JPH02147923A - Light detection circuit - Google Patents

Abstract

PURPOSE:To correct increase in a dark current generated by a radiation by setting a photodiode provided with a light shielding means near a photodiode for detection for detecting a scintillation light while a subtraction means is provided to output a difference between outputs of the two photodiodes. CONSTITUTION:A photodiode 1 detects a scintillation light generated with the incidence of a radiation into a scintillator 2. A photodiode 3 is shielded by a light shielding means 4 to keep the scintillation light of the scintillator 2 from being incident. Outputs of the photodiodes 1 and 2 are inputted into a subtraction means 5, an output of which 5 is made incident into a current- voltage conversion means 6. An output current of the photodiode 1 by the scintillation light can be measured as voltage value between output terminals 9 and 10.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a photodetector, and particularly to a photodetection circuit suitable for detecting scintillation light in a radiation g environment.

[Conventional technology]

CT equipment using T-ray sources and accelerators is being developed in order to non-destructively inspect the insides of waste drums of nuclear power plants, fuel tanks of space rockets, etc. In the CR apparatus, it is considered that radiation passing through the object to be inspected is converted into light by a scintillator, and detected by a first diode.

In addition to the light to be detected from the scintillator, radiation also enters the photodiode. Therefore, in industrial CT systems where the object to be inspected is a large object such as a drum can or fuel tank, the amount of radiation irradiated to the photodiode per - times of inspection light is extremely large, and the characteristics of the photodiode may deteriorate due to radiation. There is a risk of deteriorating measurement accuracy.

Deterioration of photodiode characteristics due to radiation is described by IE, Transactions on Nuclear Science, NS 32.6, (19135).
Pages 4453 to 4460 (IEEE Transac
tions on Nuclear 5aience,
Vol, N5-32°No, 6. (1985) pp4
453-4460) and others. The characteristic parameters of the photodiode change due to radiation irradiation. The main parameters are sensitivity characteristics, capacitance,
One example is dark current.

Although the sensitivity characteristics and capacitance hardly change up to an integrated dose of 10'rad, the dark current increases by two fingers.

In the case of a 5i-PIN photodiode sensitive to the scintillator's emission wavelength of 400 to 600 nm, a dark current of several tens of picoamperes before irradiation increases to several nanoamperes after irradiation of 108 rad. CT'! In A[, since the photodiode is used for current output, an increase in dark current leads to measurement errors.

[Problem to be solved by the invention]

This kind of photodiode characteristic deterioration is caused by medical C
With the T device, this was not a problem because the subject being tested was a human and the radiation dose was low. However, in an industrial CT apparatus that uses a large dose of radiation, the integrated dose of a photodiode during use reaches 107 to 10'rad, so measurement errors due to an increase in dark current are a big problem.

An object of the present invention is to provide a photodetection circuit that can measure the output current of a photodiode due to scintillation light without deteriorating measurement accuracy even if the dark current of the photodiode increases due to radiation.

[Means to solve the problem]

The above purpose is to install a photodiode having the same characteristics as the photodiode and provided with a light shielding means in the vicinity of a detection photodiode that detects scintillation light, and to provide a subtraction means for outputting the difference between the outputs of the two photodiodes. This is achieved by providing

[Effect]

The output current of a detection photodiode that detects scintillation light is the sum of the photocurrent due to scintillation light and dark current, but since scintillation light does not enter a photodiode with a light shielding means, its output current is equal to the dark current. The degree of deterioration of the characteristics of the two photodiodes due to radiation, that is, the amount of increase in dark current, is the same, and by subtracting the difference between the output currents of the two photodiodes, an accurate photocurrent value due to scintillation light can be determined. I can do it.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. Photodiode 1 detects scintillation light generated when radiation is incident on scintillator 2 . Of course, means is provided to prevent ambient light from entering the scintillator 2 and photodiode 1. (Omitted) The photodiode 3 is shielded by a light shielding means 4 to prevent the scintillation light from the scintillator 2 from entering. The outputs of the photodiodes 1 and 2 are input to the subtraction means 5;
The output is input to the current/voltage conversion means 6. The output current of the photodiode 1 due to scintillation light can be measured as a voltage value between output terminals 9 and 10. The current/voltage conversion means 6 is composed of a P amplifier 7 and a resistor 8.

Next, the present invention will be explained in detail. When the photodiode] receives scintillation light output from the scintillator 2, it outputs a current 11. The current 11 includes a photocurrent io generated by receiving scintillation light and a dark current ini of the photodiode 1 (i l = i o + i l) 1
-(1) Since the first diode 3 is shielded from light by the light shielding means 4, it does not receive scintillation light. Therefore, the output current 17 is the photodiode 3
is equal to the dark current ioz.

