METHOD AND SYSTEM FOR IMAGING A RADIATION SOURCE

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

[0001] This invention was made with Government support under

Contract DE-AC0576RLO1830 awarded by the U.S. Department of Energy. The

Government has certain rights in the invention.

PRIORITY

[0002] This invention claims priority from a provisional patent application

entitled Implementation of Image and Energy Calculations for CZT based Radiation Detector filed December 19, 2006, Application No. 60/670,651 , the

contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

[0003] This invention generally relates to radiation sensing and more

particularly to methods and systems for imaging a radiation source.

Background Information

[0004] In the field of radiation sensing, a need exists for devices that allow

for the improved detection, location and characterization of various suspected

radiation emitting sources. While various methods and devices have been

created to attempt to accomplish these tasks, various problems related to these

prior art methods have limited the functionality, versatility, reliability, and

implementation of such methods and the use of particular devices for various

types of applications. The present invention is a new method and system that

overcomes these prior art problems and provides a method that, for example, reduces data processing, energy, and space requirements for a variety of

functions including detecting, imaging, and characterizing a radiation source,

distinguishing between multiple radiation sources, mapping these sources, and

screening potential sources. The present invention thus provides attractive features for portable, mobile, or remote operation applications.

[0005] Additional advantages and novel features of the present invention

will be set forth as follows and will be readily apparent from the descriptions

and demonstrations set forth herein. Accordingly, the following descriptions of

the present invention should be seen as illustrative of the invention and not as limiting in any way.

SUMMARY

[0006] The present invention includes improved methods for imaging a

radiation source, and a device that utilizes such methods. In one embodiment of the invention the method includes the steps of: calculating at least one Compton

cone of a first parameter of a radiation emission from information received from

a sensor occurrence; and tracing this Compton cone on to a unit sphere having preselected characteristics using an estimated angular uncertainty to limit at

least a portion of said tracing. In another embodiment of the invention at least

two Compton cones are calculated and then intersected upon a predefined

surface such as a sphere. These intersection points can then be iterated over a

preselected series of prior events. Various implementations, modifications and

alterations of these methods are contemplated within the scope of the attached

claims and are set forth in part in the detailed description of the preferred

embodiments of the invention which is described hereinafter. In one embodiment the method is deployed inside of a device (for example, a small

lightweight handheld device) that has a FPGA (field programmable field array)

arrangement that performs the steps of the previously described methods.

[0007] The method, system and arrangement of the present invention

allows for the creation of devices that are capable of providing ready results in a

timely fashion with decreased energy consumption. These devices may also

typically occupy less space and provide faster results than other devices that are

currently known. This allows for highly efficient portable, mobile, or remotely- operated radiation detectors that may be used in a variety of applications including nuclear security, medicine, astronomy, physics and other scientific

research, as well as in nuclear safety and other applications depending upon the

exact desires and necessities of the user. Therefore the embodiments and disclosures of this application should be seen as illustrative and not limiting in

any way.

[0008] The purpose of the foregoing abstract is to enable the United States

Patent and Trademark Office and the public generally, especially the scientists,

engineers, and practitioners in the art who are not familiar with patent or legal

terms or phraseology, to determine quickly from a cursory inspection the nature

and essence of the technical disclosure of the application. The abstract is neither

intended to define the invention of the application, which is measured by the

claims, nor is it intended to be limiting as to the scope of the invention in any way.

[0009] Various advantages and novel features of the present invention are

described herein and will become further readily apparent to those skilled in this art from the following detailed description. In the preceding and following

descriptions we have shown and described only the preferred embodiment of

the invention, by way of illustration of the best mode contemplated for carrying

out the invention. As will be realized, the invention is capable of modification in various respects without departing from the invention. Accordingly, the

drawings and description of the preferred embodiment set forth hereafter are to

be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a view of a single Compton imaging cone.

[0011] Figure 2 is a view of the method of the present invention wherein a

Compton cone is imaged upon a sphere.

[0012] Figure 3 is a view of the two cone method embodiment of the

present invention.

[0013] Figure 4 is a perspective view of a portable radiation detector of the

present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] The following description includes the preferred best mode of one

embodiment of the present invention. It will be clear from this description that the invention is not limited to these illustrated embodiments but that the

invention also includes a variety of modifications and embodiments thereto. Therefore, the present description of the preferred embodiments of the invention

should be seen as illustrative and not as limiting. While the invention is

susceptible to various modifications and alternative constructions, it should be understood that there is no intention to limit the invention to the specific form

disclosed, but, on the contrary, the invention is to cover all modifications,

alternative constructions, and equivalents falling within the spirit and scope of

the invention as defined in the claims.

