JP5649044B2 - Radiation detector - Google Patents

Description

  The present invention relates to a radiation detection apparatus that detects radiation.

  Some radiation detection devices that detect radiation use scintillators and photodetectors. A scintillator converts radiation into light by a scintillation phenomenon. The radiation detection apparatus detects incident radiation by detecting light converted by the scintillator with a photodetector. The radiation includes alpha rays, beta rays, gamma rays, X rays, neutron rays, charged particle rays and the like.

  FIG. 1 is a diagram illustrating an example of radiation incident on a radiation detection apparatus. The radiation detection apparatus 3 in FIG. 1 includes a radiation scintillator 1 and a photodetector 2. The radiation scintillator 1 converts radiation into light. The photodetector 2 detects the light converted by the radiation scintillator 1. The radiation incident on the radiation detection device 3 is converted into light by the radiation scintillator 1, and the converted light is detected by the photodetector 2.

  Further, there is a radiation detector that can identify the position where the radiation is incident by detecting light converted by the scintillator with a plurality of photodetectors.

JP 2008-8675 A JP 2009-121929 A JP 2009-25308 A JP 2004-264078 A JP 61-225683 A JP 2000-180551 A JP-A-6-347557 JP 60-73484 A JP-A-60-135883

In a radiation detector, when detecting radiation by light converted by a scintillator, a position where the radiation is incident may be specified by using a plurality of photodetectors by a charge division method or the like. Moreover, when detecting a position in a wide area | region, more photodetectors may be used. However, when a plurality of photodetectors are used, stationary noise generated in proportion to the number of photodetectors increases. On the other hand, the total amount of light incident on the photodetector does not change even if the number of photodetectors is increased. Therefore, as a result, the signal-to-noise ratio (SNR) in the radiation detector is reduced by increasing the number of photodetectors. Therefore, it is required to reduce the noise output from each photodetector.

  It is an object of the present invention to provide a radiation detection apparatus that reduces noise output from a photodetector.

  The disclosed radiation detection apparatus employs the following means in order to solve the above problems.

That is, the first aspect is
A scintillator having an incident surface on which the radiation is incident and generating light from the incident radiation;
A plurality of photodetectors arranged in a one-dimensional manner for detecting the generated light;
Extending a predetermined resistance value portion a plurality of times, and comprising a linear resistance portion capable of connecting a branch line for each resistance value portion;
The photodetector includes an optical detection element that generates an electric signal from light, and a signal extraction circuit that suppresses a signal component that does not reach a reference signal intensity in the electric signal and extracts a signal component that reaches the reference signal intensity. Have
An output signal of each signal extraction circuit included in the plurality of photodetectors arranged in a one-dimensional manner is input to the linear resistance unit through the branch line, and the one-dimensionally arranged photodetectors The position is a radiation detection apparatus that is associated with the resistance value of the linear resistance portion at the connection position of the branch line to which the output signal of the signal extraction circuit of each photodetector is input.

  ADVANTAGE OF THE INVENTION According to this invention, the radiation detection apparatus which reduces the noise which a photodetector outputs can be provided.

FIG. 1 is a diagram illustrating an example of radiation incident on a radiation detection apparatus. FIG. 2 is a diagram illustrating a configuration example (1) of the neutron detection apparatus. FIG. 3 is a diagram illustrating a configuration example (2) of the neutron detection apparatus. FIG. 4 is a diagram illustrating an example of a photodetector and a resistance of the neutron detection apparatus. FIG. 5 is a diagram illustrating an example (1) of the photodetector of the neutron detection apparatus. FIG. 6 is a diagram illustrating an example (2) of the photodetector of the neutron detection apparatus. FIG. 7 is a diagram illustrating an example of the result of calculating the position by the charge division method. FIG. 8 is a diagram illustrating an example of the relationship between the bias voltage and the peak half width in the graph of FIG. FIG. 9 is a diagram illustrating an example (3) of the photodetector of the neutron detection apparatus. FIG. 10 is a diagram illustrating an example (4) of the photodetector of the neutron detection apparatus. FIG. 11 is a diagram illustrating an example (5) of the photodetector of the neutron detection apparatus. FIG. 12 is a diagram illustrating an example (6) of the photodetector of the neutron detection apparatus. FIG. 13 is a diagram illustrating an example (7) of the photodetector of the neutron detection apparatus.

  Hereinafter, embodiments will be described with reference to the drawings. The configuration of the embodiment is an exemplification, and is not limited to the configuration of the disclosed embodiment.

  Here, a radiation detection apparatus that detects neutrons, that is, a neutron detection apparatus will be mainly described.

Embodiment 1
(Configuration example)
FIG. 2 is a diagram illustrating an example (1) of the neutron detection apparatus. In the example of FIG. 2, the neutron detection apparatus 10 includes a neutron scintillator 100, a plurality of photodetectors 300, and a plurality of resistors 400.

  The neutron detection apparatus 10 in FIG. 2 includes a flat neutron scintillator 100, a photodetector 300 disposed on one surface side of the neutron scintillator 100, and a resistor 400. Neutrons to be detected are incident on the surface of the neutron scintillator 100 opposite to the surface on which the photodetector 300 is disposed.

  The neutron scintillator 100 is a medium that converts neutrons into light. As the neutron scintillator 100, for example, ZnS, Li glass, or LBO single crystal can be used. The neutron scintillator 100 is not limited to these.

  The photodetector 300 detects the light converted by the neutron scintillator 100. The photodetector 300 converts the incident light into an electrical signal or the like and outputs it. The photodetector 300 outputs an electrical signal or the like depending on the amount of incident light. For example, the photodetector 300 outputs a charge proportional to the amount of incident light. For example, a semiconductor photodetector such as an MPPC (Multi-Pixel Photon Counter) or a photomultiplier tube can be used as the photodetector element of the photodetector 300. The neutron detection apparatus 10 may be provided with a plurality of photodetectors 300. The plurality of photodetectors 300 are provided on a straight line. That is, the plurality of photodetectors 300 are arranged one-dimensionally. By using a plurality of photodetectors 300, the position where the neutron is incident can be calculated. When radiation is incident on a position away from the photodetector 300, the light detected by the photodetector 300 becomes weak and the output from the photodetector 300 becomes small. Further, when radiation is incident at a position close to the light detector 300, the light detected by the light detector 300 becomes strong, and the output from the light detector 300 increases.

