U.S. patent application number 12/692448 was filed with the patent office on 2011-09-01 for apparatus and method for detecting high-energy radiation.
This patent application is currently assigned to Radiation Watch Limited. Invention is credited to Michael Anderson, Christopher Boyce, Wayne Cranwell, William Croydon, Paul Downes, Trevor McAlister, David Prendergast, Zhuo Zhang.
Application Number | 20110210262 12/692448 |
Document ID | / |
Family ID | 44504808 |
Filed Date | 2011-09-01 |
United States Patent
Application |
20110210262 |
Kind Code |
A1 |
Prendergast; David ; et
al. |
September 1, 2011 |
APPARATUS AND METHOD FOR DETECTING HIGH-ENERGY RADIATION
Abstract
An apparatus for spatially correcting an image frame is
disclosed. In some embodiments, the apparatus stores a frame of
pixel values and scans a multi-pixel correction window across the
frame. Spatial correction is performed on pixels within the window
at correction positions during the scan. The spatial correction
comprises estimating pixel values at value estimation positions
based on one or more pixel values within the window for pixels
satisfying a logical condition. The value estimation positions
correspond to pixel values which do not fall within the window
again during the scan. Further disclosed is an apparatus for
detecting high-energy radiation, in which integration circuitry is
used for integrating charge responsive to radiation photon
interaction events. The circuits are controllable in accordance
with an exposure control signal to vary an exposure window duration
according to an operating parameter of the apparatus.
Inventors: |
Prendergast; David; (East
Cowes, GB) ; Anderson; Michael; (East Cowes, GB)
; Downes; Paul; (Newport, GB) ; Croydon;
William; (Queen Bowen, GB) ; Boyce; Christopher;
(East Cowes, GB) ; Cranwell; Wayne; (Cowes,
GB) ; McAlister; Trevor; (Ryde, GB) ; Zhang;
Zhuo; (Gloucester, GB) |
Assignee: |
Radiation Watch Limited
East Cowes
GB
|
Family ID: |
44504808 |
Appl. No.: |
12/692448 |
Filed: |
January 22, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12304755 |
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PCT/GB2007/002152 |
Jun 12, 2007 |
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12692448 |
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Current U.S.
Class: |
250/394 |
Current CPC
Class: |
G01T 1/247 20130101;
G01T 1/246 20130101; G01T 1/24 20130101; G01K 17/00 20130101 |
Class at
Publication: |
250/394 |
International
Class: |
G01T 1/16 20060101
G01T001/16 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 12, 2006 |
GB |
0611621.4 |
Jun 12, 2006 |
GB |
0611623.0 |
Claims
1-51. (canceled)
52. An apparatus for detecting high-energy radiation, the apparatus
comprising: a detector substrate configured to generate charge in
response to a radiation photon interaction event induced by
high-energy incident radiation, said detector substrate comprising
an array of high-energy radiation sense volumes; and a circuit
substrate supporting an array of read-out circuits corresponding to
said array of sense volumes and operative to collect charge from
corresponding sense volumes, each of said read-out circuits
including charge integration circuitry configured to integrate the
collected charge, wherein said read-out circuits are automatically
controllable in accordance with an exposure control signal to vary
an exposure window duration according to an operating parameter.
Description
FIELD
[0001] The present invention relates to a high-energy radiation
detector apparatus and method of operation. In particular, but not
exclusively to high-energy radiation detector apparatus configured
to adjust an exposure window duration of the apparatus dependent on
one or more operating parameters of the apparatus, and also to
apparatus configured to carry out spatial correction of an image
frame.
BACKGROUND
[0002] In general ionising radiation is considered to be radiation
within the energy range 5 KeV to 6 MeV and includes gamma rays,
x-rays, beta-rays, alpha-rays and neutron beams. Devices for
detecting ionising radiation are well-known for radiological
protection and metrology, such as in health or nuclear physics as
well as national/homeland security and anti-terrorist applications.
The devices are one of two types, either passive detectors or
electronic-based active detectors.
[0003] Passive detection systems use film (film-badges),
thermo-luminescent detection (TLD) or photochromatic technologies
(PC) as detector materials. Common to these detector technologies
is that they register the presence of ionising radiation by a
change of state. For example, a film exposed to ionising radiation
goes dark when developed, TLD materials emit light when heated
having previously been exposed to ionising radiation and PC
materials change colour when irradiated with ionising radiation.
However, the change of state of these materials requires special
processing in order to be determined, for example developing the
film or heating the TLD material. Consequently, only an historic
monitoring and evaluation of radiation exposure can be obtained. It
is not possible to achieve real-time monitoring and evaluation.
Since no direct real-time monitoring or analysis is possible it is
therefore necessary to infer what type of radiation exposure caused
the change of state. Although such inference can be drawn based on
experience, nevertheless it is not possible to precisely determine
what type of radiation (spectroscopic information) has been sensed
nor an estimate of radiation dose which takes into account such
information. Additionally, known passive detection systems
generally have poor sensitivity to ionising radiation.
[0004] Active detectors may be based upon silicon technology and
generally comprise one, two or three PIN-diodes, each PIN-diode
having a preset threshold level to signal an alarm relating to a
minimum energy level of incident radiation.
[0005] If more than one PIN-diode is used then different threshold
levels may be preset corresponding to different radiation and
energy levels thereby providing crude spectroscopic analysis of
incident radiation. However, silicon has poor sensitivity to
ionising radiation since it does not have a high atomic number (Z),
therefore there is inefficient conversion of the incident radiation
to electric current and devices using such technology suffer from
poor signal to noise ratio.
[0006] Another drawback of known active detectors is that the
electronic signals are generated remote from the detector
substrate, leading to signal losses and signal mis-shaping due to
the impedance of connecting wires and circuitry.
[0007] In order for accurate measurement of absolute values such as
the amount of exposure to incident radiation (dose), the rate of
that exposure (dose rate) and the type of radiation exposure
(radiation isotope) it is important that the charge value collected
by an electrode, e.g. each pixel contact pad, is properly
representative of the energy of the incident radiation giving rise
to a photon-interaction event which generates the charge. However,
The Applicant has appreciated that errors may be introduced into
the measurement of charge by the very nature of the radiation that
is being measured, and not only due to errors and noise in the
detectors themselves.
[0008] One example is where the radiation is incident at the mid
point between two pixels, which is likely, then a photon
interaction event may cause charge to propagate towards two
adjacent pixels in the same frame. This would be recorded as two
separate hits. However, a lower energy hit would be measured than
that which actually occurred because the energy of the event has
been spread across two pixels rather than one. Note, for the
purpose of this description a "hit" means charge collected from a
single photon interaction event, plural hits corresponding to
plural photon interaction events.
[0009] Other examples of causes of error are where: [0010] (a) two
hits in the same pixel in a single frame are recorded. This would
result in the measurement of a higher energy hit than really
occurred since the summation of the two hits within the pixel would
look like a single higher energy hit. [0011] (b) two hits in
adjacent pixels in the same frame are recorded. This is the effect
that "spatial correction" assumes will not happen. Such events are
summed together via "spatial correction". An error condition would
be present in the presence of two actual adjacent photon hits.
Spatial correction would cause a higher energy hit than really
occurred to be measured, since the two hits will be combined as
though they were two interactions from the same energy hit. [0012]
(c) two hits in the same pixel in successive frames cause a problem
as they may be separate hits corresponding to separate interaction
events. For such a situation, fixed pattern noise removal using
frame differencing will result in the difference frame recording
the difference in the hits from successive frames, rather than "hit
minus no-hit".
[0013] Embodiments of the present invention were devised with the
foregoing in mind.
SUMMARY
[0014] Viewed from a first aspect in accordance with the invention
there is provided apparatus for detecting high-energy radiation,
comprising a detector substrate for generating charge responsive to
high-energy incident radiation, the detector substrate being
configured to form an array of high-energy radiation sense volumes;
and a circuit substrate supporting an array of read-out circuits
corresponding to the array of sense volumes and operative to
collect charge from corresponding sense volumes. Each of the
read-out circuits include charge integration circuitry for
integrating charge responsive to a radiation photon interaction
event in a corresponding sense volume. The read-out circuits are
automatically controllable in accordance with an exposure control
signal to vary an exposure window duration dependent on an
operating parameter of said apparatus. Each read-out circuit is
coupled to a corresponding pixel contact pad electrode to which
charge drifts under influence of a bias applied across the detector
substrate, and from which charge is collected by the read-out
circuits during the exposure window.
[0015] Viewed from a second aspect in accordance with the invention
there is provided a method for controlling an exposure window
duration for a high-energy radiation detector including read-out
circuitry for collecting charge generated by a photon interaction
event in a detector substrate. The method comprises automatically
controlling the read-out circuitry to collect charge over an
exposure window having a duration dependent on an operating
parameter of said detector.
[0016] By controlling the timing of the read-out circuits, the
exposure window for which charge is being accumulated for each
pixel may be controlled and the number of photon interaction events
likely to detected during an exposure can be controlled.
[0017] For a given incident radiation flux level, the likelihood of
occurrence of the error inducing events described above in
paragraphs (a), (b) and (c) may be reduced by reducing the exposure
window duration, i.e. the duration over which the charge from each
pixel contact electrode is collected. Furthermore, by reducing the
likelihood of type (b) events, the accuracy of spatial correction
for charge shared events may be improved.
[0018] In a particular embodiment, the exposure control signal is
dependent on the number of photon interaction events detected by
the apparatus in a given time period. For example, a current
exposure time for the apparatus. Thus, the number of photon
interaction events ("hits") recorded for each frame of data may be
controlled and maintained at a substantially constant level
independent of the radiation flux level incident on the detector
substrate. Thus, not only may the source of errors be reduced, but
also the likelihood of charge saturation of circuitry due to high
flux levels may also be reduced.
