U.S. patent number 3,902,089 [Application Number 05/267,111] was granted by the patent office on 1975-08-26 for channel plate matrix of tubes having twisted septa.
This patent grant is currently assigned to U.S. Philips Corporation. Invention is credited to Robert Malcolm Beasley, Derek Washington.
United States Patent |
3,902,089 |
Beasley , et al. |
August 26, 1975 |
Channel plate matrix of tubes having twisted septa
Abstract
The channels in a channel plate are divided into sub-channels by
longitudinal septa which are twisted along the length of the
channels in order to prevent ion feedback. An angle of twist of
360.degree. may be used in order to permit each sub-channel to
represent a separate picture element.
Inventors: |
Beasley; Robert Malcolm
(Salfords, near Redhill, EN), Washington; Derek
(Salfords, near Redhill, EN) |
Assignee: |
U.S. Philips Corporation (New
York, NY)
|
Family
ID: |
10334414 |
Appl.
No.: |
05/267,111 |
Filed: |
June 28, 1972 |
Foreign Application Priority Data
|
|
|
|
|
Jul 8, 1971 [GB] |
|
|
32171/71 |
|
Current U.S.
Class: |
313/105R;
313/105CM; 65/402; 57/248; 65/393; 65/403 |
Current CPC
Class: |
C03B
37/15 (20130101); C03B 37/027 (20130101); H01J
43/24 (20130101); C03B 37/01211 (20130101); C03B
37/02745 (20130101); C03B 2203/04 (20130101); C03B
2205/06 (20130101); C03B 2203/10 (20130101); C03B
2203/14 (20130101); C03B 2203/20 (20130101) |
Current International
Class: |
H01J
43/24 (20060101); H01J 43/00 (20060101); C03B
37/02 (20060101); C03B 37/027 (20060101); C03B
37/15 (20060101); C03B 37/10 (20060101); C03B
37/012 (20060101); H01j 043/22 (); N01j
029/41 () |
Field of
Search: |
;313/105,104 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Segal; Robert
Attorney, Agent or Firm: Trifari; Frank R. Drumheller;
Ronald L.
Claims
What we claim is:
1. An improved matrix for a channel plate electron multiplier
having improved image resolution, comprising: a side by side
stacked array of substantially identical tubes, each tube having an
interior surface and a substantially geometrically centered
longitudinal axis thereof, each tube having an identical number of
substantially equally spaced septa extending from said axis to said
interior surface, said septa spiralling about each other in the
longitudinal direction to define within each of said tubes the same
number of substantially identical longitudinal channels spiralling
about each other, said number of channels being within the range of
three to six inclusive, the inside surfaces of said defined
channels being at least slightly conductive and secondary emissive,
said septa being set back from the end of said tubes intended to be
the input end and spiralling through an angle of substantially
360.degree. or an integer multiple thereof, whereby the ends of
said defined channels are in the same relative position with
respect to adjacent tubes on both sides of said stacked array
thereby improving the image resolution of said array.
Description
This invention relates to electron multipliers and more
particularly to electron multipliers of the channel plate type. The
invention is applicable to channel plates for use in electronic
imaging and display tube applications.
The type of device now known as a "channel plate" can be defined as
a secondary-emissive electron-multiplier device comprising a matrix
in the form of a plate having a large number of elongate channels
passing through its thickness, said plate having a first conductive
layer on its input face and a separate second conductive layer on
its output face to act respectively as input and output
electrodes.
The invention relates more particularly to channel plates of the
continuous dynode type. This is a convenient term for channel
plates having what is at present the conventional form of
construction. Such channel plates can be regarded as continuous
dynode devices in that the material of the matrix is continuous
(though not necessarily uniform) in the direction of thickness,
i.e. in the direction of the channels.
Continuous dynode channel plates are described, for example, in
British Pat. No. 1,064,073 and in U.S. Pat. Nos. 3,260,876,
3,387,137, 3,327,151 and 3,497,759, while methods of manufacture
are described in British Pat. Nos. 1,064,072 and 1,064,075.
In the operation of continuous dynode plates a potential difference
is applied between the two electrode layers of the matrix so as to
set up an electric field to accelerate the electrons, which field
establishes a potential gradient created by current flowing through
resistive surfaces formed inside the channels or (if such channel
surfaces are absent) through the bulk material of the matrix.
