JPH02195293A - Semiconductor device for measuring radiation - Google Patents

Abstract

PURPOSE:To measure accurately even a radiation of low energy under noise of low level by providing a counting rate determining means which outputs a high counting rate signal when the result of counting of a counting means shows a high counting rate exceeding a prescribed counting rate. CONSTITUTION:A counting rate determining element 18a is provided in a counting circuit 18. In this circuit, analog switches 32 and 34 as wave-shaping constant switching means for switching a wave-shaping constant tau to a small value are fitted to capacitors C2 and C4 respectively. When a semiconductor detector detects a strong radiation and a counting rate turns to be a high counting rate exceeding a prescribed value, a high counting rate signal is outputted as a switching signal from the counting rate determining element 18a. On the occasion, the value of said wave-shaping constant tau turns to be small and the pulse width of a pulse signal narrows. By this constitution, the pulse signal can be prevented from overlapping at the time of a high counting rate, omission in counting of the number of pulses is prevented and execution of accurate counting is enabled.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a radiation measuring device, and particularly to a semiconductor radiation measuring device that detects radiation using a semiconductor.

[Prior art] Radiation measuring devices include 0M counter, ionization chamber, Na
I (TJ scintillation, etc.) are known. These devices are relatively large and require a high bias current when driving, so it is difficult to miniaturize them and it is difficult to configure them as portable devices. was difficult.

Therefore, a radiation measuring device using a semiconductor detector that detects radiation using a semiconductor has been proposed as a device that has six small detection sections and is suitable for being configured as a portable device.

In such a semiconductor radiation measuring device, when radiation is incident on a region called a depletion layer or an intrinsic semiconductor layer on the surface of a semiconductor, pairs of electrons and holes are created. It is based on the principle that by attracting these carriers to the respective electrodes and collecting charges, it is possible not only to detect radiation but also to obtain electrical pulse signals whose number is proportional to the energy loss of the radiation.

[Problems to be Solved by the Invention] (a) In the conventional portable radiation semiconductor measuring device described above, the size of the battery is an important factor in making the shape smaller. There is a need to make batteries smaller.

Therefore, in order to operate the device for a long time with a small battery, there is a problem that the current consumption of the circuit must be made as small as possible.

(b) Next, how low energy radiation a device can measure depends on the noise of the system. That is, low-energy radiation that is buried in noise cannot be measured.

Therefore, in order to reliably measure even low-energy radiation, there is a problem that system noise must be reduced.

System noise is mainly due to noise inherent to the semiconductor detector and noise generated from the preamplifier that amplifies the minute pulse signal from the semiconductor detector. Therefore, since the noise of the semiconductor detector itself is already determined for each detector, the problem is how to reduce the noise of the preamplifier.

The noise of the preamplifier is determined by the characteristics of the first stage FET (field effect transistor) and is expressed by the following equation (1).

is expressed by the following equation (2) using the current lD.

Here, E, N, and C are equalized noise charges (C), and ε is 2.7
18, τ is the pulse waveform shaping constant (S), 0 is the electron ff11. [l XIO” (C), 1g is FET gate current (A), k is Boltzmann constant 1.38X10” (J/K),
T is the absolute temperature (K), Rg is the gate input resistance (Ω), C1n is the input capacitance (F), 8 is the FET forward potential conductance (8), and At' is the flicker noise constant (v2).

Note that ga is approximately the drain current flowing through the FET, where "DSS" is the drain current at the time of "as-0", and Vp is the pinch-off voltage.

In the above formula (1), the third and fourth terms are input capacitance QC
[Since n (mainly the capacitance of the semiconductor detector) is included in the molecule in the form of a square, if this capacitance is large, for example 1009
Above F, compared to the 'IJ1 term and the 2nd term of equation (1),
The values of the third and fourth terms become large, and the noise becomes largely dependent on the values of the third and fourth terms.

The fourth term can be reduced by narrowing the band of the amplifier and cutting the low frequency region from the characteristics of flicker noise.

