U.S. patent application number 17/522267 was filed with the patent office on 2022-03-03 for strip pixel detector.
The applicant listed for this patent is SHENZHEN XPECTVISION TECHNOLOGY CO., LTD.. Invention is credited to Peiyan CAO, Yurun LIU.
Application Number | 20220066057 17/522267 |
Document ID | / |
Family ID | 1000005962339 |
Filed Date | 2022-03-03 |
United States Patent
Application |
20220066057 |
Kind Code |
A1 |
CAO; Peiyan ; et
al. |
March 3, 2022 |
STRIP PIXEL DETECTOR
Abstract
Disclosed herein is a detector, comprising: a plurality of strip
pixels, wherein each of the strip pixel is configured to count
numbers of radiation photons incident thereon whose energy falls in
a plurality of bins, within a period of time.
Inventors: |
CAO; Peiyan; (Shenzhen,
CN) ; LIU; Yurun; (Shenzhen, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SHENZHEN XPECTVISION TECHNOLOGY CO., LTD. |
Shenzhen |
|
CN |
|
|
Family ID: |
1000005962339 |
Appl. No.: |
17/522267 |
Filed: |
November 9, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16919539 |
Jul 2, 2020 |
11204433 |
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17522267 |
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PCT/CN2018/074048 |
Jan 24, 2018 |
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16919539 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 23/223 20130101;
G01T 1/247 20130101; G01T 1/366 20130101; G01T 1/241 20130101 |
International
Class: |
G01T 1/36 20060101
G01T001/36; G01T 1/24 20060101 G01T001/24 |
Claims
1. A detector, comprising: a plurality of strip pixels, wherein
each of the strip pixels is configured to count numbers of
radiation photons incident thereon whose energy falls in a
plurality of bins, within a period of time; wherein the detector is
configured to add the numbers of radiation photons for the bins of
the same energy range counted by all the strip pixels.
2. The detector of claim 1, wherein the strip pixels are arranged
in an array.
3. The detector of claim 1, wherein the strip pixels are configured
to receive radiation photons from a sidewall of an absorption layer
of the detector.
4. The detector of claim 1, wherein each of the strip pixels
comprises an analog-to-digital converter (ADC) configured to
digitize an analog signal representing the energy of an incident
radiation photon into a digital signal.
5. The detector of claim 1, wherein the strip pixels are configured
to operate in parallel.
6. A system comprising the detector of claim 1, and a radiation
source, wherein the system is configured to perform radiation
radiography on human chest or abdomen.
7. A system comprising the detector of claim 1, and a radiation
source, wherein the system is configured to perform radiation
radiography on human teeth.
8. A cargo scanning or non-intrusive inspection (NII) system,
comprising the detector of claim 1, and a radiation source, wherein
the cargo scanning or non-intrusive inspection (NII) system is
configured to identify elements by energy dispersive analysis using
radiation transmitted through an object inspected.
9. A cargo scanning or non-intrusive inspection (NII) system,
comprising the detector of claim 1, and a high-energy X-ray source,
or gamma ray source, wherein the cargo scanning or non-intrusive
inspection (NII) system is configured to identify elements by
energy dispersive analysis using backscattered radiation.
10. A full-body scanner system comprising the detector of claim 1,
and a radiation source, wherein the full-body scanner is configured
to identify elements.
Description
TECHNICAL FIELD
[0001] The disclosure herein relates to a strip pixel detector
suitable for detecting radiation such as X-ray and gamma ray.
BACKGROUND
[0002] X-ray fluorescence (XRF) is the emission of characteristic
fluorescent X-rays from a material that has been excited by, for
example, exposure to high-energy X-rays or gamma rays. An electron
on an inner orbital of an atom may be ejected, leaving a vacancy on
the inner orbital, if the atom is exposed to X-rays or gamma rays
with photon energy greater than the ionization potential of the
electron. When an electron on an outer orbital of the atom relaxes
to fill the vacancy on the inner orbital, an X-ray (fluorescent
X-ray or secondary X-ray) is emitted. The emitted X-ray has a
photon energy equal the energy difference between the outer orbital
and inner orbital electrons.
[0003] For a given atom, the number of possible relaxations is
limited. As shown in FIG. 1A, when an electron on the L orbital
relaxes to fill a vacancy on the K orbital (L.fwdarw.K), the
fluorescent X-ray is called K.alpha.. The fluorescent X-ray from
M.fwdarw.K relaxation is called K.beta.. As shown in FIG. 1B, the
fluorescent X-ray from M.fwdarw.L relaxation is called L.alpha.,
and so on.
