JP6497803B2 - Scintillation type radiation measuring method and radiation measuring apparatus - Google Patents

Description

  The present invention relates to a scintillation type radiation measurement method and a radiation measurement apparatus that perform self-calibration.

  A scintillation type radiation measuring apparatus is known as a measuring apparatus that easily measures a radiation dose in a natural environment. The scintillation type radiation measuring apparatus includes a scintillator detector including a scintillator and a photodetector, and a signal processing unit that processes an output signal from the photodetector and calculates a radiation dose.

  When the scintillator receives gamma rays, the scintillator generates fluorescence having an intensity corresponding to the energy, and the photodetector receives the generated fluorescence and generates a detection signal (electric signal) having an intensity corresponding to the fluorescence intensity. In the signal processing unit, the number of detections of the signal of each energy intensity is counted for a fixed time, and the radiation dose is calculated based on this. Patent Document 1 proposes a radiation measuring apparatus having this configuration.

  In the radiation measuring apparatus, calibration is necessary so that the measurement error is within a predetermined range. In general, at the time of initial setting of the radiation measurement apparatus, for example, calibration is performed based on a detection result when gamma rays from cesium 137 are received. However, the radiation measurement apparatus has a large variation in measurement due to use conditions (temperature, use state, etc.), secular change, and the like. Therefore, it is desirable to perform calibration every measurement.

  For this reason, conventionally, a radiation measuring apparatus incorporating a self-calibration mechanism has been proposed. Patent Document 2 proposes a radiation detector in which a calibration light source is disposed, calibration light from the calibration light source is emitted toward an index scintillator, and calibration is performed based on the output of the index scintillator.

JP 2013-195254 A JP 2008-122111 A

  However, separately providing a self-calibration mechanism including a calibration light source and a calibration scintillator for calibration leads to an increase in the size and cost of the apparatus. In particular, it is not suitable for a portable simple radiation measuring apparatus.

  An object of the present invention is to propose a scintillation-type radiation measurement method and a radiation measurement apparatus that can perform self-calibration without separately providing a calibration light source, a calibration scintillator, and the like.

  In order to solve the above problems, in the scintillation type radiation measurement method of the present invention, a crystal body mainly composed of lutetium is used as a scintillator, and a self-radiating gamma ray of a radioisotope contained in the lutetium is used for calibration. It is characterized by being used as a radiation source.

  That is, the self-radiation energy information contained in the energy region of the self-radiation gamma ray of the lutetium radioisotope is separated from the gamma-ray energy information measured using a scintillator, and the measurement self-calibration is performed based on the self-radiation energy information. Like to do.

  A scintillator with a high density and a large amount of light has a high gamma ray measurement efficiency, and lutetium is the best example. Lutetium contains 2.6% radioactive isotopes in nature, and gamma rays are self-emitted from the radioactive isotopes. When lutetium is used as a scintillator, the self-radiating gamma ray becomes background noise and adversely affects the measured value.

  The present inventor has paid attention to the self-radiating gamma rays of radioisotopes contained in lutetium, which has been regarded as unnecessary noise in the past, and if this is actively used, the self-calibration of the radiation measuring apparatus can be simplified. They came up with what they could do with the configuration.

  That is, the energy region of the self-radiating gamma ray of the radioactive isotope of lutetium does not overlap the energy region with respect to nuclides such as cesium 134, cesium 137, and sodium 24, which are general measurement targets. Located at the lower position. Therefore, it is possible to extract or separate light components caused by self-radiating gamma rays from scintillation light. Also, the half-life of the lutetium radioisotope is extremely long, and it can be considered that the radiation dose of the self-emitting gamma rays is substantially constant. Therefore, it is possible to calibrate with high accuracy by using the self-radiating gamma ray of the radioactive isotope of lutetium as a radiation source for calibration of the measuring apparatus.

  Specifically, as the self-radiation energy information, information on an energy region including two peaks of 201 keV and 306 keV in the gamma ray spectrum of lutetium 176 may be sorted to perform self-calibration of the measuring apparatus.

In addition, as a scintillator containing lutetium, a LYSO single crystal scintillator, LFS (
(Lutetium silicate) scintillator or LSO scintillator can be used.

  In the self-calibration, the self-radiation dose calculated based on the self-radiation energy information may be calibrated so as to coincide with the radiation dose measured in advance.

