IDENTIFICATION OF LOCATION OF SOURCE OF GAMMA RAY RADIATION

IDENTIFICATION D'EMPLACEMENT DE SOURCE DE RAYONNEMENT GAMMA

English Abstract

A method and a system for the measurement of gamma-ray radiation from a gamma
ray Source
based on using one or more sensors positioned at differing ranges from the
Source. The
sensors conduct a series of radiation measurements with the sensors in a
series of different
spatial positions relative to the radiation sources. Analysis of the radiation
measurement data
through use of non-linear equations provides an indication for the location of
the radiation
Source. The method and system allows the optional use of a real-time automatic
control of an
optimized search strategy.

Claims

CLAIMS
1. A method of providing an indication of the location of a source of gamma-
ray
radiation comprising the steps:
1) detecting with a gamma ray sensor the intensity of gamma rays incident upon
the sensor at a series of data acquisition points, each point being at
differing
ranges between the source and the sensor with the intervening space being
occupied by air;
2) establishing the intensity level of background radiation in the intervening
space;
3) establishing a value for the Air density in the intervening space;
4) subtracting the value for the intensity of background radiation from the
measured value of the gamma ray intensity established at each data acquisition
point to provide a detected intensity of the Source radiation;
5) inputting a determinate number of values established by 1), 2), 3) and 4)
above
into a programmable computer configured to solve an equation that states a
value
for the detected intensity of the Source radiation as a function of:
a) the coordinates of the source
b) the coordinates of the sensor at each data acquisition point;
c) the air density
d) the energy of the Source radiation
e) the Euclidian distance between the sensor and the Source of gamma-ray
radiation;
f) the mass attenuation coefficient for the reduction of initially uncollided
gamma-rays by
the intervening medium between the Source and the sensor;
g) the source multiplication Buildup factor for secondary radiation arising
from gamma
rays which collide with collaterally located air molecules en route, and are
incident upon
the sensor;
h) the collision multiplication factor associated with Build-up
i) the sensor response function;
j) the gamma-ray radiation source strength;
k) the gamma-ray constant for the type of radioisotope emitting the gamma-
rays, and
17

l) T the number of mean free paths of the estimated type of gamma-rays in air
to provide the co-ordinate values for the location of the Source, and
6) cause the computer to output the estimate for the location of the Source as
established,
thereby providing the estimate for the location of the Source of gamma
radiation.
2. A method of estimating the location of a source of gamma radiation of an
estimated
type located on the surface of the Earth comprising the following steps:
a) introducing a gamma ray sensor carried by an aerial vehicle into a
detectable range within the region of the Source;
b) causing the aerial vehicle to follow a path within such range;
c) establishing values corresponding to the strength of gamma ray
background signals within the region of the Source;
d) recording sensor outputs corresponding to the strength of gamma ray
signals detected once the sensor is within detectable range of the Source
at multiple locations along the flight path sufficient to solve the equations
following below;
e) recording values corresponding to the spatial coordinates of the sensor at
which the sensor outputs are recorded
f) providing a value for the density of air in the region around the aerial
vehicle;
g) inputting a determinate number of values established by c), d), e) and f)
above into a programmable computer configured to solve the following
equations for the x,y, and z co-ordinate values for the location of the
Source:
Equation 1 r n = [ <IMG> + <IMG> + <IMG> ] -2
Equation 2 .tau. n = r n/k
Equation 3 D n = <IMG>
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wherein
D the gamma-ray sensor response in units of counts per second (cps)
S the estimated gamma-ray radiation source strength in units of Curies (Ci)
G the gamma-ray constant for the estimated radioisotope emitting gamma-
rays in units of pSv / hr per Curie at 1 meter (µSv / hr / Ci @ 1m)
R the response function of the gamma-ray sensor in units of counts
per
second per µSv per hour (cps / µSv / hr)
k linear attenuation coefficient for the estimated gamma-ray
radiation of a
specified energy in the intervening attenuating medium in units of
inverse meters ( m -1)
r the Euclidian distance between the source of gamma-ray radiation
and
the gamma-ray radiation sensor in units of meters (m)
A1 source multiplication factor (dimensionless)
(1+.alpha.1 ) collision multiplication factor (dimensionless)
.tau. number of mean free paths of gamma-rays in air (dimensionless)
wherein the subscript n labels the time series of spatially separated
radiation
measurements; the superscript dr labels the coordinates of the location of the
radiation sensor; the superscript rs labels the coordinates of the location of
the
anomalous source of radiation; and x, y, and z label latitude, longitude and
altitude
respectively (m), and
h) causing the computer to output the co-ordinates of the Source as
established by f), above,
and thereby providing an estimate the location of the Source of gamma
radiation.
