JPH06265392A - Gamma-ray level meter - Google Patents

Abstract

PURPOSE:To make the gain of a signal amplifying loop constant when a bar-like scintillator, is used for a gamma-ray measuring instrument which measures the level of a fluid or powder by optically connecting an inorganic scintillator having large light emitting energy to the top of a plastic scintillator and irradiating both scintillators with gamma rays, and then controlling the gain of the loop so that optical signals can become constant. CONSTITUTION:An inorganic scintillator 17 is connected to the top of a plastic scintillator 5 and both scintillators 17 and 5 are irradiated with gamma rays from a gamma-ray source 4. Optical signals from both scintillators 17 and 5 are amplified by means of a photomultiplier 6 and amplifier 10 and the signals from the inorganic scintillator 17 are discriminated by means of an energy discriminator 18. Then a second comparator 13 and computing element B14 perform arithmetic operation so that the signals can become constant and control an HV control circuit 16 and high-voltage power source 15 so that level signals can be always amplified constantly.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to non-contact measurement of liquid level, and to a plant requiring flammable chemical substances and explosion-proof performance, or measurement in a bad environment such as high temperature.

[0002]

2. Description of the Related Art A level meter, a densitometer, a thickness meter, or the like that utilizes a γ-ray source and γ-ray attenuation measures the change in physical quantity by utilizing the γ-ray attenuation emitted from the γ-ray source. This attenuated γ-ray is converted into an electric signal by an γ-ray detector combined with an ionization chamber or a scintillator and a photomultiplier, and can be measured as a change in physical quantity. Currently in common use.
Liquids and solids are often used as scintillators, and especially NaI (T
1), CsI (Tl), etc., and the amount used is large. or,
In recent years, a plastic scintillator, which is a mixture of plastic and thallium, is often used for high-accuracy measurement and large-sized detectors. FIG. 2 shows the structure of a conventional level meter disclosed in Japanese Patent Laid-Open No. 2-74827. In the container 1, there is a liquid 2 whose level is to be measured, and the level changes depending on the operating conditions of the plant. There is a γ-ray source 4 such as cobalt or cesium contained in a radiation source container 3 on one side of the container, and the container is irradiated at an angle indicated by θ. On the other side of the container, there is a detector 7 shown by a one-dot chain line, which comprises a plastic scintillator 5 and a photomultiplier tube 6.
However, depending on the shape of the container, the γ-ray source may be provided at an intermediate position of the measurement level.

