U.S. patent number 3,838,273 [Application Number 05/257,538] was granted by the patent office on 1974-09-24 for x-ray image intensifier input.
This patent grant is currently assigned to General Electric Company. Invention is credited to Dominic A. Cusano.
United States Patent |
3,838,273 |
Cusano |
September 24, 1974 |
X-RAY IMAGE INTENSIFIER INPUT
Abstract
A barrier layer of an electrically conductive material such as
indium oxide (In.sub.2 O.sub.3), optically transparent to x-ray
phosphor luminescence, is disposed between the phosphor layer and
photocathode film of an x-ray image intensifier tube to provide
sufficient electrical sheet conductance relative to the
photocathode film. The barrier layer provides electron
replenishment to the photocathode at all points of electron
emission therefrom to thereby reduce potential drop laterally
across the photocathode from the ring electrode to the center of
the photocathode, and also minimizes surface irregularities on the
phosphor layer to thereby significantly reduce electron-optic image
distortion in the image intensifier tube.
Inventors: |
Cusano; Dominic A.
(Schenectady, NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
22976700 |
Appl.
No.: |
05/257,538 |
Filed: |
May 30, 1972 |
Current U.S.
Class: |
250/214VT;
976/DIG.439; 250/483.1 |
Current CPC
Class: |
G21K
4/00 (20130101); H01J 29/385 (20130101) |
Current International
Class: |
G21K
4/00 (20060101); H01J 29/38 (20060101); H01J
29/10 (20060101); H01j 031/50 () |
Field of
Search: |
;250/213VT,483,486,487,488 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Borchelt; Archie R.
Attorney, Agent or Firm: Moucha; Louis A. Cohen; Joseph T.
Squillaro; Jerome C.
Claims
What I claim as new and desire to secure by Letters Patent of the
United States is:
1. An improved x-ray image intensifier tube input screen
comprising
a substrate member for supporting an x-ray sensitive phosphor layer
thereon,
an x-ray sensitive phosphor layer having a first major surface
disposed along a major surface of said substrate member opposite
from a source of x-ray photons, the phosphor layer being a
multilayer of granular phosphors of relatively large grain size of
particle diameter greater than 0.3 mil embedded in a silicone resin
binder, and
a photocathode film in optical communication with said phosphor
layer for producing emission of photoelectrons therefrom in
response to x-ray photons being converted to light photons by
luminescence in said phosphor layer,
the improvement consisting of
a relatively thick barrier layer of a relatively electrically
conductive material, optically transparent to the phosphor
luminescence, chemically compatible with the photocathode film
material and with the phosphor and providing chemical isolation
between the phosphor layer and photocathode film, said barrier
layer having a first major surface disposed along a second major
surface of said phosphor layer opposite the first major surface
thereof, said photocathode film disposed along a second major
surface of said barrier layer opposite the first major surface
thereof, the thickness of said barrier layer being in the range of
0.5 to 3 microns so that it is sufficient to provide substantial
smoothing of surface irregularities on the second major surface of
the phosphor layer and permitting a more uniform thickness of said
photocathode film to be deposited thereby reducing electron-optic
image distortion due to nonuniform potential variations across said
photocathode film resulting from the nonuniform thickness of
nonuniform resistance thereof, the barrier layer material having an
electrical resistance in the range of 10 to 10.sup.6 ohms per
square to provide sufficient electrical sheet conductance relative
to said photocathode film so that said barrier layer upon being
connected to a source of electric potential provides electron
replenishment to said photocathode film to thereby significantly
reduce electron-optic image distortion due to undesired relatively
high potential drops across said photocathode film laterally
thereof, and
a barrier layer of a relatively electrically non-conductive
material disposed between said electrically conductive barrier
layer and said phosphor layer.
2. The improved input screen set forth in claim 1 wherein
said nonconductive barrier layer is of thickness in the range of
0.1 to 1.0 micron.
3. The improved input screen set forth in claim 1 wherein
said relatively electrically nonconductive material is aluminum
oxide.
