EP0558494A1

EP0558494A1 - Process and device for gamma-spectrometric autoradiography

Info

Publication number: EP0558494A1
Application number: EP91915470A
Authority: EP
Inventors: Michael KRÖNING; Hartmut Baumbach; Carola Winkler; Sabine Just
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1990-11-07
Filing date: 1991-09-16
Publication date: 1993-09-08
Also published as: WO1992008950A1; DE4035412A1

Abstract

Le procédé et le dispositif servent à la mesure d'une variation d'épaisseur de paroi, notamment d'une réduction d'épaisseur de paroi (14), sur la surface intérieure (6) d'un tube (2), à partir de l'extérieur. A cet effet, on applique sur la surface intérieure (6), un revêtement radioactif (10), de préférence sous forme d'un émetteur gamma. Pour des composants nucléaires provenant du circuit primaire d'une centrale nucléaire, un tel revêtement (10) est de toute façon présent. Pour la recherche de l'emplacement (xo) de la réduction d'épaisseur de paroi (14), la densité de flux de rayonnement (I) en fonction de l'emplacement (x) est mesurée le long de l'objet à contrôler (2). On mesure alors, sur ledit emplacement (xo) sur la surface extérieure (4), la densité de flux de rayonnement (I) en fonction de l'énergie (E); on obtient ainsi une courbe comportant des crêtes photoélectriques (P) et des crêtes Compton (C). On établit enfin une relation prédéterminée (V1, V2) entre une grandeur caractéristique de la crête photoélectrique (P) et de la crête Compton (C). Au moyen de courbes d'étalonnage (V1*, V2*), on en déduit la profondeur (t) de la réduction de paroi (14). Il est possible, de cette manière, de détecter quantitativement, non seulement des fissures situées à l'intérieur, mais également des surépaisseurs de paroi. Un procédé analogue peut être également utilisé par mesure des crêtes photoélectriques de deux nucléides différents.The method and the device are used to measure a variation in wall thickness, in particular a reduction in wall thickness (14), on the inner surface (6) of a tube (2), from from the outside. To this end, a radioactive coating (10) is applied to the interior surface (6), preferably in the form of a gamma emitter. For nuclear components originating from the primary circuit of a nuclear power station, such a coating (10) is anyway present. To find the location (xo) of the wall thickness reduction (14), the radiation flux density (I) as a function of the location (x) is measured along the object to be checked. (2). The density of radiation flux (I) is then measured as a function of energy (E) at said location (xo) on the exterior surface (4); a curve is thus obtained comprising photoelectric peaks (P) and Compton peaks (C). Finally, a predetermined relationship (V1, V2) is established between a quantity characteristic of the photoelectric peak (P) and the Compton peak (C). By means of calibration curves (V1 *, V2 *), the depth (t) of the wall reduction (14) is deduced therefrom. In this way, it is possible to quantitatively detect not only cracks located inside, but also excess wall thicknesses. A similar method can also be used by measuring the photoelectric peaks of two different nuclides.

Description

Method and device for gamma spectrometric autoradiography

The invention relates to a method for measuring a change in wall thickness, in particular a weakening of wall thickness, in a test object, which has an outer surface and an inner surface. It further relates to a device for measuring a local change in wall thickness in a test object, which has an outer surface and an inner surface.

The invention can be used, in particular, for measuring changes in wall thickness due to cracks or abrasion on the inside of pipes exposed to media, e.g. for testing radioactive components of the primary circuit in nuclear power plants or for testing components in conventional power plants, chemical plants, etc. It can advantageously be used wherever the possible change in wall thickness is inaccessible, that is to say on the inside of the test object, but where the measurement is to take place from outside, that is to say from the outside of the test object. The term "change in wall thickness" is generally understood here to mean any local increase or decrease in the wall thickness, for example a crack, e.g. due to stress corrosion, a

Leaching, detachment, material removal or thinning, but also welding, e.g. on a pipe later clad with asbestos and thus not easily accessible. The rapid and reliable localization and measurement (width, depth or height) of such wall thickness changes in a test object of otherwise the same wall thickness is of great importance for monitoring and service work, particularly in the nuclear sector.

Cracks can occur on the inside of test objects that are exposed to media, such as metallic walls and, for example, pipes in the primary circuit of nuclear power plants eg stress corrosion cracks, abrasion and other wall thickness reductions arise. The ultrasonic test method is a common non-destructive test method for finding such phenomena from the outside (outer surface of the pipe). With this test method, crack depths of more than 10% of the total wall thickness can be detected and measured if the test requirements (such as accessibility, acoustic isotropy, etc.) are met. In the case of pipes made of ferritic steels, for example, this method also works. In the case of pipes made of austenitic cast iron, however, the ultrasonic test method has to fail because there is an acoustic anisotropy. The ultrasonic test method must also fail where measurements cannot be made in direct contact between the test piece (pipe, etc.) and the detector.

