WO1999024812A1

WO1999024812A1 - Method and apparatus for monitoring wear, corrosion, and related problems

Info

Publication number: WO1999024812A1
Application number: PCT/US1997/020160
Authority: WO
Inventors: Andrzej K. Drukier
Original assignee: Biotraces, Inc.
Priority date: 1997-11-07
Filing date: 1997-11-07
Publication date: 1999-05-20
Also published as: AU5169098A

Abstract

A method of monitoring removal of material from an object comprises providing a solid object comprising a radioactive tracer in a predetermined pattern, providing an ambient medium with which the solid object is in contact, monitoring the ambient medium for a change in level of the radioactive tracer corresponding to removal of material from the solid object, and determining the condition of the solid object based on the change in level of the radioactive tracer in the ambient medium. The radioactive tracer may be concentrated in an outer or inner layer of the solid object and preferably comprises a characteristic signature combination of multiple radioisotopes. A second aspect of the invention is a solid component of a system comprising a radioactive tracer having a characteristic signature, the tracer comprising a multiphoton emitter, and being located in a predetermined pattern such that removal of material from the solid object results in a change in level of the radioactive tracer in the ambient medium. The component can be used in a system with a detector, an analyzer that compares the level of radioactive tracer to an expected level, and an indicator that issues a warning when the removal of material from the object is at a critical level, and identifies the object. In another aspect of the invention, a method for labeling an object comprises applying a radioactive tracer to the object in a predetermined pattern, such as combining the radioactive tracer with a coating medium and applying the coating medium to the object.

Description

METHOD AND APPARATUS FOR MONITORING WEAR, CORROSION, AND RELATED PROBLEMS

BACKGROUND OF THE INVENTION The invention relates to monitoring the removal of material from component parts by doping them with radioisotopes and monitoring the level of radioactivity in an ambient medium for a change over time. The invention relates to an apparatus for detecting and monitoring wear or corrosion, tagging materials, and methods of doping component parts with isotopes.

The problems associated with wear, friction, corrosion and disintegration of materials, breakage, rupture and leakage continue to be of major and growing importance in industrial society. Ever more sophisticated machinery and other types of systems, with or without moving parts, suffer great damage and incur incalculable costs every day when a component or part of some system breaks or malfunctions.

This is a particularly significant consideration when the part or component in question is difficult to access or cannot be examined without stopping or dismantling the machine, process or system of which it is a part. Such measures lead to great disruption and cost. On the other hand, attempts to prevent breakdowns by extensive preventive maintenance schedules also involve costs and stoppages. These are often nevertheless dictated by considerations of safety and security, for example in the case of aircraft engines. In U.S. patent 3,818,227, radioactive krypton is introduced into bearing grease to facilitate detection of drill bit wear. This approach is suitable only for large and massive parts. In U.S. patent 4,683,070 a tracer metal is added to lubricating oil to help identify metals arising from engine wear. These methods are not generally applicable to predicting failure of solid components. In U.S. patent 3,898,459, the seal of a rotary engine is rendered radioactive by neutron irradiation, and then wear is measured by cooling the exhaust and collecting wear debris in a filter. This method is valid only for laboratory tests and could not be used for routine maintenance.

For these and other reasons, it remains of the greatest interest to know when a part of a system has worn, corroded or otherwise been damaged to some critical point before actual breakdown occurs, and furthermore to be able to obtain this knowledge without stopping or dismantling the system in question. In addition it would be highly beneficial if it were possible to do this quite accurately, so that necessary maintenance is performed at the proper time, and not before it is actually necessary. SUMMARY OF THE INVENTION

The principal object underlying the present invention is to provide methods and apparatus for accomplishing these aims. More specifically, it is the object of the present invention to provide a tool, i.e. a method or an apparatus for monitoring materials, machines, systems, and devices of diverse kinds to establish whether particular changes such as wear, corrosion, leakage and related damage have occurred.

According to the invention, a method of monitoring removal of material from an object comprises: providing a solid object comprising a radioactive tracer in a predetermined pattern, providing an ambient medium with which the solid object is in contact, monitoring the ambient medium for a change in level of the radioactive tracer corresponding to removal of material from the solid object, and determining the condition of the solid object based on the change in level of the radioactive tracer in the ambient medium. When the radioactive tracer is concentrated in an outer layer of the solid object, the change in level of the radioactive tracer in the ambient medium is a reduction when the outer layer is worn away. When the radioactive tracer is concentrated in an inner layer of the solid object, the change in level of the radioactive tracer in the ambient medium is an increase when the inner layer is exposed to the ambient medium. The radioactive tracer preferably comprises a characteristic signature combination of multiple radioisotopes. Preferably there is a plurality of solid objects, each comprising a unique radioactive tracer, there is an ambient medium with which the solid objects are in contact, and the ambient medium is monitored for a change in the level of the unique radioactive tracer for at least one of the solid objects, corresponding to removal of material from the at least one of the solid objects, and the condition of the solid objects is determined based on the change in level of the unique radioactive tracer in the ambient medium.

The determining step preferably comprises associating the change in level of radioactive tracer with the condition of the corresponding solid object, and has a sensitivity of 100 picoCuries. Preferably, the object has an expected working life and the radioactive tracer comprises a radioisotope having a half life within an order of magnitude of the expected working life. Preferably, the radioactive tracer comprises a multiphoton emitter and the determining step comprises multiphoton detection. The ambient medium may comprise a lubricant for the solid object, or a fluid employed in a chemical process. The part may be inert in the process, or may be corroded or partially consumed in the process, such as a catalyst. The solid object may be a valve in a transport vessel containing the ambient medium.

The method may comprise taking a sample of the ambient medium for monitoring, and the monitoring of the ambient medium may be performed continuously. The method may involve concentrating the radioactive tracer in the ambient medium for example by centrifuging, filtering, evaporating, or distilling the ambient medium, or chromatography, selective absorption, or selective adsorption.

Another embodiment involves processing the solid object to remove material and monitoring the progress of the processing by monitoring the ambient medium, such as by cutting, polishing, grinding, machining, dry etching, or wet etching. Preferably the radioactive tracer is located in a marker pattern of the solid object, and further comprising guiding the processing based on the change in level of the radioactive tracer.

The method may comprise correlating the object, the radioactive tracer, and the predetermined pattern, and may comprise a means for applying the radioactive tracer to the object in a predetermined pattern.

A second aspect of the invention is a solid object for contacting an ambient medium, comprising a radioactive tracer having a characteristic signature, the tracer comprising a multiphoton emitter, and being located in a predetermined pattern such that. removal of material from the solid object results in a change in level of the radioactive tracer in the ambient medium.

The predetermined pattern is preferably a coating.

The object may be a stationary part of a system, such as a gasket, seal, valve seat, cylinder, printing or lithographic plate, pipe, tube, valve, medical prosthesis, structural member, or protective layer or coating thereof. The object may be a moving part of a system, for moving in contact with the ambient medium, such as a gear, bearing, turbine, wheel, piston, piston ring, switch, thread, cutting edge, or protective layer or coating thereof.

In another aspect of the invention, an apparatus comprises a solid object doped with a radioisotope, and an ambient medium in contact with the solid object. Preferably, the apparatus comprises a detector capable of detecting a change in level of the radioactive tracer corresponding to removal of material from the solid object. The radioisotope is preferably a long life electron capture, positron-gamma, or nuclear cascade isotope, and the detector is optimized for emissions from such isotopes. The apparatus preferably comprises a means for sampling the ambient medium, such as a container made of low atomic number, and a detector encompassing the container. The container may be a small diameter bypass comprising an adsorption/absorption element.