[0004] Analyzing the fluorescent X-ray spectrum can identify the
elements in a sample because each element has orbitals of
characteristic energy. The fluorescent X-ray can be analyzed either
by sorting the energies of the photons (energy-dispersive analysis)
or by separating the wavelengths of the fluorescent X-ray
(wavelength-dispersive analysis). The intensity of each
characteristic energy peak is directly related to the amount of
each element in the sample.
[0005] In one type of detector suitable for energy dispersive
analysis of X-ray, when an X-ray photon incident on an absorption
layer of the detector, it can ionize a large number of atoms in the
absorption layer, with the amount of charge carriers produced being
largely proportional to the energy of the X-ray photon. The charge
carriers are collected and counted to determine the energy of the
X-ray photon. A spectrum may be compiled based on the number of
X-ray photons as a function of their energy.
[0006] Other types of radiation (e.g., gamma ray) can also be used
for elemental analysis of a sample by causing fluorescence in the
sample, in a similar fashion.
SUMMARY
[0007] Disclosed herein is a detector, comprising: a plurality of
strip pixels, wherein each of the strip pixel is configured to
count numbers of radiation photons incident thereon whose energy
falls in a plurality of bins, within a period of time.
[0008] According to an embodiment, the strip pixels are arranged in
an array.
[0009] According to an embodiment, the strip pixels are configured
to receive radiation photons from a sidewall of an absorption layer
of the detector.
[0010] According to an embodiment, the detector is further
configured to add the numbers of radiation photons for the bins of
the same energy range counted by all the strip pixels.
[0011] According to an embodiment, each of the strip pixels of the
detector comprises an analog-to-digital converter (ADC) configured
to digitize an analog signal representing the energy of an incident
X-ray photon into a digital signal.
[0012] According to an embodiment, the strip pixels are configured
to operate in parallel.
[0013] According to an embodiment, the detector further comprises:
a radiation absorption layer comprising the strip pixels, the strip
pixels comprising an electric contact; a first voltage comparator
configured to compare a voltage of the electric contact to a first
threshold; a second voltage comparator configured to compare the
voltage to a second threshold; a controller; a plurality of
counters each associated with a bin and configured to register a
number of radiation photons absorbed by one of the strip pixels
wherein the energy of the radiation photons falls in the bin;
wherein the controller is configured to start a time delay from a
time at which the first voltage comparator determines that an
absolute value of the voltage equals or exceeds an absolute value
of the first threshold; wherein the controller is configured to
determine whether an energy of a radiation photon falls into the
bin; wherein the controller is configured to cause the number
registered by the counter associated with the bin to increase by
one.
[0014] According to an embodiment, the detector further comprises a
capacitor module electrically connected to the electric contact,
wherein the capacitor module is configured to collect charge
carriers from the electric contact.
[0015] According to an embodiment, the controller is configured to
activate the second voltage comparator at a beginning or expiration
of the time delay.
[0016] According to an embodiment, the controller is configured to
connect the electric contact to an electrical ground.
[0017] According to an embodiment, a rate of change of the voltage
is substantially zero at expiration of the time delay.
[0018] According to an embodiment, the radiation absorption layer
comprises silicon, GaAs, CdTe, CdZnTe, or a combination
thereof.
[0019] Disclosed herein is a system comprising a detector described
above, and a radiation source, wherein the system is configured to
perform radiation radiography on human chest or abdomen.
[0020] Disclosed herein is a system comprising a detector described
above, and a radiation source, wherein the system is configured to
perform radiation radiography on human teeth.
[0021] Disclosed herein is a cargo scanning or non-intrusive
inspection (NII) system, comprising the detector described above,
and a radiation source, wherein the cargo scanning or non-intrusive
inspection (NII) system is configured to identify elements by
energy dispersive analysis using radiation transmitted through an
object inspected.
[0022] Disclosed herein is a cargo scanning or non-intrusive
inspection (NII) system, comprising the detector described above,
and a high-energy X-ray source, or gamma ray source, wherein the
cargo scanning or non-intrusive inspection (NII) system is
configured to identify elements by energy dispersive analysis using
backscattered radiation.
[0023] Disclosed herein is a full-body scanner system comprising
the detector described above, and a radiation source, wherein the
full-body scanner is configured to identify elements.
[0024] Disclosed herein is a method comprising: exposing a detector
with a plurality of strip pixels to a radiation, wherein the
radiation incidents on a sidewall of the strip pixels; determining
a number of radiation photons for each strip pixel for one of a
plurality of bins, wherein energy of the radiation photon falls in
the one bin; and compiling the numbers of radiation photons as an
energy spectrum of the radiation photons.
