Method and apparatus for measuring the condition of degradable components

An apparatus and method for the in-situ measurement of the condition of degradable components such as electrical cable insulation, valve internals, and gaskets. A stream of energetic subatomic particles is directed to the component under test, thereby inducing the emission of secondary gamma radiation which is detected by one or more radiation detectors positioned in proximity to the component. The secondary radiation emission spectrum is recorded and analyzed to identify features and/or changes resulting from the application of one or more stressors to the component. In the specific case of aging, the radiation spectra taken from the same component at different intervals during its lifetime are compared to identify changes in the component which then may be correlated with artificially (or naturally) aged specimens to estimate the relative level of aging of the component.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the field of material aging research and 
management, specifically to the in-situ monitoring and estimation of the 
condition of various degradable components used in a wide variety of 
applications (including, inter alia, electrical cable, process system 
valves, aircraft, spacecraft, and automobiles) via neutron activation 
techniques. 
2. Description of Related Technology 
The aging of degradable components (particularly those constructed in whole 
or in part of organic compounds such as polymers) is of great importance 
to modem society. Such degradable components comprise a significant 
fraction of what may be termed as "critical" components in use in many 
industrial, aerospace, and automotive applications, both commercial and 
military. Included in this category are components such as electrical 
cable insulation, valve internals, bushings, seals, and gaskets. 
Degradation and ultimate failure of these so-called critical components is 
of paramount importance in that such failures may result in the 
unanticipated maintenance costs, loss of operational capability and 
availability, and even loss of human life. 
Several different approaches to managing the aging of such components 
exist. One approach involves 1) subjecting laboratory or in-situ specimens 
of a given component to a progressive regimen of aging stressors such as 
heat, radiation, electrical potential, chemicals, and/or oxygen present in 
the anticipated operating environment (known generally as "artificial 
aging"); 2) identifying a critical parameter of the component's function 
in the desired application (such as dielectric strength for an insulator); 
3) determining a maximum or minimum acceptable value for the chosen 
parameter; 4) correlating the maximum or minimum acceptable value to a 
given installed lifetime (for example, via aging models such as the 
Arrhenius equation); and 5) removing the component from service when the 
installed lifetime is reached. Note, however, that this approach has the 
distinct disadvantage of not directly monitoring the condition of a given 
component, thereby introducing potentially significant variations in 
component condition across various applications. Specifically, some 
applications may have aged more or less than expected (due to a variety of 
factors such as radiant heat or radiation shielding, variations in 
oxygen/inert gas concentration, aging prior to installation, inaccuracies 
in the aging model used, etc.), and hence are being replaced either 
prematurely or too late. More effective condition monitoring programs will 
utilize a similar approach as that outlined above, yet instead of rotely 
replacing a component at a given point in life, will monitor the 
degradation of the component as a function of time to determine it's rate 
of aging as compared to the artificially (or naturally) aged specimen. The 
primary drawbacks of these latter condition monitoring programs include 
the costs of monitoring, component inaccessibility, and component/device 
downtime. For example, the condition monitoring of a fluoropolymer valve 
seat requires either remote inspection or disassembly of the valve, 
thereby removing the valve from service for a period of time. In such 
cases, simple periodic replacement of the component during other scheduled 
maintenance may be more cost effective. In some instances (such as 
electrical cable, described further below), no periodic maintenance or 
replacement is ever scheduled; hence condition monitoring of some sort is 
almost a necessity. The enormity of cost associated with replacement of 
cable in, for example, a commercial nuclear power facility, underscores 
the need for effective aging assessment and monitoring techniques. 
Electrical Cable 
As previously indicated, the aging and unanticipated failure of power, 
control, instrumentation, and data transmission cable may have significant 
adverse effects on plant operation and maintenance (O&M) costs and 
downtime. Electrical and optical cables have traditionally been considered 
long-lived components which merit little in the way of preventive 
maintenance or condition monitoring due to their generally high level of 
reliability and simplicity of construction. Like all other components, 
however, such cables age as the result of operational and environmental 
stressors. Aging effects may be spatially generalized (i.e., affecting 
most or all portions of a given cable equally, such as for a cable located 
completely within a single room of uniform temperature), or localized 
(i.e., affecting only very limited portions of a cable, such as in the 
case of a cable routed near a highly localized heat source). The severity 
of these aging effects depends on several factors including the severity 
of the stressor, the materials of construction and design of the cable, 
and the ambient environment surrounding the cable. Detailed discussions of 
electrical cable aging may be found in a number of publications including 
SAND96-0344 "Aging Management Guideline for Commercial Nuclear Power 
Plants--Electrical Cable and Terminations" prepared by Sandia National 
Laboratories/U.S. Department of Energy, September 1996. Discussions 
regarding optical cable aging may be found, inter alia, in Electric Power 
Research Institute (EPRI) publications and telecommunications industry 
literature. The following description will be limited to electrical cable, 
although it can be appreciated that the principles of aging and analysis 
described herein may also be largely applicable to optical cabling as well 
as many other types of polymeric components. 
Electrical cables come in a wide variety of voltage ranges and 
configurations, depending on their anticipated uses. Existing prior art 
low- and medium-voltage power and control cables such as that shown in 
FIGS. 1a-1d are typically constructed using a polymer or rubber dielectric 
insulation 200 which is applied over a multi-strand copper or aluminum 
conductor 202. The insulation is often overlaid with a protective polymer 
jacket 204. In multi-conductor cables (such as those used in three-phase 
alternating current systems, as shown in FIGS. 1a and 1b), a plurality of 
these individually insulated conductors are encased within a protective 
outer jacket 206 along with other components such as filler 208 and drain 
wires (not shown). These other components fulfill a variety of functions 
including imparting mechanical stability and rigidity to the cable, 
shielding against electromagnetic interference, and allowing for the 
dissipation of accumulated electrostatic charge. This general arrangement 
is used for its relatively low cost, ease of handling and installation, 
comparatively small physical dimensions, and protection against 
environmental stressors. 
Current methods of evaluating electrical cable component aging generally 
may be categorized as electrical, physical, and microphysical. Electrical 
techniques involve the measurement of one or more electrical parameters 
relating to the operation of the cable, such as the breakdown voltage, 
power factor, capacitance, or electrical resistance of the dielectric. 
