U.S. patent number RE28,592 [Application Number 05/471,271] was granted by the patent office on 1975-10-28 for terbium activated rare earth oxyhalide phosphors containing ytterbium for reduced afterglow.
This patent grant is currently assigned to General Electric Company. Invention is credited to Evelyn W. Bradshaw, Jacob G. Rabatin.
United States Patent |
RE28,592 |
Rabatin , et al. |
October 28, 1975 |
Terbium activated rare earth oxyhalide phosphors containing
ytterbium for reduced afterglow
Abstract
Phosphors of reduced afterglow having the inclusion of a
selected impurity in the phosphors lattice. More specifically, rare
earth oxyhalide phosphors of reduced phosphorescence comprising
lanthanum and gadolinium oxyhalide phosphors activated with terbium
including the selected impurity ytterbium.
Inventors: |
Rabatin; Jacob G. (Chardon,
OH), Bradshaw; Evelyn W. (Cleveland, OH) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
26797169 |
Appl.
No.: |
05/471,271 |
Filed: |
May 20, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
100448 |
Dec 21, 1970 |
03666676 |
May 30, 1972 |
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Current U.S.
Class: |
252/301.4H |
Current CPC
Class: |
C09K
11/777 (20130101) |
Current International
Class: |
C09K
11/77 (20060101); C09K 011/24 (); C09K
011/46 () |
Field of
Search: |
;252/31.4H |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Blasse et al., "Investigation of Tb.sup.+.sup.3 Activated
Phosphors" Philips Research Reports 22(S), 1967, pp.
481-504..
|
Primary Examiner: Cooper; Jack
Attorney, Agent or Firm: McDevitt; John F. Truesdell; Henry
P. Neuhauser; Frank L.
Claims
What is claimed as new and desired to be secured by Letters Patent
of the United States is:
1. A rare earth oxyhalide phosphor of reduced afterglow having the
general formula:
M.sub.1.sub.-w.sub.-y OX:Tb.sub.w Yb.sub.y
wherein M is an element selected from the group consisting of La
and Gd;
X is an element selected from the group consisting of Cl and
Br;
w is from 0.0005 to 0.03 moles per mole of the selected oxyhalide;
and
y is from 0.00005 to 0.005 moles per mole of the selected
oxyhalide.
2. A rare earth oxyhalide phosphor of reduced afterglow according
to claim 1 wherein:
M is La
X is Br
w is 0.002
y is 0.0005.
Description
BACKGROUND OF THE INVENTION
The present invention relates to rare earth oxyhalide phosphors
activated with terbium, and particularly, to such rare earth
oxyhalide phosphors having reduced afterglow or
phosphorescence.
The rare earth oxyhalides activated with terbium are finding
increasing use as phosphors in X-ray intensifier screens and
similar radiographic devices, replacing past conventional phosphors
of less desirable properties. For example, LaOBr:Tb has about a 50
percent greater absorption of incident X-rays and about three to
four times greater blue emission than the previously used
CaWO.sub.4. Currently intensifier screens are provided with
photographic films which tend to fog if a phosphor is used having
an afterglow or long persistent phosphorescence of appreciable
intensity. With the use of these long afterglow phosphors, any
movement of the film relative to the phosphor screen immediately
after exposure will result in blurred images. Attempts to eliminate
the problem of blurred images in X-ray intensifier screens have
been generally unsuccessful because any reduction in duration and
intensity of the phosphor afterglow normally has been at the
expense of the overall phosphor efficiency.
The present invention eliminates the problem of blurred images by
providing rare earth oxyhalide phosphors with significantly reduced
afterglow without an appreciable loss in phosphor efficiency.
Accordingly, it is an object of the present invention to provide a
rare earth oxyhalide phosphor activated with terbium having a
reduced afterflow.
It is a further object of the present invention to provide a
terbium-activated lanthanum or gadolinium oxychloride or oxybromide
phosphor including a selected impurity to achieve reduced
afterglow.
Further objects and attendant advantages will be apparent from the
following summary and detailed description.
SUMMARY OF THE INVENTION
The present invention relates to terbium-activated rare earth
oxyhalide phosphors having a selected impurity for reducing the
phosphors' afterglow. More specifically, the present invention
relates to rare earth oxyhalide phosphors including ytterbium as an
impurity and having the general formula:
M.sub.1.sub.-w.sub.-y OX:Tb.sub.w Yb.sub.y
Wherein M is an element selected from the group consisting of La
and Gd; X is an element selected from the group consisting of Cl
and Br; and w is from 0.0005 to 0.03 moles per mole of the selected
oxyhalide; and y is from 0.00005 to 0.005 moles per mole of the
selected oxyhalide.
DETAILED DESCRIPTION OF THE INVENTION
The phosphors of the present invention are useful in various
radiographic intensifier screens and include certain lanthanum and
gadolinium oxyhalide phosphors activated with terbium. A small
amount of a selected impurity, ytterbium, is incorporated in the
phosphor lattice in order to reduce the afterglow of the phosphor
without any appreciable decrease in the overall phosphor
efficiency.