4. An improved x-ray image intensifier tube input screen
comprising
a substrate member for supporting an x-ray sensitive phosphor layer
thereon,
an x-ray sensitive phosphor layer disposed along a major surface of
said substrate member, and
a photocathode film in optical communication with said phosphor
layer for producing emission of photoelectrons therefrom in
response to x-ray photons being converted to light photons by
luminescence in said phosphor layer,
the improvement consisting of
said phosphor layer being a single layer of a plurality of slightly
spaced apart relatively thick phosphor structures providing a light
piping effect to the phosphor luminescence and having base portions
disposed on said substrate member along a major surface thereof
opposite from a source of the x-ray photons, and
a relatively thick barrier layer of a relatively electrically
conductive material, optically transparent to the phosphor
luminescence, chemically compatible with at least the photocathode
film material, and disposed between said phosphor layer and said
photocathode film, the thickness of said barrier layer being
sufficient to provide substantial smoothing of surface
irregularities on the phosphor layer, the barrier layer material
having an electrical resistance in the range of 10 to 10.sup.6 ohms
per square to provide sufficient electrical sheet conductance
relative to said photocathode film so that said barrier layer upon
being connected to a source of electric potential provides electron
replenishment to said photocathode film to thereby significantly
reduce elctron-optic image distortion due to undesired relatively
high potential drops across said photocathode film laterally
thereof.
5. The improved input screen set forth in claim 4 wherein
said barrier layer deposited along the nonbase portions of the
phosphor structures to thereby coat at least the surface portion of
the phosphor structures opposite the base portion thereof, and
said photocathode film disposed along the barrier layer deposited
on the surface portions of the phosphor structures opposite the
base portions thereof.
6. The improved screen set forth in claim 5 wherein
said relatively thick phosphor structures are of columnar shape and
the axes thereof are substantially normal to said substrate
member,
said barrier layer being of thickness in the range of 1 to 3
microns.
Description
My invention relates to an x-ray image intensifier tube, and in
particular, to an electrically conductive barrier layer in the
input screen thereof.
The x-ray image intensifier tube is especially useful in the
medical field for obtaining brighter x-ray images, particularly the
images of body organs which generally are of low contrast. The
conventional input screens of x-ray image intensifiers are of two
types, a first employing a uniform layer of a dense high atomic
number phosphor along a surface of a glass substrate adjacent the
source of the x-ray photons for absorbing the incident x-rays which
have traversed through a patient's body, and a thin photoemitting
film deposited on the opposite surface of the glass substrate. The
x-ray photons absorbed in the phosphor layer are converted to light
photons in the order of approximately 1,000 light photons for each
x-ray photon and emitted in all directions from the point of x-ray
photon absorption. The light photons incident on the photocathode
film are converted to photoelectrons and accelerated by means of an
anode electrode and electron-optically focussed onto a second
phosphor screen at the output end of the image intensifier in close
proximity to the anode, resulting in a brighter image than at the
input phosphor screen.
The second conventional input screen utilizes a substrate
fabricated of glass or aluminum, and the phosphor layer is formed
along the opposite surface of such substrate from the source of
x-ray photons, and the photocathode film is deposited on the free
surface of the phosphor layer.
Image intensifier tubes utilizing either of the hereinabove
described input screens experience significant electron-optic image
distortion resulting from symmetrical and uniform drops in the
electrical potential of the photocathode film laterally thereof
from the ring electrode to the center of the photocathode due to
the high resistance of the photocathode. Further, with the second
type of input screen, an electron-optic image distortion also
arises due to a nonsymmetry, local variation in the photocathode
potential due to the nonuniform thickness and nonuniform
resistivity of the photocathode film resulting from the irregular
surface of the phosphor layer on which the photocathode film is
deposited.
In the case of the hereinabove second conventional input screen, it
is conventional to deposit a thin barrier layer of an electrically
insulating, phosphor luminescence transparent material between the
phosphor layer and photocathode film in order to obtain chemical
isolation therebetween. Such thin barrier layer results in only
slight smoothing of the irregular surface of the phosphor layer and
thus is only slightly effective in reducing the electron-optic
image distortion due to the nonuniform thickness and resistance of
the photocathode film, but it has no effect on the distortion
arising from the uniform drop in potential laterally across the
photocathode film. Also, there has been considerable difficulty in
the prior art in suitably coupling a thick phosphor layer with a
very thin photocathode film; and since the future trend in x-ray
image intensifier technology appears to be toward even thicker
phosphor layers or intentionally structured input phosphor screens
for achieving higher image resolution and higher local image
contrast, the phosphor layer-photocathode film interface problem
will become even more acute.
Therefore, one of the principal objects of my invention is to
provide an x-ray image intensifier tube having a new and improved
input screen and which has substantially reduced electron-optic
image distortion.
Another object of my invention is to provide an interface for
efficiently coupling a thick phosphor layer with a very thin
photocathode film.
A further object of my invention is to provide an interface between
the phosphor layer and photocathode of future generation high
resolution-high contrast input screens.
