Bariumfluorohalide phosphor comprising calcium ions at the surface

A stimulable phosphor panel is provided comprising a bariumfluorohalide phosphor characterized in that in said phosphor at least 1 mole % of the Ba-ions are replaced by Ca-ions and at least 10% of the total amount of Ca-ions are located closely to the surface of the phosphor particles. A method for producing said phosphor is also provided. The use of said phosphor in a method for recording and reproducing a radiation image is disclosed.

DESCRIPTION 
1. Field of the Invention 
This invention relates to a photostimulable alkaline earth fluorobromide 
phosphor especially a phosphor being stimulable with flight with 
wavelength lower than 600 nm and having improved erasability. 
2. Background of the Invention 
In a method of recording and reproducing an X-ray pattern disclosed e.g. in 
U.S. Pat. No. 3,859,527 a special type of phosphor is used, known as a 
photostimulable phosphor. The phosphor is incorporated in a panel is 
exposed to incident pattern-wise modulated X-rays and as a result thereof 
temporarily stores therein energy contained in the X-ray radiation 
pattern. At some interval after the exposure, a beam of light scans the 
panel to stimulate the release of stored energy as light that is detected 
and converted to sequential electrical signals which are processable to 
produce a visible image. For this purpose, the phosphor should store as 
much as possible of the incident X-ray energy and emit as little as 
possible of energy until stimulated by the scanning beam. 
As described in U.S. Pat. No. 4,239,968 europium-doped barium fluorohalides 
are particularly useful for application as stimulable phosphors for their 
high sensitivity to stimulating light of a He-Ne laser beam (633 nm), ruby 
laser beam (694 nm). The light emitted on stimulation, called stimulated 
light is situated in the wavelength range of 350 to 450 nm with its main 
peak at 390 nm (ref. the periodical Radiology, September 1983, p. 834). 
Most of the phosphor plates are stimulated by laser light with wavelength 
longer than 600 nm. It can be beneficial to have the possibility to 
stimulate the storage phosphor with light of lower wavelengths. Phosphors 
that can be stimulated with light of wavelengths below 600 nm have the 
advantage that the dark-decay of the stored energy is lower. This means 
that the user does not have to read the phosphor plate as fast as possible 
after the exposure, but that instead further exposures on other plates can 
be made before the reading of all the plates, without the risk that 
information on the first plate will be lost or will be more difficult to 
retrieve. Apparatus for reading phosphor plates using laser sources 
emitting light of wavelengths below 600 nm, can be built smaller, since 
relatively high power lasers emitting light of wavelength below 600 nm, 
are smaller than high power lasers emitting light of wavelength above 600 
nm. 
Phosphor compositions have been formulated showing a stimulation spectrum 
in which the emission intensity at the stimulation wavelength of 550 nm is 
higher than the emission intensity at the stimulation wavelength of 650 
nm. A suitable phosphor for said purpose is described in U.S. Pat. No. 
4,535,237 in the form of a divalent europium activated barium 
fluorobromide phosphor having the bromine-containing portion 
stoichiometrically in excess of the fluorine. 
A photostimulable phosphor has been disclosed wherein the energy stored in 
said phosphor can be freed efficiently, as fluorescent light, by 
photostimulation with light in a wavelength range below 550 nm, so that 
light of an argon ion laser corresponding with its main emission lines of 
514 and 488 nm and frequency doubled light (532 nm) of a solid state 
Nd:YAG laser originally emitting at 1064 nm can be more efficiently used 
in photostimulation than He-Ne laser light of 633 nm. It has been 
disclosed that said phosphors could even be photostimulated with light of 
He-Cd laser emitting at 442 nm. 
