U.S. patent application number 09/931545 was filed with the patent office on 2001-12-27 for thin layer nuclear density gauge.
This patent application is currently assigned to Troxler Electronic Laboratories, Inc.. Invention is credited to Dep, Wewage H. L., Eagan, John T., Jordan, Alfred W., Troxler, Robert E..
Application Number | 20010055363 09/931545 |
Document ID | / |
Family ID | 22404185 |
Filed Date | 2001-12-27 |
United States Patent
Application |
20010055363 |
Kind Code |
A1 |
Troxler, Robert E. ; et
al. |
December 27, 2001 |
Thin layer nuclear density gauge
Abstract
The present invention provides an improved thin layer nuclear
density gauge comprising a gauge housing having a vertical cavity
therethrough and a base, a first radiation detector located at a
first position within said housing and adjacent to said base of
said housing, a second radiation detector located at a second
position within said housing and adjacent to said base of said
housing, a vertically moveable source rod extending into said
cavity of said gauge housing, a radiation source operatively
positioned within a distal end of said source rod, at least one
bearing operatively positioned to guide said source rod within said
cavity, and means for vertically extending and retracting said
source rod to a plurality of predetermined source rod positions so
as to change the spatial relationship between said radiation source
and said first and second radiation detectors. The source rod has a
maximum radial movement of less than about 0.003 inch at each
predetermined position. The present invention also provides a gauge
with an improved radiation shield assembly comprising a sliding
block operatively positioned to move laterally between said first
position and said second position, a spring engaging said sliding
block and biasing said sliding block into said first position, and
a fixed block, said fixed block including a track engaging said
sliding block and guiding movement of said sliding block.
Inventors: |
Troxler, Robert E.;
(Raleigh, NC) ; Dep, Wewage H. L.; (Chapel Hill,
NC) ; Eagan, John T.; (Cary, NC) ; Jordan,
Alfred W.; (Raleigh, NC) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA
101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
Troxler Electronic Laboratories,
Inc.
|
Family ID: |
22404185 |
Appl. No.: |
09/931545 |
Filed: |
August 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09931545 |
Aug 16, 2001 |
|
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|
09518397 |
Mar 2, 2000 |
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6310936 |
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60122694 |
Mar 3, 1999 |
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Current U.S.
Class: |
378/55 ;
378/54 |
Current CPC
Class: |
G01N 23/203 20130101;
G01B 15/02 20130101; G01N 23/06 20130101 |
Class at
Publication: |
378/55 ;
378/54 |
International
Class: |
G01B 015/02; G01N
023/06 |
Claims
That which is claimed:
1. A nuclear gauge suitable for measuring the density of a thin
layer of material overlying a base material, comprising: a gauge
housing having a vertical cavity therethrough and a base; a first
radiation detector located at a first position within said housing
and adjacent to said base of said housing; a second radiation
detector located at a second position within said housing and
adjacent to said base of said housing; a vertically moveable source
rod extending into said cavity of said gauge housing; a radiation
source operatively positioned within a distal end of said source
rod; at least one bearing operatively positioned to guide said
source rod within said cavity; and means for vertically extending
and retracting said source rod to a plurality of predetermined
source rod positions so as to change the spatial relationship
between said radiation source and said first and second radiation
detectors; wherein said source rod has a maximum radial movement of
less than about 0.003 inch at each predetermined position.
2. A nuclear gauge according to claim 1, wherein said source rod
has a maximum vertical movement of less than about 0.003 inch at
each predetermined position.
3. A nuclear gauge according to claim 2, wherein said source rod
has a maximum vertical movement of less than about 0.002 inch at
each predetermined position.
4. A nuclear gauge according to claim 1, wherein said source rod
has a maximum radial movement of less than about 0.002 inch at each
predetermined position.
5. A nuclear gauge according to claim 1, wherein said means for
vertically extending and retracting said source rod comprises an
index rod operatively positioned adjacent to said source rod, said
index rod including a plurality of notches, each of said notches
corresponding to one of said predetermined source rod
positions.