BRIEF DESCRIPTION OF FIGURES
[0025] FIG. 1A and FIG. 1B schematically show mechanisms of
XRF.
[0026] FIG. 2A schematically shows a detailed cross-sectional view
of a detector.
[0027] FIG. 2B schematically shows an alternative detailed
cross-sectional view of the detector.
[0028] FIG. 3 schematically shows a functional block diagram for
the detector, according to an embodiment.
[0029] FIG. 4A and FIG. 4B each show a component diagram of the
electronic system of the detector, according to an embodiment.
[0030] FIG. 5 schematically shows a temporal change of the electric
current flowing through an electric contact (upper curve) caused by
charge carriers generated by a radiation photon incident on a strip
pixel associated with the electric contact, and a corresponding
temporal change of the voltage of the electric contact (lower
curve).
[0031] FIG. 6 shows an example flow chart for step 151 in FIG. 3,
according to an embodiment.
[0032] FIG. 7 schematically shows a system comprising the detector
described herein, suitable for medical imaging such as chest X-ray
radiography, abdominal X-ray radiography, etc., according to an
embodiment.
[0033] FIG. 8 schematically shows a system comprising the detector
described herein suitable for dental X-ray radiography, according
to an embodiment.
[0034] FIG. 9 schematically shows a cargo scanning or non-intrusive
inspection (NII) system comprising the detector described herein,
according to an embodiment.
[0035] FIG. 10 schematically shows another cargo scanning or
non-intrusive inspection (NII) system comprising the detector
described herein and a high-energy radiation source, according to
an embodiment.
[0036] FIG. 11 schematically shows a full-body scanner system
comprising a plurality of the detectors described herein, according
to an embodiment.
DETAILED DESCRIPTION
[0037] FIG. 2A schematically shows a detector 100, according to an
embodiment. The detector may include a radiation absorption layer
110 and an electronics layer 120 (e.g., an ASIC) for processing or
analyzing electrical signals generated in the radiation absorption
layer. The radiation absorption layer 110 comprises a plurality of
semiconductor strips, each of which comprises an electric contact
119B and a discrete region 114 shown in FIG. 2A. The plurality of
semiconductor strips may be arranged parallel with one another, or
any other suitable patterns. The detector 100 may have an "edge-on"
configuration, wherein the detector 100 is configured to receive
radiation from a radiation source 200 at a sidewall 160 of the
radiation absorption layer 110. The "sidewall" of the radiation
absorption layer 110 is a surface of the radiation absorption layer
110, where the surface traverses the semiconductor strips. In the
edge-on configuration, radiation incident on the sidewall propagate
in the radiation absorption layer 110 roughly along the
longitudinal direction of the semiconductor strips, and is absorbed
by the radiation absorption layer 110 along the way. This
configuration is helpful in increasing radiation absorption
efficiency because the radiation can propagate in and be absorbed
by the radiation absorption layer 110 along a length of millimeters
or more. The detector 100 may be in a configuration other than the
edge-on configuration, wherein the detector 100 is configured to
receive radiation from the radiation source 200 not at the sidewall
160 of the radiation absorption layer 110.
[0038] The detector 100 may have a plurality of "strip pixels" 150.
A strip pixel 150 comprises a semiconductor strip and a portion of
the radiation absorption layer 110 around the semiconductor strip,
where substantially all (more than 98%, more than 99.5%, more than
99.9%, or more than 99.99% of) charge carriers generated by a
radiation photon absorbed by the portion flow to that semiconductor
strip. Each strip pixel 150 is configured to detect a radiation
photon incident on a portion of the sidewall of the radiation
absorption layer 110. For example, each strip pixel 150 is
configured to count numbers of radiation photons incident on the
portion of the sidewall of the radiation absorption layer 110 whose
energy falls in a plurality of bins, within a period of time. All
the strip pixels 150 may be configured to count the numbers of
radiation photons incident on the portion of the sidewall of the
radiation absorption layer 110 within a plurality of bins of energy
within the same period of time. Each strip pixel 150 may have its
own analog-to-digital converter (ADC) configured to digitize an
analog signal representing the energy of an incident radiation
photon into a digital signal. The strip pixels 150 may be
configured to operate in parallel. For example, when one strip
pixel 150 measures an incident radiation photon, another strip
pixel 150 may be waiting for a radiation photon to arrive. The
strip pixels 150 may not have to be individually addressable.