The last mentioned stimulable phosphor is within the scope of the following 
empirical formula: 
EQU Ba.sub.1-x-y-p-3q-z Sr.sub.x M.sub.y.sup.II M.sub.2p.sup.I M.sub.2q.sup.III 
F.sub.2-a-b Br.sub.a X.sub.b :zA 
wherein: 
X is at least one halogen selected from the group consisting of Cl and I, 
M.sup.I is at least one alkali metal selected from the group consisting of 
Li, Na, K, Rb and Cs; 
M.sup.II is at least one alkaline earth metal selected from the group 
consisting of Ca and Mg; 
M.sup.III is at least one metal selected from the group consisting of Al, 
Ga, In, Tl, Sb, Bi, Y or a trivalent lanthanide, e.g. La, Ce, Pr, Nd, Sm, 
Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; 
a is a number satisfying the conditions of 0.85.ltoreq.a.ltoreq.0.96 when x 
is 0.17.ltoreq.x.ltoreq.0.55 and 0.90.ltoreq.a.ltoreq.0.96 when x is 
0.12.ltoreq.x.ltoreq.0.17; 
y is in the range 0.ltoreq.y.ltoreq.0.10; 
b is in the range 0.ltoreq.b&lt;0.15; 
p is in the range 0&lt;p.ltoreq.0.3; 
q is in the range 0.ltoreq.q.ltoreq.0.1; 
z is in the range 10.sup.-6 .ltoreq.z.ltoreq.10.sup.-2, and 
A is Eu.sup.2+. 
The main drawback of stimulation by lasers emitting light of shorter 
wavelengths, is the difficulty of reaching sufficient erasure depth in a 
short time. As described in Radiology, September 1983, p. 834, the imaging 
plate containing the stimulable phosphor can be used repeatedly to store 
X-ray images simply by flooding it with light to erase the residual energy 
it contains. This erasure of residual energy has to proceed both very 
rapidly (the imaging plate has to be rapidly available for repeated use) 
and very thoroughly because the imaging plate can not carry so called 
"ghost images" of the previous exposure when used for a new exposure. 
The slow erasure of all remaining stored energy from phosphors, being 
stimulable with light of lower wavelength, has hitherto prevented the use 
of short stimulation wavelengths, in spite of the advantages outlined 
above. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the invention to provide a stimulable phosphor panel 
(supported as well as self-supporting) that can be stimulated with light 
having wavelengths shorter than 600 nm and that can repeatedly be used 
without "ghost-images". 
It is a further object of the invention to provide a method for producing 
stimulable phosphors that can be used in stimulable phosphor panels and 
that can be stimulated with light having wavelengths shorter than 600 nm 
and that can repeatedly be used without "ghost-images". 
Further objects and advantages of the invention will become clear from the 
detailed description hereinafter. 
The objects of the invention are realised by providing a stimulable 
phosphor panel comprising a bariumfluorohalide phosphor characterised in 
that in said phosphor at least 1 mole % of the Ba ions are replaced by 
Ca-ions and at least 10% of said Ca-ions are located closely to the 
surface of the phosphor particles. 
In a preferred embodiment the phosphor corresponds to the general formula 
I: 
EQU Ba.sub.1-x-y-y'-z-r Sr.sub.x Ca.sub.y Pb.sub.y,Cs.sub.2r Eu.sub.z 
F.sub.2-a-b Br.sub.a I.sub.b I 
wherein 0.ltoreq.x.ltoreq.0.30, 0.01.ltoreq.y&lt;0.1, 
0.ltoreq.y'.ltoreq.10.sup.-3, 10.sup.-7 &lt;z&lt;0.15, 0.ltoreq.r&lt;0.05, 
0.75.ltoreq.a+b.ltoreq.1.00 and 0.02&lt;b&lt;0.20 
In a preferred embodiment the phosphor corresponds to the general formula: 
EQU Ba.sub.1-x-y-y'-z-r Sr.sub.x Ca.sub.y Pb.sub.y,Cs.sub.2r Eu.sub.z 
F.sub.2-a-b Br.sub.a I.sub.b, II 
wherein 0&lt;x.ltoreq.0.30, 0.01&lt;y&lt;0.10, 3.10.sup.-5 &lt;y'&lt;3.10.sup.-4, 
10.sup.-7 &lt;z&lt;0.15, 0.ltoreq.r&lt;0.05, 0.75.ltoreq.a+b.ltoreq.1.00, and 
0.02&lt;b&lt;0.20. 
In a most preferred embodiment the phosphor corresponds to general formula 
II, wherein 0.02.ltoreq.y.ltoreq.0.06

DETAILED DESCRIPTION OF THE INVENTION 
During the further investigation it was surprisingly found that further 
replacement of barium ions, already partially replaced by strontium ions, 
provided stimulable phosphors that were still well suited for stimulation 
at wavelengths lower than 600 nm, but that were better erasable. The best 
results were obtained when the further replacement of barium ions 
proceeded either by calcium ions alone or by calcium ions and lead ions. 