The apparatus may include an analyzer that compares the level of radioactive tracer to an expected level. The apparatus may comprise an indicator that issues a warning when the removal of material from the object is at a critical level, and identifies the object.

In another aspect of the invention, a method for labeling an object comprises applying a radioactive tracer to the object in a predetermined pattern, such as combining the radioactive tracer with a coating medium and applying the coating medium to the object, or manufacturing the object from plastic and forming a radiolabeled plastic and admixing into the plastic from which the object is manufactured, or forming a radiolabeled plastic and applying it to the surface of the object. If the object is rubber the applying preferably comprises contacting the object with the radioactive tracer in a liquid or a gas and allowing it to penetrate the object. The applying may comprise combining the radioactive tracer as a powder with a liquid to form a suspension and forming the object from the liquid suspension. If the object comprises metal the applying may comprise electrodeposition of the radioactive tracer from a plating solution onto the object, or vacuum deposition, sputtering, or metal diffusion.

Further objectives and advantages will become apparent from a consideration of the description and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is better understood by reading the following detailed description with reference to the accompanying figures, in which like reference numerals refer to like elements throughout. Figure 1 is a schematic view of a part with a doped exterior portion.

Figure 2 is a schematic view of a part with a doped interior portion. Figure 3 is a schematic view of a monitoring system for an industrial plant pipeline having doped valves and a bypass for detecting isotopes in the material in the pipeline.

Figure 4 shows a schematic view of a cylinder of an internal combustion engine where piston rings are labeled with a radioisotopic label.

Figure 5 is a schematic view of a part with a doped exterior pattern to be removed by machining.

Figure 6 shows a schematic view of a T-like element permitting in-situ concentration by means of absorption and subsequent remote measurement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In describing preferred embodiments of the present invention illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.

Each of the references cited herein is hereby incorporated by reference in its entirety, as if each were separately incorporated by reference.

The basic principle involved is the following: by the doping of small amounts of an unusual but identifiable substance in a predetermined way into the part or element whose wear or damage is to be observed, the wearing off or the appearance or disappearance of a layer of material may be determined. This can be advantageously done with the use of radioactive tracers.

The present invention exploits the new possibilities which have been opened up by the development of a new ultra-sensitive measuring method for radioactive tracers, known as Multiple Photon Detection (MPD), which allows the measurement of very small amounts of radioactivity, well below the level of the natural background of radioactivity as described in United States Patent No. 5,083,026, United States Patent No. 5,532,122, and U.S. Patent Application Serial No. 08/669,970, which are incorporated herein by reference. Equipment of this kind is available from BioTraces Inc., Fairfax, Virginia, under the designation MPD Analyzer.

The sensitivity of MPD technology means that a relatively small number of radioactive atoms appearing due to wear, corrosion and so forth can be detected. Because the radioactivity is well below the natural background, there are no health or regulatory impediments to use of the invention. The doping of a radioactive tracer into a part is done so that its appearance or disappearance in a system indicates a pre-deter mined level of wear or damage. Fig. 1 illustrates a simple explanatory example: let a metal surface, subject to wear in a machine, be doped with a radioactive tracer 1 in an outer region to a certain predetermined depth defining undoped region 2. Due to the normal processes of wearing down of the surface, the tracer 1 will normally appear in the lubricating oil. With a sufficiently sensitive measurement procedure for the radioactive tracer (radioisotope), it is possible to determine if the tracer is present, even taking into account the small amounts in question and its dilution in the lubricating oil. When the tracer ceases to appear in the oil, this indicates that the layer in which the doping was present has worn away, exposing region 2. By choosing the depth of doping correctly this can indicate that it is time to change the part or component before it has actually broken down.

This method may be accomplished without interruption, opening up, or dismantling of the system. The procedure is accurate, timely, and can be implemented without stopping or dismantling the system.

Fig. 2 illustrates a variant of the same technique. Consider a part with a protective shell or coating 3 as, for example, is often the case with bearings. Let the doping 1 be present in the material underneath the protective surface or layer, while the protective region 3 itself is not doped. In normal operation there is now no tracer in the lubricating oil, but when the protective region is worn or breached in some way, the radioactivity will begin to appear in the system, signaling the problem. Many variants of this basic principle may be envisioned. Instead of lubricant we may have the other liquids of a system, gases or even granulated solids. Instead of moving parts we may consider stationary elements, especially when corrosion and related problems are in question. An example here is the problem of valve integrity, which is important in chemical and pharmaceutical installations. By the doping of appropriate valve parts, critical corrosion or damage may be detected by the appearance of the tracers in the fluids of the installation, before the valve actually fails.

At this juncture, one more basic principle of the technique must be explained, namely the creation and implementation of a "radioactive signature" . A large number of radioactive isotopes are available for use by the new ultra-sensitive MPD detection technology, in particular double- and triple- gamma isotopes, as described in the Tables 1 and 2. These isotopes can not only be detected in small amounts, but can also be distinguished from one another, by using the energies of the photons emitted and other characteristics. This capability means that it is possible to create an individualized signature by using a doping consisting of combinations of distinguishable tracers. To take an example, we use three tracers: Signature A might be 10% of radioisotope 1, 80% of radioisotope 2, and 10% of radioisotope 3. Signature B might be 20% of radioisotope 1, 50% of radioisotope 2, and 30% of radioisotope 3, and so on. By varying the composition in this manner and also the number of isotopes used, an enormous number of potential signatures is possible. Such signatures may be used to establish exactly which part or element of a system is in question. The mere appearance/disappearance of a tracer may not be adequate to give this information. However, if a part or component is doped with a specific radioactive signature of the type just described, then d e identification of the radioactive signature will communicate the information as to precisely where the difficulty is, wid out the need to open up the whole system. Fig. 3 illustrates the above example of valve integrity in a large chemical installation where there may be hundreds of valves, three of which are shown as 5, 6 and 7. Each valve or part of a valve has its own signature. The appearance of the radioactive signature of one or more valves in the fluids of the installation indicates which valve or valves are in difficulty and allow isolation of the problem. A bypass 8 presents the ambient liquid to a multiphoton detector 9. It will be evident that the same principle applies to a vast number of other problems, ranging from the pistons of motors to the components of structures.

Fig. 4 shows a piston 10 with multiple piston rings 11 labelled with different labels 12 and 13. This permits detection of the critical wear point of either of the piston rings. A number of radioisotopes are particularly useful in conjunction with the MPD technique in the present context. This is helpful in making a large variety of potential dopants available, e.g. for the fabrication of signatures, to have different half-lifes available, to optimize the doping procedures the concentration procedures, and to meet various other conditions. Also the variety of different energies for the quanta emitted can be helpful in the elimination of various specific backgrounds, and in avoiding cross-talk when different radioisotopes are used in combination to make a signature. It is important that there is a plurality of radioisotopes available with different chemical characteristics. Also a variety of radioisotopes are available which can readily be incorporated into metals, such as lathanides and platinides. Furthermore within given groups with similar chemical and metallurgical properties, there are many radioisotopes distinguishable by their photon energies.

