U.S. patent application number 11/511078 was filed with the patent office on 2009-05-21 for methods and apparatus for performance verification and stabilization of radiation detection devices.
Invention is credited to Michael Iwatschenko-Borho.
Application Number | 20090127449 11/511078 |
Document ID | / |
Family ID | 39636507 |
Filed Date | 2009-05-21 |
United States Patent
Application |
20090127449 |
Kind Code |
A1 |
Iwatschenko-Borho; Michael |
May 21, 2009 |
METHODS AND APPARATUS FOR PERFORMANCE VERIFICATION AND
STABILIZATION OF RADIATION DETECTION DEVICES
Abstract
The rare earth metal Lutetium in compound form is used in check
sources of various shapes and sizes to calibrate and tune radiation
detection devices. Radioactive Lutetium-176, a naturally occurring
(non man-made) isotope forming part of the Lutetium compound,
produces gamma energies of approximately 90, 200, and 300
kilo-electron Volts which are used in the calibration. Such gamma
energies are close to the predominant spectral lines of special
nuclear materials such as U-235 and Pu-239, which is to be
monitored by radiation detection devices. Lutetium in a radioactive
calibration source (which is either integrated into the radiation
detection device or positioned close to it during calibration)
provides benefits including that no reactor or accelerator is
required during production or use, for the creation of man-made
radioactivity, no dangerous radiation exposure occurs and (because
of the long half-life of Lu-176) the radioactive calibration source
essentially never needs to be replaced. Moreover, the handling of
such a source is much less restrictive and costly than that of a
conventional man-made radioactive isotope.
Inventors: |
Iwatschenko-Borho; Michael;
(Erlangen, DE) |
Correspondence
Address: |
BARRY W. CHAPIN, ESQ.;CHAPIN INTELLECTUAL PROPERTY LAW, LLC
WESTBOROUGH OFFICE PARK, 1700 WEST PARK DRIVE, SUITE 280
WESTBOROUGH
MA
01581
US
|
Family ID: |
39636507 |
Appl. No.: |
11/511078 |
Filed: |
August 28, 2006 |
Current U.S.
Class: |
250/252.1 |
Current CPC
Class: |
G01T 1/40 20130101; G01T
7/005 20130101 |
Class at
Publication: |
250/252.1 |
International
Class: |
G01D 18/00 20060101
G01D018/00 |
Claims
1-19. (canceled)
20. A method comprising: receiving an amount of material in powder
form, the material including Lu-176; filling a cavity of a mold
with the material; applying pressure to the material in the cavity
of the mold to form a fused mass of material including Lu-176; and
forming the fused mass to be a radiation source that is movable
with respect to a radiation detector, the fused mass including the
Lu-176 to produce radiation to calibrate the radiation detector;
wherein the radiation detector is a gamma radiation detector, the
method further comprising: forming the fused mass to perform
calibration of the gamma radiation detector at one or more peak
count values between around 90 and 300 kilo-electron volts.
21. A method as in claim 20 further comprising: forming at least a
portion of the fused mass into a shroud-like structure for encasing
multiple surfaces of a radiation detection element of a radiation
detection device.
22. A method as in claim 20 further comprising: forming at least
one surface area of the fused mass to have a thickness in which the
fused mass of material including the Lu-176 has a weight to surface
area of less than five grams per square centimeter.
23. A method as in claim 20 further comprising: forming the fused
mass into a ring.
24. A method as in claim 20 further comprising: forming the fused
mass into a rod.
25. A method as in claim 20 further comprising: forming the fused
mass into a disk.
26. A method comprising: receiving an amount of material in powder
form, the material including Lutetium-176; applying pressure to the
material to change a density of the material from a first density
to a second density, the second density being greater than the
first density; and forming the received amount of material into a
unified mass as being movable with respect to a radiation detector,
the unified mass including the Lu-176 to produce radiation to
calibrate the radiation detector; wherein receiving the amount of
material includes receiving an amount of Lutetium compound and
wherein applying pressure to the material includes compressing the
Lutetium compound, the method further comprising: producing the
unified mass by exposing the compressed Lutetium compound to a
solution that fills pores of the compressed Lutetium compound and
makes the compressed Lutetium compound resistant to breaking into
pieces; filling a container made from low Z material with the
unified mass of Lutetium compound along with other unified masses
of Lutetium compound; and adding stabilizer material to the
container to secure the unified masses to each other and form a
radiation calibration source for calibrating a radiation detection
device.
27. A method as in claim 26 further comprising: producing the
unified mass by heating the material in powder form.
28. (canceled)
29. A method comprising: receiving an amount of material in powder
form, the material including Lu-176; producing a unified mass by
applying pressure to the material to change a density of the
material from a first density to a second density, the second
density being greater than the first density; filling a container
made from low Z material with the unified mass and other unified
masses including Lu-176; and adding a stabilizer material to the
container to secure the unified masses to each other and form a
radiation calibration source for calibrating a radiation detection
device.
30. (canceled)
31. A method as in claim 27 further comprising: coating the unified
mass with a low Z material.
32. A method as in claim 26 further comprising: coating the unified
mass with a low Z material.
33. A method as in claim 20 further comprising: forming at least a
portion of the fused mass to encapsulate a portion of the radiation
detector.
34. A method as in claim 20 further comprising: forming the fused
mass as being attachable and removable with respect to the
radiation detector.
35. A method as in claim 34 further comprising: forming the fused
mass as being attachable to the radiation detector; and coating the
fused mass with a low radiation absorbing protective material
enabling the radiation from the source to pass through the coated
fused mass to the radiation detector.
36. A method as in claim 20 further comprising: forming the fused
mass to include at least one surface area shaped to match a
corresponding at least one surface area of the radiation detector
to removably attach the fused mass to the radiation detector.
37. A method comprising: receiving an amount of material in powder
form, the material including Lu-176; filling a cavity of a mold
with the material; applying pressure to the material in the cavity
of the mold to form a fused mass of material including Lu-176; and
forming the fused mass to be a radiation source that is movable
with respect to a radiation detector, the fused mass including the
Lu-176 to produce radiation to calibrate the radiation detector;
wherein applying pressure to the material changes a density of the
material in powder form from less than 3 grams per cubic centimeter
to a density of greater than 4 grams per cubic centimeter.
38. A method as in claim 20 further comprising: forming the fused
mass to include at least one surface area for passing of radiation
through the fused mass to the radiation detector.
39. A method comprising: receiving an amount of material in powder
form, the material including Lu-176; filling a cavity of a mold
with the material; applying pressure to the material in the cavity
of the mold to form a fused mass of material including Lu-176;
forming the fused mass to be a radiation source that is movable
with respect to a radiation detector, the fused mass including the
Lu-176 to produce radiation to calibrate the radiation detector;
forming the fused mass to include at least one surface area for
passing of radiation through the fused mass to the radiation
detector; and limiting a thickness of the at least one surface area
to be less than 2 grams of fused material per square centimeter of
the at least one surface area.
40. (canceled)
41. A method as in claim 27 further comprising: coating the fused
mass with a low Z material.
42. A method as in claim 41, wherein the coating is between 0.5 and
2.0 millimeters.
43. A method as in claim 20 further comprising: coating the fused
mass with a low radiation absorbing protective material enabling
radiation from a source other than the fused mass to pass through
the coated fused mass to the radiation detector.
