DE102013022357B3 - Radiometric measuring system - Google Patents

Abstract

Radiometric measurement system that is designed to - determine a first count rate of events that have an energy that is above a first energy threshold (E1), the radiometric measurement system being designed to determine a measured variable as a function of the first count rate ,- to determine a slope of the first count rate (ZR) at the first energy threshold (E1), wherein determining the slope of the first count rate at the first energy threshold (E1) comprises the steps:- determining a second count rate of events whose energy is above a second energy threshold (E2) which is different from the first energy threshold (E1),- forming a count rate difference between the first count rate and the second count rate,- forming an energy difference between the first energy threshold (E1) and the second energy threshold (E2) , and- determining the gradient as a function of the count rate difference and the energy difference, and- at least one operat to set the operating parameters of the radiometric measuring system depending on the gradient,- wherein one of the operating parameters is a warning state operating parameter, wherein the warning state operating parameter is set to a value that causes a warning if the gradient exceeds a warning threshold, and/or- wherein one of the operating parameters is a alarm condition operating parameter, wherein the alarm condition operating parameter is set to a value that causes an alarm if the slope exceeds an alarm threshold, and/or- wherein one of the operating parameters is the first energy threshold, the first energy threshold (E1) changing as a function of the slope becomes.

Description

The invention relates to a radiometric measuring system.

In process measurement technology, radiometric measurement systems, which include scintillation counters or scintillation detectors for measuring radiation, are often used to measure process variables or material properties, for example for level measurement, moisture measurement, density measurement, etc.

Scintillation counters are used, for example, to determine the spectrum of ionizing radiation, i.e. to determine the intensity as a function of the energy of the ionizing radiation, with a scintillation counter comprising a scintillator which is excited when radiation in the form of high-energy charged particles or photons passes through and the excitation energy in form of light pulses (usually in the UV range or visible range), which is referred to as scintillation. The passage of radiation in the form of an energetic charged particle or photon is also called an event.

The light pulses generated in this way are converted into electrical signals and amplified using a suitable optical sensor or optical detector. Such a sensor is typically a photomultiplier or a photodiode. So-called silicon photomultipliers or SiPM (semiconductor components with internal electron amplification) have also been available for this measurement task for several years.

The optical sensor emits pulses, with a number of pulses per unit of time, i.e. the count rate, being a measure of the intensity of the radiation and a pulse height or pulse amplitude (more precisely an integral over the course of the pulse over time) being a measure of the energy of the radiation is.

The optical detectors, especially the semiconductor-based detectors, show a strong temperature dependency. This is also reflected in an increase in noise pulse amplitudes and a leakage current at high temperatures. Another reason for increasing noise contributions is the aging of the optical coupling and/or the scintillator, which necessitates that a signal generated by the sensor has to be electronically amplified more, thereby reducing the signal-to-noise ratio. As soon as the noise pulse amplitudes in an integral measurement are greater than the measurement threshold, they are incorrectly interpreted as a real count rate or useful count rate.

the EP 2 584 379 A1 shows a method for operating a scintillation counter and a scintillation counter.

the EP 2 237 073 A1 shows a method and a device for monitoring an automatic drift compensation.

the DE 100 48 559 A1 shows a device for determining and/or monitoring the density and/or the filling level of a medium in a container.

the DE 1 809 520 A shows a device for automatic drift stabilization in nuclear radiation probes.

the DE 33 28 256 C2 shows a method and an arrangement for automatic stabilization of a scintillation detector.

the DE 28 26 484 C2 shows a control method for automatic drift stabilization in a radiation measurement.

the DE 102 12 638 A1 shows a computer tomograph and a method for detecting X-ray radiation with a detector unit consisting of a large number of detectors.

the DE 10 2006 022 596 A1 shows an X-ray detector.

the US 6,051,830A and the US 3,515,878A each disclose a radiometric measurement system in which an amplification of the measurement system is set as a function of a slope of a count rate at a predetermined energy threshold.

the DE 197 11 124 A1 shows a method and a device for detecting artificial gamma radiation, in which measured data of a pulse amplitude distribution are compared with at least one reference parameter derived from a characteristic reference pulse amplitude distribution of natural gamma radiation.

The invention is based on the object of providing a radiometric measuring system which makes it possible to determine the measured variable as reliably as possible even in the case of noisy sensors.