  The output of each photodetector 300 is connected to a charge dividing circuit 40 in which a resistor 400 is connected in series. The resistance values of the resistors 400 are equal resistance values. As shown in FIG. 2, the detection signal of each photodetector 300 is input to the connection portion between two adjacent resistors 400 in the charge dividing circuit 40. A connection portion between two adjacent resistors 400 in the charge dividing circuit 40 is connected to the photodetector 300 through a branch line. As shown in FIG. 2, connecting each photodetector 300 between the series resistors of the charge dividing circuit 40 is referred to as “connecting the photodetector 300 to the series resistor circuit by dividing the resistance”. The charge dividing circuit 40 can be a linear resistance portion that can extend a predetermined resistance value portion a plurality of times and connect a branch line connected to the photodetector 300 for each of the resistance value portions.

The output of each photodetector 300 is resistance-divided by the charge dividing circuit 40. The resistor 400 is a charge dividing resistor. As shown in FIG. 2, outputs are obtained from both ends of the charge dividing circuit 40. Based on these outputs, the position where neutrons are incident can be calculated by the charge splitting method or the like. For example, a charge amplifier is connected to both ends of the charge dividing circuit 40. The output of the charge amplifier is converted into digital data by, for example, an AD (Analog to Digital) converter or the like and input to a computer. The computer calculates the position where the neutron is incident by the charge division method or the like.

  FIG. 3 is a diagram illustrating an example (2) of the neutron detection apparatus. In the example of FIG. 3, the neutron detection apparatus 10 includes a neutron scintillator 100, a light transmission plate 200, a plurality of photodetectors 300, and a plurality of resistors 400. Here, differences from the neutron detection apparatus 10 of FIG. 2 will be mainly described.

  The neutron detection apparatus 10 of FIG. 3 includes a flat neutron scintillator 100, a flat light transmission plate 200 disposed on one surface side of the neutron scintillator 100, and a neutron scintillator 100 of the light transmission plate 200. The photodetector 300 and the resistor 400 are disposed on the surface side opposite to the surface. Neutrons to be detected are incident on the surface of the neutron scintillator 100 opposite to the surface on which the transmission plate 200 is disposed.

The neutron scintillator 100 and the light transmission plate 200 may be in contact with each other. Further, the light transmission plate 200 and the photodetector 300 may be in contact with each other. Here, the light transmission plate 200 may be configured to diffuse incident light (for example, light diffusion glass). When the light transmission plate 200 is a light diffusing glass, the light incident on the light transmission plate 200 is diffused by the light transmission plate 200. When the light is diffused, the light is easily detected by more photo detectors 300. Further, by using borosilicate glass that absorbs neutrons as light diffusion glass, the light transmission plate 200 can absorb neutrons while diffusing light. The thickness of the light transmission plate 200 is set to be equal to or larger than the installation interval of the photodetectors 300. When the thickness of the light transmission plate 200 that diffuses light is increased, the distance from the position where the light is diffused to the photodetector 300 becomes longer, so that the light incident on the photodetector becomes weaker. In general, the intensity of light is inversely proportional to the square of the distance from the light source. However, as the distance from the position where the light is diffused to the photodetector 300 increases, the range in which the light is diffused increases, and the number of photodetectors 300 that detect the light increases. By detecting the light with many photodetectors 300, the accuracy of the position when calculating the position where the light is incident increases. For example, when the thickness of the light transmission plate 200 is the same as the installation interval of the photodetectors 300, if light is diffused by 90 degrees on the incident side surface of the light transmission plate 200, two or three lights The detector 300 detects light. Therefore, for example, the thickness of the light transmission plate 200 is set to be equal to the installation interval of the photodetectors 300 so that the light is detected by the plurality of photodetectors 300 and the light incident on the photodetectors 300 is not weakened. The same degree is desirable.

  The light transmission plate 200 allows incident light to pass therethrough. The light transmission plate 200 absorbs incident neutrons. As the light transmission plate 200, for example, borosilicate glass is used. Borosilicate glass is a glass containing boron. Boron-containing glass can absorb neutrons. The thickness of the transmission plate 200 can be determined by the amount of incident neutrons and the amount of neutrons that affect the photodetector 300. The light transmission plate 200 is not limited to borosilicate glass. Neutrons that have not been converted by the neutron scintillator 100 (neutrons that have passed through the neutron scintillator 100) are absorbed by the light transmission plate 200 that absorbs neutrons. Since neutrons that have passed through the neutron scintillator 100 are absorbed by the light transmission plate 200, neutrons that enter the photodetector 300 are reduced. The light transmission plate 200 protects the photodetector 300 from neutrons.

  When the light converted by the neutron scintillator 100 is not diffused and the light is detected by only one photodetector 300, the position resolution when calculating the position where the light is incident (position where the neutron is irradiated) is: It stays at about the installation interval of the photodetector 300. On the other hand, when light is diffused and light is detected by the plurality of photodetectors 300, the position resolution at the position where the light is incident becomes smaller than the installation interval of the photodetectors 300. This is because, when light is detected by a plurality of photodetectors 300, the positions can be specified by values obtained by apportioning between the photodetectors 300 by their respective output charges.

<Charge splitting method>
A method for detecting the position where radiation is incident by the charge division method will be described.