[0039] The detector 100 may have at least 100, 500, 1000, or more
strip pixels. The detector 100 may be configured to add the numbers
of radiation photons for the bins of the same energy range counted
by all the strip pixels. For example, the detector 100 may add the
numbers the strip pixels stored in a bin for energy from 20 KeV to
21 KeV, add the numbers the strip pixels stored in a bin for energy
from 21 KeV to 22 KeV, and so on. The detector 100 may compile the
added numbers for the bins as a spectrum of the radiation photons
incident on the detector 100.
[0040] As shown in a detailed cross-sectional view of the detector
100 in FIG. 2A, according to an embodiment, the radiation
absorption layer 110 may include one or more diodes (e.g., p-i-n or
p-n) formed by a first doped region 111, one or more discrete
regions 114 of a second doped region 113. The second doped region
113 may be separated from the first doped region 111 by an optional
the intrinsic region 112. The discrete regions 114 are separated
from one another by the first doped region 111 or the intrinsic
region 112. The first doped region 111 and the second doped region
113 have opposite types of doping (e.g., region 111 is p-type and
region 113 is n-type, or region 111 is n-type and region 113 is
p-type). In the example in FIG. 2A, each of the discrete regions
114 of the second doped region 113 forms a diode with the first
doped region 111 and the optional intrinsic region 112. Namely, in
the example in FIG. 2B, the radiation absorption layer 110 has a
plurality of diodes having the first doped region 111 as a shared
electrode. The first doped region 111 may also have discrete
portions.
[0041] When a radiation photon hits the radiation absorption layer
110 including diodes, the radiation photon may be absorbed and
generate one or more charge carriers by a number of mechanisms. A
radiation photon may generate 10 to 100000 charge carriers. The
charge carriers may drift to the electrodes of one of the diodes
under an electric field. The field may be an external electric
field. The electric contact 119B may include discrete portions each
of which is in electric contact with the discrete regions 114.
[0042] As shown in an alternative detailed cross-sectional view of
the detector 100 in FIG. 2B, according to an embodiment, the
radiation absorption layer 110 may include a resistor of a
semiconductor material such as, silicon, GaAs, CdTe, CdZnTe, or a
combination thereof, but does not include a diode. The
semiconductor may have a high mass attenuation coefficient for the
radiation of interest.
[0043] When a radiation photon hits the radiation absorption layer
110 including a resistor but not diodes, it may be absorbed and
generate one or more charge carriers by a number of mechanisms. A
radiation photon may generate 10 to 100000 charge carriers. The
charge carriers may drift to the electric contacts 119A and 119B
under an electric field. The field may be an external electric
field. The electric contact 119B includes discrete portions.
[0044] The electronics layer 120 may include an electronic system
121, suitable for processing or interpreting signals generated by
radiation photons absorbed by the radiation absorption layer 110.
The electronic system 121 may include an analog circuitry such as a
filter network, amplifiers, integrators, and comparators, or a
digital circuitry such as a microprocessor, and memory. The
electronic system 121 may include components shared by the strip
pixels or components dedicated to a single strip pixel. For
example, the electronic system 121 may include an amplifier
dedicated to each strip pixel and a microprocessor shared among all
the strip pixels. The electronic system 121 may be electrically
connected to the strip pixels by vias 131. Space among the vias may
be filled with a filler material 130, which may increase the
mechanical stability of the connection of the electronics layer 120
to the radiation absorption layer 110. Other bonding techniques are
possible to connect the electronic system 121 to the strip pixels
without using vias. For example, the electronics layer 120
including the electronic system 121 may be in a substrate
positioned side by side with the radiation absorption layer 110 and
the strip pixels may be connected to the electronic system 121 by
wire bonding.
[0045] FIG. 3 schematically shows a block diagram for the detector
100, according to an embodiment. In step 151, each strip pixel 150
may measure the energy of a radiation photon it absorbs. The energy
of the radiation photon is digitized (e.g., by an ADC) in step 152
into one of a plurality of bins 153A, 1536, 153C . . . . The bins
153A, 1536, 153C . . . each have a corresponding counter 154A, 1546
and 154C, respectively. When the energy is allocated into a bin,
the number stored in the corresponding counter increases by one.
The detector 100 may added the numbers stored in all the counters
corresponding to bins for the same energy range in the strip pixels
150. For example, the numbers stored in all the counters 154C in
all strip pixels 150 may be added and stored in a global counter
100C for the same energy range. The numbers stored in all the
global counters may be compiled into an energy spectrum of the
radiation incident on the detector 100.
[0046] FIG. 4A and FIG. 4B each show a component diagram of the
electronic system 121, according to an embodiment. The electronic
system 121 may include a first voltage comparator 301, a second
voltage comparator 302, a plurality of counters 320 (including
counters 320A, 320B, 320C, 320D . . . ), a switch 305, an ADC 306
and a controller 310.