The following are a few of the rare earth oxyhalide phosphors
included in this invention: La.sub.0.997
OBr:Tb.sub.0.002,Yb.sub.0.001 La.sub.0.994
OCl:Tb.sub.0.005,Yb.sub.0.001 Gd.sub.0.994
OBr:Tb.sub.0.005,Yb.sub.0.001 Gd.sub.0.994
OCl:Tb.sub.0.005,Yb.sub.0.001
The ytterbium impurity addition of the present invention is made by
the oxalate precipitation method. This method is outlined below for
the preparation of the La.sub.0.9975 OBr:Tb.sub.0.002,
Yb.[..sub.0.005 .]. .sub.0.0005.
In this first step of the preparation, lanthanum oxide
(.Badd.2,330g), terbium oxide (5.4g), and ytterbium oxide
.[.(0.0141g).]. .Iadd.(1.41g) .Iaddend.are dissolved in a mixture
of concentrated nitric acid (3,055ml) and water (11,615ml). This
mixture is then coprecipitated with 10 percent oxalic acid
(40,600ml) to form the oxalates of lanthanum, terbium and
ytterbium. The oxalates are then fired to effect the chemical
conversion of the oxalates to the mixed oxides of lanthanum,
terbium and ytterbium at 1,000.degree.C for 2 hours in open
boats.
The mixed oxides of lanthanum, terbium and ytterbium (2,230g) are
blended thoroughly with ammonium bromide (1,440g) and fired 2 hours
at 425.degree.C in covered boats to chemically form the phosphor,
terbium-activated lanthanum oxybromide containing ytterbium. This
phosphor (3,000g) is recrystallized by thoroughly blending it with
potassium bromide (531g) and refiring at 1,000.degree.C for 2 hours
in covered boats to form recrystallized larger particle-sized,
terbium-activated lanthanum oxybromide containing ytterbium. Molten
potassium bromide serves as a crystal-growing medium in this firing
in order to produce well-crystallized terbium-activated lanthanum
oxybromide containing ytterbium. The material is then washed to
remove the potassium bromide. It is subsequently dried in an oven
for 2 hours at 80.degree.C, and then sifted through 325 mesh.
The effect of ytterbium on the reduction of afterglow in
LaOBr:Tb.sub.0.002 is shown in Table I.
TABLE I
__________________________________________________________________________
The Effect of Ytterbium on the Reduction Of Afterglow Properties of
LaOBr:Tb.sub.0.002 Relative Moles of Phosphor Phosphorescence
Intensity Ytterbium Brightness In Volts After Indicated Per Mole
Under X-ray Time in Sec. Of Phosphor Excitation 0.01 1 3 9 52
__________________________________________________________________________
.00 100% .51 .41 .38 .32 .15 .0005 100% .35 .17 .10 .05 .00 .0010
100% .30 .04 .01 .00 .00 .0020 94% .25 .00 .00 .00 .00
__________________________________________________________________________
As can be seen from the above table, the addition of ytterbium
results in a marked decrease in afterglow intensity. However, as
can also be seen from the above Table, the addition of up to 0.001
mole ytterbium results in no reduction in the relative phosphor
brightness under X-ray excitation. Even the addition of 0.002 mole
ytterbium results in a phosphor of 94 percent phosphor brightness
of LaOBr:Tb.sub.0.002 without ytterbium.
Table II shows the effectiveness of ytterbium in reducing afterglow
intensities in other rare earth oxyhalide phosphors of the present
invention.
TABLE II
__________________________________________________________________________
Effect of Ytterbium on the Afterglow Intensities Of Various Rare
Earth Oxyhalides Activated with Terbium Moles of Phopsphorescence
Intensity Ytterbium In Volts After Indicated Phosphor Per Mole Time
in Sec. Composition of Phosphor 0 1 3 52
__________________________________________________________________________
LaOCl:Tb.sub..sub.[0.00005.sub.]0.005 .0000 .28 .18 .12 .02 " .0002
.23 .09 .04 .00 " .0010 .17 .03 .01 .00
GdOBr:Tb.sub..sub.[0.00005.sub.]0.005 .0000 .31 .25 .18 .00 " .0010
.24 .02 .00 .00 GdOCl:Tb.sub..sub.[0.00005.sub.]0.005 .0000 .37 .15
.10 .01 " .0010 .25 .00 .00 .00
__________________________________________________________________________
MEASUREMENTS OF PHOSPHORESCENCE DECAY
All decay measurements were made with an X-ray generator set at
90KV peak and at a milliampere setting to give 15 volt constant
output as read on an image storing oscilloscope when an S-11
photomultiplier was set at 800 volts. 0.8 gram of phosphor was
pressed into a plastic sample holder of 30 mm diameter. The sample
was placed about 3 inches above the generator target. The
photomultiplier was set about 4 inches above the sample holder. The
decay curves were recorded on the oscilloscope face with the
photomultiplier set at 1400 volts and the oscilloscope set at a
sweep of 5 seconds per cm and at a sensitivity of 0.05 volts per
cm. The phosphor was exposed to the X-rays for 10 seconds after
which the X-rays were turned off and the decay measurements were
made.
* * * * *