Note: Descriptions are shown in the official language in which they were submitted.
PHN 77~0
597~
The invention relates to a method of preparing
a luminescent alkaline earth metal fluorohalide activated
by bivalent europium. The invention further relates to
the resulting alkaline earth metal fluorohalide and to
an X-ray image intensifier screen provided with a lu-
minescent alkaline earth metal fluorohalid made in
such manner.
The said alkaline earth metal fluorohalides
may be defined by the general formula MeFX, where Me
represents one or more of the alkaline earth metals
Ba and Sr, and X represents one or more of the halogens
Cl, Br and I. The crystal structure of these materials
is known as the PbFCl structure and has a tetragonal
symmetry. It is known from our Canadian Patent 1,003,206
wh;ch ;ssued on January 11, 1977, that these fluorohalldes
are highly efficient luminescent materials when act;vated
by bivalent europium. The europium then replaces part
of the alkaline earth metal in the fluorohalide base
lattice. These known luminescent fluorohalides can be
verY satisfactorily excited both by ultraviolet radiation
and by electrons and X-rays. The spectral distribution
of the emitted radiation consists of a narrow band
(half-Yalue width approximately 30 nm) with a maximum
- 2 - ~
", ),.
~ P~IN 7740
1~59747
at approximately 380 nm. For the materials which contain
a considerable quantity of iodine the emission maximum
is shifted to longer wavelengths. Notably for the fluoro-
iodide this maximum is found at approximately 410 nm. It
was found that in these materials up to 20 mole % of
the Ba and Sr represented by Me can be replaced by Ca
whilst retaining the crystal structure.
An important use of the known fluorohalides
activated by b;valent europium is found in the so-called
X-ray image intensifier screens. Such intensifier screens
comprise a material which luminesces under X-ray radiation
and serve to shorten the exposure time when taking X-ray
images on photographic material. In general they take
the form of film cassettes which contain a support
coated with the luminescent material which is in con-
tact with the photographic film when taking the X-ray
image. The known luminescent fluorohalides when excit-
ed by X-rays have a very high luminous flux which may
be up to five times that of the known calcium tungstate
which is frequently used in X-ray image intensifier
; screens.
The above-mentioned Canadian Patent
1,003,206 describes a method of manufacturing the
fluorohalides activated by bivalent europium,
starting from a dry mixture of halides (for example
alkaline earth metal chloride, alkaline earth metal
fluoride and europium fluoride) which by a reaction
PHN 7740
~597~'7
at a high temperature is conver~ed into the desired
fluorohalide. It has also been found possible to
start from a mixture of oxides or carbonates of the
alkaline earth metals and of europium together with
ammonium halides.
A serious disadvantage of the luminescent
fluorohalides obtained in the aforedescribed manner is
that they have a high persistence level. This means
that these substances have a comparatively high-in-
tensity afterglow for a comparatively long time on ces-
sation of the excitation. For these materials the
said persistence level may, for example, be 20 to 100
times that of the known calcium tungstate. Persistence
of the luminescent material in an intensifier screen
is particularly objectionable because for some time
after exposure each movement of the screen relative to
the photographic film (when opening film cassettes)
results in blurred images. An even more serious draw-
back is that owing to the persistence considerable
waiting periods must be observed before a new film
can be placed in the cassette.
~nited Kingdom Patent Specification 1,254,271
describes a method of preparing alkaline earth metal
fluorohalides in which these materials are obtained
by precipitation from solutions. During the precipi-
tation a solution containing alkaline earth metal
cations, for example an alkaline earth metal chloride,
PHN 7740
3L~5974~7
and a solution containing the fluorine anions, for
example hydrogen fluoride, potassium fluoride or ammo-
nium fluoride, are added to the reaction vessel. It has
now been found that if alkaline earth metal fluorohalides
activated by bivalent europium are prepared by this
; method, materials are obtained which have a very high
afterglow level.
It is an object of the present invention t~
provide a method of preparing fluorohalides activated
by bivalent europium by which materials having a very
low afterglow level are obtained.