[0047] The first voltage comparator 301 is configured to compare
the voltage of a discrete portion of the electric contact 1196 to a
first threshold. The first voltage comparator 301 may be configured
to monitor the voltage directly, or calculate the voltage by
integrating an electric current flowing through the diode or
electric contact over a period of time. The first voltage
comparator 301 may be controllably activated or deactivated by the
controller 310. The first voltage comparator 301 may be a
continuous comparator. Namely, the first voltage comparator 301 may
be configured to be activated continuously, and monitor the voltage
continuously. The first voltage comparator 301 configured as a
continuous comparator reduces the chance that the system 121 misses
signals generated by an incident radiation photon. The first
voltage comparator 301 configured as a continuous comparator is
especially suitable when the incident radiation intensity is
relatively high. The first voltage comparator 301 may be a clocked
comparator, which has the benefit of lower power consumption. The
first voltage comparator 301 configured as a clocked comparator may
cause the system 121 to miss signals generated by some incident
radiation photons. When the incident radiation intensity is low,
the chance of missing an incident radiation photon is low because
the time interval between two successive photons is relatively
long. Therefore, the first voltage comparator 301 configured as a
clocked comparator is especially suitable when the incident
radiation intensity is relatively low. The first threshold may be
1-5%, 5-10%, 10%-20%, 20-30%, 30-40% or 40-50% of the maximum
voltage one incident radiation photon may generate on the electric
contact 119B. The maximum voltage may depend on the energy of the
incident radiation photon (i.e., the wavelength of the incident
radiation), the material of the radiation absorption layer 110, and
other factors. For example, the first threshold may be 50 mV, 100
mV, 150 mV, or 200 mV.
[0048] The second voltage comparator 302 is configured to compare
the voltage to a second threshold. The second voltage comparator
302 may be configured to monitor the voltage directly, or calculate
the voltage by integrating an electric current flowing through the
diode or the electric contact over a period of time. The second
voltage comparator 302 may be a continuous comparator. The second
voltage comparator 302 may be controllably activate or deactivated
by the controller 310. When the second voltage comparator 302 is
deactivated, the power consumption of the second voltage comparator
302 may be less than 1%, less than 5%, less than 10% or less than
20% of the power consumption when the second voltage comparator 302
is activated. The absolute value of the second threshold is greater
than the absolute value of the first threshold. As used herein, the
term "absolute value" or "modulus" |X| of a real number x is the
non-negative value of x without regard to its sign. Namely,
x = { x , .times. if .times. .times. x .gtoreq. 0 - x , .times. if
.times. .times. x .ltoreq. 0 . ##EQU00001##
The second threshold may be 200%-300% of the first threshold. For
example, the second threshold may be 100 mV, 150 mV, 200 mV, 250 mV
or 300 mV. The second voltage comparator 302 and the first voltage
comparator 310 may be the same component. Namely, the system 121
may have one voltage comparator that can compare a voltage with two
different thresholds at different times.
[0049] The first voltage comparator 301 or the second voltage
comparator 302 may include one or more op-amps or any other
suitable circuitry. The first voltage comparator 301 or the second
voltage comparator 302 may have a high speed to allow the system
121 to operate under a high flux of incident radiation. However,
having a high speed is often at the cost of power consumption.
[0050] The counters 320 may be a software component (e.g., numbers
stored in a computer memory) or a hardware component (e.g., 4017 IC
and 7490 IC). Each counter 320 is associated with a bin for an
energy range. For example, counter 320A may be associated with a
bin for 70-71 KeV, counter 320B may be associated with a bin for
71-72 KeV, counter 320C may be associated with a bin for 72-73 KeV,
counter 320D may be associated with a bin for 73-74 KeV. When the
energy of an incident radiation photon is determined by the ADC 306
to be in the bin a counter 320 is associated with, the number
registered in the counter 320 is increased by one.
[0051] The controller 310 may be a hardware component such as a
microcontroller and a microprocessor. The controller 310 is
configured to start a time delay from a time at which the first
voltage comparator 301 determines that the absolute value of the
voltage equals or exceeds the absolute value of the first threshold
(e.g., the absolute value of the voltage increases from below the
absolute value of the first threshold to a value equal to or above
the absolute value of the first threshold). The absolute value is
used here because the voltage may be negative or positive,
depending on whether the voltage of the cathode or the anode of the
diode or which electric contact is used. The controller 310 may be
configured to keep deactivated the second voltage comparator 302,
the counter 320 and any other circuits the operation of the first
voltage comparator 301 does not require, before the time at which
the first voltage comparator 301 determines that the absolute value
of the voltage equals or exceeds the absolute value of the first
threshold. The time delay may expire after the voltage becomes
stable, i.e., the rate of change of the voltage is substantially
zero. The phase "the rate of change is substantially zero" means
that temporal change is less than 0.1%/ns. The phase "the rate of
change is substantially non-zero" means that temporal change of the
voltage is at least 0.1%/ns.