These methods have to the present been considered largely ineffective or 
impractical, in that they either do not show a good correlation between 
the parameter being measured and the aging of the dielectric, or are 
difficult to implement under normal operations. Furthermore, such 
techniques are often deleterious to the longevity of the insulation, and 
have difficulty determining localized aging within a given conductor. 
Physical techniques including the measurement of compressive modulus, 
torsional modulus, or rigidity under bending often show a better 
correlation between the aging of the cable and the measured parameter 
(especially for low-voltage cable), and are more practical to apply during 
operational conditions. However, they generally suffer from a lack of 
access to the most critical elements of the cable, the individual 
electrical conductors and their insulation. For example, the measurement 
of compressive modulus by way of instruments such as the Indenter Polymer 
Aging Monitor are effective primarily with respect to the outer, 
accessible surface of the cable such as its outer jacket. Although 
correlations of the aging of the outer jacket to that of the underlying 
conductors have been attempted, these correlations are generally quite 
imprecise and are subject to a large degree of variability based on the 
specific configuration of the cable being tested (i.e., its materials of 
construction, insulation/jacket thickness, etc.), the presence of 
ohmically induced heating, shielding of the conductors against stressors 
by the outer jacket, and differences in the oxygen concentration at the 
conductor insulation versus that at the outer jacket. See EPRI TR-104075, 
"Evaluation of Cable Polymer Aging Through Indenter Testing of In-Plant 
and Laboratory Aged Specimens," prepared by the Electric Power Research 
Institute, January, 1996 for a discussion of the correlation between outer 
jacket and conductor physical measurements. 
Other physical techniques such as the measurement of the tensile strength 
or elongation-at-break of the insulation material are inherently 
destructive and require a specimen of the aged cable for testing. 
Another potential drawback to many of the physical techniques described 
above is disturbance of the bulk cable run during testing. In some 
applications, the dielectric of the cable being evaluated may be highly 
aged and embrittled, yet still completely functional. However, substantial 
movement of the cable (such as picking the cable up and clamping on a test 
device) may produce localized elongation stresses beyond those 
corresponding to the elongation-at-break for the insulation and/or jacket 
material, thereby inducing unwanted cracking of the insulation and/or 
jacketing and potential electrical failure. 
Microphysical techniques such as the measurement of insulation oxidation 
induction time (OIT), density, gel or plasticizer content, infrared 
absorption spectroscopy, UV spectroscopy, and NMR are generally quite 
accurate, yet require samples of the cable insulation and/or jacket for 
analysis. For jacketed conductors, such samples are generally only 
available at the ends of the cable where the conductors are terminated to 
a source or load, and not anywhere between. Furthermore, as with the 
physical techniques described above, the results of any such testing are 
necessarily applicable only to the localized area of the cable from which 
the specimen was taken, which may or may not be representative of the rest 
of the cable. Hence, one can either take a small sample of material from 
the outer jacket of the cable and attempt to extrapolate the results of 
the aging analysis to the underlying conductors, or alternatively take a 
sample at the ends of the conductor itself near its terminations and 
extrapolate these results to the rest of the unexposed conductor. Under 
either alternative, a substantial degree of uncertainty and imprecision 
exists. Plant operators are also generally reticent to allowing the 
removal of even small samples of material from their cables, especially in 
applications where plant safety and continuity of electrical power are 
critical. 
Another common problem in applying either physical or microphysical 
techniques to a localized portion of cable is the existence of conduit. In 
the typical power or industrial plant, many miles of cable may be encased 
within metallic or plastic conduit, thereby rendering it all but 
inaccessible. While it is true that such conduit also affords the cable 
additional protection from most stressors (such as heat and radiation), it 
also may preclude any effective estimation of aging using existing 
techniques. For example, the aging of a portion of nuclear plant 
safety-related cable contained in a conduit running directly over a large 
radiant heat source may be for all intents and purposes immeasurable 
during it's installed lifetime. While the remainder of the cable not in 
direct proximity to the heat source may be largely unaffected, the 
insulation of the cable in the region directly adjacent to the heat source 
may undergo dramatically accelerated aging and ultimately failure well in 
advance of the rest of the cable. 
Fast Neutron Activation 
The technique of fast neutron activation (FNA) is well known in the nuclear 
arts. Generally speaking, this technique employs a stream of energetic 
(fast) neutrons to induce secondary gamma ray emission from a target 
object via inelastic scattering with nuclei in the target. The gamma ray 
spectrum associated with a given element is unique and identifiable given 
sufficient energy resolution. Heretofore, FNA systems have been used 
exclusively in the detection and identification analysis of organic 
materials in obstructed locations (such as in contraband detection or bore 
hole exploration; see for example U.S. Pat. No. 5,098,640, "Apparatus and 
Method for Detecting Contraband using Fast Neutron Activation"). Such 
techniques, however, have not been applied to the in-situ analysis of 
changes in the atomic structure of a material resulting from the 
application of stressors (such as heat, nuclear radiation, oxygen/ozone, 
etc.). Furthermore, existing neutron scanning and detection systems 
necessarily utilize very high neutron fluxes (&gt;1E10 n/s-4 .pi.) in order 
to minimize analysis time. Such systems can induce significant damage to 
both inorganic (such as metals) and organic materials. While neutron 
radiation primarily results in atomic displacement effects (which are 
highly detrimental to inorganics), it also induces a substantial degree of 
ionization within organic materials. 
Based on the foregoing, it would be most desirable to provide an apparatus 
and method which allows an operator to more accurately assess the aging an 
in-situ degradable component in a substantially non-destructive manner and 
without requiring direct access to the component. Such apparatus and 
method could, for example, be used to estimate the aging of an electrical 
cable within a metallic conduit, or similarly to estimate the aging of a 
valve internal component while still installed within its host valve. 
SUMMARY OF THE INVENTION 
The present invention satisfies the aforementioned needs by providing an 
improved apparatus and method for the in-situ estimation of polymeric 
component degradation and aging. 