The invention solves this problem with a radiometric measuring system according to claim 1.

The radiometric measurement system is designed to be operated as described below.

A first count rate of events having an energy above a first energy threshold or within a first energy range is measured, the first energy range being defined by an associated lower energy threshold and an associated upper energy threshold. The events are typically due to radiation in the form of energetic charged particles or photons hitting a detector. However, the events can also be caused by noise processes.

A measured variable is determined as a function of the first count rate. The measured variable can be material properties, a fill level, moisture, density, etc., for example.

Furthermore, a slope of the first count rate is calculated at least at the first energy threshold. For example, the slope is the first derivative of the count rate with respect to the energy. Of course, the gradient can also be determined or measured over the entire energy spectrum of interest.

Depending on the calculated gradient, one or more operating parameters or operating states of the radiometric measuring system is/are adjusted or set, for example a warning or an alarm is/are output depending on the gradient.

Determining the slope of the first count rate at the first energy threshold comprises the steps of: determining a second count rate of events whose energy is above a second energy threshold that is different from the first energy threshold, or that is within a second energy range that is different from the first energy range is different, the second energy range being defined by an associated lower energy threshold and an associated upper energy threshold, forming a count rate difference between the first count rate and the second count rate, forming an energy difference between the first energy threshold and the second energy threshold, and determining the slope in Dependence on the count rate difference and the energy difference. The first energy threshold can be greater than the second energy threshold.

Events with energies above the first or second energy threshold can be counted, for example, by means of electronic comparators and downstream counters, the comparators being subjected to voltage pulses that are generated by the sensor. A respective comparator voltage level can be assigned to the energy thresholds, so that a respective comparator generates a pulse when the amplitude of the input pulse exceeds the comparator voltage level.

The second energy threshold can be set in such a way that a predefined functional relationship f is established between the second count rate and the first count rate.

wobei a eine reelle Zahl ist und einen vorgebbaren Faktor bezeichnet und b eine reelle Zahl ist und einen vorgebbaren Offset bezeichnet.The following can apply to the functional relationship f: $$\text{f}=\text{a}*\text{first z}\ddot{\text{a}}\text{rate}+\text{b},$$

where a is a real number and designates a specifiable factor and b is a real number and designates a specifiable offset.

A drift compensation can be carried out, in particular by means of a ratio control. In the case of drift compensation, for example, an amplification that may be subject to drift effects is tracked as a function of different variables. Aging and/or drift effects can be reliably corrected based on the drift compensation, which leads to a more precise determination of the process variable. With regard to the drift compensation and the ratio control is, for example, on the EP 2 237 073 A1 refer, which is made in this respect by reference to the content of the description.

In order to optimize a resolution of the measured variable, the first energy threshold should be selected as low as possible. However, below a certain energy threshold, referred to below as the noise edge, the probability of events caused by noise and thus the count rate increases sharply. In the area of the noise edge, the slope of the first count rate consequently shows significantly higher values than at a certain distance from (above) the noise edge. The value of the slope can therefore be used as an indicator that the first energy threshold tends to be too low, so that a warning can be issued in this case or the first energy threshold can be increased. If the slope exceeds an alarm threshold, which is typically greater than the warning threshold, an alarm may still be issued. This makes sense in particular when drift compensation is carried out, which under certain circumstances results in the first energy threshold being shifted in the direction of the noise edge, so that the measured variable would be very noisy. This condition can be signaled by a warning or an alarm.

• - Durchführen einer automatischen Driftkompensation, insbesondere mittels einer Verhältnisregelung,
• - Bestimmen einer Zählrate von Ereignissen, die eine Energie aufweisen, die über einer (ersten) Energieschwelle liegt,
• - Bestimmen einer Steigung der Zählrate an der (ersten) Energieschwelle, und
• - Bestimmen der Driftkompensation als fehlerhaft, wenn die Steigung (bzw. deren Betrag) einen Schwellenwert übersteigt.

An automatic drift compensation of the radiometric measuring system can be monitored as follows:

• - Carrying out an automatic drift compensation, in particular by means of a ratio control,
• - determining a count rate of events that have an energy that is above a (first) energy threshold,
• - determining a slope of the count rate at the (first) energy threshold, and
• - Determining the drift compensation as faulty if the slope (or its magnitude) exceeds a threshold value.

