Ionization detector, electrode configuration and single polarity charge detection method

An ionization detector, an electrode configuration and a single polarity charge detection method each utilize a boundary electrode which symmetrically surrounds first and second central interlaced and symmetrical electrodes. All of the electrodes are held at a voltage potential of a first polarity type. The first central electrode is held at a higher potential than the second central or boundary electrodes. By forming the first and second central electrodes in a substantially interlaced and symmetrical pattern and forming the boundary electrode symmetrically about the first and second central electrodes, signals generated by charge carriers are substantially of equal strength with respect to both of the central electrodes. The only significant difference in measured signal strength occurs when the charge carriers move to within close proximity of the first central electrode and are received at the first central electrode. The measured signals are then subtracted and compared to quantitatively measure the magnitude of the charge.

TECHNICAL FIELD 
The present invention relates to the field of ionization detection and, 
more specifically, to single polarity charge carrier sensing in ionization 
detectors. 
BACKGROUND ART 
Ionization detectors such as radiation detectors which use simple planar 
electrodes and which are based on ionization measurements often suffer 
from poor collection of charge carriers of certain polarity types. Such 
detectors include, for example, semiconductor detectors, liquid ionization 
detectors, and gas ionization detectors. The poor collection 
characteristics of these detectors can be due to such factors as intrinsic 
material properties, defects in the detector medium, or radiation damage. 
Semiconductors having high atomic numbers and wide band-gaps, such as 
HgI.sub.2, CdTe and CdZnTe, have long been under development as potential 
room-temperature .gamma.-detectors. These detectors have high detection 
efficiency, freedom from cryogenic cooling and potentially achievable 
excellent energy resolution decided by the small ionization energy needed 
to generate one electron-hole pair. Although these semiconductors have 
been successfully employed in various applications, the widespread use of 
these devices has been hindered by their charge trapping problem which 
causes incomplete charge collection, and therefore, very poor energy 
resolution. 
Referring now to FIGS. 1 and 2, U.S. Pat. No. 5,530,249 to Luke discloses a 
semiconductor ionization detector 10 including coplanar electrodes which 
use parallel strip electrodes 12 and 14 connected in an alternate manner 
to give two sets of inter-digital grid electrodes. When charges- move 
within the bulk of the detector, they induce the same amount of charge on 
both electrodes 12 and 14. One of the electrodes 12 and 14 is a collecting 
electrode which is biased at a higher voltage compared to that of the 
non-collecting electrode. 
In this way, the charge that drifts toward the coplanar electrodes will be 
collected by only the collecting electrode. An electrical terminal 18 
provides a common electrical terminal for the electrodes 14 and an 
electrical terminal 20 provides a common electrical terminal for the 
electrodes 12. By reading out a difference signal between these two 
terminals 18 and 20, the same amount of charges induced by holes moving 
toward a cathode 16 can be cancelled out. 
Therefore, the difference signal from the terminals 18 and 20 is mainly 
proportional to the number of electrons arriving on the collecting 
electrode. In other words, by reading the difference signal between these 
two sets of electrodes, pulses induced by one type (electrons or holes) of 
charge carriers can be obtained. This method is also called 
single-polarity charge sensing. For commonly used semiconductor devices, 
electron trapping is much less severe than that of holes. Good energy 
resolution can be obtained if signals induced by electrons only can be 
picked out so that the hole trapping problem can be eliminated resulting 
in significant improvement in energy resolution. 
In order to connect alternative strip electrodes, Luke uses a wire bounding 
technique which makes the structure very complex and fragile, and 
therefore, very difficult to commercialize. Furthermore, the coplanar 
electrode is an intrinsic non-symmetric configuration for limited detector 
area. This causes unequal amount of charges induced on coplanar electrodes 
when charges move within the bulk of the detector. This effect degrades 
the energy resolution, especially when the .gamma.-ray interaction is 
close to the coplanar electrodes. 
Referring to FIG. 5, there is illustrated another prior art ionization 
detector which, however, needs high precision mounting and is difficult to 
keep reliable contact (due to different thermal expansion of coefficients 
of detector and substrate materials and other factors). 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved ionization 
detector and electrode configuration and signal polarity charge detection 
method including first and second central electrodes and a boundary 
electrode to minimize non-symmetric effects of the first and second 
central electrodes to thereby improve energy resolution (i.e. measurement 
accuracy). 
Another object of the present invention is to provide an improved 
ionization detector and electrode configuration including first and second 
central electrodes and a boundary electrode in a single structure which is 
not only more rugged and reliable, but is also relatively inexpensive to 
make using conventional metalization processes. 
Still another object of the present invention is to provide an improved 
ionization detector and electrode configuration including first and second 
central electrodes and a boundary electrode wherein each electrode is 
self-connecting so that only three contacts are need to connect the 
electrodes in a measuring circuit. 