The object of the invention is to provide a method and a device of the type mentioned at the outset with which - as an alternative to the known ultrasound methods and ultrasound devices - also a change in wall thickness, in particular a weakening of the wall thickness such as a crack or a Ablation, can measure. In particular, it should also be possible to locate an internal, and therefore not readily accessible, reduction in the thickness of the wall of a test specimen from the outside and to measure the depth of this reduction in thickness. This should be possible in particular with a pipeline. It should also be possible to design the method and the device such that they are suitable for nuclear service.

With regard to the method, the stated object is achieved according to the invention in that a) that a radioactive coating is provided on the inner surface or a radioactive component is provided in the inner wall, and b) that the energy spectrum emanating from the radioactive coating or the radioactive component affects the Size of the change in wall thickness is closed. An implementation which is important in practice provides that a) that the covering or the component contains at least two different radioactive emitters (in particular a nuclei mixture), for example cobalt 60 and cesium 137, b) that at the location of the change in wall thickness on the outer surface of the test object, the radiation flux density of the two radioactive radiators is measured as a function of the energy, resulting in two energetically separated photopeaks, each of which is assigned to one of the radiators, and c) that a predetermined relation between the two then occurs Photopeaks is formed, which relation is a measure of the size of the change in wall thickness.

In this case, the procedure can preferably be such that the ratio or the reciprocal ratio of the maximum value of the photopeaks or of the areas under the photopeaks is formed, which ratio is a measure of the size of the change in wall thickness.

In general, one will proceed in such a way that the relation or relationship formed is compared with stored calibration curves, from which the size of the change in wall thickness is derived, the calibration curves preferably being the ratio of the areas under the photopeaks as a function of irradiated light Show material thickness.

Another implementation which is important in practice provides: a) that at the location of the change in wall thickness on the outer surface of the test object, the radiation flux density of the radioactive

Occupancy or the radioactive component depending on the energy is measured, resulting in a photopeak and a Compton back, and b) that then a predetermined relationship between the photopeak and the Compton back is formed, which is a measure of the size of the change in wall thickness. The size of the change in wall thickness can then also be most easily determined here from the relationship formed with the aid of calibration curves (for example stored in a computer).

It is therefore assumed in the invention that a radioactive coating (or a radioactive component in the inner wall of the test object), preferably in the form of a gamma emitter, is present on the inner surface of the test object, including the weakening of the wall thickness. Now there are cracks on the inside of media-exposed pipes, e.g. in components of the primary cycle of nuclear power plants, occasionally already radioactive anyway, because technologically-related corrosion products have been deposited in them; their gamma radiation is then used in the present case for crack depth measurement. In other words: This embodiment of the invention is based on the fact that the radioactive occupancy or the radioactive component has already arisen through the operation of the test object in a radioactive environment, in particular in a nuclear power plant. If this is not the case, then according to another embodiment, the radioactive coating (or the radioactive component) is applied to the inner surface (or introduced into the inner wall) before the radiation flux density is measured. In other words, suitable radio tracers, in particular short-lived ones, can also be used to mark the cracks. This applies e.g. for testing components in conventional power plants, chemical plants, etc.

In the case of the other implementation which is of importance for practice, the already existing or newly applied covering can contain a plurality of different emitters, in particular a plurality of different nuclides of different radiation energy. The same also applies to the radioactive component that is already present in the inner wall or previously formed there. The photopeak and the Compton back of a selected one of these emitters are then used to form the predetermined relation. According to a further embodiment it is provided that the coating or the component is brought about by applying an inactive component to the inner surface or by introducing it into the inner surface, and then activating the inactive component, for example by means of neutrons or photos radiation.

It has already been shown that the radioactive covering or the radioactive component preferably comprises a mast emitter. In particular, this can be one with an energy greater than 1 MeV, such as cobalt 60 (Co-60). Cobalt 60 (half-life t _H = 5.27 a) also has two gamma radiation energies, namely at 1.17 MeV and 1.33 MeV, both of which can be used for the present purposes. But also cesium 137 (Cs-137) with a gamma radiation energy of 0.66 MeV and a half-life of t _H = 30a can still be considered. In-133m with a gamma radiation energy of 0.39 MeV can also be used as a tracer, since it has the short half-life t _H = 1.7 h.

The background radiation is preferably subtracted before the relation mentioned is formed. This can be done by measurement.