It was found that a phosphor corresponding to the general formula I: 
EQU Ba.sub.1-x-y-y'-z-r Sr.sub.x Ca.sub.y Pb.sub.y,Cs.sub.2r Eu.sub.z 
F.sub.2-a-b Br.sub.a I.sub.b I 
wherein 0.ltoreq.x.ltoreq.0.30, 0.01.ltoreq.y&lt;0.1, 
0.ltoreq.y'.ltoreq.10.sup.-3, 10.sup.-7 &lt;z&lt;0.15, 0.ltoreq.r&lt;0.05, 
0.75.ltoreq.a+b.ltoreq.1.00 and 0.02&lt;b&lt;0.20, gave a very acceptable speed 
when stimulated with light having a wavelength below 600 nm and that the 
residual energy contained in such a phosphor could be erased easily. 
It was further found that, when using a phosphor corresponding to the 
general formula II: 
EQU Ba.sub.1-x-y-y'-z-r Sr.sub.x Ca.sub.y Pb.sub.y,Cs.sub.2r Eu.sub.z 
F.sub.2-a-b Br.sub.a I.sub.b, II 
wherein 0&lt;x.ltoreq.0.30, 0.01&lt;y&lt;0.10, 3.10.sup.-5 &lt;y'&lt;3.10.sup.-4, 
10.sup.-7 &lt;z&lt;0.15, 0.ltoreq.r&lt;0.05, 0.75.ltoreq.a+b.ltoreq.1.00, and 
0.02&lt;b&lt;0.20, i.e. a bariumfluorohalide phosphor wherein some of the Ba 
ions are replaced by Sr, Ca, Pb and Cs, high speed was achieved when 
stimulating said phosphor with light having a wavelength below 600 nm. The 
residual energy contained in said phosphor could be erased even more 
easily. 
When the Ca-ions were located close to the surface of the phosphor 
particles, corresponding to general formula II, the speed, when 
stimulating said phosphor with light having a wavelength below 600 nm, was 
not diminished, but the residual energy could be erased still better and 
faster. 
In the most preferred embodiment, the amount of Ca-ions is between 2 and 6 
mole percent, i.e. formula II with 0.02.ltoreq.y.ltoreq.0.06. It is not 
necessary that all Ca-ions are located close to the surface of the 
phosphor particles. Also when at least 10% of the total amount of Ca-ions, 
preferably at least 25% of the total amount of Ca-ions are located in the 
vicinity of the surface of said phosphor particles, good speed combined 
with high erasability is achieved. 
The location of the Ca-ions close to the surface of the phosphor particles 
is achieved by including at least two firing steps in the phosphor 
production and adding the Ca-ions containing precursors in the last firing 
step. The method for preparation of the phosphor comprises the steps of: 
(i) intimately mixing phosphor precursors, with at most 90%, preferably at 
most 75% of the total amount of the Ca-ions containing precursors, to have 
a raw mix 
(ii) firing said raw mix to obtain a phosphor, 
(iii) cooling said fired raw mix, recovering said phosphor and grinding it, 
(iv) optionally adding further phosphor precursors to said ground phosphor 
and repeating steps (ii) and (iii), 
(v) optionally repeating step (iv) one or more times and 
(vi) in a last firing step, before recovering the final phosphor, firing 
said ground phosphor in the presence of Ca-ions containing phosphor 
precursors. 
Between steps (iii) and (iv) it is possible, if desired, to mix other ion 
containing precursors to the phosphor recovered in step (iii) and firing 
said mixture, cooling, grinding it (this is step (iv) mentioned above). 
Said step (iv) may optionally be repeated one or more times before finally 
executing step (vi). The inclusion of optional steps (iv) and (V), makes 
it possible to provide a phosphor with "layered" crystals, i.e. phosphor 
crystal having a different composition from the surface to the core. 
In a preferred embodiment the Ca-ions containing phosphor precursor is 
CaF.sub.2. 
After the final firing and cooling, the sintered block of phosphor is 
milled into fine phosphor particles. The milling operation continues until 
phosphor particles with the appropriate average particle size and size 
distribution is obtained. Optionally, the milled phosphor powder can be 
classified in separate fraction with a specific particle size 
distribution. During the preparation of the phosphor any known flux 
materials can be added to the reaction mixture. Flux materials useful for 
use in the preparation of the phosphors according to the invention are, 
e.g., halides, metasilicates of alkali metals or alkaline earth metals. 