  FIG. 4 is a diagram illustrating an example of a photodetector and a resistance of the neutron detection apparatus. As shown in FIG. 4, a plurality of photodetectors 300 are arranged on a straight line at equal intervals. The interval between the photodetectors 300 is a distance W. The distance from the position of the photodetector 300 at one end (position 0) to the outside from the position of the photodetector 300 at one end on the straight line where the photodetector 300 is arranged, and the distance from the position of the photodetector 300 at the other end to the outside. The distance L is the distance up to W (position L). The position of the photodetector 300 refers to the center position of the light incident surface of the photodetector 300, for example. The output of each photodetector 300 is resistance-divided by a charge dividing circuit. Output charge is obtained from both ends of the charge dividing circuit 40.

  When light is incident on the photodetector 300, electric charges are output. The output charge (referred to as charge Q0) from the photodetector 300 is distributed in a ratio of resistance values from the position of the photodetector 300 to both ends. Here, the output charge obtained at one end is defined as charge Qa, and the output charge obtained at the other end is defined as charge Qb.

Further, the sum of resistance values from the photodetector 300 from which the charge Q0 is output to one end is R1, and the sum of resistance values to the other end is R2. The charge Qa obtained at one end is expressed as follows.

同様に、他方の端で得られる電荷Ｑｂは、次のように表される。

Similarly, the charge Qb obtained at the other end is expressed as follows.

複数の光検出器３００で光が同時に検出された場合は、それぞれの光検出器３００からの出力電荷が、両端への抵抗値の比で分配される。両端では、それぞれの光検出器３００からの出力電荷の和として、電荷Ｑａ及び電荷Ｑｂが得られる。

When light is detected at the same time by a plurality of photodetectors 300, the output charges from the respective photodetectors 300 are distributed in a ratio of resistance values to both ends. At both ends, a charge Qa and a charge Qb are obtained as the sum of the output charges from the respective photodetectors 300.

  When light is scattered, the photodetector 300 detects stronger light as it is closer to the light emission position. The photodetector 300 outputs a charge corresponding to the intensity of light. Therefore, when light is detected by a plurality of photodetectors 300, the position where the light is incident can be calculated with a position resolution smaller than the interval between the photodetectors 300.

  From the obtained charge Qa and charge Qb, the position x where light is incident is expressed as follows. The position x is a linear point where the photodetector 300 is disposed.

ここで、電荷Ｑａ及び電荷Ｑｂにそれぞれ雑音ｎが含まれているとすると、位置ｘは、次のように表される。

Here, assuming that noise n is included in each of the charge Qa and the charge Qb, the position x is expressed as follows.

ここで、位置ｘを求めるには、雑音ｎが次の条件を満たさなければならない。

Here, in order to obtain the position x, the noise n must satisfy the following condition.

この式を満たさないときは、信号が雑音に埋もれ、位置を正確に算出できないからである。

If this equation is not satisfied, the signal is buried in noise and the position cannot be accurately calculated.

  At this time, the position x can be expressed as follows.

よって、位置ｘは、次の式のように近似できる。

Therefore, the position x can be approximated by the following equation.

ここで、第２項のｎＬ／（Ｑａ＋Ｑｂ）が、おおよその位置のゆらぎとなる。光検出器３００の光検出デバイスの暗電流に基づく信号は、雑音ｎの原因の１つである。

Here, nL / (Qa + Qb) of the second term is an approximate position fluctuation. The signal based on the dark current of the light detection device of the light detector 300 is one of the causes of the noise n.

<< Photodetector (1) >>
FIG. 5 is a diagram illustrating an example (1) of the photodetector of the neutron detection apparatus. The photodetector 300 includes a signal extraction circuit that extracts an incident light signal. Here, MPPC is used as the light detection device of the light detector 300. The photodetector 300 in FIG. 5 includes a variable resistor VR01, a resistor R01, a resistor R02, a resistor R03, a capacitor C01, a capacitor C02, and MPPC. The photodetector 300 detects light when light is incident on the MPPC. The output of FIG. 5 is connected to the charge dividing circuit 40 of FIG.

  In order to efficiently supply electric charges to the resistor 400, the resistance value of the resistor R03 is set to a value as large as possible without causing any trouble in flowing dark current. The resistance value of the charge dividing circuit 40 is a value that becomes about 10 kΩ as a whole. If the resistance value of the charge dividing circuit 40 is small, the signal is relatively small with respect to thermal noise, and a good SN ratio cannot be obtained. The thermal noise is proportional to the reciprocal of the square root of the resistance value of the entire resistor 400. Therefore, thermal noise becomes smaller as the resistance value increases. Therefore, since the resistance value of the charge dividing circuit 40 is about 10 kΩ as a whole, the resistance value of the resistor R03 is set to about 10 kΩ to 100 kΩ.

  By adjusting the variable resistor VR01, the voltage applied to the MPPC can be finely adjusted. The adjustment range of the voltage applied to the MPPC is determined by the ratio of the resistance value of the resistor R02 and the resistance value of the variable resistor VR01. For example, if the resistance value of the resistor R01 is 4.7 kΩ, the resistance value of the resistor R02 is 500 kΩ, the maximum resistance value of the variable resistor VR01 is 10 kΩ, and the capacitance of the capacitor C01 is 0.1 μF, the resistance value of the resistor R02 And the resistance value of the variable resistor VR01, the voltage applied to the MPPC can be adjusted within a range of 2%. Since the operating voltage of many MPPCs is in the vicinity of 70V and the variation is within 1V, it can be sufficiently adjusted by the resistance value of the resistor R02 and the variable resistor VR01. For example, using a reference LED (Light Emitting Diode) light source or the like, the variable resistor VR01 is adjusted in each photodetector 300 so that the output values match.

  A power supply voltage V0 is applied to the photodetector 300. When light is incident on the MPPC, a potential is instantaneously generated in the resistor R03. The electric charge due to this potential is output as an output pulse of the photodetector 300 via the capacitor C02. The capacitor C02 cuts the DC component of the signal.