[0019] Suitably, the exposure control signal is configured to
decrease the exposure window duration for an increase in the number
of photon interaction events detected by the apparatus in the given
time period. Additionally, the exposure window may be increased for
a decrease in the number of photon interaction events detected by
the apparatus, which leads to a substantially constant hit level if
the flux level decreases.
[0020] Optionally or additionally, the exposure control signal is
dependent on the temperature of the detector substrate. This allows
account to be taken of the increase in charge generation for a
photon interaction event of a given energy that occurs with an
increase in temperature of the detector substrate.
[0021] Thus, exceeding the dynamic range of an analogue to digital
converter (ADC) for example due to increased charge generation as a
consequence of increased temperature may be avoided by reducing the
exposure window duration for increases in temperature.
[0022] Suitably, the temperature may be determined by monitoring
dark current leakage in the detector substrate which avoids the
need for a separate temperature sensor and is directly indicative
of the temperature of the detector substrate.
[0023] Typically, the apparatus comprises signal processing
circuitry for configuring the exposure control signal in dependence
on the operating parameter.
[0024] The signal processing circuitry may comprise calibration
data for modifying a charge value derived from a read-out circuit
in correspondence with a change in the temperature of said detector
substrate. The calibration data compensates for changes in the
amount of charge that is generated by the detector substrate
according to temperature in order to maintain a substantially
constant charge value level for a photon interaction event
independent of the temperature of the detector substrate.
[0025] Viewed from a third aspect of the invention there is
provided apparatus for spatially correcting an image frame
comprising an array of pixel values. The apparatus is configured to
store a frame of pixel values and scan a multi-pixel correction
window across the frame of pixel values and carry out spatial
correcting on pixels values within the window at correction
positions during the scan. The spatial correcting comprises
estimating a pixel value at a value estimation position within the
window at a correction position based on one or more pixels values
within the window for pixel values within said window satisfying a
logical condition; and the value estimation position corresponds to
a pixel value in the frame which will not fall within the
correction window again during the scan of the frame.
[0026] Viewed from a fourth aspect of the invention there is
provided a method for spatially correcting an image frame
comprising an array of pixel values. The method comprises storing a
frame of pixel values; scanning a multi-pixel correction window
across the frame of pixel values; and spatially correcting pixels
values within the window at correction positions during the scan.
The spatially correcting comprises estimating a pixel value at a
value estimation position within the window at a correction
position based on one or more pixels values within the window for
pixel values within the window satisfying a logical condition. The
value estimation position corresponds to a pixel value in the frame
which will not fall within the window again during the scan of the
frame.
[0027] Embodiments in accordance with the third and fourth aspect
of the invention address the problem of errors arising in
considering oblique incidence of radiation by assuming that hits
observed in adjacent pixels are caused by a single oblique event.
Values measured from the pixels are combined into one higher value
representing the single event that occurred. This is known as
Spatial Correction and is a mathematical correction based on pixel
to pixel geometry.
[0028] One embodiment is further configured to provide an estimate
of the pixel value at the value estimation position by calculating
the sum of the pixel values of all pixels within the window for the
logical condition being satisfied, and assigning the sum as the
estimate of the pixel value.
[0029] Optionally, an embodiment may be configured to provide an
estimate of pixel value at the value estimation position by
calculating the sum of the pixel values for which a logic condition
is true for logical condition being satisfied.
[0030] Typically, the array of pixels is a rectangular (which
includes square) array which is a common arrangement, but other
arrays such hexagonal or other geometry may be used.
[0031] One embodiment is configured to scan the window across said
frame in a raster pattern which is a common scanning pattern for
image arrays. Suitably, one embodiment is configured to scan in
steps of one pixel which ensures that all pixels are spatially
corrected for.
[0032] Since edge pixels are set to have zero value as their charge
generation characteristics are adversely affected by the slicing of
the detector substrate during manufacture, the correction window
will be able to scan over and correct for all pixel values which
will contribute to the image frame and are not set to zero due to
being at an edge.
[0033] Typically, the pixel values are representative of charge
collected from a high-energy detector substrate. In particular, the
detector substrate is configured to form an array of high-energy
radiation sense volumes, and there is further provided a circuit
substrate supporting an array of read-out circuits including charge
collection electrodes corresponding to the array of sense volumes
and operative to collect charge from corresponding sense volumes,
each of the read-out circuits including charge integration
circuitry for integrating charge responsive to a radiation photon
interaction event in a corresponding sense volume. The pixel values
correspond to charge collected from corresponding sense
volumes.
[0034] The charge collection electrodes form a pixellated charge
collection substrate forming a corresponding pixellated array of
the sense volumes, which can provide a dense high spatial
resolution array of sense volumes.
[0035] In a particularly suitable embodiment for real-time
applications the image frame undergoing spatial correction is
itself modified with spatially corrected pixel values. Thus no
intermediate spatially corrected result frame is required, and the
spatial correction can be carried out as the frame is scanned
because the corrected pixel is not used in further corrections.
This is in contrast to convolution correction which needs use
original pixel values of pixels that have already been corrected,
and therefore the original frame has to remain unchanged during the
correction process, and a separate corrected frame created.
Furthermore, convolutional encoding is computationally more
intensive than logical analysis and not so suitable for
implementation in a Field Programmable Gate Array for example.
[0036] An embodiment in which spatial correction may be performed
within the frame undergoing correction itself may achieve spatially
correction of an image frame within an exposure window duration for
capturing an image frame. Thus, spatial correction may be done in
real-time.
[0037] One embodiment is configured to carry out procedures for:
[0038] fixed pattern noise removal on an image frame and modifying
the image frame to correct for fixed pattern noise; [0039] pixel
gain correction on the noise corrected image frame and modifying
the noise corrected image frame to correct for pixel gain; [0040]
threshold filtering on the pixel gain corrected frame and modifying
the pixel gain corrected frame in accordance with the threshold
filtering; and [0041] spatially correcting the threshold filtered
frame and modifying the threshold filtered frame in accordance with
the spatial correcting; wherein the procedures are carried out
within an exposure window duration for capturing an image
frame.
[0042] A particular embodiment is configured to provide a value
corresponding to the number of non-zero pixel values in the
spatially corrected frame which provides a frame "hit" count.
LIST OF FIGURES
[0043] FIG. 1 is a schematic illustration of a detector substrate
for a detector device in accordance with an embodiment of the
invention;
[0044] FIG. 2 is a schematic illustration of the cross-section of a
detector device in accordance with an embodiment of the
invention;
[0045] FIG. 3 is a schematic illustration of charge collection
circuitry of a detector device in accordance with an embodiment of
the invention;
[0046] FIG. 4 schematically illustrates a detector system
module;
[0047] FIG. 5 is a schematic illustration of a system module for a
detector device in accordance with an embodiment of the
invention;
[0048] FIG. 6 is a schematic illustration of an nth and n+1th frame
store;
[0049] FIG. 7 illustrates a raw data frame and a data frame after
fixed pattern noise removal;
[0050] FIG. 8 schematically illustrates spatial correction in
accordance with an embodiment of the invention;
[0051] FIG. 9 illustrates a PID control mechanism operative for an
embodiment of the invention;
[0052] FIG. 10 graphically illustrates pixel mean values against
frame mean values for different exposure times;
[0053] FIG. 11 is a graph of air kerma against energy; and
[0054] FIG. 12 schematically illustrates stages in an energy
compensation calibration process.
DETAILED DESCRIPTION
[0055] Detector Substrate
[0056] A detector substrate 2 in accordance with an embodiment of
the present invention has a semi-conductor crystal 4 clad on one
surface thereof with a plurality of conductive contact pads 10
which may act as charge collection electrodes and on an opposing
side surface clad with a layer of conductive material for forming a
biasing electrode. The array of contact pads forms a pixellated
surface 7.
[0057] In the illustrated example the semi-conductor crystal 4 is
CdTe but other suitable semi-conductor materials may be used, such
as CZT, Si, GaAs, CdMgTe or a halide-metal compound with a high
atomic number, by way of non-limiting example.
[0058] Each conductive pad 10 is electrically isolated from the
other contact pads. The array of pads 10 forms an array of ionising
radiation sense volumes 12. In the illustrated example an array of
50.times.50 sense volumes is created from the array of conductive
contact pads, each pad having dimensions 100 microns by 100
microns. Typically the contact pads are square, but they could be
any other suitable shape such as triangular, hexagonal or other
polygonal shape or circular, for example.
[0059] The conductive material for the conductive contact pads may
be any suitable material for depositing on a semi-conductor, in
particular a high Z (atomic number) semi-conductor, and may
comprise aluminium (Al), gold (Au), or platinum (Pt) for
example.
[0060] Detector Structure
[0061] A cross-section of a detector device 13 as illustrated in
FIG. 1 comprising a detector substrate 2, as illustrated in FIG. 2,
and semi-conductor circuit substrate 14 is illustrated in FIG. 2.
In use a bias voltage, for example 300 volts (other bias levels may
be used suitable for the detector substrate material in use), is
applied between the arrays 7 of conductive pads 10 and the
conductive layer 6. For example, circuit substrate 14 and contact
pad array 7 may have a reference potential of -150V whilst the
conductive layer 6 may have a reference potential of 150V.
[0062] The applicant has coined the term "voxor" (volume sensor) to
refer to a sense volume comprising the three dimensional energy
collection cell within the detector alone or with the circuit
substrate collection circuitry and one or other meaning may apply
depending on the context in which the term "voxor" is used.