Briefly stated, and in accordance with my invention, I provide a
barrier layer of an electrically conductive material which is
optically transparent to x-ray phosphor luminescence and which
functions as an interface between the phosphor layer and
photocathode film of an x-ray image intensifier tube. The barrier
layer material is compatible with both the phosphor and
photocathode materials and provides chemical isolation therebetween
and is sufficiently thick to provide substantial smoothing of
irregularities on the surface of the phosphor layer. Most
importantly, the significantly lower resistance of the barrier
layer relative to the photocathode results in the barrier layer
providing electron replenishment to the photocathode at all points
of electron emission therefrom to thereby substantially reduce a
uniform potential drop from the ring electrode to the center of the
photocathode due to the high resistance of the photocathode film.
The resultant reduction in uniform potential change laterally
across the photocathode film as well as a reduction in nonuniform
variation due to minimization of irregularities on the surface of
the phosphor layer results in a significant reduction in
electron-optic image distortion due to those factors in the image
intensifier tube. A particularly suitable barrier layer material is
indium oxide In.sub.2 O.sub.3.
The features of my invention which I desire to protect herein are
pointed out with particularity in the appended claims. The
invention itself, however, both as to its orgainzation and method
of operation, together with further objects and advantages thereof
may best be understood by reference to the following description
taken in connection with the accompanying drawings wherein like
parts in each of the several figures are identified by the same
reference character, and wherein:
FIG. 1 is an elevation sectional view of a conventional x-ray image
intensifier tube;
FIG. 2 is an enlarged elevation sectional view of a portion of a
first conventional input screen in an x-ray image intensifier
tube;
FIG. 3 is an elevation sectional view of a second conventional
input screen;
FIG. 4 is an elevation sectional view of a relatively new type
input screen utilizing evaporated phosphor material;
FIG. 5 is a graphical representation of the wavelength responses of
the phosphor, barrier layer and photocathode materials of an input
screen in accordance with my invention;
FIG. 6 is an elevation sectional view of the FIG. 2 input screen
with the addition of a new barrier layer in accordance with my
invention;
FIG. 7 is an elevation sectional view of the FIG. 3 input screen
with the addition of my barrier layer;
FIG. 8 is an elevation sectional view of the FIG. 4 input screen
with the addition of my barrier layer;
FIG. 9 is an elevation sectional view of a future generation input
screen utilizing my barrier layer; and
FIG. 10 is an elevation sectional view of a second type future
generation input screen utilizing my barrier layer.
Referring now in particular to FIG. 1, there is shown a
conventional x-ray image intensifier tube comprised of a glass
envelope 10 having an input window 10a through which the x-ray
photons pass after having traversed through a patient's body. A
conventional type input screen comprising a substantially uniform
phosphor layer 11 on which is deposited a thin photocathode film 12
may be formed on the inner surface of input window 10a or,
alternatively, as illustrated in FIG. 1, the phosphor layer 11 is
formed on a separate substrate member or face plate 13 which is
slightly spaced from the inner surface of input window 10a in the
order of 1/2 inch. Face plate 13 is supported within glass envelope
10 and oriented parallel to the input window 10a. The photoemitting
film 12 forms the cathode of the x-ray image intensifier tube and
is electrically connected to the negative polarity terminal of a
power supply which energizes the tube. The phosphor may be of the
granular type such as zinc cadmium sulfide or of the transparent
type such as cesium iodide as typical materials and the thickness
of the layer (or multi-layers) is generally in the range of 5 to 15
mils. The photocathode film 12 has a thickness typically in the
range of 50 to 250 Angstroms. The photoemitting material in film 12
is typically of the multi-alkali type and has a relatively low
electrical sheet conductance thereby presenting a relatively high
electrical resistance.
An electrically conductive ring 14 having its inner surface in
contact with the outer edges of the photocathode film 12 (or an
electrically conductive strip in contact with the edges of film 12)
is utilized for applying the negative potential symmetrically to
such film. The ring electrode 14 may also serve to support the
input screen assembly (11, 12, 13) in its proper orientation within
glass envelope 10. In the case of the input screen being formed
directly on the input window 10a, a film of electrically conductive
material such as evaporated aluminum may be deposited on the inner
surface of the glass envelope along the edge of the input screen
and in contact with the photocathode film for applying the
potential thereto. Alternatively, and in both cases of the input
screen being formed directly on input window 10a or on a separate
substrate member 13, the inner major surface of window 10a or
member 13 may be completely coated with the evaporated aluminum to
form a light-reflective surface to any rearward traveling light
photons in the phosphor layer and thereby increase the number of
light photons directed to the photocathode film 12. Obviously, in
the case of a separate substrate member 13 as shown in FIG. 1, it
may be fabricated of glass or a low atomic number metal such as
aluminum, in the latter case the evaporated aluminum coating not
being required. In any case, the (photo) cathode electrode 12 is
electrically connected to the negative polarity terminal of the
power supply by means of a suitable electrical conductor 15 passing
through the wall of glass envelope 10 via a vacuum seal.