44. A method comprising: receiving an amount of material in powder
form, the material including Lu-176; filling a cavity of a mold
with the material; applying pressure to the material in the cavity
of the mold to form a fused mass of material including Lu-176;
forming the fused mass to be a radiation source that is movable
with respect to a radiation detector, the fused mass including the
Lu-176 to produce radiation to calibrate the radiation detector;
and forming the fused mass to include an opening through which
radiation from a source other than the fused mass is able to pass
and strike the radiation detector without having to pass through
the fused mass.
Description
FIELD OF THE INVENTION
[0001] This invention relates to radiation sources for calibrating
radiation detection devices, and more particularly to
Lutetium-containing check sources, their manufacturing, and their
use.
BACKGROUND
[0002] Requirements of the Untied States Department of Homeland
Security include a need for devices capable of sensitive detection
of gamma rays originating from hidden radioactive material (e.g. in
accordance with standards of the American National Standards
Institute (ANSI) such as ANSI N42.32). As well, many steel plants
and scrap yards are concerned about the potentially dangerous
melting of so-called orphane sources which might be included in the
in or outbound scrap material. Even landfills and waste
incineration plants equip their gates and personal with monitors
for the detection of such radioactivity. Commercially available
high sensitivity portable or mobile gamma radiation meters can be
deployed to detect very small amounts of radioactivity.
[0003] Some conventional radiation detection instruments simply
display the number of detected gamma rays sensed (i.e., counted) by
the device, while other conventional radiation detection devices
are capable of measuring and displaying the dose rate of the gamma
radiation field detected by the device. Operators of such devices
can set alarm thresholds on absolute numbers of the detected
particles per time unit or on the measured dose rate, depending
upon the device used. Some radiation detection systems are
configured to generate an alarm when the respective count or dose
rate of gamma radiation exceeds a predetermined threshold related
to background level.
[0004] Prior to actual use, and preferably on a regular basis
during their useful life, radiation detection devices typically
should be calibrated against a known standard. Calibration can
require at least periodic exposure of a radiation detection device
to a radioactive source exhibiting a similar spectra of energy as
those radioactive sources of concern.
[0005] Conventional manufacturing of radioactive sources for
calibration of radiation detectors (e.g., so-called check sources)
typically requires access to a reactor or an accelerator to produce
the radioactive material. The man-made isotopes used as check
sources typically exhibit a half-life between a few minutes and
several years; those with short half-lives require frequent
replacement.
[0006] For many reasons, radioactive sources often need to be very
strong (e.g., emitting a high amount of radiation). Accordingly,
such sources require special handling during use as well as
storage. Government authorities have established rules and
regulations in order to protect workers and the public from any
possible danger from these sources. Unfortunately, this can hamper
the possession and usage of even small amounts of such radioactive
material.
[0007] Commercially available high sensitivity, stationary,
portable or mobile gamma radiation meters can easily detect very
small increases in the strength of a gamma radiation field.
However, a problem arises when such devices are deployed to users
who normally do not handle radioactive materials and who therefore
do not own corresponding check sources to properly test the
performance of the detectors.
[0008] As an alternative to the use of man-made radioactive
material, certain naturally occurring radioactive materials have
been used to verify the performance of radiation detection devices.
However, the only natural materials known to be used as check
sources today are K-40, isotopes of the Th-232 decay chain, and
isotopes of the U-238 decay chain.
[0009] Material such as incandescent mantles (Thorium), old watches
(Radium) and fertilizer (Potassium K-40) can emit suitable levels
of radioactivity for testing purposes. The elements Thorium and
Uranium exhibit multiple spectral energies ranging up to 3
Megaelectron Volts; K-40 produces a single spectral line at about
1.5 Megaelectron Volts. However, these isotopes are not well suited
to test portal monitors or pocket size scintillation detectors
because their average gamma energy is significantly higher than the
typical gamma energies of those isotopes of concern.
SUMMARY
[0010] Conventional radiation calibration sources suffer from a
number of deficiencies as discussed above. For example, most known
radiation sources for calibrating radiation detection devices have
rather short half-lives. Accordingly, when used for calibration,
this type of radiation detector calibration source must be
frequently replaced. Also, as discussed above, certain conventional
material used for calibration purposes is highly regulated by
governmental agencies. Accordingly, it can be difficult to obtain
appropriate governmental clearance for use of certain radioactive
material even for the legitimate purpose of calibrating sensitive
and sophisticated devices used for detecting the presence of highly
controlled matter such as enriched Uranium or Plutonium or
dangerous orphane industrial sources which might be shielded by a
transport container or other surrounding material.
[0011] Techniques and apparatus of the present invention as
discussed herein differ from those discussed above as well as other
techniques known in the prior art. In particular, embodiments
herein include use of the natural radioactivity contained in the
rare earth metal Lutetium as a source for calibrating radiation
detection devices.
[0012] Lutetium typically occurs in very small amounts in nearly
all minerals containing yttrium, and is present in monazite to the
extent of about 0.003%. Lutetium can be prepared by the reduction
of anhydrous LuCl.sub.3 or LuF.sub.3 by an alkali or alkaline earth
metal. Naturally occurring Lutetium contains 2 different isotopes:
stable Lutetium-175 with an abundance of 97.4%, and radioactive
Lutetium-176 with an abundance of 2.6% and a half-life of around
3.7.times.10.sup.10 years (i.e., 37 billion years). Accordingly, a
Lutetium-based calibration source essentially never needs to be
replaced.
[0013] Another reason to produce test sources based on natural
Lutetium is its low specific radioactivity (e.g. approximately 48
Becquerels/gram for Lu.sub.2O.sub.3), which is low so as not to be
a health concern. For comparison, a conventional man-made isotope
Cs-137 (Half life 30 years) shows a specific activity of about
3.2.times.10.sup.12 Becquerels/gram (87 Ci/gram), which can cause
severe health damage to anybody who gets exposed to this radiation
even for a short period of time.
[0014] Additionally, use of material including Lutetium-176 (e.g.,
a Lutetium compound and/or Lutetium-176 in its elemental form) as a
calibration source provides advantages concerning the gamma
spectra. Its energies at about 300 kilo-electron Volts, about 200
kilo-electron Volts, and about 90 kilo-electron Volts are close in
approximation to the predominant spectral lines expected from
highly regulated nuclear materials such as U-235 and Pu-239.
Conventional techniques involve use of "substitutes" (rather than
U-235 or Pu-239) such as Co-57 and Ba-133 as calibration sources,
but these latter materials unfortunately have a short
half-life.
[0015] Use of gamma energy in this spectral region around 90, 200,
and 300 kilo-electron Volts (as provided by gamma radiation from
Lutetium) enables radiation detection devices to be more precisely
calibrated than when using non-matching spectral regions. Thus,
embodiments herein enable radiation detection devices to be tested
under more realistic conditions such as an expected spectral energy
associated with materials to be detected.