Secondary-emissive multiplication takes place in the channels and
the output electrons may be acted upon by a further accelerating
field which may be set up between the output electrode and a
suitable target, for example a luminescent display screen.
Channel plates can be used in imaging tubes of various kinds, for
example scanning tubes such as cathode-ray tubes and camera tubes
and non-scanning image intensifier tubes (this Specification will,
for convenience, refer to an image intensifier tube in those terms
rather than as an "image converter" tube even in applications where
the primary purpose is a change in the wavelength of the radiation
of the image).
It is an object of the present invention to overcome or mitigate
the problem of ion feed-back which arises in the practical use of
channel plates. In an individual channel, ions are fed back from
parts of the wall and interior space of the channel (these will be
referred to as "channel ions") and in single channel multipliers
this problem has been solved by considerable curvature of the
channel tube (e.g. a curvature of 360.degree. so that the tube is
in the form of one complete helical turn).
In the case of a channel plate used in an image intensifier having
a phosphor display screen located near the output end of the
channels, ion feedback may also derive from such screen.
Furthermore, it now appears that many ions are generated also in
the gap between such screen and the channel plate (such ions will
be referred to as "gap ions").
When ion feedback occurs, the ions are accelerated by the field in
the channels and can cause spurious secondary emission further back
in the channels and/or at the photo-cathode, quite apart from
damage to the photo-cathode.
Reduction of ion feedback from channel plate to photo-cathode can
lead to improved life. In addition, reduction of ion feed-back
permits operation in the pulse saturation mode. This can result in
a more favourable pulse height distribution (P.H.D.) and reduced
noise.
With channel plates used in multi-channel electron multipliers and
image intensifiers attempts have been made to overcome or reduce
ion feedback in the following ways:
A. U.S. Pat. No. 3,603,832 describes the provision of
electron-permeable conductive membranes provided to obturate the
entrances to the channels and thus prevent the passage of ions to
the photo-cathode. This technique is relatively difficult and
expensive especially for plates of large area.
B. U.S. Pat. No. 3,374,380 describes what is sometimes referred to
as a "chevron" construction in which two separate channel plates
are arranged in series with each other with the channel axes of one
plate disposed at an angle to the channel axes of the other plate.
This arrangement has the disadvantage that individual channels of
one plate are not aligned with individual channels of the other
plate so that definition is lost, and this loss may be increased by
the gap which appears to be present between the two plates in the
practical arrangements available.
However, since the publication of these prior patent specifications
further studies of the ion feedback effect have been carried out by
Applicants.
First, it is now clear that reduction of spurious secondary
electron cascades resulting from ion feedback to the channel wall
near the input can allow a plate to be operated at higher gain (for
use in photomultipliers for example).
For a channel having a 50:1 length-to-diameter (L/D) ratio
Applicants have discovered that about 75-90% of ions formed inside
the channels and which escape from the input may be formed in the
last (i.e. output) 30% of the length of the channel (the terms
"input and output" are used herein exclusively with reference to
electrons). For a single channel of a channel plate Applicants
have, on this basis, made an estimate of the two-dimensional
channel curvature required to mask this 30% end region from the
input, i.e. to ensure that such region cannot be "seen" from the
input aperture. Such curvature turns out to be much less than the
curvatures used in single-channel multipliers, One reason being the
lower gains normally used in channel plates.
On the basis of this discovery, Applicants have described in
British Application No. 12780/71 a matrix for a channel in which
the axes of the channels are curved in one plane (the term "axis"
is thus used to denote the finite centre-line of a channel and not
a straight line in the normal geometrical sense). Such curvature is
applied to the axes of the channels of a channel plate as an
alternative to the conductive membrane and "chevron" arrangements
referred to above.
The main object was, as before, to prevent ion feed-back or, to use
an alternative expression, to render the channels "ion blind" and a
secondary object (for some applications) is to render them also
"optically blind" to prevent optical feed-back from the display
screen (for the latter purpose the matrix must be made opaque).
The present invention provides a further alternative solution to
the ion-feedback problem in channel plates by adopting a
modification of the "Spiraltron" principle. The Spiraltron is
described in Trans. I.E.E.E., NS-15, No. 3, June 1968 where the
electron multiplier is composed of a parallel stack of individual
elements, that are termed Spiraltron multipliers, fused together.