The photoelectrons emitted from photocathode film 12 are focussed
by electrode 16 which is maintained at a potential of several
hundred volts positive with respect to photocathode film 12 and are
accelerated to approximately 25 kilovolts as one typical example by
means of anode electrode 17 positioned within the output end of
glass envelope 10. Focussing electrode 16 is generally oriented
either along the inner surface of glass envelope 10 in the region
between the cathode and anode electrodes, or may be slightly spaced
therefrom. Electrodes 16 and 17 are suitably shaped to provide the
desired electron-optical focussing of the accelerated
photoelectrons onto the output phosphor screen 18 which is formed
either on the inner end 10b surface of glass envelope 10 or
slightly spaced therefrom. The image appearing on the second
phosphor screen 18 is a much brighter version of the image on the
input phosphor screen 11 and can be viewed directly by the
physician or be subjected to further processing. The paths of two
photoelectrons between the photocathode film 12 and output phosphor
screen 18 are indicated by dashed line and arrowheads.
The thickness of the phosphor layer in conventional image
intensifiers is a compromise between a thick layer necessary for
high x-ray absorption and a thin layer necessary for high image
resolution and local image contrast. As a result, the conventional
5 to 15 mil thickness phosphor layer has a relatively low x-ray
absorption in the order of 15 to 35 percent of the incident x-rays
and future generation input phosphor screens to be described
hereinafter with reference to FIGS. 9 and 10 can result in thicker
phosphor layers to thereby increase the x-ray absorption, and thus
the sensitivity, but with less loss in resolution and local
contrast than occurs in conventional image intensifiers, or
alternatively, will utilize conventional thickness phosphor layers
but obtain increased resolution and contrast.
FIG. 2 illustrates a second embodiment of a conventional input
screen of an x-ray image intensifier wherein the major surface of
the support member 13 adjacent the source of x-ray photons is the
support for phosphor layer 11, and the opposite major surface of
the support member 13 is the support for photocathode film 12. In
view of the orientation of phosphor layer 11 and photocathode film
12 relative to support member 13, member 13 must be fabricated of a
phosphor luminescence transparent material such as glass and is
typically of 5 mils thickness. The phosphor layer 11 is illustrated
as consisting of a relatively thick multi-layer of relatively large
size phosphor grains (i.e., of particle diameter greater than 0.3
mil in order to have high light transmission characteristics).
Although the phosphor grains are illustrated for convenience as
being spherical, it is to be understood that the grains generally
are not quite spherical in shape. Also, the grains are not
necessarily equal in size due to the process of formation of such
granular phosphor. The granular phosphors in the FIG. 2 embodiment
are very thinly coated with a silicone resin such that upon
compaction of the phosphors the resin provides an adhesive effect
for retaining the phosphors on the surface of substrate member 13.
The granular phosphors 11 may be zinc cadmium sulfide or gadolinium
oxysulfide as two suitable granular phosphors. In some cases, a
suitable activator is added to the phosphor host material, typical
activators for zinc cadmium sulfide being silver and for gadolinium
oxysulfide being terbium. The photoemitter material forming
photocathode film 12 may be of the common types known as S-20 (a
compound of antimony, cesium, sodium and potassium) or S-11 (a
compound of cesium, antimony and oxygen) as two typical examples,
and is of thickness in the hereinabove recited range of 50 to 250
Angstroms, and typically may be 150 Angstroms.
The FIG. 2 input screen has an advantage over the input screen
depicted in FIG. 1, and in greater detail in FIG. 3, in that the
photocathode film 12 is deposited upon a smooth surface of the
glass support member 13 thereby eliminating electron-optical image
distortion due to a nonuniformly thick or nonuniformly resistive
photocathode film resulting from irregularities along the output
surface of the phosphor layer. However, since only the edges of the
photocathode film 12 in the FIG. 2 embodiment are in contact with
the electrically conductive ring member 14 for applying the proper
potential to the photocathode (as in the case of FIG. 1), the
electron-optic image distortion due to variation of electric
potential laterally across the photocathode film remains as a
source of electron-optic image distortion in this conventional
input screen structure.