[0016] Via calibration of a radiation detection device to 90, 200,
and/or 300 kilo-electron Volts using a Lutetium calibration source,
the radiation detection device can be (periodically, occasionally,
repeatedly, etc.) calibrated or stabilized to correct for "fading"
or "drifting" out of specification due to environmental factors
such as temperature changes. For example, one typical radiation
detection device includes a common inorganic scintillation detector
such as one made from Thallium doped Sodium Iodide (NaI(Tl). Such a
detector can exhibit significant drift effects as a result of
temperature changes, aging, and exposure to elements. Known methods
for a stabilization of such detectors include the insertion or
injection of man-made radioactive isotopes (e.g., Am-241 or Cs-137)
to the scintillation detector. A drawback of this conventional use
of these isotopes is the fact that, even though these materials
emit a fairly low amount of radiation, they are man-made and may
not be taken to certain places having a specific site regulation
that strictly forbids the presence or usage of artificial
radioactive material on such premises. Furthermore, use of Am-241
or Cs-137 permanently attached to a detector can prevent the
detection of small amounts of these isotopes in an environment
under test.
[0017] As an alternative to Am-241 or Cs-137, Potassium Chloride
(e.g., KCl) can be used as a source for generating gamma radiation.
Unfortunately, due to the low specific activity of KCl, to produce
an appreciable amount of gamma energy to "interact" with a
scintillator detector, would require use of rather large amounts of
KCl as a calibration source. Thus, use of KCl as a calibration
source is rather inconvenient.
[0018] In certain cases, a non-radioactive device such as an LED
(Light Emitting Diode) can be used to calibrate portions of a
radiation detector. For example, pulsed or continuous light can be
used to stabilize a photo-detector device (e.g., a
photo-multiplier) to compensate for amplification drift of a
photo-multiplier and other circuit inaccuracies. However, a light
source cannot be used to account for "inhomogeneities" in a crystal
or scintillator (of a radiation detection device) that is used to
convert gamma energy to corresponding photon energy detected by a
photon detector.
[0019] Certain embodiments of the present invention include use of
Lutetium in compound form as a check source for calibration of a
gamma detector or a beta detector. For example, a radioactive
calibration source according to embodiments herein includes
non-toxic compounds such as Lu-Oxide, Lu-Carbonate, Lu-Chloride,
etc. Such non-metallic compounds can be formed into a variety of
useful shapes to calibrate radiation detection devices. For
example, the radioactive calibration source can be formed into
tablets, rods, discs, caps, rings, etc. In certain cases, the
radioactive Lutetium calibration source is shaped to enable it to
be in close proximity to or even in contact with detector material
(e.g., a scintillator) of a radiation detection device.
[0020] Certain embodiments herein involve controlling a thickness
of the Lutetium-based radioactive calibration source. For example,
thickness of the Lu-compound can be limited in order to reduce or
minimize self-absorption of radiation, which would prevent the
radiation from reaching a detection device (e.g., a scintillator)
of a radiation detection device. A scintillator of a radiation
detection device can be encapsulated with a shroud-like structure
made from Lutetium compound. The shroud-like Lutetium-based
radioactive calibration source at least partially encapsulates a
scintillator detector and provides gamma energy to calibrate the
radiation detection device. The shroud can be thin enough to allow
gamma radiation to pass through the shroud (e.g., the
Lutetium-compound) and strike the scintillator. In certain cases,
the shroud can be thin enough to enable detection of even low
levels of external radiation in the presence of the Lutetium-based
calibration source. Accordingly, a Lutetium-based radioactive
calibration source can be continuously present during an operation
of testing for the presence of radiation from other sources.
Alternatively, a calibration source can be temporarily positioned
or held in close proximity to a radiation detection device only
during a calibration mode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The foregoing and other objects, features and advantages of
the methods and apparatus will be apparent from the following
description of particular embodiments, as illustrated in the
accompanying drawings in which like reference characters refer to
the same parts throughout the different views. The drawings are not
necessarily to scale, emphasis instead being placed upon
illustrating the principles of the methods and apparatus.
[0022] FIG. 1 is a diagram illustrating a radiation detection
device and a calibration source according to embodiments
herein.
[0023] FIG. 2A is a flow chart illustrating calibration and usage
of a radiation detection device according to embodiments
herein.
[0024] FIG. 2B is a flow chart illustrating calibration and usage
of a radiation detection device according to embodiments
herein.
[0025] FIG. 3 is a flow chart illustrating production of a
radioactive calibration source according to embodiments herein.
[0026] FIG. 4 is a graph illustrating a hypothetical example of
calibrating a radiation detection device via use of K-40.
[0027] FIG. 5 is a graph illustrating a hypothetical example of
calibrating a radiation detection device according to embodiments
herein.
[0028] FIG. 6 is a graph illustrating results of calibrating a
radiation detection device according to embodiments herein.
[0029] FIG. 7 is a diagram of a radioactive calibration source in
the form of a disk according to embodiments herein.
[0030] FIG. 8 is a diagram of a radioactive calibration source in
the form of a rod according to embodiments herein.
[0031] FIG. 9 is a diagram of a radioactive calibration source in
the form of a ring according to embodiments herein.
[0032] FIG. 10 is a diagram of a radioactive calibration source in
the form of a shroud according to embodiments herein.
[0033] FIG. 11 is a diagram of a radioactive calibration source in
the form of a container including multiple pellets according to
embodiments herein.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0034] One embodiment of the present disclosure is directed toward
use of a rare earth element Lutetium in compound form to calibrate
radiation detection devices. This rare earth metal contains a
radioactive isotope Lutetium-176 with a natural abundance of about
2.6% that produces measurable radiation energy.
[0035] Radioactive Lutetium-176 produces gamma energies of about
90, 200, and 300 kilo-electron Volts. Such gamma energies are close
to predominant spectral lines normally produced by so-called
special nuclear materials such as U-235 and Pu-239 whose detection
is desired, and also by their respective surrogates Co-57 and
Ba-133, which have conventionally been used for calibration
purposes. Radioactive Lutetium-176 has a half-life of 37 billion
years. Accordingly, use of Lutetium in a radioactive calibration
source provides a benefit that the radioactive calibration source
essentially never needs to be replaced. Additionally, via exposure
to a radioactive Lutetium calibration source, a radiation detection
device can be precisely calibrated or tuned to detect highly
regulated material such as enriched Uranium and Plutonium.
[0036] FIG. 1 is a block diagram illustrating a radiation detection
device 102 operating in environment under test 100 and which
includes a radioactive calibration source 110 integrated into/with
the detection device according to embodiments herein. The radiation
detection device 102 includes a radioactive calibration source 110,
detector 115, photo-detector 120, amplifier 125, processor 113,
memory 112, user interface 119, and display 130. This configuration
shown is a typical example of a radiation detector based on
scintillating material. In an alternate configuration using a
semiconductor (e.g. Germanium) detector, amplifier 125 would be
directly coupled to the detector 115 without the need to include a
photo-detector 120 (photo-multiplier or photodiode).
[0037] During general operation of the radiation detection device
102, a radiation source 105 emits gamma rays that pass through
radioactive calibration source 110 and strike detector 115. When
present, radioactive calibration source 110 emits radiation (e.g.,
gamma rays) that also strike detector 115. As will be discussed
later in this specification, radiation detection device 102 can be
calibrated in the presence as well as in the absence of radiation
source 105.
[0038] Detector 115 converts the gamma rays into photons.
Photo-detector 120 detects at least a portion of photons emitted by
detector 115. Based on an amount of photons striking photo-detector
120, photo-detector 120 generates an electrical signal 109 to drive
the input of amplifier 125. For example, a higher number of photons
produce a higher pulse amplitude of electrical signal passed to the
amplifier 125 along path 109.
