JP2014211380A - Dose rate measurement device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a level difference at a switching point between a low dose rate range and a high dose rate range in a dose rate measurement device less conspicuous.SOLUTION: Provided is a dose rate measurement device comprising: a scintillator; a photoelectron multiplier into which light emitted by the scintillator enters; a preamplifier for amplifying the output of the photoelectron multiplier via a capacitor; a main amplifier for amplifying the output of the preamplifier; an A/D converter for converting the output of the main amplifier into digital quantities; a V/F converter into which the output voltage of the photoelectron multiplier is inputted; a counter for counting the number of pulses outputted from the V/F converter; a low range dose rate arithmetic unit for calculating a low range dose rate RL from the output of the A/D converter; and a high range dose rate arithmetic unit for calculating a high range dose rate RH from the output of the counter. A dose rate switching arithmetic unit switches a display unit from a low range to a high range when a dose rate ratio RH/RL exceeds 1+k1, and switches the display unit from the high range to the low range when the dose rate ratio RH/RL declines to 1+k2.

Description

  The present invention relates to a dose rate measuring apparatus installed in a nuclear reactor facility, a spent fuel reprocessing facility, and the vicinity thereof.

  The dose rate measurement apparatus includes a NaI (Tl) scintillation detector and a measurement unit. The NaI (Tl) scintillation detector absorbs the energy of the incident radiation and converts it into a current proportional to the dose rate, outputs discrete current pulses, and converts a direct current into a direct current voltage for output. The measurement unit is composed of a preamplifier, a main amplifier, a bias modulation wave height discrimination circuit (DBM circuit: Discriminated Bias Modulation), a voltage / frequency converter (V / F converter), a gate circuit, and a calculation unit. Input DC voltage, convert to dose rate and output.

  The preamplifier converts the current pulse into an analog voltage pulse and outputs it. The main amplifier amplifies the analog voltage pulse and removes high frequency noise. The bias modulation pulse height discrimination circuit weights the amplified analog voltage pulse by the dose rate, and outputs a pulse having a repetition frequency proportional to the dose rate. The voltage / frequency converter receives a DC voltage, converts it into a pulse having a repetition frequency proportional to the voltage, and outputs the pulse. The gate circuit inputs these pulses, switches them, and outputs them. The calculation unit inputs the output pulse of the gate circuit, calculates the low range dose rate based on the pulse of the bias modulation wave height discrimination circuit or the high range dose rate based on the pulse of the voltage / frequency converter, and outputs it. The range of the gate circuit is switched at the dose rate.

  Gamma rays are incident on the scintillation detector. The energy characteristics of the dose rate measuring device are different between the low range dose rate and the high range dose rate due to errors that occur depending on the energy of γ rays. Further, the switching point between the high range and the low range differs depending on the energy of γ rays, and the optimum switching point is different between the low range dose rate and the high range dose rate. A dose rate measuring apparatus is known in which a NaI (Tl) scintillation detector is provided with a complex filter so that a large step generated at a switching point can be made as small as possible and can be switched at a fixed dose rate ( For example, see Patent Document 1).

Japanese Patent Laid-Open No. 61-104282 (FIG. 1)

  The dose rate measuring apparatus can be reduced to some extent both by the difference in the energy characteristics of the high dose rate range and the step generated at the switching point by providing a filter having a complicated structure in the NaI (Tl) scintillation detector. Due to the wide range, the original energy characteristics of the low range dose rate deteriorate. The present invention has been made to solve such problems, and does not deteriorate the original energy characteristics of the low dose rate range by widening the range, and the switching point between the low dose rate range and the high dose rate range. An object of the present invention is to provide a high-accuracy dose rate measuring apparatus having good energy characteristics in which the level difference is inconspicuous.

