JP2693203B2 - Semiconductor radiation measuring device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a radiation measuring apparatus, and more particularly to a semiconductor radiation measuring apparatus for detecting radiation by a semiconductor.

[Prior Art] As a radiation measuring device, a GM counter, an ionization chamber, Na I
(Tl) scintillation is known. Since these devices are relatively large and require a high bias current during driving, it is difficult to miniaturize them and it is difficult to configure them as portable devices.

Therefore, a radiation measuring apparatus using a semiconductor detector that detects radiation by a semiconductor has been proposed as an apparatus having a small detection unit and suitable for being configured as a portable apparatus.

In such a semiconductor radiation measuring apparatus, when a semiconductor surface and a region called a depletion layer or an intrinsic semiconductor layer are irradiated with radiation, pairs of electrons and holes are formed. It is based on the principle that not only the detection of radiation but also the number of electric pulse signals proportional to the energy loss of radiation can be obtained by attracting this carrier to each electrode and collecting charges.

[Problems to be Solved by the Invention] (a) In the above-mentioned conventional portable radiation semiconductor measuring device, the size of the battery is important as a factor for making the shape smaller, and it is important to reduce the size of the device. It is necessary to miniaturize the battery.

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 minimized.

(B) Next, how low energy radiation can be measured by the device 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 the system noise must be reduced.

The system noise is mainly due to the noise peculiar to the semiconductor detector and the 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 detection, how to reduce the noise of the preamplifier becomes an issue.

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).

Here, ENC is equalization noise charge (C), ε is 2.718, τ is pulse waveform shaping constant (S), e is electron quantity 1.6 × 10 ^-19 (C), Ig is FET gate current (A), k Is Boltzmann constant 1.38 × 10 ²³ (J / K), T is absolute temperature (K), Rg gate input resistance (Ω), Cin is input capacitance (F), gm is FET forward potential conductance () Af is flicker noise A constant (V ² ) is shown.

It should be noted that the above gm is approximately the drain current I _D flowing in the FET.
Is expressed by the following equation (2).

Here, I _DSS is the drain current when V _GS = 0, and Vp is the pinch-off voltage.

In the above formula (1), the input capacitance is added to the third and fourth terms.
Since Cin (mainly the capacity of the semiconductor detector) enters the numerator in the form of a square, if this capacity is large, for example, 100 pF or more, the third term is larger than the first and second terms of equation (1). The value of the term and the fourth term becomes large, and the noise is
It depends greatly on the value of the term.

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, in order to reduce the noise of the preamplifier, it is necessary to reduce the value of the third term of the equation (1), and increase the pulse waveform shaping constant τ or increase the FET forward transfer conductance gm. There is a need to.

Here, gm is the drain current I flowing to the FET according to the steam formula (2).
It can be increased by increasing _D.

When τ is increased, the pulse width of the output pulse from the waveform shaper becomes wider. Further, 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 the current consumption, the drain current flowing in the FET is minimized (the gm value is also reduced), and the waveform shaping constant τ is increased (pulse pulse) to prevent an increase in noise. Need to be wider).

However, since the semiconductor detector outputs a number of electric pulses proportional to the intensity of the radiation, when the radiation to be detected is strong, that is, when the count rate is high, the number of pulses increases. Then, since the output event of the detector is random, if the waveform shaping constant τ is increased and the pulse width is increased, pulse overlap occurs in a radiation field with a high count rate,
There is a problem that accurate counting cannot be performed.

Therefore, in order to perform accurate measurements at high count rates (strong radiation fields), the waveform shaping constant τ must be reduced to narrow the pulse width, reduce the frequency of pulse overlap, and prevent counting errors. I have to. In this case, since noise is increased by reducing τ (from equation (1)), a problem arises in that this increase in noise must be prevented.

In the conventional semiconductor radiation detector, it was difficult to measure low energy radiation at a high count rate due to the noise.

OBJECT OF THE INVENTION The present invention has been made for the purpose of solving the above-mentioned problems, and its object is to reduce the current consumption and the noise at any of the high counting rate and the low counting rate. It is an object of the present invention to provide a semiconductor radiation measuring apparatus capable of accurately detecting radiation without increasing the number of radiation.

