U.S. patent application number 12/763807 was filed with the patent office on 2010-10-21 for plasma panel based ionizing-particle radiation detector.
This patent application is currently assigned to INTEGRATED SENSORS, LLC. Invention is credited to Peter S. Friedman.
Application Number | 20100265078 12/763807 |
Document ID | / |
Family ID | 42980599 |
Filed Date | 2010-10-21 |
United States Patent
Application |
20100265078 |
Kind Code |
A1 |
Friedman; Peter S. |
October 21, 2010 |
PLASMA PANEL BASED IONIZING-PARTICLE RADIATION DETECTOR
Abstract
A plasma panel based ionizing-particle radiation detector
includes a first substrate and a second substrate coupled to the
first substrate by a hermetic seal. The second substrate is an
ultra-thin substrate. The detector further includes a discharge gas
between the first and second substrate and at least one second
electrode electrically coupled to a first electrode and defining at
least one pixel with the first electrode. The second electrode is
coupled to the first substrate and a first impedance is coupled to
the first electrode. The detector further includes a power supply
coupled to at least the first or second electrode and a first
discharge event detector circuitry is coupled to at least one of
the first or second electrodes for detecting a gas discharge
counting event in the electrode. The detector further includes a
plurality of pixels, each pixel capable of outputting a gas
discharge pulse upon interaction with ionizing-radiation. Each gas
discharge pulse is counted by the detector as having approximately
an equal value and circuitry detects if a gas discharge pulse is
output from the pixels, and counts each gas discharge pulse as an
individual event.
Inventors: |
Friedman; Peter S.; (Toledo,
OH) |
Correspondence
Address: |
SQUIRE, SANDERS & DEMPSEY L.L.P.
8000 TOWERS CRESCENT DRIVE, 14TH FLOOR
VIENNA
VA
22182-6212
US
|
Assignee: |
INTEGRATED SENSORS, LLC
Toledo
OH
|
Family ID: |
42980599 |
Appl. No.: |
12/763807 |
Filed: |
April 20, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61214149 |
Apr 20, 2009 |
|
|
|
Current U.S.
Class: |
340/600 |
Current CPC
Class: |
G01T 1/2935 20130101;
A61N 2005/1087 20130101; H01L 31/115 20130101; A61B 6/4258
20130101; A61N 5/1075 20130101; H01J 47/08 20130101; G01T 1/26
20130101 |
Class at
Publication: |
340/600 |
International
Class: |
G08B 17/12 20060101
G08B017/12 |
Claims
1. A plasma panel based ionizing-particle radiation detector
comprising: a first substrate; a second substrate coupled to the
first substrate by a first hermetic seal, wherein the second
substrate comprises an ultra-thin substrate; a discharge gas
between the first and second substrate; at least one second
electrode electrically coupled to a first electrode and defining at
least one pixel with the first electrode, wherein the second
electrode is coupled to the first substrate; a first impedance
coupled to the first electrode; a power supply coupled to at least
the first or second electrode; a first discharge event detector
circuitry coupled to at least one of the first or second electrodes
for detecting a gas discharge counting event in the electrode; a
plurality of pixels, each pixel capable of outputting a gas
discharge pulse upon interaction with ionizing-radiation, wherein
each gas discharge pulse is counted as having approximately an
equal value; and circuitry for detecting if a gas discharge pulse
is output from the pixels, and for counting each gas discharge
pulse as an individual event.
2. The detector of claim 1, further comprising a third electrode
coupled to the first substrate.
3. The detector of claim 1, wherein the ultra-thin substrate
comprises a metal foil.
4. The detector of claim 1, wherein the ultra-thin substrate
comprises at least one of ceramic, glass or a semiconductor.
5. The detector of claim 3, wherein the metal foil comprises at
least one of the metals or metal alloys of aluminum, titanium,
molybdenum, stainless steel, nickel, iron, copper, cobalt,
beryllium, magnesium, Arnavar.TM. .or Inconel.
6. The detector of claim 1, further comprising: a third substrate
coupled to the second substrate by a second hermetic seal.
7. The detector of claim 1, wherein the first substrate and second
substrate define a gas gap region that contains the discharge gas,
and wherein the first electrode extends vertically into the gas gap
region and the second electrode is substantially flat on the first
substrate.
8. The detector of claim 1, wherein the first impedance is a
quenching resistor.
9. The detector of claim 1, further comprising a bus-bar coupled to
the first electrode.
10. The detector of claim 1, wherein the first electrode is an
X-electrode and the second electrode is a Y-electrode.
11. The detector of claim 1, further comprising at least one first
driver coupled to the first electrode.
12. The detector of claim 7, wherein the first electrode comprises
a stack of electrodes alternatively separated by a stack of
insulators.
13. The detector of claim 1, further comprising a photo-cathode
layer coupled to the second substrate.
Description
RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application No. 61/214,149 filed Apr. 20, 2009, the specification
of which is herein incorporated by reference.
FIELD
[0002] One embodiment of the present invention is directed to the
detection and imaging of ionizing-particle radiation. More
particularly, one embodiment of the present invention is directed
to a plasma panel based apparatus for the detection and imaging of
ionizing-particle radiation.
BACKGROUND INFORMATION
[0003] Many useful applications, such as the detection of
radioactive material and computer-assisted tomography ("CAT"), rely
on the detection of photon radiation, known as X-ray and/or
gamma-ray radiation. Both of these types of high-energy photon
radiation cause ionization and for the purposes of this disclosure
the two terms, X-ray and gamma-ray, are used interchangeably. In
terms of the detection of such ionizing radiation, the spectral
region of greatest interest for most of these applications
generally falls between the energies of about 20 keV to 20 MeV.
Other applications, including the detection of particle radiation
from ion beam accelerators/colliders, cosmic ray generated minimum
ionizing particles (MIP's), and neutrons from special nuclear
materials (SNM) used in nuclear weapons (e.g., plutonium), rely on
the detection of ionizing particles that can be either atomic
(e.g., radioactive ion beams) or subatomic (e.g., neutrons, protons
and muons) in nature, and which can vary over a very broad energy
range from less than 1 MeV to well beyond 1 TeV.
[0004] In order to detect ionizing radiation in the above spectral
range of interest, a number of known sensing devices are commonly
used. One of the earliest known electronic devices is the
ionization chamber. Detection of radiation in an ionization
chamber, such as a Geiger-Mueller ("GM") tube, is based upon
electrical conductivity induced in an inert gas (usually containing
argon and neon) as a consequence of ion-pair formation. One
currently widely used type of ionizing-particle radiation detector
is the micropattern gas detector. These devices have been under
continuous development for many years in high energy and nuclear
physics. Detectors such as the Microstrip Gas Chamber ("MSGC"), Gas
Electron Multiplier ("GEM") and Micromegas have many desirable
properties as proportional gas detectors, but are operationally
limited to gains within the proportional region in the range of
.about.10.sup.3 to 10.sup.6.
SUMMARY
[0005] One embodiment is a plasma panel based ionizing-particle
radiation detector that includes a first substrate and a second
substrate coupled to the first substrate by a hermetic seal. The
second substrate is an ultra-thin substrate. The detector further
includes a discharge gas between the first and second substrate and
at least one second electrode electrically coupled to a first
electrode and defining at least one pixel with the first electrode.
The second electrode is coupled to the first substrate and a first
impedance is coupled to the first electrode. The detector further
includes a power supply coupled to at least the first or second
electrode and a first discharge event detector circuitry is coupled
to at least one of the first or second electrodes for detecting a
gas discharge counting event in the electrode. The detector further
includes a plurality of pixels, each pixel capable of outputting a
gas discharge pulse upon interaction with ionizing-radiation. Each
gas discharge pulse is counted by the detector as having
approximately an equal value and circuitry detects if a gas
discharge pulse is output from the pixels, and counts each gas
discharge pulse as an individual event.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a perspective view of a surface-discharge plasma
panel sensor ("PPS") and/or plasma panel photosensor ("PPPS")
detector with a parallel/rectilinear surface-discharge electrode
pattern incorporating individual cell quenching resistors and an
orthogonal back electrode pattern in accordance with one embodiment
of the present invention.
[0007] FIG. 2 is a perspective view of a columnar-discharge PPS
and/or PPPS detector with an orthogonal electrode pattern that
includes electrode line quenching resistors in accordance with one
embodiment of the present invention.
[0008] FIG. 3 is a cross-sectional view of a barrier wall vertical
electrode structure in accordance with one embodiment that can be
used with the disclosed detectors.
[0009] FIG. 4 is a cross-sectional view of an asymmetric elevated
vertical electrode structure in accordance with one embodiment that
can be used with the disclosed detectors.
[0010] FIG. 5 is a cross-sectional view of an asymmetric vertical
wall electrode structure in accordance with one embodiment that can
be used with the disclosed detectors.
[0011] FIG. 6 is a perspective view of an asymmetric
multi-potential vertical wall electrode structure in accordance
with one embodiment that can be used with the disclosed
detectors.
[0012] FIG. 7 is a cross-sectional view of a surface-discharge PPS
and/or PPPS detector with a metal-foil front substrate and a
parallel/rectilinear surface-discharge electrode pattern on the
back substrate with an orthogonal back electrode pattern in
accordance with one embodiment of the present invention.
[0013] FIG. 8 is a cross-sectional view of a double
surface-discharge plasma panel radiation detector with a central
conversion layer shared between and the top and bottom sets of
parallel/rectilinear surface-discharge electrodes in accordance
with one embodiment of the present invention.
[0014] FIG. 9 is a perspective view of a flat dielectric insulator
window barrier structure in accordance with one embodiment that can
be used with the disclosed detectors.
[0015] FIG. 10 is a perspective view of a barrier structure in
accordance with one embodiment that can be used with the disclosed
detectors.
[0016] FIG. 11 is a perspective view of a surface-discharge PPS
and/or PPPS detector with a segmented surface-discharge electrode
pattern on top of vertical quenching resistors and with dielectric
barrier strips that cover and electrically insulate the bus bar
electrodes in accordance with one embodiment of the present
invention.
[0017] FIG. 12 is a perspective view of a surface-discharge PPS
and/or PPPS detector with a segmented surface-discharge electrode
pattern on top of vertical quenching resistors and with a flat
dielectric window insulator barrier structure that covers and
electrically insulates the bus bar electrodes in accordance with
one embodiment of the present invention.
[0018] FIG. 13 is a perspective view of a surface-discharge PPPS
Cherenkov detector with a segmented surface-discharge electrode
pattern on top of vertical quenching resistors and with a flat
dielectric window insulator barrier structure that covers and
electrically insulates the bus bar electrodes in accordance with
one embodiment of the present invention.
[0019] FIG. 14 is a perspective view of a multilayered
surface-discharge PPS and/or PPPS detector with a segmented
surface-discharge electrode pattern and vertical quenching
resistors, and with a flat dielectric insulating layer over the bus
bar electrode layer in accordance with one embodiment of the
present invention.