  A dose rate measuring apparatus according to the present invention includes a scintillator that emits light when incident radiation, a photomultiplier tube that receives light emitted from the scintillator, a load resistance through which an output current of the photomultiplier tube flows, and a photomultiplier tube A preamplifier that amplifies the output of the preamplifier, a main amplifier that amplifies the output of the preamplifier, an A / D converter that digitally converts the output of the main amplifier, and a V / F that receives the voltage generated in the load resistance A converter, a counter for measuring the number of pulses output from the V / F converter, a low range dose rate calculation unit for obtaining a low range dose rate RL from the output of the A / D converter, and a high from the output of the counter A high range dose rate calculation unit for obtaining the range dose rate RH and a dose rate switching calculation unit for setting the display range of the display unit to a low range or a high range are provided. When the dose rate ratio RH / RL exceeds 1 + k1, the dose rate switching calculation unit switches the display range of the display unit from the low range to the high range, and the dose rate ratio RH / RL is 1 + k2. When it falls, the display range of the display unit is switched from the high range to the low range.

  The dose rate measuring apparatus according to the present invention has good energy characteristics over the entire energy to be measured, and also has good linearity over the entire measurement range. For this reason, the level | step difference of a switching point becomes inconspicuous and a highly accurate wide-range dose rate measuring apparatus can be provided. Since the radiation detector is not provided with a lead filter and does not need to be switched at a fixed switching point, the originally good energy characteristics of the low range dose rate are not deteriorated.

1 is a diagram illustrating a configuration of a dose rate measuring apparatus according to Embodiment 1. FIG. It is a figure which shows the outline | summary of the energy of a radiation, and the optimal switching point. It is the front view and side view which show the structure of the scintillation fiber bundle concerning Embodiment 2. It is the front view and side view which show the structure of the scintillation fiber bundle concerning Embodiment 3. It is a figure which shows the structure of the dose rate measuring apparatus concerning Embodiment 4. FIG. It is a figure which shows the outline | summary of the temperature compensation of the peak value concerning Embodiment 4. FIG. FIG. 10 is a diagram illustrating an outline of temperature compensation of a current value according to the fourth embodiment. It is a figure which shows the structure of the dose rate measuring apparatus concerning Embodiment 5. FIG.

  Embodiments of a dose rate measuring apparatus according to the present invention will be described below in detail with reference to the drawings. In addition, this invention is not limited to the following description, In the range which does not deviate from the summary of this invention, it can change suitably.

Embodiment 1 FIG.
Embodiment 1 of the present invention will be described with reference to FIG. The dose rate measuring apparatus 100 includes a radiation detector 1 and a measurement unit 2. The radiation detector 1 absorbs the energy of the incident radiation and converts it into a direct current proportional to the dose rate, outputs discrete current pulses, and converts the direct current into a direct current voltage for output. The measurement unit 2 includes a low range dose rate measurement unit, a high range dose rate measurement unit, a dose rate switching unit, and a display unit / operation unit 27. The low range dose rate measuring means includes a preamplifier 21, a main amplifier 22, an A / D converter (analog / digital converter) 23, and a low range dose rate calculation unit 261. The high range dose rate measuring means includes a V / F converter (voltage / frequency converter) 24, a counter 25, and a high range dose rate calculation unit 262. The calculation processing unit 26 includes a low range dose rate calculation unit 261, a high range dose rate calculation unit 262, and a dose rate switching calculation unit 263.

  In the measurement unit 2, the preamplifier 21 converts the current pulse into an analog voltage pulse and outputs the analog voltage pulse. The main amplifier 22 amplifies the analog voltage pulse and removes high frequency noise by a filter characteristic in which gain decreases at high frequency. The A / D converter 23 measures the peak value of the amplified analog voltage pulse. The V / F converter 24 converts the input DC voltage into a pulse having a repetition frequency proportional to the input voltage and outputs the pulse. The counter 25 counts the pulses and outputs a count value. In the arithmetic processing unit 26, the low range dose rate calculation unit 261 receives the peak value of the output of the A / D converter 23, converts it to a low range dose rate, and outputs it. The high range dose rate calculation unit 262 receives the count value of the output of the counter 25, converts it into a high range dose rate, and outputs it. The dose rate switching calculation unit 263 switches and outputs the low range dose rate and the high range dose rate at the switching point. The display unit / operation unit 27 displays the dose rate and sets the calculation processing unit 26.