[0015] The present invention is a method for imaging a radiation source,

to determine various features such as location, direction and/or characterization

of that source. In one preferred embodiment of the invention the methods which

are utilized to interpret data and characterize this data are incorporated within a

handheld radiation identifier device. In addition to isotope identification, the

device performs basic Compton imaging in accordance with the present method

to determine the location of radiation sources. In a first embodiment, individual Compton cones are projected onto a unit sphere, while in a second embodiment

the intersection of two Compton cones and the unit sphere are calculated.

Simulations demonstrate that these methods are suitable for determining the

directionality, even with features such as uncertainty calculations omitted. The

one-cone method generally works more efficiently at high count rates, and the two-cone method generally causes fewer artifacts. While these particular

descriptions related to the implementation of these methods have been

provided, it is to be distinctly understood that the invention is not limited to the

particular embodiments or implementations set forth in this description of the preferred embodiment.

[0016] The methods described in this preferred description may be

implemented by any type of device or configuration but in this description of the preferred embodiment are implemented within an arrangement that includes an

FPGA (field programmable gate array) because these types of devices typically

offer better performance than microprocessors for signal processing applications.

In addition, the methods and algorithms that are utilized rely on basic functions

such as addition and multiplication as much as possible in order to reduce power consumption. Some steps may be parallelized in order to further reduce

power consumption. Depending upon the needs and necessities of the user,

either or both of the methods described in the present application may be

utilized with or without the FPGA to achieve the desired results.

[0017] Referring now to Fig. 1, a representation of a gamma ray γ

interacting with a detector is shown. The gamma ray generally undergoes a

sequence of interactions i = 1, 2, ..., n in which it deposits energy E/ at location

d = (dix, diy, da). If all events are captured inside the detector, the incident energy

Eo equals the sum of the deposited energies. In a typical embodiment the deflection angle θ for the first interaction can be computed using the Compton

scatter formula , where m _e c ² = 511.00 keV is the rest mass energy of an electron. Although the incident direction of the gamma ray cannot

be measured directly, it generally falls on a cone with opening angle θ and d, - d,

C = I normalized axis direction I ' ² H . In a typical embodiment each Compton cone is typically expected to have different values for c and θ. By intersecting the

cones for multiple gamma rays the probable location of the radiation source is

revealed. Furthermore, many applications do not require the distance between

the detector and the source to be computed. In this case, any or all of the

Compton cones can be projected onto a two or three dimensional surface, such

as a plane or a sphere, to create an image. The resulting image can be processed

to estimate the direction to the radiation source but typically contains no depth

information. Computationally, this method is much simpler than intersecting

arbitrary cones in three dimensions. However, modifications to this method

have also been developed which provide additional advantages.

[0018] In order to perform the imaging method described above some

processing of the raw energy and position data from each gamma-ray event that

is identified may be required. First, the true energies and positions of the interactions in the detector array should be determined. Then, the correct sequence of events is typically established. In the preferred embodiment of the

invention, the gamma rays interact with one or more solid detectors, each of

which features a common cathode and pixelated anode. While this particular

embodiment is set forth and described it is to be distinctly understood that the

invention is not limited thereto but may be variously employed with a variety of

other types of detectors including positron sensitive detectors such as double

sided strip detectors, and arrays of standard types of detectors. The front-end

electronics provide four values for each interaction i within the detectors: the anode charge amplitude Ai, the anode pixel coordinates (d,χ, d, _y ), and the time

difference τ, between the anode and cathode pulses. The depth of the event da is

roughly proportional to the time difference τ>. To account for second-order

effects, it is assumed that d,z generally follows a piecewise linear approximation

which can be described as: d _a = a _z τ _ι +β _z , τ _low ≤ T ₁ ≤ τ _hιgh , where a _* and β are the gain

and offset of the line segment. Likewise, it is assumed that the deposited energy

Ei generally follows E ₁ = χ _E (a _E λ, +β _E ), A _low < A _l < A _hlgh , where aε and β are as

before, and χε is an adjustment for crosstalk between multiple events. These piecewise linear corrections encompass a wide range of effects, including

material variations, geometric asymmetries, and so forth. Parameters ctz and β

are functions of the pixel coordinates {da, d, _y ). Parameters aε and β depend on

the three-dimensional event location (d«, d, _y , d, _z ) and the current temperature.