FIG. 3 is an enlarged view of a portion of the input screen
depicted in FIG. 1. In addition to the distinction between the
FIGS. 2 and 3 embodiments in the relative orientation of the
substrate member 13, the substrate member 13 in FIG. 3 may be
fabricated of glass, aluminum or other low atomic number metal
whereas substrate member 13 in FIG. 2 is restricted to a phosphor
luminescence optically transparent material such as glass. The
phosphor layer 11 in FIG. 3 consists of a granular phosphor which
may be of the same type and size as in FIG. 2, but is embedded in a
silicone resin, that is, the resin occupies a substantial part of
the volume in the phosphor layer in order of 15 to 30 percent
whereas any resin employed in the FIG. 2 embodiment is merely for
causing adherence of the phosphor grains to each other and to the
substrate member. One of the main reasons for using granular
phosphor in a silicone resin binder is to eliminate the several
mils separation between phosphor and photocathode represented by
the thin glass substrate in FIG. 2, which separation causes a
degradation of local image contrast and resolution (i.e., a
degradation of the image modulation transfer function), and it
should be understood that the granular phosphors in the FIG. 2
embodiment can also be in a resin binder.
As stated hereinabove, a thin barrier layer 30 of an electrically
insulating, phosphor luminescence transparent material has
conventionally been utilized as an interface between the phosphor
layer and photocathode film to obtain chemical isolation
therebetween. The barrier layer 30 has a thickness typically in the
range of 0.1 to 1.0 micron and is fabricated of materials such as
aluminum oxide or silicone dioxide. The thinness of this barrier
layer is sufficient to obtain the desired chemical isolation
between the phosphor and photoemitter materials, but such thinness
results in only a slight smoothing of irregularities on the
phosphor surface. This conventional barrier layer 30 is not used in
the FIG. 2 input screen since there is no interface between the
phosphor layer 11 and the photocathode film 12.
Referring now to FIG. 4, there is shown a relatively new type of
input screen which utilizes evaporated transparent phosphor
material rather than the granular type utilized in the FIGS. 2 and
3 embodiments. The transparent phosphor material may be thallium
activated cesium iodide as one typical example, is of thickness in
the same range as the layers in FIGS. 2 and 3 (5 to 15 mils) and
has the advantage over the granular phosphors in that it has a
higher light transmission characteristic and theoretically sould
have no surface irregularities thereby permitting a more uniform
photocathode film 12 to be deposited thereon. However, in practice,
a difficulty arises in depositing evaporated phosphor in that there
invariably develops a grain growth or general cracking due to
thermal stress thereby developing an undesired physical network of
phosphor islands formed by cracks which progress from the free
surface of the evaporated phosphor 11 toward the aluminum or glass
substrate 13 and may even develop completely to such substrate
surface. The electrically nonconductive thin barrier layer 30 may
be utilized as a chemical isolating interface between the
evaporated phosphor material 11 and photocathode film 12, and
additionally may result in some smoothing or the unintentionally
structured phosphor layer by bridging across, hopefully, at least
some of the cracks developed from the free surface of the
evaporated phosphor. Thus, the use of electrical insulating barrier
layer 30 in the FIG. 4 prior art embodiment may also result in some
reduction in the electronoptic image distortion arising from a
nonuniform thickness or nonuniformly resistive photocathode film
deposited on an irregular surface on the phosphor layer, and more
specifically in the case of the FIG. 4 embodiment, arising from
microscopic and macroscopic electrical island formation on the
phosphor surface due to the cracks developed from the free surface
thereof. However, as in the case of the FIG. 3 embodiment,
conventional electrically insulating, thin barrier layer 30 has no
effect on the electron-optic image distortion arising from the
uniform drop in potential laterally across the photocathode film
due to the high electrical resistance thereof.
In accordance with my invention, I provide a barrier layer as a
chemically isolating interface between the phosphor layer and
photocathode film which is formed of a material completely
different from the materals utilized in the above-described prior
art barrier layers to thereby obtain a significantly improved x-ray
image intensifier input screen. In particular, I utilize a material
which has the desirable characteristics of the prior art barrier
layer materials, namely, low vapor pressure such that it can be
deposited at low pressure by a simple process and at low cost,
phosphor luminescence transparency for good optical coupling and
image conversion and chemical compatibility with the phosphor and
photoemitter materials both during fabrication and throughout the
lifetime of the image intensifier. But most importantly, and the
prime distinguishing features between my barrier layer material and
the prior art, I utilize a material which is relatively
electrically conductive, and the layer is of substantial thickness.
The electrically conductive characteristic of my barrier layer
material is defined as a material having an electrical resistance
in the range of 10 to 10.sup.6 ohms per square, and is typically
1,000, to provide sufficient electrical sheet conductance relative
to the photocathode film whereby the resistance of my barrier layer
is significantly less than the resistance of the photocathode film.
As a result, my barrier layer which is electrically connected to
the negative polarity terminal of the power supply, effectively
electrically short-circuits the photocathode film and thereby
provides electron replenishment to the photocathode film at all
points of electron emission therefrom. This electron replenishment
substantially reduces the potential drops developed laterally
across the photocathode film and thereby results in a significant
reduction in electron-optic image distortion due to such factor
which cannot be obtained with the conventional type conductive
barrier layer. My barrier layer is also sufficiently thick (0.1 to
25 microns) to afford substantial smoothing or bridging of surface
irregularities on the phosphor layer and thereby also obtain a
significant reduction in electron-optic image distortion due to
such factor.