[0020] FIG. 15 is a block diagram of a counting circuit in
accordance with one embodiment of the present invention.
[0021] FIG. 16 is a cross-sectional view of a surface-discharge PPS
detector with ultra-thin front and back substrates in accordance
with one embodiment of the present invention.
DETAILED DESCRIPTION
[0022] One embodiment is a plasma panel sensor ("PPS") device that
provides enhanced detection, tracking, identification and imaging
of ionizing particles generated in nuclear accelerators and
particle colliders, as well as the detection of such particles
occurring naturally or created by other means. The particles to be
detected are numerous and can encompass new classes of ionizing
particles as yet to be discovered. Embodiments used with
accelerator/colliders provide detection, monitoring and profiling
of protons and heavier ions such as carbon and neon ions in the
treatment of cancer by hadron particle beam therapy, the detection,
monitoring, tracking and profiling of very large ionizing particles
generated in radioactive ion beam ("RIB") accelerators, the
detection and tracking of subatomic ionizing particles such as
muons generated in facilities such as the Large Hadron Collider
("LHC") at CERN, and the use of such particles for materials
characterization, testing and non-destructive testing at facilities
such as the Spallation Neutron Source ("SNS") at Oak Ridge National
Laboratory.
[0023] A plasma display panel ("PDP") based sensor, or "plasma
panel sensor" (PPS") and light-sensitive plasma panel photosensor
("PPPS") used for radiation detection has been disclosed, for
example, in U.S. Pat. Nos. 7,332,726 and 7,564,039 and 7,683,340,
and utilizes some of the structure and manufacturing techniques
used in producing plasma display panels for television. These
devices can be considered hybrid gaseous solid state sensors that
encompass some of the best features of Geiger-Mueller ("GM") tubes
and micropattern gas detectors. However, they can provide a much
greater sensitivity than micropattern detectors, operating beyond
the proportional region and in what is generally called the
Geiger-Mueller region with internal gains on the order of
.about.10.sup.6 or higher (depending upon pixel resolution, etc.),
with position resolutions that can approach 10 .mu.m, a temporal
resolution estimated at about 100 ps or better, and insensitivity
to magnetic fields. Unlike conventional micropattern detectors,
PPS's are inherently digital devices and can greatly expand the
capability of micropattern detection technology for both charged
and neutral species over the full range from minimum to maximum
ionizing particles (e.g., muons to radioactive ion beams).
[0024] Fabrication of the PPS can utilize low cost
photolithographic techniques for construction of the electrodes,
and a gaseous media for electron amplification. Further,
fabrication can take advantage of a huge multi-billion dollar
technology base developed over a 40 year period for the
manufacturing and materials infrastructure of PDP modules. Such
PDP's are mass-produced as large area (e.g., 1-2 meter diagonal),
flat panel HDTV-sets and currently sell with electronics for less
than $0.30 per square inch, which is about two orders-of-magnitude
less expensive than the lowest cost photomultiplier tubes ("PMT"s).
PPS-based detectors address many of the properties sought in the
next-generation of position-sensitive, minimum and maximum
ionizing-particle detectors (e.g., for muons, pions, kaons, fast
electrons, protons, etc.) for high energy and nuclear physics, to
provide significant position, tracking and timing resolution
improvement, radiation hardness, high sensitivity and enhanced rate
capability, improved stability, compactness and low cost. In
addition, the combination of high pixelation with ultra-fast
response time allows the PPS to serve simultaneously as both a fast
triggering device and a high resolution positional detector for
both low and high luminosity sources.
[0025] The PDP was invented in the early 1960's as a flat panel
display to replace the cathode ray tube ("CRT"). The PDP is the
device that makes the plasma-television ("TV") possible. It is
composed of millions of cells per square meter, each of which
typically can initiate and sustain a plasma discharge for at least
a few hundred nanoseconds. The cells are quite small, generally
about 100 to 200 .mu.m in each dimension for both the pitch and gas
gap in a 1 meter diagonal display. Because of their small size,
large electric fields are easily produced with only a few hundred
volts of bias. The plasma discharges, which are well-controlled and
confined, employ various Xe gas mixtures (e.g., .about.5% Xe in 95%
Ne) at less than an atmosphere of pressure (e.g., 500-600 torr). In
the PDP, the discharge produces both ultraviolet ("UV") and
vacuum-ultraviolet ("VUV") light that strikes phosphors in the
cells and produces the bright colors characteristic of plasma TV's
(each pixel typically contains a red, green and blue cell). At any
given time in a TV picture, many of the pixels have at least one
cell on, so the PDP electronics must individually address, refresh
and sustain these discharges, while quickly suppressing cells that
must change state by "erasing" their stored charge on the surface
dielectric.
[0026] In a PDP configured for radiation detection, each cell can
be biased to discharge when a free-electron is generated in, or
emitted into, the gas. The PPS, as a reconfigured PDP, thus
functions as a highly integrated array of parallel
pixel-sensor-elements or cells, each independently capable of
detecting free-electrons within the cell that are due to incident
ionizing-particle radiation (since all PPS devices are
"monochrome", the terms "pixel" and "cell" are synonymous for these
devices). For example, free-electrons and ions generated by
interaction of the gas with a minimum ionizing particle ("MIP")
(e.g., a muon) passing through the PPS cell can in turn lead to
rapid electron multiplication resulting in an avalanche that can be
confined to the local pixel cell space. For known plasma panel
based devices, this avalanche process is self-limiting and
self-contained. The discharge current of an individual triggered
cell is unimportant, only the fact that a cell is either "ON" or
"OFF" (i.e., pixel switching). The PPS is therefore intrinsically
digital, and, with a gain of .about.10.sup.6 is large enough that
for many applications it can potentially avoid having to use
external signal amplification electronics, especially considering
that the PPS is a digital, particle-counting device and thus
achieves its sensitivity because it can operate in the Geiger
region, as compared to the proportional region for most known
detectors which are proportional detectors. Thus, PPS-based
detectors, including light-sensitive PPS-based detectors (i.e., the
PPPS), can have a much higher gain than proportional detectors.
[0027] For a plasma panel operating as a TV monitor in the display
mode, the pixel switching is initiated by video signals applied
through a network of driver circuits. As a PPS detector the
switching can be triggered either by direct ionization of the gas
from incident ionizing particles, or indirectly via the incident
ionizing radiation interacting with an internal conversion layer
within the PPS that emits electrons into the gas. When a pixel/cell
is turned "ON" by absorbed or inelastically scattered radiation, a
signal can be collected by the readout electronics and an image or
tracking point generated. A large number of device structures are
possible with a variety of readout options (both electronic and
optical), including the columnar structure (with anode and cathode
on opposite substrates) and the surface-discharge structure (with
anode and cathode on the same substrate), the latter used on
virtually all PDP's sold today.
[0028] Some key characteristics of the PPS are its high gain, fine
positional resolution, fast response, radiation hardness,
insensitivity to magnetic fields, and potentially high electron
detection efficiency. In terms of device resolution, a pixel pitch
of 50 to 100 .mu.m should be more than sufficient for most high
luminosity accelerators in preventing signal pileup associated with
two independent incident ionizing-particles falling within the same
pixel discharge cell space at essentially the same time (i.e.,
within the cell gas discharge response period). In fact, a pixel
pitch of about 200 .mu.m should be more than adequate for many
applications in nuclear physics. Regarding fabrication of such
devices, 21-inch diagonal AC-PDP's with a cell pitch of 108 .mu.m
have been fabricated in the past. The pixel structure of these
AC-PDP's was quite complex, significantly more so than for most PPS
devices, as these display panels required RGB phosphor patterning
and alignment via screen-printing and were fabricated on a high
coefficient of thermal expansion (i.e. 90.times.10.sup.-7/.degree.
C.) float glass substrate. In contrast, PPS devices do not require
difficult-to-control, thick film, screen-printing and phosphor
pattern pixel alignment on "thermally unstable" float glass
substrates. PPS devices fabricated on glass can use much more
stable, non-alkali, lower expansion substrate materials such as
alkaline earth boro-aluminosilicate flat panel display glass (e.g.,
Corning Eagle-XG) with a thermal expansion of
.about.32.times.10.sup.-7/.degree. C., as compared to
90.times.10.sup.-7/.degree. C. for the above 21-inch AC-PDP's.
Finally, embodiments of PPS detectors will almost always be
operated as DC devices, and therefore will more closely resemble a
DC-PDP than an AC-PDP. The electrode resolution in DC-PDPs can
therefore be much higher because the electrodes do not have to be
coated and fired with a chemically reactive and corrosive
thick-film dielectric that tends to undercut and erode the AC-PDP
electrode-material line width. For these reasons it is feasible to
fabricate embodiments of PPS devices with a pixel pitch approaching
10 .mu.m. Also in terms of high resolution electrodes, LCD's on
glass with a 10-30 .mu.m pixel pitch are now being manufactured, so
for PPS devices the fabrication process could utilize similar
technology. Finally, as a collateral benefit, minimizing the
electrode width also raises the intrinsic firing voltage, thereby
increasing the pixel electric-field strength and hence the device
sensitivity.
[0029] For most of the embodiments of the present invention, each
plasma panel cell can in some ways be thought of as a miniature
GM-tube or micro-Geiger cell, and GM-counters have very long
recovery times. However, PPS rise times can be significantly less
than a nanosecond (e.g., in the picosecond range), with recovery
times many orders-of-magnitude faster than GM-tubes. This large
difference in discharge/recovery time is in large part due to
geometry, which directly relates to field gradients and
space-charge. In both the GM-tube and plasma panel detector, the
operating voltage is similar--approximately 400-1000 volts for the
plasma panel (depending on cell geometry) and about 500-1500 volts
for GM-tubes. However, the anode to cathode gap in a GM-tube is
typically about 10-20 mm, whereas in a plasma panel it is on the
order of 0.10-0.20 mm, a reduction of two orders-of-magnitude. All
other things being equal, this translates into a field gradient
that is 100.times. greater for the plasma panel. However, the
result is much greater because the cathode "wire" in a plasma panel
is a narrow electrode, as is the anode, whereas in a GM-counter the
cathode comprises the entire inner surface of the cylindrical tube,
which could easily be 10.sup.4 mm.sup.2. The difference between the
cathode area for a medium size GM-tube, and a plasma panel
detector, could thus be four orders-of-magnitude (i.e., 10.sup.4).
It follows that the "slow-moving" ions in a plasma panel can be
"cleared-out" very quickly, because they need only travel tens of
microns to the cathode and are being pulled by a very strong
electric field. Contrast this to the GM-tube, where the cathode
field strength is many orders-of-magnitude smaller than for the
plasma panel, and where the ions have a much longer distance to
travel to reach the cathode--perhaps 10 mm. Because of the very
weak field in the vicinity of the GM-cathode, the ion movement
towards the cathode is almost a random-walk compared to the
movement of ions near the plasma panel cathode. Thus for all gas
discharge devices (e.g., GM-tubes and plasma panels) the rise time
primarily reflects the electron efficiency in going from the
cathode region to the anode, whereas the fall time primarily
reflects clearing-out the space-charge volume of slow moving ions
from the drift region typically located much further away.