  The radiation detector 1 includes a columnar plastic scintillator 11, a photomultiplier tube 12, and an output circuit 13. The plastic scintillator 11 emits fluorescence by absorbing the energy of incident radiation. The photomultiplier tube 12 converts the fluorescence into electrons, multiplies it, and converts it into a charge amount proportional to the fluorescence amount. The output circuit 13 extracts and outputs a current pulse as a change in the amount of charge with time by the coupling capacitor 131, and converts a DC current into a DC voltage by the load resistor 132 and outputs the DC voltage. The plastic scintillator 11 is tissue equivalent because the effective atomic number is close to the atomic number of the human soft tissue. Since the radiation detector 1 converts the current into a current proportional to the dose rate and outputs it, the energy characteristics of the low range dose rate and the high range dose rate match well and basically. According to the dose rate measuring apparatus 100 according to the present invention, it is not necessary to reduce the difference in energy characteristics between the low range dose rate and the high range dose rate, particularly in the low energy region, with the lead filter.

  Next, calculation processing of the calculation processing unit 26 will be described. The low range dose rate calculation unit 261 inputs and integrates the peak value of the output of the A / D converter 23, divides the integrated value by the measurement time, obtains an integrated peak value per unit time, and determines the dose rate. Multiply the conversion factor to find the low range dose rate (RL). Here, the integrated peak value per unit time is a parameter corresponding to the average current as the charge carried by the current pulse in the radiation detector 1 per unit time. The high range dose rate calculation unit 262 inputs the count value of the output of the counter 25 and integrates it, obtains the count rate by dividing the integrated value by the measurement time, and multiplies it by the dose rate conversion factor to obtain the high range dose rate. (RH) is obtained. Here, the count rate is a parameter corresponding to the direct current in the radiation detector 1.

  The dose rate switching calculation unit 263 automatically switches the display range based on the dose rate ratio RH / RL between the low range dose rate RL and the high range dose rate RH. The dose rate switching calculation unit 263 automatically performs switching from the low range dose rate RL to the high range dose rate dose rate RH when the dose rate ratio RH / RL exceeds 1 + k1. The dose rate switching calculation unit 263 automatically executes switching from the high range dose rate RH to the low range dose rate dose rate RL when the dose rate ratio RH / RL decreases to 1 + k2. Assuming that coefficient k1> coefficient k2> 0, hysteresis is given to automatic switching.

  FIG. 2 is a diagram showing an outline of radiation energy and switching points. Characteristic a1 and characteristic a2 conceptually show the input / output response of the dose rate of Am-241 (effective energy: 57 keV). The characteristic a1 is the input / output response of the low range dose rate. The low dose rate side has good linearity, and the high dose rate side tends to decrease following saturation as shown by the dotted line on the right side of the straight line. The characteristic a2 is an input / output response of a high range dose rate. On the low dose rate side, the dark current of the photomultiplier tube 12 is dominant as shown by the dotted line on the left side of the straight line, and shows good linearity as the input increases. A switching point A1 indicates a point where RH / RL = 1 + k1. A switching point A2 indicates a point where RH / RL = 1 + k2.

  Characteristic b1 and characteristic b2 conceptually show the input / output response of the dose rate of Cs-137 (effective energy: 660 ke). The input / output response of Cs-137 is such that the characteristics of Am-241 are linearly shifted by about 1 decard to the high dose rate side. The switching point B1 indicates a point where RH / RL = 1 + k1. A switching point B2 indicates a point where RH / RL = 1 + k2. In this way, the dose rate switching calculation unit 263 sets a switching point that varies according to the average energy of the radiation incident on the radiation detector 1 based on the dose rate ratio RH // RL between the low range dose rate and the high range dose rate. Search and execute switching.