In accordance with my invention I have found that indium oxide
(In.sub.2 O.sub.3) is a first material which is especially suitable
for forming my electrically conductive barrier layer. It can be
applied in either of two ways: it can be evaporated onto a room
temperature substrate at low oxygen pressure (e.g., 10 microns for
example) and subsequently fully oxidized in air at 200.degree.C and
1 atmosphere for times from 30 minutes to 2 hours; or, it may be
applied in a one step bell jar process by evaporating indium onto a
200.degree. to 300.degree. C substrate in the presence of 50 - 150
microns oxygen, no further oxidation being necessary. FIG. 5
illustrates the wavelength responses of the indium oxide and two
materials for which it functions as the barrier layer, namely, a
thallium-activated cesium iodide phosphor and the S-20 type
photoemitter material. The response curve designated "PHOSPHOR
CsI:Tl" is a plot of the relative spectral emission of the
activated cesium iodide vs. wavelength in Angstroms. THe curve
designated "PHOTOCATHODE S-20" is the relative photocathode
response for the photocathode material S-20, a compound of an
antimony, cesium, sodium and potassium. Finally, curve "BARRIER
LAYER In.sub.2 O.sub.3 " is a plot of (1-a) where "a" is the
absorbtivity of indium oxide In.sub.2 O.sub.3. A comparison of the
three curves indicates that the response of indium oxide is very
well matched to that of the cesium iodide phosphor and S-20
photocathode in that its peak transmission is over a sufficiently
broad wavelength band to include the significant wavelength
responses of cesium iodide phosphor and S-20 photocathode
materials. In fact, the broad spectral transmission of the indium
oxide makes it suitable for use with virtually any of the x-ray
sensitive phosphors which have spectral emission in the visible
band of wavelengths and with other newer negative electron affinity
photocathodes such as gallium arsenide GaAs:Cs. My invention is
therefore basically the use of indium oxide, or other suitable
electrically conductive, x-ray luminescence optically transparent
materials to be described hereinafter as a barrier layer between
the phosphor layer and photocathode film of an x-ray image
intensifier tube. The use of this new and improved barrier layer is
not limited to the conventional type x-ray image intensifier input
screens but is also equally important for use with future
generation type input screens to be described hereinafter.
Referring now to FIG. 6, there is shown a first embodiment of the
use of my electrically conductive barrier layer in an X-ray image
intensifier input screen. The input screen in the FIG. 6 embodiment
corresponds to the conventional input screen illustrated in FIG. 2
wherein the phosphor layer 11 and photocathode film 12 are disposed
on opposite sides of a smooth glass (or other phosphor luminescence
transparent material) substrate member 13. Due to the glass
substrate 13 presenting a smooth surface to the photocathode film,
the electon-optic image distortion is due primarily to the
symmetrical changes in potential developed laterally across the
(high resistance) photocathode film from the ring electrode to the
center of the film as described hereinabove. My electrically
conductive, x-ray phosphor luminescence optically transparent
barrier layer 60, which may be indium oxide as one example, is
deposited on the surface of the glass substrate member 13 such that
it is disposed between the glass substrate and the photocathode
film 12 as illustrated in FIG. 6. Barrier layer 60 is of
substantially uniform thickness such that photocathode film 12 also
retains its smooth surface as it had in the FIG. 2 conventional
embodiment. The various thicknesses of elements 11, 12 and 13 in
the FIG. 6 (and also FIGS. 7-10) embodiments may be the same as in
the FIGS. 2 (and 3 and 4) embodiments. Barrier layer 60 in all of
the embodiments herein described is of thickness in the range
between 0.1 to 25 microns, and in the FIG. 6 embodiment would
generally be in the lower portion of the thickness range since it
does not have to provide the smoothing of surface irregularities
function in the phosphor layer as in the embodiments to be
described hereinafter. Thus, the barrier layer thickness in the
FIG. 6 embodiment is more generally in the range between 0.1 to 1.0
microns. Barrier layer 60 is deposited on glass substrate member 13
in a pattern such that the edges of layer 60 slightly overlap the
side surfaces of substrate 13 and therefore come in contact with
ring electrode 14 (see FIG. 1). Alternatively, an aluminum or other
highly electrically conductive film may be formed along the edges
of barrier layer 60 to provide electrical connection thereof to the
ring electrode. Since barrier layer 60 is connected to the source
of potential (via conductor 15 shown in FIG. 1) there is no need
for photocathode film 12 to be also so connected, and my barrier
layer 60 provides electron replenishment to the photocathode film
at all points of electron emission therefrom to thereby
significantly reduce electron-optic image distortion due to
undesired relatively high potential drops developed laterally
across the photocathode film.
