An ionization chamber for use in determining the spatial distribution of x-ray photons in tomography systems comprises a plurality of substantially parallel, planar anodes separated by parallel, planar cathodes and enclosed in a gas of high atomic weight at a pressure from approximately 10 atmospheres to approximately 50 atmospheres. The cathode and anode structures comprise metals which are substantially opaque to x-ray radiation and thereby tend to reduce the resolution limiting effects of x-ray fluoresence in the gas. In another embodiment of the invention the anodes comprise parallel conductive bars disposed between two planar cathodes. Guard rings eliminate surface leakage currents between adjacent electrodes.

BACKGROUND OF THE INVENTION 
This invention relates to ionization chamber, x-ray detectors. More 
specifically, this invention relates to multicellular detectors comprising 
high pressure gas for use in computerized tomography systems. 
In a computerized, x-ray tomograph a spatial distribution of x-ray 
intensities must be translated into electrical signals which are processed 
to yield image information. Detectors for use in such systems must 
efficiently detect x-ray, electromagnetic energy with a high degree of 
spatial resolution. The x-ray pulse repetition rate in tomograph systems 
is generally limited by the recovery time of the x-ray detectors. It is 
desirable, therefore, to utilize x-ray detectors characterized by fast 
recovery times, high sensitivity, and fine spatial resolution. Proposed 
x-ray tomography systems employ hundreds of such x-ray detectors. A 
multicellular construction, wherein multiple, spatially separated 
detection cells are incorporated in a single detector assembly, provides 
an economic means for the production of such systems. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, x-ray electromagnetic radiation 
is detected in a high pressure gas of high atomic weight. X-ray photons 
interact with the gas to produce photoelectron-ion pairs in the presence 
of an electric field. The electrons thus produced are collected on an 
array of positively charged electrodes to produce electric currents in 
proportion to the x-ray intensity in the vicinity of the electrodes. In 
one embodiment of the invention the positive electrodes comprise a linear 
array of parallel metal rods disposed midway between a pair of flat, 
parallel, negative electrodes. 
The electrons and positive ions which are produced by the interaction of 
the x-ray photons and the gas drift along the electric field lines and are 
collected respectively on the positive and negative electrodes. 
Substantially all of the electrons and ions produced by the interaction of 
an x-ray pulse with the gas must be collected and removed from the 
detector before a subsequent x-ray pulse may be unambiguously detected. 
High pulse repetition rates are required for efficient computerized 
tomography so that detectors with short ion-electron collection times are 
desirable for use in such equipment. One embodiment of the present 
invention comprises a high pressure ionization chamber having a plurality 
of closed spaced parallel plate electrodes which lie substantially 
parallel to an incident x-ray beam. This electrode configuration allows 
prompt removal of the electron-ion pairs and permits the use of high x-ray 
pulse repetition rates at relatively low electrode potentials. 
Heavy gas atoms, which are used in the ionization x-ray detectors of the 
present invention, tend to fluoresce; radiating photons at low energy, 
x-ray frequencies. These low energy, x-ray photons have a relatively long 
range in the detector gas and tend to degrade detector spatial resolution. 
The parallel plate electrodes of the present invention may be constructed 
of high atomic weight material which acts to absorb those low energy, 
secondary photons at the detector cell boundaries and, thus, improve the 
spatial resolution of the detector. 
Highly efficient x-ray detectors are required to make maximum use of the 
information available from each x-ray exposure and to thereby minimize the 
total radiation exposure. Tomography detectors must, therefore, detect at 
least 50 percent of incident x-ray photons. Safe and efficient system 
operation typically requires detectors capable of detecting more than 70 
percent of the incident x-ray beam which typically has an energy in the 
range from 30 to 100 KEV. 
It is, therefore, an object of this invention to provide a multicellular, 
high pressure x-ray detector having high efficiency. 
