JPH08152478A - Radiation detector - Google Patents

Abstract

PURPOSE: To provide a radiation detector by which a radiation can be measured without a detection error and precisely. CONSTITUTION: An electric charge Q corresponding to energy of photons by X-rays which are incident on an X-ray sensor element is stored in respective capacitors 3a, 3b at Q/2 and -Q/2, and it is output as a voltage V across output terminals 1c, 1d for a differential amplifier 1. Then, switches 4a, 4b are turned on by a control means 6 in order to discharge the stored electric change, and both switches 4a, 4b are then turned off simultaneously. Then, electric charges which have been stored in parasitic capacitors of both switches are discharged simultaneously, and an influence on the voltage V across the output ends 1c, 1d by the changeover operation of the switches is removed. In addition, the capacitance Cp of a correction capacitor 5 is set in such a way that a capacitance across a negative voltage source 7 and an input terminal 1a becomes nearly equal to a capacitance across the negative voltage source 7 and an input end 1b. As a result, even when the potential of the negative voltage source 7 is changed, equal electric charge, amounts are injected into both signal lines for the differential amplifier 1, and an influence on the output voltage V is removed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to, for example, a radiation detector used in an X-ray imaging apparatus for forming an X-ray image by radiation that has passed through a subject, particularly X-rays that have passed through the subject.

[0002]

2. Description of the Related Art In recent years, as an X-ray image pickup apparatus for a chest, a line sensor or a plurality of X-ray detection elements arranged side by side instead of a method of directly irradiating a film with a transmitted X-ray to obtain an X-ray image. A method of obtaining an X-ray photographed image by converting a transmitted X-ray into an electric signal by a dimension sensor has been developed.

As shown in FIG. 2, this X-ray detector includes an X-ray sensor element 12 for detecting X-rays, an operational amplifier 11 which is an amplifier, a capacitor 13 and a switch 14 which are connected in parallel to the X-ray sensor element 12. The X-ray sensor element 12 is connected to the negative voltage source 17.

Here, each time an X-ray enters the X-ray sensor element 12, a predetermined amount of electric charge Q is stored in the capacitor 13, and as shown in FIG. 3, the output voltage V of the operational amplifier 11 is Q / C. (C is the electrostatic capacitance of the capacitor 13), and the temperature rises stepwise. Then, the output voltage of the operational amplifier 12 is differentiated by a shaping amplifier or the like (not shown) connected to the next stage and becomes a count pulse, and the X-ray incident amount is measured by counting this. On the other hand, in order to prevent the capacitor 13 from being saturated, the switching element 1
4 is turned on, and the electric charge stored in the capacitor 13 is discharged.

[0005]

However, since the switch 14 of such a radiation detector is usually composed of a MOSFET and has a parasitic capacitance C'as shown in FIG. 4, the switch 14 is controlled to discharge the accumulated charge. Voltage V '
When the voltage is applied to turn on, the parasitic capacitance C ′ accumulates a charge Q ′ = C ′ × V ′ which is the product of the two. Therefore, the switch 14 is turned off again to start the measurement of the radiation.
When it is set to F, the electric charge Q'stored in the parasitic capacitance C'is discharged and injected into the signal line, which causes noise as shown in FIG. 3 to cause a detection error.

Further, in FIG. 2, when the potential fluctuation V ″ of the negative voltage source 17 occurs, the charge Q ″ = C ″ on the signal line of the operational amplifier 11 due to the parasitic capacitance C ″ between the negative voltage source 17 and the input terminal of the operational amplifier 11.・ V ”will be injected,
As a result, a detection error of Q ″ / C occurs. Particularly, when the connection between the X-ray sensor element 12 and the operational amplifier 11 is long, the parasitic capacitance C ″ generated in the connection increases, and Q ″ / C. The detection error of is not negligible.

In order to solve such a problem, the present invention solves the above problem by switching a switch for discharging accumulated charge,
An object of the present invention is to provide a radiation detector capable of accurate radiation measurement without a detection error even when a potential fluctuation of a negative voltage source connected to a radiation sensor element occurs.

[0008]

In order to achieve the above object, the radiation detector according to the invention of claim 1 has first and second input ends and first and second output ends. , The first,
A differential amplifier that amplifies a voltage between the second input terminals and outputs the amplified voltage as a voltage between the first and second output terminals; and a radiation sensor element connected to the first input terminal of the differential amplifier, A first capacitor and a first switch element connected in parallel between a first input terminal and a first output terminal of the differential amplifier, and a second input terminal and a second output of the differential amplifier. A second capacitor and a second switch element connected in parallel between the ends respectively, and a control means for simultaneously turning off the first switch element and the second switch element are provided.