<< Photodetector (2) >>
FIG. 6 is a diagram illustrating an example (2) of the photodetector of the neutron detection apparatus. Here, the photodetector 300 includes a bipolar transistor in order to remove dark current noise. The photodetector 300 in FIG. 6 includes a variable resistor VR11, a resistor R11, a resistor R12, a resistor R13, a capacitor C11, and MPPC. The photodetector 300 in FIG. 6 further includes a resistor R14, a resistor R15, a capacitor C12, a capacitor C13, and a transistor Q11. The photodetector 300 detects light when light is incident on the MPPC. The output of FIG. 6 is connected to the charge dividing circuit 40 of FIG.

  The variable resistor VR11, resistor R11, resistor R12, resistor R13, capacitor C11, and MPPC of the photodetector 300 in FIG. 6 are the variable resistor VR01, resistor R01, resistor R02, and resistor of the photodetector 300 in FIG. It operates in the same manner as R03, capacitor C01, and MPPC.

  The photodetector 300 of FIG. 6 uses a bipolar transistor Q11 for output amplification. As shown in FIG. 6, an output resistor R15 is connected to the emitter of the bipolar transistor Q11 to form an emitter follower. The emitter follower is known as a circuit having an amplification factor of 1. Further, the bias voltage Vb is adjusted to the vicinity of the forward voltage of the transistor Q11. If the voltage drop between the base emitter due to the dark current does not exceed the voltage at which the bipolar transistor is turned on, the dark current will not be amplified. The amplifier circuit of the photodetector 300 in FIG. 6 corresponds to a class D amplifier circuit of a transistor. The amplifier circuit of the photodetector 300 in FIG. 6 allows the MPPC signal that has detected the light to pass therethrough. That is, the amplifier circuit of the photodetector 300 in FIG. 6 does not output a signal based on dark current noise.

Here, a specific example is shown. The resistance value of the resistor, the capacitance of the capacitor, the characteristics of the transistor, and the like are not limited to those described here. Variable resistor VR11 is 10 kΩ, resistor R11 is 4.7 kΩ, resistor R12 is 470 kΩ, resistor R13 is 47 kΩ, resistor R14 is 470 kΩ, resistor R15 is 4.7 kΩ, capacitor C11 is 0.1 μF, capacitor C12 is 22 pF, and capacitor C13 is 220 pF. Further, as the characteristics of the transistor Q11, it is assumed that the base emitter voltage V _BE = 500 to 600 mV and the collector current I _C = 0. The transistor Q11 has h _FE = 100. Therefore, the bias voltage Vb is adjusted to be 500 to 600 mV so that when the MPPC is not detecting light, the photodetector 300 does not output a signal based on dark current noise.

At this time, it is assumed that the MPPC detects light and a 1 V potential is generated in the resistor R13 for 1 μs. Then, a maximum current of 22 μA can flow through the capacitor C12 to the base terminal of the transistor Q11. Further, when h _{FE of} the transistor Q11 is 100, a maximum current of 2 mA flows through the resistor R15. However, since the resistor R15 is 4.7 kΩ, the current due to the input voltage 1V from the capacitor C12 is saturated at 0.2 mA, and the base current I _B is suppressed to a pulse signal of about 2 μA. Then, the bipolar transistor Q11 amplifies the input signal (voltage of the resistor R13) with an amplification factor of 1 as an emitter follower, and the photodetector 300 outputs a potential of slightly lower than 1V that is slightly lower than the resistor R13. As a result, noise due to dark current when MPPC is not detecting light is removed. Therefore, even if a plurality of photodetectors 300 are connected, noise does not increase.

FIG. 7 is a diagram illustrating an example of the result of calculating the position by the charge division method. The graph of FIG. 7 irradiates the neutron detection apparatus 10 having the photodetector 300 of FIG. 6 with light from the LED light source that is actually a reference, and obtains the output charges Qa and Qb of FIG. The position is calculated by the charge division method and counted (counted) for each channel (for each position). Here, 59 channels are used per 1 cm. That is, the straight line on which the photodetector 300 is arranged is divided into channels of 1/59 cm, and the position is counted for each channel. The width of this division can be freely set. Here, only the sum of Qa and Qb exceeds a predetermined value as a threshold is counted. This predetermined value is the lowest value among the values that the photodetector 300 is considered to detect light. In the graph of FIG. 7, the measurement is performed when the bias voltage Vb is 462 mV, 509 mV, 518 mV, 589 mV, 624 mV, 817 mV, and 1138 mV. The bias voltage Vb corresponds to the base bias signal of the bipolar transistor Q11. In the measurement of each bias voltage Vb, measurement is performed until the total count number of each channel reaches a predetermined count number. Further, here, the neutron detection apparatus 10 includes 16 photodetectors 300 (MPPC). The MPPCs are arranged on a straight line at intervals of 5 mm.

  The horizontal axis in FIG. 7 is the channel, and the vertical axis is the count number. The channel corresponds to a distance on a straight line where the photodetector 300 is arranged. The graph of FIG. 7 is an excerpt from 620 ch to 660 ch. From 620ch to 660ch, it is almost 7mm.

  When the bias voltage Vb is large, the base potential of the transistor Q11 rises and even a small signal is amplified. That is, when the bias voltage Vb is large, the potential due to the dark current is amplified even when the MPPC is not detecting light.

  When the bias voltage Vb is small, the base potential of the transistor Q11 is lowered and a small signal is not amplified. That is, when the bias voltage Vb is small, the potential due to the dark current is not amplified. However, if the bias voltage Vb is too low, the weak light signal detected by the MPPC cannot be detected.

  For example, in the graph of FIG. 7, when the bias voltage Vb is 509 mV, the channel is not counted in any channel other than the vicinity of the peak near 635 ch. This means that the influence of the dark current of the MPPC at a position where no light is detected is almost eliminated. Similarly, when the bias voltage Vb is 509 mV and when the bias voltage Vb is 462 mV, the influence of the dark current of the MPPC at a position where no light is detected is almost eliminated.

  Further, when the bias current Vb is 462 mV, the bias voltage is too low, so that the weak light signal detected by the MPPC cannot be detected. The signal of the photodetector 300 that has detected weak light is not output. Also, the detected strong light signal is slightly attenuated and output. Due to these influences, the accuracy of the position when calculating the position where the light is incident is lowered.

