Philips Reseàrch Reports

Transcription

1 VOL. 30 No. 5 OCTOBER 1975 Philips Reseàrch Repors EDITED BY THE RESEARCH LABORATORY OF N.V. PHILIPS' GLOEILAMPENFABRlEKEN. EINDHOVEN, NETHERLANDS R928 Philips Res. Reps 30, , 1975 BaFCI :Eu2+, A NEW PHOSPHOR FOR X-RAY- INTENSIFYING SCREENS Absrac 1. Inrodueion by A. L. N. STEVELS and P. PINGAULT *) A number of phosphors for Xsray-inensifying screens have been evaluaed by calculaing figures of meri. On use in combinaion wih sandard ("blue"-sensiive) X-ray film, BaFCl:Eu 2 + and BaFBr:Eu 2 +, give beer performance han he radiional CaW0 4 and more recenly developed UV- or blue-emiing maerials (e.g. sulphaes and y-oxysulphides), The calculaed figures of meri of BaFCl:Eu 2 + or BaFBr:Eu 2 + Isandard-film combinaions are comparable o hose of Gd S:Tb/green-sensiive X-ray film sysems. The preparaion of opimal fîuorohalideeuê+ phosphors involves proper formaion of he hos laice, complee reducion of Eu3+ ions as well as eliminaion of aferglow. Measuremens on powders and experimenal screens indicae ha, by using BaFCl:Eu 2 + screens in radiography, imporan dose reducions can be achieved wihou he necessiy of using oher han sandard ("blue"-sensiive) X-ray films. In he pas several aemps have been made o replace Ca W04, as a maerial for X-ray-inensifying screens. Among hese phosphors, ZnS :Ag, BaSi 2 0s :Pb and BaS04:Pb had some emphemeral success. In he long run, however, heir advanages did no surpass he drawbacks in such a way ha hese maerials rendered CaW04 obsolee, even for special applicaions. More recenly a large number of new maerials have been found, which, in heory, can be used in radiography. These phosphors include: - alkali-earh sulphaes and phosphaes: (Ba, Sr)S04:Eu2+, Ba3(P04)2:Eu2+; - cesium iodide doped wih Na or Tl; - alkali-earh :Iluorohalides (BaPCI :Eu 2 +, BaPBr :Eu2+); - rare-earh oxysulphides (Gd S:Tb, Y202S:Tb). Some of hese maerials have been discussed more or less exensively in lieraure 1-6), bu no sysemaic evaluaion of he phosphors has been effecuaed. On he one hand, pracical problems such as he preparaion of opimal maerials and he measuremen of absorpion as a funcion of X-ray energy make his a cumbersome ask. On he oher hand, such a large number of properies have o be aken ino accoun ha i is difficul o define a figure of meri covering hem all. *) Massio-Philips, Ailly-Le-Hau-Clocher, France.,"

