Note: Descriptions are shown in the official language in which they were submitted.
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Scintillator Based Radiation Detection
[0001]
BACKGROUND
1. Field
= [0002] This patent specification relates to improved
scintillator based radiation
detection. More particularly, this patent specification relates to methods and
systems
for using improved energy calibration and resolution monitoring using
intrinsic
radiation sources.
= 2. Background =
[0003] Scintillation detectors featuring a scintillator crystal and a
photodetector
(for example a PMT tube) are widely used different industries, and in
particular in the
field of oilfield services. A common problem in the use of scintillation
detectors for
nuclear spectroscopy or similar energy sensitive measurements is that the
detector
response function changes for example with changing environmental conditions.
Typically, the sensitivity of the photodetector element will vary with time
(drift) and
with changing environmental conditions such as temperature and magnetic
fields.
[0004] Conventionally, the gain of the detectors can be stabilized by
using a
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control circuit that keeps the peak of an external stabilization source signal
in the
same channel of the multi-channel analyzer. A disadvantage of this method is
the
need to supply an external stabilization source. Additionally, the external
source is
often only irradiating part of the crystal which may not give average results.
Other
techniques are used were the stabilization is based on a measured signal of an
external
radiation source. Such an external source may not primarily be used for
stabilization.
Such techniques may use thresholds, windows ratios, or more complex
algorithms.
The disadvantage is that the radiation may be weak or absent at least part of
the time
which can result in stabilization loss in particular with changing source
strength.
100051 Scintillator materials are widely used to build detectors for
measuring X-
ray and 7-radiation. Dense materials with high atomic numbers are preferred to
measure 7-rays, since the stopping power of the materials increases with these
parameters and thus the size of the detector can be reduced without loss of
sensitivity.
However, many of the heavier scintillator materials have an intrinsic
background
radioactivity due to the presence of radioactive isotopes in the heavier
elements of the
crystal matrix. In particular Lutetium has been found to be a valuable
constituent in
scintillator materials, but suffers from the presence of a radioactive
isotope. In large
detectors this background count rate might contribute significantly to the
maximal
achievable count rate and thus negatively affect the precision and accuracy of
the
measurement. For example lutetium oxyorthosilicate (LSO) has been established
as a
useful scintillator for medical imaging, but its intrinsic radioactivity
affects the count
rate in large scintillator crystals. More recently LuAP:Ce and LuAG:Pr have
been
used as matrix materials for scintillators. Typical intrinsic count rates for
material
containing a large fraction of Lu are around a few hundred counts per second
per
cubic centimeter (cm-3s-1). For example a 2" x 4" crystal contains about 200
cm3 of
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material, so that the count rate reaches around 50,000 s-1. This is about 5-
10% of the count
rate capability of a fast conventional detector and thus creates a loss in
statistical precision of
several %. Intrinsic radioactivity is therefore conventionally regarded as a
disturbance.
SUMMARY
[0005a] According to an aspect of the present invention, there is provided
a =system for
the downhole detection of nuclear radiation, the system comprising: a tool
body adapted to be
deployed in a wellbore; a scintillator component comprising a scintillator
material that
intrinsically generates radiation, the scintillator component mounted within
the tool body; a
photodetection system coupled to the scintillator component, mounted within
the tool body,
and adapted to generate electrical signals based on light emitted from the
scintillator
component according to a response function; and a processing system adapted
and
programmed to receive the electrical signals and determine one or more aspects
of the
response function of the scintillator component based at least in part on
electrical signals from
the intrinsically generated radiation within the scintillator component.
[0006] = According to some embodiments, a system for the downhole detection
of
nuclear radiation is provided The system includes a tool body adapted to be
deployed in a
wellbore and a scintillator material that intrinsically generates radiation.