Finally, parameter χε depends on the depth da and the distance between multiple interactions.

[0019] In some embodiments of the invention the parameters are calibrated against known standards. To simplify the calibration process, the

system divides the z-axis into a number of virtual layers. The system then stores

Ch. and β for each pixel and time interval, aε and βε for each pixel, virtual layer,

and amplitude interval, and χε for each virtual layer and approximate distance

between multiple interactions. Since the temperature changes relatively slowly, the system can adjust CCE and βε as necessary. Once the energy of each interaction

has been determined, and the incident energy Eo estimated, windowing

techniques may be applied to select gamma rays of certain energies. For

example, the operator might want to mask a natural radiation source to search

for other, hidden threats.

[0020] In embodiments where the front-end electronics do not have

sufficient timing resolution to capture individual events as they occur within the

detector, the imaging method takes the list of interactions reported by the electronics and arranges them in time order. There are a variety of heuristics for

performing this task. For two interactions, the most popular technique uses

energy information alone to determine the most likely sequence order. For three

or more interactions, one approach starts with an assumption about the final

interaction and reconstructs the track backwards. An alternative method uses

Compton kinematics to reject non-physical sequences and calculates the

probability of observing the remaining sequences. Yet another approach applies

Bayesian methods to compare a given sequence order against a

multidimensional simulated dataset. Finally, the actual scatter angles are

compared with the angle predicted from the Compton scatter formula. In

addition to these techniques other techniques may also be utilized to reconstruct

sequences of two or more interactions.

[0021] In this preferred embodiment of the invention, when there are two

interactions, the method includes the step of comparing deposited energies to

determine which event comes first. If the incident energy Eo is less than a certain

threshold Et, the events are ordered so that Ei < £2. Otherwise, the method takes

£1 > Ei. This empirical technique is consistent with observations that gamma rays

that deposit most of their energy in the initial scatter are more likely to be

absorbed in the second interaction. Thus the optimal value of E^ can be derived via a simulation package such as GEANT4. In one embodiment of the invention, Et is approximately 400 keV for a single 2.25-cm ³ CdZnTe crystal.

[0022] In those instances where there are three interactions, the method

takes each permutation of the three interactions and examines the deflection angle φ of the second interaction. The value of cos φ calculated from the

Compton scatter formula, cosφ _E = 1 — ¹ ^- — 2 — , is compared to the value of cos φ

(E ₂ + E ₃ )E ₃

calculated from the event locations within the detector, cos a) , = ,, ^{d3 ~ d2} ,,

Permutations that produce invalid values for the cosines are discarded. Finally,

the permutation that minimizes the absolute difference σ = is

selected. One could also compute the uncertainties in energy in position and

refine the metric to reflect those values.

[0023] In those instances where there are more than three interactions/ the

method could use the same technique to check every permutation of three events

and piece together the most likely sequence. However, this method involves

significant computational effort, and the results become less reliable as n increases. Simulations of the 18-crystal array with a 662-keV source indicate that

only 9.4% of the sequences contain four or more events. Hence, this method is

not employed in the preferred embodiment, but still may be considered and

utilized within the scope of the claims of the present invention.

[0024] The imaging methods of the present invention project Compton

cones onto a sphere of unit radius. This approach is particularly useful when

used in conjunction with three-dimensional position-sensitive detectors. The

technique is sometimes referred to as "4π Compton imaging" since a source at any angle (within all of 4π steradians) relative to the detector can be

reconstructed. The following discussion summarizes the first method which

traces individual cones onto the sphere. An estimated angular uncertainty determines the width of the trace.

[0025] The objective of the one-cone method is to trace individual

Compton cones onto the unit sphere. The result of the projection of the Compton

cone onto the unit sphere is simply a circle. The method of the present invention includes computing the cone axis direction c and opening angle θ as described

previously.

[0026] Referring now to Fig. 2, let s be the direction vector from the origin

to an arbitrary point on the sphere, and let a be the angle between c and s. Since c and s are unit vectors, cosα = c s . The circle can be described as the set of points

for which a = θ. To create the image the sphere is divided into a mesh of r points.

