GB2326232A - Gamma ray density profiling for filling level measurement - Google Patents

Abstract

In a density profile sensor 1 a spatially resolved density measurement (particularly determination of the position of the oil/water boundary 6c) is carried out using a plurality of gamma radiators 2 arranged vertically above one another and an elongate optically conducting scintillator 10. The vertical position of sensed gamma rays is determined from comparison of the delayed coincidence of the two portions of the scintillation flash 11 with the photo detector(s)and thence a density profile of the oil 6b and the water 6a is determined from gamma absorption. The scintillator can have photo detectors at both ends or one photo detector and a reflector. Calibration methods, important system parameters for configuring the sensor 1 and measures for reducing crosstalk between the gamma radiators 2 (eg shielding, energy discrimination of Compton scattered gamma rays and arrangements of multiple scintillators 10) are further specified.

Description

1 2326232

TITLE OF THE INVENTION Method and device for measuring filling levels using gamma radiators and a virtual linear detector arrangement

BACKGROUND OF THE INVENTIO

Field of the invention is The invention relates to the field of fillinglevel displays. It is based on a method and device for measuring filling levels according to the preamble of claims 1 and 10.

Discussion of Backgroun The prior art contains a considerable number of methods and devices for determining the filling level in a container, which are based on widely varying physical measurement principles. These comprise electrical (capacitive or resistive) and optical methods, radar-reflection methods, ultrasound delay- time methods and gamma-absorption methods.

Offshore exploitation in the oil industry involves the use of so-called separation tanks, in which the different phases (sand, water, oil and gas) which occur during drilling or production separate on account of their difference in density into individual layers lying on top of one another. It is in this case very important to ascertain the level at which the separation layer between the water and oil is located, in order to permit controlled opening and closure of the discharge valves for the two media on the tank.

This requires reliable filling-level gauges. If such a filling-level gauge does not work or goes wrong, then for example oil can get into the water discharge and cause extensive environmental damage and expense.

Separation tanks have recently been developed 40 which are suitable for operation on the sea bed some hundreds of metres below the surface of the sea. The extracted oil can then firstly be separated from the contaminating water, sand, etc. and only after this has been done be pumped to the surf ace of the sea with greatly reduced energy expenditure. However, the demands on such separator tanks are very great. Externally, they must withstand the water pressure at the sea bed, and internally the pressure of the extracted oil, typically 60 - 180 bar, as well as temperatures of 50 - 1200C. The system for measuring the filling level is also subjected to these severe operating conditions. Nonetheless, functional integrity for years on end, substantially without maintenance and with the utmost reliability, must be guaranteed, since operational failure and premature replacements would entail exorbitant costs. It is therefore necessary to measure the filling level using at least two redundant systems. In a prior German patent application (Reference No. 20 19704975.3), not yet published at the priority date of the present application, one solution proposed for this problem is a capacitive measuring probe which, in particular, exploits the abrupt change in the dielectric constant at the interface between oil and water. The advantage over commercially available systems consists, amongst other things, in the fact that the measuring probe is self- contained, measures the ambient medium without contact by means of stray capacitance and in this way determines the filling level.

An example of an additional redundant measuring method is the measurement of density through gamma-ray absorption. There are commercially available instruments with a gamma source (caesium, cobalt, etc.) and a scintillator (NaI with thallium-doping (NaI:Tl), plastic, etc.) as a gamma detector. From its nucleus, the gamma radiator emits high-energy photons, or gamma rays, which are absorbed in matter. The absorption depends exponentially on the length travelled, the - 3 absorption coefficient being proportional to the density (at least for monochromatic gamma rays). In the scintillation detector, the gamma quantum produces a shower of photons in the visible or adjacent spectral range. The photons are converted by a photodetector (photomultiplier, PIN photodiode, etc.) into an electrical signal. As an alternative to a scintillator, it is also possible to use a Geiger-MfAller tube. For the measurement of filling levels, the source and the scintillation detector are moved synchronously up and down on opposite walls of the separator tank, and the vertical density profile of the content of the tank is recorded. The positions of the interfaces between the different media are determined on the basis of the differences in density. one disadvantage of this system consists in the fact that it is suitable only for lowpressure separator tanks at the surface of the sea, because the mechanism needs to be inspected and serviced, and the very great demands which are made in this case in terms of operating reliability can only be met with difficulty.