19

Description

CA 02951664 2016-12-14
Title: IDENTIFICATION OF LOCATION OF SOURCE OF GAMMA RAY RADIATION
FIELD OF THE INVENTION
The present invention relates generally to determining an estimate for the
location of a
source of anomalous gamma-ray radiation when the radiation sensor or sensors
and
the radioactive radiation source or sources are located at different ranges
with respect
to each other. In particular, it relates to the use of gamma-ray radiation
sensors
deployed on aerial vehicles and single or multiple stationary gamma ray
sensors
positioned to detect emissions from moving or stationary sources.
BACKGROUND TO THE INVENTION
It is known to estimate the location of sources of radiation using a
distributed array of
sensors. It is also known to do so using sensors mounted on aerial vehicles,
particularly on unmanned aerial vehicles - UAVs, also known as "drones".
Examples of
references of this character include:
US 8,355,818
US 8,820,672
US 7,465,924
This invention addresses a new procedure for carrying-out this process and an
apparatus and system by which such procedures are implemented.
There is a need for a system which can locate sources of gamma-ray radiation
that may
be used as Radiological Threat Agents for malicious purposes. Additionally,
there is a
need for a system to effect the identification and localization of illicit
gamma-ray
radiation sources being transported by vehicles, particularly within ground
transport,
within ships and by UAVs or drones, before they can effect damage. This is in
addition
to the determination of the source strength and other parameters such as type
of source.
This invention addresses those needs amongst other objectives.
The use of "sensor" and "sensors" herein is intended to cover the use of
single or
multiple sensors as the context requires or permits. Multiple sensors may be
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CA 02951664 2016-12-14
employed both to detect different parameters and for redundancy. For example,
one
sensor could be highly sensitive and suitable for detecting signals long
distances but
because it is sensitive will saturate at closer ranges, requiring a 2nd sensor
for closer
range operations.
The invention in its general form will first be described, and then its
implementation in
terms of specific embodiments will be detailed with reference to the drawings
following
hereafter. These embodiments are intended to demonstrate the principle of the
invention, and the manner of its implementation. The invention in its broadest
and more
specific forms will then be further described, and defined, in each of the
individual
claims which conclude this Specification.
SUMMARY OF THE INVENTION
According to one aspect of the invention an indication of the location of a
source of
gamma-ray radiation is obtained by detecting such radiation at differing
ranges
between the source of the gamma-ray radiation and the detecting radiation
sensor(s). The primary sensed data includes the intensity of the gamma rays
incident upon a gamma ray sensor sensitive volume e.g. scintillation counts
per
second. Preferably it also includes the energy of such detected gamma-rays,
e.g.
the wavelength or energy content of the individual detected gamma ray photons
based upon the brightness of scintillations. Analysis of this acquired data
assumes
that the source and sensor are separated by an intervening space that is
occupied
by air.
The location of the source is determined in whole or in part by taking into
account,
inter alia, the relative geometry of the disposition of the sensor and source;
the
response function of the sensor; the intensity and energy of the gamma-ray
radiation; the level of background radiation and the physical processes
involved in
the interaction of gamma rays with the intervening air. Optionally the type of
source
may be initially estimated to provide an energy value or the spectrum of the
gamma
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ray radiation as detected may be measured directly by a sensor to determine
the
type of source. Air density at the location of the sensor may be estimated or
may be
measured by detecting air pressure and temperature. Air density may also be
established by measurements based upon multiple data acquisitions extracted by
a
sensor at different ranges between the relative locations of the source and
sensor.
Such ranges may be changing with time. This latter condition may occur, for
example, when the sensor is carried by a drone and the source is stationary or
moving. Or it may occur where the radiation source is being carried by a
seaborne
vessel, a moving ground vehicle or an aerial vehicle and the sensor(s) is/are
stationary. In such cases uncertainties will be introduced into the location
information provided by the invention, but when the differential motion is
small the
determined location will still be meaningful.