[0063] As illustrated in FIG. 2 ionising radiation 8 incident on
the detector 13 forms an electron-hole pair 18 in a sense volume 12
(referred to herein as a photon interaction event) and the bias
voltage causes the positive and negative charges (holes e+ and
electrons e-) to migrate to contact pads 10 in array 7.
[0064] In the illustrated embodiment the electrically isolating
space between contact pads 10 is filled with a passivation material
20, for example aluminium nitride, to enhance the electrical
separation and isolation of the contact pads 10 from each
other.
[0065] The circuit substrate 14 supports an array of read-out
circuits 16, there being a corresponding number of read-out
circuits 16 on each circuit substrate to the number of sense
volumes 12.
[0066] Each read-out circuit 16 includes a circuit contact 22 for
electrically coupling the read-out circuitry 16 to the detector
substrate 2. A conductive bond 24 couples the detector substrate 2
to circuit substrate 14 to form a hybrid detector 13.
[0067] In the illustrated embodiment, bonding of the detector
substrate 2 to circuit substrate 14 is by way of bump-bonding. The
bump-bonds 24 both mechanically and electrically couple the
detector and circuit substrates together. The mechanical coupling
of the bump-bonding is often augmented by the practice of "under
filling" in such detectors e.g. a low viscosity insulating epoxy
resin is introduced into the space between bumps. The bump-bonds 24
are made of a low temperature solder such as a tin-bismuth mixture,
which is particularly suitable for use with the CdTe detector
material used in the described embodiment, since CdTe (and CdZnTe)
is sensitive to heating and can be damaged if subjected to high
temperatures, for example over 200.degree. C. The chemicals
suitable for growing bumps which fulfill this low temperature
criterion are generally available from industrial sources.
[0068] The read-out substrate in the described embodiment supports
CMOS circuitry and is configured as an ASIC.
[0069] However, embodiments of the invention need not be limited to
CMOS ASICs, but may use other substrate technologies including
printing circuit board (PCB) technologies.
[0070] An advantage of having an array of relatively small
cross-section sense volumes is that "hole trapping" is reduced.
"Hole trapping" is the phrase used to describe the phenomenon of
holes becoming locked in deep levels within the semi-conductor
forbidden band. It is a common problem observed with
semiconductors. The resultant partial charge collection results in
low resolution of the high-energy, e.g. gamma energy, radiation.
According to the small pixel theory (see papers by Barrett et al.
[1] and Eskin et al. [2]) the signal contribution related to
electrons dominates over the contribution from the holes in
detectors having small detector volume cross-section such as
pixellated detectors. This leads to an improvement in energy
resolution with reduction of the aspect ratio of the sensing volume
side to its thickness. The rationale is as follows. Due to hole
trapping and field effects the induced charge relates to the
electron flow from interactions relatively close to the read-out
circuit input (e.g. conductive pad of the detector substrate).
However, the holes flow towards the common negative contact.
Consequently, their cumulative contribution is distributed over a
number of sense volumes, thereby effectively excluding the hole
contribution from a single sense volume signal.
[0071] The net effect is that detectors formed of an array of sense
volumes ("pixellated") will generally provide better energy
resolution than slab based approaches.
[0072] Charge Collection Circuitry
[0073] Turning now to FIG. 3, there is illustrated a schematic
circuit diagram for a read-out circuit 16 in accordance with an
embodiment of the invention. In the described embodiment the
read-out circuit 16 is a CMOS integrated circuit including
capacitance circuitry for integrating charge pulses received from
the direct dose detection radiation detector substrate 2.
[0074] Charge collection is carried out by charge integration
circuitry 30 which includes two capacitances, variable capacitance
Cd 32 and capacitances CpA 34. Also included in the charge
integration circuitry are reset switches 38 and 40 for respectively
discharging capacitances Cd and CpA.
[0075] Switches 44 and 46 may be operated to couple capacitance CpA
to capacitance Cd. Each capacitance includes a capacitative circuit
element which may be a discrete capacitor component or comprise
parasite capacitances of other circuit elements, or a combination
of both discrete and parasitic capacitances, for example. The
capacitances also include resistive circuit elements which may
again be discrete components, parasitics or a combination of both
types of resistance.
[0076] Capacitance Cd is coupled between the circuit contact 22,
which is coupled to the CdTe detector substrate 2 by a bump-bond
24, and the reference potential for the circuit substrate, 14. It
will be evident to the person of ordinary skill that reference
potentials other than those referred to herein may be used
depending upon circuit implementation.
[0077] The charge integration circuitry 30 may be operated by
closing switch 44 to couple Cd and CpA together to form a
capacitance suitable for capturing charge relating to a single
detection event. In the described embodiment the total capacitance
for a combination of Cd and CpA is 150 fF.
[0078] Variable capacitor Cd 32 may be varied to take account of
charge generation of different detector substrates 2 so that a
single circuit substrate 14, may be used for different detector
substrates 2.
[0079] Charge Read-Out
[0080] For the purposes of providing an illustrative example only,
the operation of read-out circuitry 16 will now be described for a
clock rate of 1 MHz and a 50.times.50 (2500) array of read-out
circuits 16. Such operational parameters provide a theoretical
maximum total charge integration time of 2.5 milliseconds per
read-out circuit, although in practice some of this time will be
used in circuit "housekeeping" such as resetting various
capacitances. The operation of read-out circuitry 16 will also be
described for ionising radiation flux densities exposure rates up
to 4 Gy/hr. For the illustrative flux density range, the variable
capacitance Cd is tunable from 50 fF to 200 fF. Evidently, for
other flux ranges the range of capacitance over which Cd may be
varied will be correspondingly modified. The capacitance needs to
be sufficient to allow for the collection of charge resulting from
the photo-electric interaction of a photon interaction event. This
is dependent on the energy of the incident photons, the mass
transfer coefficient for the detector material at this given energy
and the energy required to generate an electron hole pair for the
material.
[0081] The 50 to 200 fF range for capacitance Cd includes detector
substrate parasitic capacitances which for the CdTe based detector
substrate 2 are about 30 to 50 fF. Switches trA, rstA and rstD are
MOSFET transistor switches, but other switch means may be used, for
example other forms of transistor switch. As illustrated, switches
trA switches capacitance CpA into Cd, whilst rstA switches
capacitance CpA to the reference potential for discharging the
capacitances.
[0082] Output buffer 48 is coupled to the output of the charge
integration circuitry 30. Buffer 48 is controllable to output a
signal derived from the capacitance CpA and CpD to an output bus
"line-out" 52.
[0083] The output buffer 48 may comprise simple tri-state buffer
circuitry, although optionally the buffer may also comprise
additional pre-amplification circuitry.
[0084] In one embodiment the output buffer 48 is configured as a
two stage amplifier consisting of a first stage charge amplifier
and a second stage differential amplifier attached to line-out 52,
where the reference for such amplifiers are taken from a reference
dummy read-out circuit, i.e. an unconnected read-out circuit. This
allows the amplification to be made relative to ASIC related offset
conditions e. g. temperature change.
[0085] The output from capacitance CpA is fed to respective charge
amplifiers of two stage amplifiers 48 to produce a pulse suitable
for input on to bus 52. This output then forms the input to the
line based differential operation amplifier of the two stage
amplifier 48, together with a reference input from the read-out
circuit structure unconnected to the detector substrate. The output
from these line amplifiers is then received by analogue to digital
conversion interface circuitry.
[0086] The amplifier 48 is configured to produce pulses having
magnitude or height proportional to the amount of charge collected
in capacitance CpA. The amplifiers have good high frequency
response in order to be able to handle the sharply-peaked pulses
from the capacitances, as well as a high input impedance and linear
response to the pulses.
[0087] The read-out circuitry 16 is operated to provide a charge
capture window for capacitance CpA in which to capture charge
generated by a single detection event in the corresponding sense
volume 12. In the described embodiment the array of charge
circuitry 30 of circuit substrate 14 corresponding to respective
arrays of contact pads 7 is driven to read-out charge in a raster
scan pattern.
[0088] System Module
[0089] The system modules for an example of ionising radiation
detection device 100 incorporating a detector device 13 are
illustrated in FIG. 4. The system module responsible for converting
the incoming radiation into digital signals is shown as "sensor
module" 101. The sensor-module contains the radiation detector 13
(ASIC circuit substrates 14 bonded to the detector substrate 2) and
a Field Programmable Gate Array (FPGA) 104 having the logic
required to provide a control interface for the ASIC. The FPGA also
controls the exposure window for the detector. Charge is collected
on a frame by frame basis from the detector, and the period for
which charge is collected is known as the exposure window. The FPGA
104 sends control signals to the ASIC for controlling the sample
and hold circuitry in the read-out circuitry 16 in accordance with
an exposure window.
[0090] The analogue data received from the ASIC circuit 14 is
converted to digital form via the A/D converter 109 which in the
illustrated embodiment are part of the FPGA 104. Typically, FGPA
104 in addition to containing the interface logic to control the
ASIC, would normally also contain an implementation of a device
calibration algorithm which ensures that the charge values are
directly related to incident photon energy and cumulative exposed
dose respectively. In general terms, the FPGA 104 produces a
digital data output corresponding to "normalised" charge values
detected at the pixel contact pad electrodes 10.
[0091] FPGA 104 also includes a temperature sensor 105, which
provides temperature data corresponding to the temperature of the
detector substrate. Based on the data from the temperature sensor
the FPGA applies different pixel sensitivity correction values. The
FPGA also monitors the dark leakage current (leakage current when
no radiation incident on the detector substrate) and bright leakage
current (leakage current when radiation is incident on the detector
substrate). Based on the values of dark and bright current the
system corrects the final frame dose estimate. This is to correct
for global changes charge collection efficiency which are related
to total current.