In a first aspect of the invention, a collimated stream of energetic 
("fast") neutrons generated by a neutron source is used to bombard the 
subject in-situ degradable component in order to induce inelastic 
scattering with various constituent atoms in the materials of 
construction. Such inelastic scattering results in the production of gamma 
rays of varying energy (the energy being dependent in part on the identity 
of the scattering atom). One or more gamma ray detectors are placed in 
proximity to the irradiated component to measure the resulting gamma ray 
spectra during bombardment. Since the relative concentrations of various 
constituent atoms within certain component material(s) change as a 
function of aging, the gamma emission spectra from the component will also 
change with aging. Scattering resulting from neutron interaction with 
metal atoms or other essentially aging-independent materials (such as 
those in the conductor, shield, or conduit of an electrical cable, for 
example) will remain effectively constant, and therefore is easily 
differentiable from scattering associated with age-variant atomic 
concentrations such as plasticizers or fire retardants present in the 
polymers. 
In a second aspect of the invention, an improved method for estimating the 
aging of a degradable component using the previously described apparatus 
is disclosed. Gamma emission spectra of an in-situ test component are 
taken at various times during its installed lifetime, and compared to each 
other as well as other spectra obtained from laboratoryaged specimens of 
similar components. In one embodiment, the analog gamma emission spectrum 
is converted to a digital representation using an analog-to-digital 
converter (ADC), electronically filtered, and then subtracted from prior 
spectra to generate "difference" spectra for the component under test. 
Such difference spectra are compared to those derived from known aged 
specimens, and may further be analyzed and compiled to generate a 
statistical model of aging within a given type of component. In this 
fashion, the relative level of aging of the in-situ component can be 
reliably estimated at any given point during its lifetime.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference is now made to the drawings wherein like numerals refer to like 
parts throughout. 
While the following description is made primarily with reference to 
electrical cable, it can be appreciated that many of the aspects of the 
present invention may generally be adapted to use on other types of 
components and devices including, without limitation, optical cable, valve 
internals, and automotive or aircraft engine components such as gaskets or 
seals. Furthermore, analyzed components need not necessarily be polymeric 
in composition, but rather may be comprised of any material whose gamma 
emission spectrum varies measurably as a function of the aging of (or 
other stressors applied to) the material. Finally, while the use of 
neutrons and gamma rays are described in detail, the use of other forms of 
radiation (both incident and secondary) for component degradation 
evaluation is contemplated by the invention. 
FIG. 2 shows a first embodiment of the improved condition monitoring 
apparatus 10 of the present invention. A neutron source 12 is positioned 
generally in relative proximity to the subject 11 being evaluated, in the 
present case an electrical cable installed within a conduit (see FIG. 1). 
The source 12 may be of any readily available type which produces 
sufficiently energetic (i.e., typically between 5 and 15 MeV) neutrons in 
quantities necessary to generate the desired net neutron flux 13 after 
collimation (described further below). A Kaman Nuclear particle generator 
utilizing a deuteron/tritium beam having 14 MeV neutrons is used as the 
source 12 in this embodiment, although it can be appreciated that other 
types of sources (such as those employing a deuterium/deuterium, 
deuterium/beryllium, or hydrogen/lithium beam) may be used with equal 
success. Furthermore, although the neutron source 12 of the present 
embodiment is continuous in nature, pulsed sources may also be employed, 
depending on the needs of the intended application. Pulsed fast neutron 
sources and systems are well known and understood in the art; see for 
example, U.S. Pat. No. 5,076,993, "Contraband Detection System Using 
Direct Imaging Pulsed Fast Neutrons." 
In the present embodiment, fast neutrons having energies on the order of 14 
MeV are utilized, although neutron energies outside this range may also be 
used depending on the specific application. For example, thermal neutrons 
(&lt;&lt;1 MeV) may be particularly useful for the detection of certain atoms 
such as nitrogen. The present invention contemplates the use of either 
fast or thermal neutrons, or both. Note that within a certain range of 
neutron energies, 1) the production of gamma rays at certain energies is 
significantly enhanced in certain materials, and 2) the energy of gamma 
rays resulting from neutron scattering are essentially constant. A 
practical consequence of varying neutron energy is the change in 
probability of creation of a given gamma event. Neutron flux is also a 
determinant of gamma ray production; higher incident neutron flux will, 
holding all else constant, generally produce a higher gamma flux. 
A desirable characteristic of the neutron source 12 is comparatively small 
size and relative portability to permit in-situ testing of components. 
However, since the scattering cross-section of energetic neutrons in air 
is quite low, the source may be placed somewhat remotely to the test 
subject if desired without appreciable reduction in efficiency. The source 
of the present embodiment is fitted with a collimator 14 on its outlet 
which spatially collimates the neutron beam 13 from the source 12 prior to 
reaching the test subject. The collimator 14 may constructed of any 
material which effectively attenuates energetic neutrons, such as 
polyethylene (formulated with or without additives such as boron) or lead. 
In the present embodiment, polyethylene is chosen due to its comparatively 
light weight, ease of manufacturing, low cost, and high neutron 
absorption/scatteringcross-section. 
As shown in FIG. 2, the collimator 14 reduces the exposure of surrounding 
portions of the test subject to the neutron beam 13, and spatially refines 
the beam such that more precise testing of specific components can be 
performed. For example, as shown in FIG. 2a, the beam may be collimated 
disproportionately in the X and Y planes such that an exposure slit 15 is 
formed. In this fashion, a "slice" of a test subject 11 (electrical cable 
in this case) may be tested. Similar to techniques employed in prior art 
axial tomography, data from several of such slices may be electronically 
fused by the analyzer 60 to provide a spatial representation of the aging 
of the cable. Alternatively, the neutron beam 13 may be collimated to a 
tightly focused beam of essentially circular cross-section to allow 
examination of a very precise area within a structure (such as the stem 
seal of a valve). 
Referring again to the embodiment of FIG. 2, one or more gamma ray 
detectors 16 are mounted on separate articulated arms 17 attached to a 
supporting frame element 18, the latter being which is moveable in 
relation to the source 12 and its pedestal 21. In this fashion, a broad 
range of detector positions relative to the source 12 can be achieved such 
that optimal test efficiency and adaptability can be supported. For 
example, the detector arms 17 may be positioned relative to the test 
subject 11 such that the maximum gamma event rate is achieved for a given 
neutron flux and energy. The frame element 18, detector arms 17, and 
articulated joint(s) 19 shown in FIG. 1 may be constructed in any number 
of well known and understood configurations, and fabricated using any 
suitable material such as steel, aluminum, or polymer. 