  As described above, the radiation detector 1 includes the tissue equivalent plastic scintillator 11 and basically matches the energy characteristics of the low range dose rate and the high range dose rate. Based on the ratio between the low range dose rate and the high range dose rate, the dose rate switching calculation unit 263 searches for an optimal switching point that varies according to the average energy of the radiation incident on the radiation detector 1 and performs switching. Although the dose rate measuring device of the system which switches a NaI (Tl) scintillation detector with a lead filter and suppresses a level difference at a fixed switching point has a problem of lowering originally good energy characteristics of a low range dose rate. On the other hand, the dose rate measuring apparatus 100 according to the present application has good energy characteristics over the whole energy to be measured, and also has good linearity over the whole measurement range. Therefore, it is possible to provide a high-accuracy wide-range dose rate measurement device in which the step at the switching point is not noticeable.

Embodiment 2. FIG.
In the second embodiment, the radiation detector 1 is configured by a scintillation fiber bundle 14 instead of the columnar plastic scintillator 11 of the first embodiment. Front view 3A and side view 3B show the structure of the scintillation fiber bundle according to the second embodiment. The scintillation fiber bundle 14 is one in which the scintillation fibers 141 are bundled and bonded and the light is blocked by the light shielding case 142. The diameter and length of the scintillation fiber bundle 14 can be easily changed. According to the second embodiment, it is possible to obtain the radiation detector 1 with good energy characteristics by correcting the dimensions of the scintillation fiber bundle 14 and performing repeated experiments.

Embodiment 3 FIG.
A front view 4A and a side view 4B show the structure of the scintillation fiber bundle according to the third embodiment. In the third embodiment, dummy plastic fibers 143 are arranged on the scintillation fiber bundle 14 of the radiation detector 1. The dummy plastic fibers 143 are discretely arranged concentrically on a part of the outside of the scintillation fiber bundle 14 based on calculations or experiments. If the radiation detector 1 according to the third embodiment is used, a slight distortion remaining in the energy characteristics in the low energy range can be easily corrected, and a dose rate measuring apparatus with better energy characteristics can be obtained.

Embodiment 4 FIG.
Embodiment 1 of the present invention will be described with reference to FIG. In the fourth embodiment, the preamplifier 21 includes an operational amplifier 211, a negative feedback capacitor 212, and a negative feedback resistor 213. A negative feedback capacitor 212 and a negative feedback resistor 213 are connected in parallel between the half-rotation input and output of the operational amplifier 211. A negative feedback resistor 213 having a positive temperature coefficient is used. In the analog voltage pulse output from the preamplifier 21, the positive temperature characteristic is converted to a half-value width related to the product of the capacitance C of the negative feedback capacitor 212 and the resistance R of the negative feedback resistor 213, and the gain of the main amplifier 22 decreases at high frequency. It acts on the frequency characteristics.

  As shown in FIG. 6, the negative temperature characteristic “a” of the gain of the photomultiplier tube 12 can cancel the influence on the current pulse measurement as the temperature characteristic “c” by the action of the temperature characteristic “b” of the main amplifier 22. Further, the load resistor 132 of the output circuit 13 has a positive temperature coefficient. As shown in FIG. 7, the negative temperature characteristic d possessed by the gain of the photomultiplier tube 12 can be offset by the positive temperature characteristic e of the load resistor 132 as the temperature characteristic f. A high-accuracy wide-range dose rate measuring apparatus with good accuracy can be obtained.

Embodiment 5. FIG.
The radiation detector 1 according to the fifth embodiment includes a Si semiconductor sensor 15 and an output circuit 13 as shown in FIG. The Si semiconductor sensor 15 collects and outputs charges generated by absorbing the energy of incident radiation to the electrode. The output circuit 13 outputs a current as a time change of the charge amount as a current pulse, and also converts a direct current into a direct current voltage and outputs it. The dose rate measuring apparatus 100 uses the Si semiconductor sensor 15 having good temperature characteristics and a current output proportional to the dose rate, and measures the dose rate based on the output current. If the radiation detector 1 according to the fifth embodiment is used, as in the first embodiment, the energy characteristics are good in the whole energy to be measured, the linearity is good in the whole measurement range, and the step of the switching point is conspicuous. A highly accurate wide-range dose rate measuring device with no temperature characteristics and good temperature characteristics can be obtained.