The scintillator
material is mounted within the tool body. A photodetection system is coupled
to scintillator
material, and mounted within the tool body. The photodetection system is
adapted to generate
electrical signals based on light emitted from the scintillator material
according to a response
function. A processing system is adapted and programmed to receive the
electrical signals
and determine one or more aspects of the response function, for example, that
are susceptible
to variations due to changing downhole environmental conditions, based on
electrical signals
from the intrinsically generated radiation.
[0007] According to some embodiments, the determined aspects of the
response
function can include energy or variation of resolution of the radiation
detector system at a
given incident energy. Additionally, gain, offset and/or non-linearity may be
parameters
describing such aspects of the response function of the radiation detector
system. Some
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embodiments may look at even higher order parameters of the response function.
The
processing system can be programmed to calculate based on the electrical
signals a spectrum,
and a derivative of the spectrum. Based on the calculated derivative, the
processing system
can monitor energy resolution and/or make an energy calibration. The
derivative can be a
first, second or higher order derivative.
[0008] According to some embodiments, the scintillator material is a
matrix a
substantial part of which is an element that contains one or more naturally
occurring
radioactive isotopes. According to some embodiments, a substantial part of the
scintillator
material matrix is one or more metallic elements, preferably rare-earth
elements, more
preferably, lanthanides. Accbrding to some embodiments, the scintillator
material is Lutetium
aluminate essentially in the perovskite or garnet phases. The scintillator
material may be
doped with an activator such as Cerium or Praseodymium. According to some
embodiments,
the tool body is adapted to be deployed as part of a drilling operation.
[0008a] According to another aspect of the present invention, there is
provided a
method for the downhole detection of nuclear radiation, the method comprising:
deploying a
tool body in a wellbore, wherein the tool body houses a scintillator component
comprising a
scintillator material that intrinsically generates radiation and a
photodetection system coupled
to the scintillator component; generating electrical signals with the
photodetection system
based on light emitted from the scintillator component material according to a
response
function; and determining one or more aspects of the response function of the
scintillator
component based at least in part on electrical signals from the intrinsically
generated radiation
within the scintillator component.
[0008b] According to another aspect of the present invention, there is
provided a
system for the downhole detection of nuclear radiation, the system comprising:
a tool body
adapted to be deployed in a wellbore; a scintillator component comprising a
scintillator
material that intrinsically generates radiation, wherein the scintillator
component is mounted
within the tool body; a photodetection system coupled to the scintillator
component, mounted
within the tool body, and adapted to generate electrical signals based on
light emitted from the
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scintillator component according to a response function; and a processing
system adapted and
programmed to (i) receive the electrical signals, (ii) determine a spectrum
based at least in
part on electrical signals from the intrinsically generated radiation, (iii)
determine a derivative
of the spectrum to generate a differentiated spectrum, and (iv) identify at
least one peak
associated with the intrinsically generated radiation within the
differentiated spectrum, and (v)
analyze the at least one peak to determine energy resolution of the
scintillator component.
10008c1 According to another aspect of the present invention, there is
provided a
method for the downhole detection of nuclear radiation, the method comprising:
deploying a
tool body in a wellbore, wherein the tool body houses a scintillator component
comprising a
scintillator material that intrinsically generates radiation and a
photodetection system coupled
to the scintillator component; generating electrical signals with the
photodetection system
based on light emitted from the scintillator material; determining a spectrum
based at least in
part on electrical signals from the intrinsically generated radiation;
determining a derivative of
the spectrum to generate a differentiated spectrum; identifying at least one
peak associated
with the intrinsically generated radiation within the differentiated spectrum;
and analyzing the
at least one peak to determine energy resolution of the scintillator
component.