2 278 A. L. N. STEVELS AND F. PINGAULT In he presen paper we define a figure of meri (which is in fac an X-ray quanum-deecion efficiency of phosphor-screen-film combinaions) calculaed from X-ray absorpion and luminescence and deecion-efficiency daa. X-ray noise and he conras rendiion of screens are no included in his figure bu i will be found ha he classificaion of he phosphors by quanum-deecion efficiencies is no changed by aking hese ino accoun. For phosphors o be used in inensifying screens we find ha BaFCI :Eu2+ and BaFBr:Eu 2 + have high figures of meri. These fiuorohalides have superior properies compared wih oher UV- or blue-emiing phosphors, like CaW04, (Ba, Sr)S04:Eu2+ or Y202S:Tb, and his promped furher invesigaion a our laboraories. CsI:Na has a figure of meri which is even beer han for BaFX:Eu2+ (X = Cl, Br) bu his maerial is no suiable for pracical applicaion in inensifying screens since is luminescence is adversely affeced by moisure in he air. The presen paper discusses he problems of preparing he fluorohalides. In addiion, a number of measuremens are presened which subsaniae he good performance of BaFCI:Eu2+ and BaFBr:Eu2 + prediced by he calculaions. 2. Calculaion of X-ray quanum-deecion efficiences. ' 2.1. Phosphors o be evaluaed In he presen calculaions we consider he following phosphors: (1) BaFBr:Eu2+ and BaFCI:Eu2+. Their luminescence upon UV and C.R. exciaion has been described recenly by Sommerdijk e al. 7). (2) (Ba, Sr)S04:Eu2+ 8). On parial replacemen of Ba by Sr opimum X-rayluminescence properies are found. These are beer han for he relaed compound Ba3(PO4)2:Eu 2 + 9), so ha he laerwill no be considered furher. (3) CaW04, he radiional phosphor for inensifying screens. (4) CsI:Na. Evaporaed layers of his phosphor have replaced (Zn, Cd)S:Ag powder screens in a new generaion of image-inensifier ubes (e.g. refs I and 2). (5) CsI:Tl. An alernaive proposed for he blue-emiing CsI:Na. The CsI:Tl, phosphor has a broad ("whie") luminescence band. (6) Gd202S:Tb. This phosphor has been proposed recenly by Buchanan e al. (see e.g. ref. 6) for inensifying screens. In conradisincion o he oher phosphors in he lis, Gd202S:Tb is mainly green- (and even red-) emiing. To make full use of properies of his phosphor, he use of oher han sandard (UV- and blue-sensiive) X-ray film is necessary. (7) Y202S:Tb. A maerial relaed o Gd202S:Tb 5). ls lower X-ray absorpion han e.g. ha of he Gd compound is compensaed by is high radian efficiency and, on use of sandard X-ray film, is favourable blue emission colour.!

3 BaFCI :Eu2+, A NEW PHOSPHOR FOR X-RAY-INTENSIFYING SCREENS The figure of meri For a given X-ray energy he figure of meri of a phosphor screen is defined as follows: where A E = he X-ray absorpion of he screen; 17 = he luminescence radian efficiency under cahode-ray exciaion of he phosphor considered; A.Em= he mean wavelengh of he emission deeced by he X-ray film; CE D... = a correcion facor for energy losses in he screen; = he efficiency of deecion of he screen emission (I;..) by a film wih response R;.; D;. is defined as J I...R...dA.fJ I;. da..... ;. The X-ray absorpion coefficiens a X-ray energies in he keV range were calculaed from he ables published by Sorm and Israël!"). Boh coefficiens!-lo,en and!-lo,abs were deermined. The former allows for he escape of all secondary radiaion, while he laer does no. In acual pracice par of he secondary X-rays generaed will be re-absorbed so ha he rue absorpion will lie beween he values calculaed from he wo coefficiens!-lo,en and!-lo,abs' Figures for he re-absorpion were calculaed wih he aid of a mehod proposed by Ooserkamp 11). X-ray-absorpion daa will be presened below for 200 (J.mhick screens. The packing densiy is aken as 50% for powder screens and equal o 100% for evaporaed screens of CsI:Na and CsI :Tl. I can be shown ha he daa obained for 200-(J.mscreens are significan in he range of hicknesses beween 50 and 400 (J.m;on comparison of he various phosphors he relaive error wih respec o he resuls for 200-(J.m screens will no exceed 20% and is less han 10% in mos cases. The laer deviaions are small compared o he differences in he acual figures of meri. The luminescence radian efficienciesunder C.R. exciaion were measured a 20 kv (see able I). In general hese daa also apply o X-ray exciaion (see e.g. refs 4 and 12) if correcions are made for ligh los in he screen (hese are included in he facor CE)' The facor A.Emconvers energy efficienciesino quanum efficiencies and is derived from he emission specra given in figs 1, 2, 3 and 4. The facor CE includes energy losses by scaeringwhich cause a difference beween energy absorpion and aenuaion of he X-ray beam, losses in he lacquer (powder screen only) and losses due o phoo-elecrons leaving he screen. A furher correcion considers he self-absorpion of he fluorescen ligh. If we ake C equal o 1 for a screen wihou losses we find, dependen on he maerial and he X-ray energy considered, C = for vapourdeposied screens and C = 0,5-0,7 for "fine-grained" powder screens. For hicknesses of 200 (J.m he figures for C for "fron-screen" applicaion (cf. ref. 13) is abou equal o ha for "back-screen" applicaion. The daa for D;.