  On the other hand, when the bias voltage Vb is 1138 mV, it has a broad peak and is counted in channels other than the vicinity of the peak. The peak count number is smaller than when the bias voltage Vb is 509 mV. Since the bias voltage Vb is high, a small signal based on the dark current in the MPPC is output from each photodetector 300 without being removed. This signal becomes noise when calculating the position.

  FIG. 8 is a diagram illustrating an example of the relationship between the bias voltage and the peak half width in the graph of FIG. When the bias voltage Vb is 509 mV, the peak half width is the smallest. This peak half width corresponds to the position resolution of the neutron detection apparatus 10 using the photodetector 300 of FIG. That is, in the neutron detector 10 used in this measurement, the bias voltage Vb is preferably set to about 509 mV. By setting the bias voltage Vb to about 509 mV, the photodetector 300 can remove the influence of the dark current of the MPPC and can output a weak light signal detected by the MPPC. Therefore, the neutron detector 10 can accurately calculate the position where the light is incident (the position where the radiation is incident). However, the above values are measurement examples, and an appropriate bias voltage Vb differs depending on the dark current value, each element, and each lot for manufacturing an element.

<< Photodetector (3) >>
FIG. 9 is a diagram illustrating an example (3) of the photodetector of the neutron detection apparatus. Here, the photodetector 300 has a unipolar transistor (FET: Field Effect) to remove dark current noise.
Transistor, field effect transistor). The photodetector 300 in FIG. 9 includes a variable resistor VR.
21, resistor R21, resistor R22, resistor R23, capacitor C21, MPPC. The photodetector 300 in FIG. 9 further includes a resistor R24, a resistor R25, a capacitor C22, a capacitor C23, and a transistor Q21. The photodetector 300 detects light when light is incident on the MPPC. The output of FIG. 9 is connected to the charge dividing circuit 40 of FIG.

  The variable resistor VR21, resistor R21, resistor R22, resistor R23, capacitor C21, and MPPC of the photodetector 300 in FIG. 9 are respectively the variable resistor VR01, resistor R01, resistor R02, and resistor of the photodetector 300 in FIG. It operates in the same manner as R03, capacitor C01, and MPPC.

  The photodetector 300 in FIG. 9 uses a unipolar transistor Q21 for output amplification. As shown in FIG. 9, the output resistor R25 is connected to the source of the transistor Q21 to form a source follower. The sole follower is known as an amplifier circuit with an amplification factor of 1. Also, the bias voltage Vb is adjusted near the cutoff voltage of the transistor Q21, and the bias voltage Vb is adjusted so that the dark current noise does not exceed the threshold value Vth of the unipolar transistor Q21, so that the dark current is not amplified. The bias voltage Vb corresponds to the gate bias signal of the unipolar transistor Q21. The amplifier circuit of the photodetector 300 in FIG. 9 corresponds to a transistor class D amplifier circuit. The amplifying circuit of the photodetector 300 in FIG. 9 allows only the MPPC signal in which light is detected to pass. That is, the amplifier circuit of the photodetector 300 in FIG. 9 does not output a signal based on dark current noise.

Here, a specific example is shown. The resistance value of the resistor, the capacitance of the capacitor, the characteristics of the transistor, and the like are not limited to those described here. Variable resistor VR21 is 10 kΩ, resistor R21 is 4.7 kΩ, resistor R22 is 470 kΩ, resistor R23 is 47 kΩ, resistor R24 is 470 kΩ, resistor R25 is 4.7 kΩ, capacitor C21 is 0.1 μF, capacitor C22 is 22 pF, and capacitor C23 is 220 pF. Further, as a characteristic of the transistor Q21, it is assumed that the drain current I _D = 0 when the gate-source voltage V _GS = −600 mV or less. Therefore, the bias voltage Vb is adjusted to be −500 to −600 mV so that dark current noise is not felt when the MPPC is not detecting light.

At this time, it is assumed that the MPPC detects light and a potential of 1 V is generated in the resistor R23 for 1 μs. Then, a potential of about 1 V is generated at the gate terminal of the transistor Q21 through the capacitor C22. Further, a drain current flows through the transistor Q21, and a potential is generated in the resistor R25, and the corresponding gate-source voltage _VGS is maintained. That is, the transistor Q21 operates as a source follower with an amplification factor of 1, and the photodetector 300 outputs a potential of slightly lower than 1V that is slightly lower than the resistor R23. As a result, only noise due to dark current is removed. Therefore, even if a plurality of photodetectors 300 are connected, noise does not increase.

<< Photodetector (4) >>
FIG. 10 is a diagram illustrating an example (4) of the photodetector of the neutron detection apparatus. Here, the photodetector 300 includes a comparator in order to remove dark current noise. The photodetector 300 in FIG. 10 includes a variable resistor VR31, a resistor R31, a resistor R32, a resistor R33, a capacitor C31, and MPPC. The photodetector 300 in FIG. 10 further includes a resistor R34, a comparator 31, a switch SW31, and a capacitor C32. The photodetector 300 detects light when light is incident on the MPPC. The output of FIG. 10 is connected to the charge dividing circuit 40 of FIG.

  The variable resistor VR31, resistor R31, resistor R32, resistor R33, capacitor C31, and MPPC of the photodetector 300 in FIG. 10 are respectively the variable resistor VR01, resistor R01, resistor R02, and resistor of the photodetector 300 in FIG. It operates in the same manner as R03, capacitor C01, and MPPC.

  A power supply voltage V0 is applied to the photodetector 300. When light enters the MPPC, an electric potential is instantaneously generated in the resistor R33. The comparator CP31 compares the predetermined voltage Vb with the potential generated in the resistor R33. The comparator CP31 is one of comparison circuits. The voltage Vb is a constant value set in advance. The voltage Vb is an example of a reference signal for the comparison circuit. The comparator CP31 conducts the switch SW31 when the potential generated in the resistor R33 is higher than the voltage Vb. When the switch SW31 is turned on, a charge due to the potential caused by the incidence of light on the MPPC is output as an output charge of the photodetector 300 via the capacitor C32. The capacitor C32 cuts the DC component of the signal.