Yet still another object of the present invention is to provide an improved 
ionization detector and electrode configuration and signal polarity 
detection method including first and second central electrodes wherein 
electronic noise between the first and second central electrodes is 
significantly reduced by using a boundary electrode. 
In carrying out the above objects and other objects of the present 
invention, an ionization detector electrode configuration is provided. The 
electrode configuration includes a first central electrode disposed at a 
first surface of an ionization substrate. The first central electrode is 
adapted to receive a first voltage potential of a first polarity type. The 
first surface is disposed opposing a second surface of the ionization 
substrate. The second surface has disposed thereat an opposing electrode 
adapted to receive a voltage potential of a second polarity type. The 
electrode configuration also includes a second central electrode disposed 
at the first surface of the ionization substrate. The second central 
electrode is adapted to receive a second voltage potential of the first 
polarity type. The first and second central electrodes are positioned at 
the first surface of the ionization substrate in a substantially 
interlaced and symmetrical pattern. Finally, the electrode configuration 
includes a boundary electrode disposed at the first surface of the 
ionization substrate surrounding the first and second central electrodes. 
The boundary electrode is adapted to receive a third voltage potential of 
the first polarity type such that signals generated by at least one charge 
carrier of the second polarity type moving within the ionization substrate 
between the first and second surfaces toward the first and second central 
electrodes and away from the opposing electrode, are of substantially 
equal strength with respect to both the first and second central 
electrodes until the at least one charge carrier moves within close 
proximity of the first and second central electrodes. 
Preferably, each of the central electrodes includes parallel conductive 
strips and a conductive connector strip for commonly connecting its 
parallel conductive strips. 
Also, preferably, the conductive connector strip of each of the central 
electrodes surrounds the parallel conductive strips of the other central 
electrode. 
Still further in carrying out the above objects and other objects of the 
present invention, a method is provided for single polarity charge 
detection. The method includes the steps of positioning a first central 
electrode at a first surface of an ionization substrate wherein the first 
surface is disposed opposing a second surface of the ionization substrate, 
positioning a second central electrode at the first surface of the 
ionization substrate in a substantially interlaced and symmetrical pattern 
with the first central electrode, positioning an opposing electrode at the 
second surface of the ionization substrate, and positioning a boundary 
electrode at the first surface of the ionization substrate surrounding the 
first and second central electrodes. Signals generated by at least one 
charge carrier of a first polarity type moving within the ionization 
substrate between the first and second surfaces toward the first and 
second central electrodes and away from the opposing electrode are 
substantially of equal strength with respect to both of the first and 
second central electrodes until the at least one charge carrier of the 
first polarity type moves to within close proximity of the first and 
second central electrodes. The method further includes the steps of 
applying a first voltage potential of a second polarity type to the first 
central electrode, applying a second voltage potential of the second 
polarity type to the second central electrode, applying a third voltage 
potential of the second polarity type to the boundary electrode, and 
applying a voltage potential of the first polarity type to the opposing 
electrode. Finally, the method includes the step of measuring at both of 
the first and second central electrodes the signals generated by the at 
least one charge carrier moving within the ionization substrate between 
the first and second surfaces. 
Preferably, the steps of positioning the first and second central 
electrodes at the first surface of the ionization detector in a 
substantially interlaced and symmetrical pattern further include the steps 
of positioning a plurality of parallel conductive strips and a conductive 
connector strip for commonly connecting the plurality of parallel 
conductive strips and arranging the parallel conductive strips of the 
first central electrode parallel to the parallel conductive strips of the 
second central electrode and interlacing the parallel conductive strips of 
the first central electrode with the parallel conductive strips of the 
second central electrode such that adjacent conductive parallel strips of 
the first central electrode have a conductive parallel strip of the second 
central electrode disposed therebetween. 
Yet still further in carrying out the above objects and other objects of 
the present invention, an ionization detector is provided. The ionization 
detector includes an ionization substrate having a first surface and a 
second surface opposing the first surface. The detector also includes a 
first central electrode disposed at the first surface, an opposing 
electrode disposed at the second surface, and a second central electrode 
disposed at the first surface in a substantially interlaced and 
symmetrical pattern with the first central electrode. The ionization 
detector also includes a boundary electrode disposed at the first surface 
of the ionization substrate surrounding the first and second central 
electrodes such that signals generated by at least one charge carrier of a 
first polarity type moving within the ionization substrate between the 
first and second surfaces toward the first and second central electrodes 
and away from the opposing electrode are substantially of equal strength 
with respect to both of the first and second central electrodes until the 
at least one charge carrier of the first polarity type moves to within 
close proximity of the first and second central electrodes. The ionization 
detector further includes a first voltage potential circuit for applying a 
first voltage potential of a second polarity type to the first central 
electrode, a second voltage potential circuit for applying a second 
voltage potential of the second polarity type to the second central 
electrode, and a third voltage potential circuit for applying a third 
voltage potential of the second polarity type to the boundary electrode. 