Another object of this invention is to provide a high pressure, ionization 
chamber x-ray detector having a short recovery time as compared to prior 
art detectors. 
Another object of this invention is to provide a high pressure, ionization 
chamber x-ray detector having improved spatial resolution as compared to 
prior art detectors. 
Another object of this invention is to provide a multicellular detector 
which is insensitive to the resolution limiting effects of gas 
fluorescence. 
Yet another object of this invention is to provide multicellular x-ray 
detectors which are suitable for use in high speed, computerized, x-ray 
tomography systems.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
X-ray photons will interact with atoms of a heavy detector gas to produce 
electron-ion pairs. The x-ray photons are, generally, absorbed by a gas 
atom which emits a photoelectron from one of its electronic levels. The 
photoelectrons move through the gas interacting with and ionizing other 
gas atoms to produce a shower of electrons and positive ions which may be 
collected on suitable electrodes to produce an electric current flow. If, 
for example, xenon gas at approximately 10 atmospheres pressure is 
irradiated with 60 KEV x-ray photons, photoelectrons will be ejected from 
the 34.5 KEV xenon k shell at approximately 25.5 KEV. The 25.5 KEV 
photoelectrons, having a range of approximately 0.1 mm in the xenon, will 
produce approximately 800 electron-ion pairs each. If these electron-ion 
pairs are produced in a region between two electrodes of opposite 
polarity, they will drift along electric field lines to the electrodes and 
yield a net electric current flow between them. The electric current flow 
between the electrodes is thus a function of the total number of x-ray 
photons interacting in the vicinity of those electrodes. 
The probability of detection of an x-ray photon is a function of the atomic 
number of the gas and of the number of gas atoms lying between the 
collector electrodes. Thus, high sensitivity detectors may be constructed 
from a gas of high atomic weight at a relatively high pressure. Detector 
sensitivity may also be increased by increasing the spacing, and therefore 
the number of gas molecules, between the electrodes. Increased electrode 
spacing, however, increases the distance the electron-ion pairs must drift 
for collection and thus tends to increase the recovery time of the 
detector. An increased electric field gradient between the electrodes will 
tend to increase the electron-ion drift velocity and thus somewhat shorten 
the detector recovery time; the drift velocity, however, increases in 
relatively small proportion with electrode voltage increases. Furthermore, 
it is well known that an excessive electric field gradient will cause 
avalanche gas breakdown and will create highly nonlinear responses in 
detection sensitivity. 
The detectors of the present invention operate with electric field 
gradients which are insufficient to cause electron multiplication: that 
is, they may be characterized as ionization chambers and not as 
proportional counters. The production of electron-ion pairs described 
above is attributable solely to energy transfer from the ejected k-shell 
photoelectrons and is not caused by collisions of electrons or ions moving 
under the influence of the impressed electric field. The values of 
electric field gradients which are suitable for use in ionization chamber 
detectors are well known in the art and are more fully described in 
Medical Radiation Physics, W. R. Hendee, Year Book Medical Publishers, 
Chicago, at chapters 4 and 17. The detectors of the present invention 
operate with electric field gradients between approximately 10 and 
approximately 1000 v/mm. 
An l-shell electron will generally drop to fill the opening which is 
produced by the emission of the k shell photoelectron from a heavy gas 
atom. The energy difference resulting from the drop of the electron from 
the l to the k shell levels is radiated in the form of a secondary x-ray 
photon. In xenon gas, for example, the l to k energy level shift produces 
29 KEV x-ray photons. The range of these secondary photons in the high 
pressure gas is generally much larger than the range of the 
photoelectrons. By way of example, in xenon at 10 atmospheres pressure 
25.5 KEV photoelectrons have a range of approximately 1 mm while 29 KEV 
x-ray photons have a range of approximately 20 mm. 
