eu 3+ is worldwide considered to be the most suitable combination.
`In the fields of projection television phosphors the situation is less satisfactory. It
`is clear that luminescent materials have difficulties in meeting the high requirements.
`At the moment the largest need is for a blue-emitting phosphor with an acceptable sat(cid:173)
`uration. The bad situation is well illustrated by the blue phosphor used, viz. ZnS: Ag+.
`Its high radiant efficiency in direct-view television tubes ("'"' 20%) decreases to less
`than 5% under the conditions of the projection-television tube. Nevertheless it has
`not been possible, up till now, to find an acceptable alternative.
`
`References
`
`I. Hase T, Kano T, Nakazawa E, Yamamoto H ( 1990) In: Hawkes PW (ed) Advances in
`electronics and electron physics, vol. 79. Academic, New York, p 271
`2. Bril A, Klasens HA ( 1955) Philips Res. Repts I 0:305; Klasens HA, Bril A ( 1957) Acta
`Electronica 2: 143
`3. Ouweltjes JL ( 1965) Modern materials, vol. 5, Academic, New York, p 161
`4 . Bredol M, Merikhi J, Ronda C ( 1992) Ber. Bunsenges. Phys. Chem. 96:1770
`5. Levine AK, Palilla FC ( 1964) Appl. Phys. Letters 5 : 118
`6. Royce MR, Smith AL ( 1968) Ext. Abstr. Electrochem. Soc. Spring Meeting 34:94;
`Royce MR ( 1968) US patent 3.418.246
`7. Kano T, Kinameri K, Seki S (1982) J. Electrochem. Soc. 129:2296
`8 . Welker T (1991) J . Luminescence 48, 49:49
`9. de Leeuw DM, 't Hooft GW ( 1983) J. Luminescence 28:275
`I 0. Klaassen DBM, van Rijn TGM, Yink AT ( 1989) J. Electrochem. Soc. 136:2732
`I I. Yamamoto H , Matsukiyo H (1991) J. Luminescence 48,49:43
`12. Raue R, Vink AT, Welker T (1989) Philips Techn. Rev. 44:335
`13. Bril A, Blasse G, Gomes de Mesquita AH, de Poorter JA ( 1971) Philips Techn. Rev. 32:125
`14. Smets B (1991) In: DiBartolo B (ed) Advances in nonradiative processes in solids, Plenum,
`New York, p 353
`
`Vizio EX1018 Page 0156
`
`

`

`CHAPTER 8
`
`X-Ray Phosphors and Scintillators
`(Integrating Techniques)
`
`8.1 Introduction
`
`The terms X-ray phosphors and scintillators are often used in an interchangeable
`way. Some authors use the term X-ray phosphors when the application requires a
`powder screen, and the term scintillator when a single crystal is required. The physical
`processes in the luminescence of these two types of materials is, however, in principle
`the same and comparable to that in cathode ray phosphors (Chapter 7).
`Therefore another subdivision of the broad field of X-ray phosphors and scintil(cid:173)
`lators is used here, viz. materials used in applications where integrating techniques
`are used (Chapter 8), and materials used in applications where counting techniques
`are used (Chapter 9). The integrating technique measures the light intensity under
`continuous excitation; it is position sensitive and yields an image; a well-known ex(cid:173)
`ample is the case of X-ray imaging in medical diagnostics. The counting technique
`digests the radiation excited by a single pulse; it yields the number of exciting events;
`a well-known example is the use of scintillators in electromagnetic calorimeters in
`order to count photons, electrons or other particles.
`X-ray phosphors can be defined as materials which absorb X rays and convert the
`absorbed energy efficiently into luminescence, in practice often ultraviolet or visible
`emission. In this paragraph we consider the phenomenon of X-ray absorption, and
`the principles of some important ways of X-ray imaging and the requirements which
`X-ray phosphors have to satisfy in order to be promising for potential application.
`In the next paragraph several aspects of materials preparation are discussed. A
`complicating factor is formed by the fact that such materials are applied as powders,
`ceramics or single crystals, depending on the application. In Sect. 8.3, the possi(cid:173)
`ble materials are considered following the several applications. The final paragraph
`presents an outlook into the future of this complicated field of materials research.
`
`8.1.1 X-Ray Absorption
`
`Figure 8.1. shows a schematic picture of the X-ray absorption coefficient a vs the
`energy E of the X rays. When X rays interact with an atom or ion, they may remove
`an electron from the K shell if the energy of the X-ray quantum is equal to or larger
`
`Vizio EX1018 Page 0157
`
`

