JPS59107204A - Radiographic thickness gauge - Google Patents

Abstract

PURPOSE:To prevent the malfunction due to noise by using plural photoelectron multipliers to detect the quantity of transmitted radiation and discriminating the simultaneity of output pulses. CONSTITUTION:gamma rays generated from a radiation source 1 stored in a ray source vessel 2 are made incident to a scintillator 4. Photoelectron multipliers 6a and 6b are arranged in parallel in the scintillator 4, and their detection signals are inputted to preamplifiers 7a and 7b. When gamma rays transmitted through an object 3 to be measured are made incident to the scintillator 4 in this manner, signals are outputted simultaneously from multipliers 6a and 6b, and a signal is outputted from a simultaneity counting circuit 15 also. Noise pulses of multipliers 6a and 6b have not simultaneity, and the signal is not outputted from the circuit 15. Consequently, all outputs of the circuit 15 are handled as effective counts.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a radiation thickness meter that measures the thickness of a specimen based on radiation transmittance, and particularly relates to a radiation thickness meter that measures the thickness of a specimen using a radiation transmittance. This relates to thickness gauges.

[Prior art]

Radiation thickness gauges, which measure the thickness of objects to be inspected using radiation transmittance, can measure the thickness of objects without contact, so they are widely used for thickness measurements in steel plate manufacturing processes, etc. It's being used. Recently, there has been a demand for higher performance of this radiation thickness meter, and in order to meet this demand, the detector has shifted to the use of a 7-terrestrial detector, which has higher sensitivity than the conventional ionization chamber.

Such scintillation detectors cannot be used with high sensitivity. Therefore, the following measures are currently being taken:

FIG. 1 is a block diagram of a conventional radiation thickness meter. A radiation source 1 is housed in a radiation source container 2, and radiation emitted from the radiation source passes through an object to be measured 3, enters a scintillator 4, and is detected. That is, the radiation incident on the scintillator 4 is converted into light, which is focused by the light guide 5, enters the photomultiplier tube 6, and is amplified. The output signal is pulse amplified by a preamplifier 7 equipped with a tuned filter,
Discriminator 8 that determines the lower limit discriminant level
The signal is divided into an appropriate value by a frequency divider 9, and sent to an arithmetic processing unit 10 for calculation.

Preamplifier 7 is used to compensate for gain fluctuations in this measurement system.
The output is passed through another discriminator 11, divided by a frequency divider 12, and input to a control circuit 13 together with the signal from the frequency divider 9. In this state, the discriminator level of the discriminator 8 is set to Dl to cut the noise component, and a pulse signal having a wave height of <Dr or more is output as shown in FIG. On the other hand, the discrimination level of the other discriminator 11 is set to Dl, and outputs a pulse with a high wave height. FIG. 2 is a graph showing pulse height and discretization level.

Figure 3 shows r when using an organic scintillator for high-speed counting.
In the diagram showing the spectrum of w, the discretization level is set at a low level, and the high level DzK is set. When the gain of the photomultiplier tube 6 decreases in this state, the spectrum in FIG. 3 shifts to the left, and the count of the discriminator 8 changes, causing an error. Unfortunately, when the gain increases, the spectrum in Figure 3 knots to the right, causing an error as well. In order to compensate for such gain fluctuations, the control circuit 13 compares the signals of the discriminator 8°11 and feeds back the change in gain to the high voltage t#t14 so that the ratio is always constant. However, the voltage applied to the photomultiplier tube 6 is controlled to compensate for the gain change.

The condition for such a control method to be established is that the shape of the energy spectrum shown in FIG. 3 does not change. if,
When the shape of this energy vector I·le changes, the count discriminated between Dl and Dl changes even though the gain does not change, and the ratio of the two does not become constant, so the control circuit 13 mistakes it as a gain change. Since the applied voltage is changed by changing the applied voltage, the control characteristics deteriorate.

The causes of changes in the energy spectrum are complex, but
One of them is due to the interaction between r-rays and the substance to be measured. 13tcs commonly used for thickness measurement of copper plates etc.
Since the radiation source has an energy of 663 K e V, most of its interactions with matter are Compton scattering.

For this reason, as the plate thickness changes, the energy and amount of scattered r-rays that pass through the object to be measured changes, which appears as a change in the energy spectrum. In general, the scattered r-rays increase as the thickness of the object to be measured increases, and since they have lower energy than the transmitted r-rays, they have the drawback of appearing as a change in the energy spectrum near the discretization level Dl.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a radiation thickness meter that eliminates the drawbacks of the prior art and provides highly accurate and stable measurement values.