  It should be noted that the present invention can be freely combined with each other within the scope of the invention, and each embodiment can be appropriately modified or omitted.

DESCRIPTION OF SYMBOLS 1 Radiation detector, 11 Plastic scintillator, 12 Photomultiplier tube, 13 Output circuit, 131 Coupling capacitor, 132 Load resistance, 14 Scintillation fiber bundle, 141 Scintillation fiber, 142 Light shielding case, 143 Plastic fiber, 15 Si semiconductor sensor, 2 measurement unit, 21 preamplifier, 211 negative feedback capacitor, 212 negative feedback resistor, 22 main amplifier, 23 A / D converter, 24 V / F converter, 25 counter, 26 calculation unit, 261 low range dose rate calculation unit, 262 high range dose rate calculation unit, 263 dose rate switching calculation unit, 27 display unit / operation unit, 100 dose rate measuring device

Claims (7)

A scintillator that emits light when radiation is incident;
A photomultiplier tube into which the light emitted from the scintillator is incident;
A load resistance through which the output current of the photomultiplier tube flows;
A preamplifier for amplifying the output of the photomultiplier tube through a capacitor;
A main amplifier that amplifies the output of the preamplifier;
An A / D converter for digitally converting the output of the main amplifier;
A V / F converter to which a voltage generated in the load resistor is input;
A counter for measuring the number of pulses output from the V / F converter;
A low range dose rate calculation unit for obtaining a low range dose rate RL from the output of the A / D converter;
A high range dose rate calculation unit for obtaining a high range dose rate RH from the output of the counter;
With a dose rate switching calculation unit that sets the display range of the display to low range or high range,
The dose rate switching calculation unit switches the display range of the display unit from a low range to a high range when the dose rate ratio RH / RL exceeds 1 + k1, assuming that coefficient k1> coefficient k2> 0, and the dose rate ratio RH / RL is A dose rate measuring device that switches the display range of the display unit from a high range to a low range when the value is lowered to 1 + k2.   The dose rate measuring apparatus according to claim 1, wherein the scintillator is made of plastic.   3. The dose rate measuring apparatus according to claim 2, wherein the scintillator includes a fiber bundle having a plurality of bundled scintillation fibers.   The dose rate measuring apparatus according to claim 3, wherein the fiber bundle includes plastic fibers arranged concentrically in a scintillation fiber.   5. The preamplifier includes an operational amplifier, a negative feedback capacitor, and a negative feedback resistor, and the load resistor and the negative feedback resistor are connected in parallel and have a positive temperature coefficient. The dose rate measuring apparatus according to item 1. A semiconductor sensor that generates charge when radiation is incident on it;
A load resistance through which the output current of the semiconductor sensor flows;
A preamplifier for amplifying the output of the semiconductor sensor via a capacitor;
A main amplifier that amplifies the output of the preamplifier;
An A / D converter for digitally converting the output of the main amplifier;
A V / F converter to which a voltage generated in the load resistor is input;
A counter for measuring the number of pulses output from the V / F converter;
A low range dose rate calculation unit for obtaining a low range dose rate RL from the output of the A / D converter;
A high range dose rate calculation unit for obtaining a high range dose rate RH from the output of the counter;
With a dose rate switching calculation unit that sets the display range of the display to low range or high range,
The dose rate switching calculation unit switches the display range of the display unit from a low range to a high range when the dose rate ratio RH / RL exceeds 1 + k1, assuming that coefficient k1> coefficient k2> 0, and the dose rate ratio RH / RL is A dose rate measuring device that switches the display range of the display unit from a high range to a low range when the value is lowered to 1 + k2.   The dose rate according to claim 6, wherein the preamplifier includes an operational amplifier, a negative feedback capacitor, and a negative feedback resistor, and the load resistor and the negative feedback resistor are connected in parallel and have a positive temperature coefficient. measuring device.

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