□z=LDZ...(
2) Since photodiode 1 and photodiode 3 are installed close to each other, the amount of radiation they receive from the environment is equal. The amount of increase in the dark current of the photodiode depends on the amount of radiation to which the photodiode is exposed. Therefore, when this embodiment is used in a radiation environment, the dark current iox and i
Dz gradually increases.

Photodiode]- and photodiode 3 are used with the same characteristics. In this embodiment, photodiodes of the same manufacturer, model, and production lot number are used. Care must be taken as characteristics tend to vary between different manufacturing lots.

Since the photodiode 1 and the photodiode 3 have the same deterioration characteristics due to radiation, the dark currents i old and lDz are equal regardless of the radiation dose to which the photodiode is exposed.

lot= l oz
(3) The outputs of photodiode 1 and photodiode 3 are input to subtraction means 5. In this embodiment, the subtraction means 5 is simply a wire connection. The output current 11 of the photodiode 1 and the output current 12 of the photodiode 3 flow in opposite directions when viewed from the subtraction means 5. Therefore, the output current 13 of the subtraction means 5 is lit"11 1□
...(4) Therefore, (1), (2),
Substituting equation (3) into equation (4), 13=io
...(5), and the output i3 of the subtraction means 5 is always the photocurrent i when the photodiode 1-1 detects the scintillation light from the scintillator 2.
Equal to o.

The output current i3 of the subtraction means 5 is input to the current/voltage conversion means 6.The current/voltage conversion means 6 is often used.
A P amplifier was used. The power supply is omitted in this diagram. If the value of the resistor 8 is R1, the output terminal 9 of the current/voltage conversion means 6
The output voltage V output between and 10 is.

V=iaXRt-ioXR1.=(6) The photocurrent io is converted into a voltage and an amplified voltage is obtained. For the OP amplifier 7, an FET input type is used in order to reduce the bias current as much as possible.

Note that photodiode 1 and photodiode 3 are reverse biased in order to improve response characteristics. Since the output of the subtraction means 5, that is, the negative input of the OP amplifier 7 and the positive input of the OP amplifier 7 are in an imaginary short-circuit state, the potential of the negative input is the ground potential. For this reason, photodiode 1. A reverse bias voltage V is applied to both photodiodes 3. When applying a reverse bias voltage, it is important to apply the same voltage to photodiode 1 and photodiode 3 as in this embodiment. If the voltage is different, the degree of deterioration of the characteristics of the photodiode due to radiation will be different, resulting in a difference in the bias current. If high-speed response is not required, the dark current will be smaller if V=O1, that is, use without applying a bias voltage.

In this embodiment, it is possible to correct not only an increase in dark current due to radiation, but also a change in dark current due to a difference in the ambient temperature of the photodiode. Since the photodiodes 1 and 3 have the same characteristics, the amount of change in dark current due to changes in ambient temperature is also the same. Therefore, correction is possible using the same operating principle as described above.

It goes without saying that the current/voltage conversion means 6 should be shielded to avoid the effects of radiation or be installed at a location where it will not be affected by radiation. Since the subtraction means 5 does not have a semiconductor element in this embodiment, the subtraction means 5 has no semiconductor element. It may be placed near the photodiode 3. Since the signal between the subtraction means 5 and the current/voltage conversion means 6 is a current signal, there is no fear that the signal will be attenuated by using a coaxial cable or the like as the cable.

FIG. 2 shows the structure of the scintillator and photodiode portion of the embodiment shown in FIG. 1. 6 A photodiode 1 is installed immediately after the scintillator 2, and a photodiode 3 is installed as close to the photodiode 1 as possible. The photodiode 3 is covered with a light shielding means 4, and the casing of the photodiode 1 also serves as a front light shield.

According to this embodiment, it is possible to correct an increase in the dark current of the photodiode caused by radiation, and further.

Changes in the dark current of the photodiode due to temperature changes can also be corrected, making it possible to accurately measure photocurrent.

FIG. 3 shows an embodiment in which two photodiodes are stored in one package in advance. Photodiode 1 and photodiode 3 are assembled in package 1 in advance. A glass window 12 is installed in the package, and the photodiode 1 can detect light through the glass window 12. In order to detect radiation, a scintillator is attached to the front of the glass window]2. The integrated photodiode shown in this figure may be applied to the embodiment of FIG.

FIG. 4 shows a second embodiment. In this embodiment, the output current il of the photodiode 1 and the output current 12 of the photodiode 3 shielded from light by the light shielding means 4.

First, the current/voltage conversion means 6 converts each into a voltage, and then the subtraction means 5 subtracts the dark current to obtain the true photocurrent.

The current/voltage conversion means 6 is an OP amplifier 13, 14° resistor 1
Consists of 6.17. Resistors 16 and 17 have the same value.