Therefore, to reduce the preamplifier noise, the formula (
It is necessary to reduce the value of the third term in 1), and it is necessary to increase the pulse waveform shaping constant τ or increase the FET forward transfer conductance gffl.

Here, gl can be increased by increasing the drain current ID flowing through the FET according to the above equation (2).

It should be noted that when the above-mentioned τ is increased, the pulse width of the output pulse from the waveform shaper becomes wider. Furthermore, when the drain current flowing through the FET is increased, this drain current occupies a large proportion of the current consumption of the device, which increases the power consumption of the device.

(c) Therefore, in order to reduce current consumption, the drain current flowing through the FET is set to the minimum necessary and the value of gm is also reduced), and to prevent an increase in noise, the waveform shaping constant τ is set large (pulse width needs to be wider).

However, semiconductor detectors output a number of electrical pulses proportional to the intensity of radiation, so if the radiation to be detected is strong,
That is, at a high count rate, the number of pulses increases. and,
The output events of the detector are random, so if you increase the waveform shaping constant τ and widen the pulse width, pulses will overlap in a radiation field with a high counting rate, making accurate counting impossible. A problem arises.

Therefore, in order to perform accurate measurements at high counting rates (strong radiation fields), it is necessary to reduce the waveform shaping constant τ, narrow the pulse width, and reduce the degree of overlapping of pulses to prevent missing counts. There must be.

In this case, reducing τ will increase noise (from equation (1)), so there is a problem that this increase in noise must be prevented.

With conventional semiconductor radiation detectors, it is difficult to measure low-energy radiation at high count rates due to the above-mentioned noise.

Purpose of the Invention The present invention has been made with the aim of solving the above-mentioned problems, and its purpose is to achieve low current consumption and low noise regardless of whether the counting rate is high or low. An object of the present invention is to provide a semiconductor radiation measuring device that can accurately detect radiation without increasing radiation.

[Means for Solving the Problems] In order to achieve the above object, a semiconductor radiation illumination device according to the present invention includes a semiconductor detector that converts incident radiation into a number of electrical pulse signals proportional to the intensity of the radiation; An FET is installed in the first stage to amplify the signal input from the semiconductor detector.
a preamplification means having a waveform shaping means for shaping the waveform of the output of the preamplification means, a wave height discrimination means for removing noise from the output from the waveform shaping means and outputting it as a rectangular pulse, and a wave height discrimination means for removing noise from the output from the waveform shaping means and outputting it as a rectangular pulse. A semiconductor radiation measuring device comprising a counting means for counting pulse signals outputted from the means, and a display means for displaying the counting result by the counting means, wherein the counting result of the counting means exceeds a predetermined counting rate. counting rate determining means for outputting a high counting rate signal when a high counting rate signal is reached; a waveform shaping constant switching means that prevents overlapping of pulses by making the pulse width narrower than that at a low counting rate that does not exceed the predetermined counting rate; The FET forward transfer conductance gl of said preamplifier means by increasing
and current switching means for preventing an increase in noise in the device due to a decrease in the waveform shaping constant τ at the time of the high count rate.

[Function] According to the above configuration, it is possible to perform waveform shaping using a large waveform shaping constant during a normal low counting rate, suppress the noise of the device, and measure even low-energy radiation. Since there is no need to increase the FET drain current and there is no need to increase the drain current of the FET, measurement can be performed with low current consumption.

Then, when the measurement location moves to a strong radiation field and the count rate becomes high, a high count rate signal is output by the count rate determination means.

Based on this high count rate signal, the waveform shaping constant switching means
Switching is performed to reduce the waveform shaping constant τ of the waveform shaping means to narrow the pulse width of the pulse signal and prevent overlapping of pulses.

At this time, reducing the waveform shaping constant causes an increase in the noise of the device, but when a high count rate signal is output, the drain current of the FET of the preamplifier is increased by the current switching means. Since the FET forward transfer conductance gm is increased, it is possible to prevent an increase in noise in the device (from equation (1)).

In this case, increasing the drain current of the FET causes an increase in current consumption, but working in a strong radiation field and measuring the dose rate takes a short time from the viewpoint of preventing radiation damage.