4 280 A. L. N. STEVELS AND F. PINGAULT phosphor TABLE I Cahode-ray efficiencies and DJ. daa ofx-ray phosphors 20-kV C.R. radian efficiencies (%) deecion efficiency DJ. co BaFCI:Eu BaFBr:Eu BaS0 4 :Eu CaW CsI:Na CsI:TI Gd S:Tb Y202S:Tb given in able I were calculaed from he emission daa presened in figs 1-4 and he response of a sandard (UV- and blue-sensiive) X-ray film and a green-sensiive X-ray film (in he case of Gd S :Tb and CsI :TI) Resuls The resuls of our calculaions are presened in figs 5-8, which show he calculaed figures of meri ploed versus X-ray energy. In order o simplify a direc comparison a a paricular X-ray energy, a relaive scale has been inroduced by aking he figure of meri of CaW0 4 equal o 1 independen of he X-ray energy. In figs 5-7 sandard X-ray film has been used as deecor of he screen emission; in fig. 8 figures are calculaed for he use of green-sensiive films. Figure 5 indicaes ha he performance of BaFCI:Eu2+ and BaFBr:Eu2+ screens is superior o ha of CaW0 4 screens beween 20 and 100 key. Beween 37 and 69 kv, ha is over almos he enire diagnosic range, F.M. rel is 3 1 and 4 7 (37 key), respecively, and increases o even 5 3 and 7 4, respecively, a 69 key. The main parameer conribuing o hese large differences is he high radian efficiency of he fluorohalides: BaFCI is almos 4 imes beer han CaW0 4 (13 vs 3 5%) in his respec and BaFBr beer sill (16 vs 3 5%). The increase is only parly due o a beer X-ray absorpion. This accouns for a facor up o 1 2 and 1 3,respecively, near 69 key. Above ha energy he X-ray absorpion of CaW0 4 is imes beer han ha of he fluorohalides bu his is more han compensaed by he beer radian efficiencyof he laer. In his energy range F.M. rel of BaFCI:Eu2+ is For BaFBr:Eu 2 + we find I can be seen in fig. 6 ha he performance of CsI :Na screens is also superior o ha of CaW0 4 screens, F.M. rel being 4-16 beween 20 and 100 key. This is

5 BaFCI :Eu2+. A NEW PHOSPHOR FOR X-RAY-INTENSIFYING SCREENS 281 oo Ire/," /'. / V\ / 1\ \ I P ~( i \/\ i J, \. i / \. \. '1.'./.~ \..' #'...,/ '..' À(nm) Fig. 1. Emission specra of BaFCl:Eu2+ and BaFBr:Eu2+ (coinciding,.-.-.), BaS04:Eu2+ (... ) and CaW04 (- - -). The specral response of a sandard X-ray film is represened by a drawn curve. Ire/,,, À(nm) Fig. 2. Emission specra of CsI:Na (- - -), CsI:Tl ( ) and CsBr:Tl (... ). The specral response of a sandard X-ray film is represened by a drawn curve. OO ,-,,, \ \ ~" \ Ire/,X J"\. 1 )\ /i I,, \'"''''r~''' \ \ , R\' À(nm) Fig. 3. Emission specrum of GdzOzS :Tb. The specral response of a sandard X-ray film is represened by he dashed curve 1, ha of a green-sensiive film by curve ,---_-----~-~-~~~ ,, I I l Ire/,>" À(nm) Fig. 4. Emission specrum of YzOzS :Tb (low Tb conen). The specral response of a sandard X-ray film is represened by he dashed curve 1, ha of a green-sensiive film by curve 2.