• - Erzeugen von verstärkten elektrischen Impulsen mit einem Betrag, der von der Energie eines beobachteten Teilchens und von einer Verstärkung abhängt,
• - Bestimmen einer ersten Zählrate von Impulsen mit einem Betrag, der über einer ersten Schwelle liegt,
• - Bestimmen einer zweiten Zählrate von Impulsen mit einem Betrag, der über einer zweiten Schwelle liegt, wobei die zweite Schwelle über der ersten Schwelle liegt, und
• - Regeln der Verstärkung derart, dass ein Verhältnis zwischen der ersten Zählrate und der zweiten Zählrate einer vorgegebenen Funktion entspricht, insbesondere konstant bleibt.

The automatic drift compensation can include the steps:

• - generating amplified electrical impulses with a magnitude that depends on the energy of an observed particle and on an amplification,
• - determining a first count rate of pulses having a magnitude greater than a first threshold,
• - determining a second count rate of pulses having a magnitude greater than a second threshold, the second threshold being greater than the first threshold, and
• - Controlling the gain in such a way that a ratio between the first count rate and the second count rate corresponds to a predetermined function, in particular remains constant.

The operating parameters can include an operating parameter in the form of the first energy threshold, where the first energy threshold can be changed dynamically as a function of the gradient. For example, the gradient can be regulated to a predefinable or predetermined desired gradient value, with the first energy threshold serving as the manipulated variable. If the gradient has values that are too high in absolute terms, the first energy threshold can be increased, for example, and vice versa.

• - Bestimmen einer Zählrate von Ereignissen, die eine Energie aufweisen, die über einer (ersten) Energieschwelle liegt,
• - Bestimmen einer Steigung der Zählrate an der (ersten) Energieschwelle, und
• - Verändern der (ersten) Energieschwelle in Abhängigkeit von der Steigung derart, dass die Steigung einen vorgegebenen funktionalen Zusammenhang aufweist, insbesondere gleich einem vorgegebenen Steigungssollwert ist.

The radiometric measuring system can be operated as follows:

• - determining a count rate of events that have an energy that is above a (first) energy threshold,
• - determining a slope of the count rate at the (first) energy threshold, and
• - Changing the (first) energy threshold as a function of the gradient in such a way that the gradient has a predetermined functional relationship, in particular is equal to a predetermined desired gradient value.

The first energy threshold can be changed, in particular in steps between a lower and an upper energy threshold. The gradient of the first count rate can be determined for the changed first energy threshold or for a respective step between the lower and the upper energy threshold value. In other words, the first derivative is formed for an energy range of interest in the energy spectrum. The gradients determined or calculated in this way or the first derivative determined or calculated in this way can be evaluated to set the at least one operating parameter and/or to define an operating point of the first energy threshold.

The radiometric measuring system can have: means for determining a first count rate of events that have an energy that is above a first energy threshold, means for determining a measured variable as a function of the first count rate, means for determining a slope of the first count rate at the first Energy threshold, and means for adjusting one or more operating parameters of the radiometric measurement system as a function of the slope.

The radiometric measuring system can have an optical sensor or optical detector in the form of a semiconductor sensor, for example in the form of a photodiode or a silicon photomultiplier.

• 1 ein integrales Impulshöhenspektrum mit einer ersten Energieschwelle und einer zweiten Energieschwelle.

The invention is described in detail below with reference to the drawing. This shows:

• 1 an integral pulse height spectrum having a first energy threshold and a second energy threshold.

1 shows an integral pulse height spectrum with a first energy threshold E1, a second energy threshold E2 and a so-called noise edge ERK. In the integral pulse height spectrum shown, the energy E is plotted on the X-axis and the count rate ZR of events with an energy greater than the energy E is plotted on the Y-axis

How out 1 shows that the slope of the curve shown increases significantly in the area of the noise edge ERK, since noise pulses contribute to the count rate for energies below the noise edge ERK.

The position of the noise edge ERK can be determined by calculating the gradient or steepness of the displayed spectrum. If the gradient exceeds a threshold value that can be determined empirically, for example, then this can be inferred sen that the events causing the count rate must essentially be noise events.