[0039] Processor 113 receives the output (e.g., an amplified
signal) of amplifier 125. The processor 113 monitors (via counts or
other method) a level of radiation emitted by radiation source 105
and/or calibration source 110 (as the case may be) based on
characteristics of the signal output from amplifier 125. Processor
113 drives an output device such as an LED or display screen 130 to
provide an indication of an amount of gamma radiation emitted by
radiation source 105.
[0040] In one embodiment, a composition of radioactive calibration
source 110 includes a rare earth metal such as Lutetium-176 to
calibrate radiation detection device 102. According to embodiments
herein, elemental Lutetium can be combined with other elements such
as Fluorine: to form LuF.sub.3, Chlorine: to form LuCl.sub.3,
Carbon: to form Lu.sub.2(CO.sub.3).sub.3.xH.sub.2O; Bromine: to
form LuBr.sub.3, Iodine: to form LuI.sub.3, Oxygen: to form
Lu.sub.2O.sub.3, Sulfur: to form Lu.sub.2S.sub.3, Tellurium: to
form Lu.sub.2Te.sub.3, Nitrogen: to form LuN, etc. Such (non-metal)
compounds are well-suited for applications in which the radiation
detection device is a portable and/or handheld device. It is
preferable to use non toxic compositions such as LuCl.sub.3,
Lu.sub.2O.sub.3; Lu.sub.2(CO.sub.3).sub.3.xH.sub.2O. Of these,
Lu.sub.2O.sub.3 has the highest relative weight amount of Lutetium
within the molecule and can be considered as first choice for all
applications where a high density is required in order to minimize
the size of the check source. This is especially important in those
cases, where the detector is rather small and the surface therefore
possesses a strong convex curvature (e.g. small size cylinders).
Using a high density Lutetium compound, significantly less material
is required to achieve the same count rate, because the
radioactivity is in closer average distance in respect to the
detector.
[0041] For example, a radioactive calibration source 110
permanently integrated into the detector 115 (as opposed to being
temporarily placed near detector 115 for calibration purposes) can
add little appreciable weight to the radiation detection device 110
or at least does not render the radiation detection device 110
prohibitively heavy. In one embodiment, the Lutetium compound used
to produce radioactive calibration source 110 can have a purity of
between about 90% and 99.99% and relatively few or no other
radioactive isotopes that generate radiation other than the
Lutetium. The Lutetium compound such as Lutetium oxide is normally
available with a purity level as low as 98%, which is suitable for
embodiments herein. Use of Lutetium compound with an even lower
purity level may be attractive because it may be available at lower
cost than Lutetium compound of a higher purity.
[0042] Note that although FIG. 1 illustrates use of a radioactive
calibration source 110 for purposes of testing radiation detection
device 102, a Lutetium-based calibration source (e.g., radioactive
calibration source 110) can be used to calibrate other types of
radiation detection equipment as well.
[0043] In one embodiment a typical volume of between 2 and 50 cubic
centimeters of Lutetium compound can be used to calibrate a
corresponding radiation detection device. However, note that
certain embodiments herein can require more or less amounts of
radioactive Lutetium compound to calibrate a radiation detection
device 102.
[0044] Since commercially available Lutetium compounds typically
are available as a loose powder with a rather low density, it can
be beneficial to compress the material when used as a calibration
source. As an example, the manufacturing of high density Lutetium
Oxide requires very high pressure typically exceeding 100
MegaPascals. For this purpose, the Lutetium Oxide powder (and/or
material including the Lutetium oxide powder or Lutetium in its
elemental form) is filled into a suitable cavity which might have
the shape (e.g. diameter) of the final source or which might be
sized and shaped as smaller parts (e.g. tablets, elements). In this
latter embodiment, the tablets then can serve as "filling" for
different types (e.g., sizes and shapes) of source casings. The
high pressure can be applied to the powder until a density of the
compressed body is at the desired value of more than 3 grams per
cubic centimeter (such as up to the theoretical limit of 9.4 grams
per cubic centimeter for Lutetium oxide compounds). In one
embodiment, application of pressure to a Lutetium compound changes
a density of the Lutetium compound (e.g., in powdered for) from a
starting density of less than 3 grams/cubic centimeter to greater
than 4 grams per cubic centimeter after application of the
pressure.
[0045] In order to transform the resulting porous body (e.g., a
compressed mass of Lutetium-compound powder) into a mechanically
stable object, various types of treatments can be applied. One
method is the exposure of the compressed mass of Lutetium to high
temperatures of 500.degree. C.-2000.degree. C. in order to sinter
the material into a mechanically stable system. In this case no
additional material is added to the compressed mass of Lutetium and
the mass becomes resistant to breaking apart. Accordingly, the
weight of the radioactive calibration source can be kept to a
minimum.
[0046] Either additionally or as an alternative, the compressed
mass of Lutetium compound can be exposed to a solution composed of
plastic material e.g. PMMA (Polymethylmethacrylat), PS
(Polystyrol), PUR (Polyurethan), epoxy or other suitable plastic
material dissolved in an organic solvent like Tetrahydrofuran or
others. During exposure, the dissolved plastic fills up the pores
of the compressed Lutetium oxide. After evaporation of the solvent
the plastic material provides structural support so that the final
shape of the compressed Lutetium oxide body is resilient to
breaking apart (i.e., the mass of compressed or potentially
uncompressed Lutetium oxide becomes mechanically stable). Further,
radioactive calibration source 110 can be encased with low Z
(atomic number) material (e.g. material which is essentially
transparent for gamma radiation) such as plastic, aluminum, etc.
for protection purposes. In one embodiment, a thickness of the
low-Z material is on the order of 0.5-2 millimeters. The low-Z
material can also potentially act as a mold or form factor aiding
in production of different forms of the radioactive calibration
source 110.
[0047] In certain circumstances, it can be useful to form a
radioactive calibration source 110 which can be at least
temporarily placed in close proximity to detector 115 for purposes
of calibrating the radiation detection device 102 based on gamma
(and/or beta) radiation emitted from the radioactive calibration
source.
[0048] The detector 115 illustrated in FIG. 1 can be a cylindrical,
protruding structure encased by a "tight fitting" calibration
source 110 with little or no gap between the detector 115 and the
radioactive calibration source 110 that encases multiple surfaces
of detector 115. Accordingly, radioactive calibration source 110 at
least partially encapsulates or shrouds detector 115.
[0049] As previously discussed, if the radioactive calibration
source 110 is permanently integrated with detector 115 the
thickness or weight per amount of surface area of calibration
source 110 is limited so that radiation (e.g., gamma and beta
radiation) from radiation source 105 is able to pass though
radioactive calibration source 110 to detector 115 without being
absorbed. In other words, the radioactive calibration source 110
should be sufficiently thin so as to limit the amount of gamma
radiation absorption (or other absorption) by the radioactive
calibration source 110. In such an embodiment, a maximum of about 2
grams of the Lutetium compound per square centimeter of surface
area of the radioactive calibration source is used. For a density
of the Lutetium compound (e.g. Lu.sub.2O.sub.3) of about 5 grams or
less per cubic centimeter, this corresponds to a thickness of about
0.4 centimeters or less. However, other embodiments may have higher
amounts of Lutetium compound such as more than about 5 grams per
square centimeter of surface area.