[0092] The conversion results are transferred over serial data bus
106 to Memory Access Control (MAC) unit logic 108 on interface
module 107. The MAC logic 108 stores the radiation data in memory
112 and interfaces with the micro-controller 110.
[0093] Micro-controller 110 controls all the elements of the
ionising radiation detection device 100, for example memory
management (102), display (116), communications (118) and user
interface (120). Additionally, micro-controller 110 is configured
by programs stored in EEPROM 114.
[0094] Under control of microcontroller 110 charge values received
into memory 112 are used to form a cumulative normalised spectrum,
incorporating information from the recent past. The spectroscopic
information is used as the basis for isotopic identification. In
the described embodiment two dose calculation modes are used. In
mode 1, the number of photon "hits" may be recorded on frame by
frame basis. The number of hits per frame may be used to indicate
the radiation flux whilst the hits over a number of frames may be
used to calculate an overall radiation dose. This can be either
carried out by the controlling microcontroller or performed off the
device, using external processing power. In mode 2 the corrected
hit data (the data corresponding to a calibrated charge) is
collected on a frame by frame basis. This data is then converted
via a non-linear (adc to dose) calibration curve to provide a per
frame dose. Given the frame exposure time this per frame dose can
then be converted to a cumulative dose or dose rate.
[0095] The data from microcontroller 110 may be transmitted to a
remote location using the communications module 118. The
communications module 118 may be a wire-based communications
module, or a wireless-based communications module, typically for a
local area network where low power radio communication is suitable,
such as Bluetooth. Optionally, wire-based communication may be over
much greater area, and the communications module 118 configured to
comprise a higher power radio unit such as a cellular telephone
transceiver or alternatively be linked to such a device via the
local short distance wireless link.
[0096] The ionising radiation detection device 100 also includes a
user interface 120, for providing user input controls to the device
such as on/off functionality, and options for displaying various
types of information.
[0097] Typically the components on the sensor-module are low power,
which is particularly important for a portable detector, and it is
particularly advantageous if powers saving techniques are
implemented in order to minimise power consumption when no
radiation is present.
[0098] Noise and Error Correction Chain
[0099] In an embodiment of the present invention, the acquisition
of raw data in the form of charge from the detector ASIC 14 and the
correction of noise and errors in the raw data is carried out in
the FPGA 104. The acquisition of the charge and correction of noise
and errors is carried out in real-time within an exposure window
period, such that the corrected data may be output to the memory
access control 108 before the next frame of charge is acquired by
the FPGA 104.
[0100] The architecture of the noise and error correction in an
embodiment of the present invention is illustrated in FIG. 5. In
general terms, raw charge is acquired from ASIC 14 on a frame by
frame basis in accordance with an exposure window time period, 150.
The raw charge is converted to a raw digital charge value by ADC
109. Fixed pattern noise, including offset errors, is then removed
from the raw charge data, 152, and the charges corrected on a pixel
by pixel basis 154 in accordance with gain calibration data to
compensate for individual pixel variations such as sensitivity.
[0101] The charge data is then threshold filtered 156 to remove
"noise" signals caused by low energy reflected radiation or other
sources of radiation "noise". The charge data is then spatially
corrected, 158, he take into account photon interaction events in
which charge is collected at adjacent pixel electrodes.
[0102] The number of pixels for which a non-zero output value was
recorded, following the noise and error reduction chain processing,
are then summed, 160 to give a total "hit per frame" count for that
particular exposure window. Finally, the charge values are
converted to energy values, 162. The energy values are then
forwarded over serial bus. 106 to memory access control 108 for
storing in memory 112.
[0103] Fixed Noise Pattern Removal
[0104] Each pixel typically has a gain and offset characteristic
different to other pixels. In order to obtain a uniform response
across the pixel array, calibration and correction for the
different gain and offset characteristics for each pixel is
performed. Fixed noise pattern removal compensates for pixel to
pixel variations in pixel offset behaviour.
[0105] In the described embodiment, compensation for pixel offset
behaviour is by way of "frame differencing". As schematically
illustrated in FIG. 6, in the present embodiment the FPGA 104
comprises at least two frame stores 172 and 174 in which the nth
and n-1th frames of ADC count values may be stored. In principle,
the analogue to digital count value (ADC count value) for a pixel
in the nth frame has subtracted from it the ADC count value for the
same pixel in the n-1th frame, that is to say
ADC.sub.x,y,n-ADC.sub.x,y,n-1. This subtraction results in the
offset of the pixel being eliminated from any subsequent
calculation. This subtraction takes place for all pixels in a
frame, and for the 50.times.50 pixel array of the described
embodiment a set of 2500 different values is generated which is
termed a difference frame. The values of the n-1th frame are
subtracted from the values in the nth frame such that the nth frame
becomes the difference frame. In such an embodiment a third
intermediate or result difference frame is unnecessary. It is the
difference frame upon which subsequent processing takes place.
Optionally, the FPGA 104 comprises a further 50.times.50 frame
store for storing the difference frame.
[0106] A graphical illustration of a frame of raw digital data and
a difference frame where the offset has been removed is illustrated
in FIG. 7.
[0107] Other techniques for fixed pattern noise removal as may be
known to ordinarily skilled person also be used.
[0108] Gain Compensation
[0109] Gain compensation uses calibration data previously obtained
for the detector 13, for example during manufacture, and stored in
FPGA 104. In general terms, the calibration process has measured
the gain of each pixel against known radiation sources at different
temperatures for example at 10.degree. C. intervals, and calculated
a correction factor. For an operating temperature range of
70.degree. C. eight sets of 2500 correction factors are generated,
and stored in the FPGA 104. Typically, the set of correction
factors are unique to a specific detector and ASIC combination.
[0110] Additionally, during the calibration process pixels at the
outer edge of the detector substrate have their gain correction
factor set to Zero. This is because a pixel at the outer edge of
the detector substrates experiences a different environment to
other pixels in that they only have three or five neighbouring
pixels, rather than the eight pixels which surround any interior
pixel. Furthermore, the "dicing" process for cutting pixel detector
substrates may adversely affect the detector behaviour such that it
is difficult to compensate for charge values generated in the outer
pixels. By setting the gain of these pixels to zero they are in
practice ignored.
[0111] The gain correction factors stored in the FPGA 104 and
corresponding to the ambient temperature are applied to each pixel
ADC count value (ADC.sub.x,y,diff) in the difference frame in order
to get a gain corrected difference frame.
[0112] Threshold Filtering
[0113] A numeric threshold is applied to each pixel ADC count value
ADC.sub.x,y,diff in the difference frame. Any count value not
exceeding the threshold is set to zero. In this way, count values
representing noise rather than real event created measurement data
may be eliminated. This results in a threshold filtered gain
compensated difference frame.
[0114] Spatial Correcting
[0115] An example of an embodiment of the invention utilising
spatial correcting to compensate for charge being generated on more
than one pixel contact pad electrode for a given photon
interaction, for example where there is oblique incident radiation,
will now be described with reference to FIG. 8 of the drawings. In
the present example the spatial correcting is conducted on the gain
corrected difference frame referred to above. Each ADC count value
in the difference frame corresponds to charge collected on a
corresponding pixel contact pad electrode during a frame
exposure.
[0116] FIG. 8(a) shows a rectangular array 180 of ADC count values
each corresponding to a "pixel" of the detector 13, which may be
stored in a frame store in FPGA 104 for example. A 2.times.2
correction window 182 starts at one corner of the array, FIG.
8(a)(i) and is moved to across the array in a raster scan of one
pixel steps to each correction position as shown in FIGS. 8(a)(ii)
to 8(a)(iii). When one horizontal scan is completed, the correction
window starts at the beginning of the next row. The scan and
spatial correction are mutually arranged so that the ADC count
value estimated by the correction at a given correction position is
not later used again for estimating other ADC count values during
the scanning and correcting the rest of the frame.
[0117] The spatial correction is based on a logical condition for
the pixels in the correction window being satisfied. For ease of
description respective pixels in the correction window 182 are
labelled a, b, c and d as shown in FIG. 8(b). In the described
embodiment, the estimate of ADC count value at each correction
position is carried out for pixel a (ADCa) only.
[0118] In one example, the spatial correction tests for the
following logical condition for the ADC count values ADCa, ADCb,
ADCc and ADCd within the correction window 182:
(ADCa< >0) OR (ADCb< >0 AND ADCc< >0) (1)
[0119] where the expression "< >" indicates a value not equal
to zero.
[0120] If condition (1) is satisfied then ADCa is set equal to the
sum of the count values in all the pixels, i.e.:
ADCa=ADCa+ADCb+ADCc+ADCd (2)
[0121] and the other ADC count values are set to zero, i.e.:
ADCb=ADCc=ADCd=0 (3)
[0122] The original value for ADCa is overwritten within the
difference frame undergoing spatial correcting.
[0123] Examples of cases where a single ADC count value within the
correction window is greater than zero are illustrated in FIG.
8(c). Only the case illustrated in FIG. 8(c)(i) satisfies logical
condition (1) and so ADC count value is assigned a value
corresponding to the sum of the ADC count values in all the pixels
which gives a value ADCa, i.e. the original value for pixel a, as
illustrated in FIG. 8(c)(v).
[0124] The cases for two pixels having non-zero ADC count values
are illustrated in FIG. 8(d). Only the cases in FIGS. 8(d)(i) and
(ii) satisfy condition (1) and result in ADCa count value being
assigned the value ADCa+ADCb or ADCa+ADCc according to the cases
shown in FIG. 8(d)(i) or 8(d)(ii) respectively. Cases shown in
FIGS. 8(d)(iii) and 8(d)(iv) result in ADCa remaining zero.