Therefore, increasing the drain current of the FET does not have a significant adverse effect on reducing current consumption.

Therefore, at normal low counting rates, radiation can be measured with low level noise and low current consumption due to the large waveform shaping constant τ and small FET forward transfer conductance gl. At high counting rates, the small waveform shaping constant τ and the increased FET forward transfer conductance gm prevent noise increase.
In addition, accurate l1ej determination without overlapping pulses is possible.

[Embodiments] Hereinafter, preferred embodiments of the present invention will be described based on the drawings.

FIG. 1 is a block diagram showing a schematic configuration of an embodiment, and a semiconductor detector 1G converts radiation incident into the detector into an electric pulse signal and outputs it. The output signal of this semiconductor detector 10 is shown in FIG. 2(A), and the number of pulse signals proportional to the intensity of radiation is output.

In FIG. 2(A), Sl and pulse signal S2 indicate a noise signal, respectively. The output signal of the semiconductor detector 10 is input to a preamplifier 12. The preamplifier 12 amplifies the signal from the semiconductor detector 10 as shown in FIG. 2(B).

The output of the preamplifier 12 is sent to a waveform shaping circuit and a main amplifier 14, where the waveform is shaped as shown in FIG. 2(C) and output.

The output from this waveform shaping circuit and main amplifier 14 is
The signal is sent to the discrete circuit 16. In order to distinguish between noise and radiation detection pulse signals, the discrete circuit 16 converts signals exceeding a predetermined threshold value X into rectangular pulse signals, as shown in FIG. 2(D). and output it. The pulse signals from the discrete circuit 16 are counted by the counting circuit 18, and the counting results are displayed on the display section 20.

The present invention is characterized by the provision of a counting rate determination means that outputs a high counting rate signal when the counting result of the pulse signal is a high counting rate in which a predetermined counting rate exceeds a predetermined value. In this embodiment, the counting circuit 18 is provided with a counting rate determining section 18a.

Another feature is that the preamplifier 12 has a first-stage FET.
The waveform shaping circuit and the main amplifier 14 are provided with current switching means 12a for increasing the drain current of the waveform shaping constant.
were established respectively.

FIG. 3(A) is a circuit diagram showing an example of the preamplifier 12, in which the pulse signal from the semiconductor detector 10 is first
The signal is input to the ET 22 and further output via the transistor 24. In the figure, Vcc indicates the power supply voltage.

In this embodiment, an analog switch 26 and a current source 28 are provided as current switching means.

A high counting rate signal from a counting rate determiner 18a provided in the counting circuit 18 (FIG. 1) is input to this analog switch 26 as a switching signal.

FIG. 3(B) is a circuit diagram showing an example of the waveform shaping circuit and the main amplifier 14, and the output signal from the preamplifier 12 is input to the negative input terminal of the main amplifier 3G via a resistor R1. Capacitors C and C2 are connected in parallel to the resistor R1. Furthermore, a resistor R2 and capacitors C3 and C4 are connected in parallel between the input and output of the main amplifier 30 as a feedback circuit.

These resistors Q, R and capacitors C1゜C2, C
3. Waveform shaping is performed by C4, and the waveform shaping constant τ is determined by the time constant of CR.

This circuit includes capacitors C and C4 as waveform shaping constant switching means for switching the waveform shaping constant τ to a small value.
Analog switches 32 and 34 are attached to each of them. A high counting rate signal from the counting rate determining section 18a of the counting circuit 18 is inputted to the analog switches 32 and 34 as a switching signal.

Next, the characteristic operation of this embodiment will be explained.

First, in normal radiation measurements at low counting rates, the waveform shaping constant τ in the waveform shaping circuit is set to a large value, and the output pulse signal is set to have a wide pulse width.

According to the above equation (1), when the waveform shaping constant τ is large, the noise of the device can be reduced, and low-energy radiation can also be measured accurately. In addition, in this case, there is no need to particularly increase the FET forward transfer conductance gIll, so the FET of the preamplifier 12
There is no need to increase the drain current of the device, and radiation measurements can be performed with low current consumption.