[0050] Note that an amount of Lutetium oxide per surface area of
radioactive calibration source 110 can vary depending on the
application. For example, in one embodiment, a manufacturing
facility forms at least one surface area of a fused mass of
Lutetium compound to have a thickness in which the amount of
Lutetium compound is less than five grams per square centimeter of
surface area.
[0051] Also, note that the thickness (weight per area) of the
radioactive calibration source 110 may be limited to a smaller
value, such as when radiation detection device 102 is manufactured
to detect lower energy radiation particles such as beta radiation.
In such an embodiment, the radiation detection device 102 is used
as a surface contamination detector (e.g., beta surface
contamination probes or sample changers). Note that radiation
detectors for measuring beta radiation are typically equipped with
a thin entrance window (<10 milligrams/square centimeter). Such
conventional detectors have been used for many decades and include
devices such as gas-filled Geiger-Mueller counters, proportional
counters, ionization chambers, and plastic scintillation detectors.
The difference between radioactive calibration source 110 being
used as a beta source and a gamma source is given the much higher
self-absorption associated with beta radiation. 50
milligrams/square centimeter corresponds to a thickness or layer of
Lutetium oxide compound of about 0.1 millimeters.
[0052] Although parameters vary depending on the application, one
embodiment herein includes a radioactive calibration source 110
having a surface area of between about fifteen and one thousand
square centimeters. A thickness can vary from a few micrometers to
several centimeters depending on the application. The radioactive
calibration source 110 including radioactive Lutetium covers at
least a portion of the detection surface of detector 115, and the
Lutetium compound is limited to less than about fifty milligrams of
Lutetium compound per square centimeter of surface area. In such an
embodiment, the Lutetium-compound can be dispersed over a flat area
of around 15 to 1000 square centimeters in order to reduce or limit
absorption of the beta radiation emitted by the radiation source
105 in the radioactive calibration source. A surface activity of up
to approximately 2.5 Becquerels/square centimeter can be achieved
in accordance with such design specification. This level of
radiation activity is sufficient for testing and calibration of
surface contamination detectors designed to alarm on for example
0.4 Becquerels/square centimeter.
[0053] In addition to a shroud-like cover (e.g., a cap), a
Lutetium-based radioactive calibration source 110 according to
embodiments herein can be formed into other shapes such as rods
and/or tablets. Multiple Lutetium-based tablets can be used to fill
a container of any shape. In one embodiment, tablet-shaped
radioactive calibration sources of about 5 millimeters.times.5
millimeters.times.5 millimeters are loaded into a larger
detector-shape container which can be positioned in close proximity
to the radiation detection device for calibration purposes. In
another embodiment, rods can be placed in proximity to (e.g.,
around a circumference of, in front of, etc.) detector 115 for
purposes of calibrating radiation detection device 102. Spacing of
the rods from one another enables radiation emitted from radiation
source 105 and/or other rods to strike detector 115 without being
absorbed.
[0054] In yet other embodiments, the radioactive calibration source
110 can be formed into a shape such as a disc or ring. A disc,
similar in shape (but not necessarily similar in dimension) to a
coin can be affixed to a surface detector 115 and provide enough
radiation for calibration purposes. As discussed above, thickness
of the disc can be controlled to limit absorption of radiation
passing through the disc-shaped radioactive calibration source 110
to detector 115. In such an embodiment, the radioactive calibration
source disc can include a knob, or screw, or other structure to
permit it to be which firmly attached (e.g., temporarily or
permanently attached) to a surface of the detector 115.
[0055] When formed as a ring, the radioactive calibration source
110 can be slid onto detector 115 like a ring on a finger. In such
an embodiment, the open portion of the radioactive calibration
source 110 does not absorb radiation and therefore prevent
radiation emitted by radiation source 105 from striking detector
115.
[0056] A radioactive calibration source made from radioactive
Lutetium-176 produces gamma energies of 90, 200, and 300
kilo-electron Volts. Such gamma energies are close to predominant
spectral lines normally produced by so-called special nuclear
materials such as U-235 and Pu-239 and their respective surrogates
Co-57 and Ba-133, which have conventionally been used for
calibration purposes. And since radioactive Lutetium-176 has a
half-life of 37 billion years, its use in a radioactive calibration
source provides a benefit that the radioactive calibration source
essentially never needs to be replaced. Additionally, via exposure
to a radioactive Lutetium calibration source 110, a radiation
detection device 102 can be precisely calibrated or tuned to detect
highly regulated material such as enriched Uranium and Plutonium.
In other words, the radiation detection device 102 can utilize a
calibration routine executed by processor 113 as an adjustment
circuit to calibrate the radiation detection device to at least one
of 90, 200, and 300 kilo-electron Volts known peak counts produced
as a result of exposure to the radioactive calibration source 110.
After calibration, the radiation detection device 102 can be used
to identify peak counts of radiation at or around these energy
values as well as in between or outside of a range of these energy
values.
[0057] Detector 115 can include "inorganic" scintillation (i.e.,
radiation detection) material such as Thallium doped sodium iodide
NaI(Tl) material. This type of material facilitates conversion of
gamma energy into light energy. The detector 115 can be configured
into a relatively compact form using this material. As previously
discussed, the detector 115 operates to convert gamma energy into
(visible or invisible) light energy. As an alternative to use of
NaI(Tl) in detector 115, detector 115 can include other types of
scintillation material such as Cesium Iodide (CsI) to convert gamma
energy into photons.
[0058] Photo-detector 120 can include a photo-multiplier tube,
which receives light emitted by detector 115 and electrically
couples to the amplifier 120. In such an embodiment, the
photo-multiplier tube operates to receive an optical signal from
the detector 115 (e.g., as caused by interaction of radiation with
the NaI(Tl) material of detector 115 as previously discussed),
generate an electrical signal or electrical pulses proportional to
the light signal (e.g., proportional to the intensity of the light
signal), and transmit the output pulses to the amplifier 125. The
amplifier 125, such as a linear amplifier, can be configured to
adjust the pulse amplitude levels of respective output pulses to
enable a discrimination of different pulse amplitude levels.
[0059] Processor 113 can include energy analysis circuitry such as
comparators and counters. For example, in one embodiment, the
radiation detection device (e.g., gamma radiation measuring
instrument) utilizes one or more comparators, each having a given
threshold, to achieve energy discrimination of the detected gamma
radiation. Typical values correspond to gamma energies in ranges
such as between 1 and 3000 kilo-electron Volts. Each comparator
includes a corresponding counter (e.g., pulse counter).
[0060] Via respective counters, the processor 113 keeps track of
the count rates for different threshold energy ranges. Additional
details associated with measuring a level or levels of radiation
and use of counters can be found in related U.S. patent application
Ser. No. 11/076,409 filed on Mar. 8, 2005, entitled "PORTABLE
RADIATION MONITOR METHODS AND APPARATUS," the entire teachings of
which are incorporated herein by this reference.