6. A nuclear gauge according to claim 5, wherein said means for
vertically extending and retracting said source rod further
comprises a handle affixed to said source rod, said handle
including a cavity therethrough and an indexer, said index rod
extending into said cavity of said handle, said indexer operatively
positioned for engaging said notches of said index rod in order to
temporarily affix said source rod in one of said predetermined
positions.
7. A nuclear gauge according to claim 6, further comprising at
least two pins affixing said handle to said source rod.
8. A nuclear gauge according to claim 6, further comprising a
spring operatively positioned to bias said indexer into engagement
with said notches, said spring having a spring rate of at least
about 20 lbs./inch.
9. A nuclear gauge according to claim 5, wherein said index rod has
a substantially cylindrical shape at the position of a notch of
said index rod corresponding to a backscatter position.
10. A nuclear gauge according to claim 1, further comprising a
safety shield coaxially mounted around said vertical cavity of said
gauge housing, said safety shield including a bearing operatively
positioned to guide said source rod through said cavity.
11. A nuclear gauge according to claim 1, further comprising a
radiation shield assembly operatively positioned to move laterally
between two positions, a first position blocking a distal end of
said vertical cavity of said gauge housing such that radiation is
shielded from exiting said cavity and a second position adjacent to
said vertical cavity and allowing vertical movement
therethrough.
12. A nuclear gauge according to claim 11, wherein said radiation
shield assembly comprises: a sliding block operatively positioned
to move laterally between said first position and said second
position; a spring engaging said sliding block and biasing said
sliding block into said first position; and a fixed block, said
fixed block including a track engaging said sliding block and
guiding movement of said sliding block.
13. A nuclear gauge according to claim 12, further comprising a
ball plunger engaging said sliding block and operatively positioned
to prevent vertical movement of said sliding block as said sliding
block moves laterally between said first and said second
position.
14. A nuclear gauge according to claim 13, further comprising a
spring engaging said ball plunger and biasing said ball plunger
towards said sliding block.
15. A nuclear gauge, comprising: a gauge housing having a vertical
cavity therethrough and a base; at least one radiation detector
located within said housing and adjacent to said base of said
housing; a vertically moveable source rod extending into said
cavity of said gauge housing; a radiation source operatively
positioned within a distal end of said source rod; means for
vertically extending and retracting said source rod to a plurality
of predetermined source rod positions so as to change the spatial
relationship between said radiation source and said first and
second radiation detectors; and a radiation shield assembly
operatively positioned to move laterally between two positions, a
first position blocking a distal end of said vertical cavity of
said gauge housing such that radiation is shielded from exiting
said cavity and a second position adjacent to said vertical cavity
and allowing vertical movement therethrough, said radiation shield
assembly comprising a sliding block operatively positioned to move
laterally between said first position and said second position, a
spring engaging said sliding block and biasing said sliding block
into said first position, and a fixed block, said fixed block
including a track engaging said sliding block and guiding movement
of said sliding block.
16. A nuclear gauge according to claim 15, further comprising a
ball plunger engaging said sliding block and operatively positioned
to prevent vertical movement of said sliding block as said sliding
block moves laterally between said first and said second
position.
17. A nuclear gauge according to claim 16, further comprising a
spring engaging said ball plunger and biasing said ball plunger
towards said sliding block.
18. A nuclear gauge according to claim 17, wherein said spring
engaging said ball plunger has a spring rate of at least about 50
lbs./inch.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional No.
60/122,694, filed Mar. 3, 1999.
FIELD OF THE INVENTION
[0002] The present invention relates to an apparatus and method for
determining the density of materials and, more particularly,
relates to an apparatus and method for measuring the density of
thin layers of materials.
BACKGROUND OF THE INVENTION
[0003] Nuclear radiation gauges have been widely used for measuring
the density of soil and asphaltic materials. Such gauges typically
include a source of gamma radiation which directs gamma radiation
into the test material, and a radiation detector located adjacent
to the surface of the test material for detecting radiation
scattered back to the surface. From this detector reading, a
determination of the density of the material can be made.