[0125] Cases where three pixels have non-zero ADC count values are
illustrated in FIG. 8(e). The illustrated table shows which part of
condition (1) are satisfied for which cases. As condition (1) is
satisfied for all cases ADC count value ADCa is equal to: [0126]
ADCb+ADCc+ADCd for case 8(e)(i); [0127] ADCa+ADCc+ADCd for case
8(e)(ii); [0128] ADCa+ADCb+ADCd for case 8(e)(iii); and [0129]
ADCa+ADCb+ADCc for case 8(e)(iv).
[0130] For four pixels non-zero as illustrated in FIG. 8(f)(i)
condition (1) is satisfied and count value ADCa is equal to
ADCa+ADCb+ADCc+ADCd.
[0131] The spatial correction process results in a difference frame
having spatially corrected ADC count values.
[0132] The logical condition may be varied to suit different
spatial correcting requirements, as may the pixel combinations
dependent on the condition being satisfied. Additionally, the
correction window need not be limited to a 2.times.2 window, but
may be of any size suitable for a desired form of spatial
correcting. Furthermore, the array of pixels (e.g. ADC count
values) may be configured in other than a rectangular array and the
correction window may be configured other than as a rectangular or
square window. A raster scan need not be utilised, but merely a
scan suitable for obtaining a desired spatial correction wherein an
estimated pixel value (e.g. ADC count value) is not used in later
correction operations in the frame.
[0133] As disclosed above, there are certain incident radiation
scenarios which may result in charge being collected at more than
one adjacent pixel contact pad electrode 10 that are not due to
oblique incidence of the radiation. These scenarios give rise to
errors in the spatial correction for oblique incidence. Briefly
these are: where two hits in the same pixel in a single frame are
recorded; two hits in adjacent pixels in the same frame are
recorded; and two hits in the same pixel in successive frames are
recorded.
[0134] The occurrence of these causes of error are probabilistic in
that they will occur and cause errors in measurements based on the
likelihood of radiation being incident in accordance with one or
other scenarios outlined above. Their likely occurrence is
dependent on the duration during which a measurement is taken, e.g.
a frame exposure, and the applicant has recognised that appropriate
control of the frame exposure window may reduce the likelihood of
one or more of the scenarios occurring.
[0135] Exposure Window Control
[0136] In order to limit the occurrence of incident radiation
detection scenarios which may give rise to measurement errors an
embodiment of the invention controls the duration of the exposure
window.
[0137] The duration of the exposure window is controlled by
controlling the time for which the sample and hold circuitry of
charge integration circuitry accumulates charge, for example by
controlling the gate signal of transistor 40. Control of the gate
signal is by way of a closed loop process which hunts for a desired
number of hits per frame. If the number of hits is too high the
exposure window duration is reduced, and likewise if it is too low
the exposure window duration is increased.
[0138] There is a trade-off between having sufficient levels of
energy in each frame for the measurements to be meaningful and
restricting the likely number of hits to a level for which the
number of error inducing scenarios is acceptable and corrections
and calculations remain valid. The number of hits for each frame is
dependent on design constraints and may be selected according to
the three error criteria a,b,c outlined previously i.e. the
possibility of double hits in a single pixel, genuine hits in
adjacent pixels and hits in the same pixel in subsequent
frames.
[0139] In the described embodiment, a level of 50 hits per frame
has been selected.
[0140] This gives an average of 1 in 50 (50/2500) pixels recording
a hit per frame.
[0141] Two Hits in the Same Pixel in a Single Frame
[0142] Embodiments of the present invention may be directed to
detecting high-energy radiation which is a result of radioactive
decay. Radioactive decay may be considered a Poisson process [3]
and thus the probability of k hits in the same pixel is given
by
p ( k ) = - .lamda. .lamda. k k ! ( 4 ) ##EQU00001##
[0143] Where .lamda. is the expected value for events per frame.
For a selected hits per frame count of 50 the expected value is
1/50 and for k=2 (two hits per pixel), the probability of two hits
in the same pixel is given by:
p ( 2 ) = - .lamda. .lamda. k k ! = - 1 50 ( 1 50 ) 2 2 ! = 1.9604
e - 4 ( 5 ) ##EQU00002##
[0144] Two Hits in Adjacent Pixels in the Same Frame
[0145] The Poisson Forest equation gives the probability of
randomly getting k hits within a distance r as [4]
p r ( r ) = 2 .pi. .lamda. r ( .lamda. .pi. r 2 ) k - 1 - .lamda.
.pi. r 2 ( k - 1 ) ! ##EQU00003##
[0146] For k=2, .lamda.=50/2500 and r=1 the probability of k hits
within one pixel is:
p r ( 1 ) = 2 .pi. .lamda. ( .lamda. .pi. ) 1 - .lamda. .pi. 1 ( 6
) p r ( 1 ) = 2 ( .lamda. .pi. ) 2 - .lamda. .pi. = 0.007420 = 7.42
e - 3 ( 7 ) ##EQU00004##
[0147] Two hits within adjacent pixels includes two hits on the
same pixel and so the result (7) is multiplied by the probability
for one hit on a same pixel, i.e. Probability of a single pixel
being hit
= p ( 1 ) = - .lamda. .lamda. k k ! = - 1 / 50 ( 1 / 50 ) 1 1 ! =
1.96 e - 2 ( 8 ) ##EQU00005##
[0148] So the probability for this case is
0.0196.times.0.007420=0.000145=1.45e-4
[0149] Two Hits in the Same Pixel in Successive Frames
[0150] This is just the probability of one hit per pixel squared
which is:
p ( 1 ) p ( 1 ) = ( - .lamda. .lamda. k k ! ) 2 = ( - 1 / 50 ( 1 /
50 ) 1 1 ! ) 2 = ( 1.96 e - 2 ) 2 = 3.841 e - 4 ( 9 )
##EQU00006##
[0151] In the described embodiment the worst case scenario is a
probability of around 7 thousandths which is of the order of the
error due to electronic RMS noise.
[0152] In the present embodiment a closed feedback loop such as a
proportional, integral, derivative (PID) controller is implemented
in the microcontroller 110 in order to control the exposure window
to maintain a pre-set number of hits per frame. The microcontroller
110 receives a signal representing the number of hits per frame
from the FPGA 104 and is configured to compare that number (plant
feedback signal in PID terminology) against a pre-set number of
desired hits per frame (command signal in PID terminology). The
difference, or "error", signal between the command and feedback
signals is utilised to change the timing of the gate signal on
transistor 44 of the read-out circuits 16 in a direction to
generate the desired number of hits per frame.
[0153] A variable gain is included in the feedback, dependent upon
which decade the exposure control is in e.g. a different feedback
gain is used to control 1 ms to 10 ms exposure window duration
compared to a 0.1 ms to 1 ms exposure window duration.
Additionally, the proportion of gain applied includes a "guard"
gain in order to provide an element of over correction so that the
exposure window duration is not changed too often.
[0154] FIG. 9 shows an example embodiment of a PID (proportional
integral derivative) control mechanism 200 applied to the problem
of hit rate control. Such feedback based control mechanisms use the
error rate 206 in the control parameter, in this case the observed
hit rate 202 minus the required hit rate 204 is used as the "plant"
conditions, in this case the exposure rate. Such algorithms
balance, smooth and provide an immediacy of response. The
proportional functions 208 and 210, integral 212 and derivative 214
functions are a matter of design choice for the ordinarily skilled
person.
[0155] The temperature of the detector substrate 4 is also used to
control the exposure window. During calibration the dark leakage
currents are measured for a series of temperatures e.g. -20 to +50
C in 10 degree steps. These leakage values provide the basis for
the maximum exposure times permitted for a particular temperature.
The exposure times are chosen such that the dark leakage charge
fills a maximum of 75% of the ADC range.
[0156] The exposure time is reduced for increases in temperature in
order to provide sufficient dynamic range in the ADC 109 for
converting collected charge into digital values. This is
advantageous since the amount of charge generated and collected by
a pixel contact pad electrode for a photon interaction event of a
given energy is dependent on the temperature of the detector
substrate. Conversely, the exposure time may be increased for
decreases in temperature.
[0157] Frame Summation
[0158] FPGA 104 is configured to count the number of non-zero ADC
count values for each pixel of the difference frame following the
spatial correction process. This difference frame has also
previously undergone fixed pattern noise removal, pixel sensitivity
correction and threshold filtering.
[0159] The number of hits counted for each frame is forwarded from
the FPGA 104 to the microcontroller 110 where it may be used to
determine the radiation flux incident on the detector and establish
the exposure window duration. Additionally, a linear sum of the ADC
count values in the difference frame provides a single value which
represents the total energy received during a frame exposure.
[0160] Energy Conversion
[0161] The (difference) frame values ADC.sub.n, represent the
energy measured for each frame at each exposure.
[0162] The microcontroller 110 is configured to take the ADC count
values and convert them to energy values and then convert the
energy values to dose values by way of a look up to energy
conversion calibration data stored in memory 112.
[0163] This conversion is performed by the use of calibration
values which are stored in the unit. The calibration values are
specific to each unit and have values for eight temperatures within
the specified temperature range. The current temperature is read
from the electronic thermometer on the sensor sub-module and is
used to derive an interpolated calibration curve. This curve is
then applied to the frame values to generate the dose readings.
[0164] The calibration values are derived by testing each device in
known fields at known temperatures. Device output data is logged,
and then externally processed to generate the calibration
curves.