  As described above, the energy region of the self-radiating gamma ray of the lutetium radioisotope does not overlap the energy region of nuclides such as cesium 134, cesium 137, sodium 24, etc. It is located at the lower side of the energy region. Therefore, in the radiation measurement method of the present invention, when measuring gamma rays incident from the outside, the measurement energy information contained in the energy region above the energy region of the self-radiating gamma rays is separated from the measured energy information. Based on the energy information for measurement, incident gamma rays incident on the scintillator from outside can be measured.

  Next, the present invention is a radiation measurement apparatus using the above-described radiation measurement method, a scintillator including a crystal having lutetium as a main component, a photodetector that photoelectrically converts scintillation light generated in the scintillator, A signal processing unit that performs self-calibration and radiation dose calculation based on the detection signal output from the photodetector.

The signal processing unit separates the calibration signal included in the energy region of the self-radiation gamma ray of the lutetium radioisotope from the detection signal, and performs self-calibration of the measurement apparatus based on the calibration signal. Further, the measurement signal in the energy region above the energy region of the self-radiating gamma ray is separated from the detection signal, and the radiation dose of the incident gamma ray incident on the scintillator is calculated based on the measurement signal.

  For example, when the power of the radiation measuring apparatus is turned on, the signal processing unit may perform self-calibration processing for a predetermined time, and thereafter, start an incident gamma ray measurement operation.

  As described above, in the present invention, since the scintillator containing high-density lutetium as a main component is used as the scintillator, the measurement efficiency of gamma rays is high. In addition, since self-calibration is performed using self-radiating gamma rays of radioactive isotopes contained in lutetium, it is possible to improve the measurement accuracy of gamma rays without arranging another radiation source for self-calibration. it can. Furthermore, since gamma rays in an energy region higher than that of radioactive cesium can be reliably measured, it is possible to realize a measuring device capable of presenting highly accurate measurement results in an energy region that has a large influence on the human body.

It is a block diagram which shows the whole structure of a radiation measuring device. It is a functional block diagram which shows the signal processing part of a radiation measuring device. It is a signal wave height distribution map of the self-radiation gamma ray of the radioactive isotope of lutetium. It is a signal wave height distribution map of incident gamma rays (cesium 137 radiation source). It is a signal wave height distribution map of incident gamma rays (cobalt 60 radiation source).

  Embodiments of a radiation measuring apparatus to which the present invention is applied will be described below with reference to the drawings. FIG. 1 is a block diagram showing the overall configuration of a radiation measuring apparatus according to the present invention. The radiation measuring apparatus 1 is based on a scintillator 2 that generates radiation upon receiving radiation (gamma rays), an optical sensor 3 that detects fluorescence (visible light) emitted from the scintillator 2, and an output signal from the optical sensor 3. And a signal processing unit 4 that performs calibration and calculation of radiation dose. In addition, a power supply unit 5 that supplies driving power to each unit, and an operation / display unit 6 including a power switch, a liquid crystal display unit that displays measurement values, and the like are provided.

The scintillator 2 is made of a crystal having lutetium as a main component, and is, for example, a LYSO single crystal scintillator, an LFS (lutetium silicate) scintillator, or an LSO scintillator. Both are high-density scintillators having a density of 7.4 g / cm ³ . A scintillator having a higher density can detect radiation more efficiently. These scintillators contain Ce as an impurity in order to increase the amount of light.

  The scintillator 2 is, for example, a rod-shaped member having an outer dimension of 3 mm × 3 mm × 15 mm and a rectangular end surface shape. An optical sensor 3 is attached to one end face of the scintillator 2 using an adhesive having a refractive index that approximates the refractive index of the scintillator 2.

  The scintillator 2 includes lutetium 176, which is a radioactive isotope of lutetium. The self-radiating gamma rays emitted from lutetium 176 include 88 keV, 201 keV, 306 keV, and 400 keV gamma rays.

As the optical sensor 3, for example, MPPC (Multi-Pixel Photo Counter: manufactured by Hamamatsu Photonics) can be used. The MPPC is an extremely small product having a light receiving portion of about 1 mm × 1 mm in which a large number of minute avalanche photodiodes are arranged vertically and horizontally. MPPC operates at a low voltage of about 80 V, has a high gain of 100,000 times, can be used at room temperature, and is not affected by a magnetic field.