[Means for Solving the Problems] In order to achieve the above object, a semiconductor radiation measuring apparatus according to the present invention includes a semiconductor detector that converts incident radiation into electric pulse signals whose number is proportional to the intensity of the radiation. Preamplification means having a FET in the first stage for amplifying the signal input from the semiconductor detector, waveform shaping means for shaping the waveform of the output of the preamplification means, and noise of the output from the waveform shaping means And a pulse height discrimination means for outputting a rectangular pulse,
In a semiconductor radiation measuring apparatus including a counting means for counting the pulse signals output from the wave height discriminating means, and a display means for displaying the counting result by the counting means, the counting result of the counting means has a predetermined counting rate. A count rate determination means for outputting a high count rate signal when the high count rate exceeds
By reducing the waveform shaping constant (τ) when the high count rate signal of the waveform shaping means is output, the pulse width of the pulse signal is made narrower than that at the low count rate which does not exceed the predetermined count rate. Waveform shaping constant switching means for preventing pulse overlap, and the above when the high count rate signal is output.
A current switching means for increasing the FET forward transfer conductance gm of the preamplifying means by increasing the drain current of the FET and preventing an increase in device noise due to the decrease of the waveform shaping constant τ at the high count rate. When,
It is characterized by including.

[Operation] According to the above configuration, at a normal low count rate, it is possible to perform waveform shaping with a large waveform shaping constant, suppress the noise of the device to a low level, and measure low-energy radiation. Since it is not necessary to increase the drain current of the FET and it is not necessary to increase the drain current of the FET, measurement can be performed with low current consumption.

When the measurement place moves to a strong radiation field and the count rate becomes high, the count rate determination means outputs a high count rate signal.

Based on this high count rate signal, the waveform shaping constant switching means performs switching for reducing the waveform shaping constant τ of the waveform shaping means and narrows the pulse width of the pulse signal to prevent overlapping of pulses.

At this time, reducing the waveform shaping constant increases the noise of the device, but when the high count rate signal is output, the drain current of the FET of the amplifier is increased by the current switching means, and the FET is thereby increased. Since the forward transfer conductance gm is increased, it is possible to prevent the device noise from increasing (from the equation (1)).

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

Therefore, the adverse effect of reducing the current consumption by increasing the drain current of the FET is not so great.

Therefore, at a normal low count rate, radiation can be measured with a low level of noise and a low current consumption due to the large waveform shaping constant τ and the small FET forward transfer conductance gm. And at high count rates,
The small waveform shaping constant τ and the increased FET forward transfer conductance gm enable accurate measurement without increasing noise and without pulse overlap.

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

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

In FIG. 2A, S ₁ indicates a pulse signal S ₂ indicates a noise signal. The output signal of the semiconductor detector 10 is input to the preamplifier 12. In the preamp 12, the second
The signal from the semiconductor detector 10 is amplified as shown in FIG.

The output of the preamplifier 12 is the waveform shaping circuit and the main amplifier.
It is sent to 14, and the waveform is shaped and output as shown in FIG.

The output from this waveform shaping circuit and main amplifier 14 is
It is sent to the discrete circuit 16. Then, in the discrete circuit 16, in order to distinguish noise from the radiation detection pulse signal, as shown in FIG.
Signals exceeding the above are converted into rectangular pulse signals and output. Then, the counting circuit 18 counts the pulse signal from the discrete circuit 16, and the counting result is displayed on the display unit.
I'm trying to show on 20.

A feature of the present invention is that a counting rate determination unit that outputs a high counting rate signal is provided when the counting result of the pulse signal becomes a high counting rate where the predetermined counting rate exceeds a predetermined value. Therefore, in this embodiment, the counting circuit 18 is provided with the counting rate determination unit 18a.

Another feature is that the preamplifier 12 is provided with a current switching means 12a for increasing the drain current of the first stage FET, and the waveform shaping circuit and the main amplifier 14 are provided with a waveform shaping constant switching means 14a for reducing the waveform shaping constant. Are provided respectively.

FIG. 3 (A) is a circuit diagram showing an example of the preamplifier 12, and the pulse signal from the semiconductor detector 10 is the first stage FET2.
2 is input 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 count rate signal from the count rate determiner 18a provided in the counting circuit 18 (FIG. 1) is input to the analog switch 26 as a switching signal.

FIG. 3B is a circuit diagram showing an example of the waveform shaping circuit and the main amplifier 14. The output signal from the preamplifier 12 is input to the negative input terminal of the main amplifier 30 via the resistor R ₁ . Capacitors C ₁ and C ₂ are connected in parallel to the resistor R ₁ . Further, a resistor R ₂ and capacitors C ₃ and C ₄ are connected in parallel as a feedback circuit between the input and output of the main amplifier 30.