[0061] Based on a determination of count levels, processor 113 can
analyze levels of radiation emitted by radiation source 105 and
provide an indication of the energy deviation ratio to a user
and/or other devices. For example, in one arrangement as shown,
processor 113 drives display screen 130 to provide an indication of
a level of detected gamma radiation. In other embodiments, the
processor 113 additionally or alternatively drives an audio device
(e.g., a speaker), vibrator, LED, etc. to warn when a respective
energy deviation ratio reaches a particular threshold value.
[0062] In one embodiment, the radiation detection device 102 is
configured as a computerized device. For example, radiation
detection device 102 includes processor 113. Memory 112 (e.g., a
computer readable medium) and/or a respective repository stores an
application, logic instructions and/or respective data that are
executed or utilized by processor 113 to carry out calibration and
radiation measurements according to techniques discussed
herein.
[0063] Memory 112 can be of any type of volatile or non-volatile
memory or, alternatively, storage system such as a computer memory
(e.g., random access memory (RAM), read only memory (ROM), or
another type of memory), disk memory, such as hard disk, floppy
disk, optical disk, etc.
[0064] The processor 113 can be any type of circuitry or processing
device such as a central processing unit, computer, controller,
application specific integrated circuit, programmable gate array,
or other circuitry that can access the radiation measuring
application encoded within the memory 112 in order to run, execute,
interpret, operate, or otherwise perform the radiation measuring
application logic instructions. In other words, in one embodiment,
processor 113 executes an application stored in memory 112 to carry
out techniques as discussed herein.
[0065] FIG. 2A is a flow chart 201 illustrating calibration of a
radiation detection device 102 using a radioactive calibration
source 110 according to embodiments herein. More specifically,
flowchart 201 illustrates a technique of verifying performance of
radiation detection device 102 in the presence of a radioactive
calibration source 110 that is temporarily attached to detector
115. Note that additional details associated with this technique
are discussed with respect to FIGS. 4 and 5.
[0066] In step 210 of FIG. 2A, the user turns on the radiation
detection device 102.
[0067] In step 215, the radiation detection device 102 measures a
radiation count rate (and eventually an energy distribution,
similar to count rates as measured by certain types of conventional
detectors) in the absence of the radioactive calibration source 110
and radiation source 105. Thus, radiation detection device 102
measures only naturally occurring background radiation present in
the surrounding environment.
[0068] In step 220, the user moves the radioactive calibration
source 110 in close proximity to or in contact with the detector
115 of radiation detection device 102.
[0069] In step 225, the radiation detection device 102 measures a
radiation count rate (and eventually the energy distribution, as is
done for certain types of conventional detectors) in the presence
of the check source 110 and background radiation coming from
natural radioactivity in the environment.
[0070] In step 230, the radiation detection device 102 subtracts
the background radiation count rate as measured in step 215 from
results obtained in step 225 to produce a net radiation count rate
produced as a result of exposure to the radioactive calibration
source 110.
[0071] In step 235, the radiation detection device 102 verifies
that the produced net radiation count rate in step 230 is within a
range of factory count rate measurements reflecting an amount of
radiation that should be produced by a particular radioactive
calibration source 110.
[0072] In step 240, the user tunes the amplification parameters
(e.g., amplifier 125) associated with detector 115 so that i)
measurement readings of the radiation detection device 102 indicate
that the radiation detection device 102 (when the radioactive
calibration source 110 is in close proximity to detector 115)
measures the expected amount of radiation actually produced by the
radioactive calibration source 110 and ii) future measurements of
radiation from radiation source 105 are accurate.
[0073] FIG. 2B is a flow chart 251 illustrating performance
verification and gain control when radioactive calibration source
110 is permanently secured in close proximity to detector 115
according to embodiments herein. Note that additional details
associated with this technique are discussed with respect to FIG.
6.
[0074] In step 255 of FIG. 2B, the user turns on the radiation
detection device 102.
[0075] In step 260, the radiation detection device 102 measures the
energy distribution detected by detector 115 in the presence of the
check source 110 and naturally occurring background radiation but
in absence of any other radiation sources such as radiation source
105.
[0076] In step 265, if a count rate as measured in step 260 is
above a predetermined value (e.g., threshold), a microcontroller in
the radiation detection device can provide an indication to a user
that there is another radiation source such as radiation source 105
nearby. This alerts the user not to continue with the gain control
in the following steps, and that the user must repeat steps 255 and
260 in the absence of such interfering radiation sources. In
another embodiment, the gain stabilization is performed fully or
automatically on a periodic basis and the microcontroller inhibits
the gain control by itself. If the count rate as measured in step
260 is below the predetermined threshold, processing (e.g., gain
control calibration) continues with the following steps.
[0077] In step 275, if the count rate is below the predetermined
threshold, the radiation detection device 102 compares the actual
count rates of energy for 90 kilo-electron Volts, 200 kilo-electron
Volts, and 300 kilo-electron Volts bands to expected count rates
for the energy bands when the radiation detection device 102 is
exposed to radiation from the radioactive calibration source
110.
[0078] In step 280, if there is a significant difference between
the actual count rates and expected count rates for the radioactive
calibration source 110 for one or more of the bands, then the user
(or microcontroller associated with the radiation detection device
102) initiates adjusting gain and/or offset parameters of the
radiation detection device 102 so that i) measurement readings
displayed by the radiation detection device 102 indicate that the
radiation detection device 102 (when the radioactive calibration
source 110 is in close proximity to detector 115) measures the
expected amount of radiation actually produced by the radioactive
calibration source 110 in the different bands and ii) that future
measurements of radiation from radiation source 105 are
accurate.
[0079] FIG. 3 is a flow chart illustrating production of a
radioactive calibration source according to embodiments herein.
[0080] In step 310, a manufacturing facility receives an amount of
radioactive lutetium compound in powder, granule, particulate, or
pulverized form. Typically, the Lutetium compound used to create a
radioactive calibration source 110 has a purity between 90 and
99.99 percent by volume or weight. Other percentage of purities can
be used for cost saving purposes or when there is limited
availability of the base material. Lutetium compound can be
obtained from sources such as Stanford Materials Corporation,
American Elements, METALL RARE EARTH LIMITED, Auer-Remy GmbH, and
ChemPur GmbH.
[0081] In sub step 320, the manufacturing facility fills a cavity
of a mold with the Lutetium compound powder.
[0082] In step 330, the manufacturing facility applies pressure
(via a device such as a piston) to the Lutetium compound in the
cavity of the mold to form a compressed compact mass (e.g., fused
mass) of Lutetium compound of a predetermined shape. Applying
pressure to the amount of Lutetium compound in the cavity change a
density of the Lutetium compound from a first density to a second
density. For example, the density of the Lutetium compound before
applying pressure can be about 1.5 grams per cubic centimeter. The
density of the Lutetium compound after applying a high pressure
(e.g., typically >100 MegaPascal) can be about 5.0 grams per
cubic centimeter up to the theoretical limit of 9.4 grams per cubic
centimeter for a Lutetium oxide compound. Based on application of
the pressure in the cavity of the mold, the manufacturing facility
forms the Lutetium compound into a calibration source (e.g.,
pellets or other described shapes) for calibrating a radiation
detection device. The compressed or fused Lutetium material
produced according to the steps 310, 320, 330 may not yet possess
sufficient mechanical stability for further handling. Again, note
that the term "fused mass" reflects a compressed state of the
Lutetium compound in which the Lutetium compound is able to retain
its shape without application of outside forces. For example, in
the absence of walls of the mold to retain its shape, the
compressed Lutetium compound may crumble and easily fall apart when
handled. It is therefore advantageous to transform the material
into a more stable form.