In the simplest case, the first energy threshold E1 is predetermined, for example constant over time, and a first counting rate of events is measured that has an energy that is above the first energy threshold E1. The first count rate measured in this way is conventionally evaluated to determine a measured variable, for example the filling level of a container.

In addition, a second counting rate of events whose energy is above the second energy threshold E2 is determined. The second energy threshold E2 is smaller than the first energy threshold E1. To calculate the gradient at the first energy threshold E1, a count rate difference between the first count rate and the second count rate is calculated, an energy difference between the first energy threshold E1 and the second energy threshold E2 is calculated and finally the gradient is calculated as a function of the count rate difference and the energy difference.

If an amount of slope exceeds a warning threshold, a warning may be issued. If an amount of slope exceeds an alarm threshold, an alarm may be issued. In other words, the quality or the influence of noise on the determination of measured values is monitored and corresponding messages are output so that suitable measures can be taken in the measurement environment.

Alternatively, the first energy threshold E1 can be dynamic, i.e. changed during operation of the radiometric measurement system.

The gradient at the first energy threshold E1 is continuously calculated in the manner described above. The gradient is then, for example, continuously regulated to a predetermined, for example empirically determined, desired value, with the first energy threshold E1 representing the variable manipulated variable of the regulation. The second energy threshold E2 can be suitably carried along.

The active dynamic noise edge monitoring according to the invention ensures that the count rate measured is not substantially corrupted by noise pulses, which would lead to an incorrect measured variable or process variable.

This enables, for example, drift compensation by means of ratio control even when using optical detectors in the form of photodiodes, without the risk of an incorrectly determined count rate due to noise pulses.

In addition to evaluating the quality of the measurement, it is possible to switch the first energy threshold E1 or measurement threshold to a higher value as soon as the noise edge ERK comes too close to the energy threshold E1.

For this purpose, for example, in addition to the measuring channel for determining the count rate of pulses with an energy greater than the first energy threshold E1, a further measuring channel could be provided which has a higher energy threshold E3 than the first energy threshold.

This measuring channel is initially not active, i.e. the count rate determined in this way is not used to determine the measured variable, but the measuring channel is already counting pulses CPS(E3).

After the gradient-dependent switchover to this measurement channel, a normalization factor CPS(E1)/CPS(E3) could be applied to CPS(E3) so that the measured variable remains constant. This normalization factor can be continuously adjusted during operation and only used when switching to the higher energy threshold E3.

In this way, for example, in the production of steel in the continuous casting process, a casting cycle can be completed with reduced accuracy before a measuring probe has to be replaced.

Claims (5)

Radiometric measurement system that is designed to - determine a first count rate of events that have an energy that is above a first energy threshold (E1), wherein the radiometric measurement system is designed to determine a measured variable as a function of the first count rate , - to determine a slope of the first count rate (ZR) at the first energy threshold (E1), wherein determining the slope of the first count rate at the first energy threshold (E1) comprises the steps: - determining a second count rate of events whose energy is above a second energy threshold (E2), which differs from the first energy threshold (E1), - forming a count rate difference between the first count rate and the second count rate, - forming an energy difference between the first energy threshold (E1) and the second energy threshold (E2) , and - determining the slope as a function of the Count rate difference and the energy difference, and - set at least one operating parameter of the radiometric measuring system depending on the slope, - wherein one of the operating parameters is a warning state operating parameter, wherein the warning state operating parameter is set to a value that causes a warning if the slope exceeds a warning threshold, and/or - wherein one of the operating parameters is an alarm state operating parameter, wherein the alarm state operating parameter is set to a value that causes an alarm if the slope exceeds an alarm threshold, and/or - wherein one of the operating parameters is the first energy threshold, the first energy threshold (E1) is changed depending on the slope. Radiometric measuring system claim 1 , characterized in that the second energy threshold (E2) is set in such a way that a predetermined functional relationship f sets in between the second count rate and the first count rate. Radiometric measuring system claim 2 , characterized in that the following applies to the functional relationship f: f=a*first count rate+b, where a denotes a predeterminable factor and b denotes a predeterminable offset. Radiometric measuring system according to one of the preceding claims, characterized in that the first energy threshold (E1) is greater than the second energy threshold (E2). Radiometric measuring system according to one of the preceding claims, characterized in that the radiometric measuring system is designed to carry out drift compensation, in particular by means of ratio control.

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