The secondary photons which are produced by the fluorescence of the heavy 
gas atoms upon excitation by incident x-ray photons will be absorbed by 
other heavy gas molecules in the detector and are indistinguishable from 
the incident x-ray photons. Thus, photons which are produced by 
fluorescence in the region of one electrode cell may travel through a 
multicell detector to the region of another electrode cell where they will 
be detected in the same manner as incident x-rays. The k-shell 
fluorescence effect may, therefore, be seen to contribute to the 
degradation of spatial resolution in multicell, ionization chamber 
detectors. 
FIG. 1 illustrates an embodiment of a multicell, x-ray detector the present 
invention. A pressure vessel 10 contains a detector gas 12 at high 
pressure. One side of the pressure vessel 10 defines a thin window 14 
which is substantially transparent to electromagnetic radiation at x-ray 
frequencies. The window 14 may be constructed from any of the materials 
which are well known and commonly used for that purpose in the radiation 
detection arts; for example, aluminum, plastic resin, or a matrix of 
plastic resin reinforced by low atomic number metals. The term 
"substantially transparent", as used herein, means that the probability of 
x-ray radiation interacting with the window material is much less than the 
probability of that x-ray radiation interacting with the detector gas 12. 
The detector gas 12 fills the pressure vessel 10 and is chosen to be 
substantially opaque to electromagnetic radiation at x-ray frequencies. As 
used herein, the term "substantially opaque" means that the probability of 
x-ray radiation interacting with the detector gas 12 is much greater than 
the probability of that electromagnetic radiation interacting with the 
window 14. The gas type, gas pressure, and electrode spacing are chosen 
using methods well known to the art so that a large fraction (typically 
more than 70 percent) of the incident x-ray photons are absorbed within 
the gas. The detector gas 12 may, typically, comprise a rare gas of high 
atomic number, for example, xenon, krypton, argon, or a molecular gas 
comprising atoms having an atomic weight greater than that of argon (i.e., 
39.9); at a pressure from approximately 10 atmospheres to approximately 50 
atmospheres. 
A first cathode 16 is positioned within the pressure vessel 10 
substantially parallel to the window 14. The first cathode 16 is 
constructed from conductive materials which are substantially transparent 
to electromagnetic radiation at x-ray frequencies; for example, aluminum 
or other low atomic number metals. A second cathode 18 is similarly 
positioned within the pressure vessel, parallel to and spaced apart from 
the first cathode 16. The second cathode may be constructed from any 
conductive material and need not be transparent to x-ray electromagnetic 
radiation. By way of illustration only, in a typical detector the second 
cathode 18 is spaced approximately 2.5 cm from the first cathode 16. 
A plurality of anodes 20, which comprise a row of parallel, conductive 
bars, are positioned midway between and parallel to the first cathode 16 
and the second cathode 18. Each of the plurality of anodes 20 is 
associated with a connecting lead 22 which passes through the pressure 
vessel 10 on an insulating feedthrough 24. The connecting leads 22 serve 
to transmit electric current signals from the anodes 20 to a signal 
processing circuit 26 which may be positioned external to the pressure 
vessel 10. The first cathode 16 and the second cathode 18 are electrically 
connected in parallel by a cathode lead 30 which passes through the 
pressure vessel 10 on an insulating feedthrough 24a. A source of direct 
current electric potential 28 is connected in series between the cathode 
lead 30 and the anodes 20 to produce an electric field between the anodes 
20 and the cathodes 16 and 18. In typical detectors of the present 
invention the electric field gradient is between approximately 100 and 
approximately 300 v/mm. 
Incident x-rays 32 enter the detector through window 14 in a direction 
substantially perpendicular to the plane of the first and second cathodes 
16 and 18. The x-rays interact with the atoms of gas 12 to produce 
electron-ion pairs which drift along the electric field to produce current 
flow between the anodes 20 and the cathodes 16 and 18. The current flow 
from a particular anode 20 is associated with and proportional to the 
number of x-ray-gas interactions occurring in the vicinity of that 
electrode. The signals from the anodes may be combined in the signal 
processor 26, using techniques well known to the tomography art, to yield 
an image from the x-ray intensity along the line of anodes. 