`

`8.1. Introduction
`
`147
`
`1
`
`('Q
`
`Fig. 8.1. The X-ray absorption coefficient a as a function of the energy E of the X-rays
`(schematic). The K and L X -ray absorption edges are indicated
`
`- - - -E
`
`X-ray source
`
`p'at i ent
`
`I
`I
`int e nsifying scr e en
`
`Fig. 8.2. Schematic representation of a medical radiog raphy syste m based on the use of an
`intens ifying screen
`
`than the binding energy EK of the K electron. This absorption yields a continuous
`absorption spectrum starting at EK and extending to higher energy. This corresponds
`to the left-hand part of the spectrum in Fig. 8.1.
`In a similar way, the weaker-bounded L electrons yield three continuous absorption
`spectra which start at the energies L~> L 11 and L 111 (Fig. 8.2., right-hand side). Even
`weaker-bounded electrons (M shell, and so on) yield absorption edges at still lower
`energy, depending on the atomic number.
`The absorption coefficient increases strongly with the atomic number. As a con(cid:173)
`sequence X-ray phosphors will necessarily contain heavy elements; another way to
`formulate this requirement is that X-ray phosphors must be high-density materials.
`
`8.1.2 The Conventional Intensifying Screen
`
`After the discovery of X rays in 1895, Rontgen realized immediately that X rays
`are not efficiently detected by photographic film . The reason for this is the weak
`
`Vizio EX1018 Page 0158
`
`

`

`148
`
`8 . X-Ray Phosphors and Scintillators (Integrating Techniques)
`
`absorption of X rays by such a film. In this way long irradiation times would be
`required which results in vague pictures when the object moves (like the human
`body generally does). In addition the negative influence of X-ray irradiation on the
`human body is nowadays sufficiently known. Immediately after Rontgen's invention
`a search for phosphors which were able to absorb X rays and to convert the absorbed
`energy efficiently into light was started [ 1 ]. As early as one year later, in 1896, Pupin
`proposed CaW04 for this purpose This material served for some 75 years in the so(cid:173)
`called X-ray intensifying screens, an absolute record for a luminescent material. In
`this way the irradiation time was reduced by about three orders of magnitude! The
`luminescence properties of CaW04 have already been discussed (see Chapter 1, and
`Sects 3.3.5 and 5.3.2).
`A medical radiography system based on the use of an intensifying screen is rep(cid:173)
`resented in Fig. 8.2. The X-ray radiation transmitted by the patient is dete cted by the
`X-ray phosphor which is applied as a screen. The emitted luminescence is detected
`by photographic film. The spectral film sensitivity should coincide optimally with the
`spectral energy distribution of the emitted luminescence. Although the medical appli(cid:173)
`cation of this principle is best known, other applications are also in use. An example
`is nondestructive materials control.
`A more realistic picture of the X-ray cassette is given in Fig. 8.3. It is seen that
`the film is surrounded on both sides by an intensifying screen for optimal sensitivity.
`This figure also shows the disadvantage of the intensifying screen, viz. it causes a
`certain blur which impairs the definition of the image. This is due to the fact that
`the direction, of the light emission is independent of the angle of incidence of the
`X-ray photon, so that the emission diverges in all directions. This is worse, since the
`
`S=screen
`F = film
`
`X- R A Y BEAM
`
`l l l l l l l l
`
`CA SSE TTE FRONT
`
`SC REEN BASE
`
`FLUORESCENT LAYER
`SUPERCOAT
`EMUL S ION
`
`FILM BASE
`
`EMULS ION
`SU P ERCOAT
`FLU ORESCE NT LAY E R
`
`SCREEN BASE
`
`CA SS ETTE B AC K
`
`Fig. 8.3. Section through an X-ray cassette containing a double-coated film (F) and a pair o f
`intensifying screens (S)
`
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`
`