Therefore, the geometric differences in the discharge volume and
field gradients for the plasma panel versus GM-tube results in
orders-of-magnitude difference in pixel response time. For example,
the GM-tube which functionally acts as a single pixel, has a
typical discharge volume of 10.sup.-1 to 10.sup.-2 liters, whereas
the PPS pixel discharge volume is in the range of 10.sup.-9 to
10.sup.-12 liters.
[0030] AC-PDP's have been successfully operated at 1 MHz refresh
rates, corresponding to a pixel "ON/OFF" time of 1 .mu.s (e.g., 2%
Xe/98% Ne gas mixture and 325 .mu.m pixel pitch). In order for the
entire panel to continuously recycle at 1 .mu.s, individual pixel
discharge and recovery should be at least an order of magnitude
faster (i.e., .about.100 ns), with pixel rise times typically being
about two orders-of-magnitude faster than the fall time, or
.about.1 ns. Free-electrons in a gas discharge, owing to their low
mass and small cross-section, achieve a much greater acceleration
between encounters with surrounding neutral gas species than
ionized atoms, with the result that their mobility can be 1000
times greater than that of ions. In addition, as the percent Xe
increases from 2% in the above AC-PDP to perhaps 90-100% in a PPS,
so does the gas discharge response speed. Also, as the pixel pitch
gets smaller, the discharge gap decreases, raising the field
gradient and further reducing the rise time. For embodiments of the
PPS, a pixel pitch of 50 to 100 .mu.m is readily achievable. Given
all of the above factors, the PPS gas discharge rise time should be
shortened by about two to three orders-of-magnitude (i.e., from
about 1 ns to approximately 1-10 ps).
[0031] Another approximation of response time for a PPS in
accordance with one embodiment can be obtained from the pixel
RC-time constant. For example, from full panel measurements, a
DC-PDP with a pixel pitch of 640 .mu.m may have an isolated pixel
capacitance of 30 fF. By comparison, a PPS with a 64 .mu.m pixel
pitch would have an isolated pixel capacitance of about 1% of this
value, or two orders-of-magnitude smaller (i.e., approximately 0.3
fF). By using a current-limiting, in-line resistor of about 100
k.OMEGA., the shortest time (.tau.) required to discharge the
capacitance would be the RC-time constant, or about 30 ps (i=RC=100
k.OMEGA..times.0.3 fF). However, since the rise time is at least an
order-of-magnitude faster than the RC discharge time, the
corresponding device rise time would be about 3 ps (or faster,
especially for a smaller resistor). Thus with a rise time on the
order of possibly .about.1 ps, the temporal resolution of the
embodiments of the PPS could be in the range of about 10 to 100 ps
for resolving a discharge tracking event resulting from a single
ionizing-particle passing through the PPS drift region.
[0032] Given the above value of .about.1 ps for the signal rise
time, with a fall time perhaps two to three orders-of-magnitude
longer, the collection time is approximately on the order of about
0.1 ns to 1 ns. Thus, the PPS particle event total recovery time is
in the range of about 1 ns, corresponding to a potential count rate
of 10.sup.9 cps per pixel (i.e., 1 GHz per pixel) or a signal
saturation rate limit of 10.sup.13 cps per cm.sup.2 for a PPS with
a pixel pitch of 100 .mu.m.
[0033] In one embodiment, excited state species (e.g., photons,
ions, electrons metastables, etc.) generated in the gas discharge
may cause secondary discharges of time-delayed new avalanches. In
one embodiment, a quenching agent is added to the gas mixture to
"absorb" high energy photons emitted by the gas discharge as well
as acting as an energy sink for gas-phase metastables, electrons
and ions. In fact, by simply adding a diatomic quenching agent
(e.g., oxygen which also served as a Penning gas dopant) to an
"open-structure" DC-PDP device, along with the addition of a
current limiting series resistor, the problem of secondary
avalanches is eliminated with the discharge confined to the nearest
pixel interaction site in previously reported PPS devices that were
configured as gamma-ray detectors. In another embodiment to prevent
gas discharge spreading, each pixel is "enclosed" within a barrier
wall structure similar to those used in all commercial PDP-TV's
(e.g., as shown in FIG. 10). In another embodiment, to prevent gas
discharge spreading, each pixel is surrounded by a "flat" layer of
dielectric material similar to that used in commercial DC-PDP's
(e.g., as shown in FIG. 9).
[0034] In another embodiment, an electrical method can be employed
to preventing secondary discharges. In this embodiment quenching
resistors are fabricated using low cost thin-film or thick-film
technology within each pixel or electrode line that can serve the
dual function of both limiting the discharge and decoupling one
electrode line or pixel from another.
[0035] FIG. 1 is a perspective view of a surface-discharge PPS
and/or PPPS detector with a parallel/rectilinear surface-discharge
electrode pattern incorporating individual cell quenching resistors
and an orthogonal back electrode pattern in accordance with one
embodiment of the present invention. PPS 10 includes a first
(front) substrate 12 and a second (back) substrate 14, separated by
a gas filled gap 18. Sensor 10 includes X-surface discharge
electrodes (cathode) 24 and Y-surface discharge electrodes (anode)
26. Detector 10 further includes Z electrodes 28 on the backside of
the back substrate 14, quenching resistors 30, and a front
conductive layer 22.
[0036] PPS 10 is based on surface-discharge, 4-electrode
configuration in which the front conductive layer 22 can serve as a
front electrode or drift electrode which can also be a thin metal
coating. In another embodiment, the front conductive layer can also
be a conversion layer or thin sheet such as gadolinium (Gd) foil
that can capture an ionizing particle such as a thermal neutron and
then emit a fast conversion electron (e.g., 72 keV) into the
discharge gas 16. For many applications the PPS front conductive
layer 22 can be combined with the front substrate 12 by making the
front substrate a metal plate or metal foil. For detector 10, the
gas gap is also known as the "drift region".
[0037] PPS 10 of FIG. 1 includes a conductive layer as the 4th
electrode (i.e., drift electrode or front electrode 22). Detector
10 can be converted to a PPPS by employing for the conductive layer
22 a thin-film photocathode coating. As disclosed above, detector
10 can be converted to a neutron detector by employing for the
conductive layer 22 a thin-film Gd neutron conversion layer or Gd
neutron conversion foil.
[0038] PPS 10 in one embodiment is a highly integrated array with
roughly 10.sup.2 to 10.sup.6 micro-detection cells per cm.sup.2,
each of which can act as an independent, position-sensitive,
radiation sensor. PPS embodiments, in general, efficiently collect
free-electrons and ions created in a gas by the passage of an
ionizing particle and then, via the drift field, "channel" the
electrons and ions into the higher field region where an avalanche
develops.
[0039] In each embodiment, the design of the cell can enhance the
efficiency. As shown in FIG. 1, in one embodiment of PPS 10, the
drift region begins at the front electrode or conversion
layer/conductive layer 22 on front substrate 12 (i.e., the cover
plate) shown above the avalanche region formed by the X and Y
surface-discharge electrodes (i.e., cathode 24 and anode 26). The
drift electrode 22 can be either a metallized or transparent
conductive coating (e.g., ITO or SnO.sub.2) on a dielectric
insulating substrate such as glass or ceramic, or the drift
electrode can be a metal foil or metal plate. The most effective
metals may be the ones with the least photocathodic activity (e.g.,
metals that are good UV-VUV reflectors with high work functions)
which should also be chemically inert with respect to the discharge
gas. The selected material should be stable in a plasma discharge
environment, so it should have a high melting point and be sputter
resistant--two such candidate electrode materials include nickel
(Ni) and chromium (Cr).
[0040] During operation of PPS 10, interesting tracks are expected
to enter the drift region (i.e., gas gap 18 of FIG. 1) somewhat
normal to the plane of the drift electrode (i.e., front substrate
conductive layer 22). Ions deposited along the track will drift in
the uniform part of the field toward the discharge electrodes and
enter a large field where an avalanche will begin. Expansion of
this avalanche into the high-field region between the cathode
(X-electrode 24) and anode (Y-electrode 26) is instrumental to the
operation. The avalanche is terminated when the avalanche or
discharge voltage drops significantly (e.g., approximately 50% or
more in one embodiment) as a result of sufficient resistance in the
path that provides high voltage to the X-electrode (i.e., cathode
or discharge electrode). The current generated during the avalanche
yields the voltage on the Y-electrode (i.e., anode or sense
electrode).
[0041] In some embodiments, the discharge cathodes protrude
vertically into the gas. PPS fabrication technology permits great
flexibility in the choice of electrode shapes including elevated
wall-like structures with differing cross-sections and heights for
the X- and Y-electrodes. Other embodiments utilize planar
electrodes having very little height above the substrate surface.
Optimization of the electrode shapes are application dependent. PPS
electrode configuration embodiments include: those with and without
embedded pixel series resistors, with and without orthogonal (Z)
strip electrodes beneath the X- and Y-electrodes (see FIG. 1) or on
the PPS bottom substrate backside, and the various alternate height
vertical electrode structures (disclosed in FIGS. 3-6). High aspect
ratio vertical electrodes will allow a larger and more focused
drift field to be moved out significantly further into the gas to
enhance the detector charged particle collection efficacy.
[0042] In one embodiment, the avalanche region is extended further
into the gas by the use of non-planar discharge electrode
structures, such as vertical electrodes extending up from the PPS
back plate into the gas towards the drift electrode. FIGS. 3-6
disclose embodiments with such an asymmetric arrangement. FIG. 3 is
a cross-sectional view of a barrier wall vertical electrode
structure in accordance with one embodiment that can be used with
the disclosed detectors. FIG. 3 includes vertical electrodes 72 and
flat electrodes 74. FIG. 4 is a cross-sectional view of an
asymmetric elevated vertical electrode structure in accordance with
one embodiment that can be used with the disclosed detectors. FIG.
4 includes vertical electrodes 92 and flat electrodes 94. FIG. 5 is
a cross-sectional view of an asymmetric vertical wall electrode
structure in accordance with one embodiment that can be used with
the disclosed detectors. FIG. 5 includes vertical electrodes 110
and flat electrodes 112. FIG. 6 is a perspective view of an
asymmetric multi-potential vertical wall electrode structure in
accordance with one embodiment that can be used with the disclosed
detectors. FIG. 6 includes flat electrodes 124 and a stack of
vertical electrodes 122 separated by alternate vertical dielectric
insulating layers 126. The multi-potential vertical electrode
structure 120 shown in FIG. 6 allows the voltage of each vertical
cathode layer to be individually set for more precise control of
the overall vertical field gradient. In another embodiment, the
vertical stack of electrodes 122 with alternate insulating layers
126 shown in FIG. 6 can be replaced by a flat top and bottom strip
conductor separated by a uniform vertical resistive wall resulting
in a vertical field gradient. In one embodiment the vertical
resistive wall can be made non-uniform with respect to its volume
resistivity as a function of wall height.