[0009] Further features and advantages will become more readily
apparent from the
following detailed description when taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
100101 The present disclosure is further described in the detailed
description which
follows, in reference to the noted plurality of drawings by way of non-
limiting examples of
exemplary embodiments, in which like reference numerals represent similar
parts throughout
the several views of the drawings, and wherein:
[0011] Fig. 1 is a block diagram of a gamma-ray spectroscopy system
in accordance
with some embodiments;
[0012] Fig. 2 is a spectrum chart showing spectra for a PR:LuAG
crystal;
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[0013] Fig. 3 is a flowchart describing a technique for stabilising
gain of a gamma-ray
spectroscopy system, according to some embodiments;
[0014] Fig. 4 is a chart showing a differentiated spectrum, according
to some
embodiments;
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[0015] Fig. 5 is a chart showing differentiated spectrum smoothed using a
smoothing filter, according to some embodiments; and
[0016] Fig. 6 is a chart showing an example of a fit function used to
approximate
the peak in a differentiated spectrum.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] The following description provides exemplary embodiments only, and
is
not intended to limit the scope, applicability, or configuration of the
disclosure.
Rather, the following description of the exemplary embodiments will provide
those
skilled in the art with an enabling description for implementing one or more
exemplary embodiments. It being understood that various changes may be made in
the function and arrangement of elements without departing from the spirit and
scope
of the subject matter disclosed in the application as set forth in the
appended claims.
[0018] Specific details are given in the following description to provide a
thorough understanding of the embodiments. However, it will be understood by
one
of ordinary skill in the art that the embodiments may be practiced without
these
specific details. For example, systems, processes, and other elements in the
subject
matter disclosed in the application may be shown as components in block
diagram
form in order not to obscure the embodiments in unnecessary detail. In other
instances, well-known processes, structures, and techniques may be shown
without
unnecessary detail in order to avoid obscuring the embodiments. Further, like
reference numbers and designations in the various drawings indicated like
elements.
[0019] Also, it is noted that individual embodiments may be described as a
process which is depicted as a flowchart, a flow diagram, a data flow diagram,
a
structure diagram, or a block diagram. Although a flowchart may describe the
operations as a sequential process, many of the operations can be performed in
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parallel or concurrently. In addition, the order of the operations may be re-
arranged. A
process may be terminated when its operations are completed, but could have
additional steps not diScussed or included in a figure. Furthermore, not all
operations
in any particularly described process may occur in all embodiments. A process
may
correspond to a method, a function, a procedure, a subroutine, a subprogram,
etc.
When a process corresponds to a function, its termination corresponds to a
return of
the function to the calling function or the main function.
100201 Furthermore, embodiments of the subject matter disclosed in
the
application may be implemented, at least in part, either manually or
automatically.
Manual or automatic implementations may be executed, or at least assisted,
through
the use of machines, hardware, software, firmware, middleware, microcode,
hardware
description languages, or any combination thereof. When implemented in
software,
firmware, middleware or microcode, the program code or code segments to
perform
the necessary tasks may be stored in a machine readable medium. A processor(s)
may
perform the necessary tasks.
[0021] According to some embodiments, features in a spectrum
associated with a
scintillation material's intrinsic radioactive decay is used for the
determination of one
or more parameter's of the detector response function. An advantage of some
such
embodiments is that the gain of the detector can be stabilized without an
external
stabilization source and in absence of any other external sources of
radiation. For
further detail in gain stabilization using a scintillation material's
intrinsic activity, see
US Patent No. 8,173,953.
100221 An additional benefit of some embodiments is that the
intrinsic material is
evenly distributed within the detector material and therefore is not affected
by source
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movement with respect to the crystal. Furthermore, according to some
embodiments,
it is possible to monitor the resolution of the detector over time without
using an
external source. In some cases, the internal spectrum contains more than one
feature.
In this case, according to some embodiments, multiple parameters such as but
not
limited to gain and/or offset and/or non-linearity can be determined at the
same time.
In certain cases, an estimate of higher order terms in the energy calibration
and
detector resolution can be provided.