  Further, the comparator CP31 does not turn on the switch SW31 when the potential generated in the resistor R33 is smaller than the voltage Vb. Since the voltage Vb is a direct current, it is cut by the capacitor C32. Therefore, the photodetector 300 outputs almost 0 as the output charge.

  Here, the predetermined voltage Vb is set to a value higher than the voltage generated in the resistor R33 by the MPPC dark current. This value may be determined experimentally and empirically. Thereby, noise due to the dark current of MPPC is not output.

<< Photodetector (5) >>
FIG. 11 is a diagram illustrating an example (5) of the photodetector of the neutron detection apparatus. Here, the photodetector 300 includes an operational amplifier in order to remove dark current noise. The photodetector 300 of FIG. 11 includes a variable resistor VR41, a resistor R41, a resistor R42, a resistor R43, a capacitor C41, and MPPC. The photodetector 300 in FIG. 11 further includes a resistor R44, a resistor R45, a capacitor C42, a diode D41, and an operational amplifier IC41. The resistor R45 is a feedback resistor. The photodetector 300 detects light when light is incident on the MPPC. The output of FIG. 11 is connected to the charge dividing circuit 40 of FIG.

  The variable resistor VR41, resistor R41, resistor R42, resistor R43, capacitor C41, and MPPC of the photodetector 300 of FIG. 11 are the variable resistor VR01, resistor R01, resistor R02, and resistor of the photodetector 300 of FIG. It operates in the same manner as R03, capacitor C01, and MPPC.

  The photodetector 300 in FIG. 11 uses an operational amplifier IC41 for output amplification, and a diode D41 is connected in parallel with the feedback resistor R45 to remove noise. Further, the bias voltage Vb is adjusted to be close to the cutoff voltage of the diode D41 so that the signal is not amplified unless the signal exceeds the dark current noise. By using the operational amplifier IC41, the polarity of the signal is inverted, but it can be amplified by the ratio of the resistor R43 and the resistor R45, and the steepness of the cutoff voltage of the diode D41 can be increased by the amplification factor. The amplifier circuit of the photodetector 300 in FIG. 11 passes only the MPPC signal that has detected the light. That is, the amplifier circuit of the photodetector 300 in FIG. 11 does not output a signal based on dark current noise.

Here, a specific example is shown. The resistance value of the resistor, the capacitance of the capacitor, the characteristics of the transistor, and the like are not limited to those described here. Variable resistor VR41 is 10 kΩ at maximum, resistor R41 is 4.7 kΩ, resistor R42 is 470 kΩ, resistor R43 is 4.7 kΩ, resistor R
44 is 10 kΩ, resistor R45 is 10 kΩ, capacitor C41 is 0.1 μF, and capacitor C42 is 220 pF. Here, the resistance value of the resistor R44 and the resistance value of the resistor R45 are the same. Further, as a characteristic of the diode D41, it is assumed that the forward current I _F = 0 when the forward voltage V _{F is} 180 mV or less. Therefore, bias voltage Vb = -200 mV to -300 m
V is adjusted so that the light detector 300 does not output a signal based on dark current noise when the MPPC is not detecting light.

  There is an imaginary short between the input terminals of the operational amplifier IC41. When the output voltage of the operational amplifier IC41 is Vout and the input potential of the resistor R43 is Vin, the following is obtained.

ＭＰＰＣが光を検出していない状態では、抵抗Ｒ４３の入力電位Ｖｉｎの絶対値が、バイアス電圧Ｖｂ（＝−２００ｍＶ〜−３００ｍＶ）の絶対値よりも小さい。このとき、オペアンプＩＣ４１の出力電圧ＶｏｕｔがダイオードＤ４１のカットオフ電圧を超えると、ダイオードＤ４１に順方向の電流が流れ、増幅がされない。暗電流による抵抗Ｒ４３の電圧Ｖｉｎとバイアス電圧Ｖｂとがオペアンプの加算回路によって反転増幅されると、抵抗Ｒ４５の電圧がダイオードＤ４１のカットオフ電圧を超え、ダイオードＤ４１に電流が流れるからである。一方、ＭＰＰＣが光を検出した場合、抵抗Ｒ４３の電位（Ｖｉｎ）が大きくなるため、電圧ＶｏｕｔがダイオードＤ４１のカットオフ電圧を超えず、抵抗Ｒ４３と抵抗Ｒ４５との比で信号が増幅される。例えば、ＭＰＰＣが光を検出したことにより、抵抗Ｒ４３に１Ｖの電位が発生したとすると、抵抗Ｒ４１には、−２．１Ｖの電圧が発生する。この電圧は、ダイオードＤ４１の逆電圧になるため、ダイオードＤ４１には電流が流れない。例えば、抵抗Ｒ４４と抵抗Ｒ４５とが等しいとき、光検出器３００は、オペアンプの加算回路により、抵抗Ｒ４３と抵抗Ｒ４５との比から増幅された電圧からバイアス電圧Ｖｂが引かれた電圧による電荷を、コンデンサＣ４２を介して、出力パルスとして出力する。コンデンサＣ４２は、信号の直流成分をカットする。バイアス電圧Ｖｂは、加算回路における基準信号の一例である。