Each Spiraltron multiplier in turn comprises six single-tube
multipliers twisted or wrapped about a solid central structural
core.
In a later version (Review of Scientific Instruments, Vol. 41, No.
5, May 1970) the six helically twisted tubes are replaced by six
twisted radial partitions which subdivide the space between the
solid core and a cylindrical outer tube.
In both cases the solid core has a cross-sectional area which is
comparable with that of each of the six channels and this renders
the "Spiraltron" arrangements unsuitable for use in channel plates
of imaging quality because of the loss of useful area (the solid
cores occupy almost one-seventh of the total area). Another
objectionable fact would be that each six-tube Spiraltron would
effectively represent only one picture element if it were used in
an imaging array instead of being used in the simpler
photo-multiplier or detector applications described in the above
references.
According to a first aspect the present invention provides a matrix
for a channel plate of the continuous dynode type wherein each
channel includes a longitudinal single or multiple septum which is
twisted along the length of the channel without the provision of
any solid structural core so that, apart from the thickness of the
septum, effectively the whole cross-sectional area of each channel
is subdivided into two or more separate sub-channels by said
septum.
With such an arrangement angles of twist t much smaller than
360.degree. can be sufficient to achieve ion blindness and also (if
the matrix is opaque) optical blindness. However, for imaging
purposes, each group of sub-channels can only represent one picture
element unless, according to a second aspect of the invention, each
septum is twisted through an angle t of 360.degree., or
approximately 360.degree. or a multiple of 360.degree., over the
length of its channel so that the output aperture of each
sub-channel is aligned with its input thereby permitting each
sub-channel to represent a separate picture element. In this case
the degree of accuracy required for the angle of twist t depends
solely on the use of the sub-channels to provide high resolution by
representing separate picture elements.
Each channel may include a plain single septum which extends
diametrically, or substantially diametrically, across the channel
from side to side so as to divide it into two sub-channels. In this
case the structure is a very simple one which can be made with
little more difficulty than a conventional matrix.
Alternatively, each channel may include a radial multiple septum
which radiates from the central axis of the channel and subdivides
the channel into n sub-channels, the number n being preferably
within the range 3 to 6 inclusive.
The input end of each septum may be set back with respect to the
input aperture in order to increase the number of secondary
electrons generated and utilized in the input region of the
channel.
A method of manufacturing matrices according to the present
invention may include the steps of forming an initial tubular
structure with a septum supported in position therein, drawing said
structure down to a single fibre, twisting said fibre during the
drawing process, forming a boule from such twisted fibres and
slicing said boule along parallel cutting planes. Preferably the
fibre is twisted with constant pitch and the cutting planes are
spaced apart substantially by 360.degree. of twist or a multiple of
360.degree.. A uniformly regular input and output pattern can be
obtained if in addition the boule is formed in a regular manner in
the sense that all septa have the same orientation in any one of a
series of parallel transverse planes. In any event, for imaging
purposes it is desirable to select the cutting planes so that the
thickness of each resulting slice corresponds substantially to one
pitch or a multiple of the pitch.
The drawing and twisting of the initial tubular structure can be
carried out without using temporary internal etchable supporting
cores since the septum can provide some internal support against
unwanted deformation during subsequent processing, but the use of
etchable cores in the sub-channels may still be desirable for
particular reasons as will be explained.
Angles of twist smaller than 360.degree. can be sufficient to mask
the output aperture of a sub-channel from its input aperture so
that gap ions are intercepted and can also be sufficient to mask
the 30% end region of the sub-channel from its input aperture so as
to intercept most channel ions. However, such considerations are
not relevant when, as hereinnafter, angles t of 360.degree. or
multiples of 360.degree. have been adopted in order to obtain
maximum resolution.