[0043] In FIGS. 3-6, only the X-electrodes (i.e., cathodes) extend
vertically into the gas gap region while the Y-electrodes (i.e.,
anodes) assume a flat-wire configuration in the plane of the back
substrate. However, in one embodiment this assignment could be
reversed with the vertical electrodes being the anodes and the flat
electrodes the cathodes. In one embodiment, given a 20:1 aspect
ratio and a PPS electrode width of 100 .mu.m, the vertical
electrode structure would extend 2.0 mm into the gas drift region.
In another embodiment the alternate vertical electrode
configuration could be eliminated and both the X- and Y-electrodes
could be vertical.
[0044] Another embodiment can enhance the collection efficiency by
reducing the gas gap distance between the drift electrode and the
discharge X- and Y-electrodes, thus increasing the field strength
between the drift and avalanche planes, and then compensating for
the reduced active gas media volume (i.e., from the reduced gas
gap) by increasing the gas pressure. Depending on the
ionizing-particles to be detected, the internal gas pressure can be
made significantly positive (i.e., well above one atmosphere)
without distorting the internal gas gap distance by the use of
sufficiently thick substrate plates to contain the gas pressure.
Alternatively, an embodiment can use thinner substrates with
external reinforcing strips, bars, rods, wires to maintain the
flatness of the substrates under a positive internal pressure.
[0045] In some embodiments, surface-discharge electrodes can be
made by employing multichip module ceramic technology (i.e., MCM-C)
on low-cost ceramic, glass-ceramic or high temperature glass
substrates (e.g., alumina, silicon nitride, fused silica,
boro-aluminosilicate glass, etc.). As one example, to fabricate
embedded resistors a thin-film LTCC (low temperature co-fired
ceramic) process can be used to produce resistors which with laser
trimming can achieve an accuracy of .+-.1%. Other thin-film and
thick film processes with photolithography and wet or dry etching
can be used to fabricate vertical resistors under the electrodes,
shown in FIGS. 11-14, and depending upon the cell resolution,
thick-film printed resistors can also be fabricated within each
cell or group of cells with an accuracy on the order of .+-.1% with
laser trimming.
[0046] FIG. 11 is a perspective view of a surface-discharge PPS
and/or PPPS detector with a segmented surface-discharge electrode
pattern 174 on top of vertical quenching resistors 182 and with
dielectric barrier strips 184 that cover and electrically insulate
the bus bar electrodes 176 in accordance with one embodiment of the
present invention. FIG. 12 is a perspective view of a
surface-discharge PPS and/or PPPS detector with a similar segmented
surface-discharge electrode pattern on top of vertical quenching
resistors and with a flat dielectric window insulator barrier
structure 192 that covers the bus bar electrodes in accordance with
one embodiment of the present invention. FIG. 13 is a perspective
view of a surface-discharge PPPS Cherenkov detector with a similar
segmented surface-discharge electrode pattern on top of vertical
quenching resistors and a flat dielectric window insulator barrier
structure that covers the bus bar electrodes in accordance with one
embodiment of the present invention. FIG. 14 is a perspective view
of a multilayered surface-discharge PPS and/or PPPS detector with a
segmented surface-discharge electrode pattern and vertical
quenching resistors, and with a flat dielectric insulating layer
over the bus bar electrode layer in accordance with one embodiment
of the present invention. For the embodiments shown in FIGS. 11-14,
the X-electrode bus bar is in all cases prevented from itself
discharging to the Y-electrode by being covered by a dielectric
insulator layer such as the barrier strip 184 in FIG. 11, or the
window barrier 192 in FIG. 12 (also shown by itself in FIG. 9). In
another embodiment, the X-electrode bus bar 218 is "buried" under
the dielectric layer 219 shown in FIG. 14 and connects via the
vertical quenching resistor 220 to the surface-discharge
X-electrode 217. All of the embodiments for the described PPS and
PPPS detectors can be hermetically sealed, gas processed and
generally fabricated in a manner similar to PDP devices.
[0047] Known PDP-TV monitors have pixel configurations that
incorporate internal vertical barrier structures to isolate each
cell from adjacent cell interactions, including avalanche spreading
but primarily to prevent UV and VUV photon "leakage" from one cell
to the next causing adjacent phosphors to be stimulated and thus
resulting in RGB phosphor "bleeding" and color de-saturation. Many
of these structures literally surround and enclose each cell,
thereby preventing all migration of excited species to adjacent
cells. Dozens of these vertical barrier structures have been
developed, some with height-to-width aspect rations as great as
15:1 to 20:1. These barriers are being developed for PDP-TV sets
with widths as small as 12-15 .mu.m (such barriers also serve as
the gas gap spacer between the front and back plates) and have been
described as broadened knife-edge structures with completely
vertical or very slightly trapezoidal-shaped walls. Most PDP
barriers have well-defined flat plateau tops while others can be
rounded at the edge or the top. Practically every shape imaginable
has been fabricated, including geometries described as: diamond,
egg-crate, delta, honeycomb, saddle-back, square, rectangular,
triangular, pentagonal, hexagonal, U-shaped, cylindrical,
hemispherical, etc. A number of barrier fabrication techniques have
been developed employing a variety of materials including
multilayer composites that could have alternate insulating and
conductive layers as shown in FIG. 6. Since most of these barrier
wall structures were developed for meter-size PDP TV-sets, they
employ low-cost fabrication techniques and low-cost materials. Some
of these structures have been made of conductive materials, and all
of them could be coated with a thin-film metallization layer to
create what the PDP industry calls vertical wall or barrier wall
electrodes which have been demonstrated to exhibit high efficiency.
In some embodiments of PPS 10, for example, the use of elevated
vertical electrode structures can result in a higher efficiency,
well-behaved, avalanche/discharge field that extends well into the
gas and thus produces a significantly improved, higher performance
detector when compared to the traditional "flat" micropattern
electrodes that essentially reside in the substrate plane.
[0048] Embodiments include a large number of PPS and PPPS electrode
device structures (both AC and DC) with a variety of X-Y (i.e.,
2-electrode, and 3-electrode if combined with a drift electrode,
conversion electrode or photocathode) and X-Y-Z (i.e., 3-electrode,
and 4-electrode if combined with a drift electrode, conversion
electrode or photocathode) configurations. One such example of a
2-electrode X-Y structure is detector 40 shown in FIG. 2, which is
a perspective view of a columnar-discharge PPS and/or PPPS detector
with an orthogonal electrode pattern that includes electrode line
quenching resistors 52 in accordance with one embodiment of the
present invention. In this embodiment, X-electrodes 46 and
Y-electrodes 48 are perpendicular to each other and on opposite
substrates 42 and 44, and in their orthogonal orientation have some
similar elements as DC- and AC-PDP's produced through the
early-1990's (and disclosed, for example, in U.S. Pat. No.
7,518,119). Both the columnar and surface-discharge structures can
include quenching resistors as shown in FIGS. 1 and 2. For the
columnar structure embodiment of FIG. 2, one quenching series
resistor 52 is used for each X-electrode line 46 (cathode). For the
surface-discharge embodiment of FIG. 1, one quenching series
resistor 30 is used for each discharge cell off of the cathode line
24. In another embodiment of the surface-discharge structure, one
quenching series resistor is used for each X-electrode line instead
of each X-electrode cell in FIG. 1.
[0049] Further, the surface-discharge electrode embodiments shown
in FIGS. 1 and 11-14 can be used not only for ionizing-particle
PPS's, but also for light-sensitive PPPS detectors by incorporating
a photocathode for the front conductive layer for most of the
embodiments disclosed above and as shown for the "Cherenkov" PPPS
detector in FIG. 13. For all of the embodiments described herein,
the designation of "front" and "back" is arbitrary, meaning that
all of the embodiments can be used with the top and bottom or front
and back reversed with respect to the incident radiation
direction.
[0050] For all PPS and PPPS embodiments, whether surface-discharge
or columnar-discharge, there is a cathode (e.g., X-electrode) and
anode (e.g., Y-electrode). Either electrode can function as the
power electrode. In many embodiments, the cathode is biased
negatively by the power supply and operated as the discharge
electrode. The anode would then generally be operated at or near
ground and would be monitored as the signal sense electrode. In a
3-electrode, surface-discharge configuration, the 3rd electrode, or
Z-electrode would be deposited orthogonal to the X and Y
surface-discharge electrodes, most conveniently on the backside of
the surface-discharge substrate (as shown in FIGS. 1, 7-8, 11-14
and 16) and capacitively couples through the substrate to the
discharge pulse created at the X-Y cell avalanche site. In order
for the Z-electrode to acquire a good signal, the substrate is a
dielectric and is generally limited to a maximum thickness of a few
millimeters, with thinner typically being better. Operationally,
the specific Z-electrode that picks up the strongest signal would
serve to locate the orthogonal intersection that defines the
specific X-Y cell discharge position. In the 4-electrode
surface-discharge embodiment, the 4th electrode can be a metal
drift electrode (i.e., surface front electrode), which serves
primarily to vertically shape the drift field and thereby "push"
the radiation generated gas ions towards the X-cathode while also
"pushing" the radiation generated free-electrons towards the
Y-anode. Normally the potential of the drift electrode would be set
a little more negative than the cathode. In other embodiments, the
absolute bias values are not fixed and so the cathode could be set
at essentially any potential value with the anode biased at a
suitably higher voltage in the positive direction. For a given
electrode discharge gap and gas pressure, there will be a minimum
voltage required for gas breakdown (i.e., the firing voltage) for a
given gas composition, and this minimum voltage can be plotted as a
function of pd (i.e., pressure times distance of anode-cathode
separation gap). The resulting plot is known as the "Paschen"
curve, and can be used to estimate the required anode-cathode bias
setting for a given PPS or PPPS device.
[0051] For the PPPS embodiment, one benefit of the
surface-discharge electrode structure is that by moving the
X-electrodes 46 from, for example, the front substrate in the
columnar embodiment of FIG. 2, to the back substrate in the
surface-discharge embodiments of FIGS. 1 and 11, the conductive
electrode layer 22 in FIG. 1, or layer 172 in FIG. 11, can function
as an efficient photocathode with essentially a 100% fill-factor.
However in one embodiment the columnar-discharge embodiment of FIG.
2 can be configured as a PPPS, albeit with a reduced fill-factor,
by the use of a robust, dual purpose X-electrode conductor that is
also a photocathode, such as a thin-film Au (e.g. .about.0.1 .mu.m
thick) or fluorine-doped SnO.sub.2 transparent photocathodic
conductor.