First of all, in the MPD technique we require radioisotopes that emit more than one photon. These fall into two main families:

Family 1: Electron capture-nuclear gamma isotopes. In this case an atomic x-ray and a nuclear gamma ray are emitted (see Table 1). The tables indicate the half-lifes of the radioisotopes. The energies of the photons emitted can be obtained from the Handbook of Chemistry and Physics, CRC Press, NYC.

Table 1: Long Life EC isotopes.

Be ⁷ ( 53.6 d)

Na²² ( 2.58 y)

Ar³⁷ ( 34.3 d) y 49 (330.0 d)

Cr⁵¹ ( 27.8 d)

Mn⁵⁴ (291.0 d)

Fe⁵⁵ ( 2.7 y)

Co⁵⁶ ( 77.3 d); Co⁵⁷ (270.0 d); Co⁵⁸ ( 71.0 d) Zn⁶⁵ (245.0 d)

As⁷³ (76.0 d); As⁷⁴ (18.0d);

Se⁷⁵ (120.0 d);

Rb⁸³ (83.0 d); Rb⁸⁴ ( 33.0 d);

Sr⁸² (25.5 d); Sr⁸⁵ ( 64.0 d);

Y ₈₈ (108.0 d);

Zr⁸⁸ (85.0 d);

Nb⁹² (10.1 d)

Tc^95m ( 60 d);

Rh⁹⁹ (16 dy, Rh¹⁰¹ (5 y); Rh¹⁰ (206 d);

Pd¹⁰³ (17 d);

Ag¹⁰⁵ ( 0 d); Ag^108m (>5 y);

Cd¹⁰⁹ (470 d);

In¹¹³ (118 d);

Sn¹¹³ (118 d);

Sb¹¹⁹ (158 d);

Te¹¹⁸ ( 60 d); Te¹¹⁹ (45 d); Te¹²¹ ( 17 d);

T 1₂5 ( 60 d); T 126 ( 13.2 d);

Ba¹³¹ (11.6 d);

Ce¹³⁹ (140 d);

Pm¹⁴³ (265 d); Pm¹⁴⁴ (440 d); Pm¹⁴⁵ ( 18 y); Pm¹⁴⁶(710 d);

Pm^158m(40.6 d);

Sm¹⁴⁵ (340 d);

Eu¹⁴⁷ (24 d);

Eu¹⁴⁸ (54 d); Eu¹⁴⁹ (120 d); Eu¹⁵⁰ ( 5 y);

Eu¹⁵² (13 y);

Gd¹⁴⁶ ( 48 d); Gd¹⁵¹ (120 d); Gd¹⁵³ (200 d);

_Tb160 (73 d);

Tm¹⁶⁸ ( 85 d);

Yb¹⁶⁹ (32 d);

Lu¹⁷³ ( 1-3 y); _Lu174m (165 d);

_Hfπs (70 d);

W¹⁸¹ (130 d)

Re¹⁸³ (71 d); Re¹⁸⁴ (50 d);

Os¹⁸⁵ (94 d);

Ir¹⁸⁹ (11 ); Ir¹⁹⁰ ( 11 d); Ir¹⁹² ( 74 d);

Au¹⁹⁵ (200 d);

Hg¹⁹⁴ (130 d);

--pι20₂ (12 d); πι204 ( 3.9 y); _βi206 ( 15.3 d); Bi²⁰⁷ (30 y);

Pu²³⁷ (45.6d);

Cm²⁴¹ (35 d).

Subtotal : 71 isotopes Family 2: Positron-nuclear gamma emitters wherein an emitted positron is followed by an nuclear gamma transition. The positron annihilates with a nearby electron, giving two high energy gamma rays in addition to the nuclear gamma ray. It also includes nuclear cascade gamma emitters. In this case a metastable nucleus cascades from a highly excited metastable state, emitting several gamma rays. The isotopes from this family are listed in Table 2.

Table 2: Long life positron-gamma and nuclear cascade isotopes.

Na²² ( 2.58 y);

Al²⁶ ( > 10⁵ y);

Co⁵⁶ ( 77.3 d); Co⁵⁸ ( 71.0 d)

Zn⁶⁵ (245.0 d)

As⁷⁴ ( 18.0 d);

Rb⁸⁴ ( 33.0 d); γ 88 (108.0 d);

_Tc95m ( 60 d);

Rh⁹⁹ ( 16 d); Rh¹⁰² (206 d)

T 126 ( 13.2 d);

Eu¹⁵² ( 13 y);

Hg¹⁹⁴ (130 d); _{β i}206 ( 15.3 d).

Subtotal: 15 isotopes

The general principle underlying the present invention comprises a method according to which the wear or damage, such as corrosion, fracture, tearing or disintegration, of a part or component of a system may be determined by the observation (appearance or disappearance) of one or more radioactive tracers which have been doped into said part or element. For practical and regulatory reasons, the amount of radiolabel should be small, preferably below the natural radioactivity associated with the machinery or component under study. Thus, the ultrasensitive radiation detectors are preferred.

There are many sources of background and noise in nuclear radiation detectors. Herein, four classes of detectors should be mentioned; * ultra low radioactive background devices (ULBD): (background < 1 cpd);

* very low radioactive background detectors (VLBD): (background < 1 cph); * low radioactive background detectors (LBD): (background < 1 cpm).

* laboratory detectors with background of about 1 cps.

The practical implementation of the disclosed methods of wear /corrosion measurement requires detectors with background lower than about 1 cph. Therefore, in the following the preferred implementation of these very low background detectors (VLBD) is described.

In a method of me above type an important aspect, enabled by the existence of a highly sensitive detection method like MPD, is the possibility of creating a characteristic signature using a doping consisting of a specific combination of radioactive tracers. A particularly useful application involves more than one of said signatures in a system consisting of a plurality of components, in order to identify each component or certain sets of components by doping mem with different radioactive signatures. Furthermore, in such a method one associates each signature with a given component or set of components of a machine or system, so that it is possible to identify which part(s) or component(s) is (are) giving rise to the radioactive signal when the signature is detected. The question of the suitable half-life of the dopant plays a role in the present methods.

An isotope wim a short half-life has more decays per second than one with a long half-life and is thus easier to detect. On the other hand the half-life should not be appreciably shorter than the anticipated lifetime of the part in question. For example, it would not make sense to dope a machine part which is expected to last ten years with an isotope with a half-life of ten hours. There is thus an optimal choice of half-life of the isotope(s), connected with the expected life of the part.

The great power and practicability of the present procedure is connected with the use of new very low background and highly sensitive detection techniques for radioisotopes. These techniques make it possible to monitor wear and related phenomena leading to activities in the sub-nanoCurie regime. For comparison, a glass of mineral water has radioactivity of about 10 nCi. Therefore, the systems of the invention may detect radioactivity well below average exposure levels.

A major application is anticipated to be to in machinery where the tracer is sampled in the lubricating oil and in lubricants of other kinds. However, motor oil is just one of the fluids in which tracers can be sampled. Major applications involving other liquids and gases are also to be anticipated. The appearance/disappearance of the radioactive tracer or tracers can also be observed in the fluids of a process such as those of a petrochemical or pharmaceutical installation. It is in particular to be noted that these applications do not necessarily involve only the moving parts of machinery. Included are control components like valves, and structural components like pipes and tanks where corrosion and damage is to be monitored. However, it is to be noted that also in the case of motors, sampling of exhaust gases is part of the present invention.