It had previously been assumed that the location of the change in wall thickness was known. In many cases, however, this will not be the case. Accordingly, according to a particularly preferred embodiment of the method according to the invention, it is initially provided that the radiation flux density is measured as a function of the location along the test object in order to find the location of the change in wall thickness. The recording will result in a diagram from which the type of change in wall thickness (eg inclined, deeply incised, narrow crack; eg wide, V-shaped crack; or eg wide notch) can be read on the inside of the wall. The combination of locating the change in wall thickness and determining its size in two successive measuring steps appears to be particularly important in the present case, since a reliable result is quickly achieved: after that, the radioactively marked inner surface is first scanned using nuclear measurement technology, for example by means of a computer-controlled manipulator. Radiation emanates from the radioactive gamma-ray covering on the crack flanks, which is measured in an energy-selective manner after it has been located and adjusted to the crack. From this energy-selective measurement, the crack depth is then determined by means of a suitable relation of characteristic sizes of the photopeaks of two radiators or else of the photopeak and the Compton peak (or Compton back) of a single radiator.

The measurement of the radiation flux density as a function of the location along the test object can be carried out in various ways. According to a first, particularly preferred variant, it is provided that a detector is moved along the outer surface for measuring the radiation flux density (sequential measurement), which is preferably equipped with a collimator. According to a second variant, it can also be provided that for the measurement of the radiation flux density, a detection system extending along the test object, e.g. comprising a film or an array of detectors is provided (simultaneous measurement). If the detection system comprises a film or an array of detectors, then a system of collimators with predetermined slots or holes should be assigned to the film or the array. The slot or hole collimators act as passive imaging elements.

It had already been pointed out that the present measuring method is based essentially on the determination of the photo peaks or else the photo peak and the Compton peak of the radiation from the area of the change in wall thickness. A predetermined relation is formed from characteristic quantities of both peaks, which is a clear measure for the

Residual wall thickness in the area under consideration and thus for the change in wall thickness. According to a preferred embodiment, photo and

Compton peaks proceeded in such a way that the ratio or the reciprocal ratio of the maximum value of the photo peak to the maximum value of the Compton printing is formed, which ratio is a measure of the size of the change in wall thickness. Instead, according to a further embodiment, the procedure can also be such that the ratio or the reciprocal ratio of the area under the photopeak to the area under the Compton spine is formed, which ratio is a measure of the size of the change in wall thickness. It is also possible that the difference between the amplitude values or the area values of the photopeak and the Compton printing is formed, which difference is a measure of the size of the change in wall thickness. The same also applies if two different photopeaks are evaluated. These operations are preferably carried out using a computer.

With regard to the device, the stated object is achieved according to the invention in that a) the inner surface is provided with a radioactive coating or the inner wall is provided with a radioactive component, the coating or the component containing at least two different radioactive emitters, for example cobalt 60 and cesium 137, b) that a detector is provided, bl) which responds to the radiation emanating from the radioactive coating or component, b2) which can be aligned with the location of the change in wall thickness, and b3) which the radiation flux density of the radiation in

Depends on the energy of the radiation, whereby a photopeak results for each radiator, c) that an evaluation device is connected to the detector, which identifies the photopeaks, which forms a predetermined relation between the photopeaks, in particular the ratio of the intensities or else Areas, taking into account calibration curves in particular, and d) that an output device is provided with which the size of the change in wall thickness can be represented quantitatively.

With regard to the device, the stated object is also achieved according to the invention in that a) that the inner surface is provided with a radioactive coating or the inner wall is provided with a radioactive component, b) that a detector is provided, b1) which detects the radioactive coating or component which responds to outgoing radiation, b2) which can be aligned with the location of the change in wall thickness, and b3) which measures the radiation flux density of the radiation as a function of the energy of the radiation, resulting in a photopeak and a Compton back, c) that at the Detector an evaluation device is connected, which identifies the photopeak and the Compton back and forms a predetermined relationship between the photopeak and the Compton back, which relationship is a measure of the size of the wall thickness change, and d) that an output device is provided with which the size the change in wall thickness can be represented quantitatively.

The evaluation device preferably comprises a multi-channel analyzer, to which a computer is assigned internally or externally. The computer ensures the formation of the predetermined relation from a characteristic size of the photo peak and a characteristic size of the Compton peak.

It is advantageous for the measuring accuracy if the evaluation device is set up in such a way that it eliminates disturbing lines, peaks and / or background radiation.

According to a further development of the device, a scanning device is provided which guides the detector over the outer surface to be tested. It is preferably provided that the Scanning device is operatively connected to the multi-channel analyzer, which can be switched to "discriminator operation".

It is of particular interest if the scanning device is set up to carry out a movement adapted to the geometry of the test object, in particular for a linear, meandering or circular movement. For reasons of cost savings, a scanning device known from ultrasonic testing technology, which is also known under the name "manipulator", can be used.

A secondary electron multiplier with scintillator crystal is preferably used as the detector.

The detector can be aligned perpendicular to the outer surface of the test specimen (tube). It preferably has a collimator, e.g. made of lead, which has a round or slot opening for the entry of radioactive radiation.

