GB2106643A - Improvements in or relating to vapour sensing - Google Patents

Abstract

A sensor capable of detecting a concentration C of impurity in a gas sample comprises an airtight container 5 having a gas inlet 6 and a gas outlet 7; the container is also light-tight. A radioactive source 1 is provided within the container and is spaced apart by a gap 3 from a detection means 2. A sensing device includes the vapour sensor and a detection circuit (not shown) to which the detection means is linked. The source emits beta particles causing the impurity vapour to fluoresce and emitted photons are selectively allowed to reach the detection means by utilization of a filter means 4, chosen for the type of impurity to be detected, between the sample and detection means. Calculations are disclosed regarding the sensitivity of the device to low impurity concentrations. <IMAGE>

Description

SPECIFICATION Improvements relating to vapour sensing This invention relates to sensors capable of impurity vapour detection.

Low concentrations of impurity vapours or gases in a gas sample may be detected by irradiation of the gas sample and simultaneous examination of the radiation absorption characteristic. Concentration variations will modify the extent to which absorption may occur. However, at low concentrations the degree of absorption requires resolution of small fluctuations against a large background, creating potential signal-to-noise problems. Induced fluorescence and detection of the reemitted photon from impurity gases can moderate these problems. Nevertheless photon sampling itself is generally asociated with a bulky assembly, particularly when the concentration of re-emitting impurity gas or vapour may be low.

It is an object of this invention to provide a compact vapour sensor and sensing device suitable for detection of impurity vapours at low concentration levels.

According to one aspect of the invention there is provided a sensor suitable for detecting impurity in a gas sample comprising a substantially light-tight container and a source of ionising radiation disposed to irradiate a gas sample introduced into said container; the sensor also including a detection means disposed to receive energy emitted from said sample in response to said irradiation and further including a filter located so as to filter said energy prior to reception thereof in the detection means.

According to a further aspect of the invention there is provided a sensing device suitable for detecting impurity in a gas sample comprising a sensor according to the immediately preceding paragraph and a detection circuit capable of responding to electrical property variation in said detection means.

In a preferred embodiment the energy source comprises a ss emitting radioactive source arranged to irradiate the sample, photons emitted by any impurity gas contained within this sample, may be detected by a photon detection means, creating a current therein which is subsequently sensed by the detection circuit.

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 only to the accompanying drawings, in which: Figure 1 illustrates a cross-sectional view of a vapour sensor.

Figure 2 illustrates a detection circuit of a sensing device in accordance with the invention.

Fluorescence occurs when an electron, energised from a low energy level to an excited energy level, emits a photon on return to the low level. Suitable energising means are radioactive radiations, for example ss particles (electrons). Referring to the vapour sensor illustrated in figure 1, a ss emitting source 1 is disposed facing a photon-sensitive detection means 2 and spaced therefrom by a gap 3. A photon filter means 4, provided in the gap and located directly adjacent to the detection means, covers the photon sensitive entrance of the detection means. Consequently only photons having the frequency range passed by the filter means 4 will reach the detection means 2.

A suitable emitting source is Krypton 85 in carbon, with an activity of 5 curies, enclosed within a box having an outer thickness of 2.4 cms and an area of 9.5 cm by 6.0 cms. It will be apparent that because of the geometrical arrangement 50% of the emitted ss rays travel outwards from the sensor and are lost, consequently reducing the effective activity by a factor of 2. The emitting area is typically 3.65 cm2. It will be apparent that other sources, activities and dimensions may be utilized. A container 5 is arranged to enclose the emitting source, detection means and filter means thereby ensuring that the gap 3 is air-tight and light-tight. A gas sample including an impurity vapour at a concentration C is introduced into gap 3 through an inlet pipe 6 and allowed to exit through an outlet pipe 7.Both inlet and outlet pipes are shaped to prohibit the entry of light into gap 3, which would otherwise cause spurious photon detection by the detection means. For example a coil or labyrinth shape may be used for the inlet and outlet pipes. It will be apparent that container 5 can also include radioactive shielding material, for example polystyrene, sufficient to satisfy the appropriate safety criteria. Therefore radiation impinges on the gas sample, exciting it, and emitted photons from the impurity gas pass through the selected filter means to be detected by detection means 4.

The emitting source described hereinabove emits ss particles (electrons) with 670 keV energy. After passage through the emitting source box the average energy of emitted particles is approximately one third of the maximum energy with typically 90% of emitted particles lost in the-box. (See for example "Nuclear Radiation Detectors" by J. Sharpe Methuen s Co. Ltd.). Consequently there are 1.85 x 1011 disintegrations within the source and allowing for the above-mentioned geometrical effects and assuming nine tenths of emitted ss particles are lost in the source, the remainder emerging with an average energy of 223 keV, then the energy available per second to excite the gas sample is 2.06 X 10'5eV.