The proposed methods converge with the increased use of microprocessors / computers in optimizing the performance of many machines and engines. First, all statistical information about the patterns of wear/corrosion and failure of the similar (the same model) parts can be used. Second, the data base of previous wear /corrosion measurements can be used. Third, the information about the usage pattern in the period between the measurements is often available. The wear and corrosion effects can then be computer modelled to establish the optimal time for the next measurements, so that both the cost of preventive maintenance is diminished and risk of failure minimized. Furthermore, the radioactive tracer or tracers may be present in solid form as in the products of corrosion, tearing, crumbling and the like. In addition these solids may be the result of an intentional wear or abrading process, as in the use of abrasives. Thus when the abrasive material is used up, say in some cutting, grinding or polishing procedure, this may be determined by the present methods. Relatively few radioactive atoms are present in a sample, particularly when they are diluted in a large volume, as in say, a large chemical installation or a large motor. This may make it difficult to detect the radiation quickly even with high-sensitivity methods. Thus it may prove helpful or necessary to precede the step of detection of the radiation with a step of concentration of the sought-for atoms. These may include filtering, centrifugation, chromatography, distillation and chemical concentration techniques.

A preferred method of the present invention is one according to which the wearing away of a surface to a given depth is ascertained from the fact that a radioactive doping which was present in the material only to that given depth no longer is observed in the system at the expected or appropriate level. In particular it is to be noted that this method has the potential for very high accuracy. This is especially important in uses where fine tolerances are significant. The accuracy (as concerning depth especially) can reach into the sub-micron range. The degree of accuracy will depend on how precisely the doped volume is defined in the process of creating the doping. If more care and expense is devoted to very high precision in the spatial definition of the doping, then a higher precision in the knowledge of the wear is possible. We draw attention to the phrase "appropriate level". Since radioisotopes become less active with time as they decay, an equivalent concentration will lead to less activity as time goes on. This effect can be compensated for by standard calculations using the known half-life of the radioisotope. Thus we say "appropriate level" or expected level instead of "same level". Note that when several radioisotopes are involved, as for a signature, a separate compensation is necessary according to the half-life of each radioisotope.

In a converse method to the one just discussed, the doping is first present after a certain depth. Note tiiat the statements of the preceding paragraph concerning accuracy and "appropriate level" apply equally well here, except that the accuracy now depends of the precision of the undoped layer. This converse method may be used to determine the wear of a surface as before; and is also particularly suitable when protective layers, coatings, shells and the like are present.

The methods apply to the entire wide variety of moving parts and surfaces, where the radioactive doping is applied to the moving parts of a machine or a system, such as gears, bearings, turbines, wheels, pistons, switches, threads, and cutting, abrading, machining edges, surfaces and materials. Note that this includes cases where wear is an intended part of their function, such as abrasive materials.

Further, the radioactive doping can be applied to the stationary parts of a machine or system such as gaskets, seals, valve seats, printing or lithographic plates, piping and tubing, and protective layers and coatings; or to structural members such as rails and parts of buildings and medical protheses. In this application to non-moving or structural parts of systems, the sampling procedures are evident for systems where fluids are normally present such as for pipes and tanks. In other systems like buildings and structures this sampling may take place through existing systems such as water, gas, air conditioning or rain runoff; in special cases it may be useful to build in sampling systems. In the case of medical prostheses, bodily fluids such as breath, blood or saliva may be sampled. Here again the very low level of radioactivity involved means that the method involves no radiation hazard. In these structural applications the capability of the technique to detect wear or damage in difficult-to-access places can prove to be of capital importance.

As in the case of wear and friction, the application to abrasives and cutting may be done in a converse manner where the signal is the appearance rather than the disappearance of a radioactive signal. In the past, the use of tracer methods to guide machinery was not practicable since the detection of the signals is in general rather slow compared to the speed of machinery.

However with the new ultra-sensitive detection procedures, this need not be the case. The doping may then also be in the workpiece, that is the object be worked. In Fig. 5, for example, the material which is being worked may be doped with radioisotope 1 by methods described below to indicate to a machine when and where to cease cutting or polishing. In general an automated tool or process can be guided by a path, profile, or markers laid down by radioactive tracers, which are then read by a highly sensitive and thus fast detection method as represented by MPD.

Scrap containing the radioactive tracer 1 is removed automatically to reveal the resulting machined piece 15, shown here as having a round cross-section. Any other pattern may be applied, and the tracer could be in the useful portion rather than the scrap, or different tracers may be used.

An apparatus for the implementation of the technique of the present invention comprises means for detecting the radioactive tracers which are present in the system, and means for analyzing the resulting signals to compare them with expectation. The means for detecting the radioactive tracers may involve taking a sample from the system or it may not. When sampling is used, the sample may be in liquid, gaseous, or solid form. In the case of sampling, it is preferable to have a portable device. In this case emitters of lower energy photons, as from Family 1, are preferred. In addition for sampling procedures where high through-put is important, the use of the electron capture-nuclear gamma emitters of Family 1 is particularly indicated since these are well adapted to parallel processing by the MPD technique.

However in some applications it may be possible to eliminate the step of taking a sample, if it can be arranged that the radiation from the tracers is detected directly as emitted from within the system, say from the lubricant or other fluids. In particular since x-rays or higher energy photons are rather penetrating, the use of thin walled containers, by-passes in the system or other special arrangements can increase the sensitivity of this implementation. This is shown in Fig. 3, where bypass 8 permits continuous monitoring via MPD device 9.

Furthermore, in cases where sampling is avoided, the radioisotopes of Family 2, for which particularly penetrating photons are emitted from the positron annihilation or nuclear cascade, are of particular interest. The use of particularly penetrating photons which therefore more easily escape from the interior of the system being monitored, can, depending on the nature of the system, make it possible to avoid special arrangements like thin walls, by-passes and so forth.

When especially penetrating radiation is used, it is preferable to use detection techniques optimized for such radiation, such as high density/high atomic number scintillators. These include the scintillators BGO, Csl, BaF, and NaΙ(Tl). Furthermore it is particularly advantageous to operate such scintillators in the MPD mode, namely where background rejection is obtained by multiple photon coincidence and on-line pulse shape analysis.

In the case where a sample is taken, it may prove advantageous, as discussed above, to perform a step of concentration by physical or chemical means to improve the signal. A step of concentration can improve the signal even if it does not increase the number of radioactive atoms actually in the sample. This is because the concentration process, by reducing the total amount of material in the sample, will tend to reduce the unwanted radioactive background radiation, thus improving the signal-to-noise ratio of the detection procedure.

Physical concentration includes centrifugation to select on density, filtering to select on size, or distillation, particularly vacuum distillation, which selects on molecular or atomic weight. Chemical concentration exploits the chemical properties of the radioisotope(s) by standard purification and concentration procedures as well as the particularly sensitive techniques of chromatography and selective adsorption.

When radioactive signatures are employed, it is necessary to have an. apparatus which can distinguish the different radioisotopes being used. This apparatus detects and distinguishes various radioisotopes in order to determine their radioactive signatures, and performs the steps of analysis necessary to determine their proportions.