6 282 A. L. N. STEVELS AND F. PINGAULT F.M.rel BaFBr:Eu2~ : " I '...I... BaFCI:Eu 2 +,.,... j.,.'" I»" I,,," : / I I I (,,... ' BaFBr:Eu z.,. ----BaFCI:Eu 2 + CáW CaW X-ray energy (kev) 120 Fig. 5. Relaive figure of meri vs X-ray energy for 200-(Lm screens of BaFCI:Eu2+, BaFBr:Eu 2 + and CaW0 4 The screens are in conac wih sandard X-ray film. 18 F M.rel CsI:Na -- CsI:TI CaW X-ray energy (kev) Fig. 6. See fig. 5. The drawn curve represen figures of meri for CsI:Na, for CaW0 4 for CsI:TI and mainly due o he beer luminescen-energy efficiency of Cs! :Na and o he fac ha his phosphor can be evaporaed. Vapour-deposied screens have approximaely wice he packing densiy of powder screens and herefore a higher X-ray absorpion and lower X-ray quanum noise. Moreover, he facor C (see above) in hese screens is higher. A furher imporan advanage of using evaporaed screens would be ha echnologies can be employed ha resul in

7 BaFC1:Eu3+. A NEW PHOSPHOR FOR X-RAY-INTENSIFYING SCREENS 283 he suppression of laeralligh leakage and hus in improvemen of he resoluion of he screen 2). The drawback ha renders CsI:Na unsuiable for use in X-ray-inensifying screens is he hygroscopiciy of he maerial. The decrease of-he luminescence in air is so fas ha even if proecive layers were employed, screens wih an accepable lifeime could no be produced *). This is why, alhough he figure of meri of CsI:Na is 1,5-3 imes higher han ha of BaFCl:Eu2+, we have no considered his phosphor furher for use in inensifying screens. CsI:Tl is much less hygroscopic, bu he ouperformance of CaW0 4 is much less and in fac resriced o he keV range. If we compare i wih he l.uorohalides no advanage exiss a all. The unfavourable emission colour of CsI:Tl compared wih he sandard X-ray film is responsible for his (cf. fig. 2). In conradisincion o he iodide, CsBr:TI has mainly a near-uv emission 14). The properies of his Tl-doped bromide are in some aspecs more similar o hose of CsI :Na. One of hese, he hygroscopic characer of he "maerial, seems o be prohibiive for pracical applicaion. In fig. 7 we have brough ogeher daa on Y2028 :Tb, Gd2028 :Tb, Ba80 4 :Eu2+ and CaW0 4 In his group Y2028:Tb ends o be he mos promising maerial wih a figure of meri 3 5 imes beer han CaW0 4 below 69 key. Compared o he l.uorohalides Y2Û28:Tb performs slighly beer below 37 key, bu is behaviour above ha energy is inferior. In he diagnosic range, BaFCI :Eu2+ ouperforms he oxysulphide 1 0 resp. 1 6 x a 40 kev, 1 4 resp. 2 0x a 60 key and 1 8 resp. 2 5x a 80 key. We noe ha in Y2028 :Tb a high luminescence radian efficiency has o F.M.rel 6 3 )2 25:Tb ~~::.-: :Tb BiJ504:EU!.>...-f,.-: CaW04 1 CaW04. (, (... _ /...J' '- :"- _ -Ba504:Eu Gd s:tb---:::.: Gd2~S:Tb % m M ro 00 m ~ - X-ray energy (kev) Fig. 7. See fig. 5. The drawn curves, represen Y202S :Tb (Iow Tb conen), he dashed curve Gd202S:Tb, he dash-doed curve BaS04:Eu2+. CaW04 is represened by a drawn line...) On using CsI:Na in vacuum ubes (e.g. as phosphor in inpu screens of image-inensifier ubes) he hygroscopy problem does no exis. CsI:Na is han clearly superior. This has been analysed in some deail in e.g. ref. 2.