The gamma detectors 16 of the present embodiment are neutron shielded high 
purity Germanium crystal detectors of the type well known in the art, the 
theory of operation of which is described in further detail below. Such 
detectors have the important advantage of high spectral resolution 
(typically less than 1%) as compared to other types of gamma detectors. It 
should be noted, however, that any type of gamma ray detector having 
sufficiently high spectral resolution may be used in the present 
invention. 
In addition to the collimator 14, neutron shielding 22 and gamma ray 
shielding 24 is optionally utilized to shield the equipment operator, 
nearby personnel, and equipment from the relatively high neutron and gamma 
radiation fluxes generated by the apparatus 10. The primary object of the 
neutron and gamma shields is to allow testing of components in a typical 
setting or environment (such as a nuclear power plant) which may be 
populated during testing. In this fashion, elaborate precautions 
associated with high dose-rate/high energy radiation sources (such as 
those utilized during radiography) can be largely obviated. It should be 
noted however that while highly desirable, neutron and gamma shielding 22, 
24 are not essential components of the present invention. For example, the 
equipment operator can be located remotely and areas adjacent to the test 
location evacuated as necessary to eliminate any potential hazards to 
personnel resulting from neutron/gamma exposure. 
A second embodiment of the condition monitoring apparatus of the present 
invention is shown in FIG. 3. The neutron shield 22 of this second 
embodiment is a two-piece device having 1) a hollowed cylinder 23 with 
shield extension element 25 attached thereto, the shield element 25 
adapted to the test subject 11 shape (in the present case, a sectioned 
cylinder for use with an electrical cable conduit), and providing 
protection against backscattered or deflected neutrons; and 2) a backstop 
element 26 having a similarly adapted shape. The backstop element 26 is 
mounted to the shield element 25 via a hinge or similar device thereby 
permitting the rapid positioning of the apparatus 10 around the test 
subject 11. The material of construction neutron shielding 22 in the 
present embodiment is again chosen to be polyethylene, although it can be 
appreciated that other types of materials may be used. The shield element 
25 and cylinder 23 also contain optional gamma shielding elements 27 to 
limit exposure of the neutron source 12, operator, and electronics to 
gamma exposure resulting from neutron irradiation. These gamma shield 
elements 27 are constructed of material similar to the main gamma shield 
24 since the attenuation of MeV-energy gammas by polyethylene is generally 
poor. 
As shown in FIG. 3, two gamma detectors 16 are mounted within recesses 32 
within the neutron shield element 25 adjacent to the gamma shield 24. To 
minimize the size and weight of the gamma shield (which is appreciably 
more dense than the neutron shield 22), the gamma shield is placed within 
the neutron shield elements 25, 26, and penetrations 28 are cut into the 
gamma shield 24 to permit the maximum system count rate efficiency. The 
gamma shield 24 is comprised of a plurality of steel or lead interlocking 
components which effectively shield the majority of the solid angle (4 
.pi.) around the test subject 11. The gamma shield 24 also contains a 
neutron beam penetration 29 which allows passage of the neutron beam 13 
from the source 12 to the test subject 11. Gamma "streaming" through the 
detectors and penetrations may be mitigated through the use of a lead 
blanket, if desired, or the detectors 16 each "capped" with a form fit 
element 35 as shown in FIG. 3. It can also be appreciated that while two 
detectors are shown in the present embodiment, any number of detectors may 
be utilized depending on the specific application. 
Note that for both the neutron and gamma shields 22, 24, lateral radiation 
streaming (i.e., out the sides of the shields longitudinally along the 
cable 11) is minimized in part by the cable and conduit (if any). This is 
due largely to the construction of these components: the cable generally 
consists of a metallic conductor(s), with polymer insulation and 
jacketing, while the conduit is often metallic in construction (typically 
either aluminum or steel). Hence, as the neutron and gamma shields 22, 24 
are made to extend laterally from the neutron beam impact point 37 on the 
cable 11, the amount of shielding provided by the cable and conduit is 
increased, since the effective gamma and neutron shielding thickness 
increases for all solid angles. 
FIG. 3a shows a cross-section of the system taken perpendicular to the 
longitudinal axis of the cable 11, illustrating the relative locations of 
the above-described components. 
Gamma Ray Spectral Analyzer 
Referring now to FIG. 4, gamma rays detected by the gamma detectors 16 
described above are processed by the system analyzer 60 in order to 
produce gamma energy spectra useful for the evaluation of component 
condition and aging. Generally, the analyzer consists of multiple detector 
channels having the following major components: 1) a pulse shaper 62; 2) 
an analog-to-digital converter (ADC) 64; 3) a pulse height discriminator 
66; 4) a first stage FIFO buffer 68 and associated buffer manager 70; 6) a 
logic gate 72; 7) a local memory 74; 8) a second stage FIFO buffer 76 and 
associated buffer manager 78 and 9) a personal or laptop computer 80. The 
function and operation of each component is described below. Note that 
while one specific embodiment of analyzer 60 is described herein, any type 
and configuration of electronic signal analyzer which performs the desired 
functions (i.e., gamma ray spectral processing) may be used without 
departing from the spirit of the invention. For example, a conventional 
multi-channel analyzer (MCA) and nuclear scalers may be used with equal 
success. 
As previously described, high purity crystalline scintillation detectors 16 
are used for the detection of gamma rays in the present invention. In a 
scintillation detector, a gamma of a given energy excites crystal to 
produce lower energy quanta (lower frequency electromagnetic radiation). 
These lower energy quanta are subsequently detected by a photomultiplier 
(PM) tube, the output of which is an analog signal representing the 
detected gamma events. Specifically, the output of the PM tube is a series 
of analog pulses, each pulse corresponding to a gamma detection event and 
having an amplitude essentially proportional to the energy of the detected 
gamma. 
Separate detector channels 82 (as opposed to a common or multiplexed 
arrangement) are utilized in the embodiment of FIG. 4 to, inter alia, 
allow single detector processing, increase the system efficiency, and 
allow coincident or near-coincident gamma detection events occurring 
within different detectors 16 to be counted by the circuitry. Note that 
coincidence circuitry for the detector channels 82 is not required in the 
present embodiment, since no neutron/alpha particle or neutron/gamma 
correlation is performed. However, utilization of such techniques (as well 
as neutron time-of-flight) for spatial resolution within the test subject 
11 is contemplated by the present invention. 