  The output signal of MPPC is the sum of the output signals of all the APD pixels constituting the light receiving surface. Therefore, when a plurality of pixels output signals at the same time, the MPPC output is a superposition of those signals. By measuring the height (intensity) of this signal and the amount of integrated charge, it is possible to know how many photons are incident.

  FIG. 2 is a conceptual functional block diagram of the signal processing unit 4. The signal processing unit 4 includes a control unit 10 that controls driving of each unit. Under the control of the control unit 10, the light detection signal S <b> 1 captured from the optical sensor 3 is amplified via the signal amplification unit 11. The signal sorting unit 12 converts the amplified detection signal (voltage signal) S2 into a digital signal (pulse signal) according to the wave height. In addition, it is determined whether to send to the calibration signal counter 13 (calibration counter) or the measurement signal counter 14 (measurement counter) according to the wave height.

  That is, when the wave height is lower than a preset threshold value, the signal classification unit 12 supplies the calibration signal S3 obtained by digitizing (pulsing) the detection signal S2 to the calibration signal counting unit 13. . When the wave height is equal to or higher than the set threshold value, the measurement signal S4 obtained by digitizing (pulsing) the detection signal S2 is supplied to the measurement signal counter 14.

  The calibration signal counting unit 13 counts the calibration signal S3, and the measurement signal counting unit 14 counts the measurement signal S4. In the calculation unit 15, the number of radiations and the radiation dose are calculated based on the calibration signal count value N 1 supplied from the calibration signal counting unit 13 and the measurement signal count value N 2 supplied from the measurement signal counting unit 14. The The calculated value is displayed on the display screen of the operation / display unit 6.

  The calibration unit 16 of the control unit 10 performs a self-calibration operation for adjusting the set threshold value of the signal classification unit 12 based on the self-calibration radiation dose obtained from the calibration signal count value.

  Here, FIG. 3 is a diagram showing a signal wave height distribution of lutetium 176 which is a radioisotope included in the scintillator 2. In the figure, the horizontal axis indicates the channel of the multi-wave height discriminator used for measurement, and the vertical axis indicates the count number. The self-radiating gamma rays emitted from lutetium 176 include 88 keV, 201 keV, 306 keV, and 400 keV gamma rays.

  On the other hand, FIG. 4 and FIG. 5 are diagrams showing signal wave height distributions when measuring a cesium 137 radiation source and a cobalt 60 radiation source, respectively. As shown in these figures, the peak of cesium 137 appears in the vicinity of 662 keV, and cobalt 60 contains 1173 keV and 1333 keV gamma rays.

  The self-radiation energy region of lutetium 176 is lower than the energy region having a large influence on the human body, that is, the energy region of radioactive cesium or higher. Therefore, as shown in these spectrum diagrams, the threshold B set at a position including the energy region of radioactive cesium is sandwiched, the self-calibration region A is set on the lower side, and the threshold B or higher is set. Can be set to the signal measurement region C.

  Therefore, the detection signal whose wave height is lower than the set threshold value B corresponds to the self-radiation energy information of the scintillator 2, and the detection signal with a wave height higher than this corresponds to the energy information of the incident gamma ray. The self-calibration can be performed based on the counting result of 13, and the incident radiation dose can be accurately measured by the measurement signal counting unit.

(Measurement operation example)
An example of the radiation measurement operation of the radiation measurement apparatus 1 will be briefly described below. First, when the power is turned on, the radiation measuring apparatus 1 performs a self-calibration operation for a predetermined time, and then starts an incident gamma ray measurement operation. Of course, if necessary, a calibration operation can be performed based on an external input from the operation / display unit 6. The operation state is displayed on the display screen of the operation / display unit 6.

  In the self-calibration operation after the power is turned on, the detection signal S2 whose wave height is lower than the set threshold B is supplied to the calibration signal counting unit 13 as the calibration signal S3 via the signal sorting unit 12. The calibration signal counting unit 13 counts the calibration signal count value N1 corresponding to the self-radiation energy information included in the energy region of the self-radiation gamma ray of the lutetium radioisotope from the calibration signal S3.

  Based on the calibration signal count value N1 (self-radiated energy information), the self-calibration operation of the radiation measuring apparatus 1 is performed. During the calibration operation, that effect is displayed on the display screen of the operation / display unit 6.