8 284. A. L. N. STEVELS AND F. PINGAULT compensae for a relaively low X-ray absorpion. The laer also gives rise o a relaively high X-ray quanum noise. Above 50 kev he relaed GdzOzS :Tb has a much beer X-ray absorpion, bu he response of he sandard X-ray film o is mainly green emission is very Iow. The calculaed P.M'rel are beer han CaW0 4 in he keV range only. Ouside his area he performance is no so good. For BaS0 4 :Eu2+ he P.M'rel curve has a similar form as for BaFCI:Euz + and Baf'BrBu-". The figures of meri are lower by a facor of more han 2 resp. more han 3, however. In fig. 8 we give he calculaed figures of meri of some phosphor screens used in combinaion wih green-sensiive :films. In order o compare wih figs 5-7, we use for CaW0 4, in fig. 8, F.M. daa derived for sandard film. For simpliciy i is assumed ha he overall sensibiliy of he green film is equal o ha of he sandard film (i may be doubed, however, wheher in pracice such sensibiliies can be reached). The performance of CsI :TI wih green film is as good as ha of CsI:Na used in combinaion wih sandard X-ray film. Since CsI:Tl is much less hygroscopic han he Na-doped phosphor i seems o be a very promising maerial. Our firs ess wih vapour-deposied CsI:Tl screens show indeed resuls as good as suggesed by fig. 8. However, i was also found ha afer X-ray exposure of CsI :Tl screens a srong aferglow occurred, which is unaccepable in radiological pracice. The calculaed performance of GdzOzS :Tb screens (used in combinaion wih green-sensiive X-ray film) is less han ha of Cs! :Tl ones, bu he applicaion problems seem o be less. In fac such screens are being markeed now all over he world. On comparison of GdzOzS:Tb (fig. 8) wih he BaFX:Eu2+ phos- ~'r ~,,, " CsI:TI ~ CaW04 %L_-L--20~~--4~O--J_-6JO---L--8~O--L--ro~o--J_~uo - X-ray energy (kev) Fig. 8. See fig. 5. Daa are given for Gd S :Tb and CsI :TI in conac wih a green-sensiive Xsray film, and for CaW0 4 in conac wih a sandard X-ray film.

9 BaFCI :Eu2+. A NEW PHOSPHOR FOR X-RAY-INTENSIFYING SCREENS 285 phors (fig. 5) we see ha in he keV range BaFBr:Eu2+ is slighly beer, whereas Gd S :Tb is in urn slighly beer han BaFCl :Eu 2 +. In he key range, he fluorohalide-buê" phosphors ouperform Gd S:Tb; beween 50 and 69 key Gd S:Tb is clearly superior. Above 69 key BaFBr:Eu2+ can compee wih he oxysulphide; he figure of meri of BaFCl:Eu 2 + is less, In pracice, where polychromaic X-rays are used, a general conclusion may be ha BaFBr:Eu2+ and BaFCI:Eu 2 + have a performance comparable o Gd S :Tb, and have he imporan advanage over he las-named maeria] ha no special mms have o be used. 3. Preparaion of BaFCI:Eu2+ and BaFBr:Eu2+ phosphors 3.1. General consideraions In he firs insance samples of BaFCl:Eu2+ and BaFBr:Eu2+ were prepared by milling dry BaF 2, BaCl 2 C.q. BaBr 2 and EuX 3 (X = F, Cl, Br) and heaing he mixure a C in a slighly reducing amosphere consising of N 2 and H 2 As a resul a large variey of whie o bluish grey producs were obained. Some samples consised of well crysallized coarse grains, ohers of irregularly inergrown small crysallies only. The measured ligh oupus ranged from pracically zero o abou he maximum values prediced by our calculaions. All samples prepared in he way described above were found o have an unaccepable aferglow afer X-ray irradiaion (see sec. 3.3). For BaFCl :Eu2+ he aferglow was imes as inense as for CaW0 4, while for BaFBr:Eu2+ his facor was up o 100 x. Moreover, afer milling in order o ge an appropriae grain size, hese high aferglow levels had increased furher. In view of he foregoing he problem of preparing fluorohalides suiable for use in X-ray-inensifying screens is wofold, namely: (1) on he one hand, o find he proper condiions for he formaion of he hos laice, he reducion of Eu3+ and incorporaion of Eu2+ ions; (2) on he oher hand, o avoid he formaion of aferglow cenres. As will be seen below, hese objecives do no conflic bu are o a large exen parallel. I is now possible o prepare samples wih ligh oupu close o he calculaed (maximum) values. In he case of BaFCI :Eu2+ he aferglow has been reduced o he CaW0 4 level. For BaFBr:Eu2+ a considerable reducion has been achieved bu a presen he aferglow is a leas four imes ha of he ungsae. From an applicaion poin of view, herefore, BaFCI:Eu 2 + is a presen he mos ineresing member of he BaFX:Eu 2 + family Preparaion experimens Firsly we observed ha even small races of oxygen in he saring maerials