According to Fig. 1, it is also prior art to make a density-profile sensor without moving parts, by arranging a pair of gamma radiators and detectors at vertical intervals and, from their gamma transmission values, determining the filling level of the differently absorbing media.

Further, scintillator detectors with one- or two-dimensional spatial resolution are known and are offered on the market. Scintillators are available in the form of rods which, for example, contain NaI:Tl. The light waves emanating from the scintillation flash are detected at both ends of the rod and the location of the gamma-photon absorption is determined from the ratio of their intensities or pulse amplitudes, which decrease exponentially with the optical path length. Spatially resolving scintillators are also made using plastic fibers in extensive parallel or crossed arrangements. As also disclosed by WO 85/04959, the 4 spatial information is readily obtained by identifying that fiber or those fibers which deliver a scintillation flash to a photodetector.

SUMMARY OF THE INVENTION

Accordingly, one object of the invention is to provide a novel gamma-ray density-profile sensor for a system and a method for measuring filling levels, which is distinguished by good vertical spatial resolution and a simplified yet very robust structure. This object is achieved according to the invention by the features of claims 1 and 10.

The essence of the invention is in fact to fit a rod-shaped, optically conducting scintillator detector facing an arrangement of a plurality of gamma radiators, and, using a photodetector at one or both ends of the scintillator, to detect jointly the gamma rays from a plurality of sources. A density profile is determined from the count rate of the scintillation flashes as a function of the difference in delay time of the light components propagating in opposite directions. The vertical spatial resolution is in this case essentially given by the distance between the gamma radiators and their emission angle or the collimation quality.

An illustrative embodiment illustrates a f irst density-profile sensor with two vertically aligned closed tubes for the gamma sources and the scintillator rod, as is particularly suitable for a separator tank in the oil production industry.

Further illustrative embodiments represent variants of the scintillator rod with end photodetectors on one or both sides.

Other illustrative embodiments show measures according to the invention for reducing the crosstalk between the gamma sources.

One advantage of the gamma-ray density-profile sensor according to the invention is its reduced complexity and susceptibility to interference. Primary advantages are the simplicity, mechanical robustness and inherent reliability of the scintillator rod or scintillator fibers in comparison with customary gamma detectors.

A further advantage consists in the fact that it is possible to achieve sufficient spatial resolution in measuring the filling levels because the crosstalk between different gamma radiators can be offset by 10 various measures.

It is especially advantageous that a likewise non-contact and sensitive method for measuring filling levels is provided, providing a redundant alternative to electrical capacitance measurement and.based on a is different, fully independent measuring principle as well as being substantially without need of maintenance.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when 25 considered in connection with the accompanying drawings, wherein:

Fig. 1 shows a section through a system for measuring filling levels, with a linear arrangement of gamma radiators and separate detectors (prior art);

Fig. 2 shows a section through a system according to the invention for measuring filling levels, with a linear arrangement of gamma radiators and a single scintillator rod; shows a section through the scintillator rod according to Fig. 2, with a photodetector at the upper and lower ends; Fig. 4 shows a section through the lower end of the scintillator rod with a retroreflector Fig. 3 6 alternative embodiment to according Fig. 3; shows a section through the scintillator rod with an input collimator according to an alternative embodiment to Fig. 2; Fig. 6 shows a section through a system according to the invention for measuring filling levels, with a linear arrangement of gamma radiators and two scintillator rods; shows the arrangement of a system according to the invention for measuring filling levels, inside a separator tank.