[0052] The controller 310 may be configured to activate the second
voltage comparator during (including the beginning and the
expiration) the time delay. In an embodiment, the controller 310 is
configured to activate the second voltage comparator at the
beginning of the time delay. The term "activate" means causing the
component to enter an operational state (e.g., by sending a signal
such as a voltage pulse or a logic level, by providing power,
etc.). The term "deactivate" means causing the component to enter a
non-operational state (e.g., by sending a signal such as a voltage
pulse or a logic level, by cut off power, etc.). The operational
state may have higher power consumption (e.g., 10 times higher, 100
times higher, 1000 times higher) than the non-operational state.
The controller 310 itself may be deactivated until the output of
the first voltage comparator 301 activates the controller 310 when
the absolute value of the voltage equals or exceeds the absolute
value of the first threshold.
[0053] The controller 310 may be configured to cause the number
registered by one of the counters 320 to increase by one, if,
during the time delay, the second voltage comparator 302 determines
that the absolute value of the voltage equals or exceeds the
absolute value of the second threshold, and the energy of the
radiation photon falls in the bin associated with the counter
320.
[0054] The controller 310 may be configured to cause the ADC 306 to
digitize the voltage upon expiration of the time delay and
determine based on the voltage which bin the energy of the
radiation photon falls in.
[0055] The controller 310 may be configured to connect the electric
contact 119B to an electrical ground, so as to reset the voltage
and discharge any charge carriers accumulated on the electric
contact 119B. In an embodiment, the electric contact 119B is
connected to an electrical ground after the expiration of the time
delay. In an embodiment, the electric contact 119B is connected to
an electrical ground for a finite reset time period. The controller
310 may connect the electric contact 119B to the electrical ground
by controlling the switch 305. The switch may be a transistor such
as a field-effect transistor (FET).
[0056] In an embodiment, the system 121 has no analog filter
network (e.g., a RC network). In an embodiment, the system 121 has
no analog circuitry.
[0057] The ADC 306 may feed the voltage it measures to the
controller 310 as an analog or digital signal. The ADC may be a
successive-approximation-register (SAR) ADC (also called successive
approximation ADC). An SAR ADC digitizes an analog signal via a
binary search through all possible quantization levels before
finally converging upon a digital output for the analog signal. An
SAR ADC may have four main subcircuits: a sample and hold circuit
to acquire the input voltage (V.sub.in), an internal digital-analog
converter (DAC) configured to supply an analog voltage comparator
with an analog voltage equal to the digital code output of the
successive approximation register (SAR), the analog voltage
comparator that compares V.sub.in to the output of the internal DAC
and outputs the result of the comparison to the SAR, the SAR
configured to supply an approximate digital code of V.sub.in to the
internal DAC. The SAR may be initialized so that the most
significant bit (MSB) is equal to a digital 1. This code is fed
into the internal DAC, which then supplies the analog equivalent of
this digital code (V.sub.ref/2) into the comparator for comparison
with V.sub.in. If this analog voltage exceeds V.sub.in the
comparator causes the SAR to reset this bit; otherwise, the bit is
left a 1. Then the next bit of the SAR is set to 1 and the same
test is done, continuing this binary search until every bit in the
SAR has been tested. The resulting code is the digital
approximation of V.sub.in and is finally output by the SAR at the
end of the digitization.
[0058] The system 121 may include a capacitor module 309
electrically connected to the electric contact 119B, wherein the
capacitor module is configured to collect charge carriers from the
electric contact 119B. The capacitor module can include a capacitor
in the feedback path of an amplifier. The amplifier configured as
such is called a capacitive transimpedance amplifier (CTIA). CTIA
has high dynamic range by keeping the amplifier from saturating and
improves the signal-to-noise ratio by limiting the bandwidth in the
signal path. Charge carriers from the electrode accumulate on the
capacitor over a period of time ("integration period") (e.g., as
shown in FIG. 5, between t.sub.s to t.sub.0). After the integration
period has expired, the capacitor voltage is sampled by the ADC 306
and then reset by a reset switch. The capacitor module 309 can
include a capacitor directly connected to the electric contact
119B.