In addition the apparatus preferably includes steps of data processing and analysis to interpret these signals, including carrying out the compensation calculations discussed above in connection with "appropriate level". Finally, steps of data processing are necessary to match the signature with the records on file, preferably electronically, of which parts or elements have been doped with which signature. The steps of data analysis and processing are in addition to those commonly involved in the radiation detector itself, such as in the pulse shape analysis of MPD instruments or more conventional radiation counters. Automated carrying out of the above steps is often preferred. Automation is advantageous in large complex installations, allowing the instrumentation to be placed in difficult locations, and reducing certain risks of error connected with human intervention. An apparatus for automated on-line monitoring of a machine or an installation, preferably also includes means for issuing a warning when the monitoring of the tracer or tracers indicates critical levels of wear or damage, and furthermore means for indicating which part or element is in question by identifying a signature, if such has been used, with the part(s) or element(s).

In order to prepare the parts or components of a system for use by the present technique, they must be doped with radioisotopes. The optimal methods for doping depends on accuracy desired, metallurgy/chemistry of the radioisotopes involved, cost, and possibly other factors. Among the possibilities for incorporating the radioisotopes are dilution in a bulk material, ion implantation, lamination, neutron activation of a prepared volume, diffusion and electroplating.

In the case of the doping with a signature, the apparatus or procedure must be able to deal with more than one radioisotope. Observe that this type of apparatus may also lay down the markers or patterns for the guidance of an automatic tool. In this case the apparatus must be suitable programmed and/or used in conjunction with masks and templates.

In cases where a radioactive signature is used in a large complex system with many components, it is necessary to assign and keep track of many signatures involved. The apparatus for doing so comprises a data processing apparatus with appropriate programs and data bases. The means for guiding an automated tool or process according to the present invention comprises a fast, high sensitivity detector for the radioactive signal or signals, a means for data processing and analysis of the signals, and means, preferably electronic, for guiding the tool or process based on the signals and their analysis. Such an apparatus must be able to detect the signals in question rapidly, as is becoming possible with new ultra-sensitive detectors. Methods for incorporating radiolahels

The majority of parts for which the wear and corrosion are of importance can be divided into four groups. The first important group are plastics which are often mechanically soft but are very resistant to corrosion. Thus they are often used as protective layers. The second group consists of soft, often elastic materials such as parts made of rubber or silicon rubber. I the following, this group of elements is called elastomers. They are essential in many industrial applications as O-rings. The third group of materials consists of ceramic material often used because of their superior thermal, chemical and electric insulation properties. The fourth group of elements are the materials used in structural function, typically metals. These have intrinsic mechanical resilience, often mitigated by the susceptibility to chemical corrosion. In practical applications, the structural parts or elements are often coated or laminated by other materials with better chemical resistance or thermal properties, e.g. steel parts surface can be either hardened or covered with protective layer of chrome or nickel. Often, the protective layer made of plastics or elastomer is used to provide the increased resistance to chemical corrosion or to provide the required physical, e.g. optical properties.

Because of d e different chemical/physical properties of these four classes of elements, as well as due to their very different functionality, the methods for doping and the choice of optimal radiolabel are quite different. For doping of the plastics and elastomers, chemical methods are more appropriate and are disclosed in the following. More specifically, in the case of many plastics and elastomers, the useful life-time is often limited, for example O-rings are typically expected to be changed every few months. For parts with an expected life-time of a few monttis, the use of ¹²⁵I is preferred. For example, radioiodinated plastics can be formulated and admixed at very low level, say less than 1 ppb into the plastic from which the part is manufactured. There is a large family of plastics including fluor, e.g. teflon and many other plastics have chlorine as important element, e.g. PVC. For all these plastics the replacement of fluor or chlorine by iodine, especially ¹²⁵I is preferred. This use of radio-iodinated plastics is especially advantageous, because they can be produced in the same process of chemical synthesis by simple addition of iodine to fluor and chlorine at a very small level, say 0.001 ppb.

Optionally, the radioiodinated material can be introduced not into the bulk of the material to monitor the wear rate, but on the surface of the part to monitor integral wear, i.e. the thickness of material already removed from the surface. In this case the radioiodinated plastic can be used to plate, laminate or simply paint the object under study. The thickness of the labeled layer can be so small, say a few microns, that the external dimensions of the part are essentially within mechanical tolerances. Also, because the radioiodinated material can be formulated with essentially the same chemical composition, there is no difference in chemical, thermal and mechanical properties of the part.

The use of natural, synthetic or silicon rubbers is necessary in many mechanical systems. The rubber is a heterogeneous material, characterized by its elasticity and some porosity. The molecules of natural and synthetic rubbers can be easily covalently labeled, e.g. with ¹²⁵I. Thus, similarly to the plastic parts, the label can be either introduced into the bulk material or introduced as a known thickness layer upon the surface of prefabricated part. In the last case, the use of physical procedures of painting, spraying, lamination and vulcanization is disclosed. Furthermore, rubbers are heterogeneous materials characterized by their porosity on the micron scale. Thus, the appropriate chemical agent containing the radioiodine in gas or liquid form can in a controlled way be introduced into the rubber down to a few micron thickness. Herein, it will covalently react with the rubber, leading to a permanently conjugated radiolabel. The methods of control of the deptii of penetration of the said radiolabel are disclosed, and involve the control of the temperature, pressure and time of the conjugation process. Note, that by using two or more chemical radiolabeled precursors, the concentrations and depth of specific radiolabeled layers can be controlled.

Almost all radioisotopes can be used to dope plastics and elastomers. Typically, the appropriate chemical compound of me used radiolabel can be obtained in solid forms and granulated, say with typical diameter of grains below one micron. It can be subsequently introduced into the bulk elastomer through a plurality of physical processes. More specifically, it can be admixed into the liquid monomers which are subsequently polymerized. For thermoplastics, the granulated radiolabeled material may be introduced into material at elevated temperature and casted or extruded subsequently. Finally, colloidal "paints" can be prepared consisting of selected elastomer and finely granulated radiolabeled material. It can than be deposed on the surface of the prefabricated part made of elastomer. In the case of rubbers, the said colloidal liquid paint can be under pressure pressed into the material partially filling the voids and retained herein by a combination of surface adsorption or polymerization methods. Once more, by control of the temperature, pressure, the paint viscosity and grains size, the depth into which the radiolabel penetrates can be reliably controlled.

The majority of industrial ceramics are obtained by die steps of thermal treatment, including firing, melting and sintering. As initial material one uses the plurality of easily mixable components, e.g. highly granulated materials in the case of sintering. Often, the highly viscus fluid is used, e.g. in porcelain fabrication process. In the case of ceramics, the radiolabels can be introduced physically, by admixing the granulated radiolabeled material into the compounds from which the ceramic material is obtained by appropriate thermal process. Furthermore, most ceramics have an absorbing surface which can be coated by a thin (a few microns) layer containing the radiolabels by means of painting or spraying. Furthermore, in the case of ceramics, the external surface can be coated by a thin layer of radiolabeled ceramic material of essentially identical properties by appropriate glazing procedures.

