GB2185814A - Radiation meter - Google Patents

Abstract

A radiation meter for measuring the radiation of flowing gases comprises at least one radiation detector (18,19,20) and a measuring filter (14) located opposite the detector(s) for the gases to flow through. An adjustable diaphragm (32) is disposed between the measuring filter (14) and the radiation detector(s) (18,19,20) and it is adjusted automatically as the T- radiation detector(s) (18,19,20) reaches its saturation limit. In the event a small aperture (34) determines the passage of the radiation from the measuring filter (14) to the detector(s) (18,19,20). Thus a single radiation meter is sufficient to cover both the normal case with little radiation of the exhaust gas of a nuclear installation and the case of an accident with intense radiation of the exhaust gas. <IMAGE>

Description

SPECIFICATION Radiation meter The present invention relates to a radiation meter suitable for measuring the radiation of flowing gases.

Radiation meters comprising at least one radiation detector and a measuring filter through which the gases flow and which is positioned opposite the detector are widely used nowadays for the purpose of measuring the radiation content of the used air or exhaust air of installations of nuclear technology. The exhaust air is passed through a measuring filter in which radiating aerosols are kept in intermediate store. The radiation emitted by this measuring filter is detected by conventional radiation detectors.

As the radiation detectors are highly sensitive, even minute radiation exposure can be detected.

However, a problem exists since it should be possible also to measure the radiation exposure in the case of an accident. The highly sensitive radiation detectors are far outside their dynamic range in the case of such an accident so that they become "overloaded" and consequently do not supply an accurate measuring result.

It has been proposed that this problem be solved by the provision of two separate radiation meters having different measuring ranges, one serving for normal operation and the other one for an accident. Of course, this means considerable extra measuring expenditure.

It is desirable to provide a radiation meter in which high measuring sensitivity is guaranteed in normal operation at the smallest possible expenditure, while the higher radiation exposure can still be measured accurately in the case of an accident.

According to the invention there is provided a radiation meter for measuring the radiation of gas, the meter comprising filtering means through which, in use, the gas flows to produce radiation on a radiating surface of the filtering means; radiation detection means having an operative surface arranged to detect radiation produced at the radiating surface; and means for changing the relationship between the radiating surface of the filtering means and the operative surface of the detection means.

Thus in accordance with an embodiment of the invention, means are provided to change the relationship, e.g. surface area exposed, between the radiating surface of the measuring filter and the effective surface of the detection means, comprising at least one radiation detector, in a defined manner.

Thus it is the basic concept to permit a defined change of the ratio between the radiating surface of the measuring filter and the effective surface of the detector positioned opposite the same. This may be accomplished either by varying the volume in front of a detector or by means of a diaphragm which shields off part of the radiation from the measuring filter to the detector. As soon as the radiation measured by the detector rises above a given value, either the volume is lessened or the width of opening of the diaphragm is reduced.

A preferred embodiment according to the invention makes use of a diaphragm: the description below relates to this particular embodiment.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example to the accompanying drawings in which: the only Figure is a diagrammatic cross-sectional view of a radiation meter according to an embodiment of the invention.

Exhaust air is supplied to a radiation meter 10 through a gas supply line 11 which is annular, and which has a tubular gas inlet 12 and a plurality of gas outlets indicated in the Figure by arrows 13. The exhaust air or gases pass through the gas outlets 13 and flow through a measuring filter 14 which is positioned opposite the gas outlets 13 and which is made, for example, of a glass fibre mat.

The measuring filter 14 in this case is held by a sintered metal plate 15 which is gas permeable and which does not present any significant resistance to the flow. A funnel-shaped gas collector 16 followed by a tubular gas exit 1 7 are located downstream of the sintered metal plate 15.

A plurality of radiation detectors, 18, 19 and 20 in the present case, stacked one above the other, are positioned opposite the measuring filter 14 (above the gas supply line 11 in the Figure). The detector 18 positioned closest to the measuring filter 14 is designed for a radiation. Above this there is a detector 19 which is designed for ss radiation, and finally, on top, there is a detector 20 which is designed for y radiation. A "Pertinax" disc 21 which is impermeable to ss radiation is positioned between the detector 19 and the detector 20. The upper side of detector 20 is covered by a plate 22 made, for example, of steel.

The basic structure and mode of operation of these detectors 18, 19 and 20 are known in principle. Each detector includes a chamber to which a so-called "counting gas" is supplied, such as a mixture of argon and methane. In the embodiment shown this counting gas is supplied to the three detectors 18, 19 and 20 by way of a common branching conduit 23. Each detector chamber has its own counting gas outlet 24, 25 and 26, respectively. In principle the counting gas is ionized by the radiation, the ions generating electrical signals at electrodes which, in the embodiment shown, may be tapped from lines 28, 29 and 30, respectively, of the corresponding detector. Furthermore, the individual detectors are subjected to high voltage supplied through a line 27. In the embodiment shown all three detectors receive the same high voltage.However, it is possible as well to apply different high voltages to the individual detectors.

