JP2008281442A - Radiation measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To detect light from a scintillator by utilizing a photodetection element array. <P>SOLUTION: Each APD element in an APD array 20 detects whether photons enter the APD element at each sampling timing or not over a plurality of sampling timings. A photon number integration part 40 calculates the spatial total number of the number of photons from the element number of each APD element entered by the photons. Further, the photon number integration part 40 extracts a plurality of sampling timings corresponding to an emission time of the scintillator 10, and integrates the spatial total number of the number of the plurality of extracted sampling timings, to thereby calculate a spatial and time integrated value of the photons during the emission time. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a radiation measuring apparatus, and more particularly to a technique for detecting radiation using a scintillator.

  As a radiation measurement apparatus, an apparatus using a semiconductor detector is known. For example, Patent Document 1 discloses a battery-driven radiation measuring apparatus using a semiconductor detector.

  An apparatus using a scintillation detector in a radiation measuring apparatus is also known. The scintillation detector uses a scintillator that generates light when radiation enters, and detects extremely weak light generated by the scintillator using a photodetector such as a photomultiplier tube (PMT).

  The PMT needs to apply a high voltage of about 1000 volts in order to amplify the light detection signal. In order to stabilize the gain in the PMT, it is desirable that the high voltage applied to the PMT is controlled to have an appropriate voltage value. However, since the voltage is high, it is not easy to stably control the voltage applied to the PMT. Therefore, in a scintillation detector using a PMT, for example, a voltage applied to the PMT is manually adjusted and then measured for a short time, and after the measurement, the voltage applied to the PMT is further adjusted as necessary. Need to maintain the accuracy of measurement.

  As described above, while various techniques relating to radiation measuring apparatuses are known, semiconductor devices such as avalanche photodiodes (APDs) have recently attracted attention as devices for detecting light. For example, a photodetector using APD can be operated with a relatively low bias voltage, and is used in the field of optical data transmission.

Japanese Patent Application Laid-Open No. 3-243884

  In the background described above, the inventor of the present application has repeatedly researched and developed a technique for detecting the light of the scintillator using APD or the like instead of the PMT. In particular, since a light detection element array having a plurality of light detection elements can be formed by APD or the like, attention has been paid to a technique for detecting light using the light detection element array.

  The present invention has been made in the course of research and development, and an object thereof is to detect light of a scintillator by using a light detection element array.

  In order to achieve the above object, a radiation measuring apparatus according to a preferred aspect of the present invention includes a scintillator that generates light when gamma rays enter, and a plurality of light detection elements that detect light generated in the scintillator for each light detection element. A space of light generated by the incidence of gamma rays by spatially adding a light detection element array including a light detection element and a light detection result detected for each light detection element to a plurality of light detection elements. And a total amount calculation unit for calculating a total amount.

  In a preferred aspect, the total amount calculation unit is generated by the incidence of gamma rays by temporally adding the detection results of light detected by the plurality of light detection elements within a period corresponding to the light emission time of the scintillator. It is characterized by calculating the total amount of light in time. In the above configuration, the temporal addition process may be performed after performing the spatial addition process, or the spatial addition process may be performed after performing the temporal addition process.

  In a preferred aspect, each photodetection element detects light generated in a scintillator for each photon, and the total amount calculation unit calculates the number of photons detected by each of the plurality of photodetection elements in the entire photodetection element array. To calculate the spatial total of the number of photons, and integrate the spatial total within a period corresponding to the emission time to obtain a spatial count of the number of photons generated by the incidence of gamma rays. In addition, a temporal integrated value is calculated.

  In a preferred aspect, each of the light detection elements detects whether or not a photon is incident on the light detection element at each sampling timing over a plurality of sampling timings, and the total amount calculation unit is at each sampling timing. The spatial total number of photons is calculated from the number of photodetection elements incident to the photon, and multiple sampling timings corresponding to the light emission time are extracted based on the spatial total calculated for each sampling timing. Then, the integrated value is calculated by integrating the spatial total of the extracted sampling timings.