[0004] These gauges are generally designed to operate either in a
"backscatter" mode or in both a backscatter mode and direct
transmission mode. In gauges capable of direct transmission mode,
the radiation source is vertically moveable from a backscatter
position, where it resides within the gauge housing, to a series of
direct transmission positions, where it is inserted into small
holes or bores in the test specimen.
[0005] Many of the gauges commonly in use for measuring density of
soil, asphalt and other materials are most effective in measuring
densities of materials over depths of approximately 4-6 inches.
However, with the increase in cost of paving materials, the
practice in maintaining and resurfacing paved roadbeds has become
one of applying relatively thin layers or overlays having a
thickness of one to three inches. With layers of such a thickness
range, many density gauges are ineffective for measuring the
density of the overlay because the density reading obtained from
such gauges reflects not only the density of the thin layer, but
also the density of the underlying base material.
[0006] Nuclear gauges capable of measuring the density of thin
layers of materials have been developed by the assignee of the
present invention. For example, thin layer density gauges are
disclosed in U.S. Pat. Nos. 4,525,854, 4,701,868, and 4,641,030,
all of which are assigned to the assignee of the present invention
and are incorporated herein by reference in their entirety. The
gauges disclosed in the above-referenced patents are referred to as
"backscatter" gauges because the radiation source does not move
outside the gauge housing, which is necessary for measurement in
the direct transmission mode.
[0007] As disclosed in the above patents, the preferred method of
measuring the density of thin layers of materials, such as asphalt,
requires two independent density measurement systems. The geometry
of these two measurement systems must be configured with respect to
one another and with respect to the medium being measured in such a
manner that they measure two different volumes of material. The two
different volumes are not mutually exclusive insofar as they
partially overlap one another. Measurement accuracy depends upon a
larger portion of the volume measured by one of the measurement
systems being distributed at a lower depth beneath the gauge than
the volume measured by the other measurement system. This is
accomplished by placing one radiation detection system in closer
spatial proximity to the radiation source than the other detection
system.
[0008] There remains a need in the art for a nuclear gauge capable
of operating in both backscatter mode and direct transmission mode,
and which is suitable for measuring the density of thin layers of
material.
SUMMARY OF THE INVENTION
[0009] The present invention provides a nuclear density gauge
capable of operating in both backscatter and direct transmission
modes and also capable of accurately measuring the density of thin
layers of materials. The nuclear gauge of the present invention
minimizes the effect of variance in radiation source positioning on
the accuracy of the density reading. The source rod of the nuclear
gauge of the present invention has a maximum radial movement of
less than about 0.003 inch at each predetermined source rod
position and, preferably, a maximum radial movement of less than
about 0.002 inch. Additionally, the source rod of the nuclear gauge
of the present invention has a maximum vertical movement of less
than about 0.003 inch at each predetermined source rod position
and, preferably, a maximum vertical movement of less than about
0.002 inch.
[0010] The nuclear gauge of the present invention is suitable for
measuring the density of the thin layer of material overlying a
base material and comprises a gauge housing having a vertical
cavity therethrough and a base. Within said housing, first and
second radiation detectors are located, both detectors being
positioned adjacent to the base of the gauge housing. The two
radiation detectors are in separate positions within the gauge
housing. The gauge further comprises a vertically moveable source
rod extending into the cavity of the gauge housing. The source rod
contains a radiation source within a distal end thereof. The gauge
further comprises at least one bearing operatively positioned to
guide the source rod within the vertical cavity of the gauge
housing. The gauge also includes means for vertically extending and
retracting the source rod to a plurality of predetermined source
rod positions so as to change the spatial relationship between the
radiation source and the two radiation detectors.
[0011] Preferably, the means for vertically extending and
retracting the source rod includes an index rod operatively
positioned adjacent to the source rod. The index rod has a
plurality of notches, each of the notches corresponding to one of
the predetermined source rod positions. The means for extending and
retracting further comprises a handle affixed to the source rod.
The handle includes a cavity therethrough and an indexer. The index
rod extends into the cavity of the handle and the indexer is
operatively positioned for engaging the notches of the index rod in
order to temporarily affix the source rod in one of the
predetermined positions. Preferably, at least two pins are used to
affix the handle to the source rod. A spring is preferably used to
bias the indexer into engagement with the notches of the index rod.