Referring again to FIG. 4, the present embodiment utilizes one or more 
pulse shapers 62 to shape the analog pulses received from each detector 16 
as required. Shaping is often necessary to account for ballistic deficit 
and charge trapping, two effects associated with scintillation detectors 
well known in the art. While the specific origins of each of these 
phenomena are documented in a number of publications, their effects are of 
more significance to the present invention since they tend to distort the 
shape, timing, and amplitude of the pulses 61 produced by the 
photomultiplier circuitry in the detectors 16. Specifically, ballistic 
deficit tends to broaden and delay the pulse, whereas charge trapping 
tends to distort the amplitude of the pulse, thereby reducing spectral 
resolution. Many commercially available detectors incorporate pulse 
shaping circuitry to allow compensation for these effects. The pulse 
shaper(s) 62 of the present invention may be of any type (such as, for 
example, those employing pulse integration, or the "two pulse" method as 
disclosed in U.S. Pat. No. 5,021,664) which sufficiently mitigates the 
effects of ballistic deficit, charge trapping, or other similar phenomena. 
The signal output 63 from the pulse shaper(s) 62 merely must be such that 
a sufficiently high degree of spectral resolution can be obtained for 
purpose of discriminating gamma lines attributable to individual elements 
(described further below). 
In order to take advantage of the great computational capability inherent 
in modern digital processors and integrated circuits, the analog pulses 
are converted to binary digital data 65 representative of gamma ray 
energy, as shown in FIG. 4. This conversion is accomplished in the present 
embodiment through a standard analog-to-digital converter (ADC) 64. For 
example, a standard 12-bit ADC, such as the TLC2500 series devices 
manufactured by Texas Instruments Corporation, will provide more than 
4,000 possible "bins" (2.sup.12 or 4096) for gamma energy resolution while 
also allowing for multiple analog inputs. Assuming a gamma energy range of 
0-15 MeV, this allows for a spectral resolution of approximately 3.66 KeV. 
This level of energy resolution is more than adequate for the purposes of 
the present invention (considering the spectral resolution capability of 
the Germanium detectors), and the 12-bit ADC is compatible with a broad 
variety of commonly available FIFO buffers, logic gates, and other digital 
hardware as described further below. 
After conversion to a multi-bit digital representation by the ADC 64, each 
pulse is passed through a digital pulse-height discriminator (PHD) 66. 
Pulse height discrimination is used to eliminate or screen ranges of the 
detected gamma spectrum which are of little or no analytical value. For 
example, the PHD 66 can screen all pulses below a given desired threshold 
energy 67. In this fashion, the processing burden on each detector 
channel, and computational burden on the logic gate 72 can be reduced. 
Alternatively, the PHD 66 can be selectively configured to pass all 
signals to the logic array such that a more complete spectrum can be 
analyzed. The PHD 66 can be embodied in any of a wide variety of hardware 
devices well know in the electronic arts, or may be conveniently 
implemented via the logic gate 72, as represented by the dashed lines 
between the PHD 66 and the logic gate 72 in FIG. 4. 
Conceptually similar to pulse height discrimination described above, 
filtering in the context of the present invention relates to the filtering 
out of specific pulses having unwanted gamma ray energies, such as those 
associated with inelastic scattering of neutrons from elements invariant 
as a function of stressor application, those associated with intervening 
materials (such as aluminum or steel conduit), or those not associated 
with inelastic neutron scattering (such as background radiation). The 
binary digital format of each pulse after conversion by the ADC 64 is well 
suited to rapid discrimination and filtering by the logic gate 72 , since 
the logic gate may be easily programmed to efficiently eliminate data 
stored in memory array addresses associated with unwanted discrete gamma 
energies. For example, if the gamma energies associated with the 
400.sup.th and 4000.sup.th bins of the spectrum must be filtered, the 
logic gate can simply "skip over" these addresses when reading data out to 
the second stage buffer, as described further below. It should be noted 
that pulse height discrimination (as well as filtering) can be 
accomplished after the first stage buffer 68, using a similar approach. 
One possible detriment to this approach, however, is that the first stage 
buffers must then process and store data associated with all gamma 
energies as opposed to a greatly reduced set when pulse height 
discrimination is performed prior to the first stage. 
The present embodiment of the analyzer 60 utilizes a form of memory 
indirect addressing as further described herein. Referring now to FIG. 5, 
the logic gate 72 stores data associated with the incoming pulse stream 
within its internal memory array 90 ("X" array) based on the binary value 
of the data produced by the ADC 64. Each memory location is then "indexed" 
(i.sub.n) or incremented upon receipt of additional data with the same 
address. In this way, the data stream is scaled using a minimum amount of 
memory space. 
A secondary array 92 ("Y" array) located within the same or different 
physical memory is created based on the gamma energy values desired to be 
filtered. This secondary array is generated based on previous observations 
and analysis of energy spectra obtained from similar or identical test 
specimens. For example, if it is known that inelastic scattering 
associated with Aluminum (a non-degradable material) in the cable conduit 
produces spectral lines at a set of different gamma energies, these 
energies can be programmed into the Y array and filtered as the X array is 
read out of memory. The counting or scaling interval (i.e., number of 
"counts" obtained in order to produce a given spectrum) can be set to any 
value consistent with the memory resources of the logic array 72 (or 
external memory, if used). 
It is further contemplated that the present invention may be configured to 
identify specific degradable materials or constituents thereof present in 
the test specimen through comparison of test data to a predetermined 
"signature" gamma spectrum associated with a given material and stored 
within the logic array 72 or other memory. In one embodiment, the logic 
array 72 is programmed to obtain signature gamma spectrum data from the 
host PC 80 or external memory array and difference the index value for 
each gamma energy bin. This produces a type of difference spectrum which 
can then be analyzed (either manually or via an algorithm within the host 
PC 80) to determine the level or quality of match between the observed 
spectrum and the signature spectrum. 
A field programmable gate array (FPGA) or application-specific integrated 
circuit (ASIC) with embedded memory is chosen as the logic gate 72 since 
it may efficiently perform the relatively simple tasks necessary to index 
and filter the digital data as described above, and no significant 
external is needed in the present application. Alternatively, if more 
sophisticated processing of the data is required (such as Fourier 
transform or other operation requiring a MAC stage), a more capable 
integrated circuit (such as a DSP having an external memory interface and 
DMA) may be utilized. 