[0023] Fig. 1 is a block diagram of a gamma-ray spectroscopy system in
accordance with some embodiments. Fig. 1 illustrates a gamma-ray spectroscopy
system 110 configured for use in nuclear well logging operations. The gamma-
ray
spectroscopy system 110 may provide spectroscopic analysis of gamma-rays or x-
rays
from a surrounding geological formation or borehole to determine, among other
things, a general composition of the formation. Rather than maintain an
external
radiation source near a scintillator for gain stabilization, the gamma-ray
spectroscopy
system 110 may employ a scintillator 112 having a natural radioactivity. Using
techniques described below, the gamma-ray spectroscopy system 110 may
stabilize
the gain of the system using the natural radioactivity of the scintillator
112.
[0024] The scintillator 112 may represent any scintillator having a
natural
radioactivity. Thus, the scintillator 112 may represent, for example, a
scintillator
based at least in part on Lutetium Silicate (LSO), or Lutetium Aluminum
Perovskite
(LuAP), or Lutetium Aluminum Garnet (LuAG), or Lanthanum Bromide (LaBi-3) or
Lanthanum Chloride (LaCI3). Such scintillators may include those by Saint
Gobain or
General Electric, as generally described in U.S. Patents Nos. 7,067,816 and
7,084,403. Alternatively, the scintillator
112 may represent any other scintillator containing a naturally occurring
radioactive
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isotope such as, for example, Bismuth Germanium Oxide (BGO) containing 207Bi.
According to some embodiments the scintillator 112 is a pure crystal such as
undoped
BGO. According to some other embodiments the scintillator material such as
LuAP
or LuAG is doped with a material such as Cerium, Praseodymium or other similar
activators.
[0025] According
to some alternate embodiments, the scintillator material can be
of a common type, for example an oxide or a halide (e.g. containing Cl, Br,
I), which
is additionally doped with a radioactive material. However, such radioactive
doped
scintillators can suffer from disadvantages, such as non-uniform distribution
of the
radioactive material. It may also negatively impact the luminescence
properties of the
scintillator material. Thus, according to the preferred embodiments rather
than
doping with a radioactive material, the scintillator material contains an
element that is
substantially part of the scintillator material matrix and incidentally also
contains a
fraction of naturally occurring radioactive isotopes. According
to many
embodiments, the scintillator material is selected from the group containing
metallic
elements. According to some more preferred embodiments, the scintillator
material is
selected from materials containing rare-earth elements. Even more preferably,
the
scintillator material is selected from materials containing lanthanides.
[0026] When a
gamma-ray strikes the scintillator 112, the energy deposited by
the gamma-ray may be converted into light and received by a photodetector such
as a
photomultiplier 114 or any other device suitable for converting light into an
electrical
signal like an avalanche photodiode (APD). Gamma-rays detected by the
scintillator
112 may arise from external radiation or from the internal radioactivity of
the
scintillator 112. Thus, as described below, an external reference source of
radiation
may be avoided for the purpose of stabilizing the gain of the gamma-ray
spectroscopy
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system 110. Moreover, the source of radioactivity within the scintillator 112
may be
uniformly distributed throughout the scintillator 112. As such, the
corresponding
response of the scintillator 112 to the internal radiation source may be
insensitive to
non-uniformities in the light generation or transport in the scintillator 112,
providing
another advantage over a discrete external radiation source.
100271 After the light output by the scintillator 112 is received by the
photomultiplier 114, the photomultiplier 114 may convert the light from the
scintillator 112 into an electrical signal 116. It should be understood that
the gamma-
ray spectroscopy system 110 may alternatively employ multi-channel plate
multipliers, channeltrons, or solid state devices such as Avalanche Photo
Diodes in
lieu of the photomultiplier 114. The electrical signal 116 may be amplified by
amplification circuitry 118, which may provide an amplified signal 120 to
signal
processing circuitry 122. The signal processing circuitry 122 may include a
general or
special-purpose processor, such as a microprocessor or field programmable gate
array,
and may perform a spectroscopic analysis of the electrical signal, which may
include
the gain stabilization techniques described herein. The signal processing
circuitry 122
may additionally include a memory device or a machine-readable medium such as
Flash memory, EEPROM, ROM, CD-ROM or other optical data storage media, or
any other storage medium that may store data or instructions for carrying out
the
following techniques.