Most preferred are fluxes comprising halides of the alkali metals or 
alkaline earth metals that are already present in the raw mix. A very 
useful and preferred method for the preparation of stimulable phosphors 
according to the present invention can be found in Research Disclosure 
Volume 358, February 1994 p 93 item 35841, that is incorporated herein by 
reference. 
An other useful method for preparation of stimulable phosphors according to 
this invention can be found in U.S. Pat. No. 5,154,360. 
The phosphor particles for use in the method according to the present 
invention, are preferably classified. This classification, ensures that 
the size distribution of the phosphor particles comprises at most 20% by 
weight, preferably at most 10% by weight, of particles with a diameter 
lower than 1 .mu.m. The absence of small phosphor particles (phosphor 
particles with diameter .ltoreq.1 .mu.m) has a beneficial effect on the 
image quality. 
For use in the method according to the present invention the phosphor can 
be present in dispersed form in a binder layer that may be supported or 
self-supporting and forms a screen or panel. 
The binder layer incorporates said phosphor in dispersed form preferably in 
(a) film forming organic polymer(s), e.g. a cellulose acetate butyrate, 
polyalkyl (meth)acrylates, e.g. polymethyl methacrylate, a 
polyvinyl-n-butyral e.g. as described in the U.S. Pat. No. 3,043,710, a 
copoly(vinyl acetate/vinyl chloride) and a 
copoly(acrylonitrile/butadiene/styrene) or a copoly(vinyl chloride/vinyl 
acetate/vinyl alcohol) or mixture thereof. 
When a binder is used, it is most preferred to use a minimum amount of 
binder. The weight ratio of phosphor to binder preferably from 80:20 to 
99:1. The ratio by volume of phosphor to binding medium is preferably more 
than 85/15. 
Preferably the binding medium substantially consists of one or more 
hydrogenated styrene-diene block copolymers, having a saturated rubber 
block, as rubbery and/or elastomeric polymers as disclosed in WO 94/00531. 
Particularly suitable thermoplastic rubbers, used as block-copolymeric 
binders in phosphor screens in accordance with this invention are the 
KRATON-G rubbers, KRATON being a trade mark name from SHELL. 
The coverage of the phosphor is preferably in the range from about 5 to 
about 250 mg/cm.sup.2, most preferably said coverage is between 20 and 175 
mg/cm.sup.2. 
The stimulable phosphor used according to the present invention is 
preferably protected against the influence of moisture by adhering thereto 
chemically or physically a hydrophobic or hydrophobizing substance. 
Suitable substances for said purpose are described e.g. in U.S. Pat. No. 
4,138,361. 
According to a preferred embodiment the phosphor layer is used as a 
supported layer on a support sheet. Suitable support materials are made of 
a film forming organic resin, e.g. polyethylene terephthalate, but paper 
supports and cardboard supports optionally coated with a resin layer such 
as an alpha-olefinic resin layer are also particularly useful. Further are 
mentioned glass supports and metal supports. The thickness of the phosphor 
layer is preferably in the range of 0.05 mm to 0.5 mm. 
When the phosphor according to the present invention is used in combination 
with a binder to prepare a screen or a panel, the phosphor particles are 
intimately dispersed in a solution of the binder and then coated on the 
support and dried. The coating of the present phosphor binder layer may 
proceed according to any usual technique, e.g. by spraying, dip-coating or 
doctor blade coating. After coating, the solvent(s) of the coating mixture 
is (are) removed by evaporation, e.g. by drying in a hot (60.degree. C.) 
air current. 
An ultrasonic treatment can be applied to improve the packing density and 
to perform the de-aeration of the phosphor-binder combination. Before the 
optional application of a protective coating the phosphor-binder layer may 
be calendered to improve the packing density (i.e. the number of grams of 
phosphor per cm.sup.3 of dry coating). 
Optionally, a light-reflecting layer is provided between the 
phosphor-containing layer and its support to enhance the output of light 
emitted by photostimulation. Such a light-reflecting layer may contain 
white pigment particles dispersed in a binder, e.g. titanium dioxide 
particles, or it may be made of a vapour-deposited metal layer, e.g. an 
aluminium layer, or it may be a coloured pigment layer absorbing 
stimulating radiation but reflecting the emitted light as described e.g. 
in U.S. Pat. No. 4,380,702. 
In order to improve resolution it is possible to provide underneath the 
phosphor layer a layer absorbing the emitted light e.g. a layer containing 
carbon black or to use a coloured support e.g. a grey or black film 
support. 