[0052] For those embodiments which do not have a photocathode, the
surface-discharge electrode structure significantly improves device
efficiency because it allows 100% surface coverage by a front plate
drift electrode. It also facilitates the use of a dual purpose
design in which a metal thin-foil could be used as both the front
cover plate and the drift electrode as shown in FIGS. 7 and 16
below. FIG. 7 is a cross-sectional view of a surface-discharge PPS
detector with a metal-foil front substrate 132 and a
parallel/rectilinear surface-discharge electrode pattern (i.e.,
X-electrodes 138 and Y-electrodes 140) on the back substrate with
an orthogonal backside Z-electrode pattern 142 in accordance with
one embodiment of the present invention. FIG. 16 is a
cross-sectional view of a surface-discharge PPS detector with both
ultra-thin front and back substrates in accordance with one
embodiment of the present invention.
[0053] By using a thin-foil cover plate as shown in FIGS. 7 and 16,
the reduction in front substrate mass and thickness leads to
enhanced device efficiency, especially for applications involving
large atom-sized, ionizing-particles (e.g., PPS Active Pixel Beam
Monitor for radioactive ion beam profile diagnostics) that are
easily stopped via interaction with the front substrate and thus
are very sensitive to the front substrate mass. However, a metal
foil cover plate embodiment as shown in FIGS. 7 and 16 is also
advantageous for the detection of smaller ionizing particles, such
as for proton, carbon ion and neon ion beams used in hadron
particle beam therapy, in which the minimization of particle beam
scattering by the detector is critical. Such applications thus
require very low mass detectors which could greatly benefit by an
ultra-thin front and back substrate structure such as that in FIG.
16 based on metal foils or flexible sheets made from ceramic,
glass, semiconductor, or crystalline type materials on the order of
about 0.001 inches thick and preferably less. Examples of suitable
foils include metals or metal alloys of aluminum, titanium,
stainless steel, nickel, iron, copper, cobalt, beryllium, magnesium
and molybdenum, and superalloys such as 0.00025'' thick Arnavar.TM.
foil (i.e., Co--Cr--Fe--Ni alloy made by Arnold Magnetic
Technologies) and Inconel foil. Examples of other ultra-thin
substrates include materials such as alumina, sapphire, fused
silica, silicon, glass, silicon nitride, etc., which can all be
fabricated in thicknesses of less than 0.001 inches. In one
embodiment, an "ultra-thin substrate" would include any substrate
having a thickness between approximately 1-300 microns, or
0.001-0.3 mm, thick.
[0054] An additional advantage of the surface-discharge design is
that with the two discharge electrodes (i.e., X and Y) located on
the back substrate, the gas gap is independent of the discharge
gap, as the latter is solely a function of the quality of the X-Y
electrode lithography on the back substrate. Thus for the
surface-discharge structure, the device uniformity should be far
better than in the columnar-discharge design. In addition, a much
larger gas-gap can be employed without compromising the device
spatial resolution since the gas gap and discharge gap are
completely decoupled in the surface-discharge configuration.
Finally for PPS and PPPS detectors, including both columnar and
surface discharge electrode embodiments, the choice of the
electrode metal may be important in terms of device performance and
device lifetime. With regard to device lifetime, for DC discharge
devices in which the bare electrode is in direct contact with the
plasma, which means subject to hot electrons and ion sputtering,
the electrode material should have a high melting point and be
sputter resistant. This means that ideally the selected metal
should have some level of refractory character, while being
chemically resistant to the discharge gas including possible
chemical species created in the plasma. Also the selected metal
should not be a catalyst for gaseous reactions in the plasma, nor
should it contain significant amounts of radioactive isotopes.
Other considerations include photocathodic activity and device
fabrication process compatibility. Given the above considerations,
two examples of suitable electrode metals for most PPS and PPPS
device applications are Ni and Cr.
Cell Dielectric Isolation
[0055] Embodiments can use a number of methods to minimize adjacent
cell crosstalk and prevent avalanche spreading to neighboring
cells. These methods include the use of avalanche quenching gaseous
components added to the discharge gas mixture, the use of internal
cell series quenching resistors, and the use of a physical cell
dielectric isolation barrier. As an example of such barriers, FIG.
10 is a perspective view of a 3-dimensional vertical barrier
structure 156 in accordance with one embodiment that can be used
with the disclosed detectors. FIG. 9 is a perspective view of a
flat dielectric insulator window barrier structure 152 in
accordance with one embodiment that can be used with the disclosed
detectors, and implemented as the flat dielectric insulator window
barrier 192 shown in FIG. 12. For detector 190 shown in FIG. 12, in
one embodiment the 3-dimensional vertical barrier structure 156 can
be substituted for the flat window barrier 192. Other examples of
dielectric isolation include the embodiment in FIG. 11 which has an
array of narrow 2-dimensional flat dielectric barrier strips
184.
[0056] The use of physical dielectric barrier structures for
achieving cell isolation (i.e., discharge confinement or
localization) is applicable to both surface-discharge and
columnar-discharge PPS and PPPS device embodiments. In terms of
physically confining the radiation induced plasma avalanche to the
initial cell gas discharge site, there are two basic approaches
used in commercial PDP's that can each be employed in two different
embodiments of the PPS and PPPS. The first method is to physically
surround each individual cell or an entire electrode line of cells
within a barrier wall structure such as wall structure 156 in FIG.
10, which if vertically tall enough could also serve as a
three-dimensional support matrix that both defines and maintains a
uniform gas gap between the front and back substrates. Dozens of
differently shaped barrier wall structures have been successfully
employed, from a simple parallel wall configuration to much more
complex polygon shaped enclosures (e.g., triangles, squares,
rectangles, diamonds, hexagons, honeycombs, saddlebacks, etc.).
Generally these 3-dimensional vertical barriers have been used
primarily on AC type PDP structures for TV-set applications. In one
embodiment the barrier wall structure can also function as a
barrier electrode such as the X-electrode 72 of FIG. 3. For the
much less complex DC type PDP structures, which for the most part
employ a simple double substrate, orthogonally configured,
columnar-electrode structure, a low profile (i.e., 2-dimensional)
"flat" dielectric pattern can be used to confine the discharge to
the local cell site. Accordingly, for the columnar-electrode PPS
shown in FIG. 2, in one embodiment, a thick-film dielectric layer
is screen-printed over the metal electrodes on the back substrate,
with an "open" window area as shown in FIG. 9 located at the X-Y
electrode intersection of each pixel site. In this embodiment the
open window in the dielectric insulator layer 154 shown in FIG. 9,
that is patterned on the back substrate 44 of FIG. 2, leaves the
"bare" Y-electrodes 48 exposed to the discharge gas in the
discharge overlap region defined by the corresponding orthogonal
X-electrodes 46 on the front substrate 42.
Vertical Quenching Resistor Structure for Pixel Isolation
[0057] One embodiment uses internal cell series quenching resistors
to achieve pixel isolation. An avalanche can be prevented from
spreading to adjacent cells by using an appropriate electrode line
resistor, for example one resistor for each discharge electrode
line connected to the power. These resistors can vary anywhere from
the k.OMEGA. to M.OMEGA. range, depending on the electrode
structure, device size and capacitance, pixel resolution, gas
composition and pressure, etc. For the more general case in which
each pixel has its own embedded series resistor to limit or quench
the discharge, there are at least two basic embodiments--the
laterally located quenching resistors 30 located in the same plane
as the cell electrodes as shown in FIG. 1, and the vertically
located quenching resistors 182 in FIGS. 11 and 220 in FIG. 14
located immediately beneath the discharge X-electrodes (174 in FIG.
11 and 217 in FIG. 14), and immediately above the bus-bar
X-electrodes (176 in FIG. 11 and 218 in FIG. 14). The more space
efficient embodiment associated with a higher cell fill-factor is
the vertically configured resistor located directly under the
discharge electrode (i.e., cathode), which can be employed for both
the various columnar-discharge and surface-discharge PPS
embodiments. However, in the case of PPPS embodiments, the
described vertically configured resistor under the discharge
electrode is most conveniently employed for the surface-discharge
structure.
[0058] For the embodiment shown in FIG. 11, the segmented top
discharge X-electrode (cathode) 174 is connected to an underlying
contiguous cathode bus-bar X-electrode 176 through the vertical
quenching resistor 182, so that each cell has its own dedicated
series resistor that effectively isolate it from every other cell
connected to the same bus-bar X-electrode line 176. Sensor 160,
referred to as the "segmented cell structure", can be used for both
PPS (e.g., FIG. 11) and PPPS detectors (e.g., FIG. 13). The most
convenient shape in terms of fabrication for the vertical resistive
strip located directly under the segmented discharge electrode is
that the dimensions of the resistive strip 182 exactly replicate
that of the segmented discharge electrode 174 as shown in FIG. 11
as well as FIGS. 12-13. However in the alternative embodiment shown
in FIG. 14, a dielectric insulator layer 219 covers the "buried"
X-electrode bus-bar 218 structure which connects to the segmented
top layer surface-discharge X-electrodes 217 through the vertical
quenching series resistor 220. One advantage of the segmented cell
structure is that the pixel quenching resistor does not take up any
lateral cell space since it is a vertical structure, thus the
discharge cell fill-factor should be extremely high, resulting in
higher device efficiency.
Primary and Secondary Conversion Layers
[0059] The PPS embodiments in FIGS. 1, 11, 12 and 14, with an
appropriate conversion layer material, used for the conductive
layers 22, 172 and 215 respectively, can be configured as an
efficient thermal neutron detector that can rival or even exceed
the performance of .sup.3He based neutron detectors in many
respects, and surpass .sup.3He based detectors in high pixel
resolution and fast response time with time-of-flight (TOF)
capability. The elements Li, B and Gd are all suitable materials
for this conversion layer due to having reasonably abundant
isotopes (e.g., .sup.6Li, .sup.10B, .sup.155Gd and .sup.157Gd) that
undergo efficient thermal neutron capture followed by detectable
particle emission in a PPS device. For the above embodiments, the
efficiency of the PPS as a neutron detector using these materials
can be further enhanced by using either isotopically enriched
mixtures or highly enriched mixtures of these materials (Li, B and
Gd) for the conversion layer. However the preferred element for the
PPS conversion layer for many neutron detector applications is Gd,
with naturally occurring isotopes .sup.155Gd and .sup.157Gd which
have amongst the largest thermal (i.e., 0.025 eV) neutron capture
cross sections of any isotopes, approximately 61,000 and 254,000
barns, respectively. The naturally available metal, in which
.sup.155Gd is 14.8% and .sup.157Gd is 15.7%, has an (n, .gamma.)
cross section of 49,000 barns. These high cross sections, much
higher than that of the popular .sup.3He (n,p) reaction (5300
barn), allow embodiments of a Gd-based PPS detector to have many
important commercial applications, including homeland security.