The ionization detector still further includes a fourth voltage potential 
circuit for applying a voltage potential of the first polarity type to the 
opposing electrode and a signal measurement circuit for measuring at both 
of the first and second central electrodes signals generated by the at 
least one charge carrier moving within the ionization substrate between 
the first and second surfaces. 
Preferably, each of the central electrodes includes parallel conductive 
strips and a conductive connector strip for commonly connecting its 
parallel conductive strips. 
The above objects and other objects, features, and advantages of the 
present invention are readily apparent from the following detailed 
description of the best mode for carrying out the invention when taken in 
connection with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
With reference now to FIGS. 6 through 8, a perspective view of a 
semiconductor ionization detector having an electrode configuration 
constructed in accordance with the present invention is generally 
indicated at 30. In the present embodiment, the detector 30 includes a 
cube-shaped, cadmium zinc telluride, CdZnTe, semiconductor ionization 
substrate 31 having a cathode (i.e. electrode 32 in FIG. 8 connected to 
terminal c in FIG. 6) formed on one side surface of the substrate 31. Two 
independent central electrodes, generally indicated at 34 and 36, are 
formed in a substantially symmetric and interlaced arrangement on another 
side surface of the substrate 31 opposing the cathode. The first central 
electrode 36 is a collecting electrode and is connected to a terminal e1, 
located on a base 37. The second central electrode 34 is a non-collect 
electrode and is connected to a terminal e2 located on the base 37. 
The second central electrode 34 includes a plurality of parallel conductive 
strips 38 and a conductive connector strip 40 for commonly connecting the 
parallel conductive strips 38. 
The first central electrode 36 also includes a plurality of parallel 
conductive strips 42 and a conductive connector strip 44. A typical width 
for the strips 38, 40, 42 and 44 is approximately 100 .mu.m. A fewer 
number of conductive strips 38 and 42 are illustrated in FIG. 6 for the 
sake of simplicity. 
A boundary electrode 46 (connected to terminal g in FIG. 6) completely 
surrounds the first and second central electrodes 36 and 34 and together 
with the independent electrodes 34 and 36 function as an anode. Although 
the electrodes 34, 36 and 46 function as an anode in the present 
embodiment, the present invention is also well suited to having the 
electrodes 34, 36 and 46 alternatively function as a cathode and the 
opposing electrode 32 function as an anode. Also, the boundary electrode 
46 need not be continuous but could be segmented. 
With reference still to FIG. 6, in the present embodiment, the electrodes 
34, 36 and 46 may be formed by any well known semiconductor metalization 
process such as photolithography and sputtering on a surface of the 
substrate 31. Thus, an ionization detector utilizing the present invention 
can be fabricated using existing technologies and without dramatically 
increasing detector fabrication costs. Furthermore, although the electrode 
configuration of the present invention is used in a cadmium zinc telluride 
semiconductor substrate, the present invention is also well suited for use 
with, for example, other compound semiconductor substrates (such as 
HgI.sub.2, CdTe, Ge, etc.), semiconductor detectors or substrates, solid 
state detectors or substrates, and even liquid or gaseous ionization 
detectors. In liquid or gaseous ionization detectors, the interlaced and 
symmetric central electrodes (i.e. electrodes 34 and 36) and the boundary 
electrode (i.e. electrode 46) would be positioned at one of the opposing 
faces of a liquid or gaseous detector. 
The present invention uses the novel electrode structure previously 
described and a signal subtraction technique of FIG. 8 (via the circuit 
elements shown thereon including a subtraction circuit 50) to obtain 
signals whose strength and signal amplitude variation is not position 
dependent. That is, the magnitudes of the measured signals do not vary 
significantly regardless of where in a plane parallel to the planes 
containing the anodes and cathode the collected charge carrier is 
generated within the substrate 31. Furthermore, even when only charge 
carriers of one polarity type are collected, the position of charge 
generation within such parallel plane in the substrate 31 does not affect 
signal strength. 
With particular reference now to FIG. 7, an end view of the electrode 
configuration of the embodiment of FIG. 6 is shown. In the present 
embodiment, the conductive strips 38 and 42 extend across most of a 
surface of substrate 31. The gap between the conductive connector strips 
40 and 44 and the boundary conductive electrode 46 is preferably as small 
as possible yet maximize interstrip resistance (i.e., e1-e2, e1-g, e2-g) 
to be .about.10.sup.9 ohms or higher. Although a pattern of two, 
alternating sets of parallel conductive strips of the electrodes 34 and 36 
is used in the present embodiment, the present invention is well suited to 
numerous other symmetrical and interlaced conductive strip configurations. 