A light guide 8 is provided between the plastic scintillator (hereinafter abbreviated as "plasmin") and the photomultiplier tube.
Therefore, an optical pulse of tens of nanoseconds (ns) emitted by plasin upon irradiation with γ-rays is efficiently transmitted to the photomultiplier tube. A light pulser 9 is optically coupled and incorporated in the vicinity of the contact surface between the plasticine and the light guide. The light pulsar emits a very strong light pulse that combines NaI scintillator and americium (Am energy 3.4 MeV) with a very small amount of radiation source, and uses an Am radiation source with a long half-life of 433 years. Therefore, it is a stable light source in the long term. The optical pulse signal of the detector having such a configuration is further amplified by the amplifier 10.
FIG. 3 shows an example in which the output signal of the preamplifier is analyzed by a multi-channel wave height analyzer. The horizontal axis shows the wave height P _H (pulse length) of the optical pulse signal, and the vertical axis shows the number N of light pulses at each wave height. In the figure, there are two wave height distribution groups, which are divided into wave height B and below and above. The light pulse having a wave height B or less is a level signal of a plasin light generated when a gamma ray emitted from a gamma ray source such as cobalt or cesium which is generally used as a gamma ray source is attenuated by a measurement object and is incident on plasin. Therefore, the number of light pulses and the wave height distribution of this wave height distribution group change depending on the level. The group of light pulses having a wave height B or higher is due to the light pulser. The light of the light pulser has an energy of 3.4 MeV, which is about 2.5 times larger than that of cobalt, so the wave height is higher than that of cobalt or cesium, and the scintillator uses an inorganic substance of NaI, so the wave height distribution is relatively wide. Has a narrow Gaussian distribution and does not mix with the level signal. Using the optical signal of this light pulser, the amplification degree of the measurement system is automatically corrected so as to eliminate the influence of the disturbance of the measurement system described below and the change over time, and stable measurement is realized. That is,
In FIG. 2, the optical pulse output of the amplifier is separated by two comparators. The first comparator 11 passes a signal having a wave height equal to or higher than a noise component when the comparison wave height is the value of C in FIG. 3, and the arithmetic unit A12 arithmetically converts the pulse number into a level signal and outputs the level signal. The second comparator 13 sets the comparison wave height to the center wave height value A of the optical pulse group by the light pulser,
Only the optical pulse whose wave height is larger than A is passed, and the arithmetic unit B
14 and the number of pulses that have passed through the second comparator and have a wave height of A or higher, that is, FIG.
Compare and calculate the number of pulses in the hatched area
A signal is output to the HV control circuit 16 that controls the high-voltage power supply 15 applied to the photomultiplier tube so that there is no difference from the set value. The number of light pulses emitted by the light pulser has a half-life of 4
Since it is very stable for 33 years, the high-voltage power supply of the photomultiplier tube is controlled so that the number of pulses passing through the second comparator becomes constant as described above, so that the light including the level signal is controlled. The amplification of the pulse measurement system can be kept constant. For example, the amplification change due to the temperature of the photomultiplier tube is about -0.2 to -0.4 (% / ° C) and the change in amplification due to secular change. Even if a change in the rate and a change in the amplification factor temperature of the amplifier occur, the amplification factor of the photomultiplier tube can be corrected by changing the high-voltage power supply as a whole. Therefore,
The level can be measured stably.

However, in order to configure the measurement system, two types of γ-ray sources, a γ-ray source for measurement and a γ-ray source for a light pulser, are required, which causes a problem in management of the γ-ray source. Furthermore, the intensity of the γ-ray source for measurement, Cs and Co, decreases with time and has a half-life of 5 years or 30 years. Correction was necessary.

[0005]

The first object of the present invention is to use γ-rays for measurement as they are as light-emission sources of optical pulses without using two types of γ-ray sources as in the prior art. I have done something good. The second problem is to eliminate the output change due to the fluctuation of the radiation intensity of the γ-ray source,
As short-term fluctuations of the order of a few minutes, output changes caused by random fluctuations of the radiant intensity, which is called statistical noise, and long-term fluctuations of the order of several months to years are compensated for by the natural attenuation of the γ-ray source. It's about making it unnecessary.

[0006]

In order to achieve the present invention, an inorganic scintillator 17 having a larger emission energy than that of Plasin is used at the top of Plasin which has been conventionally used.
The γ-ray source for measurement is used to simultaneously irradiate and emit light.

[0007]

The light emission energy of the scintillator is represented by the product of the light emission luminance and the light emission pulse width. As the light emission energy larger than that of plasin, there are inorganic scintillator sodium iodide NaI and cesium iodide CsI. The table below shows a comparison of the properties of NaI and plasin.

[0008]

[Table 1]

With such a structure, when irradiating plasin and NaI with a γ-ray source for measurement, the light pulse due to plasin and NaI incident on the photomultiplier has a pulse height distribution as shown in FIG. By counting only the pulses having a pulse height equal to or higher than that indicated by and controlling the gain of the signal amplifier loop so that the value is always constant, the same result as the conventional device can be obtained. The difference between the pulse height distributions of plasin and NaI does not become the difference in luminescence energy because the transmittance of plasin is abruptly bad near the wavelength of 410 nm and the reflection at the interface between NaI and plasin.

Further, since the γ-ray source for measurement irradiates NaI, even if the γ-ray source radiation fluctuates for a short time, it is automatically corrected by the signal amplification loop. It is possible to eliminate the influence of statistical noise and attenuation due to.

