Method for calibrating a radiation measuring density measuring device

Abstract

The invention relates to a method for calibrating a radiometric density measuring device. The radiometric density measuring device is used to determine and / or monitor the density of a medium (6) located in a container (1). The method comprises the following method steps: determining the count rate N0 of the radioactive radiation after it has passed through an empty container (1) on the basis of the activity of a sending unit (3); determining the measured count rate N of the radioactive radiation after it has passed through the container (1) when a calibration medium of known density is located in the container (1); determining the mass attenuation coefficient (μ) according to the formula μ = -(ln(N / N0))(ρ1D), wherein D: beam path of the radioactive radiation or inner diameter of the container (1), ρ1: density of the calibration medium; calculating a calibration curve which represents the dependence of the density of the medium on the count rate of the measured radiation density after the radiation has passed through the container (1).

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for calibrating a radiation measuring device for determining and / or monitoring the density of a medium located in a container, a transmitting unit being provided which emits radioactive radiation of a predetermined intensity, and a receiving unit which receives the radioactive radiation emitted by the transmitting unit after it has passed through the medium, and a control / evaluation unit being provided which determines the density of the medium located in the container on the basis of the measurement values determined by the receiving unit. BACKGROUND

[0002] Radiation measuring level or density measurements are used whenever the usually used measurement methods fail or can no longer be used. Radiation measuring density measurements are used, for example, in the process for producing aluminum from bauxite and for measuring the density of sludge, which usually contains rock, in the case of dredging work in marine or river environments. Radiation measuring density measurements are usually carried out in combination with flow measurements.

[0003] In radiation measuring density measurements, the medium located in a container (tank, silo, pipe, etc.) is usually irradiated with gamma radiation. The radiation is emitted from a gamma source and detected by a receiving unit (scintillator) which is positioned such that it detects the gamma radiation emitted by the transmitting unit after it has passed through the medium. Depending on the application, for example, a Cs137 or Co60 source is used as the gamma source. The receiving unit consists of a plastic or crystal, a photomultiplier and a receiving element.

[0004] The gamma radiation emitted by the transmitting unit is at least attenuated or weakened when passing through the medium and / or the container. The attenuation or weakening of the gamma radiation exhibits a functional dependence on the density of the medium present in the container. The attenuated or weakened gamma radiation impinges on the detector material of the detector unit and is converted there into light pulses which are detected by a detector, for example a photodiode. In order to determine the density, the number of light pulses generated by the gamma radiation when impinging on the detector material is counted.

[0005] Attenuation F of the gamma radiation when passing through the medium D This can be described by the following Beer-Lambert law:

[0006] F D = N / No = e -μi·D

[0007] Here, μί is the linear attenuation coefficient and D is the beam path. In the case of a radiation density measuring device with a pipe having a bend, the beam path for example corresponds to the inner diameter of the pipe, increased by twice the wall thickness value of the container. If the inner diameter of the pipe is much larger than the thickness of the wall of the pipe, the attenuation or weakening of the gamma radiation caused by the material of the wall of the pipe can be neglected. Alternatively, the weakening of the gamma radiation caused by the wall of the container can be determined experimentally with an empty container. This path is problematic in practice due to the high radiation incident on the detector. The weakening of the gamma radiation when it passes through the material of the wall of the container can also be calculated.

[0008] The linear attenuation coefficient is a function of the energy of the incident gamma radiation, the chemical composition of the irradiated medium and the density of the medium. By introducing a mass attenuation constant μ, which is derived from the linear attenuation coefficient μί, the dependence of the attenuation of the gamma radiation on the properties of the medium can be virtually completely eliminated. Under ideal conditions, the mass attenuation constant is independent of the density and the nature of the medium and thus depends only on the energy of the incident gamma radiation. This fact can be explained by the fact that the gamma radiation used in density measurements lies in the energy range of 0.5 to 0.6 MeV. In this energy range, the Compton effect, i.e. the inelastic scattering of photons at the electrons of the scattering medium, is the dominant effect. Therefore, the mass attenuation constant μ is constant when the energy of the photons that are irradiated and interact with the medium remains constant. Thus, μ = μί / ρ and N = No e -μ·ρ·D Result:

[0009] - In(N / No) = (μ·ρ·D).

[0010] Since the beam path D is a calculable constant size with respect to the predetermined container and since the mass attenuation constant is constant, -In(N / No) is proportional to ρ.