100281 Because the output of the scintillator 112, the photomultiplier 114,
and the
amplification circuitry 118 may depend highly upon external factors, such as
temperature, the age of internal components, or gamma-ray count rate, to name
a few,
the signal processing circuitry 122 may stabilize the gain of the amplified
signal 120.
Stabilizing the gain of the amplified signal 120 may ensure a consistent gain
across
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variable conditions, such as variances in temperature or the age of the gamma-
ray
spectroscopy system 110, i.e. the electrical signal will have the same pulse
height for
a given amount of energy deposited in the scintillation crystal independent of
temperature, age, detector count rate and other factors that can affect the
total gain of
the system. The gain stabilization approaches employed by the signal
processing
circuitry 122 may rely not on an external radiation source, but rather the
natural
radioactivity of the scintillator 112.
100291 As noted above, the scintillator 112 may include a naturally
radioactive
material that may serve as a reference source of radiation. For explanatory
purposes,
the scintillator 112 may be a Lutetium Aluminum Perovskite (LuAP) or Lutetium
Aluminum Garnet (LuAG) scintillator. The LuAP (or LuAG) scintillator may have
a
natural radioactivity as a certain isotope of Lutetium decays within the LuAP
(or
LuAG) scintillator. The decay of the Lutetium generates beta and gamma
radiation
that may interact with the scintillator 112 to generate a corresponding
scintillation
signal, and the resulting energy spectrum may be used to stabilize the gain of
the
gamma-ray spectroscopy system 110.
[0030] A number of properties may make LuAP and LuAG very well suited for
logging operations. LuAP and LuAG are non-hygroscopic, and have very high
stopping power due to their high density and effective Z. Additionally, LuAP
and
LuAG have excellent temperature characteristics and show very little loss (or
even
gain) of light output with temperature. There are two isotopes of Lutetium:
175Lu
(97.4%) and 176Lu (2.6%). The latter, being radioactive, decays with a half-
life
2.6x101 y to 176Hf. The radioactivity results in several hundred counts per
second per
cubic centimeter (cps/cm3) of the LuAP or LuAG material. With the known
radioactivity of LuAP (or LuAG) in the scintillator 112 as a reference, the
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processing circuitry 122 may stabilize the gain of the amplified signal 120.
100311 Fig. 2 is a spectrum chart showing spectra for a PR:LuAG crystal. In
particular, shown is a 100s spectrum 210 of a 50mmx100mm Pr:LuAG crystal,
overlaid with a spectrum 212 that has undergone a 5% gain shift. Note that
although
100s spectra are shown in Fig. 2, much shorter acquisition times could be used
for
some applications. Fig. 3 is a flowchart describing a technique for
stabilising gain of a
gamma-ray spectroscopy system, according to some embodiments. A processing
system is used to compare a previously recorded 'standard' spectrum SPCa with
a
gain (G) and/or offset (0) corrected measured spectrum SPCb(G,O). Here the
term
SPCb(G,O) indicates symbolically that the original shape of the measured
spectrum
SPCb is modified by applying gain and offset and thus the resulting spectrum
SPCb(G,O) is a function of these two parameters. In step 310, a 'standard'
spectrum
is generated using a much longer acquisition to minimize statistical errors.
In step
312, a current spectrum is measured, which is not yet gain or offset
corrected. In step
314, the spectrum is then is normalized to the acquisition time of the
measured
spectrum. This normalization makes use of the fact that the intrinsic activity
is
constant over time within the limits of statistical variation. In step 316, a
residual res
is then be computed between the 'standard' spectrum and the current measured
spectrum where a gain and/or offset is applied to the measured spectrum. In
step 318,
the gain and/or offset corrections are determined. In the simplest case of no
interference with other radiation source a minimization routine would be
applied to
determine the gain (Gmin) and possibly also offset (Offmin) at which the
residual is
minimal. In step 320, the gain is then be stabilized with the methods known
from
prior art.