  In a desirable mode, the apparatus further includes a spectrum forming unit that forms a spectrum reflecting a distribution of energy obtained from gamma rays based on the integrated value calculated for each incidence of gamma rays.

  In a desirable mode, the spectrum forming unit forms a frequency distribution associating a plurality of integrated values calculated for each incidence of gamma rays and a count value indicating an appearance frequency of each integrated value as the spectrum. And

  According to the present invention, a technique for detecting light of a scintillator using a photodetecting element array is provided. For example, according to a preferred aspect of the present invention, it is possible to obtain a spatially and temporally integrated value of the number of photons with a relatively simple device configuration. Further, according to a more preferable aspect, it is possible to form a spectrum reflecting the energy distribution obtained from gamma rays based on the integrated value.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings.

  FIG. 1 shows a preferred embodiment of a radiation measuring apparatus according to the present invention, and FIG. 1 is a functional block diagram showing the overall configuration thereof. The radiation measurement apparatus of FIG. 1 is embodied as, for example, a dose monitor or a survey meter, but may be embodied in other forms.

  The scintillator 10 generates light when gamma rays (γ rays) are incident. That is, the scintillator 10 emits a number of photons proportional to the energy lost by the gamma ray by interacting with the incident gamma ray.

  The APD (Avalanche Photo Diode) array 20 includes a plurality of APD elements. The plurality of APD elements are arranged in a two-dimensional lattice pattern, for example. For example, the APD array 20 is formed by 10000 APD elements of 100 vertical and 100 horizontal. A one-dimensional APD array 20 in which a plurality of APD elements are arranged in a line may be formed. In addition, each APD element is formed in a square shape that is about 0.1 mm in length and width. Of course, each APD element may be a rectangle or other shapes.

Each APD element is a semiconductor device that detects light (photons) generated in the scintillator 10 and outputs a detection signal. Each APD element has an n ⁺ pπp ⁺ semiconductor structure, and a reverse bias voltage is applied. When a photon is incident on each APD element, the photon is absorbed in a relatively thick π region and a hole-electron pair is generated. The generated electron is accelerated by an electric field and further generates a hole-electron pair by impact ionization. To do. Newly generated electrons are also accelerated by the electric field, and further generate hole-electron pairs by impact ionization. In this way, pairs of electrons and holes are avalancheally increased, and finally output as a saturated electric signal (detection signal).

  The photon determination unit 30 determines whether a photon is detected in each APD element. The photon determination unit 30 includes a plurality of diodes corresponding to each of the plurality of APD elements. Each diode has a threshold level corresponding to one photon, whereby noise is discriminated and a photon is detected. Each diode determines whether a photon is detected in the APD element based on a detection signal output from the corresponding APD element. Each diode outputs an electrical signal (for example, a current signal) when a photon is detected in the corresponding APD element.

  The photon number integrating unit 40 adds the number of photons detected by the plurality of APD elements spatially and temporally based on the electrical signals output from the plurality of diodes of the photon determining unit 30. Photon detection is performed at each sampling timing over a plurality of sampling timings. The photon number accumulating unit 40 calculates the spatial total number of photons from the number of APD elements determined to have entered photons at each sampling timing. Furthermore, the photon number integration unit 40 extracts a plurality of sampling timings corresponding to the light emission time of the scintillator based on the spatial total calculated for each sampling timing, and spatially extracts the plurality of sampling timings extracted. The integrated value is calculated by integrating the total number.

  Therefore, the calculation process of the integrated value by the photon number integrating unit 40 will be described with reference to FIG. In the description using FIG. 2, the reference numerals in FIG. 1 are used for the portions (configurations) already shown in FIG.

  FIG. 2 is a diagram for explaining the integrated value calculation processing by the photon number integrating unit 40. In FIG. 2, time indicates each sampling timing. The total number of spatial photons is the total number of photons at each time and corresponds to the number of APD elements that have detected photons at each time. If two or more photons are incident on the same APD element at each sampling timing, the number of APD elements that have detected the photon and the total number of photons do not exactly match. Therefore, it is desirable that the sampling interval is such that a plurality of photons do not enter each ADP element. The sampling interval is, for example, about 10 ps (picosecond), and preferably about 1 ps.