Referring again to FIG. 4, a first stage FIFO (first-in, first-out) buffer 
68 and associated buffer manager 70 are used in each detector channel 82 
to allow asynchronous storage and retrieval of spectral data obtained from 
each detector. This architecture is utilized primarily to prevent data 
loss during data collection when using a comparatively high neutron flux, 
which produces a high gamma detection event rate. Crystal detectors 
generally saturate at count rates on the order of 1E05-1E06 cps, and may 
begin to suffer severe degradation of spectral resolution at lower count 
rates. Based on a neutron flux of 1E06 n/s-4 .pi., the gamma count rate 
for the present invention (each detector, including background) is 
calculated to be well below saturation and spectral degradation levels. 
However, backend signal processing as described herein may, under certain 
circumstances, act as a "bottleneck" to data output from the ADC 64 at 
very high ADC sampling rates. The sample rate (SR) of the ADC(s) 64 is set 
higher than the maximum anticipated event or data rate (DR) rate to 
prevent data loss. Use of a first stage buffer allows for the accumulation 
of data between the ADC output and logic gate 72, thereby permitting use 
of a lower MIPS processor or logic gate 72 or alternatively, use of a high 
MIPS device and much additional processing of each data pulse. Use of 
second stage buffer 76 allows for the accumulation of data between the 
logic gate output and the storage/display device (personal computer) 80. 
Data output from the PHD 66 is input to the first stage FIFO buffer 68 
under control of the buffer management module (BMM) 70. Each buffer 68 may 
be further equipped with a separate overflow buffer 69, the buffer manager 
70 monitoring the level within each primary buffer 68 and allocating data 
as necessary to the overflow buffer(s) 69 to prevent data loss. Such 
arrangement may be embodied in separate physical devices, or incorporated 
within a single piece of silicon. A Texas Instruments SN74ACT series 
device is chosen for the FIFO buffer of the present embodiment, although a 
wide variety of devices may be used with equal success. The logic gate 72 
provides the necessary control signals to the buffer manager via it's 
control port to read out data from the buffer(s) 68 for further processing 
by the gate 72. 
Data output from the logic gate 72 is input to the second stage buffer 76 
under control of the second stage buffer management module (BMM) 78. 
Again, the buffer(s) 76 may be provided with a separate overflow buffer 
77, the buffer manager 78 monitoring the level within the primary buffer 
76 and allocating data as necessary to the overflow buffer(s) in similar 
fashion to the first stage. The PC 80 provides the necessary control 
signals to the second stage buffer manager 78 to read out data from the 
buffer(s) 76 for further processing, storage, or display by the PC 80. A 
standard parallel data interface 89 or I/O adapter board of the type well 
known in the computer art is used to interface the analyzer 60 with the PC 
80. 
Degradation of Materials 
For purposes of the present disclosure, the term "degradable" shall mean 
any material or object whose chemical or physical composition changes, in 
whole or in part, as a result of the application of one or more stressors. 
Stressors as used herein refers to any chemical, electrical, physical or 
other force or influence including, without limitation, heat (whether by 
conduction, convection, or radiation), nuclear and cosmic radiation, 
electrical potential or current, chemicals, oxygen and other gases, 
volatization, or any combination thereof. 
The primary constituent atoms within most commercially available polymers 
include carbon, hydrogen, oxygen, nitrogen, sulfur, chlorine, and 
fluorine. For example, in electrical cable insulation and jacketing, 
materials such as Hypalon.TM. (CSPE, or-chlorosulfonated polyethylene), 
EPR (ethylene propylene rubber), PVC (polyvinyl chloride), Tefzel.TM. 
(ethylene tetraflouroethylene) and XLPE (crosslinked polyethylene) are 
quite common. In addition to the base polymers listed above, many 
materials contain a variety of other substances or compounds which perform 
various ancillary functions. For example, lamp black (carbon) is commonly 
added to polyethylene in order to increase its resistance to cracking and 
degradation due to ultraviolet radiation. Clay (or other similar material) 
is commonly used as filler, often comprising the majority component within 
electrical cable insulation/jacketing in order to reduce cost. 
Plasticizers are commonly added to polymer formulations (including most 
notably PVC and CSPE) to increase their pliability and resistance to 
fatigue cracking. A typical formulation of EPR might consist of EPDM 
(ethylene propylene diene monomer), parrafin wax, zinc salts and oxides, 
vinylsilane, diadduct of hexachlorocyclopentadiene, dicumyl peroxide, SRF 
black, and antimony oxide. 
Many polymer formulations also contain additives specifically designed to 
reduce the flammability of the cable insulation/jacketing under certain 
conditions. These so-called "fire retardants" volatize to varying degrees 
under exposure to heat and radiation, and are emitted from the cable at a 
rate related at least in part to the temperature/radiation dose rate to 
which the material is exposed. In many materials, the volatization of fire 
retardants roughly parallels the volatization of other flammable 
compounds; hence, the overall flammability of the material remains roughly 
constant as a function of thermal and/or radiation aging. However, as the 
fire retardants and flammable compounds are removed from the material, the 
relative concentration of the constituent atoms of these substances within 
the material as a whole change. 
Similar to fire-retardants discussed above, plasticizers used in various 
polymer formulations are lost from the material as a function of aging and 
stressor application. Plasticizers are lost via both volatization and 
scission of the molecule. Plasticizer content has been shown to have a 
good correlation with, inter alia, elongation-at-break of certain 
materials in the early stages of component aging. Later in life, however, 
plasticizer content remains essentially constant for many materials, 
thereby limiting the effectiveness of these compounds as aging indicators 
during this period. 
Ozone (O.sub.3) is another stressor which may act on certain materials. 
Ozone is generated in the air as a result of the interaction of ionizing 
radiation with monatomic or diatomic oxygen, or by corona discharge 
ionization. Similar to oxygen diffusion. ozone effects occur predominately 
at the surface of the object where the ozone concentration is highest. 
Generally, however, most modern polymer formulations are resistant to the 
effects of ozone. 
Cable components may also be exposed to chemical by-products of the thermal 
or radiolytic decomposition of cable jacketing, insulation, fire-resistant 
coatings, or other organic components. Many materials commonly used in 
cable construction either contain or are manufactured using potentially 
corrosive chemicals such as chlorides, peroxides, or sulfurous compounds. 