When the MPPC is not detecting light, the absolute value of the input potential Vin of the resistor R43 is smaller than the absolute value of the bias voltage Vb (= −200 mV to −300 mV). At this time, if the output voltage Vout of the operational amplifier IC41 exceeds the cut-off voltage of the diode D41, a forward current flows through the diode D41 and is not amplified. This is because when the voltage Vin of the resistor R43 and the bias voltage Vb due to the dark current are inverted and amplified by the adder circuit of the operational amplifier, the voltage of the resistor R45 exceeds the cutoff voltage of the diode D41, and the current flows through the diode D41. On the other hand, when the MPPC detects light, since the potential (Vin) of the resistor R43 increases, the voltage Vout does not exceed the cutoff voltage of the diode D41, and the signal is amplified by the ratio of the resistor R43 and the resistor R45. For example, if the MPPC detects light and a potential of 1V is generated in the resistor R43, a voltage of -2.1V is generated in the resistor R41. Since this voltage is a reverse voltage of the diode D41, no current flows through the diode D41. For example, when the resistor R44 and the resistor R45 are equal, the photodetector 300 uses the adder circuit of the operational amplifier to charge by the voltage obtained by subtracting the bias voltage Vb from the voltage amplified from the ratio of the resistor R43 and the resistor R45. Output as an output pulse via the capacitor C42. The capacitor C42 cuts the DC component of the signal. The bias voltage Vb is an example of a reference signal in the adding circuit.

<< Photodetector (6) >>
FIG. 12 is a diagram illustrating an example (6) of the photodetector of the neutron detection apparatus. Here, the photodetector 300 includes a bipolar transistor and a diode in order to remove dark current noise. 12 includes a variable resistor VR51, a resistor R51, a resistor R52, a resistor R53, a capacitor C51, and MPPC. The photodetector 300 in FIG. 12 further includes a resistor R54, a resistor R55, a resistor R56, a resistor R57, a capacitor C52, a capacitor C53, a transistor Q51, and a diode D51. The photodetector 300 detects light when light is incident on the MPPC. The output of FIG. 12 is connected to the charge dividing circuit 40 of FIG.

  The variable resistor VR51, resistor R51, resistor R52, resistor R53, resistor R54, resistor R55, capacitor C51, capacitor C52, MPPC, and transistor Q51 of the photodetector 300 in FIG. 12 are the same as those in the photodetector 300 in FIG. , The variable resistor VR11, the resistor R11, the resistor R12, the resistor R13, the resistor R14, the resistor R15, the capacitor C11, the capacitor C12, the MPPC, and the transistor Q11. Here, differences from the photodetector 300 in FIG. 6 will be mainly described.

Here, a specific example is shown. The resistance value of the resistor, the capacitance of the capacitor, the characteristics of the transistor, and the like are not limited to those described here. Variable resistor VR51 is 10 kΩ, resistor R51 is 4.7 kΩ, resistor R52 is 470 kΩ, resistor R53 is 4.7 kΩ, resistor R54 is 47 kΩ, resistor R55 is 1 kΩ, resistor R56 is 4.7 kΩ, resistor R57 is 100 kΩ, and capacitor C51 is 0.1 μF, capacitor C52 is 220 pF, and capacitor C53 is 22 pF. Further, as a characteristic of the transistor Q51, it is assumed that the base emitter voltage V _BE = 500 to 600 mV and the collector current I _C = 0. The transistor Q51 is
h _FE = 100.

At this time, it is assumed that the MPPC detects light and a 1 V potential is generated in the resistor R53 for 1 μs. Then, a maximum current of 220 μA can flow through the capacitor C52 to the base terminal of the transistor Q51. Further, when h _{FE of} the transistor Q51 is 100, a maximum current of 22 mA flows through the resistor R55. However, since the resistor R55 is 1 kΩ, the current due to the input voltage 1V from the capacitor C52 is saturated at 1 mA, and the base current I _B is suppressed to a pulse signal of about 10 μA. Then, the bipolar transistor Q51 amplifies the input signal (voltage of the resistor R53), and the potential of the resistor R55 becomes slightly lower than 1V, which is slightly lower than that of the resistor R53. Further, a reverse potential voltage multiplied by 4.7 is output to the resistor R56 connected to the collector of the transistor Q51.

  Furthermore, a power feeding circuit that feeds an output based on the voltage of the reverse potential to the charge dividing circuit 40 is a diode D51, a resistor R57, and a capacitor C53. A diode D51 and a resistor R57 are connected in parallel between the output and the capacitor C53. When the MPPC detects light, the potential of the resistor R56 swings in the direction of decreasing. At this time, an output pulse based on the voltage of the reverse potential is output to the charge dividing circuit 40 through the diode D51 and the capacitor C53.

  Here, the forward voltage of the diode D51 contributes to noise removal. When the signal from the MPPC disappears, the potential of the resistor R56 fluctuates in the increasing direction, so that a reverse potential is applied to the diode D51 and no current flows through the diode D51. Therefore, the capacitor C53 is discharged only by the resistor R57.

  In addition, when there is no signal in MPPC and there is a signal in MPPC of another photodetector 300, the signal leaks from the charge dividing circuit 40 through the capacitor C53 in the direction in which the potential decreases. At this time, since a reverse potential is applied to the diode D51, no current flows through the diode D51. Therefore, since only the resistor R57 enters the capacitor C53 in series, the influence on the amplification of the signal from the MPPC and the influence on the charge dividing circuit 40 can be suppressed. Therefore, even if a plurality of photodetectors 300 are connected, noise does not increase.

<< Photodetector (7) >>
FIG. 13 is a diagram illustrating an example (7) of the photodetector of the neutron detection apparatus. Here, the photodetector 300 includes an operational amplifier and a diode in order to remove dark current noise. The photodetector 300 in FIG. 13 includes a variable resistor VR61, a resistor R61, a resistor R62, a resistor R63, a capacitor C61, and MPPC. The photodetector 300 of FIG. 13 further includes a resistor R64, a resistor R65, a resistor R66, a capacitor C62, a diode D61, a diode D62, and an operational amplifier IC61. The resistor R65 is a feedback resistor. The photodetector 300 detects light when light is incident on the MPPC. The output of FIG. 13 is connected to the charge dividing circuit 40 of FIG. The photodetector 300 in FIG. 13 has common points with the photodetector 300 in FIG. 11 (photodetector (5)). Here, differences from the photodetector 300 of FIG. 11 will be mainly described.