10 286 A. L. N. STEVELS AND F. PINGAULT or in he gas flow during heaing were faal for boh he ligh oupu and he aferglow of he sample. In fac, an almos linear correlaion was found beween aferglow level and oxygen conen. On doping wih Eu i was found ha fluorohalides acivaed wih Eu in he form of EuF3 did give producs inferior o samples where Eu was added as EuCl3 or EuBr3' This is ascribed o he high meling poins of EuF3 and EuF 2 (namely 1390 and 1380 C) in relaion o EuCI3/EuCI 2 (850 C/727 0c) and EuBr3/EuBr 2 (702 C/677 0C). Analysing in more deail he reducion of rivalen Eu and is subsequen incorporaion ino he hos laice, led us o conclude ha hese processes require high reacion emperaures and a srongly reducing amosphere. However, he proper formaion of he hos laice was found o be badly affeced. I was found ha BaCl 2 and more especially BaBr 2 were much more unsable under hese condiions han BaFCI and BaFBr, respecively. This led o he conclusion ha he formaion of he hos laice and he reducion/incorporaion of he Eu ions has o be achieved by wo separae seps, he firs a emperaures as low as possible, and he second a emperaures as high as possible. In he low-emperaure process BaFCI and BaFBr were obained byevaporaing o dryness a suspension of BaF 2 o which BaCl 2 2H 2 0 and BaBr 2.2H 2 0 respecively had been added. X-ray diffracion showed ha he produc obained in his way was free of BaF 2 and BaCl 2 C.q. BaBr 2 Afer addiion of EuCl3 (EuBr3) he samples were fired a C in an amosphere conaining e.g. 1% of H 2 The coarse-grained phosphor obained in his way has o be milled o obain he grain size needed for making inensifying screens. This had he effec of increasing he aferglow, bu he laer appears o be of a differen naure from he aferglow discussed above. In our opinion i is relaed o crysallie perfecion raher han o chemical defecs, since i can be resored again o is original level by prolonged heaing a e.g. 700 C in N 2 Under he preparaion condiions described above (high emperaure, srongly reducing amosphere) he formaion of HCl and HBr seems o be favoured and halide vacancies may herefore exis in BaFCI. By doping experimens we have found ha hese chlorine vacancies - if presen - do no increase he aferglow. For example, addiion of small amouns of KCI in BaFCI:Eu 2 + resuled raher in a lower han in a higher aferglow. On he oher hand he presence of meal vacancies seems o favour he formaion of aferglow cenres. Addiion of e.g. LaCl3 o BaFCI :Eu2+ resuled in grealy increased aferglow. A similar effec was also observed on heaing BaFCI in HCl gas, dilued wih nirogen. 4. Measuremens 4.1. Ligh oupu of powder samples The ligh oupus on X-ray exciaion were measured by comparing powder layers of abou 500 (.Lmhickness (1 4 g/cm 2, packing densiy is 60 %). The resuls

11 BaFCl :Eu2+, A NEW PHOSPHOR FOR X-RAY-INTENSIFYING SCREENS 287 TABLE Il Measured and calculaed ligh-oupu daa phosphor measured ligh adjused ligh calculaed ligh oupu oupu oupu CaW BaFCl:Eu *) ("fine" -grained) BaFCI:Eu *) ("coarse" -grained) BaFBr:Eu *) The effec of grain size on he ligh oupu can only be esimaed in an inaccurae way. are given in able Il. The firs column represens measured values of he quanum oupu, while he second refers o daa adjused for comparison wih he calculaed figures in fig. 1. Wih a ube volage of 70 kv and a 20-mm filer he effecive radiaion disribuion is peaking a kev. ForBaFCI:Eu 2 + here... is reasonable agreemen beween he measured daa and he values calculaed (4 4 imes beer han CaW0 4 ) for hese energies. For BaFBr:Eu2+ he agreemen is less saisfacory (he ligh oupu was calculaed o be 6 5 imes "higher), In fac he measured ligh oupus are even lower han for BaFCI :Eu 2 +. A presen we have no explanaion for his phenomenon, since he effeciveness of BaFBr:Eu2+ on film is quie good (see below). The effeciveness in conac wih sandard film was measured by comparing he X-ray doses required o obain a uni densiy. For his purpose powder phosphors were placed in a cuvee measuring 10 X 10 xo 35 mm". The film in direc conac wih he cuvee is placed a he X-ray-ube side. Wih a ube volage of 90 kv and a 3'5-cm Al filer he effeciveness of BaFCI:Eu 2 + was found o be 5-7 imes ha of CaW0 4, while values up o 10 X were measured for BaFBr:Eu Aferglow of powder samples The aferglow measuremens consised in inegraing par of he decay of he luminescence as a funcion of ime. A film was placed in conac wih he cuvee described above, one minue afer irradiaion of he sample, and lef in ha posiion for five minues. The aferglow figure is expressed by he raio of he film densiy obained o ha ofa sandard CaW0 4 screen measured in he same way. For 30 Röngen X-ray radiaion from a ube a 90 kv (no filering) we found for our bes BaFCI :Eu2+ an aferglow figure of 1. For BaFCI :Eu2+ obained by direc synhesis a high emperaures (cf. sec. 3.1) he aferglow figure was 4-6.