The sharpness of panels comprising a bariumfluorohalide phosphor according 
to the present invention can be enhanced by diminishing the void ratio in 
the panel (e.g. air entrapped between the phosphor particles and the 
binder(s). This can be achieved by, e.g. applying a compression procedure 
to the panel as described in, e.g., EP-A 102 085 and EP-A 113 656. 
The electrostatic properties of the panels can be tuned to the needs at 
hand by adding polyethyleneoxides to the phosphor layer and or the 
protective layer. A preferred polyethyleneoxide derivative to fine tune 
said electrostatic properties corresponds to the formula: 
EQU H.sub.37 C.sub.18 (O--CH.sub.2 CH.sub.2).sub.n OH. 
Bariumfluorohalide phosphors according to the present invention, can be 
used in either supported or selfsupporting stimulable phosphor screens. 
The stored energy can be stimulated by the light of an HE-Cd laser (442 
nm), the light of the main emission lines of an argon laser (488 and 514 
nm) and the light of a frequency doubled Nd:YAG laser (532 nm). It is 
preferred to use light of wavelength between 480 and 550 nm to stimulate 
stimulable phosphor plates, comprising a phosphor according to the present 
invention. It is most preferred to use the light of a frequency doubled 
Nd:YAG laser (532 nm) to stimulate stimulable phosphors according to the 
present invention. 
The panels, comprising a phosphor, according to the present invention, are 
advantageously used in a radiation image recording and reproducing method 
comprising the steps of: 
i. causing a radiation image storage panel containing a photostimulable 
phosphor to absorb radiation having passed through an object or having 
been radiated from an object, 
ii. exposing said image storage panel to stimulating rays to release the 
radiation energy stored therein as light emission, the stimulating rays 
being electromagnetic waves having a wavelength lower than 600 nm 
iii. detecting the emitted light, 
iv. erasing the residual stored energy. 
Preferably said stimulating rays have a wavelength between 480 nm and 550 
nm. 
The erasure of the residual stored energy can, with panels comprising a 
phosphor according to the present invention, proceed in any way known in 
the art. Ways and means, useful for erasing the residual stored energy of 
phosphor panels comprising a phosphor according to the present invention, 
are disclosed in e.g. EP-A 022 564, EP-A 079 791, EP-A 056 599, U.S. Pat. 
No. 5,065,021, etc. Very useful methods for erasing the residual stored 
energy of phosphor panels comprising a phosphor according to the present 
invention, are disclosed in EP-A 598 949 and EP-A 586 744. The invention 
is illustrated by the examples and comparative examples given below, 
without however restricting the invention thereto. 
EXAMPLES 
Preparation of the Stimulable Phosphors 
All stimulable phosphor samples have been prepared in the following way: 
The phosphor precursors forming a raw mix, in proportions chosen so as to 
yield a particular phosphor, were collected in a PE container, and the mix 
was homogenized for 15 minutes on a jar rolling mill. Next, the powder mix 
was transferred to a rotating blade mixer (Henschel - Germany) and milled 
for 5 minutes at 2,000 rpm (rotations per minute) under Ar atmosphere. 
Three crucibles containing 130 g of the mix each, were placed in a quartz 
tube. The quartz tube was sealed with a flange with a water lock at the 
gas outlet side. 
The sealed quartz tube was placed in an oven at 850.degree. C., and the 
temperature was kept constant at this temperature during the three hour 
firing. During the firing the tube was flushed with Ar at a rate of 1.5 
l/min. 
After the firing, the tube was taken out of the furnace and allowed to 
cool. 
After the cooling, the flange was removed and the three crucibles were 
taken out of the tube. 
The powder was milled and homogenized. 
A second firing was performed at 750.degree. C., for 6 hours, under a 1.5 
l/min 99.8% N.sub.2 /0.2% H.sub.2 gas flow rate. 
Finally, the powder was deagglomerated with a pestle and mortar. 
The proportions of the phosphor precursors are given under the headings of 
the specific examples. 
Measurements 
2.1. Measurement A: Phosphor composition. 
Since the cations do not evaporate during the firing, the Ba, Sr, Ca, Cs, 
Pb and Eu contents of the phosphors were not measured, and it was assumed 
that the cation ratios were equal to those in the raw mix. 
The halides being in molar excess over the non-evaporating cations, 
evaporate partly during the firing. 