[0165] Calibration
[0166] During manufacture of the detector 13 calibration is carried
out in order to compensate for differences between the detector
substrate material forming sense volumes for respective pixel
contact pad electrodes 10, and any differences in respective charge
collection and read-out circuitry. The calibration is conducted on
a pixel by pixel basis in order that variations between pixels may
be compensated for on an individual pixel basis, and that charge
values from each pixel may be treated in exactly the same way. This
allows the pixel ADC values to form the basis of dose calculations.
These ADC values are converted to a per photon dose estimate via a
non-linear ADC to dose calibration curve (energy compensation). The
pixel dose estimates are summed for a particular frame, and knowing
the frame exposure time allows these values to be converted to dose
rate and cumulative dose values.
[0167] Calibration is carried out in a calibration configuration in
which a detector 13 undergoing calibration is coupled via
communications interface 118 to external data processing apparatus
such as a personal computer. ADC values representative of charge
values collected from the detector substrate to forwarded from FPGA
104 to a personal computer. The personal computer is configured to
analyse the ADC values in accordance with the calibration processes
described below to derive sets of calibration data. The calibration
data is then stored in the FPGA 104 for use to operation of the
detector.
[0168] There are three aspects to the detector calibration: bad
pixel calculation; gain correction which compensates for different
detector sensitivity and energy compensation.
[0169] A two phase approach is taken to calibrating for gain and
offset, and energy compensation. For gain compensation: [0170] A
set of gains is calculated which normalises the response of each
pixel. [0171] Equalising the pixel response allows subsequent pixel
processing to treat the pixel data as being equivalent e.g. the
spatial filtering in the read chain described above may add pixel
values together linearly. [0172] In practice, if a pixel
differenced approach is utilised only the gain portion of the
calibration calculation is required as the offsets are compensated
for in the pixel differencing of the read chain process described
above.
[0173] For the second phase, energy compensation, the non-linear
response between induced pixel charge and external dose in
compensated for.
[0174] Bad Pixel Calculation
[0175] Dark Frame Bad Pixel Process
[0176] A dark current leakage frame is obtained with the bias
voltage for the detector substrate 2 turned on, but without
exposing the detector undergoing calibration to incident radiation.
Charge values are collected by the read out circuitry 16 and
converted into ADC count values in FPGA 104 and forwarded to the
personal computer for inclusion in the calibration analysis
[0177] Any hit that is observed due to background radiation is
excluded e.g. hits can still be detected by frame differencing.
[0178] The process collects dark leakage current over a series of
frames at a short exposure of 0.5 ms, and over a series of frames
at a long exposure of 10.0 ms. Typically, several hundred frames of
dark leakage current are obtained, the exact number being a matter
of choice for the operator of the calibration process.
[0179] For each pixel leakage current gradient is calculated. A
leakage current gradient is defined by:
g x , y = p l ( x , y ) - p s ( x , y ) l - s ( 10 )
##EQU00007##
[0180] Where
[0181] g.sub.x,y is the leakage current gradient for the x,yth
pixel,
[0182] p.sup.l(x,y) is the mean leakage for the x,yth pixel for the
long exposure,
[0183] p.sub.s(x,y) is the mean leakage for the x,yth pixel for the
short exposure, and
[0184] l,s are the long and short exposure times respectively.
[0185] The mean leakage current is the mean for each pixel over the
series of dark frames.
[0186] The mean and standard deviation of the 2500 gradients
(50.times.50 pixel detector) is then calculated and all the pixels
with gradients which are outside n standard deviations from the
mean (for example in one embodiment n is 4) are designated bad
pixels.
[0187] The personal computer creates a bad pixel mask in which a
"1" indicates that the pixel data should be retained and a "0"
indicates the pixel is bad and the values should be rejected. This
mask is used in further calibration processes to avoid calibrating
using a bad pixel.
[0188] Bright Frame Pixel Values
[0189] Bright frames are obtained by operating the detector under
radiation sources representative of the type of radiation that the
detector will be used to detect. In one example the bright pixel
values are obtained using .sup.241Am (Americum) and .sup.137Cs
(Caesium). The activities are not particularly important.
[0190] In one example the sources output the following radiation
levels: [0191] The Am source outputs 1.67 GBq=18.3 .mu.Sv/hr=120
cps/.mu.Sv/hr=2000 cps approximately; and [0192] The Co source
outputs 64 MBq=77.5 .mu.Sv/h=1.1 cps/.mu.Sv/h=85 cps approximately,
where
[0193] cps denotes counts per second and .mu.Sv/h micro Sieverts
per hour.
[0194] For each of the Am and Cs sources 200,000 frames of data are
obtained. For each pixel the mean, standard deviations, skew,
kurtosis, max, min, median and number of hits is calculated for the
frames corresponding to a particular isotope.
[0195] For each set of frames two sets of bad pixel masks are
generated.
[0196] Hit Based Mask
[0197] The number of hits are distributed as a Poisson process as
defined by equation (4) above, and repeated here for clarity:
p ( x , y ) = - .lamda. .lamda. k k ! ( 4 ) ##EQU00008##
[0198] Where k is the number of hits and lambda indicates the mean
hit value.
[0199] Taking mean hit values the probability associated with
particular hit values is calculated.
[0200] Using this relationship hit count thresholds associated with
a particular probability may be calculated, i.e. pixels may be
excluded which have a number of hit associated with small
probabilities i.e. they represent outlier in terms of too many of
too few hits. In the described embodiment, the probability
threshold is set at 1 in 1,000,000. Masks msk.sub.pois,am and
msk.sub.pois,cs are created in which excluded pixels are given a
"0" value and the rest a "1" value.
[0201] Hotellings t.sup.2 Mask
[0202] The remaining statistics mean, standard deviations, skew,
kurtosis, max, min and median are used as the basis of a
multivariate outlier test. For each pixel a hotelling's t.sup.2
value is calculated. Calculation of hotelling t.sup.2 values is
known and is described in a text book "Johnson, R. A. and Wichern,
D. W. (1992), Applied Multivariate Statistical Analysis. 3rd. ed.
New-Jersey: Prentice Hall. pp. 180-181,199-200". The probability
associated with each hotellings values is calculated, and any
pixels which represent outliers i.e. which look less like the bulk
of the pixels, are excluded. Again, this embodiment uses a
probability threshold of 1 in 1 million. Masks msk.sub.hot,am and
msk.sub.hot,cs are created in which excluded pixels are set value
"0" and the rest value 1".
[0203] A final mask is formed on the basis of;
Msk=msk.sub.1|(msk.sub.pois,am|msk.sub.hot,am) &
(msk.sub.pois,cs|msk.sub.hot,cs)
[0204] Where "|" denotes "or" and "&" denotes "and".
[0205] The final mask is used further in the calibration process to
avoid calibrating using bad pixels. For example, it may be loaded
into every pixel gain mask prior to further processing of the
frame. The pixel values are multiplied by the gain mask for each
frame. Consequently, excluding the bad pixels whose mask value is
"0".
[0206] Additionally, the bad pixel mask is combined with a pixel
gain mask, described later in the calibration process, and stored
in the detector 12 memory 112 and accessed by the FPGA 104 to
compensate charge values collected in a frame. Thus, the combined
bad pixel and gain mask only has gain values not equal to zero for
"good" pixels, the bad pixels having a gain of zero by virtue of
being combined with the bad pixel value "0".
[0207] Gain Calibration
[0208] A problem with calibrating gain values and in order to
normalise the gains in a meaningful way energy compensation should
be applied. However, the applicant has recognised that there are
two ways to calculate calibration gain values not dependent on
energy compensation: [0209] Leakage current based gain calculation;
and [0210] Hit count based gain calculation.
[0211] An advantage of a leakage current based calculation is that
the apparatus characteristics do not differ between signal and
leakage current operation. Normalisation of leakage is a good thing
in that it partially ameliorates the "in exposure" decay problem
described below.
[0212] The "in exposure" decay problem results from the fact that a
signal from each pixel is the result of a number of interacting
processes. There is for example: [0213] Induced charge from
incident radiation; [0214] Charge stored as a result of bulk
substrate leakage current; and [0215] Charge leaked as a result of
ASIC leakage current.
[0216] For long exposure periods the ASIC leakage component can
become a significant factor.
[0217] When no incident radiation charge is present the signal from
the pixel is the result of the equilibrium between the incoming
bulk leakage and the out going ASIC leakage. Using the frame
difference approach as described above this is what is subtracted
from a pixel containing an incident radiation charge. However, a
problem may arise when the ASIC leakage is significant during the
frame exposure time when compared to the induced photon charge, and
if this occurs the charge value should be compensated.
[0218] Knowing what the expected ASIC leakage was would allow for
the adjustment of the resultant signals accordingly.
[0219] Estimating the rate at which current leaked away may be
achieved by ASIC simulation but is as only reliable as the
simulation model. An experimental approach would be more
reliable.
[0220] The second approach is based on the number of hits. This is
based on the following hypothesis: [0221] All pixels "see" the same
flux (photons per unit area per unit time N.B. fluence is merely
the integral of flux); and [0222] Consequently over a "long" period
of time exposed to same overall fluence each pixel should measure
the same number of hits.