[0083] One embodiment involves converting the powdered form of
Lutetium compound into a unified, or coherent mass via mixing the
Lutetium compound with a stabilizing material such as epoxy or
performing additional processing steps. For example, in step 340,
the manufacturing facility further processes the compressed
Lutetium compound into a unified mass of Lutetium compound (e.g., a
form in which the compressed Lutetium compound is resistant to
breaking apart) by heating the compressed Lutetium compound to a
temperature of greater than 500.degree. C. This sinters the
Lutetium compound. The step of sintering causes the powder to form
a unified mass (i.e., ceramic) by heating without melting. Also as
previously discussed, the manufacturing facility can (as an
alternative or in addition to sintering) expose the compressed
Lutetium compound to a solution (e.g., plastic) that fills pores of
the mass of compressed Lutetium compound to make it more resistant
to crumbling and falling apart after the solution hardens.
[0084] Further processing to form a radioactive calibration source
110 according to embodiments herein can continue at step 350 and/or
360. For example, in step 350, the manufacturing facility fills a
container with the unified masses of Lutetium compound (e.g.,
pelletized versions of the Lutetium compound resistant to breaking
apart) in a shape such as a tablet or pelletized form. The
container can be many times larger than a size of a single unified
mass of Lutetium compound pellets that fill the container. Epoxy or
similar material can be added to the container of pellets to secure
the pellets to each other in order to avoid movement of the
"pelletized" Lutetium compound material in the container.
[0085] In step 360, in lieu of or in addition to step 350 of
creating a radioactive calibration source 110 based on a container
of tablets or pellets created from the powder Lutetium compound,
the manufacturing facility can apply a protective coating (e.g.,
outer shell or coating) to a radioactive calibration source 110
(e.g., pellets or other forms) such as those shown in FIGS. 6-9.
Accordingly, one embodiment herein includes production of check
sources (e.g., a planar shaped check source) containing natural
Lutetium for testing of plane large area plastic detectors. Unlike
a corresponding small size of conventional gamma check sources
containing only a few micrograms of radioactive material enclosed
in a holder of the size and shape of a small coin (e.g., disk),
radioactive calibration source 110 according to embodiments herein
must be adapted to the size of the detector in order to achieve a
sufficient net radiation signal versus the natural background to
calibrate the detector.
[0086] Note again that the radioactive calibration source can be
made into a predefined shape that can be integrated into detector
115 (e.g., a scintillator) of a radiation detector device as in
FIG. 1. As discussed above, the detector 115 enables conversion of
gamma radiation into photon energy.
[0087] A typical surface area of radioactive calibration source 110
for at least partially flat-shaped detectors is up to about 90% of
the detector surface. In such an embodiment, a mass per unit
surface area of radioactive calibration source 110 should not
exceed approximately 5 grams/square centimeter, which corresponds
to 250 Becquerels/square centimeter in order to avoid self
absorption of the gamma radiation in the Lutetium compound.
Accordingly, one embodiment herein includes forming at least one
surface area of the radioactive calibration source to have less
than five grams of radioactive Lutetium compound per square
centimeter.
[0088] This limited mass per unit surface area prevents significant
self-absorption in the radioactive calibration source 110 itself.
For plane large area detectors, even a low density Lu-compound can
be used, since the detection efficiency is not highly dependent on
distance is small compared to the dimensions of the large area
detector.
[0089] As discussed above, the radioactive calibration sources
(i.e., check sources) containing natural Lutetium can have a
geometry adapted to the size and shape of high sensitive gamma
detectors. Depending on the embodiment, a typical volume of the
radioactive calibration source 110 can be in a range from about
5-50 cubic centimeters of Lutetium compound. Assuming an achievable
density of about 5 grams/cubic centimeters of a compressed Lu salt,
an amount of natural Lutetium for use in a radioactive calibration
source would be on an order of 25 to 250 grams. Such a volume would
yield a radiation activity value on the order of 1 to 10
kBecquerels.
[0090] Rod-like sources containing natural Lutetium (typical volume
of 5-500 cubic centimeters, 25-2500 g natural Lutetium) compound
can be inserted into detector chambers and detector arrays of a
radiation detection device 102. Since the gamma radiation from
radioactive calibration source 110 is coincident within 1
nanosecond, coincidence types of monitors (Patent document EP 1 131
653) also can be tested via use of radioactive calibration source
110. For coincidence monitoring, the self-attenuation of gamma rays
is especially important, since it enters via both factors into the
product of the detection probabilities of both gammas. That means,
as an example, if the first gamma is attenuated by 50% and the
other gamma by 50% as well, the gross number of detected gammas is
attenuated by 50% while the coincidence rate is attenuated by 75%.
Thus, in such an embodiment, the specific weight of the Lutetium
filling desirable should not exceed 1 gram per each centimeter of
length of the rod. In case the corresponding activity associated
with a single rod is insufficient for calibrating a radiation
detection device 102, an array of multiple Lutetium rods can be
used for calibration purposes. In such an embodiment, these rods
can be sufficiently spaced from each other to minimize absorption
by a rod of the gamma rays originating from a neighboring rod.
[0091] As previously discussed, embodiments herein can include use
of a natural Lu-compound for economical reasons. Thus, although
possible, it is far less desirable to use metallic Lutetium (in its
pure metallic form) to produce radioactive calibration source 110,
since this would be quite expensive. Furthermore, Lutetium metal or
metal powder is very reactive in the presence of oxygen, and would
require special safety measures (i.e., handling under inert gas
atmosphere), in order for Lutetium in pure metallic form to be used
in producing check sources.
[0092] One embodiment herein includes increasing the density of
Lu-oxide or other Lutetium salts from normal density (of as
purchased powder) of about 1.5 grams/cubic centimeter up to the
theoretical maximum of 9.4 grams/cubic centimeter (for
Lu.sub.2O.sub.3) by applying high pressure via device such as a
compaction tool. This more dense form of Lutetium-oxide (when so
compressed) can then be used to form radioactive calibration source
as discussed above but with a lesser volume of material to achieve
the same result. In addition to or in lieu of utilizing a low-Z
material to encase the Lu compound and thus prevent damage to the
source, compressed and/or non-compressed Lu-oxide (powder) can be
mixed with liquid epoxy, which eventually sets into a desired form
as discussed herein.
[0093] FIG. 4 is a hypothetical example illustrating the difficulty
of detecting of fading of a radiation detector using a conventional
K-40 calibration source. In general, the graphs (e.g., graph 401
and graph 402) show the measured distribution of detected events N
as a function of the gamma energy for a detector (e.g., a large
area plastic scintillator which is used in an airport gate monitor)
with limited energy resolution.
[0094] More particularly, graph 401 shows (as an example) a
distribution of counts (e.g., count region 20) based on exposure of
a radiation detection device to a known natural gamma radiation
source K-40 with a gamma energy of 1460 kilo-electron Volts. This
peak is far above an energy threshold 410, so that the count rate
of all events above this threshold 410 is very insensitive to the
amplification of the detector. Note that in graph 402, where the
amplification of the detector (as a result of drifting) is
significantly reduced, all events are still above the threshold
410, and hence the fading condition of the detector cannot be
detected by measuring the total count rate. Fading of more than
5-10% is enough reason to apply a corrective action such as
recalibrate the radiation detection device 102 in accordance with
techniques discussed above.