A method according to the invention of pre-
paring a luminescent alkaline earth metal fluorohalide
activated by bivalent europium and defined by the for-
mula Mel pFupFX, where Me represents at least one of
the alkaline earth metals Ba and ~r whilst up to 20
mole% of these metals can be replaced by Ca, X re-
presents at least one of the halogens Cl, Br and I
and 0.001 _ p C 0.20, is characteri~ed in tha~ an
aqueous suspension of MeF2 is made which contains 1
mole of MeX2 in dissolved form per mole MeF2 and may
further contain europium in the form-of europium
halide in an amount at most equal to the amount desired
in the fluorohalide, in that the suspension is evaporat-
ed to dryness at a temperature from 50C to 250C, the
resulting product is mixed with the amount of europium
halide which still may be required, the mixture is sub-
PHN 7740
~C~597~
jected to at least one temperature treatment at 600C to
1000C in a weakly reducing atmosphere and the obtained
reaction product, after cooling, is subjected to a final
heating at a temperature of 600C to 850C in an inert
or weakly reducing atmosphere.
A method according to the invention starts
from an aqueous suspension of MeF2, i.e. a mixture of
fine-grained MeF2, which is substantially insoluble
in water, and an aqueous dissolving medium. Prior or
subsequent to the preparation of this suspension MeX2
is dissolved in the dissolving medium in the stoichio-
metric amount of 1 mole per mole of the MeF2 used.
Small deviations from the stoichiometry are found to
be permissible and in general have little effect on
the properties of the fluorohalide obtained. When
preparing this suspension the required amount of
europium can entirely or partly be added either as
insoluble EuF3 or as soluble EuX3. The suspension
then ;s evaporated to dryness at a temperature of
50C to 250C. X-ray diffraction analyses show that
the obtained product already has the characteristic
PbFCl structure, and this product is mixed with the
amount of europium which still may be required in the
form of europium halide. The mixture is subjected to
one or more temperature treatments at 600C to 1000C
in a weakly reducing atmosphere. During this treatment
the already incorporated europium is reduced to the
PHN 7740
~6:)5~7~7
bivalent state and/or the admixed europium is incorporated
as bivalent europium in the crystal lattice. The result-
ing reaction product is subiected to a final heating
at 600C to 850C in an inert or weakly reducing at-
mosphere to improve the crystal structure and to ob-
tain optimum grain size of the powder.
Formation of the fluorohalide compound, in-
corporation of europium in the desired bivalent state
and recrystallization of the obtained pulverulent
product take place separately and successively in a
method according to the invention. This is highly ad-
vantageous, because thus these three steps in the pre-
paration can separately be optimized and a luminescent
material having optimum luminescence properties can be
obtained. In particular, with respect to the afterglow
level it proves to be an essential requirement that
the fluorohalide compound is formed at a comparatively
low temperature (during evaporation to dryness at 50C
to 250C). It is assumed that under these cond;tions
the incorporation of traces of oxygen which may be
the cause of a h;gh afterglow level is avoided as
far as possible.
It was -found that by means of a method ac-
cording to the invention a luminescent fluorohalide
is obtainable the afterglow level of which has been
reduced to an amount about equal to that of the known
calcium tungstate. It was further found that, in com-
PHN 7740
~L135~7~7
.
par;son to the fluorohalide prepared in the known man-
ner, the luminous flux obtained by the said materials
when excited by X-rays has the same high value or even
a higher value.
Although it is possible to add the desired
amount of the europium activator in its entirety to
the suspension in the form of EuX3 or EuF3, a method
according to the invention is preferred ;n which the
suspension contains no europium at all, p moles of EuX3
being added per mole of the MeFX obtained by evaporation
to dryness, for it has been found that the best results
are obtained, in particular in respect of the afterglow
level, if a europium halide (except the fluoride) is
used as the activator compound and if this activator
compound is added after the synthesis of the fluoro-
halide compound.
It was found to be of advantage for the
reaction product obtained after the temperature
treatment at 600C to 1000C in a weakly reducing at-
mosphere to be ground before it is subjected to the
final heat;ng, for if the conditions during the tem-
~ perature treatment are optimized for the incorporation
; of bivalent europium in many cases a reaction product
may be formed which is too coarse-grained for practical
uses. Any deleterious effect of the grinding operation
on the afterglow level is entirely eliminated by the
final heating.