When the semiconductor detector detects strong radiation and the count rate reaches a high count rate higher than a predetermined value, a high count rate signal is output as a switching signal from the count rate determination section 18a, and the Brian Bu 12 and the waveform shaping circuit and the analog switches 32 and 34 of the main amplifier 14, respectively. By inputting this switching signal, the analog switches 32 and 34 of the waveform shaping circuit are turned off, and the capacitors C2 and C4 are respectively disconnected.

As a result, the value of the waveform shaping constant τ becomes smaller, and the pulse width of the output pulse signal becomes narrower. By reducing the pulse width, it is possible to prevent the pulse signals from overlapping at a high counting rate, thereby preventing the number of pulses from being omitted and allowing accurate counting.

If the waveform shaping constant τ is made smaller, the noise of the device will increase according to the above equation (1), but at the same time, the analog switch 26 of the preamplifier 12 is turned on, and the FET 22 is connected to the current source 28. connected, FET2
2 drain current increases.

Since the FET forward transfer conductance gm increases due to this increase in drain current, it is possible to prevent an increase in noise in the device due to a decrease in the waveform shaping constant τ. That is, at a normal low count rate, there is little possibility that the pulse signals will overlap even if the pulse width of the pulse signals is widened, so the waveform shaping constant τ can be increased.

Therefore, it is possible to reduce the noise of the device (Equation (1)
), one drain current of the FET 22 can be kept at the minimum necessary value, so low current consumption drive can be achieved.

In addition, at high count rates, the waveform shaping constant τ is switched to a small value in order to narrow the pulse width of the pulse signal, but at the same time, the drain current of the FET is increased and the forward transfer conductance gi of the FET is increased. There is no increase in device noise.This increase in the drain current of the FET 22 is an operation that is performed in an emergency of a strong radiation field and for a short time, so this increase in current has very little negative effect on the reduction of current consumption. .

[Effects of the Invention] As explained above, according to the semiconductor radiation measuring device according to the present invention, even low-energy radiation can be accurately measured under low-level noise at both low count rate and high count rate. can be measured. As a result, the device can be downsized by using a small battery, and can be used for a longer period of time.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the overall configuration of an embodiment of the present invention, Figures 2 (A) to (D) are output signal waveform diagrams of each component, and Figure 3 (A) is an example of a preamplifier. FIG. 3(B) is a circuit diagram showing an example of a waveform shaping circuit and a main amplifier. 10... Semiconductor detector 12... Preamplifier 12a... Waveform shaping constant switching means 14... Waveform shaping circuit and main amplifier 14a...
Current switching means 16...Discrete circuit 18...Counting circuit 18a...Counting rate determination section 20...Display section 22...FET 26.32 and 34...Analog switch 28...
current source.

Claims (1)

[Claims] (1) A semiconductor detector that converts incident radiation into a number of electrical pulse signals proportional to its intensity; a preamplifier having an FET in the first stage to amplify the signal input from the semiconductor detector; A waveform shaping means for shaping the waveform of the output of the preamplification means, a pulse height discrimination means for removing noise from the output from the waveform shaping means and outputting it as a rectangular pulse, and counting the pulse signal output from the pulse height discrimination means. and a display means for displaying the counting results of the counting means.
A semiconductor radiation measuring device comprising: a counting rate determining unit that outputs a high counting rate signal when the counting result of the counting unit reaches a high counting rate exceeding a predetermined counting rate; and the high counting rate signal is output. waveform shaping to prevent overlapping of pulses by reducing the waveform shaping constant (τ) of the waveform shaping means to make the pulse width of the pulse signal narrower than at a low counting rate that does not exceed the predetermined counting rate. constant switching means; increasing the FET forward transfer conductance (gm) of the preamplifying means by increasing the drain current of the FET when the high counting rate signal is output; A semiconductor radiation measurement device comprising: current switching means for preventing an increase in noise in the device due to a decrease in the waveform shaping constant (τ).