Multiple data samples of gamma ray intensity and energy obtained while the
sensor
and the anomalous source of radiation are at different ranges from each other
are then
used to solve the following equations for the location of the source, e.g.,
solving for the
x, y, z co-ordinates.
Equation 1 r n = [ xgr xnrs )2 + ygr ynrs )2 4_ ( 4.1Ir
zli.s )2 }-2
Equation 2 T n = rn/k
A1 S G R exp (¨(1+ al ) T n )
Equation 3 D n =
"2
n
wherein
D is the gamma-ray sensor intensity response in units of counts per second
(cps)
S is the gamma-ray radiation source strength in units of Curies (Ci)
G is the gamma-ray constant for the type of radioisotope emitting gamma-rays
in
units of pSv / hr per Curie at 1 meter (pSv / hr / Ci @ 1m)
R is the response function of the gamma-ray sensor in units of counts per
second
per pSv per hour (cps / pSv / hr)
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k is the linear attenuation coefficient for the gamma-ray radiation of a
specific
energy in the intervening attenuating medium, i.e. air, in units of inverse
meters
m--1)
r is the range or Euclidian distance between the source of gamma-ray
radiation
and the gamma-ray radiation sensor in units of meters (m)
A1 source multiplication factor associated with Build-up (dimensionless)
(1+a,) collision multiplication factor associated with Build-up
(dimensionless)
number of mean free paths of the estimated type of gamma-rays in air
(dimensionless)
and wherein the subscript n labels the time series of spatially separated
radiation
measurements; the superscript dr labels the coordinates of the location of the
radiation sensor; the superscript rs labels the coordinates of the location of
the
anomalous source of radiation; and x, y, and z label latitude, longitude and
altitude
respectively (m).
Equation 3 is a condensed version of more elaborate equations wherein the
terms A1,
(1+al ) and T are taken from a Taylor expansion in order to simplify the
equations.
In place of Equation 3 more terms may be employed from the Taylor expansion.
Finding the location (x, y and z) from a solution of the above n equations for
D n will
require at least as many data samples as there are unknowns. As progressively
more
sets of data samples are acquired more exact values for these parameters can
be
obtained once the equation set becomes over determined.
The equations assume that the Euclidian distance separation between source and
sensor is effectively unchanging during the data sampling interval. This
condition will
be essentially met when the relative velocity between source and sensor is
small.
Preferably the sampling interval at each data acquisition point should be as
short as is
consistent with obtaining meaningful data. And the delay between acquiring
data at
distinct data acquisition points should be similarly minimized. The motion of
the
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Source relative to the Sensor will show up as a progressively changing
solution for the
location of the Source every time the set of equations are solved.
Some of the parameters may be estimated or predetermined. For example, the
technique of the invention can be applied where the source type and strength
of the
radiation are known, as where the source has been lost or stolen. Standard
values for
air density may be initially employed as an estimate. The values for these
parameters
may be iteratively adjusted as data acquisition allows improved estimates to
be
applied.
As a preferred feature of the invention, an estimate for the co-ordinates for
a Source of
gamma radiation of a specific type located on the surface of the Earth may be
determined using an aerial vehicle by a method comprising the following steps:
a) introducing a gamma ray sensor carried by an aerial vehicle into a
detectable
range within the region of the Source;
b) causing the aerial vehicle to follow a preferably curvilinear flight path
within such
range;
c) establishing values corresponding to the intensity and preferably energy of
gamma ray background signals within the region of the Source;
d) detecting and recording values for the intensity and preferably energy of
gamma
rays originating from the Source as provided by the sensor, correlated with
the
spatial coordinates of the sensor at corresponding multiple data acquisition
points along the flight path;
e) providing a value for the density of air in the region around the aerial
vehicle;
f) inputting a determinate number of values established by c), d), and
e) above
into a programmable computer configured to solve the above equations and
provide the co-ordinate values for the location of the Source:
and lastly
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CA 02951664 2016-12-14
g) causing the computer to output the estimate for the co-ordinates of the
Source as established by f), above, and thereby provide the estimate for the
location of the Source of gamma radiation.