As an alternative to a single detector, it can also be provided that, in order to save scanning time, the detector comprises a number of energy-selective sensors which can be queried sequentially or simultaneously.

It has already been mentioned that often radioactive

Occupancy or the radioactive component on the inner surface or in the inner wall must first be generated before the actual measurement. For this purpose, a suitable device is provided on the test object.

Further advantageous configurations result from the following description of the figures.

The described method according to the invention and the device according to the invention have a number of significant advantages: a) In contrast to the ultrasound test methods, no direct contact between the radiation detector and the test specimen is required (such a contact is of course also possible). Measurements can therefore also be carried out at a certain distance, for example from the pipe to be tested, if the pipe is corroded, painted, thermally insulated on the outside or otherwise covered.

b) In contrast to the ultrasound test methods, the test specimen can consist of an acoustically anisotropic material. He can e.g. consist of a weld metal or austenitic steel at the point to be tested. This is particularly important for pipeline tests in the context of nuclear service or when testing chemical plants.

c) The method and device according to the invention can be used during the operation of the test specimen if there is a radioactive coating or a radioactive wall component.

d) Experiments have also shown that the depth of cracks can still be determined with sufficient accuracy if not only the inside crack, but also the inside of the test specimen is covered with an appreciable radioactive layer.

Embodiments of the invention are explained in more detail below with reference to 8 figures. Show it:

1 shows a device for gamma spectrometric car radiography of a tube; 2 shows a diagram in which the transmission intensity of two

Steel plates of different thickness depending on the energy of the transmitted gamma radiation is shown; 3 to 5 each show the transmission intensity of a steel plate with different forms of wall thickness weakening; 6 shows the dependence of the ratio V, the sum of the

Heights I of the two photopeaks P to height I des

P ^c

Compton peaks C from the crack depth t at different

Steel thicknesses d for Co-60 (calibration curves); 7 shows the dependence of the ratio V _{2 of} the sum of the

Areas of the two photopeaks P to area A of the

P ^c

Compton peaks C from the crack depth t at different

Steel thicknesses t for Co-60 (calibration curves); and

8 shows a calibration curve for depth resolution, the for a Cs-137 and a simultaneous Co-60 marking

Photo area ratio of the two emitters as a function of the remaining wall thickness y = d - t is shown.

According to FIG. 1, a test object 2, which has an outer surface 4 and an inner surface 6, is examined with the method presented here for gamma spectroscopic autoradigraphy for weakened walls. In the present case, test object 2 is a tube made of steel of thickness d, the longitudinal axis of which is designated by 8. The tube 2 is in normal operation, e.g. in a nuclear power plant, through which a radioactive medium such as water with corrosion products flows, which has left a gamma-emitting radioactive coating 10 on the inner surface 6, which is shown in the form of small dots. Instead, the assignment 10 for carrying out the present measuring method can also have been introduced in a preceding method step. Alternatively, however, it is also possible to introduce a radioactive component into the inner wall parts or into the inner wall 11 of the tube 2, that is to say in the area of the inner surface 6. This is illustrated by several small crosses 12.

In the present case, it is further assumed that there is a change in the wall thickness 14 in the wall of the tube 2, especially a weakening of the wall thickness or a crack in the depth t. In the example, the crack 14 has a rectangular cross section, is relatively thin and is oriented perpendicular to the tube axis 8. Of course, the crack 14 is also inside with the radioactive Assignment 10 provided. The present measuring method is not limited to this rectangular shape of cracks.

Further crack variants are shown later in FIGS. 4 and 5.

The crack 14 lies here at the point x, the coordinate x defining the direction parallel to the pipe axis 8.

A gamma emitter, in particular a radiator whose energy E is greater than 1 MeV, is preferably used as the radioactive coating 10 or component 12. As already explained, this can be Co-60, but also Cs-137 or In-113m.

At the location x of the wall thickness change 14, namely close to or directly on the outer surface 4, the radiation flux density I of the radioactive coating 10 or the radioactive component 12 is measured as a function of the radiation energy E by means of a measuring device 16. This measuring device 16 comprises a detector 18 which responds to the gamma radiation u * ^* - emanating from the radioactive assignment 10 or from the radioactive component 12. In the measurement I (E, x) mentioned here, the detector 18 is aligned with the location x of the change in wall thickness 14.

The detector 18 can be constructed differently. However, it preferably comprises a scintillation crystal 20 with a photomultiplier 22 downstream in a known manner.

It can be, for example, a NaJ (Tl) scintillator crystal. In the scintillation counter 20, 22, the gamma radiation is absorbed in the crystal 20, which is usually - as mentioned - a NaI or CsJ crystal activated with Tl and having a diameter of 30 to 70 mm and the same height. It is converted into a flash of light, the intensity of which is proportional to the energy of the absorbed quantum. This flash of light is conducted to the photocathode of the photo secondary electron multiplier 22. There is an output pulse p, which is counted, in one Multi-channel analyzer 24. It is counted here in a memory location under a channel number KN, which is proportional to the energy. Such a multi-channel analyzer 24 has, for example, 256 or 1024 channels.