An average energy of formation for an ion pair within a fluorescent type gas is of the order of 33eV, and typically three photons are emitted for each ionizing event. Therefore the energy incident on the gas sample could produce approximately 1.87 x 1014 photons each second. It will be apparent to those skilled in the art that the detection means can be tuned to the relevant photon wavelength to achieve a quantum efficiency (internal conversion of photons to electrons) of 70%, in addition to a detection means collection effeciency of 50%.

Therefore the current produced within the detection means for an impurity concentration C is of the order of C x 1.05 x 10-5 amperes.

A suitable detection means comprises a photo-voltaic silicon diode detector, for example an EMI type S30520, the dimensions of which, are chosen to optimize photon detection. Gas containing the impurity concentration of C is irradiated with ss particles in gap 3 causing fluorescence. The gap dimensions are chosen to optimize the photon detection. The frequency sensitive filter 4 allows primarily only the impurity re-emitted photons to impinge on detection means 2. A detector of the above type can detect a current of 10-12 amps, allowing concentration levels of 9.5 X 10-8 to be detected.

A detection circuit for inclusion in a sensing device including the above-mentioned vapour sensor is coupled across terminals A and B of the detection means 2, a suitable circuit being illustrated in figure 2. Terminal A of a detection means comprising a photo-voltaic diode detector is connected to a gate of a first FET transistor 8 having a common drain with a second FET transistor 9, thus forming the FET equivalent of a long tailed pair. The common drain is connected to a constant current supply 10, while the gate of FET transistor 9 and terminal B of the photo-voltaic diode detector 2 are connected to a zero voltage line 11. The source of FET transistor 8 is connected, firstly through a resistor 1 2 to a voltage line 13, and secondly to a positive input of a noninverting voltage amplifier 14.In a similar manner the source of FET transistor 9 is connected, firstly through a resistor 1 5 to the voltage line 13, and secondly to a negative input of the amplifier 14. An element of feedback is fed through very high impedence resistor 16, from an output 1 7 of amplifier 14 to the terminal A of photo-voltaic diode detector 2.

The impingement of photons on the photovoltaic diode.detector produces a current therein, which affects the conductance of the FET transistors 8 and 9. The resultant shift in voltage levels at the imputs of amplifier 14, is amplified and the amplifier output voltage with respect to zero voltage line 11 is representative of the current flowing in the photovoltaic diode detector, and can be calibrated to impurity vapour concentration accordingly.

Therefore a compact vapour sensor and sensing device can be produced, sensitive to low concentration of impurity vapour. Naturally the photon filter means may be need to be altered for differing impurity vapours. Alternative detection means may be utilized, for example an avalanche diode. However, the small responsive area may necessitate an element of photon channelling, requiring highly reflective surfaces, comprising for example, magnesium oxide.

It will be understood that the embodiment illustrated shows an application of the invention in one form only, for the purposes of illustration. In practice the invention may be applied to many different configurations, the detailed embodiments being straight-forward for those skilled in the art to implement.

Claims (9)

1. A sensor suitable for detecting impurity in a gas sample comprises a substantially light-tight container and a source of ionising radiation disposed to irradiate a gas sample introduced into said container; the sensor also including a detection means disposed to receive energy emitted from said sample in response to said irradiation and further including a filter located so as to filter said energy prior to reception thereof in the detection means.

2. A sensor according to Claim 1, wherein said source of ionising radiation is a ss particle emitting source disposed inside said container.

3. A sensor according to Claim 1 or 2, wherein said detection means comprises a photo-voltaic diode detector disposed inside said container.

4. A sensor according to Claim 1 or 2, wherein said detection means comprises an avalanche diode disposed inside said container.

5. A sensor according to any preceding claim, wherein said container comprises photon reflective interior surfaces and radiation absorbent walls.

6. A sensing device suitable for detecting impurity in a gas sample comprises a sensor according to any preceding claim and a detection circuit capable of responding to electrical property variation in said detection means.

7. A device according to Claim 6, wherein said detection circuit responds to current production in said detection means and includes at least one pair of field effect transistors having a common drain connected to a constant current supply, and said detection means is connected across the gates of said pair of transistors.

8. A sensor substantially as hereinbefore described with reference to and as illustrated in Fig. 1.

9. A sensing device substantially as herein described with reference to an as illustrated in Figs. 1 and 2.