Monitoring shipping and ground vehicles
As a further feature of the invention estimated spatial co-ordinates for a
Source of
gamma radiation of an estimated type moving on the surface of the Earth may be
determined using a series of stationary sensors deployed as an array on the
surface of
the Earth by a method comprising the steps set-out above, the stationary
sensors
being located at the corresponding data acquisition points. For the purposes
of this
disclosure "surface of the Earth" is to be understood as including both
surfaces on land
and the surface of water, as in the case for example of rivers and harbours
with
shipping present on the water. This series of stationary sensors deployed as
an array
on the surface of the Earth may be augmented by sensors carried in aerial
vehicles.
Dispersed Radioactive on Ground
According to another aspect of the invention radiation from a dispersed
anomalous
radiation sources may be detected and analyzed. This variant is relevant where
a
radiation spill or intentional dispersal may have occurred. The location of
the
dispersed Source will be indicated by a spread of location values within the
dispersal
zone.
Determining Uncertainties in Location, Energy and Source Strength
Another useful aspect of the present invention resides in its ability to
provide data on
the uncertainties in the identified location, energy and radioactive source
strength of
said anomalous radiation sources.
Graphical User Interface
Optionally but preferably, the method and system of the invention may include
a real-
time graphical user interface providing system operators with information
including
map overlays, collateral video images, and computer-generated optimized search
strategies. This permits meaningful and rapid human interpretations of and
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CA 02951664 2016-12-14
interventions in radiation measurements being obtained from an aerial vehicle.
Such
interventions could adjust the otherwise automated flight path of a UAV-
carried
sensor or otherwise allow adjustment of an automatic search strategy.
Collateral imaging of Sensor field of view
In conjunction with obtaining radiation information from a region of interest,
contemporary or archival still, video or Radar images of the region may be
correlated
with the radiation data. This will assist in distinguishing amongst multiple
possible
vehicles that may be carrying the Source. This can also assist on-site
personnel in
response and inspection of a Source. Once a visual identification is made of a
Source carrier, it may be tracked visually as by a drone with image-following
capability.
The foregoing summarizes the principal features of the invention and some of
its optional
aspects. The invention may be further understood by the description of the
preferred
embodiments, in conjunction with the drawings, which now follow.
Wherever ranges of values are referenced within this specification, sub-ranges
therein are
intended to be included within the scope of the invention unless otherwise
indicated or are
incompatible with such other variants. Where characteristics are attributed to
one or another
variant of the invention, unless otherwise indicated, such characteristics are
intended to apply to
all other variants of the invention where such characteristics are appropriate
or compatible with
such other variants.
BRIEF SUMMARY OF THE FIGURES
The present invention will now be described in more detail with reference to
the
accompanying Figures, as follows:
Figure 1 is a schematic depiction of a volume of space showing the geometry
wherein
a gamma-ray radiation sensor is located in space at a distance r from a single
gamma-
ray radiation point Source at rest at a specified location, e.g., on the
surface of the
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CA 02951664 2016-12-14
Earth. The precise location of the radiation Source is unknown but the co-
ordinates for
the sensor are known (as by GPS).
Figure 2 is a graph showing the loss of gamma ray signal strength as a
function of the
distance from the Source due to the 11r2 loss or inverse square law effect in
a
hypothetical vacuum where the Source is Cesiumi32.
Figure 3 is a graph showing the loss of gamma ray signal strength with
increasing
distance from a Source based upon the progressive absorption of gamma rays by
air
over the intervening distance from the Source where the Source is Cesium-137.
In this
graph the curve of values appears as a straight line due to the use of a semi-
log scale
for the Y axis.
Figure 4 is a graph showing the combined effects of the effects depicted in
Figures 2
and 3.
Figure 5 is a graph showing the enhancing effect of "Build-up" on the measured
strength of gamma rays detected by a sensor as a function of the distance of
the
sensor from the Source where the Source is Cesiumi32.
Figure 6 is a graph showing the curve of Figure 4 and an additional curve
showing the
enhancing effect arising from including the "Build-up" factor depicted in
Figure 5 in
establishing the measured strength of the gamma ray signal strength as a
function of
the distance from the Source.
Figure 7 is a reproduction of Figure 1 wherein the vehicle carrying the gamma
ray
sensor is following a flight path and data based on detection of gamma ray
signals is
being acquired at multiple data acquisition points along the flight path.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
Premises
In the following description, location and spatial location measurements are
made in a
system of coordinates which provide latitude, longitude and altitude with
respect to the
center of the Earth. It is understood that latitude, longitude and altitude
may be stated and
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CA 02951664 2016-12-14
expressed in units of meters with respect to an origin different from the
center of the earth
or in any other appropriate frame of reference.