Waveform shaping is performed by these resistors R ₁ , R ₂ and capacitors C ₁ , C ₂ , C ₃ , C ₄ , and the waveform shaping constant τ is determined by the time constant of CR.

This circuit includes capacitors C ₂ and C ₄ as a waveform shaping constant switching means for switching the waveform shaping constant τ small.
Analog switches 32 and 34 are attached to the respective switches. A high count rate signal from the count rate determination unit 18a of the counting circuit 18 is input to the analog switches 32 and 34 as a switching signal.

Next, the characteristic operation of this embodiment will be described. First, in the measurement of radiation at a normal low count rate, the waveform shaping constant τ in the waveform shaping circuit has a large value, and the output pulse signal has a wide pulse width.

When the waveform shaping constant τ is larger than the above equation (1),
Since the noise of the device can be reduced, low-energy radiation can be accurately measured. Further, in this case, since it is not necessary to particularly increase the FET forward transfer conductance gm, it is not necessary to increase the drain current of the FET of the preamplifier 12, and radiation measurement can be performed with low current consumption.

Then, the semiconductor detector detects strong radiation, and when the count rate is at a high count rate of a predetermined value or more, a high count rate signal is output as a switching signal from the count rate determination unit 18a, and the analog of the preamplifier 12 is used. The signals are sent to the switch 26, 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 C ₂ and C ₄ are not connected.

As a result, the value of the waveform shaping constant τ becomes small, and the pulse width of the output pulse signal becomes narrow. Due to this reduction in pulse width, it is possible to prevent the pulse signals from overlapping at a high count rate, and it is possible to prevent counting down of the number of pulses and to perform accurate counting.

When the waveform shaping constant τ is reduced, the above equation (1)
Although the noise of the device is further increased, at this time, the analog switch 26 of the preamplifier 12 is simultaneously turned on, the FET 22 is connected to the current source 28, and the drain current of the FET 22 increases.

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

Therefore, the noise of the device can be reduced (from the equation (1)), and the drain current of the FET 22 can be a minimum required value, so that low current consumption driving can be achieved.

Also, at a high count rate, the waveform shaping constant τ is switched to a small value in order to narrow the pulse width of the pulse signal.
At the same time, the drain current of the FET is increased to increase the FET forward transfer conductance gm, so that the device noise is not increased. Since the increase of the drain current of the FET 22 is an operation that is performed in a short time in an emergency of a strong radiation field, the adverse effect on the reduction of the consumption current due to the increase of the current is extremely small.

[Effects of the Invention] As described above, according to the semiconductor radiation measuring apparatus of the present invention, even at low count rate and high count rate, even low-energy radiation is accurately generated under low level noise. Can be measured. As a result, the size of the device can be reduced by using a small battery, and the device can be used for a long time.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an overall configuration of an embodiment of the present invention, FIGS. 2 (A) to (D) are output signal waveform diagrams of respective components, and FIG. 3 (A) is an example of a preamplifier. FIG. 3 (B) is a circuit diagram showing an example of the waveform shaping circuit and the 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 …… Count rate judging unit 20 …… Display 22 …… FETs 26, 32 and 34 …… Analog switch 28 …… Current source.

Claims (1)

(57) [Claims] 1. A semiconductor detector for converting incident radiation into a number of electric pulse signals proportional to the intensity of the radiation, and a preamplifying means having an FET in the first stage for amplifying the signal input from the semiconductor detector. A waveform shaping means for shaping the waveform of the output of the preamplifying means, a wave height discriminating means for removing noise of the output from the waveform shaping means and outputting it as a rectangular pulse, and a pulse signal outputted from the wave height discriminating means In a semiconductor radiation measuring apparatus comprising: a counting means for counting, and a display means for displaying a counting result by the counting means, when the counting result of the counting means becomes a high counting rate time exceeding a predetermined counting rate. A count rate determination means for outputting a high count rate signal; and a pulse width of the pulse signal is reduced by reducing the waveform shaping constant (τ) of the waveform shaping means when the high count rate signal is output. By changing the waveform shaping constant switching means that prevents the pulse overlap by narrowing the pulse count when the low count rate does not exceed the predetermined count rate, and by increasing the drain current of the FET when the high count rate signal is output. FE of the preamplification means
A semiconductor radiation measurement, comprising: a current switching means for increasing T forward transfer conductance (gm) and preventing an increase in device noise due to a decrease in the waveform shaping constant (τ) at the high count rate. apparatus.