[0060] The design parameters of a Gd-based detector are derived
from the physics of the capture cross section and the resulting
decay of the daughter nucleus. The optimum conversion layer
thickness will depend on the isotopes chosen, including designing
for the naturally available metal. For example, if pure .sup.157Gd
were to be used, then its cross section of 254,000 barns
corresponds to a mean free path for the neutron of 1.3 .mu.m. To
stop 99% of the neutrons would require a thickness of 6.0 .mu.m,
which can be deposited for example by conventional sputtering on an
ultra-thin, low density, low atomic number, substrate (e.g.,
thin-film polymer or aluminum foil) to minimize scattering losses
from the emitted conversion electrons (see below). For .sup.155Gd
or .sup.natGd, the corresponding 99% neutron absorption thickness
is .about.0.001'', which is within the range of commercially
available Gd-foils. Several embodiments of the PPS detector can
utilize a metal foil electrode that also serves double-duty as the
device front substrate. Two of these embodiments are shown in FIGS.
7 and 16, which are ideal for a Gd-foil based PPS neutron detector,
but could also be used for .sup.6Li and .sup.10B foil based PPS
neutron detectors. For neutron detector 130, shown in FIG. 7, the
Gd-foil electrode 132 would be hermetically sealed directly to a
significantly thicker surface-discharge back substrate 134 that
would provide the required mechanical support. In one embodiment,
the fabrication of the Z-electrodes 142 is optional and may not be
needed. For neutron detector 300, shown in FIG. 16, the Gd-foil
electrode would serve as the ultra-thin front plate 312 and would
hermetically seal to a thin or ultra-thin back plate 318, thus
enabling high transmission of gamma radiation through the device
that could be efficiently detected behind it, and then by timing
coincidence used to help discriminate incident thermal neutrons
from incident gamma radiation. It is noted that for detector 300,
mechanical support for the ultra-thin front and back plates is
provided by sealing the thick front support perimeter plate 302 to
the thick back support perimeter plate 304 as shown in FIG. 16.
Also, as disclosed above for detector 130 in FIG. 7, the
fabrication of the bottom Z-electrodes 311 for detector 300 is
optional and may not be needed.
[0061] In terms of detecting the neutron capture for .sup.157Gd,
the daughter nucleus, .sup.158Gd, decays by emitting a number of
gamma rays that easily escape the foil. However, 59% of the neutron
capture events cause an electron to be emitted from the nucleus,
through internal conversion, with energy around 75 keV. About 7% of
the time, a second electron is emitted with energy of 181 keV. An
electron with 75 keV will lose 12 keV, on average, of energy in
traversing the entire 6 .mu.m foil. If the foil is at the window of
the detector as in FIG. 7, about half the electrons will enter the
sensitive gas volume, for a net efficiency of about 30%. This means
that of all the neutrons stopped in the foil, about 30% can
potentially be detected. This efficiency, however, can be
approximately doubled for the embodiment shown in FIG. 8 of a
double surface-discharge PPS detector 150 with a central conversion
layer 149 shared between a top and bottom set of
parallel/rectilinear surface-discharge X and Y-electrodes 139. Thus
for PPS detector 150, the .sup.157Gd-foil conversion layer 149
efficiency can essentially be doubled to nearly 60% when located
between the two surface-discharge PPS back plates so as to detect
electrons emitted into the gas volume from the Gd-foil in all
directions. These values compare very favorably with .sup.3He
detectors, which can be as efficient as 70%. Additional embodiments
could employ multiple layers of PPS detectors utilizing thinner
Gd-foil to increase the detection efficiency above 60% by detecting
in addition to the conversion electrons, other fast electrons
between 29 and 79 keV produced by Auger transitions. For detector
150, the Z-electrodes 142 located on both the front and back
substrates are application specific and therefore optional.
[0062] Regarding the response of the above detector embodiments, Gd
is an excellent detector for thermal neutrons, far more efficient
than .sup.3He (n,p). Yet for energies above 1 eV, the Gd neutron
cross section falls below .sup.3He. Nevertheless, the use of Gd
near and below thermal energies should be an excellent replacement
for .sup.3He for many critical applications. It is noted that a
hydrogenous moderator such as paraffin or polyethylene will be
utilized to convert the incident fast neutrons to slow thermal
neutrons for maximum detection efficiency. The disclosed Gd-foil
based PPS neutron detectors, require detecting the fast conversion
electrons emitted by the Gd in the PPS discharge gas, which is
symmetrical to detecting a positively charged muon in the PPS.
Careful system design including critical materials selection will
result in a high efficiency system with high gamma discrimination.
Embodiments of The PPS detector, because of its extremely fast
response time, could be used to advantage with active interrogation
techniques to reduce background signals by coupling the
interrogation pulse to the PPS detector response so as to record
only the coincidence readings. In addition, with a pixel pitch of
50-100 .mu.m, its very high pixelation granularity allows
discrimination of different size discharge cluster patterns from
different incident source radiations. These capabilities are
further aided by the addition of a gamma filter such as lead in
front of the detector should it prove necessary.
[0063] In one embodiment for the detector shown in FIG. 8, in which
the discharge gas 136 is at a significant positive pressure (e.g.,
10 atmospheres), then the front substrate 148 and back substrate
134 would need to be thick enough and strong enough to contain this
pressure without significant physical/dimensional distortion. For
this detector embodiment, the front and back substrates could be a
heavy metal such as steel which would also have the benefit of
being a partial gamma filter. Given such a metal substrate, a
dielectric insulator layer (e.g., enamel on steel) would be coated
on the inside surface facing the gas on which the X and
Y-electrodes would then be fabricated.
[0064] Materials selection for device fabrication will utilize a
low neutron absorption cross-section conductor with low background
emission such as copper (or Ni or Cr), with a flash coating of a
sputter resistant low neutron absorption cross-section metal such
as Mo, Ni or Cr for the device electrodes, in combination with a
low neutron absorption and low background emission substrate such
as fused silica or possibly an alumina, zirconia (i.e., MgO or
yttria stabilized zirconia--YSZ) or a magnesia based ceramic with
low alkali content (e.g., cordierite or mullite).
[0065] The disclosed Gd-based PPS neutron detector embodiments can
utilize thin-film deposited Gd coatings from several microns thick,
to thin foils in the range of 0.001'', with a pixel pitch from
about 0.1 mm to 10 mm, an external thick polyethylene or other such
hydrogenous moderator, and a gas discharge mixture composition
including gas components such as Ne, Ar, Kr, Xe, N.sub.2, Hg,
CO.sub.2, CH.sub.4, C.sub.2H.sub.6, CF.sub.4 and C.sub.2F.sub.6,
with gas pressures over the range from about 0.1 to 10 atmospheres.
Other embodiments not shown include the same primary Gd conversion
material as described above, but with the addition of a thin-film
(e.g., less than 1 .mu.m) of high secondary electron emitter
material coated on top of the Gd primary conversion layer to
generate a cluster of slower secondary electrons, from each Gd
primary conversion electron, into the discharge gas volume that can
more easily be detected and thereby further enhance the detector
efficiency. Examples of secondary electron emitter thin film
coatings for this application include: CsI, MgO, BaO,
La.sub.2O.sub.3, Eu.sub.2O.sub.3, LiF, CaF.sub.2 and BaF.sub.2. The
Gd-based PPS neutron detector is suitable for both passive and
active interrogation applications and could be used to detect
special nuclear materials (SNM) such as plutonium.
[0066] The PPS detector shown in FIG. 8 and disclosed above can
also be constructed to be a high efficiency PPPS detector. In one
such embodiment, the conversion layer 149 would consist of a very
thin photocathode layer deposited on both the top and bottom
surfaces of an optically-transparent substrate. The dual
photocathode conversion layer 149 would therefore be emitting
photoelectrons on both surfaces towards both sets of
surface-discharge electrodes 139. For this dual surface
photocathodic coating, the top photocathode layer would be
functioning primarily as a reflective photocathode, whereas the
bottom layer would be functioning primarily as a transmissive
photocathode. For all PPPS embodiments, the front substrate is
optically transparent and could be made of either glass or fused
silica, although other transparent substrate materials such as
sapphire and MgF.sub.2 could also be employed. For maximum PPPS
efficiency, the front substrate X, Y and Z electrodes should also
be transparent and so would employ a transparent conductor such as
ITO or SnO.sub.2.
Cherenkov Detector
[0067] PPPS embodiments disclosed above can be readily configured
as Cherenkov detectors by careful selection of the PPPS
photocathode to the selected Cherenkov radiator, and optically
coupling the two components together. However a more efficient
Cherenkov detector can be fabricated by effectively making the
Cherenkov radiator the PPPS front substrate window--that is
integrating the two components into a single PPPS Cherenkov
Detector. In this combined device, disclosed in FIG. 13, the PPPS
front substrate window will as a result be much thicker. However,
the choice of window material for the Cherenkov radiator will be
largely governed by the choice of photocathode. For example, if a
robust VUV photocathode such as CsI is chosen, then the PPPS front
substrate window/Cherenkov radiator will likely be MgF.sub.2 or
possibly CaF.sub.2, but the radiator could also be sapphire or
fused silica if the emitted photons are in the VUV region closer to
the UV. If on the other hand the PPPS photocathode is a longer
wavelength UV or blue sensitive material, then the PPPS front
substrate/Cherenkov radiator can be either fused silica or a highly
blue transmissive glass.
Vertical Electrode Structures
[0068] In order to enhance the drift field effectiveness and more
efficiently channel the radiation generated free-electrons to the
sense anode (Y-electrode) and the ions to the discharge cathode
(X-electrode), in one embodiment the drift field can be custom
configured by use of vertical electrodes that extend well up into
the drift field region. Such vertical electrodes structures have
been demonstrated in PDP's developed for TV-sets and are
generically known as barrier electrodes. A variety of vertical
electrode or barrier electrode shapes and cross sections have thus
been fabricated by well-established commercial processes, such as
direct thick-film printing, or thick-film coating followed by
pattern etching or pattern sandblasting, etc. These techniques also
allow multiple layers of different conductivities or resistivities
to be fabricated, including alternate layers of conductors,
resistors and insulators so that the vertical electrode can have
different layers biased at different potentials to optimize the
drift field shape to the application, such as shown in FIG. 6.
Alternatively, a single vertical conductive barrier can be
thick-film fabricated or a vertical dielectric barrier fabricated
and then thin-film coated with a conductor to make it into a
vertical electrode. Finally, the vertical electrode can be
configured to extend either partially into the drift field (as
shown in FIGS. 4 and 5) or all the way up to the front cover plate.
In the latter case, the vertical electrode in one embodiment has a
top insulator layer (see FIG. 3) so that the vertical electrode
does not electrically short out the drift electrode in the case of
a PPS, or electrically short out the photocathode in the case of a
PPPS. However in one embodiment, the drift electrode or
photocathode is at the same potential as the vertical cathode, so
making electrical contact between the vertical electrode and the
drift electrode or photocathode in this case is acceptable.