The physical quantities, equations and units of measurement employed herein
are
useful for the present purposes. It is well-known that other quantities and
units of
measurement can be used equivalently with the appropriate conversion factors.
Alternative equations can be used to describe the physical processes of the
emission,
transport and sensing of gamma-ray radiation.
Transport of gamma-ray radiation through air
The wording "gamma-ray sensor" used herein refers to a sensor which provides a
real-time response to gamma rays interacting with the sensor. This response of
the
sensor to gamma-ray radiation may provide data including:
1). the intensity of radiation incident on the sensor or equivalently the
number of
gamma-ray photons interacting with the sensor per unit time expressed in
suitable
units such as counts per second (cps) or micro-Sieverts per hour (pSv/hr)
2). the energy values of individual gamma ray photons included in the spectrum
of the
radiation incident on the sensor.
Well-known examples of such gamma-ray radiation sensors may include but are
not
limited to plastic scintillation sensors, Sodium Iodide scintillation sensors,
Geiger tube
sensors, etc. For brevity, "sensor" as used herein denotes any type of gamma-
ray
sensor that records the intensity and, optionally, the energy content of gamma-
ray
radiation.
Physical processes
The physical processes determining the response of a gamma-ray sensor to a
gamma-ray source in an infinite attenuating medium include:
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CA 02951664 2016-12-14
1) The gamma-ray radiation Source strength expressed in units of Curies;
2) the Euclidian distance between the sensor and the Source of gamma-ray
radiation
in units of meters;
3) the reduction of initially uncollided gamma-rays by the intervening medium
between
the Source and the sensor as expressed through the mass attenuation
coefficient in
units of cm2/gm;
4) secondary radiation arising from gamma rays which collide with collaterally
located
air molecules en route, and are incident upon the sensor expressed through the
dimensionless Buildup Factor, and
5) the gamma-ray sensor response function expressed in counts per second per
microSievert.
The sensor response function may be determined by a calibration of the sensor.
The
sensor response function may be omnidirectional or may have a dependence on
the
angle of incidence of the radiation on the sensor. In the absence of a
significant
collimating configuration, the Response function need not be strongly or
significantly
dependent on the angle of incidence. In such cases variations in the angle of
incidence within the Sensor's field of view may be ignored. Further, the
Sensor
response function may be dependent on gamma-ray energy and therefore the type
of
source.
The invention does not require that the Sensor be collimated and have the
capacity to
be pointed at a radiation Source in order to detect the location of the
Source.
However, shielding to reduce background noise may give the Sensor a preferred
field
of view in which it is more sensitive to the detection of gamma rays. Within
the field of
view the Sensor is multidirectional.

CA 02951664 2016-12-14
Calculations
For a sensor at a distance r - 31 from a source of gamma-ray radiation as
shown in
Figure 1, the gamma rays incident on the sensor are proportional to the total
number
of gamma ray photons given off by the source. Further, for a sensor at a
distance r
from a source of gamma-ray radiation, the gamma rays incident on the sensor
are
proportional to the ratio of the intercepting area of the sensor divided by
the area of
the sphere with radius r. This the well-known 117-2 signal diminution
parameter that
depends on range. This effect is shown in Figure 2.
Still further, for a sensor at a distance r from a source of gamma-ray
radiation, the
uncollided gamma rays incident on the sensor are proportional to the fraction
of
gamma rays which do not collide with the medium and are not thereby absorbed
or
otherwise attenuated by interaction with the medium. This fraction may be
expressed
as e-kr wherein the constant k is the linear attenuation constant and is
determined by
the composition and density of the medium and the gamma ray energy. For air a
typical value for k is 0.08 cm2/gm at 600 KeV and varies slowly over the gamma-
ray
energy region of interest. The value also varies slightly with air pressure
and moisture
content but such variations may be ignored when satisfactory accuracy is
otherwise
being achieved. The effect is shown in Figure 3.
Figure 4 then shows the net consequences of these two effects combined.
Yet further, for a sensor at a distance r from a source of gamma-ray
radiation, the
gamma rays incident on the sensor are proportional to the gamma rays which are
scattered into the detector ¨ Figure 6. These can be rays that are initially
directed
collaterally from the direct path to the sensor but which scatter from the
medium to be
redirected towards and become incident on the sensor. This factor of
proportionality,
known as Build-up may be expressed as B and is determined by the composition
and
density of the medium and the gamma ray energy. An example of the dependency
of
Buildup on separation between the Source and the sensor in air is shown in in
Figure
5.