Embodiments of the invention will now be described by way of
example with reference to the diagrammatic drawings in which:
FIG. 1 (a) shows in cross-section a channel tube divided by septa
into two sub-channels;
FIG. 1 (b) shows the twisted septa of FIG. 1 (a);
FIG. 2 shows in cross-section a fused stacked array of tubes of the
type shown in FIG. 1 (a) with all the septa parallel;
FIG. 3 (a) shows in cross-section a channel tube divided by septa
into three sub-channels;
FIG. 3 (b) shows the twisted septa of FIG. 3 (a);
FIG. 4 shows in cross-section a fused stacked array of tubes of the
type shown in FIG. 3 (a) with the septa having the same
orientation;
FIG. 5 shows in cross-section a channel tube divided into four
sub-channels;
FIG. 6 shows a side cross-sectional view of a septum set back from
the input end of the channel tube;
FIG. 7 (a) shows the structure of FIG. 1 (a) in one stage of
manufacture;
FIG. 7 (b) shows the various component parts of the structure of
FIG. 7 (a);
FIG. 8 (a) shows a modification of the structure of FIG. 7 (a);
FIG. 8 (b) shows the various component parts of the modified
structure of FIG. 8 (a);
FIG. 9 schematically shows a machine for drawing fibers according
to the present invention;
FIG. 10 schematically shows a boule or stacked array of channel
tubes;
FIG. 11 illustrates the use of a channel plate according to the
present invention in an image tube of the proximity type; and
FIG. 12 illustrates the use of a channel plate according to the
present invention in an image tube of the "electron-optical diode"
or "inverter" type.
Referring now to the drawings, FIG. 1 (a) shows in cross-section a
glass channel tube C which includes a plain single septum D of
glass which extends diametrically across the channel from side to
side so as to divide it into two sub-channels. Such tubes can be
assembled and fused to form a regular matrix structure and the
structure is a very simple one which can be made with little more
difficulty than a conventional matrix.
FIG. 1 (b) shows the twisted form of the septum D which in these
examples is twisted through 360.degree. to achieve high resolution,
as aforesaid, by ensuring that each sub-channel will represent a
separate picture element in its correct location. Such an angle of
twist is at the same time more than sufficient to achieve ion --
and (if the matrix is opaque) optical -- "blindness". If it is
desired, in addition, to have all the septa D orientated in the
same direction, this can be achieved by more methodical stacking of
the tubes during assembly, and the input and output pattern will
then be completely regular as shown in FIG. 2.
FIGS. 3 (a) and 5 show single-channel units which include a radial
multiple septum R which radiates from the central axis of the
channel and sub-divides the channel into 3 or more
sub-channels.
In particular, FIG. 3 (a) shows a tube with a triple septum R which
defines three sub-channels, and FIG. 3 (b) shows the twisted form
of the septum. If such units are assembled to form a regular matrix
array and are fused under compression to an approximately hexagonal
form, uniform definition can be obtained. In addition, a completely
regular pattern of apertures can be obtained at the faces of the
plate as shown in FIG. 4 if the tubes are assembled so that all the
septa have the same orientation.
The further example given in FIG. 5 shows a tube C with a quadruple
septum R defining four sub-channels.
As shown in FIG. 6, the input end of each septum may be set back to
a depth d with respect to the input aperture in order to increase
the number of secondary electrons generated and utilized in the
input region of the channel. Primary electrons e1 and secondaries
e2 are shown schematically and E1 represents the input electrode of
the completed channel plate with a re-entrant conductive extension
E1a in the mouth of each channel. With this arrangement it is
easier to prevent metallization of the input edge of the septum and
it is more likely that the secondaries therefrom will travel down
the sub-channels and thus be utilized which action represents
effectively an increase in the "open area" of the channel plate,
i.e. the area capable of detecting incident electrons. The
arrangement may be achieved by etching back the end of the septum
and is applicable to the septa D of FIG. 2 and septa R of FIGS. 4
and 5.
A method of manufacture will now be described and it will be
assumed that, apart from the provision of twisted septa required by
the present invention, the basic process involves the drawing,
stacking and fusing together of single tubular glass fibres with or
without temporary supporting cores e.g. as described in the
aforesaid British Pat. No. 1,064,072. When such cores are used (to
control the deformation of the tubes during drawing and fusing) the
cores will be etched out or otherwise removed after the formation
of each plate. In particular, such cores may be of a glass which
can be etched more readily than the tube and septum glass, and said
cores may have a passage or bore to faciltate such etching in known
manner.
Preferably the septa are of the same glass and have
secondary-emissive and conductivity surfaces of the same kind as
those of the channel walls of the tubes C. If so, etching back the
septa according to FIG. 6 can be facilitated by making the septa
thinner than the tube walls.