In FIG. 2, for example, at time t ₀ , the total number of spatial photons is 0 (zero), which indicates that no photon was detected by the APD array 20 at time t ₀ . At time t ₁ , the total number of spatial photons is 1200, indicating that only 1200 photons were detected by the APD array 20 at time t ₁ . It has also been shown time t ₂ after the data in Figure 2. The data (table) shown in FIG. 2 is stored in a memory in the photon number integrating unit 40, for example.

  When gamma rays are incident on the scintillator 10, the scintillator 10 generates light (photons) by the gamma rays each time the gamma rays are incident. Therefore, photons are detected continuously in time within the time (light emission time) in which the scintillator 10 emits light due to the incidence of gamma rays.

In FIG. 2, times t _{1 to} t ₃ indicate data of light emission time A by gamma rays incident at a certain timing. That is, since the scintillator 10 does not emit light at time t ₀ before time t _{1 to} t ₃ , the total number of photons is 0 (zero). During the period from time t _{1 to} t ₃ , the scintillator 10 emits light. Therefore, photons are continuously detected during the period (light emission time A). Also, the total number of photons emission and end times t ₄ in the scintillator 10 after the time t ₁ ~t ₃ is 0 (zero).

Times t _{n + 2 to} t _{n + 4} indicate data of the light emission time B by gamma rays incident at different timings. In the light emission time B, as in the case of the light emission time A, photons are detected continuously in time within that period.

The photon number integrating unit 40 extracts a plurality of times corresponding to the light emission time based on the total number of spatial photons calculated at each time. The photon number integrating unit 40 extracts, for example, a plurality of times within a period in which photons are continuously detected from the time when one or more photons are detected. Thereby, the times t _{1 to} t ₃ and the times t _{n + 2 to} t _{n + 4} are extracted as a plurality of times corresponding to the light emission time. In consideration of the influence of noise and the like, a plurality of times within a period in which photons are continuously detected from a time when a predetermined number (for example, 100) or more photons are detected are represented as a plurality of times corresponding to the light emission time. May be extracted as Alternatively, only the start timing of the light emission time may be detected, and a period preset from that timing may be set as the light emission time. It is desirable that the preset period is set to be long enough to cover the light emission time and short enough not to include another light emission time. The period is set to about 100 ps to 500 ps, for example.

When a plurality of times corresponding to the light emission time are extracted, the photon number integration unit 40 calculates the integrated value by integrating the total number of photons within the light emission time. In FIG. 2, the integrated value of photons is a value obtained by integrating the total number of photons within a period corresponding to the light emission time. That is, by integrating the total number of photons at times t _{1 to} t ₃ extracted as the period corresponding to the light emission time, 3300 photon integrated values at the light emission time A are calculated. Similarly, by integrating the total number of photons at times t _{n + 2 to} t _{n + 4} extracted as the period corresponding to the light emission time, 1600 photon integrated values at the light emission time B are calculated. In this way, the photon number integrating unit 40 calculates the integrated value one after another for each light emission time.

  Returning to FIG. 1, the integrated values calculated one after another by the photon number integrating unit 40 are sequentially stored in the memory in the counting unit 50. The counting unit 50 divides the integrated value stored in the memory into a plurality of channels according to the size, and counts the number of appearances of the integrated value for each channel. In this way, a count distribution in which a plurality of channels and count values (counts) of the respective channels are associated is obtained. The count distribution formed in the counting unit 50 is displayed on the display unit 60.

  FIG. 3 is a diagram showing a count distribution in which a plurality of channels and counts of each channel are associated with each other. The horizontal axis in FIG. 3 indicates the channel c, and the vertical axis indicates the count n. Each channel is associated with the magnitude of the integrated value output from the photon number integrating unit (reference numeral 40 in FIG. 1). The integrated value corresponds to the total number of photons generated by gamma rays incident at a certain timing, and reflects the energy obtained from the gamma rays. Therefore, the count distribution shown in FIG. 3 reflects the distribution of energy obtained from gamma rays. The count distribution in FIG. 3 is called a spectrum.