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In summary, Figure 6 depicts the effect of combining this Build-up effect with
the
attenuating factors of Figures 2 and 3. Further Figure 6 contrasts the
inclusion of
Buildup with the effects of only 1/r2 and attenuation.
A consequence of these considerations is that, at relatively further ranges
from a
Source, the rate of fall-off or diminution of a signal available to be
detected is reduced,
or colloquially, the curve is flattened. Accordingly, the refinements in the
sensitivity
and precision of identifying the location of a Source which have been
disclosed can
improve the range at which a Source may be reliably located. This is a
valuable
benefit of the invention.
Math ¨ assuming no Background radiation
Figure 7 depicts a vehicle carrying the gamma ray sensor 20 along a flight
path
having 6 data acquisition points. The Source 7 is stationary. At each data
acquisition
point 1 ¨ 6 the intensity, and optionally, the energy of the gamma rays 18 is
measured
along with the location of the data point, e.g. acquired by a GPS receiver.
Time may
also be recorded for use in cases where the Source is moving.
In instances wherein a radioactive source of gamma-ray radiation is lost or
stolen, then, the
radioactive isotope and thereby the gamma-ray energy is known and the
radioactive source
strength is known. These known factors then reduce the number of, or place
restrictions on,
the values of some of the unknown variables in the set of simultaneous
equations. The
knowledge of the gamma-ray energy relieves the requirement for a measured
determination
of that parameter and its attendant measurement uncertainties. The radioactive
source
strength can set an upper bound on that parameter, as determined by for
example a least
squares solution of the simultaneous equations. The possibility of unknown
shielding of the
lost or stolen Source results in an unknown apparent or effective radioactive
Source strength
outside the possible shielding; however, that apparent Source strength may not
exceed the
known lost or stolen Source strength.
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This data is then applied to the equation 3, above, to generate a set of
simultaneous
nonlinear equations which may be solved for the location of the Source.
Methods for
solution are well known. A determined set of equations will normally give one
solution.
In some cases, as where, for example, the sensor 20 is moving along a straight
line,
dual or mirror solutions may be delivered. This can be addressed by having the
sensor
20 depart from following a straight course to follow a curvilinear flight
path.
Alternately the sensor 20 may be configured to distinguish between dual
solutions, as
by bifurcating the sensitive volume to create left and right fields of view.
Poisson statistics
Gamma-ray radiation measurement data are the result of stochastic processes
and are
described by Poisson statistics. Specifically, the measurement uncertainty in
each
measurement of sensed gamma-ray radiation in the method of the present
invention may be
described by Poisson statistics. Additionally, there is measurement
uncertainty in other
measurements in the method of the present invention including sensor position
and gamma
ray photon energy. Finally, there is uncertainty inherent in the measurement
data and
analytic methods underlying tabulated parameters used in the method of the
present
invention and in their interpolation.
Over Determined equations
Additionally, the number of measurement points along a flight path may exceed
the number
of unknown variables in the set of simultaneous non-linear equations. Thus,
the set of
simultaneous non-linear equations is overdetermined. Multiple values for the
location of the
Source may then be presented. As a consequence it may be appropriate to use a
least
squares procedure, or other applicable methodology, to provide a preferred
solution for the
over determined case
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Background radiation (real world)
The above analysis assumed the absence of background radiation. In fact, the
total
sensed gamma-ray radiation measurement data at any location and at any time
will
consist of the sum of counts arising from measurement data from normally
occurring
background gamma-ray radiation and gamma-ray radiation measurement data from
any anomalous Source or Sources of gamma-ray radiation.
The gamma ray natural background radiation intensity and energy spectrum may
be
measured or estimated from other measurements carried out in the absence of an
anomalous Source or Sources of gamma-ray radiation. It is well-known that the
background gamma-ray radiation intensity data must be subtracted from the
total
sensed radiation measurement intensity data in order to obtain measurements of
the
intensity of any anomalous gamma-ray radiation at any measurement location.
This
represents the simplest method of addressing Background radiation.