The detector output signals are passed through lines 28, 29 and 30, respectively, to an evaluating circuit 31 containing display and/or recording instruments. It is likewise possible to provide for signal processing to take place there so as to determine the radiation proportions of the a, ss, and y radiation from the individual signals. (The output signal of detector 18 also may contain signal proportions caused by y and y radiation. Similarly, the output signal of detector 19 still may contain signal proportions caused by y radiation.

The individual radiation proportions then may be determined by signal processing, such as differentiating.) In one embodiment of the invention a diaphragm 32 is disposed between the measuring filter 14 and the next adjacent detector 18. In the particular embodiment shown it is located between the detector 18 and the gas supply line 11. In the present case the diaphragm 32 has two apertures 33 and 34 of different widths of opening. The larger aperture 33 corresponds to the full surface of the measuring filter 14 or of the detectors. The smaller aperture 34, on the other hand, may be much smaller, amounting for instance to one thousandth of the surface area of aperture 33.

The diaphragm 32 is displaceable by a mechanism 35 actuated by a drive means 36 such that in one limit position the aperture 33 will be positioned opposite the measuring filter 14, whereas in the other limit position aperture 34 will be effective. The displacement may be a linear movement. It is also possible to pivot, tilt or move the diaphragm in any other way.

An evaluating circuit 31 comprises limit value warning elements for the signals transmitted on lines 28, 29 and/or 30. As soon as such a limit value is exceeded, as sensed for example by a comparator, an elec tricai signal will appear on line 37 causing the drive means 36 (for example an electric motor) to shift the diaphragm from one limit position into the other limit position. This switches over the measuring range of the detectors. In the embodiment shown with which the smaller aperture 34 is effective, this means that radiation will reach the detectors only through this aperture 34. The respective position of the diaphragm is noted in the evaluating circuit 31 so that all that has to be done is to multiply the result of measurement of the detectors 18, 19 and 20 by a factor corresponding to the ratio of the opening cross sections of apertures 33 and 34.

According to one variant of the invention the diaphragm consists of aluminium of a thickness of about 3 mm which will shield off only a and ss radiation. In this case, therefore, a "switchover of ranges" takes place only as regards the detectors 18 and 19.

According to another variant the diaphragm is made of lead of a thickness of about 7 to 8 cm whereby also y radiation is shielded off.

The diaphragm shown may be replaced by an infinitely adjustable diaphragm, such as an annular diaphragm similar to those ordinarily used in photographic cameras. It is likewise possible to move or swing one or two plates without any opening laterally into the beam path in a manner similar to a slit diaphragm so as to cut out part of the radiation.

A radiation meter which has the desired high sensitivity in normal operation and also supplies precise measuring results in case of an accident can be realized by very simple means with the structure described above.

All the technical details presented in the claims, specification, and drawings may be inventive features both individually and in any desired combination.

Claims (12)

1. A radiation meter for measuring the radiation of gas, the meter comprising filtering means through which, in use, the gas flows to produce radiation on a radiating surface of the filtering means; radiation detection means having an operative surface arranged to detect radiation produced at the radiating surface; and means for changing the relationship between the radiating surface of the filtering means and the operative surface of the detection means.

2. A radiation meter as claimed in claim 1, comprising evaluating circuitry operable to control said means for changing the relationship between the surfaces in response to the radiation detected by the detection means exceeding a predetermined value.

3. A radiation meter as claimed in claim 1 or 2, wherein the means for changing the relationship of the surfaces comprises a diaphragm disposed between said surfaces and having an aperture through which radiation from the radiating surface can each the operative surface.

4. A radiation meter as claimed in claim 3, wherein the diaphragm comprises a plate supported for displacement and having first and second apertures of different dimensions, there being means for displacing the diaphragm between first and second positions in which the first or second aperture respectively is arranged to define the relationship between the surfaces.

5. A radiation meter as claimed in claim 3, wherein the dimension of the aperture of the diaphragm is continuously variable.

6. A radiation meter as claimed in claim 3, 4 or 5, wherein the diaphragm is made of aluminium.

7. A radiation meter as claimed in claim 6, wherein the diaphragm has a thickness, measured in the direction of radiation, of 3 mm.

8. A radiation meter as claimed in claim 3, 4 or 5, wherein the diaphragm is made of lead.

9. A radiation meter as claimed in claim 8 having a thickness, measured in the direction of radiation, in the range of from 70 to 80 mm.

10. A radiation meter as claimed in any preceding claim, wherein the detection means comprises three radiation detectors for detecting respectively different types of radiation and arranged one after the other in the direction of radiation, the means for changing the relationship between the surfaces being such as to control the passage of at least one of the types of radiation.

11. A radiation meter as claimed in claim 10 in which the different types of radiation are alpha, beta and gamma radiation.

12. A radiation meter substantially as hereinbefore described with reference to, or as shown, in the accompanying drawing.