Chemical by-products originating from decomposition of cable components 
may result in several degradation mechanisms, including softening, 
swelling, or decomposition of other organics within the cable structure. 
Plasticizer migration (PVC) can also result in swelling of adjacent 
elastomers. 
For example, neoprene rubber (chloroprene), PVC (polyvinyl chloride), CSPE 
(chlorosulfonated polyethylene), and CPE (chloropolyethylene) may all 
produce chlorine ions (and hydrochloric acid) upon decomposition. 
Additionally, elastomers including EPR/EPDM are cured using peroxide or 
sulfur compounds that can be leached from the material as it ages or is 
subjected to certain environmental conditions (such as heat or wetting). 
Copolymers such as ethylene vinyl acetate (semiconducting shield material) 
may also decompose to produce by-products such as weak acids. 
Degradation resulting from copper-catalyzed oxidation reactions may occur 
in certain polymers as well. A catalyst is defined as a substance that 
affects the rate or the direction of a chemical reaction, but is not 
appreciably consumed in the process. Because of its proximity to the 
insulation, ions from copper-based conductors may act as catalysts for 
oxidation reactions within the insulation, thereby accelerating its 
degradation. This will occur primarily in areas where the insulation is in 
direct contact with the conductor. 
By-products are also generated from chemically crosslinked XLPE as a result 
of the high temperature/pressure curing process. By-products such as 
acetophenone, cumene, and alpha methyl styrene are produced as the 
chemical crosslinking agent (dicumyl peroxide) decomposes. 
Another potential aging mechanism is hydrolytic degradation of mylar 
(polyethylene terephthalate) shield film under exposure to high 
temperature and moisture. Under this mechanism, water increasingly reacts 
with the mylar polymer as temperature is increased. 
In sum, there are a substantial number of different possible aging 
mechanisms for electrical cable components (and more broadly, degradable 
components), each of which may ultimately vary the concentration of 
various atomic species within the material. The effects of these aging 
mechanisms are specific to each class or even formulation of material, and 
hence generally must be analyzed individually. For example, as discussed 
above, it can be shown for some materials that the rate of fire retardant 
loss is roughly proportional to the thermal aging applied to the material 
(at least for certain aging intervals). Hence, the signature gamma lines 
associated with the specific fire retardant present in that material are 
used as an indirect indicator of thermal aging. Since fire retardant 
volatizes, the atomic concentration and hence inelastic scattering of 
neutrons by the constituent atoms (which may include carbon, hydrogen, 
bromine, fluorine, or chlorine) will also vary as a function of aging. As 
the atomic concentration (N) of a given element is reduced, the associated 
gamma yield at specific energies is reduced as well (assuming a measurable 
gamma yield for the incident neutron energy selected). Hence, a reduction 
in atomic concentration due to aging stressors is ultimately reflected as 
a reduction in detected counts at those energies as compared to prior 
spectra of the same sample; see FIGS. 6a and 6b, which show typical gamma 
spectra taken from the same specimen at two different levels of aging. The 
following generalized formula represents the approximate gamma counting 
rate for a given material, gamma energy, and neutron energy in the 
apparatus of the present invention (assuming no detector or processing 
circuit saturation): 
EQU CR=S.multidot.d.phi..multidot.E.sub.d .multidot..gamma..sub.i .multidot.AF 
Where: 
CR=Counting Rate (cps) 
S=Total neutron source strength (neutrons/s-4 .pi.) 
d.phi.=Uncollimated solid angle subtended by active detector area 
(steradians) 
E.sub.d =Detector efficiency at selected gamma energy 
.gamma.i=Gamma yield for ith material for selected gamma energy and 
incident neutron energy 
AF=Attenuation factor for interposed materials for selected gamma energy 
Note that the gamma yield .gamma..sub.I as shown in the above relationship 
is a complex function of the inelastic scattering cross-section (.sigma.) 
of the various constituent atoms, their atomic concentrations (N), and the 
incident neutron energy. Obviously, the yield at different gamma energies 
will vary for each material. 
The present invention further contemplates the evaluation of multiple 
degradation processes during the installed lifetime of the degradable 
component as required. For example, while changes in the gamma spectrum 
associated with plasticizer loss may be useful during the earlier stages 
of component life, fire retardant volatization may be a better indicator 
of component condition later in life. 
Neutron and Gamma Attenuation in Surrounding Materials 
One of the principal benefits of the present invention is its ability to 
"look through" most any components or materials interposed between the 
test subject 11 and neutron source 12. This unique property results from 
the use of energetic neutrons which have a very low scattering/absorption 
cross-section in most materials of relatively low thickness (i.e., less 
than a few inches). Obviously, some attenuation of the incident neutron 
beam will occur. Unlike the interaction of gamma rays with matter 
(described below), the energy and spatial distribution of incident 
neutrons will vary as a function of the attenuating material. 
Specifically, a fraction of the neutrons in the incident beam 13 will be 
reduced in energy, and a fraction will be scattered at angles relative to 
the beam centerline. This characteristic is due not to coulombic 
interaction, but rather the inelastic scattering of the comparatively 
massive neutron with other particles in a given nucleus. The neutron 
spatial and energy distributions after passage through an intervening 
material are not critical in the present embodiment of the invention, 
since 1) a sufficient population of sufficiently energetic neutrons will 
exist after passing through most any material in most contemplated 
applications; 2) gamma rays (and not neutrons) are detected upon their 
egress from the test subject; and 3) the spatial position of the test 
subject is not being measured, hence, any errors induced by alteration of 
the spatial distribution of neutrons will only affect the scope of 
material within the test subject 11 which is analyzed. This affords the 
invention the ability to analyze test subjects shielded behind any number 
of types and configurations of materials. The aforementioned secondary 
gamma emissions are prompt (occur on the order of femtoseconds after 
scattering) and spatially distributed around the target atom(s). 