In the case of structural parts, physical and metallurgic methods are more efficient than chemical methods of conjugation. Fortunately, also the list of possible radioisotopes is very large. For example, in the family of lanthanides there are about 65 EC isotopes characterized by very different life-times and emission energies. Also, among platinides there is a plurality of useful radiolabels. ^Ti is an example of EC isotopes with very long life-time (t_1/2 = 1,000 years). Some of the available metallic radioisotopes emit multiple very high energy gammas, e.g. ²²Na, ^Co and ⁶⁵Zn. Additionally, we note that an isotope of chromium ⁵¹Cr, can be used in MPD technique. Thus, the bulk of the metal parts can be easily doped with appropriate radioisotopes by the standard means of metallurgic processes, including alloying the metal with microtraces of the metal whose radioisotope is used. Furdiermore, a plurality of processes exist which permit coating the surface of the metallic part with a thin, physically resistant layer of another metal or alloy. One process is the use of an elecfrodeposition process in which the radiolabel is introduced in trace concentration into the solution. In this process, the use of electrochemically deposed radioisotopes of iron, nickel, chrome, cobalt, vanadium, cooper, silver, lanthanides, gold and platinides is disclosed.

Similarly, in some cases, the use of vacuum deposition is useful, especially for coating the surface of smaller, either flat or essentially cylindrical objects. Platinides and heavy metals are especially practical. Finally, the surface treatment by sputtering and ion implementation is very useful, especially for lower atomic number radioisotopes, such as ²²Na. In the case of small atoms, by the appropriate choice of accelerating potential, the density of doping can be modified inside a very thin, sub-micron depth of radio-ion penetration. By use of two radio-ions with different masses and atomic radius, the radioisotope density of such two tracers can reliably modulate the doped diickness. Thus, by measuring the ratio of these two radioisotopes, a few percent precision on the wear/corrosion depti can be obtained. Similarly, the surface diffusion of some small radioactive atoms in metal can be used. Herein, me metallic object is heated to close to melting temperature in the environment of the high pressure gas containing the traces of radiolabel which leads to controllable process of gas diffusion across the metal surface. Typically, the exponential penetration profile can be obtained, with typical penetration depth of a part of microns. Once more, by using a plurality of diffusing radiolabels additional information about the wear /corrosion pattern can be obtained. Finally, we disclose the use of a plurality of processes to introduce the different radiolabels into the same metallic element. For example, the surface of the metallic element can be doped with ²²Na and then coated with thin elecfrodeposed layer of chrome including trace amount of ^Co.

There are a number of practical techniques by which the radiolabels can be introduced into a bulk part. Even larger is the number of techniques by which the surface is modified to introduce the radiolabels. The number of combinations of these techniques permit some level of understanding of the pattern of wear and/or corrosion and is claimed in this invention.

Sampling of liquids

In fl is implementation, the wear of the part or the component is monitored by periodic sampling of the lubricant, cooling fluids or other liquids. Mechanical wear or corrosion leads to constant shedding of some material from the surface of the part under study into the liquid, e.g. lubricant. The turbulent flow of the material carries the detritus leading to an essentially homogeneous distribution in the liquid. After a preset time a small fraction of the liquid, (typically between 0.1 and 1 %) is sampled and presented to the measurement system. Please note that the sampling may be done either periodically or following a statistical model for "expected" preventive maintenance time based on the calculated load of die part. This calculation can be quite sophisticated.

In die further processing of the sample, the important information is whether mechanical wear or chemical corrosion dominates. In the first case, the appearance of microscopic granulate of the material, often with sub-micron size, is expected. Typically, mis material will have different physical parameters, e.g. density, dian the liquid itself. Thus, a step of physical concentration by either filtration or centrifugation can be used. In some cases, me physical concentration step can be facilitated by the exchange of liquid, e.g. dilution of oil in heavier but lower viscosity liquid such as frichloroethylene. Thus, in the simplest case of physical concentration, the about 1 ml sample of oil is diluted in 100 ml of frichloroethylene and after short decantation the bottom 1 ml of me liquid is subsequently measured with very low background radiation detector. With a simple centrifugation step this technique can be shortened to less than a minute.

On the other hand, when the dominating effect is chemical corrosion, chemical concentration methods are preferred. In this case, d e difference between the chemistry of the corroded material, e.g. metal, and hydrocarbons are used; chemical adsorption is a good example. Finally, me appropriately concentrated sample with typical volume of a less than 1 ml is placed into a detector. When the step of evaporation of the sample is used, much lower volumes, say less than 50 microliters can be obtained leading to the possible use of spatially resolving detectors and considerably increasing the throughput of the method.

Another disclosed implementation uses the in-situ concentration by means of filters or other means of adsorption. Subsequently, the said absorbing element is removed and measured in a low radioactive background detector. In the case of mechanical wear, such an absorption element may be simply a filter trapping all sub-micron particles. In case of chemical corrosion, more sophisticated absoφtion elements should be used, e.g. using the chemical affinity of the corroded material and specially prepared surface of the absorbing device. In machinery, the lubricant is forced to circulate within the system. Thus, by appropriate placement of the filter, the radioisotope can be integrated over a long time upon me absorbing element surface. With the typical preventive maintenance time of weeks/months, each fraction of lubricant flows through/by the absoφtion element hundreds of thousands of times. Up to 99% of all radioactive signal can be trapped in such a configuration.

This implementation has a considerable advantage in that diere is no liquid radio-waste handling, i.e. almost all radioactive waste is solid and easy to remove. Such in-situ concentration allows much higher throughput because the absorbing element can be physically very small, say only a few millimeters in diameter. Thus a plurality of absorbing elements each coming from diverse devices can be ordered in an appropriate pattern and quantitated using a spatially resolving, low background detector. When using in-situ concentration the absorbing element can be removed in one place and quantitated in an other place. Thus in-field preventive maintenance is facilitated and is a preferred mode of wear /corrosion testing.

Use of MPD technique

The proposed method of wear and corrosion detection is practicable only if die total amount of radioactivity used is negligible, so that there is no radiation hazard to personnel and no environmental contamination. Thus, ulfralow radioactive background detectors should be used. In the Multi-Photon Detection (MPD) technique, for enhanced performance mode, which is based on coincidence and more stringent pulse shape analysis, the detection efficiency (DE) is somewhat lower (10% vs 50%), while the background is lower by a few orders of magnitude. For this enhanced mode the performance depends on die sample size. For standard 12 mm diameter sample tubes the DE is 6.5 % and the background is 0.25 cph, yielding an effective background of 4 dph. For small samples (4 mm in diameter or less) me DE is 5 % and the background is 1 count per week which is equivalent to an effective background of 3 dpd (decays per day). In this enhanced sensitivity mode it is thus possible to detect ¹²⁵I sources with activities of about 10 dpd, i.e. containing less than a thousand ¹²⁵I atoms. MPD techniques also provide a broad dynamic range. Linearity over many orders of magnitude is a very important advantage for all industrial applications. Unfortunately, the majority of currently used radioanalytical instruments have limited dynamic range. Typically, the response of die detectors is limited both at low levels and at high counting rates. Thus, instead of the desired linear response, the characteristic detector's response is an S-shaped curve. For example, at low count rates, well detectors are seriously limited by the intrinsic background. At the high activity levels, pile-up or optical interference distorts the linearity of prior-art detectors. MPD instruments are close to ideal even for high count rates, up to about 1,000,000 cpm. MPD devices features linearity over 9 logs dynamical range. At higher count rates (above 500,000 cpm) saturation may be caused by pulse pile-up in me scintillator. Although the response is no longer linear in this range, the dead time of the MPD is non-extendable, so that counting can be performed and the results corrected for pile-up. At low count rates the linearity of response is limited by the background.