In order to obtain a higher resolution (in particular for the locus curve detection described later, in which the count rate I (x) is determined as a function of the path x), it is advantageous to use a collimator 26. This consists e.g. made of lead and shields the detector surface except for a small window 28 in the form of a slot or a round hole. Thus, only the gamma radiation that comes directly from the source 10 or 12 to the detector 18 is detected.

On the display or screen 30 and / or printed out by the printer 32 of the multi-channel analyzer 24, representations I (E, x ₀ ) or I (KN) are obtained, which are shown qualitatively in FIG. In photopeak P, gamma quanta are detected which have come directly to the scintillation detector 18 from the source, ie the marked crack 14 on the inside of the tube 6. And in the Compton peak, in the Compton back or the Compton flank C gamma quanta are detected, which have received less energy due to the Compton effect in the material (eg steel) of the tube 2.

The present method is therefore based on the energy-selective detection of the gamma radiation which originates from the radioactive substance 10 which has penetrated into the crack 14 or is deposited on the surface 6 and which reaches the detector 18 after corresponding weakening in the material of the tube 2.

The following can be said about FIG. 2: The energy spectrum I (E) or I (KN) of a flat Cs-137 gamma source is shown, namely in curve a behind a steel plate of 10 mm thickness and in curve b behind one Steel plate 40 mm thick. The measurement time for the selected marking of 2 x 0.4 MBq / 7 mm ^{2 was} about 2 minutes for the entire spectrum. It became a Multi-channel analyzer with 256 channels used (KN = channel number).

An evaluation device 36 is connected to the detector 18 and consists on the one hand of the multi-channel analyzer 24 and on the other hand of a computer 38 connected to it. This evaluation device 36 is able to identify the photopeak P and the Compton back C and also to form a predetermined relationship between a characteristic size of the photopeak P and the same characteristic size of the Compton printing C. For example, the computer 38 forms the ratio V-, = I / I, where I is the height of the photopeak P and I is the height of the Compton peak C. Or the computer 38 forms the ratio V ₂ = A / A, where A is the area of the photopeak P and A _{c is} the area of the Compton peak C.

Calibration curves VI * or V2 * previously measured on samples are stored in the computer 38. Such calibration curves are shown for Co-60 in FIG. 6 and FIG. 7. These are calibration curves VI * or V2 * measured on steel for different steel thicknesses d (d = 20, 30, 40, 50, 60 and 70 mm) and for different crack depths t (t from 0 to 20 mm).

If the computer 38 has e.g. a ratio V, = 0.20 determined from the measurement in a steel tube 2 with a thickness d = 30 mm for Co-60 radiation, the comparison of the computer 38 with the stored calibration curves VI * shows that V-, heard a crack depth t of about t = 13 mm. This is illustrated by two arrows in FIG. 6 and is recorded as a result.

An output device 40 is also provided on the computer 38, with which the size t = 13 mm of the change in wall thickness 14 determined from C and P can be represented quantitatively, for example as a value on a paper strip or as a bar on a screen. The following generally applies to the storage of the calibration curves: The energy spectra for different nuclides are recorded at different transmission thicknesses d and the ratio of the height I of the photopeak P to the height I of the Compton peak C or the ratio of the area A of the photopeak P is determined Area A _c of Compton peak C. Since the pipe wall thickness d is known, the difference (dy) between the pipe wall thickness d and the radiation thickness y gives the crack depth t: t = d - y. The calibration curves obtained from this are independent of other influences, such as activity and form of error, because of the ratio formation. In principle, ratios other than I _Q I _C or A / A are also conceivable, for example on the basis of a focus or another weighting.

In the present case, the computer 38 fulfills another important function: it controls a scanning device 44, which guides the detector 18 over the outer surface 4 of the pipe 2 to be tested. This can be, in particular, a scanning device 44 which is known per se from ultrasound testing technology and which is sometimes also referred to as a “manipulator”. In the present case, it ensures a step-by-step linear movement of the detector 18 in the x-direction parallel and at some distance from the outer surface 4 of the tube 2. A meandering run along the tube 2 is of course also possible.

The detector 18 can in particular be intelligence-led. That is, where little or no intensity I is registered, the detector 18 runs comparatively quickly in order to save measurement time.

The computer 38 can thus be designed in such a way that it contains material data such as the attenuation coefficients for the gamma radiation energies present for the radioactive marking of the crack flanks, wall thicknesses etc., as well as test specimen data, Geometry, material etc. can be entered. A computer program can be written for the computer 38, with which the computer 38 uses this to determine the test steps for the computer-controlled manipulator 44.