Upon barrier layer 60 having been deposited on substrate 13, the
photocathode film 12, which may be of the multi-alkaline types
hereinabove-described, is deposited on barrier layer 60 by
conventional techniques.
Referring now to FIG. 7, there is shown a second embodiment of an
input screen utilizing my barrier layer, and in particular is an
improvement over the conventional input screen illustrated in FIG.
3. Thus, in FIG. 7, my barrier layer 60 is deposited directly on
the irregular surface of phosphor layer 11. Alternatively, an
electrically nonconductive barrier layer 30, as defined with
reference to FIG. 3, may be deposited on the irregular surface of
the phosphor layer 11 for additional chemical compatibility of the
phosphor and photocathode materials, and my electrically conductive
barrier layer 60 is then deposited on top of barrier layer 30 such
that the electrically conductive barrier layer 60 is in contact
with the photocathode film 12. Thus, if desired, both type barrier
layers can be utilized in the input screen with the electrically
conductive barrier layer being in contact with the photocathode
film and connected to the source of potential in order to provide
electron replenishment thereto. In the FIG. 7 embodiment, and
assuming that the nonconductive barrier layer is not utilized, my
electrically conductive barrier layer 60 provides the advantages
described with reference to the use of such barrier layer in the
FIG. 6 embodiment as well as providing chemical isolation between
phosphor (or phosphor-in-resin) layer 11 and photocathode film 12,
being compatible with both materials during preparation and
throughout the life thereof and affording substantial smoothing or
bridging of surface irregularities in the phosphor layer 11 such
that electron-optic image distortion from this second factor is
also substantially reduced in addition to the reduction in such
distortion due to uniform drops in potential laterally across the
photocathode film. The electrically conductive barrier layer 60
thickness in my FIG. 7 embodiment is generally in the mid portion
of the 0.1 to 25 micron range and therefore is more generally of
thickness in the range of 0.5 to 3 microns, especially in the case
wherein nonconductive barrier layer 30 is not employed.
Referring now to FIG. 8, there is shown my improved version of the
input screen depicted in FIG. 4 wherein phosphor layer 11 is an
evaporated transparent phosphor such as cesium iodide. The network
of microscopic and macroscopic electrical islands formed by the
cracks progressing from the free surface of the evaporated phosphor
layer 11 which progress toward or actually to the aluminum (or
other low atomic number metal) or glass substrate member 13 are
substantially eliminated by depositing my electrically conductive
barrier layer 60 (of thickness in the range of 1 to 3 microns)
along the entire free surface of evaporated phosphor layer such
that surface continuity exists over the phosphor surface and the
physical network of islands is no longer present and the edges of
layer 60 are connected to the source of potential. However, barrier
layer 60 bridges the cracks developed from the surface of the
phosphor layer and also penetrates into the voids developed by such
cracks, and in the case of the cracks extending to (i.e.,
contacting) the surface of an aluminum substrate which has its side
surfaces connected to the ring electrode, it may not be necessary
to provide an electrical connection from the outer edges of barrier
layer 60 to the ring electrode. Alternatively, and especially in
the case wherein substrate member 13 is formed of electrically
insulating material such as glass, the conventional electrical
insulating barrier layer 30 may be deposited on the cracked surface
of the evaporated phosphor layer 11 and my electrically conductive
barrier layer 60 then deposited on top of the insulating layer. In
this latter embodiment, the electrically conductive barrier layer
60 obviously must be electrically connected to the ring
electrode.
Referring now to FIG. 9, there is shown what may be described as a
future generation input screen wherein the phosphor layer is an
intentionally fabricated array of columnar, honeycomb, or other
generally symmetrically arranged phosphor structures which provide
a light piping effect to the phosphor luminescence. The phosphor
layer is thus a single layer of a plurality of spaced apart
relatively thick phosphor structures 11 having base portions
disposed along and in adherence with a low atomic number metal
substrate member 13 on the major surface thereof that is opposite
from the source of the x-ray photons. The columnar phosphor
structures 11 illustrated in FIG. 9 may be generally square,
hexagonal or even circular in cross section as typical shapes and
have a height (distance normal from substrate member 13)-to-width
ratio in the range of 2:1 to 10:1. Although the phosphor structures
11 are preferably equally spaced apart and are of identical
dimension, this may not occur in a practical sense due to possible
difficulties in the fabrication process thereof. Thus, as
illustrated in FIG. 9 the columnar structures 11 are not equally
spaced apart and the shapes, although similar, are not identical.