The subtraction means 5 includes an OP amplifier 15 and resistors 18, 18, 20.
．． It consists of 21. In this embodiment, since the subtraction means 5 does not have a gain, the resistors 18, 19, 20.21 all have the same resistance value.

If the value of the resistor 1.6.17 is R, the output V+ of the OP amplifier 13 is V = 1t Since equations (1) and (2) hold true for 11.17, the output voltage V between the output terminals 22 and 23 is V=Vz-
Vt: i o X R (9), and an output voltage corresponding to the photocurrent io is obtained. Therefore, an increase in the dark current of the photodiode caused by radiation can be corrected, and a change in the dark current of the photodiode due to a temperature change can also be corrected, making it possible to accurately measure the photocurrent.

FIG. 5 shows a third embodiment of the present invention. This embodiment is an example in which only one photodiode is used. A movable light shield 32 is provided between the scintillator 31 and the photodiode 34, and the movable light shield 32 is moved by a drive unit 33. When the movable light shielding body 32 is inserted between the scintillator 31 and the photodiode 34 (state 1, solid line in this figure), the photodiode 34 cannot receive scintillation light generated by radiation incident on the scintillator 31. Therefore, the output current of the photodiode 34 is only a dark current. When the movable light shielding body 32 is not located between the scintillator 31 and the photodiode 34 (state 21 shown by the broken line in the figure), the photodiode 34 can receive scintillation light.

The output of the photodiode 34 in this case is the sum of the photocurrent and the dark current.

The output current of the photodiode 34 is converted into a voltage output by the current/voltage conversion means 35, and the output of the current/voltage conversion means 35 in state 1 is stored in the analog memory 36.

The control circuit 37 is a circuit that operates the drive unit 33 and analog memory 36 in synchronization. In state 2, that is, when measuring scintillation light, the current/voltage conversion means 3
By subtracting the output of the analog memory 36 stored in the state 2 from the output of the output terminal 3 by the subtraction means 38,
At 9, a voltage corresponding to the photocurrent is output.

Therefore, just before measuring the scintillation light, the control circuit 37 causes the control circuit 37 to perform the operation of state], and stores the amount of dark current that has increased due to the deterioration of the characteristics of the photodiode 34 in the analog memory 36. Even if the dark current changes due to deterioration of the characteristics of 34 due to radiation, the correction amount can be changed accordingly.

Therefore, according to this embodiment as well, it is possible to correct the increase in the dark current of the photodiode caused by radiation, and it is possible to accurately measure the photocurrent.

〔Effect of the invention〕

According to the present invention, it is possible to accurately measure the optical output current of a photodiode due to scintillation light.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of one embodiment of the present invention, Fig. 2 is a cross-sectional view of the scintillator and photodiode portion, Fig. 3 is a cross-sectional view of the second embodiment of the photodiode portion, and Fig. 4 is the present invention. FIG. 5 is a circuit diagram of a third embodiment of the present invention. 1.3... Photodiode, 2, 31... Scintillator, 4... Light shielding means, 5, 38... Subtraction means, 6... Current/voltage conversion means, 7, 13, 14.15
...op amp, 8,16-21...resistance, 9,1
0...Output terminal, 11...Package, 12...
glass window. 32...Movable light shielding body. 1st factor Figure 3 Figure 4 V Figure 2 and t Lz

Claims (1)

[Claims] 1. In a photodetection circuit installed in a radiation environment, a first photodiode for detecting light is provided near the first photodiode and has the same characteristics and has a light shielding means. A photodetection circuit comprising: a second photodiode; and subtraction means for outputting a difference between an output signal of the first photodiode and an output signal of the second photodiode. 2. In a photodetection circuit installed in a radiation environment, a photodiode for detecting light and a movable light-shielding body for a light-receiving surface are provided, and an output signal is generated when the movable light-shielding body blocks light from the light-receiving surface of the photodiode. and subtracting the output signal of the signal storage means from the output signal of the photodiode when the movable light shielding body does not shield the light-receiving surface of the photodiode, and outputting the calculation result. A photodetection circuit characterized in that it is provided with subtraction means. 3. In claim 1, a scintillator is installed in front of the light-receiving surface of the first photodiode, and the first photodiode detects scintillation light generated within the scintillator by radiation. radiation detection device. 4. In claim 2, a scintillator is installed in front of the movable light shield in front of the light-receiving surface of the first photodiode, and scintillation light generated within the scintillator by radiation is transmitted to the first photodiode. A radiation detection device that uses a photodiode to detect radiation. 5. A photodetector characterized in that two photodiodes are housed in one package, and the package is provided with a transparent portion that allows only one of the two photodiodes to receive external light. .