Illustrative Case: Aerial Vehicle and known Source with Unknown Location
Reference is made to Figure 7. A gamma-ray radiation 18 sensor 20 is carried
by an
aerial vehicle 11 such as, but not restricted to, an unmanned aerial vehicle,
commonly
known as a drone 21, along a flightpath 8. A series of measurements of gamma-
ray
radiation intensity are made by the sensor 20 at a series of spatially
distinct points,
numbered as 1 through 6, along the flightpath. All flightpath spatial location
measurements can be made with respect to a system of coordinates which provide
latitude 11, longitude 12 and altitude 13 with respect to the center of the
Earth. The
radiation intensity measurement data are the result of sensed natural
background
radiation together with sensed radiation 18 from the known anomalous gamma-ray
radiation source 7.
The present invention provides a method for determining an estimate of the
location
and the source strength of the found source of gamma-ray radiation together
with an
estimate of the uncertainty in the location estimate 9 as shown in Figure 1.
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Aerial vehicle measurement data as system of simultaneous nonlinear equations
Further reference is made to Figure 7 in which the radiation measurement data
locations
are shown as a series of six points, labeled 1 through 6. Each data
acquisition point is at a
differing range 31 from the Source 7. The number of points at which gamma-ray
radiation
measurements are made, shown in Figure 7 as six points, is one example. Data
can be
acquired continuously and used to solve sets of determined or over determined
equations as
is most preferred.
At each of the data acquisition points the intensity of the sensed gamma-ray
radiation 18 is
measured. From the known energy of the Source of gamma-ray radiation 18, if
known, the
energy-dependent multiplicative factors A1 and al may be obtained by
interpolation of data
contained in well-known tables of these factors. Further, the gamma-ray
constant G for the
radioisotope is contained in well-known tables of these factors. Still
further, from the known
value for the maximum gamma-ray energy, the energy dependent mean free path
factor T
may be interpolated from published tables of the linear attenuation
coefficient k.
Alternately, this data may be acquired by solving the set of equations using
the necessary
larger number of data acquisition samples.
The intensity of background gamma-ray radiation from previously recorded data
made by
others or estimated from background measurements made prior to the search is
subtracted
from the total sensed gamma-ray radiation data to obtain the gamma-ray
radiation data from
the unknown anomalous Source 7 at the measurement points on the flightpath 1 ¨
6. From
this data sufficient information is obtained to specify the D i, values at
each data point
which are due to the signal of interest from the anomalous Source of gamma-ray
radiation by subtracting the Background level from the apparent value of D.
Reference is made to Equations 1, 2 and 3. A substitution is made of the
factors obtained
from the measurements of gamma-ray radiation intensity and the energy, as
above, and
further, substitution is made of the position measurements of location of the
sensor 20 for
each of the corresponding radiation measurements. This results in a set of
equations with
each equation of the set in a one to one correspondence to a sensor 20
measurement
location.

CA 02951664 2016-12-14
Solve for source strength and location
Once sufficient measurements have been made at multiple data acquisition
points 1 ¨6 this
data may be used to create a set of equations based on equations 1 ¨ 3. This
provides a set
of simultaneous non-linear equations which may be solved by well-known
analytic or numeric
methods for the remaining unknown four quantities which are:
the gamma-ray radiation source strength outside of any shielding
in units of Curies (Ci)
xrs, yrs, zrs the latitude, longitude and altitude of the anomalous radiation
source
Figure 7 depicts a stationary Source 7 and a moving sensor 20 carried by a
drone
21. It can equally depict buoys in a harbour, each carrying a sensor 20 and a
stationary vessel carrying the Source 7. The buoys would be positioned at each
of
the data acquisition points 1 ¨ 6. Even when the Source 7 is moving, data
acquired
simultaneously at each buoy location can be applied to equations 1 ¨ 3 to
produce
the necessary set of simultaneous equations. Thus, the case of a moving Source
7
and multiple stationary sensors 20 has also been addressed.
Conclusion
The foregoing has constituted a description of specific embodiments showing
how the
invention may be applied and put into use. These embodiments are only
exemplary.
The invention in its broadest, and more specific aspects, is further described
and
defined in the claims which now follow.
These claims, and the language used therein, are to be understood in terms of
the
variants of the invention which have been described. They are not to be
restricted to
such variants, but are to be read as covering the full scope of the invention
as is implicit
within the invention and the disclosure that has been provided herein.
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Representative Drawing