For example, a spring having a spring rate of at least about 20
lbs. per inch may be used. In a preferred embodiment, the index rod
has a substantially cylindrical shape extending from a distal end
of the rod to the position of a notch of the index rod
corresponding to the backscatter position.
[0012] The gauge also comprises a safety shield coaxially mounted
around the vertical cavity of the gauge housing. The safety shield
includes a bearing operatively positioned to guide the source rod
through the vertical cavity.
[0013] Additionally, the nuclear gauge preferably comprises a
radiation shield assembly operatively positioned to move laterally
between two positions; a first position blocking a distal end of
the vertical cavity of the gauge housing such that radiation is
shielded from exiting the cavity and a second position adjacent to
the vertical cavity and allowing vertical movement therethrough. In
a preferred embodiment, the radiation shield assembly comprising a
sliding block operatively positioned to move laterally between the
first position and the second position, a spring engaging the
sliding block and biasing the sliding block into the first
position, and a fixed block. The fixed block preferably includes a
track engaging the sliding block and guiding movement of the
sliding block. Advantageously, a ball plunger engages the sliding
block, and is operatively positioned to prevent vertical movement
of the sliding block at the sliding block moves laterally between
the first and second positions. A spring engages the ball plunger
and biases the ball plunger towards the sliding block. This spring
preferably has a spring rate of at least about 50 lbs. per
inch.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Having thus described the invention in general terms,
reference will now be made to the accompanying drawings, which are
not necessarily drawn to scale, and wherein:
[0015] FIG. 1 is a perspective view of a nuclear gauge according to
the present invention;
[0016] FIG. 2 is a cross-sectional view of a gauge according to the
present invention taken along line 2-2 of FIG. 1;
[0017] FIGS. 3A and 3B illustrate the top and side views of the
fixed block portion of the gauge of the present invention;
[0018] FIGS. 4A and 4B illustrate two side views of the sliding
block portion of the gauge of the present invention; and
[0019] FIGS. 5A and 5B shows two side views of the index rod
portion of the gauge of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The present invention now will be described more fully
hereinafter with reference to the accompanying drawings, in which
preferred embodiments of the invention are shown. This invention
may, however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. Like numbers refer to like
elements throughout.
[0021] The present invention provides a nuclear gauge 10 as shown
in FIG. 1. The gauge 10 is capable of accurately measuring the
density of thin layers of materials, such as asphalt. The gauge 10
can operate in both backscatter and direct transmission modes.
[0022] Operation of the gauge is more clearly illustrated by FIG.
2. As shown in FIG. 2, the gauge 10 of the present invention
includes a vertically moveable source rod 20 containing a radiation
source 22 in a distal end thereof. The radiation source 22 may be
any suitable radiation source, such as .sup.137Cs radiation source.
The source rod 20 extends into a vertical cavity 11 in the gauge
housing 12. Bearings 44 are operatively positioned to guide the
source rod 20 through the cavity 11 in the gauge housing 12.
[0023] As shown, the gauge 10 of the present invention contains two
separate density measurement systems. The gauge 10 contains a first
pair of radiation detectors 16 and a second pair of radiation
detectors 18, wherein the first radiation detectors are located in
closer spatial proximity to the radiation source 22. The radiation
detectors, 16 and 18, may be any type of gamma ray radiation
detector known in the art. Preferably, the radiation detectors, 16
and 18, are Geiger Mueller tubes. The radiation detectors, 16 and
18, are preferably located adjacent to the base 14 of the gauge
housing 12.
[0024] The gauge 10 also includes means for vertically extending
and retracting the source rod 20 to a plurality of predetermined
source rod positions so as to change the spatial relationship
between the radiation source 22 and the radiation detectors, 16 and
18. The predetermined positions may include, for example, a
backscatter position as well as a plurality of direct transmission
positions, wherein the radiation source 22 is positioned below the
base 14 of the gauge housing 12.