FIGS. 3 to 5 show diagrams I (x) for steel with a thickness of d = 30 mm and for three different crack variants A, B and C with a crack depth of t = 10 mm. Variant A corresponds to the shape of crack 14 known from FIG. 1. Variant B is an oblique crack, and variant C is a V-shaped crack. The recordings were made with a scintillation detector 18 with a collimator 26 with a slot 28. There was a measuring point at a distance x = 5 mm, and the measuring time per measuring point was 60 sec.

It can be seen from FIGS. 3 to 5 that this slow collimator scanning technique can even be used to make statements about the shape of the crack 14 in question, because the three measurement curves reflect the shape of the crack in question quite well and characteristically.

With the aid of the scanning device 44, the tube 2 can therefore be scanned quite quickly for changes in the wall thickness 14. In addition, with repeated slow scanning at the point of interest x, this time with collimator 26, information about the type of wall thickness change 14 in question can be obtained.

In summary, it can be said that one will generally proceed in two steps: First, the intensity I (x) will be measured (with or without collimator 26) in order to find the location x _{Q of} the change in wall thickness 14. Then one will measure the intensity I (E, x _Q ) at this location x _{0 in} order to determine the depth t (with a decrease) or also height (with an increase) from the characteristic sizes of the Compton peak C and the photopeak P. Close wall thickness change 14 quantitatively. A somewhat more sophisticated testing technology, especially for nuclear components, provides for the following five steps:

1. Measurement or calculation of the radiation flux densityj determination of the scanning speed.

2. One-time spectro etration on the tube 2 to select the photo peak P of a suitable nuclide which is present in the coating 10 or as an inclusion 12 in the tube material. For further work, the measuring device is set to the energy of this photopeak P.

3. Scanning the tube 2 with scintillator crystal at a constant speed in the area of the photopeak P of the selected nuclide in order to quickly find any

Cracks 14 or any other wall thickness reduction. The scanning can e.g. be carried out in a test grid (= distance of the measuring points) of 10 mm x 10 mm. The gamma radiation flux density I is determined as a function of the location x.

4. When such an error is detected, ie when a predetermined count rate I _m is exceeded at a specific location X _Q , there is punctiform scanning with scintillator detector 18 and collimator 26 to detect the shape of the crack and the local extent of the crack.

5. Recording the gamma spectrum I (E, x _Q ) at this location XQ to determine the crack depth t from a relation of characteristic sizes of Comptonpeak C and Photopeak P by means of calibration curves; this recording is expediently also carried out with a collimator 26 to increase the local resolving power.

It is advisable to record the background spectrum and the spectrum at an error-free point x and then subtract them from the individual measured spectra. The method, like the device, can also be used for pipeline systems other than in nuclear technology, for example in chemical plants, if these systems are marked with a radioactive tracer (radiotracer).

In principle, halogen silver films (which detect non-energy selectively) and gamma counter tubes are also suitable for the position-sensitive measurement of gamma radiation. As a result of the higher detection probability, priority is given to the scintillation counter 18. Another

The advantage is its small mass and mechanical insensitivity. With a scintillation counter 18, electronically working amplitude classification makes it possible to work in an energy-selective manner in a simple and direct manner, which is necessary for the present gamma spectrometric autoradiography. The principle of which is most easily implemented by less readily available components, namely by a scintillation counter 18 (including high-voltage supply), a multi-channel spectrometer 24 and by computer-controlled manipulator technology 44.

The further implementation, which is important in practice, will be briefly explained with reference to FIG. It should be noted here that the comments made above in FIGS. 1 to 7 are to be applied analogously to this further implementation. This implementation assumes that the source of the radiation detection is a nuclide mixture or isotope mixture of at least two emitters. With these emitters it can be, for B. are Cs-137 and Co-60. Here, the two energetically far apart lines (photopeaks) of the two radioactive nuclides are used. The ratio of the photopeaks Pl, P2 of the two nuclides as a function of the irradiated material thickness is used to determine the residual wall thickness or crack depth. The ratio (maxima, areas) of the two photo lines is therefore measured via the background radiation. Since the attenuation coefficient is energy-dependent, this ratio will change at the crack location x; it will be a measure of the crack depth t. The apparatus shown in FIG. 1 can be used with minor modifications (see claim 21).

FIG. 8 shows the calibration curve from the ratio of the photopeaks Pl, P2 of Cs-137 and Co-60. W * = Ap (Cs) / Ap (Co) is plotted as a function of the residual wall thickness y = d - t, where Ap (Cs) means the area under the photopeak of cesium and Ap (Co) the area under the photopeak of cobalt. Because of the different gamma energies of the nuclides, these nuclides Cs, Co have different attenuation coefficients. This means that the nuclide with lower gamma energy is weakened more when passing through the same wall or material thickness than that with higher energy. That is, Cs-137 is weakened more than Co-60. As a result, the ratio W of the photopeak area Ap (Co) of Co-60 to the photopeak area Ap (Cs) of Cs-137 increases with increasing irradiated layer thickness. Analogously to FIG. 6, the course of the reading of y with a measured relation W is represented by two arrows.