A method of manufacturing matrices according to the present
invention may include the step of forming an initial tubular
structure with a septum supported in position therein. The
structure for the arrangement of FIG. 1 (a) can comprise merely the
tube C and a flat (untwisted) septum element D. Preferably,
however, the structure also comprises two "half-round" solid cores
c1 - c2 as shown in FIGS. 7 (a) and 7 (b), such cores being etched
or dissolved away at a later stage in accordance with the solid
core principle described and claimed in the aforesaid British Pat.
No. 1,064,072. The two solid cores may have passages to facilitate
etching as shown in FIGS. 8 (a) and 8 (b).
Other arrangements such as those of FIG. 3 (a) and FIG. 5 can
employ corresponding initial untwisted tubular structures in which
three or four solid cores may be used. In such cases the cores are
less important than in the plain septum case since a multiple
septum is self-locating and can act to a greater extent as internal
support during subsequent processing, but cores may still be
desirable in many cases when cylindrical tubes C are forced
together into a hexagonal pattern as in FIGS. 2 and 4 (this may
occur when the septa are made thinner than the tube walls as
aforesaid).
The initial tubular structure thus formed is then subjected to the
step of drawing down to a single fibre and twisting of said fibre
during the drawing process. This step can be carried out by a
drawing machine as shown schematically in FIG. 9, the machine
being, if desired, conventional except for the provision of a motor
Mr to rotate the initial stock while it is being drawn in the oven
Mo and taken up as fibre by take-up rollers Mt (Mf represents the
feed mechanism for the stock).
The process includes the subsequent steps of forming a boule B
(FIG. 10) from twisted fibres all having the same pitch and slicing
said boule along parallel cutting planes. A regular pattern such as
those of FIGS. 2 and 4 can be obtained at each face of the matrix
if in addition the boule is formed in a regular manner in the sense
that all fibres have the same orientation in any one of a series of
parallel transverse planes S1, S2, S3 etc. spaced apart by equal
distances .lambda.. Such planes are then used as the cutting planes
so that the thickness of each resulting slice corresponds
substantially to one pitch (.lambda.) as shown or a multiple of the
pitch.
Of course, in addition to the above processes, the slice or matrix
may have to have its internal channel surfaces rendered
secondary-emissive and slightly conductive and it has to be made
into a channel plate by the addition of a first conductive layer on
its input face and a separate second conductive layer on its output
face to act respectively as input and output electrodes.
FIGS. 11 and 12 illustrate the use of channel plates in accordance
with the invention in imaging tubes. In the examples given, a
channel plate I having electrodes E1-E2 is shown inside the
envelope of an image intensifier tube containing also a
photo-cathode P and a luminescent screen S. FIG. 11 shows a tube of
the "proximity" type while FIG. 12 shows a tube of the
"electron-optical diode" or "inverter" type having a conical anode
A.
When a display screen S is used, the plate I can be made opaque so
as to prevent optical feedback from S as well as ion feedback.
The invention may also be used for other imaging tubes, for example
cathode-ray display tubes and camera tubes.
As will be appreciated, all the constructions and methods described
with reference to the drawings can be carried out without the
restriction to an angle of twist t of 360.degree. (or a multiple of
360.degree.) in which case ion-blindness can still be achieved but
the individual sub-channels will no longer represent separate
picture elements in their correct positions.
Alternative twisted structures are described in co-pending U.S.
patent application Ser. No. 515,322.
As was stated in the preamble, the required current flows through
resistive surfaces formed inside the channels ("surface conduction"
type of channel plate) or through the bulk material of the matrix
("bulk conduction" type). Suitable glasses exist for both types.
The usual way of obtaining resistive surfaces inside the channels
of an insulating matrix is to use a lead-glass and, as one of the
last steps in the manufacturing process, to reduce some of the lead
oxide to lead at the channel surfaces. As for performance, it can
be said generally that the geometries given in this Specification
are appropriate for surface-conduction plates; if bulk conduction
is used with the same geometries, the performance will be at least
equal.
The tube of FIG. 12 may replace one of the type described in U.S.
Pat. No. 3,487,258 in which the channels are straight and have
angles of tilt with respect to the electron paths such that it is
possible to substantially avoid a "black spot" on the screen due to
electrons passing straight through the channels without
multiplication. Thus a channel plate having curved channels
according to the present invention can be arranged to suppress the
"black spot" effect while also counteracting ion feedback.
* * * * *