Ground Planes and Shielded Electrode Traces
[0069] Since embodiments of the PPS and PPPS are very fast response
detectors with some applications requiring subnanosecond and
picosecond scale temporal resolution, such as for time-of-flight
(TOF) detection and Cherenkov detectors, the circuit layout for
these PPS and PPPS devices can take advantage of standard industry
practice by incorporating the addition of appropriately located
ground planes and the aggressive shielding of electrode lines,
traces, wiring, ribbon cabling and connectors and other components
wherever feasible.
Electronic Circuitry and Readout
[0070] Embodiments of PPS and PPPS devices operate as
highly-pixelated digital radiation detectors by flashing "ON" each
pixel (which is normally "OFF") as a direct consequence of a gas
discharge avalanche stimulated within the cell by incoming
radiation, and so at their most basic level functionally behave as
digital radiation counters and not as proportional counters. Each
such gas discharge pulse is counted as having an approximately
equal value and is therefore counted by the circuit as simply an
individual event. The amount of detected radiation is thus based on
how many individual gas discharge events are outputted from the
pixels. The electronic readout circuitry is thus designed to detect
if and when a gas discharge pulse is outputted from the pixel--i.e.
when a pixel has turned "ON". In order to maximize the temporal
resolution, the readout circuitry preserves the cell discharge
output pulse rise time.
[0071] On both an operational and functional basis, all such
radiation induced pixel discharges begin (i.e., are turned "ON") by
initially maintaining the panel at a voltage just below its
spontaneous discharge setting, such that any free-electron upon
entering the gas can "immediately" set off a discharge (at the
nearest pixel site) that can very quickly grow into a localized
high-gain avalanche. The rise time for such fast discharges can
typically vary from a few nanoseconds to a few picoseconds,
depending upon the effective cell capacitance, which includes the
contributions from the matrix of capacitively coupled surrounding
cells. The previously disclosed embodiments include a variety of
electrode structures and material choices including special gas
mixtures and physical barriers to optimize performance and localize
the discharge including minimizing plasma generated interference
from photons, electrons, ions and metastables.
[0072] In one embodiment, the pixel sensing mechanism includes
digital (i.e., photon/particle counting) acquisition electronics to
store time-tagged pixel discharge information and correlated X-Y or
X-Y-Z events. Recording of X-Y or X-Y-Z positions and histogramming
counting rates versus positional locations are implemented in one
embodiment by field programmable gate array (FPGA) logic devices.
FIG. 15 is a block diagram of readout electronics coupled to a PPS
or PPPS cell matrix 282 in accordance with one embodiment. The
readout electronics includes a discriminator 284 to condition the
signals for FPGA processors 286. Since the PPS and PPPS signal
integration is inherently digital, with an electron gain on the
order of 10.sup.6 or greater for a cell pitch of 0.10 mm, ND
converters and/or amplification electronics are not required for
most of the applications for which these detectors are designed.
For lower resolution panels with a larger pitch, or for panels
having a large effective cell capacitance, the electron gain will
be greater than 10.sup.6. Also the duty cycle should be a few
orders-of-magnitude less than for PDP video monitors which are AC
devices with high capacitance, thus the DC type PPS and PPPS
detectors described here have low capacitance will consequently
have very low power consumption.
[0073] In a number of embodiments, the detector can operate
synchronously in a clocked mode with timing intervals that can be
less than a nanosecond. The cells of the detector can provide a
single digital bit for each time interval. Timing at this rate is
within the capability of a typical FPGA which is well suited and
has the flexibility to examine the temporal and positional
information in the bit streams. The speed of these embodiments also
makes possible their use as trigger elements in an accelerator when
adjacent layers are combined to define a trajectory compatible with
particles of interest. This usually represents the selection of
tracks of near normal incidence to the plane of the detector. Less
interesting tracks are bent away from normal incidence by
magnets.
Fast-Timing Readout Electronics
[0074] Embodiments disclosed herein may include readout electronics
to determine the number of cells fired and the mean location of a
"hit" within the detector. The electronics also measure the time
between an "external" event and the discharge of a pixel or a close
grouping of pixels. Individual pixel discharge rise times (e.g.,
10% to 90%) for PPS and PPPS embodiments configured to have the
minimum effective cell capacitance may be on the order of a few
picoseconds at the low end, but could also be in the upper
subnanosecond range depending upon the detector resolution,
materials and device structure. However most "meaningful" discharge
events generally consist of the superposition of many pixels
firing, so the time resolution readout for that superposition of
pulses is longer, but can still be better than 100 ps.
[0075] In a PPS or PPPS embodiment, an event can occur when a
particle or photon entering the device creates ionization in the
gas volume. The extremely high field generated across the discharge
electrodes (i.e., X- and Y-electrodes), on the order of 10 MV/m,
results in fast electron drift velocities across the small
discharge gap thus producing the fast rise times associated with
these devices. The huge multiplication effect in the gas, with a
gain on the order of 10.sup.6, dramatically increases the size of
the initial ionization signal. For the PPS/PPPS embodiments shown
in FIGS. 1, 7-8, 11-14 and 16 there are three distinct sets of
electrodes which combine to form a pixel site. They are the
Y-electrode (anode), the X-electrode (cathode), and the orthogonal
Z-electrode which is located beneath the X-Y electrode pair and
isolated by either a thin dielectric substrate or a dielectric
layer. When triggered by an ionization event in the gas, the high
field associated with the X- and Y-electrodes generates a plasma
avalanche in the shape of a surface-discharge with characteristics
somewhat similar to the surface-discharge created at each pixel
site in a PDP-TV. However, in the PPS and PPPS embodiments, the
discharge is much faster primarily because of the much lower pixel
capacitance of the discharge cell (a DC device), as compared to the
PDP (an AC device), but also due to the higher field and different
gas mixtures. The signals from all three electrodes thus generate
position, pixel count and timing information.
[0076] A given pair of surface discharge X- and Y-electrodes is
generally shared by many pixels. The output signal pulses will be
an aggregate of the charge deposited by multiple discharge events
in the pixels. Either anode or cathode signals can be used to
generate a trigger for events in the PPS or PPPS embodiments.
However the component of the signal which is generated by the
movement of electrons to the Y-electrode/anode (sense) will
typically be very fast, while the component of the signal that is
generated by the movement of positive ions to the
X-electrode/cathode (discharge electrode) will be slow. The total
quantity of charge generated is due to electron multiplication, and
the time to collect this charge on the anode/cathode pair will
determine the duration of the pulse. With a single emitted
free-electron experiencing a gain on the order of 10.sup.6, the
fast component of the generated charge, the electrons, reaching the
anode will thus generate a large current spike due to the very
short rise time associated with this electron charge migration. The
much longer fall time associated with the same pixel discharge is
primarily due to the secondary current component arising from the
slow ion drift to the cathode. As a general rule the ion drift
component tends to be about two orders-of-magnitude slower than the
electron drift. Thus a 10 ps rise time might be associated with a 1
ns fall time. The large current pulse per pixel discharge event
associated with the high gain and short rise time, multiplied by
the number of such events occurring within a period of perhaps a
few nanoseconds in different pixels on a given electrode, will
produce a correspondingly larger output pulse.
[0077] To collect tracking location/positional information of
activated (i.e., discharging) pixels, each electrode in one
embodiment includes a current-limiting impedance, typically a
"quenching resistor", to prevent run-away discharges and to
determine the location of each discharge event by measuring the
electrode voltage drop. When a discharge occurs, it produces a
rapid change in voltage on the X- and Y-electrodes. The sense of
the change on the cathode will be the opposite to the sense of the
change on the anode. The amplitude of the discharge (i.e., current)
can be limited as necessary, either by adding a quenching agent to
the gas or by the use of a series quenching impedance. To determine
the precise pixel discharge location in two-dimensions, a third,
orthogonal strip electrode can be fabricated either under the X-
and Y-electrodes (i.e., electrically isolated by a dielectric
insulator layer as shown in FIG. 14) or on the PPS or PPPS bottom
substrate backside as shown in 1, 7-8, 11-13 and 16 (assuming that
the bottom substrate is of a dielectric insulator material and no
more than a few millimeters thick). This third electrode (i.e.,
Z-electrode), which runs orthogonal to the discharging X- and
Y-electrodes, serves a function similar to the "back strip"
electrodes in a Cathode Strip Detector or large area MicroStrip Gas
Chamber (MSGC) detector, that is to fix the discharge position in
the orthogonal direction. For a given discharge event, the rapid
change in the associated discharge electrode voltage will be
capacitively coupled into the Z-electrode producing a somewhat
similar voltage pulse. The magnitude and shape of this pulse will
be determined by the capacitance between the X- and Z-electrodes,
the Y- and Z-electrodes, and the other impedances attached to the
Z-electrode. Simple field calculations assuming that the dielectric
is glass suggest that the induced signal could be 10% to 30% that
of the anode. The output signal from the Z-electrode will thus be
smaller than the output from the X-and Y-electrodes, but should
still be significant and can therefore be used to obtain the
positional location. In one embodiment, the Z-electrode in the
above referenced cathode strip detector has a pulse height about
one-quarter that of the Y-electrode (anode) and a position
resolution of better than 100 .mu.m. In terms of the Z-electrode
circuit, a charge division network can be connected to a group of
Z-electrodes to form a segment. In order to maximize the capacitive
coupling between the X- and Z-electrode, in one embodiment the
thickness of the dielectric which separates them is minimized, but
not so much as to incur dielectric breakdown. Finally, instead of
an external electrode line impedance as shown for the
columnar-discharge embodiment of FIG. 2, an internal impedance
element (e.g., resistor) can be patterned into each pixel to reduce
the discharge dead-time and to allow each pixel to operate
independently of other pixels on the same electrode line. The use
of embedded pixel resistors as shown in FIGS. 1 and 11-14 also
helps control avalanche spreading, and for a number of PPS and PPPS
detector embodiments the use of embedded pixel resistors are all
that is needed to achieve avalanche cell localization. Although not
shown, various embodiments of FIGS. 7, 8 and 16 also include
embedded pixel resistors similar to those in FIGS. 1 and 11-14.
[0078] The quantity of charge produced by a single discharge event
in an embodiment having a gain of 10.sup.6 will be
1.6.times.10.sup.-13 coulombs, and for a high resolution PPS or
PPPS embodiment this charge could be generated and move across the
discharge gap in a time period on the order of a nanosecond (or
faster) thus corresponding to approximately 0.1 ma of current. The
effect of multiple discharges (pixel hits) on a single electrode
will produce a correspondingly larger output pulse, as will a
higher effective pixel capacitance corresponding to a higher gain.
The output from many Y-electrodes can be combined using a charge
division network. The pulse signals generated by the charge
division networks can be processed and used to estimate arrival
time, energy, and location of the incident particle. The voltage
pulse output from the Z-electrodes can also be combined using a
charge division network. The Z-electrode output will be smaller but
sufficient to estimate the second position coordinate of the
incident particle.