The radiation detector according to the invention of claim 2 is
A differential amplifier having first and second input terminals for amplifying and outputting a voltage between the first and second input terminals, and a radiation sensor connected to the first input terminal of the differential amplifier. An element, a capacitor connected to a second input terminal of the differential amplifier, and a negative voltage source connected to the other ends of the radiation sensor element and the capacitor, respectively, the capacitor including the negative voltage source and the negative voltage source. The parasitic capacitance between the second input terminals of the differential amplifier has a capacitance that is substantially equal to the parasitic capacitance between the negative voltage source and the first input terminal of the differential amplifier.

[0010]

The operation of the present invention will be described with reference to FIG.
The charge Q corresponding to the energy of the photons of the X-rays incident on the line sensor element 2 is accumulated in the first capacitor 3a and the second capacitor 3b by Q / 2 and -Q / 2, respectively.
The voltage V is output between the first and second output terminals 1c and 1d. Then, the control means 6 causes the capacitors 3a, 3
The first and second switches 4a, 4a for discharging the accumulated charge of b
After turning on the switch 4b, the switches 4a and 4b are turned off at the same time to start the X-ray measurement.
, And the charge Q'stored in the parasitic capacitance C'of the switch 4b is also discharged at the same time and injected into both signal lines of the differential amplifier 1 equally. The effect on the voltage V across the second output is eliminated.

Further, the capacitance between the negative voltage source 7 and the first input terminal 1a of the differential amplifier 1, and the negative voltage source 7 and the differential amplifier 1
Since the electrostatic capacitance Cp of the correction capacitor 5 is set so that the electrostatic capacitances between the second input terminals 1b of
Even if the potential of the negative voltage source 7 fluctuates, the same charge amount is injected into both signal lines of the differential amplifier 1, and the output voltage V is not affected.

[0012]

An embodiment of the present invention will be described below with reference to FIG.

FIG. 1 is a schematic view showing an embodiment of an X-ray detector which is one of the radiation detectors according to the present invention. In the figure, the differential amplifier 1 includes a first input terminal 1a and a second input terminal 1a.
Input end 1b, first output end 1c, and second output end 1
d, and is configured to amplify the voltage between the first and second input terminals 1a and 1b and output it as the voltage V between the first and second output terminals 1c and 1d.

The X-ray sensor element 2 generates a current according to the number of photons of the incident X-ray, and is formed as a P / N junction element on a semiconductor, for example. The output terminal of this X-ray sensor element 2 is the first input terminal 1 of the differential amplifier 1.
The other end is connected to the negative voltage source 7 and is held at a potential of about -200V.

Between the first input terminal 1a and the first output terminal 1c of the differential amplifier 1, there is a first capacitor 3 having a capacitance C.
a and the first switch 4a are respectively connected in parallel, and similarly, between the second input end 1b and the second output end 1d of the differential amplifier 1, the same electrostatic capacitance as that of the first capacitor 3a is applied. The second capacitor 3b having the capacitance C and the second switch 4
b are connected in parallel. An integrating circuit S is formed by the first capacitor 3a, the second capacitor 3b, and the differential amplifier 1, and electric charges corresponding to the X-ray dose detected by the X-ray sensor element 2 are accumulated in the respective capacitors. Thus, the output voltage V is obtained.

First and second parallel-connected differential amplifiers 1
The switches 4a and 4b in FIG. 4 are composed of MOSFETs having a parasitic capacitance C'as shown in FIG.
The control voltage V'is applied to control the opening and closing of the switch. In particular, the control means 6 controls both switches 4
It has a function of simultaneously turning on and off a and 4b. In addition,
Both switches 4a and 4b can be configured to have substantially the same parasitic capacitance C'by forming them as semiconductor circuits on a silicon substrate, for example.

Next, the operation of the X-ray detector having the above structure will be described. When the control unit 6 turns off the first switch 4a and the second switch 4b, when an X-ray enters the X-ray sensor element 2, a charge Q corresponding to the energy of the photon is generated and the operational amplifier 1 From the characteristics of the above, Q / Q is applied to the first capacitor 3a and the second capacitor 3b, respectively.
Charges of 2, -Q / 2 are accumulated. Due to the charges accumulated in these capacitors 3a and 3b, the potentials of the first and second output terminals 1c and 1d become V / 2 and -V / 2, respectively, and the voltage V between these output terminals becomes an output. .