Unlike neutrons, gamma rays (photons) generally retain their initial energy 
regardless of intervening material; rather, such materials act to 
attenuate the gamma flux, yet not alter the spectral or spatial 
distribution. Lower energy gammas are attenuated much more severely by 
relatively thin materials than are higher energy gammas. For example, the 
attenuation of 100 KeV gamma flux in 1 inch of steel is almost complete, 
whereas the attenuation of a 10 MeV gamma flux in the same material is 
fairly minimal. A common measure of this property is so-called "tenth 
thickness", defined as the thickness of a given material required to 
attenuate an incident gamma flux of a given energy to one-tenth of it's 
initial value. 
Damage to the test subject 11 and any intervening material resulting from 
incident neutron radiation during testing with the apparatus 10 described 
herein is not considered significant, since the total integrated dose 
(TID) applied to a given test specimen even over several in-situ tests is 
well below the threshold dose necessary to result in measurable property 
changes in the target. Most elastomers thermoplastics, and thermosets have 
estimated neutron threshold doses on the order of 1E14 n/cm2, whereas the 
neutron dose imparted during a standard testing protocol of the present 
invention is several orders of magnitude below this value. Even when 
accounting for differences in neutron energy, the dose to a given 
component resulting from even frequent periodic testing during its 
lifetime is well below the threshold value cited above. Furthermore, with 
certain test components (such as cable), slightly different physical 
locations on the cable with essentially identical environmental conditions 
may be used for subsequent tests in order to distribute the 
neutron/gammadose within the component. A tightly collimated neutron beam 
(given the same flux emitted from the source 12 prior to collimation by 
the collimator 14) allows greater spatial control, yet generally at the 
expense of slower counting rates and longer integration times. 
Aging Analysis and Method 
As is presently known in the art, specimens of similar or identical 
construction to the in-situ components to be analyzed may be aged in a 
controlled fashion in order to observe changes in various physical, 
chemical, electrical, or atomic characteristics. Typically, the aging is 
accelerated in nature (in order to make the results available more 
immediately), and attempts to replicate the environmental conditions to 
which the component will be exposed as closely as possible. These 
laboratory or artificially aged specimens are then used as "yardsticks" 
against which in-situ components are compared in order to evaluate the 
condition (level of aging) of the component. 
The present invention utilizes the foregoing general approach to evaluate 
in-situ specimens as depicted in FIG. 7. First, a specimen similar or 
identical to the in situ component being evaluated is aged, either 
naturally or artificially, in a manner consistent with the natural aging 
of the in-situ component. For example, if the in-situ component is exposed 
to heat and radiation, similar aging is applied to the specimen to induce 
similar types of degradation. The specimen is aged to or beyond the 
maximum expected level of aging anticipated for the in-situ component to 
provide a complete aging profile. The techniques used to artificially and 
naturally age specimens for purposes of in-situ component aging analysis 
are well known and understood in the art, and accordingly need not be 
explained further herein. At selected intervals during the specimen aging 
process, gamma spectra are obtained from the specimen using the 
above-described apparatus and stored within the memory of the analyzer 60 
or host PC 80 for later use. For example, the spectra (or sets of spectra) 
may be obtained at 25, 50, 75, and 100% of anticipated aging of the 
specimen. The spectra are also analyzed to identify gamma energies 
associated with non-degradable components within the specimen, such as the 
copper conductors of the cable. These gamma energies are recorded for 
later use with the pulse height discrimination/filterfunctions of the 
analyzer 60 previously described. It should be noted that essentially the 
entire gamma spectrum obtained from the specimen is used in the analysis, 
since as previously described, different spectral lines may be more or 
less useful as a function of the level of aging. 
Next, the apparatus of the present invention is again used to obtain a 
first spectrum (or set of spectra if averaging or more complex statistical 
analysis is used) of the in-situ component at a given time in the lifetime 
of that component. In the case of intervening materials such as conduit or 
valve bodies, the representative spectra obtained from the insitu 
component is differenced, using the analyzer 60 described above, from that 
of the laboratory aged specimen for a comparable level of aging in order 
to identify the spectral lines associated with the intervening material. 
The choice of specimen spectrum for comparison to that obtained from the 
in-situ component is not critical (so long as a spectrum representing a 
roughly comparable level of aging is chosen), since variations in the 
aging of the degradable materials will be minimal in comparison to the 
more salient differences relating to the non-degradable materials. The 
gamma energies associated with these "salient" differences are then 
entered into the filtering algorithm (i.e., the "Y" array values) to 
permit filtering of subsequent spectra obtained with the intervening 
material in place. Note that multiple arrays (filter values) may be stored 
in memory and accessed as desired depending on the specific application. 
Lastly, a second spectrum (or set of spectra) is obtained from the in-situ 
specimen at a later time in life, or after the application of a 
significant stressor. This second spectrum is then filtered as necessary 
and compared to the first spectrum in order to identify changes in the 
material as a function of aging/stressor application. Specifically, the 
differences between the first and second spectra from the in-situ specimen 
are compared to the difference between the gamma spectrum for the unaged 
laboratory specimen and those taken at subsequent times during the 
artificial (or natural) aging regimen as shown in FIG. 8. For example, if 
the difference spectrum for the in-situ specimen indicates a change of 
1000 cps for a gamma energy of 3 MeV, and the 25% and 50% aging spectra 
for the laboratory specimen indicate changes at 3 MeV of 500 cps and 2000 
cps, respectively, the aging of the in-situ component can be inferred to 
be between 25% and 50%. Obviously, more sophisticated differencing and 
analytical interpolation techniques may be employed to more precisely 
estimate the level of aging of the component; the foregoing example is 
merely illustrative of the general methodology of the present embodiment. 
Note also that such comparisons may be functionally implemented entirely 
in software operating on the PC processor 80, such software being easily 
produced using techniques well known in the computer arts. 
It should be recognized that while the foregoing discussion has described a 
specific sequence of steps necessary to perform the method of the present 
invention, other sequences (such as obtaining the in-situ measurements 
prior to conducting artificial aging of the laboratory specimens) of steps 
may be used depending on the particular application. 
While the above detailed description has shown, described, and pointed out 
novel features of the invention as applied to various embodiments, it will 
be understood that various omissions, substitutions, and changes in the 
form and details of the device or process illustrated may be made by those 
skilled in the art without departing from the spirit of the invention. The 
foregoing description is of the best mode presently contemplated of 
carrying out the invention. This description is in no way meant to be 
limiting, but rather should be taken as illustrative of the general 
principles of the invention. The scope of the invention should be 
determined with reference to the claims.