  A power feeding circuit that feeds an output based on the output potential of the operational amplifier IC61 to the charge dividing circuit 40 is a diode D62, a resistor R66, and a capacitor C62. When the MPPC detects light, the output potential of the operational amplifier IC 61 swings in the direction of decreasing. At this time, an output pulse based on this potential is output to the charge dividing circuit 40 through the diode D62 and the capacitor C62. The power supply circuit including the diode D62, the resistor R66, and the capacitor C62 has the same function as the power supply circuit including the diode D51, the resistor R57, and the capacitor C53 of the photodetector 300 in FIG.

  Further, the power supply circuit including the diode D62, the resistor R66, and the capacitor C62 can be applied to the photodetector 300 in FIG. 9 or FIG. 10 in the same manner as the photodetector 300 in FIG. At this time, the direction of the diode D62 can be changed depending on whether the output pulse from the photodetector 300 is positive or negative.

(Effect of embodiment)
The neutron detector 10 converts incident neutrons into light by the neutron scintillator 100. The neutron detection apparatus 10 detects the converted light with a plurality of photodetectors 300, and calculates the position where the neutron is incident. The neutron detection apparatus 10 reduces noise due to dark current or the like generated in a light detection device such as MPPC with a circuit using a bipolar transistor, a unipolar transistor, a comparator, an operational amplifier, or the like. In the neutron detection apparatus 10, a signal due to dark current is less likely to be output to the charge dividing circuit 40 by a power supply circuit including a diode, a resistor, and a capacitor (for example, a power supply circuit including a diode D 51, a resistor R 57, and a capacitor C 53). Further, in the neutron detection device 10, the power supply circuit makes the photodetector 300 less susceptible to the signal from the charge dividing circuit 40. Since the noise output from each photodetector 300 is reduced, the output noise obtained from both ends of the charge dividing circuit 40 does not increase even when many photodetectors 300 and resistors 400 are connected. According to the neutron detection apparatus 10, the position where the neutron is incident can be calculated with high accuracy by removing the influence of dark current or the like generated in the light detection device.

(Modification)
In the above embodiment, the neutron detection apparatus 10 that detects neutrons has been described. The configuration of the neutron detection apparatus 10 can be applied to a radiation detection apparatus that detects radiation other than neutron beams. Other radiation includes, for example, alpha rays, beta rays, gamma rays, X-rays, charged particle rays and the like.

  In the radiation detection apparatus, a radiation scintillator corresponding to another radiation to be detected is used instead of the neutron scintillator 100. The radiation scintillator converts incident radiation into light. A radiation scintillator corresponding to the radiation to be detected is selected.

  In the radiation detection apparatus, for example, lead glass or acrylic glass is used as the light transmission plate 200. As the light transmission plate 200, one that absorbs radiation to be detected is selected. Lead glass allows light to pass through and absorbs radiation such as alpha rays, beta rays, gamma rays, X-rays, and charged particle rays. Acrylic glass allows light to pass through and absorbs radiation such as alpha rays. The light transmission plate 200 may be a low-pass filter or a band-pass filter that transmits visible light and shields X-rays and gamma rays having a higher frequency than visible light. As the light transmission plate 200, a plate that diffuses light may be used.

The radiation detection apparatus absorbs radiation that has not been converted by the radiation scintillator by a light transmission plate that absorbs radiation. Since the radiation is absorbed by the light transmission plate, the radiation incident on the photodetector is reduced. Thereby, damage to the photodetector due to radiation can be reduced, and a photodetector that is easily affected by radiation can be used.

1 Radiation scintillator
2 Photodetector
3 Radiation detector
10 Neutron detector
40 Charge division circuit
100 Neutron scintillator
200 Light transmission plate
300 photodetector
400 resistance
C01 capacitor
R01 resistance
VR01 variable resistance
V0 supply voltage
Vb bias voltage
Q11 Bipolar transistor
Q21 Unipolar transistor
CP31 comparator
SW31 switch
IC41 operational amplifier
D41 Diode

Claims (6)

A scintillator having an incident surface on which the radiation is incident and generating light from the incident radiation;
A plurality of photodetectors arranged in a one-dimensional manner for detecting the generated light;
Extending a predetermined resistance value portion a plurality of times, and comprising a linear resistance portion capable of connecting a branch line for each resistance value portion;
The photodetector includes an optical detection element that generates an electric signal from light, and a signal extraction circuit that suppresses a signal component that does not reach a reference signal intensity in the electric signal and extracts a signal component that reaches the reference signal intensity. Have
An output signal of each signal extraction circuit included in the plurality of photodetectors arranged in a one-dimensional manner is input to the linear resistance unit through the branch line, and the one-dimensionally arranged photodetectors The position is associated with the resistance value of the linear resistance portion at the connection position of the branch line to which the output signal of the signal extraction circuit of each photodetector is input ,
The radiation detection apparatus , wherein the signal extraction circuit includes a power supply circuit that suppresses input of a signal from the linear resistance unit and includes a diode, a resistor, and a capacitor .   The radiation detection apparatus according to claim 1, wherein the signal extraction circuit controls the reference signal intensity with a base bias signal of a bipolar transistor.   The radiation detection apparatus according to claim 1, wherein the signal extraction circuit controls the reference signal intensity with a gate bias signal of a unipolar transistor.   The radiation detection apparatus according to claim 1, wherein the signal extraction circuit controls the reference signal intensity with a reference signal of a comparison circuit. The signal extraction circuit includes an adder circuit that adds a first input terminal to which the electric signal is input and a second input terminal to which a reference signal is input, and the reference signal strength is set to the electric signal and the reference The radiation detection apparatus according to claim 1 , wherein when the added value with a signal exceeds a predetermined value, control is performed by amplifying the electrical signal . The radiation detection apparatus according to any one of claims 1 to 5 , further comprising a light transmission plate that is installed on a surface of the scintillator facing the incident surface and transmits light generated by the scintillator.

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