12 288 A. L. N. STEVELS AND F. PINGAULT In he laer case he aferglow is so brigh ha i can even be measured wih a phoomuliplier. The aferglows ofcaw0 4 and well prepared BaFCI:Eu2+ are close o he phoomuliplier noise, so ha proper comparison wih hese sandards was no possible in his way. I was, however, esimaed ha he aferglow of direcly synhesized BaFCI :Eu2+ is more han en imes ha of CaW0 4 Phoomuliplier measuremens of he aferglow of BaFBr :Eu (prepared in a similar way) gave a raio up o 100. For BaFBr:Eu2+ obained wih our improved preparaive mehod, his value is reduced o abou Measuremens on experimenal screens In figs 9 and 10 we compare new BaFCI:Eu z + screens (4 4 gfdmz ~ 200 (.Lm hickness), Philips Universal screens (3'7 g/dm" ~ 120 pm hickness), Philips Ulra-S screens (8 9 gfdm Z ~ 290 (.Lmhickness), and commercially obained YzOzS:Tb screens of abou 350 (.Lmhickness. These screens were evaluaed using a UV- and blue-sensiive sandard film. For measuremens on a commercially obained GdzOzS :Tb screen (abou 190 (.Lmhickness) a green-sensiive film (cf. figs 3 and 4) was used. For he screen-film combinaions he relaive speed was defined as he raio of X-ray doses required o obain uni film densiies for each radiaion qualiy considered. The laer was characerized by he hickness HVT of Al which reduces he X-ray beam o half is original energy. I can be concluded from fig. 9 ha he screen consising of he new BaFCI:Eu2+ phosphor is superior o he blue-emiing phosphors over he whole energy range. A very low HVT he y-oxysulphide is sill comparable bu is relaive speed is much lower in he acual medical range. Boh CaW0 4 la r 4r 2r B- Relaive speed f- 6fr f- f- a 2 5 q.~ 2+ ~BaFCI: u (... - Gd202S:Tb :. I _ _._ Ulra-S I }:zd.zs:tb HVT(mmAl) l!pj.versal Fig. 9. Relaive speed of X-ray-inensifying screens for various X-ray-energy disribuions. The laer are characerized by HVT (mm AI), he hickness of Al reducing he X-ray beam o half is original energy. The curves include a BaFCI :Eu 2 + screen, a Gd202S :Tb screen, a Y202S:Tb screen, and Philips Ulra-S and Philips Universal CaW0 4 screens. For screen hicknesses, see ex