This embodiment of detector yields high sensitivity and relatively fast 
response time. Electron-ion pairs produced between the anodes and cathodes 
must traverse only half the distance separating the first cathode from the 
second cathode. The volume of gas 12 available for the detection of x-rays 
in the vicinity of a particular anode 20 is equal to the sum of the volume 
of gas lying between that anode and the first cathode 16 and the volume of 
gas lying between that anode and the second cathode 18. The volume of gas 
available for detection is thus twice that available in a simple planar 
detector. 
Another embodiment of an anode structure 20 suitable for use in the 
detector of FIG. 1 is illustrated in FIG. 2. In this embodiment the anodes 
comprise a row of parallel metal strips 34 disposed on the surface of a 
sheet of dielectric material 36. The dielectric sheet 36 may be 
constructed, for example, from ceramic, mica, plastic resin, or any other 
material of the type commonly used for this purpose in the electrical 
arts. The metal strips 34 may be attached to the dielectric sheet 36 in 
any conventional manner, for example, by vapor deposition, by screen 
printing, or by adhesive bonding. Lead wires 22 are bonded to the 
individual metal strips 34 and pass through the pressure vessel 10 in the 
manner described above. 
FIGS. 3 and 3a illustrate another embodiment of the detector of the present 
invention. A pressure vessel 10 having an x-ray transparent window 14 is 
filled with a detector gas 12 in the manner and of the type described 
above. A plurality of flat anodes 42 are aligned within the pressure 
vessel 10 in a direction substantially perpendicular to the window 14. The 
anodes 42 are individually connected to a plurality of leads 22 which pass 
through the pressure vessel on dielectric feedthroughs 24. A metal plate 
cathode 38 is positioned equidistant between each of the anodes 42. The 
cathodes 38 are connected in parallel by a lead 30 which passes through 
the pressure vessel 10 on a dielectric feed-through 40. 
The anode plates 42 and the cathode plates 38 are constructed from metals 
which are substantially opaque to electromagnetic radiation at x-ray 
frequencies. Metals of high atomic number, for example, molybdenum, 
tantalum, or tungsten, are suitable for use as the anodes 42 and the 
cathodes 38. By way of illustration only, in a typical detector the anode 
and cathode plates are constructed from 0.05 mm molybdenum or tungsten 
sheets. The cathode lead 30 and the anode leads 22 are electrically 
connected to a signal processor 26 and a potential source 28 in the manner 
described above. 
Photons of x-ray radiation 32 enter the detector through the window 14 in 
directions substantially parallel to the anode plates 42 and the cathode 
plates 38. The photons interact with the fill gas 12 in the regions 
between the anode plates 42 and the cathode plates 38. Electron-ion pairs 
which are produced by interaction of the gas 12 with the photons 32 drift 
along electric field lines between the anodes and cathodes and are 
collected thereon to produce electric current signals. The electric 
current flowing from a particular anode 42 is proportional to the number 
of x-ray photons interacting with the gas 12 in the space between that 
anode and the adjacent pair of cathodes 38. 
This embodiment of the detector is insensitive to the resolution limiting 
effects of k-band x-ray fluorescence. Any x-ray photons which are produced 
by fluorescence in the region between an anode plate 42 and a cathode 
plate 38 must pass through a cathode plate 38 before they would be capable 
of producing electron-ion pairs which would drift to an adjacent anode. As 
indicated above, the cathode plates 38 are constructed from material which 
is substantially opaque to x-ray photons and the incidence of fluorescent 
x-ray photons with sufficient range to produce current in adjacent anode 
cells is thereby greatly reduced. The anode 42 and cathode 38 structures 
of the present embodiment lie parallel to the direction of photon 
incidence. The plates of the anodes 42 and the cathodes 38 may, therefore, 
be spaced relatively close together yielding a detector with a short 
recovery time, while the length of the plates may be increased to produce 
a detector of high sensitivity. By way of illustration only, in a typical 
detector, the anode and cathode plates are mounted on 2 mm centers. The 
parallel electrode plates of this detector embodiment also serve to absorb 
incident photons which are scattered from external objects (i.e., tissue 
under examination) and which enter the detector at an oblique angle. 