[0025] Preferably, the means for extending and retracting comprise
an index rod 24 operatively positioned adjacent to the source rod
20. The index rod 24 includes a plurality of notches 26. Each notch
26 corresponds to a predetermined source rod position. For example,
one notch 30 corresponds to the "safe" position wherein the
radiation source 22 is raised and shielded from the test material.
The safe position is used to determine the standard count. Another
notch 32 corresponds to the backscatter mode wherein the radiation
source 22 is located adjacent to the surface of the test material
underlying the gauge 10. Advantageously, the index rod 24 includes
flat side 42 where a resistive depth strip (not shown) may be
affixed.
[0026] The means for vertically extending and retracting the source
rod 20 also includes a handle 28 affixed to the source rod. The
index rod 24 extends into a cavity 40 in the handle 28. The handle
further comprises an indexer 36 operatively positioned for engaging
the notches 26 of the index rod 24 in order to temporarily affix
the source rod 20 in one of the predetermined positions. The
indexer 36 is biased into engagement with the notches 26.
Preferably, the indexer 36 is biased into engagement by a spring
38. A trigger 34 allows the user to move the indexer 36 into and
out of engagement with the notches 26.
[0027] The gauge 10 also includes a safety shield 46 coaxially
mounted around the vertical cavity 11 and operatively positioned to
minimize the user's exposure to radiation when the radiation source
22 is in the safe position. Preferably, the safety shield 46 is
constructed of lead or tungsten. However, other radiation shielding
material may be used without departing from the present
invention.
[0028] It has been discovered that slight inconsistencies in
radiation source 22 positioning can result in unacceptable levels
of variance in thin layer densities measured by a nuclear gauge.
These inconsistencies can manifest themselves both during the
actual density measurement counts and also during acquisition of
the standard counts. It is believed that gauges having vertically
moveable source rods are sensitive to slight changes in radiation
source position due to the close proximity of the radiation source
22 to the first radiation detector 16. Due to the relatively long
distance or path length between the radiation source 22 and the
second radiation detector 18, variability problems are not normally
associated with the second radiation detector due to a dampening
effect caused by the relatively long distance.
[0029] The nuclear gauge 10 of the present invention minimizes the
variance in nuclear count rate, and the subsequent variance in
measured thin layer density due to this variance, that is
attributable to minor source rod positioning changes. Specifically,
it is desirable, for a four minute thin layer density measurement,
that the first radiation detector 16 count uncertainty due to minor
source rod positioning inconsistencies should contribute no more
than twenty-five percent of the total observed count
uncertainty.
[0030] The gauge 10 of the present invention provides a maximum
radial movement of less than about 0.003 inch, most preferably less
than about 0.002 inch, at any given predetermined source rod 20
position. In other words, the gauge 10 is designed so that the
distal end of the source rod 20 containing the radiation source 22
cannot move in a radial or lateral direction more than about 0.003
inch, most preferably less than about 0.002 inch, from the central
axis of the vertical cavity 11. Additionally, the gauge 10 of the
present invention has a maximum vertical movement of less than
about 0.003 inch, and preferably less than about 0.002 inch, at any
given source rod position. Thus, for each source rod position, the
radiation source position will vary less than about 0.003 inch,
preferably less than about 0.002 inch, from the desired radiation
source depth. The gauge handle 28 may be constructed of any
suitable material, such as aluminum or stainless steel. Preferably,
the gauge handle 28 is constructed of stainless steel. The
stainless steel minimizes distortion of the hole into which the
source rod 20 is pressed. The machined hole of the gauge handle 28
into which the source rod 20 is pressed is preferably sized as
0.6235.+-.0.0005 inches in diameter to ensure a good fit.
[0031] Additionally, to ensure that there is no movement of the
source rod 20 within the hole in the gauge handle 28, at least two
fasteners 64, such as spring pins, are inserted through the handle
28 and source rod 20 to affix the source rod to the handle. Unlike
the use of a single fastener, the use of at least two fasteners
does not provide a pivot point about which a source rod 20 may
move.