Fig. 5 Fig. 7 is to an DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, Fig. 2 shows an excerpt of a first illustrative embodiment of a gamma- ray density-profile sensor according to the invention. The gamma rays 5 from a plurality of radioactive sources. or gamma radiators 2, are collimated by output collimators 3, pass through the medium 6 to be measured and, with a given response probability, cause scintillating light flashes 11 in a scintillator 10. By virtue of the particular design of the scintillator 10 as an elongate optical wave guide, the scintillation flashes are split into two light components which propagate to the two ends of the scintillator. As can be seen in Figure 3, each light wave is received by a photodetector or photodetector array 14, 15, converted into an electrical signal and fed via lines 9 to measuring electronics 16. In a so-called delayed coincidence measurement, the precise time difference of the pairs of associated photodetector signals are determined by the measuring electronics 16. From the time delay or propagation time difference of the light components, the place scintillation flash 11 is determined, of origin of and two the the - 7 responsible gamma radiator 2 is identif ied theref rom. For each time delay or gamma radiator position, the frequency of the light pulses is also counted, and on account of the strong densitydependence of the gamma absorption, this makes it possible to determine the medium 6 through which the radiation has passed. In particular, the difference in density between water 6a(1015 kg/m3) and oil 6b(850 kg/m3) is detectable. The presence of gas and sand may also be demonstrated in this way. The frequency distribution of the pairs of successive photodetector signals, ordered according to their time delay, is therefore a direct measure of the density profile of the medium in the separator tank 17. Through dynamic determination of this frequency distribution, an instantaneous density profile can be measured even for a relatively fast-flowing medium, and the sedimentation process can be monitored.

Fig. 4 shows a detail view of another embodiment of the scintillator 10. At the lower end of the scintillator 10 there is, instead of the photodetector 15, a retroreflector 18 which, for example, may be designed as a cubic or plane mirror'. Both light rays emanating from the scintillation flash 11 are fed via different paths to a single photodetector or photodetector array 14 at the upper end of the scintillator, and are processed as before by the measuring electronics 16. Owing to the reflection, the wave originally propagating downward undergoes stronger attenuation, as well as a greater propagation time delay. The spatial resolution can be doubled by means of this. Further, the scintillator 10 is simplified by omitting one of the photodetectors 15 and the associated electrical signal line 9, and is particularly suitable for being installed in applications which are accessible from only one side.

Fig. 3 illustrates a further embodiment, in the case when the scintillator 10 is composed of two rods, which are sep arated by a layer (not shown) which reflects on both sides. A scintillator 10 composed in 8 this way therefore has an upper half and a lower half, in which the light propagates entirely independently. The reflecting layer is preferably half-way up in the scintillator 10. When there is a scintillation event in one half of the scintillator, the optical pulses are evaluated as in the illustrative embodiment according to Fig. 4. The advantage over Fig. 4 consists in the fact that, for the same spatial resolution, the optical paths are halved and therefore the optical attenuations are considerably reduced. In general, positional determination using propagation time measurement instead of by comparing optical attenuations is also advantageous because the latter method is very insensitive to ageing-induced changes in the optical attenuation in the scintillator 10.

The absorption of high-energy gamma photons is based on the photoelectric effect, Compton- scattering and pair creation. In the photoelectric effect, the gamma photon is fully absorbed by the electron cloud of an atom, and detaches a preferably strongly bound electron. Through further collisions, the electron loses its kinetic energy, so that the full gamma energy Ey is given up in the medium. Above the strongest bond energy of the electrons, the probability for the photoelectric effect decreases greatly with increasing incident gamma energy.

In the range between Sy = 100 keV and 1 MeV, the Compton effect is dominant: in the event of a collision between a gamma photon and an electron, the gamma photon continues on at a scattering angle a with residual energy Ryl (&). With an increasing angle of deviation &, the relative residual energy of the gamma photon R(&) = Ey' (4)/E. decreases according to the equation R(G) = [1 + ú - (1 COSGH-1 (1) 1 E = Ey/S11 keV denoting the ratio of the gamma energy to the rest-mass energy of the electron.

- 9 Further, above 1 MeV, the gamma photon can also, by interaction with the electric field of an atomic nucleus, or of an electron, decay into an electron/positron pair. With increasing atomic number, the probabilities for the photoelectric effect and pair creation increase greatly, and moderately for the Compton effect. Further, the absorption coefficient is proportional to the density of atoms. The resulting material-specific density dependence of the gamma absorption is used as a measuring principle in gammaray density sensors. In principle, all three processes for the dissipation of the gamma energy play a part as a detection effect in the scintillation detector and as an interference effect, for example in the container walls 4, 24.