  The calibration unit 16 stores and holds self-radiant energy information of the same energy region measured in advance as a reference value for self-calibration. The calibration unit 16 adjusts the set threshold value B in the signal classification unit 12 so that the calculated radiation dose of self-radiation energy matches the reference value.

  After the self-calibration operation is finished, the incident gamma ray measurement operation is started. In the measurement operation, the measurement signal S4 included in the signal measurement region C (see FIG. 3) is separated from the detection signal S2 (measured energy information) and supplied to the measurement signal counter 14. The measurement signal count value N2 counted by the measurement signal counter 14 is measurement energy information included in the energy region above the energy region of the self-radiating gamma ray, and based on this, the number of radiations and the radiation dose are Calculated with high accuracy.

  In addition, when measuring a radiation dose simply like measuring the radiation dose in a natural environment, the analysis of the energy spectrum of a radiation is not required. However, in order to identify the nuclide, an energy spectrum of radiation may be required.

  In this case, it is only necessary to connect a pulse height analyzer to the output side of the optical sensor 3 in the radiation measuring apparatus 1 and perform the pulse height analysis. Various known analyzers are provided as wave height analyzers, and the energy spectrum of radiation can be obtained by using these analyzers. Therefore, it is also possible to configure the measurement device so that the energy spectrum can be analyzed together with the measurement of the dose of natural radiation. Even in this case, the accuracy of nuclide identification can be increased by adjusting the upper and lower thresholds of the wave height for signal classification in the wave height analyzer by self-calibration.

DESCRIPTION OF SYMBOLS 1 Radiation measurement apparatus 2 Scintillator 3 Optical sensor 4 Signal processing part 5 Power supply part 6 Operation / display part 10 Control part 11 Signal amplification part 12 Signal classification part 13 Calibration signal counting part 14 Measurement signal counting part 15 Calculation part 16 Calibration part S1 Photodetection signal S2 Amplified detection signal S3 Calibration signal S4 Measurement signal N1 Calibration signal count value N2 Measurement signal count value A Self calibration area B Setting threshold C Signal measurement area

Claims (6)

As a scintillator, a crystal body mainly composed of lutetium is used,
From the gamma ray energy information measured using the scintillator, the self-radiation energy information contained in the energy region of the self-radiation gamma ray of the lutetium radioisotope is separated,
Perform self-calibration of measurement based on the self-radiant energy information,
From the energy information of the gamma rays measured using the scintillator, the measurement energy information contained in the energy region above the energy region of the self-radiating gamma rays is separated,
A scintillation type radiation measurement method for measuring incident gamma rays incident on the scintillator based on the measurement energy information.   The scintillation-type radiation measurement method according to claim 1, wherein information on an energy region including two peaks of 201 keV and 306 keV in the gamma ray spectrum of lutetium 176 that is the radioisotope is classified as the self-radiation energy information.   The scintillation-type radiation measurement method according to claim 1, wherein the scintillator is a LYSO single crystal scintillator, an LFS (lutetium silicate) scintillator, or an LSO scintillator. A scintillator including a crystal containing lutetium as a main component;
An optical sensor that photoelectrically converts scintillation light generated in the scintillator;
Based on the detection signal output from the optical sensor, a signal processing unit for performing self-calibration processing and radiation dose calculation processing,
Have
In the self-calibration process, the calibration signal included in the energy region of the self-radiation gamma ray of the lutetium radioisotope is separated from the detection signal, and self-calibration is performed based on the calibration signal.
In the calculation process, a measurement signal included in an incident gamma ray energy region above the energy region of the self-radiating gamma ray is separated from the detection signal, and an incident gamma ray incident on the scintillator is based on the measurement signal. A scintillation type radiation measuring apparatus characterized by calculating a radiation dose.   5. The scintillation equation according to claim 4, wherein in the self-calibration process, energy region information including two peaks of 201 keV and 306 keV in the gamma-ray spectrum of the radioisotope lutetium 176 is classified as the calibration signal. Radiation measurement device.   The scintillation type radiation measuring apparatus according to claim 4 or 5, wherein the scintillator is a LYSO single crystal scintillator, an LFS (lutetium silicate) scintillator, or an LSO scintillator.