  In FIG. 3, a waveform 80 is a count distribution curve corresponding to gamma rays emitted from the reference source. In the case of gamma rays, the interaction in the scintillator (reference numeral 10 in FIG. 1) includes photoelectric effect, Compton scattering, electron pair generation, and the like. That is, energy is obtained from gamma rays by these interactions. Among these interactions, the photoelectric effect or the like is an action that transfers the total energy of gamma rays to electrons or the like.

In FIG. 3, the total absorption peak P _B of the waveform 80 corresponds to the energy absorption of gamma rays due to the photoelectric effect or the like. That is, the total absorption peak P _B corresponds to the total energy of gamma rays, and the energy of gamma rays can be known from the position (channel).

  According to the above-described embodiment, it is possible to obtain a spatially and temporally integrated value of the number of photons with a relatively simple device configuration. For example, the integrated value of the number of photons can be calculated without using means for digitizing the detection signals output from the APD array 20 (such as an analog-digital converter). Further, according to the above-described embodiment, it is possible to form a spectrum reflecting the energy distribution obtained from gamma rays based on the calculated integrated value.

  As mentioned above, although preferred embodiment of this invention was described, embodiment mentioned above and its effect are only a mere illustration in all points, and do not limit the scope of the present invention. The present invention includes various modifications without departing from the essence thereof.

It is a functional block diagram which shows the whole structure of the radiation measuring device which concerns on this invention. It is a figure for demonstrating the calculation process of the integrated value by a photon number integration part. It is a figure which shows the count distribution which matched each channel and the count.

Explanation of symbols

  10 scintillators, 20 APD arrays, 30 photon determination units, 40 photon number integration units, 50 counting units, 60 display units.

Claims (6)

A scintillator that generates light by the incidence of gamma rays;
A photodetecting element array comprising a plurality of photodetecting elements for detecting light generated in the scintillator for each photodetecting element;
A total amount calculation unit that calculates a spatial total amount of light generated by the incidence of gamma rays by spatially adding the detection results of light detected for each light detection element for a plurality of light detection elements;
Having
A radiation measuring apparatus characterized by that. The radiation measurement apparatus according to claim 1,
The total amount calculation unit temporally adds the detection results of light detected by a plurality of light detection elements within a period corresponding to the light emission time of the scintillator, thereby temporally calculating the light generated by the incidence of gamma rays. Calculate the total amount,
A radiation measuring apparatus characterized by that. The radiation measurement apparatus according to claim 2,
Each of the light detection elements detects light generated in the scintillator for each photon,
The total amount calculation unit calculates a spatial total number of photons by adding the number of photons detected by each of the plurality of photodetecting elements over the entire photodetecting element array. By integrating the total number within the period corresponding to the emission time, the spatial and temporal integration value of the number of photons generated by the incidence of gamma rays is calculated.
A radiation measuring apparatus characterized by that. The radiation measurement apparatus according to claim 3.
Each of the light detection elements detects whether a photon enters the light detection element at each sampling timing over a plurality of sampling timings,
The total amount calculation unit calculates a spatial total number of photons from the number of photodetecting elements on which photons are incident at each sampling timing, and emits light based on the spatial total calculated at each sampling timing. Extracting a plurality of sampling timings corresponding to time, and calculating the integrated value by integrating the spatial total of the extracted sampling timings,
A radiation measuring apparatus characterized by that. The radiation measurement apparatus according to claim 4,
A spectrum forming unit that forms a spectrum reflecting a distribution of energy obtained from gamma rays based on the integrated value calculated for each incidence of gamma rays;
A radiation measuring apparatus characterized by that. The radiation measurement apparatus according to claim 5,
The spectrum forming unit forms, as the spectrum, a frequency distribution in which a plurality of integrated values calculated for each incidence of gamma rays and a count value indicating an appearance frequency of each integrated value are associated with each other.
A radiation measuring apparatus characterized by that.

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