Claims

1. A method for measuring a change in wall thickness, in particular a weakening of the wall thickness (14), in a test object (2) which has an outer surface (4) and an inner surface (6), characterized in that a) that on the inner surface (6 ) a radioactive occupancy (10) or in the inner wall (11) a radioactive component

(12) is provided, and b) that the size (t) of the change in wall thickness (14) is inferred from the energy spectrum emanating from the radioactive occupancy (10) or the radioactive component (12).

2. The method of claim 1, d a d u r c h g e k e n n ¬ z e i c h n e t, a) that the coating (10) or the component (12) contains at least two different radioactive emitters (in particular a Nuclide mixture), for example. Cobalt 60 and cesium 137, b) that at the location (X) of the change in wall thickness (14) on the outer surface (4) of the test object (2) the radiation flux density (I) of the two radioactive radiators as a function of the energy (E ) is measured, resulting in two energetically spaced photopeaks (P1, P2), each of which is assigned to one of the emitters, and c) that a predetermined relation (W) is then formed between the two photopeaks (P1, P2) which relation (W) is a measure of the size (t) of the change in wall thickness (14).

3. The method according to claim 2, characterized in that the ratio or the reciprocal ratio of the maximum value (1 ^, I ₂ ) of the photopeaks (Pl, P2) or of the areas (A ^, A ₂ ) among the photopeaks (Pl , P2), which ratio is a measure of the size (t) of the wall thickness change (14).

4. The method according to claim 2 or 3, dadurchge ¬ indicates that the relation (W) or the ratio formed is compared with stored calibration curves (W *), from which the size (t) of the change in wall thickness (14) is derived , the calibration curves (W *) preferably representing the ratio of the areas under the photopeaks (P1, P2) as a function of the irradiated material thickness (d).

5. The method according to claim 1, characterized in a) that at the location (x) of the change in wall thickness (14) on the outer surface (4) of the test object (2) the radiation flux density (I) of the radioactive assignment (10) or the radioactive component (12) is measured as a function of the energy (E), resulting in a photopeak (P) and a Compton back (C), and b) that a predetermined relation (V ,, V ₂ ) is formed between the photopeak (P) and the Compton back (C), which is a measure (y) for the size (t) of the change in wall thickness (14).

6. The method according to any one of claims 2 to 5, characterized in that to find the location (x _Q ) of the change in wall thickness (14), the radiation flux density (I) is measured depending on the location (x) along the test object (2).

7. The method of claim 6, d a d u r c h g e k e n n - z e i c h n e t that for measuring the radiation flux density

(I) a detector (18) is moved along the outer surface (4) (sequential measurement), which is preferably equipped with a collimator (26).

8. The method according to claim 6, characterized in that for measuring the radiation flux density (I) a along the test object (2) extending Nach- white system, e.g. B. a movie or an array of

Comprising detectors, is provided (simultaneous measurement).

9. The method of claim 8, d a d u r c h g e k e n n - z e i c h n e t that the detection system comprises a film or an array of detectors, and that the film or the array is associated with a system of collimators with predetermined slots or holes.

10. The method according to any one of claims 5 to 9, d a d u r c h g e k e n n z e i c h n e t that the ratio (v or the reciprocal ratio of the maximum value

(I) of the photopeak (P) to the maximum value (I) of the Compton-P c back (C) is formed, which ratio (V-) is a measure of the size (t) of the wall thickness change (14).

11. The method according to any one of claims 5 to 9, characterized in that the ratio (V ₂ ) or the reciprocal ratio of the area (A) under the photopeak (P) to the area (A) under the Compton¬ back (C) is formed which ratio (V ₂ ) is a measure of the size (t) of the change in wall thickness (14).

12. The method according to any one of claims 5 to 9, that the difference in the amplitude values (Ip, Ic) or the area values (Ap,

A _c ) of the photopeak (P) and Compton printing (C) is formed, which difference is a measure of the size (t) of the change in wall thickness (14).

13. The method according to any one of claims 5 to 12, characterized in that the covering (10) or the component (12) contains several different radioactive emitters (in particular nuclides), and that to form the relation of the photopeak (P) and Compton back (C) of a selected one of these radiators is used.

14. The method according to any one of claims 1 to 13, that the radioactive coating (10) or the radioactive component (12) is applied to the inner surface (6) or introduced into the inner wall (11) before the measurement.

15. The method according to any one of claims 1 to 14, that the radioactive occupancy (10) or the radioactive component (12) by the operation of the test object (2) in a radioactive one

Environment has arisen, especially in a nuclear power plant.