The number of gamma radiators 2 and their spacing are preferably chosen in such a way that a density profile can be determined with the desired length and spatial resolution. In general, the gamma radiators 2 are arranged essentially vertically above one another and preferably equidistant. If the position of the oil/water boundary layer 6c is known, then 'a smaller spacing can be chosen for the gamma radiators, in order to improve the spatial resolution locally. Factors involved in the choice of the radioactive elements for the gamma radiators 2 are, in particular, the half-life, gamma energy and activity. An example of a well-suited element is 137Cs, with a half-life of 30 years and a monochromatic gamma line at 30 660 keV. It is also possible to use 60CO with 5.3 years and gamma emission at 1.17 MeV and 1.33 MeV, or other radiators or mixtures. The activity of the sources, that is to say the number of gamma photons emitted per second, depends on the amount and decay probability of the radioactive substance. The activity is chosen to be large enough so that, on the one hand, enough scintillation events occur at the scintillation detector 13 per second for accurate and fast density determination, and on the other hand the pairs of - 10 successive photodetector signals can still be readily discriminated. In this case, the losses due to collimation and gamma absorption on the path to the scintillator '10 and the response probability of the 5 scintillation detector 13 should be taken into account.

The centerpiece of the scintillation detector 13 is the scintillator 10 which has to detect gamma rays from a plurality of gamma radiators 2. According to the invention, the scintillator has for this purpose an elongate shape, conducts light, is optically connected at its ends to photodetectors 14, 15 and delivers temporarily separated light pulses or electrical photodetector signals for delayed coincidence measurement to the measuring electronics 16. In the scintillation process, a gamma photon excites the scintillator 10 through the aboVedescribed interaction processes to emit short scintillation light flashes 11 whose intensity is proportional to the energy given up. The scintillator 10 may contain an inorganic material, in particular NaI:Tl in crystalline or polycrystalline form, or an organic material in crystalline, liquid or plastic-like form or a preferably doped glass. Important design parameters for a desired scintillator length and spatial resolution are availability in lengths of up to 2 m and mechanical stability, optical attenuation and the decay time of the scintillation, i.e. the optical pulse width. Round or angular rods of plastic, with NaI:Tl are particulary suitable, as well as optical fibers or optical fiber bundles, in particular plastic fibers or plastic fiber bundles, or a combination of rods and fibers. Plastic scintillator rods may also be bonded together in the desired length from a plurality of parts, possibly with optical junction adaptors. The optical wave guides have typical optical attenuations of about 10-2cm-1. In order to reduce the light losses, the rods may be provided on their lateral surface with a reflecting coating. For a spatial resolution of 10 cm, differences in propagation time of about 1 ns must be detectable, 11 which makes it necessary to have pulse widths in the ns range or smaller. Suitable examples f or this are, in particular, plastic scintillators and correspondingly fast photodetectors or photodetector arrays 14, 15 with detector surf aces matched to the cross section of the optical wave guide. PIN photodiodes are preferred in order to save space.

Lastly, a maximum and minimum optical pulse rate may be indicated for the density-profile sensor according to the invention. For a scintillator length of 2 m, the maximum difference in propagation time is ns. A minimum time difference between the pairs of light signals of about 0.1 gs then corresponds to a maximum pulse rate of 107s-1. For a scintillator length 8 1 of 20 cm, the maximum value is 10 s-. On the other hand, for measuring the density profile with an accuracy of 11k, it is necessary to detect 104 pairs of light signals per gamma radiator 2. For one densityprofile measurement per 10s, this gives a minimum pulse rate of 103s-1 multiplied by the number of gamma radiators 2 which are detected by one scintillator 10. One density-profile measurement per 100 s may still be sufficient, which gives a lower limit of 102s-1 for the minimum pulse rate. The total activity of the gamma radiators 2 pointed at a scintillator 10 is thus preferably chosen in such a way as to produce an optical pulse rate in the region of 102s-1 _ 108s-1, in 3 -1 - 7 -1 particular in the range 10 s 10 S.

The calibration of the gamma-ray density- profile sensor comprises two steps. The synchronicity of the measuring electronics 16, i.e. of the two measuring channels for determining the propagation time delays, can be monitored, and if appropriate can be corrected, even during operation, using a calibration source 12 which, according to Fig. 2, is fitted close to the scintillator 10. This guarantees that the measuring electronics 16 are free of drift and stable in the long term. Suitable calibration sources 12 include pulsed light or scintillation sources with 12 - optical input coupling in both directions of the scintillator 10. In particular, weak gamma or alpha radiators, for example americium with an equivalent gamma energy of 60 keV, may be used.