The cone is then traced onto the image by finding all points on the mesh for

≤a ≤θ+δ . The difference angle δ compensates for the fact that points

on the mesh will not always coincide with the projection in general. While this value could be related to the uncertainties in energy and position, in this

embodiment of the invention δ is set to a constant that is proportional to the

angle between adjacent points on the mesh. In the preferred embodiment, implementing the cosα = c s directly is not computationally efficient on an FPGA

since only the cosines of a and θ are known. However, it can be assumed without loss of generality that a and 6>fall within the range [0, π\. The value of δ

can also be limited to the range [0, π/2] without imposing undue restrictions on

the imaging algorithm. Simplifying these interrelationships produces the

equation

{c ■ s < cos θ cos δ + sin θ sin δ OR cos θ > cos δ) AND {c ■ s > cos θ cos δ - sin θ sin δ OR cos θ < - cos <5}

which can then be evaluated for each point on the mesh.

[0027] Each point on the mesh is associated with a value that is initially

set to zero. In this preferred embodiment, the direction histogram is updated by

incrementing by one the values corresponding to the points that satisfy the

conditions set forth above. In an alternative embodiment, said values are

incremented by a factor 1/fc, where k is the total number of points that meet said

conditions. This normalization would ensure that all Compton cones contribute

equally to the direction histogram in summation. In addition, in other

embodiments, the number of computations required by this one-cone method

could be reduced by computing latitude and longitude bounds of the circular projection. Rather than checking every point, the method could iterate over all points in the spherical rectangle inside these bounds.

[0028] In the preferred embodiment of the one-cone method, the directionality

to suspected sources is also calculated. Rather than having the operator interpret

the peaks in the raw image, the instrument will display a horizontal azimuth and

in some instances a vertical altitude for any or all suspected sources. These values can be calculated in several ways. A simple method is to sum along lines

of altitude to determine the intensity versus the azimuth. Another approach is to

identify peaks in the image and compute the centroid of each peak. Neither

technique requires a large amount of computation.

[0029] In this preferred embodiment of the invention, in order to allow for

frequent updating of the directionality display, a sliding-window buffer

arrangement, in which individual cones are not immediately discarded, but

rather remain for a preselected period of time, allows for various cone images to

be compared and allows for the directionality of the source to be continually

updated. The method could also be used in conjunction with an accelerometer to detect when the operator is moving, and discard sequences more rapidly in that

case.

[0030] In addition to this one-cone method, a second method has also

been developed which provides directionality information more directly and is computationally more efficient. In the method the intersection points between

two Compton cones and the unit sphere are calculated. This approach eliminates

the need to trace individual cones onto the sphere. Rather than iterating over points on the unit sphere, the two-cone algorithm iterates over the last m sequences captured by the detector. Referring now to Fig. 3, let c« and & be the

parameters of the Compton cone for one of the previous sequences, and let Cb

and θb be the corresponding parameters for the current sequence. Projecting

these two cones onto the unit sphere might produce the result in Fig. 3.

Assuming the two cones do not coincide, they mutually intersect the sphere at

no more than two points. Now the intersection of the unit sphere with one

Compton cone is equivalent to the intersection of the unit sphere with a certain

plane. This plane is normal to the cone axis c, and located a distance cos θ away from the origin. For two Compton cones, the corresponding planes will intersect

in a line, if they intersect at all. The intersection of this line with the unit sphere

equals the mutual intersection of the two Compton cones with the unit sphere.

[0032] The two-cone method begins with the equations of the two planes

in point-normal form: c _u u = cos6> _α , c _λ u = cos6> _λ . Here u is an arbitrary position

vector. AU vectors are represented in Cartesian coordinates with x, y, and z components, so one could rewrite the equations above as : c _lLX u _κ + c _ay u _y + c _az u _z = cosθ _a

c _bx u _x + c _by u _y + c _hz u _z = cosθ _b .

To determine the intersection line, the algorithm needs to find a point p that

lies on both planes, and hence on the line. The simplest method is to solve the two equations in with the z-coordinate set to zero. Using Cramer's rule,

_p _ ^c _hy cosg,, - c _ay cosfl, ^ c _ax cosfl, - c _hx cosg _fl ^ λ _

^C iLX ^C by ~ ^C ay ^C bx ^C ιa ^C by ~ ^C ay ^C bϊ J

The notation c« refers to the x-component of vector &. Given p, the method can

determine the direction L of the intersection line by taking the cross product

Ca X CK

L ⁼ ( ^C ay ^C hz ~ ^c ia ^c hy ' ^C az ^C hx ~ ^c <ιx ^C bz ' ^C ax ^C by ~ ^C ay ^C bx ) ^•

Then the equation of the line is

u = ^{AL + p} _^ where λ represents an arbitrary real number. Since the unit sphere

is described by ^{u u} = ' , intersecting the line with the sphere produces

(AL + p) • (AL + p) = 1 ,

which expands to the quadratic equation

(L- Lμ ² + 2(L-pμ + (p -p) - l = 0 .