16. The method according to any one of claims 1 to 15, characterized in that the coating (10) or the component (12) comes about in that an inactive component is applied to the inner surface (6) or introduced into it, and that the inactive component is then activated, for example by neutron or photon radiation.

17. The method according to any one of claims 1 to 16, so that the radioactive coating (10) or component (12) comprises a gas emitter, in particular one with an energy greater than 1 MeV such as Cobalt 60.

18. The method according to any one of claims 2 to 17, characterized in that the background radiation is subtracted before the formation of said relation (V-V ₂ ).

19. The method according to any one of claims 5 to 18, characterized in that the formed relation (V ^, V ₂ ) is compared with stored calibration curves (VI *, V2 *), from which the size (t) of the wall thickness change ( 14) is derived, the calibration curves (VI *, V2 *) preferably the ratio of the area (Ap) under the photopeak Represent (P) to the surface (Ac) under the Compton back (C) depending on the irradiated material thickness (d).

20. Device for measuring a change in wall thickness in a test object (2), which has an outer surface (4) and an inner surface (6), characterized in that a) that the inner surface (6) with a radioactive coating (10) or the inner wall (11) is provided with a radioactive component (12), the covering (10) or component (12) containing at least two different radioactive sources, for example cobalt 60 and cesium 137, b) that a detector (18 ) is provided, bl) which responds to the radiation (y ") emanating from the radioactive coating (10) or component (12), b2) which can be directed to the location (x _Q ) of the change in wall thickness (14), and b3) which measures the radiation flux density (I) of the radiation as a function of the energy (E) of the radiation, resulting in a photopeak (P1, P2) for each radiator, c) that an evaluation device (18) is connected to the detector (18) 36) is connected, which identifies the photopeaks (P1, P2) which forms a predetermined relation (W) between the photopeaks (Pl, P2), in particular the ratio of the intensities or the areas, and which takes into account calibration curves in particular, and d) that an output device (40) is provided with which the size (t) the change in wall thickness (14) can be represented quantitatively.

21. Device for measuring a change in wall thickness in a test object (2), which has an outer surface (4) and an inner surface (6), characterized in that a) that the inner surface (6) with a radioactive coating (10) or the inner wall (11) with a radioactive component 1 nente (12) is provided, b) that a detector (18) is provided, bl) which responds to the radiation (V ") emanating from the radioactive coating (10) or component (12),

5 b2) which can be aligned with the location (x _Q ) of the change in wall thickness (14), and b3) which measures the radiation flux density (I) of the radiation as a function of the energy (E) of the radiation, where, ₀ is A photopeak (P) and a Compton back (C) result in c) that an evaluation device (36) is connected to the detector (18), which identifies the Photopeak (P) and the Compton back (C) and that a predetermined relation (V,, V ₂ ) between the photopeak (P) and the Compton back (C)

, - det which relation is a measure of the size (t) of the change in wall thickness (14), and d) that an output device (40) is provided with which the size (t) of the change in wall thickness (14) can be quantitatively represented . 0

22. The apparatus of claim 20 or 21, d a d u r c h g e k e n n z e i c h n e t that the evaluation device (36) comprises a multi-channel analyzer (24) to which a computer (38) is assigned internally or externally. 5

23. The device according to claim 20, 21 or 22, so that the evaluation device (36) is set up in such a way that it eliminates disruptive lines, peaks and / or background radiation. 0

24. The device as claimed in one of claims 20 to 23, that a scanning device (44) is provided which guides the detector (18) over the outer surface to be tested (4). 5

25. The apparatus of claim 22 and 24, characterized in that the scanning device (44) is operatively connected to the multi-channel analyzer (24) which can be switched to "discriminator operation".

26. Device according to one of claims 24 or 25, since ¬ characterized in that the scanning device (44) is designed to carry out a movement adapted to the geometry of the test object (2), in particular for a linear, meandering or circular movement, and that it is in particular a scanning device (manipulator) known from ultrasonic testing technology.

27. Device according to one of claims 20 to 26, d a - d u r c h g e k e n n z e i c h n e t that the detector

(18) comprises a secondary electron multiplier (22) with scintillator (20).

28. Device according to one of claims 20 to 27, d a - d u r c h g e k e n e z e i c h n e t that for saving scanning time the detector (18) comprises a number of energy selective sensors which can be queried sequentially or simultaneously.

29. Device according to one of claims 20 to 28, that the detector (18) can be aligned perpendicularly to the outer surface (4) and preferably has a collimator (26).

30. Device according to one of claims 20 to 29, since ¬ characterized in that a device is provided on the test object (2) via which the radioactive assignment (10) on the test object (2) or the radioactive component layer (12) in the test object (2) can be generated.

31. The device according to any one of claims 20 to 30, that the test object (2) is a tube which consists in particular of an austenitic material.