[0223] However, such an approach may encounter problems due to:
[0224] The distribution affect; the bulk of the probability
distribution is towards the low energy photons (because of Compton
scattering etc). This means that gain variations are primarily
influenced by this part of the spectra i.e. increase probability
means more hits. [0225] Threshold affect; the hits are defined by
the energy values falling above the threshold value. In practice,
this can only work by moving hits below the threshold to above the
threshold. Therefore the only photon values which will be taken
into consideration by the hit count based gains are those
immediately below the threshold. Consequently, hit count based
gains are a function of the threshold value used. [0226] Shot noise
due to the photons being quantised. Hence, there exists natural
uncertainty which implies large sample size in order to reduce
[0227] However, the applicant has appreciated that the gains should
be adjusted to make the spectral response of each pixel as similar
as possible. Using this insight, an embodiment of the invention
uses the threshold affect to advantage i.e. the threshold is varied
and the gains adjusted for each threshold value. This produces a
set of gains for each different bit of the energy spectrum. The
mean gain for each pixel is calculated, which provides a maximum
likelihood estimate of the gain values which maximise the
similarity of the individual pixel spectra.
[0228] In the described embodiment the two methods discussed above
may be used for calculating pixel gain corrections.
[0229] In a first method gain correction calculation is based on
dark current leakage. The graph in FIG. 10 shows leakage based gain
calculation results as a calibration graph for an individual pixel.
The average pixel gain (pixel mean gain) over a series of frames is
plotted against the average frame gain (frame mean gain) for
different exposure times. A graph of the type illustrated in FIG.
10 is generated for different temperatures as the detector
substrate behaves differently at different temperatures.
[0230] For example, the ADC values for 1000 dark field frames at a
particular exposure time are obtained. The 1000 ADC values per
pixel are averaged to find the pixel mean gain. The term pixel mean
refers to the mean of these 1000 values. The frame mean is the mean
of the pixel means.
[0231] The gain graph defines an input output relationship for an
individual pixel, and data is collected to produce a gain graph for
each pixel. The gradient of the graph defines the pixel gain
deviation from a normalised gain defined by the frame mean. In
effect, the graph maps for each pixel how the individual pixel
leakage varies in comparison to the average pixel leakage defined
by the frame mean. Effectively the average leakage represents the
standard "gain" i.e. a gain of 1. Pixels with leakage values less
than 1 are assigned a gain greater than 1 to bring their post gain
leakage in line with the average, those with leakage values greater
than this will be given gains of less than 1. In the described
embodiment, the gain value assigned to a pixel is the inverse of
the gradient of the pixel's pixel mean to frame mean graph. Thus, a
pixel having a gradient of 0.8 would be assigned a gain value of
1.25 to bring it into conformity with the normalized gain.
[0232] The gain values, gradients of the graphs, are calculated by
line fitting a gradient to the pixel mean/frame mean data point. In
this embodiment, line fitting is achieved by robust linear
regression. Calibration gains for each pixel may then be stored in
the detector 13 for use in gain compensation of collected charge
values.
[0233] A second method is based on collecting charge values for
bright frames. The average pixel ADC value is calculated for many
frames, for example several thousand frames. The average of all the
pixel averages is then calculated to obtain a bright frame mean.
For each pixel the ratio of the frame mean to the pixel mean is
calculated and stored as the pixel gain compensation value.
Optionally, these values may be stored in detector 13.
[0234] In one embodiment, both methods are utilised in calibrating
devices. The bright frame approach results in values which may
reflect all aspects of the charge collection process, for example
the charge transport properties of the detector, any mechanical
defects in the bump or interface between ASIC and detector, and the
variations in the pixel.
[0235] However, the dark leakage approach is not dependent on the
charge transport properties of the detector. Consequently, the two
gain factors provide complementary information. The leakage based
gains may be used to check the bright frame gains; the summed
overall discrepancy between the gains providing an indication of
sensor quality.
[0236] The foregoing calibration methods provide for the
compensation of pixel to pixel variations in sensitivity. This
allows the output response from each pixel value to be treated in
the same way in the read chain. Thus, the results from the detector
are x,y position independent.
[0237] Inherent in the leakage based gains in the assumption that
the device leakage is the same for every pixel the difference we
perceive are due to capacitive differences.
[0238] Energy Compensation
[0239] The problem is that the charge induced and collected by in
the ASIC has a non-linear relationship to the actual radiation
dose. In general terms this has been solved by the applicant by
breaking down the non-linear relationship.
[0240] There is a known empirical non-linear relationship between
the exposure in terms of "air kerma" and the energy of the energy
of the incoming photon. This non-linear relationship is embodied in
the "mass energy transfer coefficient". Theoretically, the amount
of charge released by a photo-electric interaction is directly
proportional to the energy of the photon. In practice this
relationship also is non-linear due to charge trapping, incomplete
charge deposition, interaction depth for example. Therefore,
embodiments of the invention treat these two non-linear
relationships separately. That is to say, a "known" empirically
deduced curve is derived from the NIST (National Institute for
Standards and Technology) data and a separate sensor dependent
relationship between induced charge and photon energy is derived.
It is the derivation of this second relationship which the energy
compensation portion of the calibration process is concerned
with.
[0241] As outlined above the energy compensation problem is reduced
to the problem of deriving a relationship between induced charge
and photon energy. The end result which is required is that the
calculated dose and dose rate are accurate and precise. The
accuracy in this case is determined as referenced to known
environmental dose rate at which empirical tests are conducted.
This may be considered a typical radiation metrology calibration
problem. In such cases the measuring instrument is placed in a
known radiation flux. A number of experimental data are obtained.
On this basis of the obtained data the instrument is adjusted to
bring the measured response of the instrument in line with the
known external environmental values.
[0242] Typically the optimisation parameters are adjusted to
minimise (and sometimes maximise) an optimisation criterion or
optimisation criteria.
[0243] The optimisation criterion to be minimised is the difference
between the calculated dose (dose rate) and the actual known dose
rate. From the experimental setup the actual dose rate is known.
The shortest time period over which experimental data may be
collected is a single frame, and each frame will contain several
hits. This embodies the credit assignment problem, and the observed
dose calculation is deconstructed in order to solve the
problem.
[0244] As the observed dose is the result of several hits and each
hit may be considered the result of a Poisson process, the observed
dose rate results from a combinatorial Poisson process.
[0245] The model underlying the calibration comprises an ADC value
to energy conversion, followed by an energy to dose conversion. The
energy to dose curve is an empirical curve defined for a particular
detector substrate type (calculated from NIST data). The ADC to
energy curve is the curve that is fitted by the calibration
process. The energy dose curve provides effectively provides a
constraint mechanism to ensure that the final calibration is
sensible from a device physics point of view. This curve is more
generally referred to as constraint curve in the following
description.
[0246] As seen from the general outline above, energy compensation
calibration provides a conversion from observed charge values to
radiation dose.
[0247] The dose per frame may be approximated as the sum of
converted photon energies, and as charge collected in a frame is
proportional to the photon energies the applicant has recognised
that there is a functional relationship between the ADC value
representative of the charge and the dose. Conversion data may be
represented as a curve that provides for conversion between adc
values to dose i.e. the functional relationship between adc dose
e.g.
Dose frm = x , y f cal ( adc ) ( 10 ) ##EQU00009##
[0248] Where f.sub.cal indicates the ADC to dose calibration
curve,
[0249] Dose.sub.frm denotes the cumulative dose for a frame,
and
[0250] adc denotes a vector which contains the ADC values for the
current frame.
[0251] Finding this curve is a classic optimisation problem. In the
described embodiment the ADC to dose curve may be defined by 32
energy level points. For each frame an optimisation problem is
defined based around scaling the 32 values which define this space
i.e. an optimisation problem based in 32 dimensional space is
defined. This is done by defining a multiplication vector .alpha.,
which is modified in order to minimise the relative differences
between the calculated and observed dose rates. The .alpha.
modified frame dose over plural frames is expressed as equation and
the minimisation as expression (11) below.
D sum ( .alpha. ) = fr i = 1 n f cal 0 ( adc f , i .alpha. ) min
.alpha. ( D act , fr - D sum ( .alpha. ) D act , fr ) ( 11 )
##EQU00010##
[0252] Where; [0253] 1. D.sub.sum is the summed dose over several
frames, and [0254] 2. D.sub.act is the actual dose.
[0255] Various standard optimisation algorithms are known. However,
it has been found in practice that not all these solutions provide
physically realistic solutions.
[0256] The error between the calculated and observed dose rates is
to be minimised for over the all the calibration data, which will
be several hundred frames (and several thousand hits), obtained at
different exposure times and fluency rates.
[0257] Optimisation
[0258] The above optimisation function, (11), can be represented in
terms of what can be measured;
E f = 1 2 i = 1 n ( o i - e i ) 2 ( 12 ) ##EQU00011##
[0259] Where E.sub.f=the observed dose for the fth frame;
[0260] o.sub.i=the observed dose for the ith pixel; and
[0261] and e.sub.i=the expected dose for the ith pixel.
[0262] The total error is defined as the sum of such error over all
the frames observed,
E tot = f = 1 N E f ( 13 ) ##EQU00012##
[0263] This frame based optimisation is directly equivalent to
mapping the ADC probability density functions (pdfs) to the dose
domain and minimising the difference between the actual dose (known
from calibration setup) and the expected dose (e.g. the mean of the
pdf).
E tot = D a - adc = 1 N adc pdf ( adc ) E tot = D a - D e ( 14 )
##EQU00013##
[0264] As mentioned previously, a constraint curve is introduced in
order to maintain a physically realistic solution. The ADC pdf is
mapped through the ADC/constraint energy/dose curves to produce a
dose pdf equivalent. If the ADC pdf is represented as a histogram
each of the ADC bin ranges has an equivalent dose curve bin
range.
[0265] The constraint curve is then adjusted, to map the observed
ADC pdf to a require dose ADC, e.g. the constraint curve is
adjusted to make the expected dose pdf (the mean of the mapped
dose) coincident with the required actually observed dose. The
mapping of the observed ADC pdf onto a dose pdf with the
appropriate expected value results in a compound dose rate that
will have the correct mean value.