The density measurement is calibrated using reference measurements with water, oil, etc. In this case, it is in principle possible to use individual calibrations for each gamma-radiator position to take into account different activities of the gamma radiators 2, different levels of gamma-photon loss or inhomogeneities in the scintillator 10. The measuring electronics 16 thus preferably comprise means for individually calibrating the optical pulse rates caused by each gamma radiator 2.

is The crosstalk between two different gamma radiators 2 further represents a considerable problem. The primary interference effect is due to Compton scattering in the steel walls 4. The irradiated parts thereby become secondary gamma sources with isotropic emission. Depending on the distance from the output collimator 3 to the scintillator 10, the deflected gamma photons can impinge on the scintillator 10 at an arbitrary position, and vitiate the position and density measurements. Countermeasures which the invention proposes are shields, input collimators 19, energy discrimination for the optical pulses and arrangements having a plurality of scintillation detectors 13. Fig. 5 shows an embodiment of an input 30 collimator 19 with an opening. The collimators 3, 19 typically consist of lead, which is a very efficient absorber of gamma radiation. The openings of the input collimators 19 face the output collimators 3. The collimators 3 and 19 are preferably mutually parallel, in particular horizontally oriented and directed at one another at each level along a common connecting line. The collimators may have any desired shape, for example round, angular, canted, etc. For the input collimators 19, b denotes the overall width and t the depth. The 13 crosstalk is considerably reduced even for a vanishingly small depth t, because the scintillator 10 receives gamma photons only at the openings. With increasing collimator depth t, the shielding against 5 scattered gamma photons can be improved further. Ignoring multiple scattering, the shielding is ideal if a collimator depth t > b - L/a is chosen, a being the distance between two nearest -neighbor gamma sources 2 directed at the same scintillator 10, and L being the path length between the output collimator 3 and the scintillator 10. A further measure for reducing the crosstalk consists in mounting horizontal shields 20, for example lead plates, in the medium 6 between two gamma radiators 2.

The detection of gamma photons scattered at a large angle of deviation S can also be prevented by energy discrimination for the scintillation flashes 11. The intensity or energy distribution of the scintillation events is given by the exciting gamma energy and the probabilities for the photoelectric effect, Compton effect and, if appropriate, pair creation in the scintillator 10. The brightest flashes are due to the photoelectric effect of the primary gamma photons which undergo no interference along their path, and form the so-called photopeak. Weaker flashes result from the secondary gamma photons, in particular from the steel walls 4, and also from primary gamma photons which undergo Compton scattering in the scintillator and then escape. By cutting off the scintillation spectrum below the photopeak, it is therefore possible to limit the detection of Comptonscattered gamma photons, albeit at the cost of the count rate. On account of the optical attenuation, the discrimination threshold must be chosen as an essentially exponentially decreasing function of the optical propagation path. The spatial resolution can be increased with an increasing discrimination threshold and gamma energy. In order to prevent overlap between the scattering cones of different gamma radiators 2, - 14 the following relationship satisfied 18 1 < arctan [a/ (2. L) 1 must approximately be (2), a again being the source separation and L the path length. As an example, a threshold 5% below the photopeak and E = 660 keV will be chosen. Equation (1) with R(G) > 95% gives 181< 160, and equation (2) with a path length L - 30 cm gives the spatial resolution a -- 17 cm.

By combining energy discrimination with input collimators 19 even of small depth t, the measuring accuracy can be improved further. For small scattering angles iGI< arctan [(a-b/2)/L)].

(3) the deviated gamma photons are blocked by the shields between the openings of the input collimators 19. In addition, a dscrimination threshold should be designed such that the residual energies of the gamma photons scattered in a neighboring input collimator 19 are below the threshold. As an example, a threshold 10% below the photopeak and Zy = 660 keV will be chosen.