[0032] Preferably, the gauge handle 28 has a cavity for the indexer
36 having a diameter of 0.503.+-.0.001 inch. This reduces variance
in the source rod position by encouraging stable indexer 36
positioning and movement. The spring rate for the spring 38 that
biases the indexer 36 towards the notches 26 of the index rod 24 is
at least about twenty pounds per inch, preferably at least about
twenty-two pounds per inch. Thus, when the indexer 36 is engaged in
a notch 26 of the index rod 24, the force pushing on the indexer is
approximately 8.3 lbs.
[0033] As shown in FIG. 5, the notches 26 of the index rod 24
comprise a first side surface roughly perpendicular to the axis of
the index rod and a bottom surface roughly parallel to the axis of
the index rod. Further, the notches 26 include a second side
surface having a sloped portion. This notch configuration, in
conjunction with the relatively high spring rate of spring 38,
allows precise placement of the indexer 36 in the notches 26. If
placed adjacent to the sloped portion of a notch 26, the
spring-loaded indexer 36 will slide into abutting contact with both
the bottom surface of the notch and the first side surface, thereby
ensuring consistent indexing of the source rod 20.
[0034] Preferably, the trigger 34 has very little clearance in the
oblong hole in the handle 28 from which it protrudes. Specifically,
the trigger 34 has a diameter of 0.496.+-.0.002 inch and moves
laterally within an oblong slot having a width of 0.500.+-.0.002
inch. This result in a clearance range of 0.002 to 0.008 inch. The
small resulting clearance of the trigger 34 within the oblong slot
prevents rotation of the trigger 34 within the slot that can cause
a poor fit of the indexer 36 in the notches 26.
[0035] Preferably, there is also very little clearance between the
index rod 24 and the hole within the handle 28 through which it
passes. The index rod 24 has a diameter of 0.625.+-.0.001 inch and
passes through a hole in the handle 28 having a diameter of
0.6270.+-.0.0005 inch, resulting in a diametrical clearance range
of only 0.0005 to 0.0035 inch. Note that the nominal diametrical
clearance of 0.0020 inch (0.0005.+-.0.0035 divided by 2) is
nominally 0.0010 inch radial clearance from true center.
[0036] The flat side 42 of the index rod 24 prevents the index rod
hole in the handle 28 through which the index rod passes from
closely following the index rod. This lack of concentric fit can be
a significant source of positioning variability, especially in the
backscatter position 32 and standard count position 30. In order to
eliminate this source of error, the index rod 24 has a full
diameter extending down at least through the position of the
backscatter position notch 32. Thus, the full diameter of the index
rod 24 extends through the backscatter position and provides a
better fit of the index rod to the handle 28. The index rod 24 is
shown in greater detail in FIGS. 5A and 5B.
[0037] The safe position corresponding to the notch 30 is
preferably located at least about 2.20 inches above the outer
surface of the base 14 of the gauge housing 12. This places the
radiation source 22 in a position that exhibits reduced sensitivity
of the standard count to slight radiation source positioning
variability in the vertical direction. Specifically, the radiation
standard count rate with the radiation source 22 in the safe
position changes only about 2.8 counts per mil of radiation source
position change in the vertical direction in the gauge 10 of the
present invention.
[0038] The bearings 44 that guide the source rod 20 through cavity
11 in the gauge housing 12 preferably provide an extremely close
fit to the source rod in order to minimize variability in radiation
source positioning. Specifically, the outer diameter of bearings 44
is preferably 1.1265.+-.0.0005/-0.000 inch and the bearing inner
diameter is preferably 0.6265.+-.0.00005/-0.000 inch. Additionally,
the bearing housing diameter is preferably 1.1265.+-.0.0005 inch.
The source rod 20 diameter is preferably 0.625.+-.0.001. This
results in a nominal bearing clearance of 0.00025 inch and a
bearing clearance range of press-fit to 0.001 inch. The nominal
source rod 20 clearance is 0.00175 inch and the source rod
clearance range is from 0.0005 to 0.0030 inch. Thus, the source rod
20 has a total range of radial movement of no more than about
0.0005 to about 0.0040. Since the desired position of the source
rod 20 is on the true centerline of the bearings 44, the movement
away from true center is actually the radial clearance, which
equals one-half of the diametrical clearance. Thus, the maximum
movement away from true center of the source rod 20 is one-half of
0.0040 inch or 0.0020 inch.