[0266] The optimization scheme maps to a dose distribution with the
correct mean. However, more complex mapping schemes can be used to
map the observed ADC values onto dose distribution with a required
distribution shape (as opposed to merely having the correct
mean).
[0267] Calibration
[0268] How to "adjust" the constraint curve to map the ADC values
to the dose domain is an important aspect of the calibration
process. The adjusting is done via a weighting curve, which
initially is set to unity. This weighting curve is iteratively
updated, each step in the iterative process being intended to
minimise the error as shown in equation (14). The final ADC to dose
curve is produced by applying this weighting curve to the
constraint curve thereby distorting the constraint curve as
appropriate for the particular sensor under consideration.
[0269] The initial constrain curve is created using a theoretical
energy to dose relationship, which is derived from the material
properties for the detector substrate material, in the described
embodiment CdTe,
D r ( .DELTA. E ) = E _ .gamma. ( .mu. km air ) ( .DELTA. E .gamma.
/ .rho. air ) A ( 1 - - .mu. CdTe ( .DELTA. E .gamma. ) t ) .phi. (
.DELTA. E .gamma. ) ( 15 ) ##EQU00014##
[0270] where: [0271] .DELTA.E.sub..gamma., is the photon energy
range, [0272] .sub..gamma., is the mean photon energy, [0273]
.phi.(.DELTA.E.sub..gamma.), is the fluence for the photon energy
range, .DELTA.E.sub..gamma., [0274] D.sub.r(.DELTA.E.sub..gamma.),
is the dose rate for the photon energy range, .DELTA.E.sub..gamma.,
[0275] .mu..sub.CdTe(.DELTA.E.sub..gamma.), the linear attenuation
coefficient for CdTe for the photon energy range,
.DELTA.E.sub..gamma. [0276]
.mu..sub.km.sup.air(.DELTA.E.sub..gamma.)/.rho..sub.air, the mass
energy transfer coefficient for CdTe for the photon energy range,
.DELTA.E.sub..gamma., and .rho..sub.air, denotes air density,
[0277] t, indicates detector thickness=0.07 cm [0278] and, A is
detector surface area=5.times.5 mm.
[0279] An example of the theoretical relationship between photon
energy a dose is illustrated in FIG. 11.
[0280] However, once a sufficient body of instruments have been
calibrated the average of all the ADC to dose curves may be used as
the constraint curves. In this way the weighting curve can be seen
as a modification of a standard calibration curve.
[0281] Standard transformation methods, for example as described in
(Press et al, "Numerical Recipes in `C`" show how to map a pdf via
any functional relationship;
y=f(x)
[0282] into the y domain merely by the relationship;
p ( y ) = p ( x ) y x . ##EQU00015##
[0283] Where
[0284] p(x) denotes the probability of x,
[0285] p(y) denotes the probability of y, and
y x ##EQU00016##
the modulus of the instantaneous gradient at x.
[0286] Iterative Process
[0287] Each step in the iterative process proceeds as follows;
[0288] 1) Calculate the pdf's, p(adc), for the 3 calibration
isotopes (.sup.241Am, .sup.137Cs, .sup.60Co. [0289] 2) Produce a
weighted version of each of these 3 pdfs, by multiplying the pdf
value, corresponding to each adc value, by the gradient of the
weighting curve i.e. using the standard transformation method
[0290] 3) Similarly transform the weighted pdf's into the dose
domain using the constraint curve. [0291] 4) Calculate the global
error between the actual dose and the average of the dose pdf.
[0292] 5) Back-propagate the error to adjust the weighting curve
and repeat the process.
[0293] Back-Propagation of Error
[0294] Back-propagation of error is a gradient decent optimisation
process, such as described by Rumulhart and Hinton [5]. A similar
approach is utilised to propagate back the overall dose error and
attribute this dose error to individual ADC values. In diagrammatic
form steps 1-4 discussed above may be represented as illustrated in
FIG. 12, where:
[0295] W(x) denotes the weighting curve; and
[0296] C(x) denotes the constraint curve.
[0297] The back-propagation utilizes an update rule to modify the
weighting curve.
W n + 1 ( adc ) = W n ( adc ) + .eta. .DELTA. W n ( adc ) where
.DELTA. = .differential. E tot .differential. W n ( adc ) ( 16 )
##EQU00017##
[0298] This is known as a delta update rule.
.differential. E tot .differential. W n ( adc ) ##EQU00018##
is obtained by the chain rule, as described in [5].
[0299] Consequently, in step 5 each entry in the ADC weighting
curve is updated using the above delta update rule expressed in
equation (16). This process continues until the E.sub.tot falls
below an acceptable e.g. <5% error. The values obtained for the
ADC weighting curve are stored in memory 112, where they are used
by microcontroller 110 in the energy compensation step of the read
out chain process to convert the composite frame ADC values to a
dose value.
[0300] Insofar as embodiments of the invention described above are
implementable, at least in part, using a software-controlled
programmable processing device such as a general purpose processor
or special-purposes processor, digital signal processor,
microprocessor, or other processing device, data processing
apparatus or computer system it will be appreciated that a computer
program for configuring a programmable device, apparatus or system
to implement the foregoing described methods, apparatus and system
is envisaged as an aspect of the present invention.
[0301] The computer program may be embodied as any suitable type of
code, such as source code, object code, compiled code, interpreted
code, executable code, static code, dynamic code, and the like. The
instructions may be implemented using any suitable high-level,
low-level, object-oriented, visual, compiled and/or interpreted
programming language, such as C, C++, Java, BASIC, Perl, Matlab,
Pascal, Visual BASIC, JAVA, ActiveX, assembly language, machine
code, and so forth. A skilled person would readily understand that
term "computer" in its most general sense encompasses programmable
devices such as referred to above, and data processing apparatus
and computer systems.
[0302] Suitably, the computer program is stored on a carrier medium
in machine readable form, for example the carrier medium may
comprise memory, removable or non-removable media, erasable or
non-erasable media, writeable or re-writeable media, digital or
analog media, hard disk, floppy disk, Compact Disk Read Only Memory
(CD-ROM), Company Disk Recordable (CD-R), Compact Disk Rewriteable
(CD-RW), optical disk, magnetic media, magneto-optical media,
removable memory cards or disks, various types of Digital Versatile
Disk (DVD), tape, cassette solid-state memory. The computer program
may be supplied from a remote source embodied in the communications
medium such as an electronic signal, radio frequency carrier wave
or optical carrier waves. Such carrier media are also envisaged as
aspects of the present invention.
[0303] It will be appreciated that any of the application programs,
or any other logical module, may be made up of more than one
functional unit that may be distributed across more than one data
processing apparatus. The one of the more than one data processing
apparatus may or may not be in the same physical location or
device.
[0304] In view of the foregoing description it will be evident to a
person skilled in the art that various modifications may be made
within the scope of the invention. For example, the pixellated
array of contact pads or electrodes need not be square or
rectangular, but may be formed in another arrangement such as
concentric circles, spirals or a hexagon for example.
[0305] Additionally, charge circuitry 30 need not be driven in a
raster pattern, but may be read-out in any suitable pattern an
application of the invention or design parameters or constraints
requires.
[0306] The exposure control need not be by way of a PID controller,
but any other suitable target hunting or variation control
mechanism.
[0307] The ADCs need not be formed in the FPGAs of the system
modules, but may be separately formed.
[0308] As used herein any reference to "one embodiment" or "an
embodiment" means that a particular element, feature, structure, or
characteristic described in connection with the embodiment is
included in at least one embodiment. The appearances of the phrase
"in one embodiment" in various places in the specification are not
necessarily all referring to the same embodiment.
[0309] As used herein, the terms "comprises," "comprising,"
"includes," "including," "has," "having" or any other variation
thereof, are intended to cover a non-exclusive inclusion. For
example, a process, method, article, or apparatus that comprises a
list of elements is not necessarily limited to only those elements
but may include other elements not expressly listed or inherent to
such process, method, article, or apparatus. Further, unless
expressly stated to the contrary, "or" refers to an inclusive or
and not to an exclusive or. For example, a condition A or B is
satisfied by any one of the following: A is true (or present) and B
is false (or not present), A is false (or not present) and B is
true (or present), and both A and B are true (or present).
[0310] In addition, use of the "a" or "an" are employed to describe
elements and components of the invention. This is done merely for
convenience and to give a general sense of the invention. This
description should be read to include one or at least one and the
singular also includes the plural unless it is obvious that it is
meant otherwise.
[0311] The scope of the present disclosure includes any novel
feature or combination of features disclosed therein either
explicitly or implicitly or any generalisation thereof irrespective
of whether or not it relates to the claimed invention or mitigate
against any or all of the problems addressed by the present
invention. The applicant hereby gives notice that new claims may be
formulated to such features during prosecution of this application
or of any such further application derived therefrom. In
particular, with reference to the appended claims, features from
dependent claims may be combined with those of the independent
claims and features from respective independent claims may be
combined in any appropriate manner and not merely in specific
combinations enumerated in the claims.
REFERENCES
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[0313] [2] J. D. Eskin, H. H. Barrett and H. B. Barber, J. Appl.
Phys. 85 647 (1999)
[0314] [3] Glenn Knoll, Radiation Detection and Measurement, John
Wiley & Sons, Inc. 1999
[0315] [4] Heikkinen, J. and Arjas, E. (1999). Modeling a Poisson
forest in variable elevation: A nonparametric Bayesian approach,
Biometrics 55: 738-745.
[0316] [5] D. E. Rumelhart, G. E. Hinton, R. J. Williams, Learning
internal representations by error propagation, MIT Press Cambridge,
Mass., USA
* * * * *