The F- and Br-content of the phosphors was determined via 
ion-chromatography. 
Measuring equipment and conditions: 
______________________________________ 
ion chromatograph 
gic analyser 
detector conductivity detector 
guard columna AG 3 
separator column 
AS 3 
injection volume 
50 .mu.l 
detector sensitivity 
100 .mu.s/l000 mV full scale 
eluence 2.8 mM NaHCO.sub.3 :2.2 mM Na.sub.2 CO.sub.3 
eluence flow rate 
2.0 ml/min 
regenerant 0.025 N H.sub.2 SO.sub.4 
regenerant flow rate 
3.0 ml/min 
reference time F 
1.65 min 
______________________________________ 
Determination of F.sup.- 
The concentration of the fluoride ions (F.sup.-) was determined from the 
height of the F-peak. 
To determine the accuracy of the measuring procedure five 1 ppm NaF 
standards were prepared: 
0.5525 g NaF p.a. (pro analysis quality) was weighed and transferred into a 
250 ml volumetric flask. The NaF was dissolved in doubly distilled water 
and water was added to get a total volume of 250 ml. The solution was 
first diluted 10-fold with doubly distilled water and then further diluted 
100-fold. The five 1 ppm NaF standards were injected and the peak height 
was measured. The average peak height was 385,068 in arbitrary values and 
the standard deviation was 914.299. This gave a coefficient of variability 
(standard deviation divided by the average value) of 0.00237. 
To measure the F-concentration in the phosphor samples, 50 mg of each 
sample was transferred into a test tube and 1 ml of analytically pure HCl 
(1N) was added followed by the addition of about 10 ml of doubly-distilled 
water. The tube was then sealed and heated for 5 to 10 min in a boiling 
water bath. The tube was then cooled in ice and 1 ml of NaOH (1N) was 
injected. The solution was then poured into a 50 ml volumetric flask and 
the solution made up to 50 ml with doubly distilled water. Finally, the 
solution was diluted 50-fold with eluence and injected into the 
ion-chromatograph. 
The correctness of the results obtained with the measuring procedure was 
tested by applying it three times to a pure BaF.sub.2 standard, that 
theoretically contains 21.7% F. The dilution factor was 100 instead of 50. 
The average percentage F measured on the pure BaF.sub.2 standard was 21.73 
with a standard deviation of 0.115. This gave a coefficient of variability 
(standard deviation divided by the average value) of 0.0053. 
The reproducibility of the F-concentration measurement, in a phosphor, was 
determined by performing the measurement in 5-fold for a standard 
phosphor. The average percentage F measured on the standard phosphor was 
8.14 with a standard deviation of 0.288. This gave a coefficient of 
variability (standard deviation divided by the average value) of 0.0354. 
Determination of Br.sup.- 
The concentration of the bromide ions (Br.sup.-) was determined from the 
height of the Br-peak. 
To determine the accuracy of the measuring procedure for determining the 
bromide ion content five 5 ppm NaBr standards were prepared as follows: 
0.3219 g NaBr p.a. (pro analysis quality) was weighed and transferred into 
a 250 ml volumetric flash. The NaBr was dissolved in doubly distilled 
water and diluted up to a total volume of 250 ml. The 1000 ppm solutions 
were diluted 200-fold with double distilled water and then injected into 
the ion-chromatograph. The peak height was measured. The average peak 
height was 200,709 in arbitrary values and the standard deviation was 
669.106. This gave a coefficient of variability (standard deviation 
divided by the average value) of 0.00333. 
The reproducibility of the results obtained with the described procedure 
was determined by performing the measurement 5-fold for a standard 
phosphor. The average percentage Br measured on the standard phosphor was 
32.076 with a standard deviation of 0.180. This gave a coefficient of 
variability (standard deviation divided by the average value) of 0.0056. 
The I-content was determined via XRF (X-ray diffraction). 
2.2. Measurement B: Erasure depth 
On the phosphor plate a 1 mm thick Funk raster was imaged with a X-ray dose 
of 6960 .mu.Gy at 70 kV.sub.p. 
The plate was scanned with a frequency doubled Nd:YAG laser (532 nm) in a 
direction perpendicular to the slits of the Funk raster. 
The modulation of the stimulated output signal at 0.025 lp/mm (line 
pairs/mm) is taken as the image output prior to erasure and designated by 
O.sub.0. 