[0032] According to some embodiments, where the more complex case of
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significant other signals interfering with the background spectrum, additional
techniques are used to account for the interference. However, in many cases it
is
possible to assume that the interfering radiation remains essentially constant
over a
time period that is significantly longer then the time used to update the
stabilization
circuit (which is typically seconds), such that the described minimization
technique
can be carried out. According to other embodiments, for some applications the
measured radiation is described by a set of standards, and the optimization is
extended
to a set of gains and relative amplitudes of the standards.
[0033] According to some embodiments, the differentiated spectrum of the
internal radiation is used. Differentiation is a very simple mathematical
process,
which can easily be handled in most acquisition systems. The resulting
differentiated
spectrum may have more prominent and localized features than the original
spectrum.
Fig. 4 is a chart showing a differentiated spectrum, according to some
embodiments.
Spectrum 410 is a differentiated spectrum computed from spectrum 210 in Fig.
2. The
differentiated spectrum 410 of this Pr:LuAG detector show three prominent
peaks
412, 414 and 416. According to some embodiments, the detector gain
stabilization
uses any of the peaks 412, 414 and 416, in combination with a known gains
stabilization technique for stabilization sources. An advantage of using a
differentiated spectrum is that the peaks can be easily separated from other
spectral
features that may interfere with the intrinsic background. Note that a full
energy or
escape peak from a gamma source would show up as a bipolar peak in the
differentiated spectrum and a Compton edge appears as a negative peak.
According
to some embodiments, higher order differentials are used, such as second
derivative or
third derivative spectra.
[0034] According to some embodiments additional known techniques may be
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used to reduce noise and digitization effects on the spectral features for
improved gain
stabilization. One example for such a technique is a triangular smoothing
filter. Fig. 5
is a chart showing differentiated spectrum smoothed using a smoothing filter,
according to some embodiments. The effect of 1/4 - 1/2 - 1/4 filtering on the
differentiated spectrum 410 of Fig. 4 is shown in the smoothed spectrum 510 of
Fig.
5.
[0035] According to some embodiments, the internal spectrum from the
intrinsic
radiation of the scintillator material can also be used to accurately monitor
the
intrinsic detector resolution. This can be important for some measurement
applications. For example in geochemical logging, environmental parameters
such as
temperature have an impact on the detector response function and indirectly
influence
the accuracy of the determined elemental concentrations. Fig. 6 is a chart
showing an
example of a fit function used to approximate the peak in a differentiated
spectrum.
In particular, the data points, shown as "+" signs, such as mark 610 are the
differentiated spectrum 510 shown in Fig. 5. The fit curve 612 is shown as
resulting
from a fit function. According to some embodiments, the width of the fit curve
612 is
used to estimate a detector resolution. In cases where the intrinsic spectrum
is
complex, according to some embodiments, the variation of resolution with peak
position is estimated. According to some embodiments, the estimated variation
is
used to determine problems such as noise or deterioration of the detector.
[0036] According to some embodiments, a more complex intrinsic spectrum is
used to determine a non-linear energy calibration. The differentiated spectrum
410 of
the Pr:LuAG detector in Fig. 4 shows three prominent peaks 412, 414 and 416.
According to some embodiments, the detector gain stabilization uses all of the
peaks
412, 414 and 414 and optimizes the parameters gain offset and non-linearity
until a
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measured spectrum is best fitted to a previously determined reference
spectrum.
Since differentiation is a very simple mathematical process, which can easily
be
handled in most acquisition systems, the use as described herein are
particularly well-
suited for downhole applications where processing power may be very limited.
For
example the described techniques are particularly well-suited for applications
carried
out during and as part of a drilling operation such as MWD and LWD operations.