Prior-art detectors can quantitate the EC sources down to about 100 pCi, wherein wim MPD instruments even 10 fCi sources are reliably detected. Thus, about 10,000-fold increase of sensitivity is possible when using MPD instruments.

Asymmetric MPD instruments

A symmetric sandwich configuration of MPD is driven by the symmetry of decay of EC isotopes; there is no directional correlation between the photons and all of them are emitted into 360 degrees. For example, for ¹²⁵I the two photons are actually of almost the same energy, which further facilitates the construction of symmetric detectors. Actually, the use of any odier geometry for symmetric decay was showed experimentally to be inefficient.

However, in applications of MPD to wear /corrosion monitoring applications, there are some additional constraints, namely the isotopes should be of long life-time, chemically compatible with specific materials and easily distinguishable one from another. Thus, the list of applicable EC radioisotopes is much shorter than the list of all EC radioisotopes compatible with the MPD technique. Between them are some radioisotopes in which the X-ray is soft, say < 50 keV, but the gamma-ray is much more penetrant, i.e. has energy higher than 100 keV. Actually, this class of soft X/hard gamma radioemitters are especially important for implementation of multi-labelling schemes, because the energy resolution of detectors is much better for energies higher than 100 keV.

However, the construction of a detector for soft X/hard gamma radioisotopes is more difficult then in the case when both X-ray and gamma ray are of low energies. It should be noted that for detecting die high energy gamma rays, the much thicker and preferably high atomic mass scintillators should be used. However, this eliminates CaF₂(Eu) for which the radioactive background is very low and background rejection especially efficient.

Industrial applications of MPD detectors with asymmetric configuration includes three classes of asymmetric devices. * the two detectors are made from different materials, e.g. two different scintillators can be used or one scintillator and one semiconducting detector are used;

* the two detectors are of different size, typically one very small/thinner to diminish background and one large to increase the detection efficiency;

* the two detectors have different functions, e.g. one is spatially resolving and other is simply a triggering device. Typically, the spatially resolved detector is partially obscured by an appropriate pattern of coded apertures and the second detector is wide open to die source.

The optimal configurations of MPD instrumentation for wear /corrosion measurements use a combination of tiiese elements. In a particular implementation of MPD detector optimized for the soft X/hard gamma EC radioisotopes, one of the detectors uses a thin, say 3 mm, CaF₂(Eu) scintillator and other detector uses thicker, say 1 cm, NaΙ(Tl) or Csl scintillator. This configuration leads to a factor two loss of detection efficiency. Both signal and signal/background are better than in "symmetric" MPD using two thin CaF₂(Eu). Also, the signal/background is better than in "symmetric" MPD using two thick NaΙ(Tl) scintillators. For such soft X/hard gamma radiolabels, a preferred implementation is one consisting of two phoswich detectors (phosphor sandwich). A phoswich detector consists of two scintillators with different characteristic scintillation time coupled to DSO. For example, a thin CaF₂(Eu) crystal with characteristic scintillation time of a few microseconds can be coupled in a diicker NaI(Tl) crystal with characteristic scintillation time of about 200 nsec. The pulse shape analysis permits to decide if the photon was stopped in CaF₂(Eu) or NaI(Tl). For industrial application of MPD we disclose the use of a phoswich made of CaF₂(Eu) and BGO scintillators.

Spatially resolving MPD CSR-MPD'l instrumentation

In many industrial applications, the throughput of wear /corrosion testing devices is very important because it diminishes the down time of often very expensive machinery. For example, it is clearly advantageous to sample the oil from many devices and quantitate the amount of apparent radiolabel(s) concurrently. Use of a multidetector MPD has considerable economical advantages and additionally, permits better calibration and comparison of measurements. Thus, an important feature of the disclosed application of MPD detectors for wear/corrosion measurements is the optimal use of their spatial resolution which permits the concurrent processing of samples from many wear /corrosion testing procedures.

A scintillator-based SR-MPD features low cost, excellent sensitivity and good spatial resolution of a few millimeters. SR-MPD devices (available from BioTraces Inc.) concurrently quantitate, with zeptomole sensitivity, up to 50 samples of material labeled with EC isotopes. An optimal SR-MPD device has background of 1 count per three days per sample, and enables detection of radioactivities as low as 5 femtoCi per sample. The spatial resolution is about 2 mm.

An SR-MPD device is an example of "asymmetric" MPD technology, wherein the spatially resolving detector consists of a NaI(Tl) crystal coupled to spatially resolving PMT.

However, the triggering detector is a CaF₂(Eu) scintillator coupled to another type of photomultiplier tube. All parts of the system are preferably optimized, e.g. type of scintillator, diameter of scintillator, thickness of scintillator, thickness of optical window, type and geometry of coded aperture mask. Scintillators convert the energy of the emitted particle into low energy photons, which are collected by photodetectors. These instruments use coincident detection and pulse shape analysis to reject background. A Digital Storage Oscilloscope (DSO), implemented as a low-cost two-channel 40 MHz 8-byte PC card permits on-line pulse shape analysis. For images of 50 samples, each with about one pCi activity, as SR-MPD device produces highly reproducible results with S/B > 100.

Remote sensing of selected radioisotopes In many cases the removal of liquid or an absoφtion element from the closed system is either too expensive or too dangerous. For example, when monitoring valve integrity in chemical production facilities, the circulating liquid is often either ultra-pure, highly corrosive or toxic. In the case of heavy machinery, e.g. ship engines, continuous rather than periodic wear/corrosion testing is more appropriate. In such cases, a variant of the in-situ concentration method disclosed above is preferred. The absorbing element is not removed, but interrogated from outside, i.e. remotely. The simplest implementation includes die absoφtion element essentially spanning the diameter of liquid/gas conduit. The low background detector is than placed close to the absorbing element.

For large diameter liquid conduits the size of the detectors should be compatible, which leads to considerable losses in detection sensitivity. Both detection efficiency and background are typically proportional to detector dimensions. Thus in the disclosed simple configuration, mere are complicated trade-offs between the diminished detection efficiency and increased radioactive background. Furthermore, in many applications, the liquid is under pressure. Wim large diameter liquid conduits, this means that the conduit walls are thick, which leads to increased absorption of photons emitted by radioisotope. Thus, in this implementation use of positron- gamma or nuclear cascade MPD compatible isotopes is favored, because EC radioisotopes always emit one X-ray, which is easily absorbed. Preferably modified liquid conduits are used, wherein the part in which the absorption element is placed is manufactured from low atomic number material to diminish the photon absoφtion. For example, beryllium, kevlar or other fiber reinforced plastic or aluminum can be advantageously used.

In industrial applications, it may be preferred to build a bypass with easy access and smaller diameter than the main liquid conduit. The absoφtion element can be placed in such bypass. The low background radiation detector can then either be affixed close to the absorption element, or can be placed there periodically. Note that the preferred use of MPD devices puts considerable constraints on the bypass geometry. Such an MPD device consists of two detectors back to back, and the bypass with resident absoφtion element should be placed between ese two detectors. Optionally, a T-like configuration can be used, as in Figure 6. In Figure 6, the absorbing or adsorbing element 20 resides inside die main flow conduit 21, but can be mechanically or elecfromagnetically moved from operational position (shown at 20) into measurement position 23 in T-structure 22. The extended part of the T-like structure is then placed inside die MPD detector 24, which may be a portable device.