[0011] In order to allow a reliable radiometric determination of the density of a medium located in the container, a two-point calibration is carried out. For this purpose, in a first step, the container is filled with a first medium having a known density ρ1. The medium is irradiated with gamma radiation and the corresponding count rate N1 is determined. In a second step, the container is filled with a second medium having a known density ρ2, wherein the density of the second medium is preferably as different as possible from the density of the first medium. The count rate N2 is determined. The mass attenuation constant μ is calculated on the basis of the determined measurement values (count rates) and the known variables (ρ1, ρ2). A calibration curve of the radiometric density measuring device is determined using the determined variables and is stored in the density measuring device.

[0012] Radiometric density measuring devices can be calibrated individually via known methods. However, this two-point calibration requires a great deal of effort. The containers, tanks, silos or pipelines usually have a considerable volume, which is why a considerable amount of medium must be available for the calibration. While the calibration with water as calibration medium for the higher density range is still relatively unproblematic, filling with a second medium, for example oil, of lower density is usually only possible with a great deal of effort. In this connection, reference is made to measurement sites, for example in desert regions, which are difficult to access. Therefore, the calibration curve is usually determined on the basis of measurements of the count rate with only one medium (one-point calibration). The standard value of 7.7 mm 2 / g is then assumed as the second mass attenuation coefficient, which is necessary for calculating the calibration curve. This value is based on experience values. Although this halves the calibration effort, in individual cases this is at the expense of the measurement accuracy of the radiometric density measuring device.

[0013] Even the known two-point calibration, that is to say the determination of the mass attenuation coefficient on the basis of the determination of the attenuation of the gamma radiation when passing through two media with known different densities, is not very reliable, since in many applications the influence of the density of the two media on the mass attenuation coefficient is significantly lower than the influence of the geometric dimensions of the container and the geometric arrangement of the sending and receiving unit. SUMMARY

[0014] It is an object of the present application to provide a simple method for precisely calibrating a radiometric density measuring device.

[0015] This object is achieved by a method for calibrating a radiometric device for determining and / or monitoring the density of a medium located in a container, a sending unit and a receiving unit being provided, the sending unit emitting radioactive radiation of a predetermined intensity and the receiving unit receiving the radioactive radiation emitted by the sending unit after it has passed through the medium, and a control / evaluation unit being provided, which determines the density of the medium located in the container on the basis of the measurement values determined by the receiving unit, the method having the following method steps:

[0016] determining the count rate N0of the radioactive radiation after it has passed through the empty container (1) on the basis of the activity of the sending unit (3),

[0017] determining the measured count rate N of the radioactive radiation after it has passed through the container (1) when a calibration medium of known density is located in the container (1),

[0018] The mass attenuation coefficient (μ) is determined according to the formula μ = -(ln(N / N0)) / (ρ1D), wherein D: the beam path of the radioactive radiation or the inner diameter of the container (1), ρ1: the density of the calibration medium,

[0019] A calibration curve is calculated which represents the dependence of the density of the medium on the count rate of the measured radiation density after the radiation has passed through the container (1).

[0020] The method according to the application thus proposes a single-point calibration for calibrating a radiation measuring density measuring device, wherein only one measurement point is determined experimentally: the first count rate N0 required for determining the mass attenuation coefficient is determined by the activity of the sending unit. When designing a radiation measuring density measuring device, it is usually first to select the dose rate of the receiving unit, then to calculate the required activity of the sending unit in order to select the sending unit accordingly. The calculation of the activity of the sending unit is known to the person skilled in the art from the respective literature. The formula for calculating the activity of the sending unit can be reversed for calculating the dose rate of the detector when the container is empty, whereby the count rate N0 is obtained. The count rate N of the gamma radiation of the sending unit used is determined experimentally by filling the container with a medium of known density. Since there are thus two count rates N0 and N, the mass attenuation coefficient can be calculated directly and the calibration curve can be determined on the basis of the two count rates and the mass attenuation coefficient.

[0021] The experimental determination of the radiation intensity when the container is empty is usually excluded, since in this case a large over-irradiation of the detector occurs. The method according to the application has been found to provide very good results compared to the experimental measurement values obtained by two-point calibration.

[0022] In a possible refinement of the method according to the application, the count rate N0 of the radioactive radiation after passing through the empty container is determined according to the formula N0 = (P K B) / (F a ·F s ), wherein F a : a squared distance factor between the sending unit and the receiving unit, F s : an attenuation factor which depends at least on the wall of the container and the isotope of the sending unit, P: the activity of the sending unit, K: an isotope-dependent correction factor, B: a correction factor for converting the pulse rate into the count rate N0. If the sending unit is arranged at an angle to the receiving unit, the distance factor F a between the sending unit and the receiving unit can optionally be the average (squared) distance. The geometric arrangement of the radiation measuring density measuring device must be taken into account fundamentally. The activity P of the sending unit is isotope-dependent and needs to be corrected by the isotope-dependent correction factor K. The attenuation factor Fs The attenuation of the radioactive radiation is taken into account when passing through the wall of the container and other layers attached to the wall of the container. The attenuation of the radioactive radiation by the air inside the empty container is negligible. The correction factor B depends on the type of receiving unit and is required to convert the dose rate F i into N0. The dose rate F i originates from F i = (P · K) / (F a · F s ).