[0011]

Embodiments of the present invention will be described below with reference to FIG. The inorganic scintillator optically connected to the top of the plasticine is always irradiated with γ-rays regardless of the level by expanding the irradiation range of the γ-ray source by θ + θ 'and the upper side by θ'. Further, conventionally, the signal amplified by the amplifier was directly separated by the second comparator, but in the meantime, a pulse energy discriminator 18 including an amplifier having a CR time constant using a capacitance and a resistor is added, When the signals before and after the pulse energy discriminator are analyzed by the multi-channel wave height analyzer, the pulse height of the NaI signal and the pulse height of the plasin with large pulse energy are as shown in Fig. 4 (input) and Fig. 5 (output). Can be separated for easy discrimination, and N
A large signal amount of aI was obtained, and more stable control became possible. Further, FIG. 6 shows another embodiment in which an inorganic scintillator is attached to a plasticine, and a part of the plasticine is replaced with a small-sized inorganic scintillator. Even with this,
Although the number of pulses is reduced, the height of the pulse does not change, so that the control is sufficiently performed. When the γ-ray source attenuates, the NaI signal in FIG. 5 also decreases in proportion, so it is not necessary to automatically increase the gain of the signal amplification system and perform the γ-ray source attenuation correction as in the conventional device. Absent. However, in order to eliminate the statistical fluctuation noise, the response time constant of the loop gain including the optical signal amplifier, the arithmetic unit, and the high-voltage power supply cannot be fed back unless it is equal to or less than the time fluctuation of the statistical fluctuation noise. Therefore, it is necessary to set it to several seconds or less.

[0012]

According to the present invention, two kinds of light signals having different emission energies can be obtained by one kind of γ-ray source, and the gain of the signal amplifier system can be made constant by controlling the light signal of the inorganic scintillator to be constant. By doing so, the level measurement could be performed stably. In addition, the effect of eliminating statistical fluctuation noise and simultaneously performing attenuation correction of the γ-ray source was also obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a conventional device.

FIG. 3 is an explanatory diagram of a conventional device.

FIG. 4 is a diagram illustrating the present invention.

FIG. 5 is a diagram illustrating the present invention.

FIG. 6 is a diagram showing another embodiment of the present invention.

[Explanation of symbols]

1 ... Container, 2 ... Liquid, 3 ... Radiation source container, 4 ... γ-ray source, 5 ...
Plastic scintillator, 6 ... Photomultiplier tube, 7 ... Detector, 8 ... Light guide, 9 ... Light pulser, 10 ... Amplifier, 11 ... First comparator, 12 ... Operation unit A, 1
3 ... second comparator, 14 ... arithmetic unit B, 15 ... high-voltage power supply, 16 ... HV control circuit, 17 ... inorganic scintillator,
18 ... Energy discriminator.

Claims (4)

[Claims] 1. An optical signal directly or via a light guide to one end of a rod-shaped plastic scintillator arranged at a position facing the γ-ray source with the γ-ray source arranged on one side of the container sandwiching the container. A photomultiplier tube for optically receiving and amplifying a photomultiplier tube is optically connected, and in a γ-ray level meter for measuring the level change in the container by the counting rate of the output pulse of the photomultiplier tube, an inorganic scintillator is provided on the top of the plastic scintillator. Is optically connected, both scintillators are irradiated with one γ source, and the gain of the optical signal amplification part is controlled so that the wave height distribution of the inorganic scintillator light is always the same shape. Total. 2. A gamma ray level meter according to claim 1, further comprising an energy discriminator provided between an amplifier for electrically amplifying an optical signal and a second comparator for discriminating an HV control signal. 3. The inorganic scintillator according to claim 1, wherein the inorganic scintillator is N.
A gamma ray level meter characterized by being aI or CsI. 4. The gamma ray level meter according to claim 1, wherein the response time of the optical signal amplification system is smaller than the change time of the statistical fluctuation noise.