Then, a predetermined amount of charge is applied to the capacitors 3a,
When the control means 6 applies a control voltage V'to turn on both the first and second switches 4a and 4b when they are accumulated in the capacitor 3b, the accumulated charges of the capacitors 3a and 3b are changed to the first and second charges. While being discharged through the switches 4a and 4b, as described in FIG. 4, the charge Q ′ due to the control voltage V ′ is accumulated in the parasitic capacitance C ′ of the first and second switches 4a and 4b. Next, when the control means 6 stops the application of the control voltage V'to turn off both switches 4a and 4b at the same time in order to start the X-ray measurement,
The charge Q'stored in the parasitic capacitance C'of the switch 4a and the switch 4b is simultaneously discharged and injected into both signal lines of the differential amplifier 1, so that the charge Q'is discharged due to the characteristics of the differential amplifier 1. The influence of the above is canceled, and the influence of the switching operation of the switch on the voltage V between the first and second output terminals is eliminated.

Further, in the present invention, not only the parasitic capacitance C'parasitic in the switch but also the detection error caused by the parasitic capacitance C "parasitic between the negative voltage source 7 and the first input terminal 1a of the differential amplifier 1 is detected. Can also be removed.

That is, the correction capacitor 5 according to the present invention shown in FIG. 1 has the parasitic capacitance generated in the X-ray sensor element 2 and the connection between the X-ray sensor element 2 and the first input terminal 1a of the differential amplifier 1. The capacitance Cp has a value obtained by subtracting the parasitic capacitance generated in the connection between the correction capacitor 5 and the second input terminal 1b of the differential amplifier 1 from the sum. For example, the parasitic capacitance generated in the connection between the X-ray sensor element 2 and the X-ray sensor element 2 and the first input terminal 1a of the differential amplifier 1 is 300 pF, and the correction capacitor 5 and the second input terminal of the differential amplifier 1 are set. If the parasitic capacitance generated in the connection between 1b is 100 pF, the difference between them is 200 pF. Therefore, even when the potential of the negative voltage source 17 changes with respect to the ground potential, the negative voltage source 17 and the first and second input terminals 1a and 1b of the operational amplifier 1 are changed.
Since the parasitic capacitances C ″ between them are equal, the amount of electric charge Q ″ = C ″ · V ″ injected into both signal lines of the operational amplifier 1 is also equal due to the potential fluctuation amount V ″. Even if the potential of the output signal fluctuates, the fluctuation component becomes an in-phase signal, and the influence of the characteristic of the differential amplifier 1 on the output voltage V is removed.

Although X-rays, which are the most representative of radiation, have been described in the above embodiments, the present invention is not limited to this, and radiation such as α-rays and γ-rays may be used.

[0022]

According to the present invention, a capacitor and a switch are connected in parallel between the input and output ends of a differential amplifier having first and second input ends and first and second output ends. Since this switch is configured to be turned off at the same time, the radiation can be accurately measured without a detection error even when the switch is switched to discharge the accumulated charge.

Further, the second input terminal of the differential amplifier and the negative voltage source are connected via a capacitor, and the capacitance of the capacitor is connected between the negative voltage source and the second input terminal of the differential amplifier. Since the parasitic capacitance is set to be substantially equal to the parasitic capacitance between the negative voltage source and the first input terminal of the differential amplifier, even if the potential of the negative voltage source with respect to the ground potential fluctuates, a detection error will occur. Without, accurate radiation measurement is possible.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a radiation detector according to the present invention.

FIG. 2 is a diagram showing a conventional radiation detector.

FIG. 3 is a diagram showing a time change of an output voltage of a radiation detector.

FIG. 4 is a schematic diagram of a switch formed of MOSFETs.

[Explanation of symbols]

 1-Differential amplifier 2--X-ray sensor element 3a-First capacitor 3b-Second capacitor 4a-First capacitor Switch 4b ... Second switch 5 ... Corrected capacitor

Claims (2)

[Claims] 1. A first and a second input end, and a first and a second output end, amplifying a voltage between the first and the second input ends, A differential amplifier that outputs a voltage between the output terminals, a radiation sensor element that is connected to a first input terminal of the differential amplifier, and a radiation sensor element between the first input terminal and the first output terminal of the differential amplifier, respectively. A first capacitor and a first switch element connected in parallel, and a second capacitor and a second switch element connected in parallel between the second input terminal and the second output terminal of the differential amplifier, respectively. A radiation detector comprising: the first switch element and a control unit that simultaneously turns off the second switch element. 2. A first and a second input terminal are provided, and the first and second input terminals are provided.
A differential amplifier that amplifies and outputs the voltage between the second input terminals, a radiation sensor element that is connected to the first input terminal of the differential amplifier, and a radiation sensor element that is connected to the second input terminal of the differential amplifier. A negative voltage source connected to the other ends of the radiation sensor element and the capacitor, wherein the capacitor has a parasitic capacitance between the negative voltage source and a second input end of the differential amplifier, A radiation detector having a capacitance substantially equal to a parasitic capacitance between a negative voltage source and a first input terminal of the differential amplifier.