[0023] Water is preferably used as the calibration medium.

[0024] In a further refinement, the sending unit and the receiving unit are positioned relative to each other such that the container is irradiated perpendicular to the longitudinal axis of the container, oblique to the longitudinal axis of the container or parallel to the longitudinal axis of the container. The container is usually a pipe or a tank. Since gamma radiation also penetrates solids, the sending unit and the receiving unit are fastened to the outer wall of the container. The actual arrangement is selected depending on the application in question.

[0025] In a further possible refinement, a pipe is used as the container, the sending unit and the receiving unit being fastened to opposite surface areas of the pipe.

[0026] The receiving unit is advantageously designed and positioned such that the sensitive components of the receiving unit are hit by the radiation. In order to obtain the best measurement results, the receiving unit is designed and positioned relative to the sending unit such that the sensitive components of the receiving unit are hit by the radiation passing through the container.

[0027] In a further refinement, the attenuation factor F s is calculated according to the formula F s = e a·q , wherein a: isotope-dependent attenuation coefficient, q: thickness of the wall of the container measured in steel equivalent, wherein the steel equivalent is defined as the density of the wall of the container relative to the density of steel. In the case of an empty container, the intensity of the emitted radioactive radiation is mainly attenuated by the wall of the container. It can also be necessary to take into account any insulation layers for blocking gamma radiation or other layers applied to the container between the sending unit and the receiving unit, since these likewise reduce the intensity of the emitted radioactive radiation.

[0028] The intensity of gamma radiation decreases exponentially with increasing penetration depth into the irradiated medium. The half-value thickness denotes the distance covered by the gamma radiation in the material / medium at which the intensity of the gamma radiation is halved due to interaction with the material / medium, essentially Compton scattering. The half-value thickness depends on the wavelength of the gamma radiation and the atomic number of the irradiated material / medium. For steel, for example, the half-value thickness is known and is approximately the thickness of 14 mm of steel. The corresponding attenuation of the radioactive radiation can thus be calculated from the actual thickness of the steel wall of the container. If the wall of the container has a density (p steel ) different from that of steel (p wall ), a conversion must be made accordingly. For the calculation of q, the following formula can be used: q = (2 · L · p wall ) / p steel , where L is the thickness of the container wall. Further layers that attenuate the radioactive radiation can also be applied to the container wall. In general, a layer i can be introduced as an additional sum into the counter of the formula, so that q = (∑ i 2 · L · p i ) / p steel . The factor 2 takes into account the fact that the radioactive radiation passes through the container wall twice and can optionally be omitted. The attenuation coefficient a is adapted to the attenuation by the steel and the gamma source in question. BRIEF DESCRIPTION OF DRAWINGS

[0029] The application is explained in more detail below with reference to the following drawings, in which: Figures 1 to 2

[0030] Figure 1 : a schematic diagram of a device for radiometrically determining the density of a medium is shown.

[0031] Figure 2 : a schematic diagram of a graphic visualizing the dependence of the count rate on the density is shown. DETAILED DESCRIPTION

[0032] Figure 1 A schematic diagram of a device for radiometrically determining the density of a medium 6 located in a container 1 (here a pipe with an arm 2) is shown. A receiving unit 4 and a transmitting unit 3 with a gamma source are arranged on opposite surface areas of the pipe 1. The two components 3, 4 are fastened to the outside of the pipe 1 via a clamping mechanism, which is not shown separately in Figure 1 .

[0033] ​The encapsulation of the gamma source by the surrounding housing is designed such that the gamma radiation only exits from the sending unit 3 in the area of the exit face A. The gamma radiation illuminates the container 1 with the medium 6 having its density p to be determined therein on the indicated beam path SP. The receiving unit 4 receives the gamma radiation attenuated due to the interaction with the container wall and / or the medium 6. The evaluation unit 7 determines the density of the medium 6 located in the container 1 on the basis of the intensity or count rate of the receiving unit 4. The applicant offers and sells corresponding radiation measurement density measurement devices. As mentioned above, the arrangement of the sending unit 3 and the receiving unit 4 relative to the container 1 can be configured differently.