[0059] FIG. 5 schematically shows a temporal change of the electric
current flowing through the electric contact 119B (upper curve)
caused by charge carriers generated by an radiation photon incident
on the strip pixel 150 associated with the electric contact 119B,
and a corresponding temporal change of the voltage of the electric
contact 119B (lower curve). The voltage may be an integral of the
electric current with respect to time. At time t.sub.0, the
radiation photon hits the diode or the resistor, charge carriers
start being generated in the strip pixel 150, electric current
starts to flow through the electric contact 119B, and the absolute
value of the voltage of the electric contact 119B starts to
increase. At time t.sub.1, the first voltage comparator 301
determines that the absolute value of the voltage equals or exceeds
the absolute value of the first threshold V1, and the controller
310 starts the time delay TD1 and the controller 310 may deactivate
the first voltage comparator 301 at the beginning of TD1. If the
controller 310 is deactivated before t.sub.1, the controller 310 is
activated at t.sub.1. During TD1, the controller 310 activates the
second voltage comparator 302. The term "during" a time delay as
used here means the beginning and the expiration (i.e., the end)
and any time in between. For example, the controller 310 may
activate the second voltage comparator 302 at the expiration of
TD1. If during TD1, the second voltage comparator 302 determines
that the absolute value of the voltage equals or exceeds the
absolute value of the second threshold at time t.sub.2, the
controller 310 waits for stabilization of the voltage to stabilize.
The voltage stabilizes at time t.sub.e, when all charge carriers
generated by the radiation photon drift out of the radiation
absorption layer 110. At time t.sub.s, the time delay TD1 expires.
At or after time t.sub.e, the controller 310 causes the ADC 306 to
digitize the voltage and determines which bin the energy of the
radiation photons falls in. The controller 310 then causes the
number registered by the counter 320 corresponding to the bin to
increase by one. In the example of FIG. 6, time t.sub.s is after
time t.sub.e; namely TD1 expires after all charge carriers
generated by the radiation photon drift out of the radiation
absorption layer 110. If time t.sub.e cannot be easily measured,
TD1 can be empirically chosen to allow sufficient time to collect
essentially all charge carriers generated by an radiation photon
but not too long to risk have another incident radiation photon.
Namely, TD1 can be empirically chosen so that time t.sub.s is
empirically after time t.sub.e. Time t.sub.s is not necessarily
after time t.sub.e because the controller 310 may disregard TD1
once V2 is reached and wait for time t.sub.e. The rate of change of
the difference between the voltage and the contribution to the
voltage by the dark current is thus substantially zero at t.sub.e.
The controller 310 may be configured to deactivate the second
voltage comparator 302 at expiration of TD1 or at t.sub.2, or any
time in between.
[0060] The voltage at time t.sub.e is proportional to the amount of
charge carriers generated by the radiation photon, which relates to
the energy of the radiation photon. The controller 310 may be
configured to determine the bin the energy of the radiation photon
falls in, based on the output of the ADC 306.
[0061] After TD1 expires or digitization by the ADC 306, whichever
later, the controller 310 connects the electric contact 119B to an
electric ground for a reset period RST to allow charge carriers
accumulated on the electric contact 119B to flow to the ground and
reset the voltage. After RST, the system 121 is ready to detect
another incident radiation photon. Implicitly, the rate of incident
radiation photons the system 121 can handle in the example of FIG.
6 is limited by 1/(TD1+RST). If the first voltage comparator 301
has been deactivated, the controller 310 can activate it at any
time before RST expires. If the controller 310 has been
deactivated, it may be activated before RST expires.
[0062] Because the detector 100 has many strip pixels 150 that may
operate in parallel, the detector can handle much higher rate of
incident radiation photons. This is because the rate of incidence
on a particular strip pixel 150 is 1/N of the rate of incidence on
the entire array of strip pixels, where N is the number of strip
pixels.