[0095] FIG. 5 is a hypothetical example illustrating the detection
of fading of a detector below a respective threshold value based on
use of a Lutetium compound as a calibration source. Graph 501
illustrates calibration of a radiation detection device at 200
kilo-electron Volts and 300 kilo-electron Volts gamma energies
based on Lutetium as a calibration source. Note that nearly all
events fall into the region above the energy threshold and
contribute to the detector count rate represented by region 22.
[0096] Graph 502 illustrates fading of the radiation detector. In
this example, the radiation detection device continues to measure a
Lutetium-based calibration source, and graph 502 illustrates that a
significant part of all events falls into region 24 below the
energy threshold 510. A detected portion of counts above threshold
510 appears in region 23. The counts in region 24 are not useful
for measurement purposes because they fall below threshold 510.
However, a comparison of the counts in region 23 with the counts in
region 22, i.e., the counts taken by a previous measurement under
the same test conditions (i.e., measuring time and position of the
Lutetium check source with respect to the detector), permits the
fading of the detector amplification to become immediately apparent
even to the unskilled user.
[0097] FIG. 6 is a diagram illustrating a hypothetical example of
applying permanent gain controlling to a radiation detection device
for higher resolution detection.
[0098] In graph 601, upon exposure to a Lutetium calibration
source, a radiation detection device is calibrated (e.g., via
amplification adjustments and/or electronic tuning of the voltage
thresholds) so that region 32 is centered (e.g., in a respective
energy window) around the 200 kilo-electron Volts energy level
produced by the Lutetium calibration source. Additionally, region
33 is centered (e.g., in an energy window) around the 300
kilo-electron Volts energy level produced by the Lutetium
calibration source.
[0099] In graph 602, detector amplification is slightly reduced so
that the counts in region 34 and region 37 fall outside window
regions centered around 200 kilo-electron Volts and 300
kilo-electron Volts respectively. Region 35 and region 38 display
counts falling in a peak window region. Region 36 indicates counts
falling far outside of a center window (e.g., 300 kilo-electron
Volts) as a result of drifting or fading. To correct for this drift
or fade, the amplification of the radiation detection device needs
to be increased by adjusting an amplification level of amplifier
125. In one embodiment, this involves tuning attributes of the
photodetector 115 (e.g., a photomultiplier) or increasing the
electronic amplification of amplifier 125.
[0100] In graph 603, detector amplification is higher than desired,
and a significant number of events 40 and 42 fall into the right
reference windows at higher energy levels than 200 kilo-electron
Volts and 300 kilo-electron Volts respectively. In this case the
amplification of the detector needs to be decreased (e.g. by tuning
the high voltage of the photomultiplier of a scintillation detector
or by decreasing the electronic amplification of an amplifier
circuitry).
[0101] Compared to the known technology of using single line gamma
sources (Cs-137 or K-40), the stabilization via the spectral
energies produced by Lutetium is more dependable (during exposure
to background radiation) since the expected ratio of the counts 32
and 33 is known and comparison with measured ratios enables an
additional verification of the proper amplification settings.
[0102] Also, via calibration of a radiation detection device 102 to
90, 200, and/or 300 kilo-electron Volts using a Lutetium
calibration source as discussed above, a radiation detection device
102 can be (periodically, occasionally, repeatedly, etc.)
calibrated to correct for "fading" or "drifting" out of
specification due to environmental factors such as temperature
changes. In other words, detector 115 can be a scintillator device
having characteristics that vary based on temperature or other
factors. In one embodiment, a radiation detection device includes a
common inorganic scintillation detector such as one made from
NaI(Tl) as previously discussed. Such a detector can exhibit
significant drift effects as a result of temperature changes,
aging, and exposure to elements. Adjustments to amplification
enable the radiation detection device 102 to be precisely
calibrated for detecting energy profiles of unknown radiation
sources. In other words, after calibration to provide correct gain
of amplifier 125 (FIG. 1) and/or drifting effects associated with
detector 115 as a result of temperature changes, the radiation
detection device 102 can be used to more accurately measure
"signatures" (e.g., energy patterns) of unknown materials for
purposes of material identification.
[0103] FIG. 7 is a diagram of a radioactive calibration source 110
in the form of a disk 640 according to embodiments herein. As
shown, the radioactive calibration source 110 includes a disk 640
of Lutetium compound formed, for example, using techniques
discussed above such as application of pressure and/or fusing of
powdered Lutetium compound into a coherent or unified mass (via a
bonding material or heating), etc. Disk 640 is encased with a
combination of protective coating 620 and protective coating 610,
each of which are made from a low Z material (e.g., plastic,
aluminum, etc.) as discussed above. Thus, even if the disk 640 of
Lutetium compound were not formed into a unified compressed mass,
the Lutetium compound (e.g., a powdered form) would be protected
from damage by such an encasing. Stem 630 (e.g., bracket) enables
the radioactive calibration source 110 to be mounted or secured to
radiation detection device 102.
[0104] FIG. 8 is a diagram of a radioactive calibration source 110
in the form of a rod 720 according to embodiments herein. As shown,
the radioactive calibration source 110 includes rod 720 of Lutetium
compound formed as discussed above based on application of pressure
and/or fusing of powdered Lutetium compound into a coherent or
unified mass (via a bonding material or heating). Rod 720 is
encased with a combination of protective coating 710 and protective
coating 730 made from a low Z material (e.g., plastic, aluminum,
etc.) as discussed above. Thus, even if the rod 720 of Lutetium
compound were not formed into a unified mass, the Lutetium compound
(e.g., not converted from a powdered form) would be protected from
damage by such an encasing.
[0105] FIG. 9 is a diagram of a radioactive calibration source 110
in the form of a ring 810 according to embodiments herein. As
shown, the radioactive calibration source 110 is formed into a ring
810 of Lutetium compound. As previously discussed, the ring 810 can
be formed based on application of pressure and/or fusing of
powdered Lutetium compound into a coherent or unified mass (via a
bonding material or heating). Similar to other embodiments as
discussed herein, ring 810 can be coated or encased with low Z
material.
[0106] FIG. 10 is a diagram of a cross section of a radioactive
calibration source 110 in the form of a shroud 940 according to
embodiments herein. The shroud 940 of Lutetium compound is encased
with protective covering 910 and protective covering 920 to protect
against damage. As shown, the radioactive calibration source 110
including shroud 940, protective covering 910, and protective
covering 920, at least partially encases detector 115 of radiation
detection device 102 (additional details shown in FIG. 1).
[0107] FIG. 11 is a diagram of a radioactive calibration source 110
in the form of a container 1010 filled with pellets of (compressed
or uncompressed) Lutetium compound according to embodiments herein.
For example, pellets 1005 (or tablets, cubes, balls, etc.) at least
partially made from Lutetium compound based on techniques as
discussed above fill container 1010. Container 1010 can be made
from a low Z material so that radiation generated by pellets passes
through the container to a radiation detection device 102 for
calibrating the radiation detection device 102.
[0108] While this invention has been particularly shown and
described with reference to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
spirit and scope of the invention as defined by the appended
claims.
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