Equation (1),with RG9)> 90% gives 1GI< 240 and equation (3) with b 2 cm and L Pz 30 cm gives a spatial resolution a 14 cm, with the width of the output collimators 3 relative to their spacing a being neglected in (3). In this configuration, the width b of the input collimators 19 and the descrimination threshold can thus be matched to one another, so that they reinforce one another in their shielding effect against scattered gamma photons. The input collimators 19 can thus be made wide and space-savingly short. In this way it is possible to achieve high spatial resolution with very little crosstalk at relatively high count rates. In practice, crosstalk is satisfactory in the context of measurement accuracy, so that relationships (2) or (3) need to be satisfied only approximately.

Fig. 6 discloses a further embodiment with twice the spatial resolution of Fig. 2 or with reduced crosstalk. In this case, the density-profile sensor 1 consists of a gamma radiator tube 21 with output collimators 3 oriented alternately in opposite directions, and two scintillator tubes or probes 22, each with an elongate scintillator 10. Each is, as before, connected to 14, 15 and measuring electronics 16 one of the preceding illustrative and can be provided with the scintillator photodetectors according to embodiments, abovementioned reduction or particular, it measures for calibration, crosstalk improvement of spatial resolution. In is also possible to use a plurality of probes 22 in a ring around a gamma radiator tube 21 with gamma radiators 2 directed at it alternately. For example, three probes 22 may be arranged at 1200.

Fig. 7 shows one possible way of fitting a gamma-ray density-profile sensor 1 in a separator tank 17. Typical dimensions of a, for example, cylindrical tank 17 are 2-3 m in diameter and 10 m in length. The sensor 1 comprises at least one gamma-radiator tube 21 and at least one probe 22 according to one of the above illustrative embodiment s. 'Only one of the tubes 21, 22 can be seen in Fig. 7. The tubes 21, 22 are connected to the separator tank 17 such that gamma photons at least partially pass through the medium 6 in the tank 17 on their way from the source 2 to the scintillator 10. During normal operation, the tubes 21, 22 are intended to extend to such a depth that, in particular, it is possible to detect the position of the boundary layer 6c between oil and water. The tubes 21, 22 are preferably mounted in tube holders 24 which project into the interior of the separator tank 17 or pass completely through the interior (not illustrated); alternatively, the tubes 21, 22 can be attached to the wall of the separator tank 17 or can be immersed directly in the medium 6 in the separator tank17, although, once again, this is not illustrated in Fig.

7. One advantage of the tube holders 24 is that the probe 22 or the gammaradiator tube 21 can be replaced even during operation of the separator tank 17.

overall, the invention discloses a measuring system for filling levels having a gamma-ray density profile sensor 1 which is robust and sensitive for measurements. The use of a scintillator 10 to detect a plurality of gamma rays from discrete sources 2 greatly reduces the complexity, and therefore the susceptibility of the sensor 1 to interference, and a more compact structure is produced. With additional measures for shielding and energy discrimination, it is possible to achieve very high spatial resolution.

obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

LIST OF DESIGNATIONS 1 2 3 4 5 6 6a 6b 6c 7 8 9 10 12 13 14 16 17 18 19 20 21 22 23 24 b t L a Ey c R(C) c Gamma-ray density-profile sensor (Section) Gamma radiators Output collimator Steel wall Gamma rays Medium Water Oil Oil/water boundary layer Gamma detectors Gamma-detector shielding Electrical signal lines Scintillator (rod, fiber, fiber bundle) Scintillation light flash Calibration source Scintillation detector Photodetector I Photodetector 2 Measuring electronics Separator tank Retroreflector Input collimators Horizontal shields Gamma-radiator tube Scintillator tube, probe Frame Tube holder Input collimator width Input collimator depth Path length Distance between gamma radiators Gamma energy Scattering angle Relative residual energy of the gamma photon Ratio of the gamma energy to the rest- mass energy of an electron

Claims (1)

1. CLAIMS:

   b) A gamma-ray density-profile sensor (1), in particular suitable for measuring the filling level in a separator tank (17), comprising a plurality of gamma radiators (2) which are arranged essentially vertically above one another, and at least one gamma detector (7), wherein a) the at least one gamma detector (7) has a scintillator (1) which is elongate and conducts light, the scintillator (10) is optically connected at one end or at both ends to a photodetector (14, 15), and the measuring electronics (16) comprise means for delayed coincidence measurement. The gamma-ray density-profile sensor (1) as claimed in claim 1, c) 2.

   wherein a) the gamma radiators (2) are arranged at equal distances along a vertical axis, b) each scintillator (10) comprises a rod or an optical fiber or an optical fiber bundle and extends along a parallel vertical axis, and C) the gamma radiators (2) have output collimators (3), each of which is oriented in the direction of a scintillator (10) and preferably horizontally.