With particular reference again to FIGS. 6 and 8, a description of the 
operation of the present invention is now given. For electron detection, 
voltage potentials V(e1) and V(e2) of positive polarity are applied to 
respective terminals e1 and e2 for electrodes 36 and 34, respectively. A 
voltage potential V(g) of positive polarity is also applied to terminal g 
for the boundary electrode 46. Finally, a voltage potential V(c) of 
negative polarity is applied to a terminal c for the opposing electrode 
32. For electron detection, the following equation governs the relative 
voltages: 
EQU V(c)&lt;&lt;V(g).ltoreq.V(e2)&lt;V(e1) 
As a result, a relatively uniform electric field is generated inside the 
semiconductor substrate 31 so that negative charge carriers drift toward 
the electrodes 34, 36 and 46 and away from the opposing electrode 32. When 
a negative charge carrier drifts from the opposing electrode 32 toward the 
electrodes 34, 36 and 46, an increasing charge signal is induced 
separately on the electrodes 34 and 36. For most of the distance traveled 
by the negative charge carrier, the signals induced at the electrodes 34 
and 36 are almost identical. The two signals will only deviate 
significantly from each other when the negative charge carrier drifts to 
within close proximity of the electrodes 34 and 36 when making its final 
approach to the collecting conducting strips 42. By making the separation 
of the electrodes 34 and 36 small compared to the thickness of substrate 
31, the difference between signals obtained at the electrodes 34 and 36 
will be extremely small for almost the entire volume of the substrate 31. 
With reference next to FIGS. 9 and 10, an energy spectrum graph obtained 
from exposing the detector 30 to 662 KeV gamma rays from a source of such 
rays is shown. By comparing the spectrums of FIGS. 3 and 4 (obtained by 
using the detector having similar electrode structure of that of FIGS. 1 
and 2) with their corresponding spectrums of FIGS. 9 and 10, it is clear 
that the present invention provides substantial improvement in energy 
resolution over the prior art of FIGS. 1 and 2. Specifically, the detector 
30 provides a clear full energy photo peak corresponding to the energy of 
the gamma rays. 
The electrode structure has a unique symmetric pattern for the central 
coplanar electrodes 34 and 36. By adding the third boundary electrode 46 
surrounding the coplanar electrodes 34 and 36, the difference of signals 
induced by charges moving within the bulk of the substrate 31 is 
minimized. This insures that the signal amplitude is only proportional to 
the number of charge carriers arriving at the collecting electrode (i.e. 
the first central electrode 36). 
Significant improvement in energy resolution has been demonstrated on a 1 
cm cube CdZnTe detector using the electrodes 34, 36 and 46. More 
importantly, since each of the electrodes 34, 36 and 46 is 
self-connecting, only three contacts are needed to connect the electrodes 
34, 36 and 46 with the electronic circuit of FIG. 8. This electrode 
structure is much more rugged and easy to manufacture using standard 
methods of semiconductor metalization processes, such as photolithography 
and sputtering. 
Several additional benefits are provided by the present invention. By 
enhancing the performance of ionization detectors, the performance of room 
temperature semiconductor detectors can be brought even closer to that of 
cryogenic Ge detectors. In so doing, the need for expensive cooling 
systems can be eliminated. 
As a result of the vastly improved energy resolution, the present invention 
can have a positive impact in many areas including, for example 
radioactive waste management, environmental monitoring material analysis, 
nuclear medicine, nuclear physics, and gamma-ray astronomy. The present 
invention can also be used to reduce the effect of radiation damage in 
semiconductor detectors such as for example, germanium detectors. 
Additionally, the present invention can also be used to determine the time 
of arrival of the charge carriers at the collecting electrode 34. 
Furthermore, the present invention can also be employed, for example, in 
time-of-flight spectrometers. 
Thus, the electrode configuration of the present invention can be readily 
formed onto the surface of semiconductor ionization substrates. The 
present invention also provides for single polarity charge carrier sensing 
ionization detectors including planar semiconductor detectors while 
achieving uniform electric field distributions within the ionization 
substrate or medium. As a result, the present invention achieves a large 
improvement in the energy resolution of semiconductor detectors, 
especially compound semiconductor detectors which can be operated at room 
temperature but currently have poor energy resolution because of the 
inefficient collection of the positive carriers. This invention also 
simplifies the fabrication of gas and liquid ionization detectors. 
Furthermore, the present invention significantly reduces position 
dependent signal amplitude variation problems within a surface parallel to 
the cathode and anode surfaces associated with the prior art. 
While the best mode for carrying out the invention has been described in 
detail, those familiar with the art to which this invention relates will 
recognize various alternative designs and embodiments for practicing the 
invention as defined by the following claims.