[0039] As discussed above, the gauge 10 of the present invention
preferably includes a safety shield 46. The backscatter position is
a position that is particularly sensitive to variations in
radiation source positioning. To minimize radial movement of the
distal end of the source rod 20, the safety shield 46 preferably
includes a bearing 48. The bearing 48 is press-fit into the safety
shield 46 and has a through diameter of 0.6265.+-.0.0005 inch. This
provides a maximum radial clearance of 0.0015 inch. Thus, there is
a maximum of 0.0015 inch radial movement from true centerline by
the distal end of the source rod 20 in the backscatter
position.
[0040] The nuclear gauge 10 advantageously includes a radiation
shield assembly that is operatively positioned to move laterally
between two positions, a first position blocking a distal end of
the vertical cavity 11 of the gauge housing 12 such that radiation
is shielded from exiting the cavity and a second position adjacent
to the vertical cavity and allowing vertical movement therethrough
by the source rod 20. The radiation shield assembly include a
sliding block 50 operatively positioned to move laterally between
the first position and the second position. The sliding block 50 is
shown in greater detail in FIGS. 4A and 4B. As shown, the sliding
block 50 preferably includes a chamfer 66 that lessens the effect
of radiation source positioning on the count rate measured by the
first radiation detector 16 in backscatter mode. A spring 54
engages the sliding block 50 and biases the sliding block into the
first position where it blocks the vertical cavity 11 within the
gauge housing 12. The spring guide 62 guides one end of the spring
54 while the other end of the spring is engaged with the sliding
block 50.
[0041] The radiation shield assembly further comprises a fixed
block 52. The fixed block 52 is located adjacent to the gauge base
14 and is shown in greater detail in FIGS. 3A and 3B. As shown, the
fixed block 52 includes a track 60 which engages the sliding block
50 and guides movement of the sliding block as it moves laterally
between the first position to the second position. The fixed block
52 shields the nearest radiation detector 16 from internal gamma
rays streaming inside the gauge housing 12 when the source rod 20
is in the backscatter position. The track 60 restricts the
side-to-side movement of the sliding block 50 such that the sliding
block follows a more stable path between the first position and the
second position. Preferably, the sliding block 50 and the fixed
block 52 are constructed of lead or tungsten, but other suitable
radiation shielding materials may be used.
[0042] Referring back to FIG. 2, the gauge 10 preferably comprises
a ball plunger 56 engaging the sliding block 50 and operatively
positioned to prevent vertical movement of the sliding block as the
sliding block moves laterally between the first and second
positions. Preferably, the ball plunger 56 is biased towards the
sliding block 50 by a spring 58. Since there is a slight gap
between the top of the sliding block 50 and the top wall of the
cavity creating the housing for the radiation shield assembly, the
sliding block has room to move in the vertical direction. The ball
plunger 56 prevents the sliding block from rocking upward and
downward as it moves between the first and second position,
particularly as it retracts when the distal end of the source rod
20 engages the sliding block and forces the sliding block to move
into the second position. The ball plunger 56 includes a ball,
preferably constructed of steel and having a {fraction (3/16)}"
diameter, that provides a point contact on the top surface of the
sliding block 50. The spring 58 preferably has a spring rate of at
least 50 lbs. per inch such that it pushes the ball plunger 56
downward with a force of about 11 to about 12 lbs. The downward
force on the top of the sliding block 50 eliminates any rocking
motion and forces the sliding block to move in a horizontal
plane.
[0043] Many modifications and other embodiments of the invention
will come to mind to one skilled in the art to which this invention
pertains having the benefit of the teachings presented in the
foregoing descriptions and the associated drawings. Therefore, it
is to be understood that the invention is not to be limited to the
specific embodiments disclosed and that modifications and other
embodiments are intended to be included within the scope of the
appended claims. Although specific terms are employed herein, they
are used in a generic and descriptive sense only and not for
purposes of limitation.
* * * * *