`

`8.1. Introduction
`
`149
`
`diffusing emission light reaches not only the film side adjacent to the emitting screen,
`but also that remote from the screen (see Fig. 8.3). This is known as the cross-over
`effect. Occasionally radiologists prefer, therefore, to make X-ray images without an
`intensifying screen. This is sometimes possible, for example, for an image of the hand
`which is a non-moving object.
`Of course the packing and the size of the crystallites in the screen and the thickness
`of the screen are another source of unsharpness in the image. It is obvious that the
`smaller the crystal size, the closer the packing, and the thinner the screen layer is, the
`better the sharpness wi 11 be.
`From this discussion it will be clear that the requirements for X-ray phosphors to be
`used in intensifying screens are the following: high X-ray absorption and high density,
`a high conversion efficiency for X-ray to light conversion, an emission spectrum which
`covers the film sensitivity (in practice blue or green), stability, and an acceptable cost
`price. The factors determining a high conversion efficiency were discussed in Sect. 4.4.
`Although CaW04 has been used for a long time, it did not satisfy these requirements,
`as we will see below. It has been replaced by rare-earth doped materials.
`
`8.1.3 The Photostimulable Storage Phosphor Screen
`
`About ten years ago the Japanese Fuji corporation introduced a new technique for
`X-ray imaging (2]. This technique is based on the use of a photostimulable storage
`phosphor screen. The operation of a storage phosphor is depicted schematically in
`Fig. 8.4. Upon irradiation electrons are promoted from the valence band to the con(cid:173)
`duction band. In a storage phosphor a number of the created free charge carriers are
`trapped in electron traps and hole traps. The traps are localized energy states in the
`bandgap due to impurities or lattice defects. If the trap depth ~E is large compared
`to kT, the probability for thermal escape from the trap will be negligibly small and a
`metastable situation is created.
`
`c b L'IE
`1 - - ·
`
`a
`
`d
`
`2----o----
`
`CB
`
`VB
`
`Fig. 8.4. Energy band model showing the electronic transitions in a storage phosphor: (a)
`generation of electrons and holes; (b) electron and hole trapping; (c) electron release due to
`stimulation; (d) recombination. Solid circles are electrons, open circles are holes. Center I
`presents an electron trap, center 2 a hole trap
`
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`
`

`

`150
`
`8. X-Ray Phosphors and Scintillators (Integrating Techniques)
`
`H e- Ne la se r
`
`screen
`hard copy
`
`X-ray
`source
`
`a
`
`patient
`
`st~rage
`phosphor
`
`I
`c omputer
`1
`!.... photomultiplier
`
`STORAGE PHOSPHOR SCREEN
`
`b
`Fig. 8.5. (a) Schematic representation of a medical radiography system based on the use of
`a photostimulable storage phosphor. (b) The image reader in greater detail. Figures 8.2, 8.4
`and 8.5 are from A . Meijerink, thesis, University Utrecht, 1990
`
`The stored energy can be released by thermal or optical stimulation. In the case
`of thermal stimulation the irradiated phosphor is heated to a temperature at which
`the energy barrier ~E can be overcome thermally. The trapped electron (or hole)
`can escape from the trap and recombine with the trapped hole (or electron). In the
`case of radiative recombination, luminescence is observed which is called thermally
`stimulated luminescence (TSL) (compare Sect. 3.5). Under optical stimulation the
`energy of an incident photon is used to overcome 6.E. The luminescence due to
`optical stimulation is called photostimulated luminescence (PSL). The phenomenon of
`stimulated luminescence from storage phosphors has been known since 1663 (Boyle) 2 •
`Storage phosphors have found a wide range of applications, e.g. as infrared detectors
`and in the field of dosimetry [3].
`The X-ray imaging system based on a photostimulable storage phosphor is de(cid:173)
`picted in Fig. 8.5. The X-ray film is replaced by a storage phosphor screen as a
`primary image receptor. The transmitted X-ray photons are absorbed by the storage
`phosphor in the screen, where a dose-proportional amount of energy is stored. The
`latent image in the screen is read out by scanning the phosphor screen with a focussed
`
`2 Boyle reported in the Register of the Royal Society (1663) 213 "a glimmering light from
`diamond by taking it to bed with me and holding it a good while upon a warm part of my naked
`body".
`
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`

`8.1 . Introduction
`
`151
`
`He-Ne laser. The red laser light (A. = 633 nm) stimulates recombination, resulting
`in photostimulated luminescence. The intensity of the photostimulated luminescence
`is proportional to the X-ray dose. For each spot of the laser beam on the screen the
`intensity of the photostimulated luminescence is measured by a photomultiplier tube
`and stored in a computer. The X-ray photograph in the computer can be visualized
`on a monitor or by making a hard copy.
`This new technique for X-ray imaging offers several important advantages over
`conventional screen-film radiography. The response of the system is linear over at
`least four decades of the X-ray dose (10- 2 -
`102 mR). This wide dynamic range
`prevents overexposure and underexposure. The sensitivity of the system is higher
`due to a higher sensitivity of the photomultiplier tube to light compared to film.
`The higher sensitivity enables a reduction of the exposure time. Finally, the digitized
`images obtained with this system can be processed by a computer which offers the
`possibility of digital manipulation and facilitates archiving.
`Apart from the high price, the main disadvantage of the digital X-ray imaging
`system is the lower resolution. Due to scat