When using a bypass or T-like structure, the absoφtion element may be geometrically much smaller than the diameter of the main liquid conduit. This is enabled by die fact diat liquid is circulating, i.e. efficient sampling is achieved even widi a small diameter absorbing element. As disclosed above, the material from which the bypass or the T-like structure is made should be made of low atomic number material such as beryllium, fiber reinforced plastic or aluminum. Note that use of a bypass or T-structure solves the problem of mechanical stability under pressure, and the walls of such structure can be quite thin, typically below 5 mm. Thus, the absorption of photons in the walls is minimized and EC radioisotopes can be used. This capability means mat a relatively small number of radioactive atoms appearing due to wear, corrosion and so forth can be detected. On the odier hand only small amounts of radioactivity, well below the natural background, need be employed so diat there are no health or legal impediments to its use.

The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art the best way known to the inventors to make and use the invention. Nothing in this specification should be considered as limiting the scope of the present invention. Modifications and variations of the above-described embodiments of the invention are possible without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the claims and dieir equivalents, the invention may be practiced odierwise than as specifically described.

Claims

160

WHAT IS CLAIMED IS:

1) A method of monitoring removal of material from an object comprising: providing a solid object comprising a radioactive tracer in a predetermined pattern, providing an ambient medium with which the solid object is in contact, monitoring the ambient medium for a change in level of the radioactive tracer corresponding to removal of material from the solid object, and determining the condition of the solid object based on die change in level of the radioactive tracer in the ambient medium.

2) A method according to claim 1, wherein the radioactive tracer is concentrated in an outer layer of the solid object, and die change in level of the radioactive tracer in the ambient medium is a reduction when the outer layer is worn away.

3) A method according to claim 1, wherein the radioactive tracer is concentrated in an inner layer of the solid object, and the change in level of die radioactive tracer in the ambient medium is an increase when the inner layer is exposed to the ambient medium. 4) A method according to claim 1, wherein the radioactive tracer comprises a characteristic signature combination of multiple radioisotopes.

5) A method according to claim 1, comprising providing a plurality of solid objects, each comprising a unique radioactive tracer, providing an ambient medium with which the solid objects are in contact, monitoring the ambient medium for a change in the level of the unique radioactive tracer for at least one of the solid objects, corresponding to removal of material from the at least one of the solid objects, and determining the condition of the solid objects based on the change in level of the unique radioactive tracer in the ambient medium. 6) A metiiod according to claim 5, wherein the radioactive tracer is a combination of multiple radioisotopes.

7) A method according to claim 5, wherein the determining step comprises associating the change in level of radioactive tracer with the condition of the corresponding solid object. 8) A method according to claim 1 , wherein the object has an expected working life and die radioactive tracer comprises a radioisotope having a half life within an order of magnitude of the expected working life.

9) A method according to claim 1 , wherein the determining step has a sensitivity of 100 picoCuries.

10) A method according to claim 1, wherein the radioactive fracer comprises a multiphoton emitter and the determining step comprises multiphoton detection.

11) A metiiod according to claim 1 , wherein the ambient medium comprises a lubricant for the solid object. 12) A method according to claim 1 , wherein the ambient medium comprises a fluid employed in a chemical process, and the object is at least partially consumed in die chemical process.

13) A method according to claim 1, wherein die solid object is a valve in a transport vessel containing the ambient medium. 14) A method according to claim 1, further comprising taking a sample of the ambient medium for monitoring according to a predetermined schedule.

15) A method according to claim 1 , wherein the monitoring of the ambient medium is performed continuously.

16) A method according to claim 1 , further comprising concentrating the radioactive tracer in the ambient medium.

17) A method according to claim 16, wherein the concentrating comprises centrifuging, filtering, evaporating, or distilling die ambient medium.

18) A method according to claim 16, wherein the concentrating comprises chromatography, selective absorption, or selective adsoφtion. 19) A method according to claim 1, further comprising processing the solid object to remove material and monitoring the progress of the processing by monitoring the ambient medium.

20) A memod according to claim 19, wherein the processing is cutting, polishing, grinding, machining, dry etching, or wet etching. 21) A method according to claim 19, wherein the radioactive tracer is located in a marker pattern of the solid object, and further comprising guiding the processing based on die change in level of the radioactive tracer.

22) A method according to claim 1 , further comprising a means for correlating the object, the radioactive tracer, and the predetermined pattern.

23) A method according to claim 1, further comprising a means for applying die radioactive tracer to the object in a predetermined pattern.

24) A solid object for contacting an ambient medium, comprising: a radioactive tracer having a characteristic signature, the fracer comprising a multiphoton emitter, and being located in a predetermined pattern such that removal of material from the solid object results in a change in level of the radioactive tracer in the ambient medium.

25) An object according to claim 24, wherein the predetermined pattern is a coating.

26) An object according to claim 24, wherein the object is a stationary part of a system. 27) An object according to claim 26 wherein the object is a gasket, seal, valve seat, cylinder, printing or lithographic plate, pipe, tube, valve, medical prosthesis, structural member, or protective layer or coating diereof.

28) An object according to claim 24, wherein the object is a moving part of a system, for moving in contact with the ambient medium. 29) An object according to claim 28, wherein the object is selected from a gear, bearing, turbine, wheel, piston, piston ring, switch, thread, cutting edge, or protective layer or coating thereof .

30) An apparatus comprising a solid object according to claim 24, and an ambient medium in contact wi i the solid object. 31) An apparatus comprising a solid object according to claim 24, further comprising a detector capable of detecting a change in level of die radioactive tracer corresponding to removal of material from the solid object.

32) An apparatus according to claim 31, further comprising means for sampling the ambient medium. 33) An apparatus according to claim 31, further comprising an analyzer that compares the level of radioactive tracer to an expected level.

34) An apparatus according to claim 33, wherein the radioisotope is a long life electron capture, positron-gamma, or nuclear cascade isotope, and the detector is optimized for emissions from such isotopes.

35) An apparatus according to claim 31 , further comprising an indicator diat issues a warning when the removal of material from the object is at a critical level, and identifies the object.

36) An apparatus according to claim 32, wherein the sampling means comprises a container made of low atomic number.

37) An apparatus according to claim 36, further comprising a detector encompassing the container.

38) An apparatus according to claim 36 wherein the container is a small diameter bypass comprising an adsorption/absoφtion element. 39) A method for labeling an object comprising applying a radioactive tracer to the object in a predetermined pattern.

40) A method according to claim 39, wherein the applying comprises combining the radioactive fracer with a coating medium and applying the coating medium to the object.

41) A method according to claim 39, comprising manufacturing the object from plastic and further comprising forming a radiolabeled plastic and admixing into the plastic from which the object is manufactured.

42) A method according to claim 39, comprising forming a radiolabeled plastic and applying it to die surface of the object.

43) A method according to claim 39, wherein the object is rubber and die applying comprises contacting the object widi die radioactive tracer in a liquid or a gas and allowing it to penetrate the object.

44) A method according to claim 39, wherein the applying comprises combining die radioactive tracer as a powder witii a liquid to form a suspension and forming the object from the liquid suspension. 45) A method according to claim 39, wherein the object comprises metal and die applying comprises electrodeposition of the radioactive tracer from a plating solution onto the object.

46) A memod according to claim 39, wherein the applying comprises vacuum deposition, sputtering, metal diffusion.