PHN 7740
5~747
The most important use of the fluorohalides
activated by bivalent europium is found, as mentioned
hereinbefore, in X-ray image intensifier screens. Ad-
vantageously the element Me then is barium because this
gives the highest luminous fluxes on excitation by X-
rays. For obtaining high luminous fluxes upon X-ray ex-
citation it is also advantageous to use bromine for the
element X. It was found, however, that the afterglow
level of BaFBr is high compared with that of BaFCl.
1~ For example, the BaFBr prepared in known manner has
an afterglow level which is up to 100 times that of
calcium tungstate. A method according to the invention
enables this level to be reduced to a few times that
of calcium tungsta~e. The best combination of a low
afterglow level and a high luminous flux is obtained
with materials of the formula Bal pEupFCl. Hence a
method according to the invention for preparing such
materials is preferred.
In a preferred embodiment of a method ac-
cording to the invention in which Bal pEupFCl is pre-
pared, evaporation to dryness is performed at a tem-
perature of 160C to 180C, for under these conditions
the formation of the BaFCl is optimal.
When preparing Bal pEupFCl the best results
are further obtained if the temperature treatment is
effected at a temperature of 800C to 950C in nitro-
gen containing from 0.1 to 1 volume percent of hydrogen
PHN 7740
~13597~7
and if the final treatment is performed at a temperature
of 700C to 800C in nitrogen. Hence these conditions are
preferred.
The invention will now be described more fully
; 5 with reference to a few Examples of preparation.
Example 1.
An amount of 8.765 9 of BaF2 is added to water
and suspended by st;rring. 12.240 9 of BaC12.2H20 are
dissolved in this suspension. The suspension then is
evaporated to dryness at a temperature of 170C. The'
resulting substance consists of BaFCl and, as is shown
by X-ray diffraction analyses, has the PbFCl structure.
The BaFCl is mixed with 1.374 9 oAF EuC13 and then heat-
ed in a quartz crucible'at 900C in a furnace in a
weakly reducing atmosphere for 1 hour. The said at-
mosphere is obtained by passing a stream of nitrogen
~about 137 1 per h~ur) containing 0.7 volume percent
of hydrogen into the furnace. After cooling the result-
ing product is ground in a ball mill.' Then the product
is subjected to final heating at 750C in a nitrogen
stream (about 18 1 per hour) for 1 hour. After cooling,
the product, which has a composition defined by the
formula BaO 95Euo 05FCl, is ready for use-
In order to compare the luminescence pro-
perties on excitation by X-rays of the material ob~
tained according to the above Example of preparation
which those of the known CaW04, screens are manufactured
- 10 -
PHN 7740
~0597~7
using the said material and identical screens which,
however, contain calcium tungstate in the same amounts
by weight. A photographic film is brought into contact
with these screens. Then the film density rate is
measured under standard conditions (primary voltage
of the X-ray tube; distance of the X-ray tubè from the
screen and the filmi filters). The fluorohalide prepar-
ed by the method according to the invention proves to
have a rate which is 7 times that of the known CaWO~.
The afterglow level of the fluorohalide also is measured
and compared with that of CaW04. For this purpose the
screens are exposed to X-rays for some time (under
standard conditions). One minute after cessation of
the excitation a photographic film is placed on the
screen and held in contact with it for 5 minutes.
The film density measured, which is a measure of the
afterglow level, proves to be equal for the fluoro-
halide and for the CaW04.
Example 2
Example 1 is repeated with the difference
that the amount of 1.374g of EuC13 is added to the
suspension and dissolved in it. The product obtained
is equal to that of Example 1 and has substantially
the same luminescence properties.
Example 3
Example 1 is repeated with the difference
that the temperature treatment in the weakly reducing
1 1
PHN 7740
1~59747
atmosphere is performed at 850C for 1 hour. The density
rate of the obtained material is found to be 6.3 times
that of CaW04 whilst the afterglow level is found to
be about twice that of CaW04.
Example 4.
Example 1 is repeated with the difference that
the temperature treatment in the weakly reducing atmos-
phere is performed at 950C for 1 hour. The density rate
of the resulting material is 5.2 times that of CaW04 and
the afterglow level is about equal to that of CaW04.
- 12 -