[Summary of the invention]

The present invention is characterized by detecting the amount of radiation transmitted through the object to be measured using a scintillator and a plurality of photomultiplier tubes, determining the simultaneity of output pulses generated from these photomultiplier tubes,
The reason is that it is configured so that only simultaneous signals are effective for thickness calculation.

In other words, the measurement principle of a thickness meter is to determine the thickness by determining the effective absorption coefficient including scattered radiation from the number of detected crosses, as shown in equation (1) below, and the detected energy The difference between them is not considered a problem.

■=Equation 0eXp (-μm・ρ・[) ・・・・・・
・・・・・・(1) Here, I: Detection coefficient of radiation transmitted through the object to be measured ■ o: Calculated count when there is no object to be measured μm = Effective absorption coefficient (Cnt/g) ρ: Measured object Density of object (g/cm3) t: Thickness of object to be measured (on) Therefore, t is determined by the following formula.

Therefore, the difference in the energy spectrum that changes depending on the thickness is not a problem in calculations, but the problem is how to separate and remove the noise generated from the detector itself from the measurement signal.

The present invention has been made with this point in mind, and is intended to remove noise components generated from a detector without using a discriminator, which is a factor that causes a change in cross counting due to a change in spectrum.

Fig. 4 is a block diagram of a radiation thickness meter that is an embodiment of the present invention, and Fig. 5 is an enlarged view of the main parts of Fig. 4, where the same parts as in Fig. 1 are given the same reference numerals. .

R-rays generated by a radiation source l housed in a radiation source container 2 enter a scintillator 4 . Photomultiplier tubes 6a and 6b are juxtaposed to this scintillator 4, and their detection signals are input to preamplifiers 7a and 7b. When the r-rays (including scattered r-rays) transmitted through the object to be measured 3 enter the scintillator 4 in this way, the photomultiplier tubes 6a and 6b simultaneously output signals, and the coincidence circuit 15 also outputs a signal. Ru.

However, since the noise pulses (noise accompanying dark current, etc.) of the photomultiplier tubes 6a and 6b are random pulses,
Since there is no simultaneity between the two pulses, the coincidence circuit 15 does not output a corresponding signal.

Therefore, the lower limit discriminator D! cant the noise pulses present in the conventional method. is no longer needed,
All outputs of the coincidence counting circuit 15 can be treated as valid counts. Therefore, the sensitivity is high. Further, even if there is a change in the spectrum, it is not affected at all, and furthermore, even if there is a change in the gain of the photomultiplier tube 6, it is not affected at all.

This means that the instability caused by energy spectrum fluctuations and gain fluctuations, which was impossible with the conventional technology, has been solved all at once, and the instability that was the drawback of the conventional technology has been solved, making it possible to create a high-performance thickness gauge. You can get it.

The radiation thickness meter of this example has a pair of photomultiplier tubes connected to a scintillator, the output of each photomultiplier tube is pulse amplified by each preamplifier, and only the simultaneously occurring pulses are counted by a coincidence counting circuit. By doing so, effects such as stability and high sensitivity can be obtained regardless of fluctuations in the energy spectrum and gain.

The above embodiment is useful not only for measuring radiation thickness meters but also for measuring signals mixed with noise using a pair of photomultiplier tubes, and has a wide range of industrial applications.

〔Effect of the invention〕

Mizumoto Φ's radiation thickness meter has the advantage of providing high performance and stable measurement values.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a conventional radiation thickness meter, Figure 2 is a graph showing pulse height and discretization level, and Figure 3 is r, 1ll when using an organic scintillator for high-speed counting.
FIG. 4 is a block diagram of a radiation thickness meter which is an embodiment of the present invention, and FIG. 5 is an enlarged view of the main part of FIG. 4. DESCRIPTION OF SYMBOLS 1...Radiation source, 2...Radiation source container, 3...Measurement object, 4...Scintillator, 6...Photomultiplier tube, 7
...Preamplifier, 8.11... Discriminator, 9.12... Frequency divider, IO... Arithmetic processing unit, 1
3...Control circuit, (1 other person) 1st area 2nd figure 30'If) 4th figure

Claims (1)

[Claims] 1. Irradiate the object to be measured with radiation and detect the amount of radiation transmitted,
In the radiation thickness meter that measures the thickness of the object to be measured,
The amount of transmitted radiation is detected using a scintillator and multiple photomultiplier tubes, and the simultaneity of the output pulses generated from these photomultiplier tubes is determined, and only the simultaneous signals are considered effective for thickness calculation. A radiation thickness meter characterized by being configured so as to be placed sideways.