Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1999-12-27
2003-05-27
Porta, David (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
Reexamination Certificate
active
06570160
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a radiation measurement technique used in a facility for handling radioactive material, such as a nuclear power plant or the like, and more particularly, to a radiation detecting apparatus which is capable of simultaneously and independently measuring radiations such as a and &bgr; rays at a same position, and is suitable to a practical use as a radiation monitor.
2. Description of the Prior Art
FIG. 20
shows a phoswich detecting apparatus (phosphor sandwich detecting apparatus) as a conventional example of a radiation detecting apparatus for simultaneously detecting an &agr; ray and a &bgr; ray.
This radiation detecting apparatus is provided with a light shielding film
1
through which the &agr; and &bgr; rays are transmitted and for shielding light from the outside of the apparatus. The radiation detecting apparatus is also provided with a first scintillator
2
and a second scintillator
3
which are piled up below the light shielding film
1
shown in FIG.
26
.
There are many cases where ZnS (Ag) detecting an &agr; ray is used as the first scintillator
2
, and plastic detecting &agr; and &bgr; rays is used as the second scintillator
3
. The first and second scintillators
2
and
3
piled into two layers are directly mounted to a photo detector
5
so as to be received in a case
6
. In general, a photo-multiplier tube having a high speed response and a high sensitivity is used as the photo-detector
5
.
A decay time constant of emission of ZnS (Ag) constituting the first scintillator
2
is &mgr; sec order, but that of emission of plastic constituting the second scintillator
3
is several tens of n sec order. Therefore, the decay time constant of emission of the plastic scintillator
3
is considerably shorter as compared with that emission of the ZnS (Ag) first scintillator
2
. When an output current signal of the photo-detector
5
is converted into a voltage signal by means of an RC integrating circuit having a sufficiently long time constant as compared with each decay time constant of emission of the scintillators
2
and
3
, a pulse rise time is substantially equal to a decay time of emission, and shows an index decay waveform of a time constant determined by a resistor R and a capacity C. This signal converting process can be carried out in a pre-amplifier unit connected to the photo-multiplier tube and included in the photo-detector
5
.
The converted voltage signal is amplified up to a voltage level which is capable of being analyzed by means of a waveform discrimination processing unit
7
, as the necessity arises. When the voltage signal is inputted in the waveform discrimination processing unit
7
, an analog-digital converter of the processing unit
7
, in order to output a pulse signal having a pulse height proportional to the rise time of the inputted signal, converts the pulse height of the inputted signal into a digital value so that a general analyzer of the processing unit
7
measures a pulse height distribution (a spectrum data) on the basis of the converted digital value.
It is possible to distinguish an emission of the first scintillator
2
and that of the second scintillator
3
on the basis of the spectrum data showing the rise time and obtained from the waveform discrimination processing unit
7
.
FIG. 21
shows, as another conventional example, an &agr;-&bgr; rays detecting apparatus using a sensor
8
for measuring energy spectrum.
For example, an Si semiconductor sensor is used as the sensor
8
for measuring energy spectrum of the above apparatus. However, the sensor
8
has a sensitivity to a room light and the like other than a radiation; for this reason, similarly to the above described radiation detecting apparatus, a light shielding film
1
is mounted on the sensor
8
so that the sensor
8
is housed in a case
6
.
An output signal of the sensor
8
is analyzed by means of a pulse height analysis system
9
, so as to be measured as an energy spectrum. In general, the analysis system
9
includes: a charge sensitive pre-amplifier for processing the sensor output signal; a linear amplifier, an analog-digital converter, a pulse height analyzer for analyzing multiple pulse heights and the like. In the energy spectrum data obtained by the analysis system
9
, the &agr;-ray data and the &bgr;-ray data show different distributions and peak shapes, respectively, and therefore, it is possible to distinguish the &agr; ray and the &bgr; ray by processing these spectrum data corresponding to the a and &bgr; rays.
However, the pulse height discrimination processing unit
7
necessary for the conventional phoswich detecting apparatus shown in
FIG. 20
is a processing unit for analyzing a pulse rise, and is very expensive. Therefore, this conventional detecting apparatus is useful to a study in an experimental level.
However, as a detecting apparatus which is mounted in a monitoring device used in an actual nuclear facility or the like, there is a problem relating to a cost. Moreover, the waveform discrimination processing unit analyzes a rise time itself, and is an over specification in the case of discriminating signals having different rise times, respectively.
Furthermore, in view of the principle, in order to obtain a rise time, for example, there is a need of carrying out a signal detection at a 10% level and a 90% level of an inputted pulse height value, so that there is a problem that it is impossible to analyze and measure a signal having a low pulse height value. This problem relates to a dynamic range of the pulse height value of the signal. For example, an emission of ZnS (Ag) scintillator generated by an &agr; ray is considerably larger than that of the plastic scintillator generated by a &bgr; ray, and actually, the output signal of the photo-multiplier tube corresponding to the emission of ZnS (Ag) is larger 10 times or more as much as that of the photo-multiplier tube corresponding to the emission of &bgr; ray of the plastic scintillator at the point of time of being converted into the voltage signals.
Therefore, since the &bgr; ray signal has a low pulse height value and is continuously distributed on a low energy side, the measurement of the &bgr; ray is disadvantageous as compared with that of the &agr; ray. In particular, a component of the &bgr; ray having a low pulse height value is not analyzed and measured so that there is a problem that an effective &bgr;-ray sensitivity gets to be low. Especially, in the case where a thickness of the plastic scintillator is made thin in order to suppress a &ggr;-ray sensitivity, the emission of the plastic scintillator is further lowered so that the aforesaid phenomenon of lowering the effective &bgr;-ray sensitivity is further accelerated.
In addition, in the case of the radiation detecting apparatus using the energy spectrum measuring sensor
8
as shown in
FIG. 21
, the pulse height analyzer which is substantially equal to the above waveform discrimination processing unit must be required; as a result, there is a problem that the cost of the radiation detecting apparatus gets to be high. Furthermore, since an effective atomic weight of a base material of the energy spectrum measuring sensor
8
is larger than the plastic scintillator, a &ggr;-ray sensitivity is high so that there is a problem that a &ggr;-ray signal is mixed into a &bgr;-ray signal.
Still furthermore, in the case where measurement is not carried out in a vacuum state, or in the case of measuring an &agr; ray from an &agr;-ray emission nuclide absorbed to a filter paper, an energy loss of the &agr;-ray is high and a fluctuation of range is large. For this reason, a Gaussian peak as obtained in vacuum is not obtained so that there is the case where the energy spectrum of the &agr;-ray overlaps with that of the &bgr;-ray, whereby, in spite of measuring the energy spectrums of the &agr; and &bgr; rays, it is hard to clearly distinguish the &agr; ray and the &bgr; ray.
SUMMARY OF THE INVENTION
The present invention is directed to
Maekawa Tatsuyuki
Morimoto Soichiro
Ohara Yoshiaki
Sumita Akio
Watanabe Kazumi
Kabushiki Kaisha Toshiba
Lee Shun
Porta David
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