Applications
[0079] In some embodiments, the combination of very high spatial
resolution along with time-of-flight (TOF) capability in the range
of 100 ps, coupled with insensitivity to magnetic fields and
radiation hardness, has the potential to yield order-of-magnitude
advances in a multitude of applications, such as detectors for
radioactive ion beam (RIB) accelerators, beam monitors for RIB
profile and target diagnostics, high energy neutron research
including neutron calorimeters, focal plane tracking detectors,
gamma-ray tracking detectors (e.g., Compton telescopes), minimum
ionizing particle (MIP) detectors (e.g., muons, etc.) such as for
the planned upgrade of the Large Hadron Collider (LHC) at CERN,
neutron and gamma-ray detectors with high discrimination capability
for active interrogation in homeland security, passive neutron and
gamma-ray detectors for homeland security, improved hadron particle
beam therapy for cancer/tumor irradiation (e.g., with protons,
carbon ions and neon ions) via better accelerator beam control and
real-time beam measurement using PPS Active Pixel Beam Monitors,
numerous gamma-ray detectors for improved nuclear medicine imaging
modalities including PET/CT, PET/MRI, SPECT, SPECT/MRI,
multislice-CT, computed tomography angiography (CTA), scintillation
mammography, bright-field and dark-field X-rays, etc. The various
detector embodiments, structures, materials, configurations and
circuitry shown in FIGS. 1-16 are directed towards addressing the
above described applications.
[0080] Many of the PPS and PPPS embodiments have properties that
make them particularly suitable for high energy and nuclear physics
research. For example, both the PPS and PPPS devices offer
extraordinary pixelation, allowing precise position measurements.
They operate at very high gains in each pixel, so they can trigger
under very low incident flux. The pixel discharge has a very rapid
rise time and thus can provide an excellent timing detector. This
combination of properties for example makes the PPPS an attractive
candidate for a Cherenkov detector, as disclosed above and in the
embodiment shown in FIG. 13. Further, most of the embodiments
described are relatively inexpensive to manufacture for large
areas. The projected costs for covering 1 m.sup.2 with PPS devices
manufactured in reasonable volumes can be comparable to the cost of
a few 2-inch diameter photomultiplier tubes.
[0081] In embodiments concentrating on charged particle breakup
reactions, better position resolution should permit embodiments to
be placed closer to the target and cover more solid-angle, reducing
the time needed for already difficult measurements. The PPPS
embodiments in these applications would need to cover areas on the
order of about 1 m.sup.2. A time resolution of .about.100 ps is
definitely within the capability of the previously disclosed PPPS
readout electronics. This time resolution capability is in addition
to the same circuit being able to read out both the incident
particle energy and its positional "hit" location (for each event)
within the scintillator, the latter with sub-millimeter resolution
(depending of course upon the choice of scintillator material and
thickness). Achieving 100 ps time resolution also enables
time-of-flight measurements that could help discriminate neutrons
and other particles from photons for a variety of applications.
Such capability is also enabling for time-of-flight temporal
filtering and image enhancement for various medical imaging
modalities, including applications such as time-of-flight PET/CT
imaging systems.
Low-Energy Radioactive Ion Beam (RIB) Profile Diagnostics
[0082] Intensity profiles and emittance analyses are among the most
critical tools used for optimizing beam transport through
accelerators. Embodiments of the PPS are highly position and
intensity sensitive (intensity via number of cells firing
repeatedly). In detecting charged particles for RIB-profile
diagnostics, the embodiments do not require any type of converter
for the range of beam energies of prime interest for most
low-energy studies. Charged particles passing through the active
gas volume of the device will create free-electrons by collisions
with the gas atoms. The probability of interaction at a given ion
velocity increases significantly with atomic number, and so there
would be a large amount of interaction at 100 MeV/nucleon for
.sup.124Sn or .sup.238U. One problem at very low beam energies is
getting the ion through the front substrate without losing a large
amount of its energy. In order to accomplish this, one embodiment
is a PPS with an ultra-thin foil front substrate, which if needed
can be strengthened by means of an external wire matrix
reinforcement grid. The basic design for such a PPS Active Pixel
Beam Profile Monitor to be used for beam diagnostics at low
energies is disclosed in FIG. 7.
Active Pixel Beam Monitors for Particle Beam Diagnostics
[0083] Embodiments can provide real-time diagnostics with very low
intensity particle beams produced by radioactive ion beam (RIB)
facilities. Embodiments can be used for beam imaging and counting
detectors (i.e., beam position monitors) that can be dropped into
the beam for a "destructive" measurement--that is, one in which the
beam is stopped. By constructing a PPS embodiment using an
extremely thin (e.g. .about.3 .mu.m) front substrate (e.g., sol-gel
formed alumina, etched ceramics such as Si.sub.3N.sub.4, etched
glass, metal foil or an ultra-thin superalloy foil), the PPS could
be used for example at RIB energies as low as 1 MeV per nucleon for
essentially any element. The exact substrate thickness can be
estimated for the beams and energies to be used, but ultra-thin
foils can be employed for most beams, especially in larger
diameters, if supported by a proper external wire reinforcement
matrix arrangement. In a low energy RIB accelerator, even a 25
.mu.m substrate/window of plastic will stop the lower energy
heavier ions, but a 3 to 6 .mu.m thick, superalloy foil such as
Arnavar.TM. provides an excellent window/substrate. More
specifically, for the described application in one embodiment the
PPS active area need only be about 1.0 to 1.5 cm in diameter. It
thus follows that a 6 .mu.m Arnavar.TM. foil window placed in
tension by hermetically sealing it to a lower expansion, relatively
thick (e.g., 3 mm) glass substrate, such as fused silica or a
non-alkali borosilicate type glass, functions in a vacuum
accelerator environment without significant physical distortion
(i.e., "bowing out" of the foil due to the positive internal gas
pressure in relation to the "outside" vacuum) and hence without the
need for external wire reinforcement, as shown in FIG. 7. By
purposely coupling (i.e., sealing) the above foil material to a
much lower expansion coefficient "thick" back-plate substrate, the
superalloy foil can be forced to be in a condition of "permanent"
tension, thus giving it enhanced mechanical strength to resist
further stretching caused by the positive internal gas pressure.
Larger active area PPS embodiments, however, would likely require
external wire reinforcement.
[0084] For the above described PPS beam monitor embodiment, in one
embodiment the materials and fabrication processing would be as
follows. FIG. 7 is a cross-sectional view of a PPS beam monitor in
accordance with one embodiment. Arnavar.TM. foil (commercially
available and tested with a thickness of 6 .mu.m) having a linear
expansion coefficient of 125.times.10.sup.-7/.degree. C. would be
sealed to a "thick" borosilicate glass substrate (e.g., .about.3
mm) having a linear expansion coefficient of
32.times.10.sup.-7/.degree. C. The Arnavar.TM. foil would thus be
put in tension with respect to the lower expansion glass substrate
after both materials are sealed at high temperature--i.e., as both
materials cool the Arnavar.TM. foil wants to contract faster than
the borosilicate glass but is restrained from doing so by the
strength of the glass seal. For small area devices at about a half
atmosphere of internal pressure (e.g., up to .about.1.5 cm in
diagonal), the Arnavar.TM. foil will resist distortion when placed
in an external vacuum environment. For maximum seal strength, a
crystallizing solder-glass frit seal should be employed. If more
tension is needed for the Arnavar.TM. foil, then instead of a
borosilicate substrate, a lower expansion and high temperature
sealing substrate can be used such as fused silica which has an
expansion coefficient of 6.times.10.sup.-7/.degree. C.
Focal Plane Detectors at Intermediate Energies
[0085] At intermediate energies, the PPS embodiments can be used as
a very high-resolution, fast position detector for many
applications. At accelerators such as the National Superconducting
Cyclotron Laboratory (NSCL), fast beams are often characterized by
"tracking", i.e. by determining their positions at various points
before and after reactions. The detectors used in these
applications typically range from 10 cm.sup.2 in the beam analysis
lines, to 0.25 m.sup.2 in the focal plane of magnetic
spectrometers. This tracking is currently done with relatively
slow, fragile gas discharge detectors with gas pressures of about
140 torr. These have position resolutions on the order of 1 mm or
less, but are restricted to low count rates by the slow charge
collection time in larger detectors. A PPS embodiment made with
thin entrance and exit windows could replace these fragile
detectors with a more robust and much faster detector with
comparable or better position resolution. Ultra-thin front
substrate alternatives exist for viable PPS devices such as 3 to 6
.mu.m Arnavar.TM. foil, as well as an ultra-thin "fused" sapphire
type material that might also be available in a thickness of
.about.3 .mu.m. All these detectors operate in vacuum and therefore
depending upon the PPS internal gas pressure and internal spacer
bonding technology, might very well require some form of external
wire reinforcement or support structure. Embodiments such as that
shown in FIG. 16 should be able to transmit the particles with only
a few percent energy-loss. To meet this requirement, embodiments
should be constructed using an ultra-thin metal foil or ceramic
front substrate (window) as described above. For example, a 100
MeV/nucleon .sup.132Sn ion loses less than 1% of its energy in
traversing a 6 .mu.m titanium-foil, and less than 2% of its total
energy traversing a detector made with this Ti-foil window, with Xe
gas at 140 torr, and a 50 .mu.m glass substrate for the
surface-discharge electrodes.
Active Pixel Beam Detectors for Hadron Beam Therapy
[0086] The ultra-thin, low mass, PPS detectors embodiments, as
shown for example in FIGS. 7 and 16, also have the requisite
characteristics needed for the next generation of particle
detectors for cancer treatment by hadron beam therapy of tumors. In
particular, the proton, carbon and neon ion beams presently being
used on these tumors require active pixel detection without
significantly altering the incident beam energy, intensity,
direction, focus or collimation, and most importantly without
introducing significant particle/photon scattering to the
patient.
Other Embodiments
[0087] For many applications, a particular ionizing radiation
detector system could benefit significantly by a vertical stacking
of the PPS and/or PPPS detector embodiments described herein, and
such detector arrangements and configurations would be obvious to
anyone skilled in the art of designing such detection systems. As
one example, the vertical stacking of PPPS Cherenkov detectors
and/or PPS particle detectors, or combinations of such PPPS and PPS
detectors (e.g., embodiments shown in FIGS. 1, 7, 11-14), would
provide significantly enhanced particle trajectory tracking and
timing (e.g., time-of-flight) information as well as providing a
means to filter or separate MIP induced discharges from a Cherenkov
photon signal.
[0088] A number of embodiments are specifically illustrated and/or
described herein. However, it will be appreciated that
modifications and variations of the disclosed embodiments are
covered by the above teachings and within the purview of the
appended claims without departing from the spirit and intended
scope of the invention.
* * * * *