The plate was then erased for 10 sec, in an erasure unit, containing quartz 
halogen lamps with total power of 4 kW. The UV emission of said quartz 
halogen lamps (i.e. all emission below 413 nm) was filtered away by the 
use of a suitable filter. 
After the erasure step, the plate was again scanned with a frequency 
doubled Nd:YAG laser (532 nm) in a direction perpendicular to the slits of 
the Funk raster. 
The modulation of the stimulated output signal at 0.025 lp/mm (line 
pairs/mm) is taken as the image output after erasure and designated by 
O.sub.1. 
The erasure depth (ED) in dB was calculated as 
EQU ED=10.times.log(O.sub.0 /O.sub.1) in dB. 
Invention Example 1 (IE1) and Non-Invention Example 1 (NIE1) 
A raw mix was prepared with the following composition: 
______________________________________ 
BaF.sub.2 : 0.86 mole 
SrF.sub.2 : 0.14 mole 
CaF.sub.2 : 0.03 mole 
NH.sub.4 Br: 0.994 mole 
NH.sub.4 I: 0.186 mole 
EuF.sub.3 : 0.001 mole 
CsI: 0.003 mole. 
______________________________________ 
For Non-Invention Example 1: 
The raw mix, given above was used as such. 
For Invention example 1: 
The raw mix as in non-invention example 1 was used, but only 0.02 mole of 
CaF.sub.2 was added to the raw mix and before the second firing an 
additional 0.01 mole of CaF.sub.2 was added. 
After the preparation procedure described above, two phosphor samples were 
obtained, and an X-ray diffraction (XRD) spectrum did not show CaF.sub.2 
lines, indicating that in both phosphors the CaF.sub.2 is incorporated in 
the phosphor crystals. The composition of each phosphor was determined 
according to measurement A. 
The Non-Invention phosphor corresponded to the formula: 
EQU Ba.sub.0.832 Sr.sub.0.135 Ca.sub.0.029 Eu.sub.0.001 Cs.sub.0.003 F.sub.1.49 
Br.sub.0.877 I.sub.0.83 (NIE 1) 
The Invention phosphor corresponded to the formula 
EQU Ba.sub.0.824 Sr.sub.0.134 Ca.sub.0.030 Eu.sub.0.001 Cs.sub.0.003 F.sub.1.41 
Br.sub.0.869 I.sub.0.091 (IE 1) 
Both powders were dispersed in a binder solution containing cellulose 
acetobutyrate dissolved in methyl ethyl ketone. The dispersions obtained 
were coated onto a 100.mu. thick transparent sheet of polyethylene 
terephthalate to give a dry coating weight of about 1,000 g/m.sup.2. 
The erasure depth (ED) was measured according to measurement B. 
Non-invention phosphor 1 (NIE) had a ED=40.6 dB and Invention phosphor IE1 
had an ED=41.8 dB. This indicates that the addition of Ca-ions during the 
last firing increases the erasure depth. 
Invention Example 2 (IE2) and Non-Invention Example 2 (NIE2) and 3 (NIE3) 
Two raw mixes were prepared with the following compositions: 
For Non-Invention Example 2: 
______________________________________ 
BaF.sub.2 : 0.86 mole 
SrF.sub.2 : 0.14 mole 
NH.sub.4 Br: 0.994 mole 
NH.sub.4 I: 0.186 mole 
EuF.sub.3 : 0.001 mole 
CsI: 0.003 mole. 
______________________________________ 
For Non-Invention Example 3: 
______________________________________ 
BaF.sub.2 : 0.82 mole 
SrF.sub.2 : 0.18 mole 
NH.sub.4 Br: 0.82 mole 
NH.sub.4 I: 0.15 mole 
EuF.sub.3 : 0.001 mole 
CsI: 0.003 mole. 
PbF.sub.2 0.0003 mole 
______________________________________ 
For Invention Example 2: 
The raw mix of non-invention example 2 (NIE2) was used, but in the last 
firing 0.03 mole of CaF.sub.2 was added. In this way the phosphor of 
invention example 2 (IE2) was prepared. 
The synthesis was performed in the way described above. Three phosphor 
samples were obtained, and an X-ray diffraction (XRD) spectrum did not 
show CaF.sub.2 lines, indicating that in phosphor IE2 the CaF.sub.2 is 
incorporated in the phosphor crystals and thus does not exist as a 
separate phase. 
The compositions of the obtained phosphors were determined in the way 
described hereinbefore (Measurement A). 