Solving this equation for λ using the quadratic formula and substituting back

into ^{u = + p} yields the desired intersection points:

These points are then recorded in a buffer and then the process is repeated for

another previous sequence, again determining its intersection points with the

last-measured sequence.

[0033] A direct implementation of the two-cone method could be used, for

example, to compute the intersection points for all possible pairs of Compton

cones. If the detector stored m valid sequences in memory, the two-cone

algorithm would intersect m(m + l)/2 pairs of cones in the worst case. Each pair

of cones would produce zero or two intersection points (disregarding the cases

where both cones coincide or intersect the sphere at the same point). However,

sequences that have different incident energies Eo are less likely to come from the same source than sequences with similar incident energies. The intersection

points between cones from different sources would only add unnecessary clutter

to the result. Thus, our implementation of the two-cone algorithm only intersects

cones whose incident energies differ at most by a fixed percentage ε.

[0034] This two cone method in particular reduces the total computational

effort, while improving the ability of a user to locate weak sources in the

presence of background. It has been generally demonstrated that the best results occur when the energy spectrum has definite full-energy peaks. One could

extend this approach to intersect cones with energies that correspond to full- energy peaks of the same isotope, full-energy peaks of the same decay chain, or

other criteria as defined by the particular desired application. In some

applications, the cones of the various articles may be selected by characteristics such as observed energy or selection of the energies from a list of radioisotopes.

[0035] The two-cone algorithm offers a number of ways to compute directionality. In one embodiment, each intersection point could be plotted on a spherical surface a direction histogram is created and the same peak-finding

techniques as the one-cone method can be utilized to image the energy source. In

another embodiment, the intersection points could be grouped into clusters and

the centroid of each cluster determined. This method would bypass the imaging

step. In yet another embodiment, the horizontal bearing of each intersection point can be computed. The system would then display the directions that

corresponded to the most points.

[0036] In one embodiment of the present invention the methods are

implemented through a handheld radioisotope identifier device, such as the

device shown in Fig. 4. In this preferred embodiment of the invention a CdZnTe

detector array measures the energy and direction of incoming gamma rays from

50 keV to 3 MeV. This device includes 18 CdZnTe crystals arranged in a two-

level 3x3 array. Each crystal is 15x15x10 mm ³ and features an 11x11 anode grid

that provides position sensitivity in two dimensions. The third coordinate is determined by measuring the time difference between the anode and cathode

pulses as described above. The detector pitch is 22 mm laterally with a 20-

mmmm spacing between layers to accommodate the readout electronics. This

arrangement provides for 40.5 cm ³ of active detector volume. The outputs of each crystal are preprocessed by a specialized ASIC, converted to digital form, and

loaded onto an embedded computing platform for further processing. The embedded computing platform combines a microprocessor with a field-

programmable gate array (FPGA) that implements two key functions:

identifying key isotopes present in the vicinity, and indicating the origin of the

detected gamma rays. The handheld device also includes an LCD display, user

inputs, and two Li-ion batteries that supply power.

[0037] In general, handheld radiation detection devices must perform

several tasks in order to indicate the origin of detected gamma rays. These tasks

include, but are not limited to, energy correction and discrimination, sequence

order reconstruction, Compton imaging, and directionality calculation. In order

for the results from these calculations to be effective, these tasks typically need to

run at or near real time. However, because of size, weight, and portability

considerations, handheld devices generally must rely on battery power when

operating in the field and thus typically have limited computational capacity.

Hence, it is generally desirable to offload the most computationally intensive

tasks onto the FPGA. This objective in turn drives the need for energy correction,

sequence order reconstruction, Compton imaging, and directionality calculation

methods that have an efficient FPGA implementation, such as those within the scope of this invention.

[0038] In one implementation of the present invention, the method of the

present invention was incorporated into an operating code that directed a typical desktop computer to perform the steps described above. These tests showed that

the methods could provide useful information in much less time than prior art

imaging methods. In addition the methods of the present invention have also

been incorporated into a handheld CZT device such as the one shown in Fig. 4,

which has been discussed previously. In addition to these descriptions a variety of other types of "systems" may be employed that utilize the methods described

herein.

[0039] While various preferred embodiments of the invention are shown

and described, it is to be distinctly understood that this invention is not limited

thereto but may be variously embodied to practice within the scope of the following claims. From the foregoing description, it will be apparent that various

changes may be made without departing from the spirit and scope of the invention as defined by the following claims.