FIG. 4 illustrates an alternate embodiment of the anode plates 42 which may 
be utilized in the detector of FIG. 3. In this embodiment, each anode 
plate comprises a thin dielectric sheet 46; which may, by way of 
illustration, be constructed from ceramic, mica, or Mylar.TM. plastic 
resin sheet. A pair of electrodes 44, constructed from metal which is 
substantially opaque to electromagnetic radiation at x-ray frequencies, 
are disposed on opposite sides of the dielectric sheet 46. Separate leads 
22 are connected to each metal electrode 44 and pass through the pressure 
vessel 10 on separate dielectric feed-throughs 23. Electron currents 
flowing to opposite sides of the anode plate 42 are thus collected on the 
separate metal sheets 44 and transmitted separately to the signal 
processor 26 (of FIG. 3). The spatial resolution of the detector is 
thereby increased by a factor of two. 
A method of construction of an assembly of anode and cathode plates is 
illustrated in FIG. 5. The anode plates 42 and the cathode plates 38 are 
alternately stacked on a plurality of insulating bolts 48. A series of 
tubular insulators 50 are threaded on the bolts 48 between the anode 
plates 42 and the cathode plates 38 and serve to position the plates. The 
plates may be mounted in parallel alignment for detection of a collimated 
x-ray beam or the thickness of the insulators 50 may be varied to produce 
a curved plate alignment suitable for detection of a diverging x-ray beam. 
The electron-ion current flowing within these ionization chambers is 
typically very small and may be of the same order of magnitude as leakage 
currents which flow on the structures. These leakage currents which may 
induce noise in or interfere with the operation of detector amplifier 
electronics may be drained from the detector circuit on guard rings which 
are spaced on the electrode support structures between adjacent electrodes 
and are operated at anode potential. 
FIG. 6 is an alternate embodiment of electrode structures for use in the 
detector of FIG. 3. Guard ring elements 52 are disposed on support rods 50 
between the cathode plates 38 and the anode plates 42 to drain surface 
leakage currents which might otherwise flow between them. Guard rings are 
connected to the positive terminal of the potential source 28 in parallel 
with the signal processor 26. 
FIG. 7 is an alternate embodiment of the anode of FIG. 2 which incorporates 
guard ring elements 54 which are disposed on the dielectric sheet 36 
between adjacent anode strips 34. The guard rings are connected and 
function to drain surface leakage currents in the manner described above. 
It may, therefore, be seen that the present invention provides x-ray 
detector structures which produce electrical signals in response to a 
linear space distribution of x-ray intensities. The structures allow the 
construction of detectors having high sensitivity, short recovery time, 
and fine spatial resolution and which are relatively insensitive to the 
adverse effects of k shell, x-ray fluorescence. 
The electrodes in the descriptions of the preferred embodiments of the 
present invention have, for ease of description, been referred to as 
"cathodes" and "anodes". It is to be understood, however, that the 
polarity of the electric potentials applied to these detectors may be 
reversed without affecting the principles of operation of the disclosed 
invention and that the "anode" structures may be operated at an applied 
potential which is negative with respect to the "cathode" potential. The 
terms "cathode" and "anode" as used herein and in the appended claims mean 
electrodes of opposite polarity. 
While the invention has been described in detail herein in accord with 
certain preferred embodiments, thereof, many modifications and changes 
therein may be effected by those skilled in the art. Accordingly, it is 
intended by the appended claims, to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.