[0037] Although
many embodiments are described herein using the first
derivative, according to some embodiments, higher order derivatives are in an
analogous fashion. For example, according to some embodiments a second
derivative
of the spectrum is combined with a search for roots based on linear fitting
around the
intersections of the second derivative with the axis. Such root finding
algorithms are
known in the art. For example they may search the function for sign changes
and then
do a local linear interpolation of the data which gives a first order
approximation of
the root value.
[0038] According
to some embodiments, the intrinsic radioactivity of certain
scintillator materials in a radiation detector is used as a count rate
reference or
'intrinsic pulser' for the apparatus containing the radiation detector.
Using the
scintillator material's intrinsic radioactivity as a count rate reference thus
avoids some
disadvantages associated with an external count rate reference. The count rate
reference can be used to measure a number of properties of the detection
system.
According to some embodiments, the count rate reference or intrinsic pulser is
used
for testing functionality of the system without adding additional parts, such
as an
electronic pulser, or other extrinsic source.
[0039] According
to some embodiments, the count rate reference or intrinsic
pulser is used for precision dead time corrections. This is possible because
the
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intrinsic count rate is perfectly random, but overall very stable in energy
and average
count rate with only the statistical spread from the total number of decay
events in a
given time period. With a large detector delivering an average count rate of
about
50,000 s-1 one has to acquire only 20s of data to get to a statistical
precision of 0.1%.
According to some other embodiments, the internal radiation can be used as
input for
pileup simulation. The randomness of the intrinsic events makes it equivalent
to
random external radiation events resulting in real pileup events. According to
some
embodiments, corrections for pileup are based on characterization measurements
made in a calibration facility. Characterization measurements could include
variable
count rate by changing external source strength, controlled variations in the
environment, etc.
100401 An advantage over using a conventional external source as a count
rate
reference is that the intrinsic radioactivity is uniformly distributed
throughout the
system and therefore the count rate is produced at the same location as the
signal.
Therefore there are no geometrical or shielding effects that would change the
count
rate in the system.
100411 As mentioned above, the dead time of the system under exposure to
external radiation can be estimated from a comparison with dead time of a
reference
spectrum of the intrinsically generated radiation. According to some
embodiments,
the comparison may be based on evaluating count rates in different regions of
the
spectrum. For example, if only peaks need to be clearly distinguished, then an
integration of counts over the region of each peak can be performed without
any
fitting. According to some embodiments, the comparison is based in part on
spectral
fitting that includes the use of standard spectra of the intrinsically
generated radiation
and at least one component form an external radiation source. According to
some
CA 02743051 2013-07-05
69897-149
embodiments, this type of characterization of the dead time of the reference
spectrum
is performed on a prototype or calibration system well before performing the
well log.
For example it could be done during the engineering phase of product
development.
According to other embodiments, the characterization is performed as a
calibration
step on each system, and can be occasionally repeated in a local workshop.
[0042] Whereas
many alterations and modifications of the present disclosure will
no doubt become apparent to a person of ordinary skill in the art after having
read the
foregoing description, it is to be understood that the particular embodiments
shown
and described by way ,of illustration are in no way intended to be considered
limiting.
Further, the disclosure has been described with reference to particular
preferred
embodiments, but variations within the scope of the disclosure will occur to
those skilled in the art. It is noted that the foregoing examples have been
provided
merely for the purpose of explanation and are in no way to be construed as
limiting of
= the present disclosure.. While the present disclosure has been described
with reference
to exemplary embodiments, it is understood that the words, which have been
used
herein, are words of description and illustration, rather than words of
limitation.
Changes may be made, within the purview of the appended claims, as presently
stated
and as amended, without departing from the scope of the present disclosure
= in its aspects. Although the present disclosure has been described herein
with
reference to particular means, materials and embodiments, the present
disclosure is
not intended to be limited to the particulars disclosed herein; rather, the
present
disclosure extends to all functionally equivalent structures, methods and
uses, such as
are within the scope of the appended claims.
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