   3. The gamma-ray density-profile sensor (1) as claimed in claim I or 2, wherein a) a retroreflector (18) is optically connected to one end of a scintillator (10), and b) one photodectector (14, 15) is optically connected to the other end of said scintillator (10).

   4. The gamma-ray density-profile sensor (1) as claimed in claim I or 2, wherein a) a scintillator (10) has a layer which reflects on both sides, preferably half-way up, and b) said scintillator (10) is optically connected to one photodetector (14, 15) at each of its two ends.

   The gamma-ray density-profile sensor (1) as claimed in any of claims 1-4, whereM a) a scintillator (10) has a calibration source (12), in particular a weak gamma or alpha radiator, for synchronizing the measuring electronics (16), the measuring electronics (16) comprise means for individual calibration of the optical pulse rate caused by each gamma radiator (2), the total activity of the gamma radiators (2) directed at a scintillator (10) produces an optical pulse rate in the range 102S -1 _ 10 's -', in particular in the range 10's -1 - 10's -'. The gamma-ray density-profile sensor (1) as claimed in any of wherein each scintillator (10) comprises a plurality of input collimators (19), and the input collimators (19) are aligned in the direction of the output collimators (3).

   7. The gamma-ray density-profile sensor (1) as claimed in any of claims 1-6, wherein the measuring electronics (16) comprise means for energy discrimination of the optical pulses, and the discrimination threshold is an essentially exponentially decreasmg fimction of the optical propagation path.

   8. The gamma-ray density-profile sensor (1) as claimed in any of claims 1-7, wherein a) one tube (21) contains the gamma radiators (2) and at least one other tube (22) contains the at least one scintillator (10), the tubes (21, 22) fit into tube holders (24) located in the separator tank (17), or the tubes (21, 22) dip directly into the medium (6) in the separator tank (17), and in particular, horizontal shields (20) are suspended in the separator tank (17) half-way up between the gamma radiators (2).

   9. The gamma-ray density-profile sensor (1) as claimed in any of claims 1-8, wherein b) c) 6. claims 1-5 15 a) a) b) b) c) a) b) the gamma radiators (2) contain 137CS and/or "Co, the at least one scintillator (10) contains an inorganic material, in particular NaLTI in crystalline or polycrystalline form, or an organic material in crystalline, liquid or plastic-like form or a preferably doped glass, in particular, the at least one scintillator (10) is a plastic rod which consists of a plurality of parts bonded together and has a reflecting coating on its lateral surface, and d) the photodetectors (14, 15) are PIN photodiodes.

   10. A method of measuring the filling level in a separator tank (17) using a gamma-ray density-profile sensor (1) as claimed in one of the preceding claims, wherein, a) scintillation flashes in the scintillator (10) are split into two components which are fed via two optical propagation paths to a photodetector (14, 15), b) accurate time measurements of Successive photodetector signals are made in the measuring electronics (16), the places of origin of the scintillation flashes are determined from the time delays of the photodetector signals, and the responsible gamma radiators (2) are identified therefrom, and the frequency distribution of the pairs of successive photodetector signals, ordered according to their time delay, is determined dynamically in the measuring electronics (16), and an instantaneous density profile of the medium in the separator tank (17) is calculated therefrom.

   11. The method for measuring the filling level in a separator tank (17) as claimed in claim 10, wherein a) the photodetector signals are discriminated according to their pulse amplitude in the measuring electronics (16), and the discrimination threshold is chosen as an exponentially decreasing function of the time delay, in such a way that the scintillation flashes (11) caused by the photopeak in the scintillator (10) are evaluated. Any of the gamma-ray density-profile sensors substantially as b) 12.

   hereinbefore described with reference to figures 2 to 7 of the accompanying drawings.

   13. Any of the methods for measuring the filling level in a separator tank substantially as hereinbefore, described with reference to figures 2 to 7 of the accompanying drawings.