[0034] Figure 2 A diagram showing the visualization of the dependence of the count rate N as a function of the density p of the medium 6 is shown. The attenuation of the gamma radiation on the beam path SP by the medium 6 can be described by the Beer-Lambert law and thus follows an e-function. The attenuation F corresponds to the ratio of the count rate of the gamma radiation after passing through the medium 6 to the count rate N0 of the gamma radiation emitted by the gamma source through the exit opening A. The count rate N is indicated by counts per second (number of events) (c / sec).

[0035] In Figure 2 , the count rate is plotted against the density. At the maximum density p max of the medium 6, the count rate N min is approximately zero; at the minimum density p min , the count rate N max substantially equals the count rate N0 of the gamma radiation emitted by the gamma source. The following mathematical relationship applies:

[0036]

[0037] According to the application, the count rate N0 is determined on the basis of the activity of the sending unit 3, only a further count rate N of the medium 6 having the defined density p1 needs to be determined. Then, the mass attenuation coefficient p can be determined and the exemplary calibration curve shown in Figure 2 can be calculated.

[0038] To illustrate the calculation of the count rate N0, an example of the calculation will be shown below. For example, a sending unit 3 with a Cs137 gamma source is used, which has a correction factor K of 95.95 pSv m2 / (h GBq), an activity P of 0.13 GBq, and an isotope-dependent attenuation coefficient a of 0.048. The receiving unit 4 has a correction factor B of 1350 h / (s pSv).

[0039] For a steel container 1 with a wall 2 having a thickness of 9.3 mm and a steel density of 7890 kg / m 3 , an attenuation factor F s = e aq= 2.4. The distance between the sending unit 3 and the receiving unit 4 is 0.45 m, resulting in a distance factor F a = (0.45 m) 2 = 0.2 m 2 This results in a dose rate F of 25.5 μSv / h i and thus a count rate N0of 344791 / s.

[0040] List of reference signs

[0041] 1 container

[0042] 2 wall of the container

[0043] 3 sending unit with gamma source

[0044] 4 receiving unit

[0045] 5 sensitive component

[0046] 6 medium

[0047] 7 evaluation unit

[0048] D inner diameter of the container

[0049] A gamma radiation exit face

[0050] SP beam path

Claims

1. A method for calibrating a radiation measuring device for determining and / or monitoring the density of a medium (6) located in a container (1), wherein a transmitting unit (3) and a receiving unit (4) are provided, the transmitting unit (3) emitting radioactive radiation of a predetermined intensity, and the receiving unit (4) receiving radioactive radiation emitted by the transmitting unit (3) after the radioactive radiation has passed through the medium (6), and a control / evaluation unit (7) is provided, the control / evaluation unit (7) determining the density of the medium (6) located in the container (1) based on the intensity measured by the receiving unit (4), the method comprising the following method steps: ● Determine the count rate of the radioactive radiation after it has passed through the empty container based on the activity of the transmitting unit (3). , ● When a calibration medium of known density is located in the container (1), determine the measured count rate of the radioactive radiation after it has passed through the container (1). N , ● According to the formula Determine the mass attenuation coefficient μ, where D: the beam path of the radioactive radiation or the inner diameter of the container (1), ρ1: the density of the calibration medium. ● Calculate a calibration curve that represents the dependence of the density of the medium on the count rate of the measured radiation density after the radioactive radiation has passed through the container (1).

2. The method according to claim 1, in, The count rate of the radioactive radiation after passing through the empty container According to the formula To determine, among which, The squared distance factor between the transmitting unit (3) and the receiving unit (4) : Attenuation factor, which depends on the wall (2) of the container (1) and the isotope of the transmitting unit (3), P: Activity of the transmitting unit (3), K: Isotope-dependent correction factor, B: Used to convert pulse rate to count rate The correction factor.

3. The method according to claim 1, in, Water was used as the calibration medium.

4. The method according to claim 1, in, The transmitting unit (3) and the receiving unit (4) are positioned relative to each other such that the container (1) is illuminated perpendicular to the longitudinal axis of the container (1), inclined to the longitudinal axis of the container (1), or parallel to the longitudinal axis of the container (1).

5. The method according to any one of claims 1-4, in, The pipe is used as the container (1), and the sending unit (3) and the receiving unit (4) are fastened to the opposite surface areas of the pipe.

6. The method according to claim 5, in, The receiving unit (4) is designed and positioned such that the sensitive component (5) of the receiving unit (4) is struck by the radioactive radiation.

7. The method according to claim 2, in, The attenuation factor According to the formula To calculate, where, : Isotope-dependent decay coefficient, q: Thickness of the wall (2) of container (1) as measured in steel equivalent, where steel equivalent is defined as the density of the wall (2) of container (1) relative to the density of steel.

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