The Non-Invention phosphor (NIE2) corresponded to the formula: 
EQU Ba.sub.0.857 Sr.sub.0.139 Eu.sub.0.001 Cs.sub.0.003 F.sub.1.039 
Br.sub.0.859 I.sub.0.103 (NIE 2) 
The Non-Invention phosphor (NIE3) corresponded to the formula: 
EQU Ba.sub.0.8165 Sr.sub.0.1792 Eu.sub.0.001 Cs.sub.0.003 Pb.sub.0.0003 
F.sub.1.047 Br.sub.0.855 I.sub.0.098 (NIE 3) 
The Invention phosphor corresponded to the formula 
EQU Ba.sub.0.7928 Sr.sub.0.174 Ca.sub.0.029 Eu.sub.0.001 Cs.sub.0.0029 
F.sub.1.075 Br.sub.0.83 I.sub.0.095 (IE 2) 
The powders were dispersed in a binder solution containing cellulose 
acetobutyrate dissolved in methyl ethyl ketone. The dispersions obtained 
were coated onto a 100.mu. thick transparent sheet of polyethylene 
terephthalate to give a dry coating weight of about 1,000 g/m.sup.2. 
The erasure depth was measured according to measurement B: 
______________________________________ 
NIE2 41.7 dB 
NIE3 42.8 dB 
IE2 43.7 dB 
______________________________________ 
Invention Example 3 (IE3) and Non Invention Example 4 (NIE4) and 5 (NIE5) 
Two raw mixes were prepared with the following compositions: 
For Non-Invention Example 4: 
______________________________________ 
BaF.sub.2 : 0.86 mole 
SrF.sub.2 : 0.14 mole 
NH.sub.4 Br: 0.994 mole 
NH.sub.4 I: 0.186 mole 
CaF.sub.2 0.03 mole 
EuF.sub.3 : 0.001 mole 
CsI: 0.003 mole. 
______________________________________ 
For Non-Invention Example 5: 
______________________________________ 
BaF.sub.2 : 0.82 mole 
SrF.sub.2 : 0.18 mole 
CaF.sub.2 0.03 mole 
NH.sub.4 Br: 0.82 mole 
NH.sub.4 I: 0.15 mole 
EuF.sub.3 : 0.001 mole 
CsI: 0.003 mole. 
PbF.sub.2 0.0003 mole 
______________________________________ 
For Invention Example 3: 
The raw mix of non-invention example 5 (NIE5) was used, but in the last 
firing 0.03 mole of CaF.sub.2 was added. In this way the phosphor of 
invention example 3 (IE3) was prepared 
The synthesis was performed in the way described above. Three phosphor 
samples were obtained, and an X-ray diffraction (XRD) spectrum did not 
show CaF.sub.2 lines, indicating that in the three phosphors the CaF.sub.2 
is incorporated in the phosphor and thus does not exist as a separate 
phase. 
The compositions of the obtained phosphors were determined in the way 
described hereinbefore (Measurement A). 
The Non-Invention phosphor (NIE4) corresponded to the formula: 
EQU Ba.sub.0.832 Sr.sub.0.135 Ca.sub.0.029 Eu.sub.0.001 Cs.sub.0.003 F.sub.1.07 
Br.sub.0.83 I.sub.0.1 (NIE 4) 
The Non-Invention phosphor (NIE5) corresponded to the formula: 
EQU Ba.sub.0.7928 Sr.sub.0.174 Ca.sub.0.029 Eu.sub.0.001 Cs.sub.0.0029 
Pb.sub.0.0003 F.sub.1.099 Br.sub.0.819 I.sub.0.082 
The Invention phosphor IE3 corresponded to the formula 
EQU Ba.sub.0.7705 Sr.sub.0.1691 Ca.sub.0.0564 Eu.sub.0.0009 Cs.sub.0.0028 
Pb.sub.0.0003 F.sub.1.125 Br.sub.0.795 I.sub.0.08 
The powders were dispersed in a binder solution containing cellulose 
acetobutyrate dissolved in methyl ethyl ketone. The dispersions obtained 
were coated onto a 100.mu. thick transparent sheet of polyethylene 
terephthalate to give a dry coating weight of about 1,000 g/m.sup.2. 
The erasure depth was measured according to measurement B: 
______________________________________ 
NIE4 40.1 dB 
NIE5 41.5 dB 
IE2 43.0 dB 
______________________________________