13 BaFCI :Eu2+. A NEW PHOSPHOR FOR X-RAY-INTENSIFYING SCREENS 289, screens, he relaively fas (and hick!) ulra-s and he Universal screens, lay sill furher behind so far as speed is concerned. On comparison of he 'BaFCI:Eu2+ /sandard-film combinaion wih he Gd202S:Tb/green-film combinaion, we see ha he BaFCl:Eu 2 + combinaion is beer a he lowes X-ray energies (HVT R:i 4). A somewha higher HVT (beween 5 and 7) Gd202S :Tb is slighly beer. Above HVT = 8 mm we have approximaely equal performances. In fig. 10 we have ploed he conras rendiion as a funcion of he spaial frequencies for he screens discussed in fig. 9. The pseudo-mtf curves were obained by measuring he square-wave response of he screens wih he help of a Funk es grid consising of lead srips a varying disances apar. The X-ray radiaion originaed from a ube a 60 kv and was filered by 3 5 cm of AI. The Philips Universal screens had he bes resoluion. Owing o he fac ha he screens are hicker and he grains in i are coarser, he pseudo-mtf of he Gd202S:Tb, BaFCI:Eu2+ and Ulra-S CaW0 4 ypes is below ha of he Universal screen. The same conclusions appear o hold even more definiely for he Y202S:Tb screen. The daa in figs 9 and 10 can be combined by defining a relaive qualiy facor F = S R 2 where S is he speed and R he resoluion measured by he mehods described above. The excellen performance of he new BaFCI:Eu2+ is illusraed by he fac ha F relaive o Universal screens ranges for 3 up o 7. Wih respec o an Ulra-S screen, Ffor BaFCI:Eu 2 + screens is Because of poor resoluion he F number of Y202S :Tb ranges from 1 5o even below 1, compared wih boh ypes of CaW0 4 screens. For he Gd20 2 S :Tb combinaion we find F wih respec o he Universal screens in he range 2 5 o 6 and on l00~ ~ S- SOf- Relaive conras ~~-..._-.~ e, """... -,,~ ',,':::-... <, 0. """'"0 <, -..~....'>::::::-... '>::.':--., <,.: 2- I 1 I 10~----~10~----~20~----~3~ ~40' - Freq. (fp/cm) Fig. 10. Relaive conras vs spaial frequencies for he screens in fig. 9.

14 290 A. L. N. STEVELS AND F. PINGAULT comparison wih Ulra-S screens beween 2 and 3. In boh cases BaFCI :Eu 2 + is equal or even beer han Gd S :Tb. 5. Conclusion Compared o radiional phosphors such as CaW0 4 and o more-recenly developed UV- or blue-emiing phosphors, BaFCI:Eu2+ and BaFBr:Eu2+ are superior for use in X-ray-inensifying screens. If also green- or whie-emiing phosphors are aken ino accoun, we find ha he ffuorohalide-euê" screens are a leas comparable wih e.g. Gd S :Tb screens, bu have he imporan advanage ha no special films have o be used. These conclusions are suppored boh by calculaed figures of meri and by measuremens on powders and experimenal screens. By using appropriae preparaive mehods he original drawback of aferglow afer irradiaion has been eliminaed for BaFCI:Eu 2 +. The high speed of BaFCI:Eu 2 + screens make us believe ha wih hem dose reducions can be achieved in full-size radiography. 6. Acknowledgemen The auhors wish o hank Mrs A. D. M. Schrama-de Pauw for her conribuions o his invesigaion. Eindhoven, March 1975 REFERENCES 1) C. W. Ba es, Adv. Elecronics and Elecron Physics 28A, 451, ) A. L. N. Sevels and W. Kühl, Paper a he XIII Congress of Radiology (Madrid 1973), Medica Mundi 19, 3, ) G. W. Ludwig and J. S. Prener, IEEE Trans. nucl. Sei. 19, 3, ) G. W. Ludwig, J. Elecrochem. Soc. 118, 1152, ) R. V. Alves and R. A. Buchanan, IEEE Trans. nucl. Sei. 19, 415, ) R. A. Buchanan, S.1. Finkelsein and K. A. Wickersheim, Radiology 105, 187, ) J. L. Sommerdijk, J. M. P. J. Versegen and A. Bril, J. Luminescence 8,502, ) Duch paen specificaion nr , May ) U.S. paen specificaion nr. 3,527,710, Sep ) E. Sorm and H. I. Israël, Na. echno Informaion Service Rep. LA 3753 (1967). 11) W. J. Ooserkamp, Philips Res. Labs, Inernal Repor (1972). 12) J. A. de Poorer and A. Bril, J. Elecrochem. Soc. 122, 1086, ) H. C. Hamaker, Philips Res. Reps 2, 55, ) S. G. Zazubovich, Phys, Sa. sol. 38, 119, 1970.