[0063] FIG. 6 shows an example flow chart for step 151 in FIG. 3,
according to an embodiment. In step 701, compare, e.g., using the
first voltage comparator 301, a voltage of an electric contact 119B
of a diode or a resistor exposed to radiation photons (e.g.,
fluorescent X-ray), to the first threshold. In step 702, determine,
e.g., with the controller 310, whether the absolute value of the
voltage equals or exceeds the absolute value of the first threshold
V1. If the absolute value of the voltage does not equal or exceed
the absolute value of the first threshold, the method goes back to
step 701. If the absolute value of the voltage equals or exceeds
the absolute value of the first threshold, continue to step 703. In
step 703, measure T=(t.sub.1-t.sub.0). In step 704, start, e.g.,
using the controller 310, the time delay TD1. In step 705, compare,
e.g., using the second voltage comparator 302, the voltage to the
second threshold. In step 706, determine, e.g., using the
controller 310, whether the absolute value of the voltage equals or
exceeds the absolute value of the second threshold V2. If the
absolute value of the voltage does not equal or exceed the absolute
value of the second threshold, the method goes to step 708. In step
708, reset the voltage to an electrical ground, e.g., by connecting
the electric contact 119B to an electrical ground. If the absolute
value of the voltage equals or exceeds the absolute value of the
second threshold, continue to step 709. In step 709, measure the
voltage after it stabilizes, at time t.sub.m, and subtract an
contribution from a dark current to the measured voltage. Time
t.sub.m can be any time after TD1 expires and before RST. The
result is provided to ADC in step 152 in FIG. 3. The time when the
reset period ends (e.g., the time when the electric contact 119B is
disconnected from the electrical ground) is t.sub.r.
[0064] FIG. 7 schematically shows a system comprising the detector
100 described herein. The system may be used for medical imaging
such as chest X-ray radiography, abdominal X-ray radiography, etc.
X-ray emitted from a radiation source 1201 penetrates an object
1202 (e.g., a human body part such as chest, limb, abdomen), is
attenuated by different degrees by the internal structures of the
object 1202 (e.g., bones, muscle, fat and organs, etc.), and is
projected to the detector 100. The detector 100 forms an energy
spectrum by detecting the intensity distribution of the
radiation.
[0065] FIG. 8 schematically shows a system comprising the detector
100 described herein. The system may be used for medical imaging
such as dental X-ray radiography. The system comprises a radiation
source 1301. Radiation emitted from the radiation source 1301
penetrates an object 1302 that is part of a mammal (e.g., human)
mouth. The object 1302 may include a maxilla bone, a palate bone, a
tooth, the mandible, or the tongue. The radiation is attenuated by
different degrees by the different structures of the object 1302
and is projected to the detector 100. The detector 100 forms an
energy spectrum by detecting the intensity distribution of the
radiation. Teeth absorb radiation more than dental caries,
infections, periodontal ligament. The dosage of radiation received
by a dental patient is typically small (around 0.150 mSv for a full
mouth series).
[0066] FIG. 9 schematically shows a cargo scanning or non-intrusive
inspection (NII) system comprising the detector 100 described
herein. The system may be used for luggage screening at public
transportation stations and airports. The system comprises a
radiation source 1401. Radiation emitted from the radiation source
1401 may penetrate a piece of luggage 1402, be differently
attenuated by the contents of the luggage, and projected to the
detector 100. The detector 100 forms an energy spectrum by
detecting the intensity distribution of the transmitted radiation.
The system may reveal contents and element composition of luggage
and identify items forbidden on public transportation, such as
firearms, narcotics, edged weapons, flammables.
[0067] FIG. 10 schematically shows another cargo scanning or
non-intrusive inspection (NII) system comprising the detector 100
described herein. The system may be used for inspecting and
identifying goods in transportation systems such as shipping
containers, vehicles, ships, luggage, etc. The system may comprise
a high-energy X-ray or gamma ray radiation source 1501, and the
detector may comprise an absorption layer comprising CdTe, CdZnTe,
or a combination thereof. Radiation emitted from the radiation
source 1501 may backscatter from an object 1502 (e.g., shipping
containers, vehicles, ships, etc.) and be projected to the detector
100. Different internal structures and composition of the object
1502 may backscatter the radiation differently. The detector 100
forms energy spectra by detecting the intensity distribution of the
backscattered radiation and/or energies of the backscattered
radiation.
[0068] FIG. 11 schematically shows a full-body scanner system
comprising a plurality of the detectors 100 described herein. The
full-body scanner system may detect objects on a person's body for
security screening purposes, without physically removing clothes or
making physical contact. The full-body scanner system may be able
to detect non-metal objects. The full-body scanner system comprises
several collimated radiation sources 1601 on the side. The
collimated radiation emitted from the pulsed radiation source 1601
may backscatter from or penetrate through a human body 1602 being
screened and objects thereon, and be detected by the detector 100.
The objects and the human body may backscatter or attenuate the
radiation differently. The detectors 100 can analyze the object
element compositions by detecting the intensity distribution of the
backscattered or transmitted radiation.
[0069] While various aspects and embodiments have been disclosed
herein, other aspects and embodiments will be apparent to those
skilled in the art. The various aspects and embodiments disclosed
herein are for purposes of illustration and are not intended to be
limiting, with the true scope and spirit being indicated by the
following claims.
* * * * *