The light piping effect produced by the intentionally structured
phosphor layer provides a high resolution-high contrast input
phosphor screen and cesium iodide is a typical transparent phosphor
which is especially suitable for this embodiment. My electrically
conductive barrier layer material, such as indium oxide, is
deposited along the entire nonbase portion of the phosphor
structures by evaporation as in the previous embodiments such that
for the more narrowly spaced apart phosphor structures, the
electrically conductive barrier layer material 60 completely fills
the voids therebetween, whereas in the more widely spaced apart
structures, the barrier layer material coats such nonbase surfaces
of the phosphor structures but a much smaller void may remain
between such structures. Thus, in the case of the narrowly spaced
apart phosphor structures 11, my conductive barrier layer 60
bridges the adjacent phosphors along the free ends thereof whereas
the more widely spaced apart phosphor structures do not have their
free ends bridged. The photocathode film 12 is then evaporated only
upon the surfaces of the barrier layer formed along the free ends
of the phosphor structures such that photocathode film is
continuous in the regions wherein the barrier layer bridges
adjacent phosphor structures, and is discontinuous wherein such
bridging does not occur. In either event, since the conductive
barrier layer material 60 extends from the base of the phosphor
structures 11 to the photocathode film, such barrier layer provides
electron replenishment to each portion of the photocathode film 12.
In the event substrate member 13 is fabricated of glass of other
x-ray transparent, electrically insulating material, the major
surface of substrate member 13 along which the base portions of the
phosphor structures 11 are disposed is first coated with an
electrically conductive film such as evaporated aluminum and this
film, or the electrically conductive substrate member, if used, are
electrically connected to the ring electrode. The thickness of
layer 60 would typically be in the range of 1 to 3 microns.
FIG. 10 illustrates a second embodiment of a future generation
input screen wherein granular phosphors are utilized with no, or
very limited amount of binder material and the electrically
conductive barrier layer material 60 is intended to cover major
portion of the outer surface presented by the multi-layers 11 of
phosphor grains. In this embodiment, the evaporation of the
electrically conductive barrier layer material at the low pressure
of several tens of microns causes such conductive layer to extend
over all of the exposed surfaces of the phosphor grains in a
thickness range of 2 to 25 microns as well as behind the grains in
the regions where the grains are adjacent to or in contact with
substrate member 13. The barrier layer does not necessarily have to
fill the larger voids between adjacent more widely spaced apart
phosphor grains as is depicted in FIG. 10.
Indium oxide adheres very well to the thick phosphor film of the
structured phosphor layer in FIG. 9, as illustrated in FIG. 8 as
well as to the phosphor grains depicted in FIGS. 7 and 10. Other
suitable electrically conductive barrier layer materials which may
be used in any of the embodiments hereinabove described are
slightly chemically reduced titantium diode TiO.sub.2, cuprous
iodide CuI, and zinc oxide ZnO.
From the foregoing description, it can be appreciated that may
invention makes available an x-ray image intensifier tube having a
new and improved input screen which, due to the use of an
electrically conductive barrier layer between the phosphor material
and photocathode material significantly reduces electron-optic
image distortion due to undesired unifrom potential drops and
nonuniform potential variations laterally across the photocathode
film by providing electron replenishment to the photocathode film
as well as smoothing surface irregularities on the phosphor layer.
The electrically conductive barrier layer provides a compatible
interface between the phosphor layer and photocathode for both
conventional type input screens and future generation high
resolution-high contrast input screens.
Having described a number of specific embodiments of my input
screen, it is believed obvious that modification and variation of
my invention is possible in light of the above teaching. Thus,
other materials suitable for my electrically conductive barrier
layer other than those enumerated above may also be used as long as
they meet the limitations of having an electrical resistance in the
range of 10 to 10.sup.6 ohms per square and are optically
transparent to the particular phosphor luminescence involved,
provide chemical isolation between the phosphor and photocathode
materials and are compatible therewith and can be deposited in
sufficient thickness to provide a smoothing or bridging of surface
irregularities or cracks in the phosphor layer. Finally, there may
be some applications wherein a very thin (approximately 10
Angstroms) film of an electrically insulating material may be
utilized between the electrically conductive barrier layer and
photocathode film, but in this latter case the insulating or
nucleating film would be sufficiently thin such that its desired
characteristics would be effective without providing substantial
electrical insulation between the electrically conductive barrier
layer and photocathode film. It is, therefore to be understood that
changes may be made in the particular embodiment described which
are within the full intended scope of the invention as defined by
the following claims.
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