GB2042253A - An electron-capture detector for analysis of substances in gaseous current - Google Patents

Abstract

An electron-capture detector for analysis of substances in gaseous current, comprising a radioactive chamber forming a first electrode (16) and a second electrode (18) which is able from the detector for cleaning purposes. In a preferred arrangement the second electrode (18) comprises an axially bored plug threadedly engaging in a metal plate (38) and removable therefrom by removal of a screw cover (52), lifting out a Belleville washer (50) and an axially bored connecting member (44) and unscrewing the electrode with a screwdriver. The axial bore through the electrode (18) is axially in line with the inlet (24) and outlet (56) for the gas flow and serves preferentially to collect impurities in the gas stream, rather than on the walls of the chamber (16). <IMAGE>

Description

SPECIFICATION An electron-capture detector for analysis of substances in gaseous current This invention relates to apparatus for the analysis of substances in a gas current, operating according to the principle of electron capture and generally known as electron-capture detectors. Such detectors record variations in electron flow caused by the passage of gaseous current containing an electroncapturing substance through a chamber between two electrodes, one of which comprises a radioactive source, whilst the other electrode is an electrode, for instance a stem electrode, which receives the gaseous current after it has crossed the chamber.

The theoretical principles and operating methods of electron-capture detectors are well known and do not need to be described herein. For the purposes of the present invention it is sufficient to record that these apparatus employ a radioactive source, generally Ni63, which is housed inside the apparatus and suitably protected in order to avoid any danger of contamination to the outside. These apparatus are well known and widely used, but it has been noticed that, after a more or less prolonged period of use, impurities unavoidably present in the gases flowing through the apparatus form a sediment on both electrodes and specially on the electrode comprising the radioactive source. This results in the build-up of an insulating layer on one or both electrodes which adversely affects the precision and sensitivity of the detector.It is therefore necessary occasionally to dismantle the detector in order to reach the electrodes and clean them. This operation, though not particularly complicated in itself, involves several problems due to the presence of the radioactive source and the consequent need of operating under conditions such as to avoid any possible contamination to things or persons. The dismantling and cleaning operations must therefore be carried out by highly specialised personnel under conditions of isolation from possible radioactive contamination by the radioactive source.

The present invention seeks to provide a detector of the above type which eliminates or at least greatly reduces the above-mentioned disadvantages by annulling or considerably reducing the frequency of the cleaning operations necessary particularly on the electrode constituting the radioactive source.

According to this invention, there is provided an electron capture detector comprising an electrode chamber, the side walls of which constitute a first electrode supporting a radioactive source and an end wall of which supports a second electrode, means for feeding a current of gas through said chamber, and means for detecting electron flow variations between said two electrodes as a result of electron capture by electron capturing substances in said gas as it passes through said chamber, wherein said second electrode is removably mounted in a seating provided in said end wall and is removable therefrom from outside said chamber.In other words, according to the invention, the proposed detector has a structure allowing the removal of the non-radioactive electrode from the apparatus without any need to involve the other electrode comprising the radioactive source and above all without any danger of radioactive contamination to the operator in charge of disassembling orto the enviroment.

This allows a considerable reduction in the number of operations involving the radioactive chamber.

However, according to a further feature of the invention, this number of operations may be further reduced and practically eliminated by a particular configuration of the second electrode or stem electrode whereby the gaseous current substantially reaches only this second electrode thereby depositing most or all the impurities contained therein on the removable electrode rather than the radioactive electrode chamber.

The invention is further described with reference to the accompanying drawings, in which Figure 1 is a partial cross-section illustrating an electron capture detector according to the invention; Figure 2 is a cross-section, corresponding to that of Figure 1, showing the removable components of the stem electrode, in a disassembled condition but in their respective positions ready for assembly; Figure 3 is an enlarged cross-section showing the relative positioning of the two electrodes.

With reference to the drawings, the electroncapture detector comprises a closed envelope 10 which is traversed by a gaseous current containing the substances to be examined. The gas current is introduced through a fitting 12 and withdrawn through a second fitting 14. Along the path of said gaseous current inside the apparatus there are two electrodes, a first electrode 16 essentially consisting of a chamber, for instance a cylindrical one, the walls of which bear a radioactive source, in particular of Ni63, and a second electrode 18, for instance a stem electrode, the two electrodes being connected by means of connections and electrical contacts 20 and 22, to a counting and detecting device, not illustrated in detail.

The radioactive chamber 16, which has a hollow inside and is connected by means of a channel 24to the inlet fitting 12, is screened from the outside by a first U-shaped block 26, made of metal, to which a contact element 28 joined to the conductor 20 is connected, the block 26 being separated by a first insulating insert 32 from a second block 30, acting as external support, made of metal and suitably shaped.

A second insulating insert 34 separates contact 28 from external block 30. The U-shaped block 26, insulating insert 32 and external block 30 are drilled to allow formation of inlet channel 24.

On the downstream side with respect to the flow of gaseous current, the radioactive chamber 16 is delimited by a second insulating element 36, backed by a metal plate 38 which receives a second contact element 40 connected to conductor 22 and insulated from block 30 as shown at 42. The metal plate 38 receives, with reciprocal electrical contact, the second electrode 18, the structure and configuration of which will be illustrated later on.

Behind the metal plate 38 is another insulating element 42 separating the plate from a substantially T-shaped conveying element 44 made of metal and having a shaped opening 46 for conveying the gaseous current into an outlet channel 48. The element 44 is kept in position by means of a Belleville washer 50 in turn held by a sealing element or cover 52 screwed into a threaded aperture 54 in the body 30. The channel 48 leads to a duct 56 which passes through an opening in the sealing element or cover 52. As can be seen in more detail in Figures 2 and 3, the stem electrode 18 is formed by an elongated element having an external surface partially threaded as indicated at 58 and a head 60 with a groove for engagement of a suitable tool, e.g. à screwdriver. The electrode 18 is screwed into a threaded aperture 64 in the plate 38.

As it can be seen from Figure 3, the electrode 18 has a central bore 66 through which the gaseous current, which has traversed the chamber 16, is conveyed. For this purpose, the lower end of the electrode 18 protrudes through a suitable opening 68 in the insulating element 36, the insulating element 36 and the plate 38 serving to provide an airtight seal around the electrode 18.

As it can be seen from Figure 2, disassembly of the electrode 18 may be performed very quickly and easily by taking off the sealing element or cover 52 and removing in sequence, the Belleville washer 50 and the T-shaped element 44. This allows access, by means of a screwdriver, to the head 60 of electrode 18 so as to unscrew it from the threaded seat 64 and remove it from the apparatus for the cleaning operation. Assembly is carried out equally simply in the reverse order. Not only that but the electrode 18 may be removed from its seat without the need of opening the access to the radioactive chamber 16 and therefore without the need of taking all the precautions which otherwise would be necessary.

The axial bore 66 through the electrode 18 furthermore creates a preferential flow path for the gaseous current from channel 24 through chamber 16 in such a way that impurities therein are deposited on said electrode 18, particularly in the bore thereof, in preference to deposition on the walls of the chamber 16. This reduces the frequency with which it is necessary to enter the chamber 16 to clean it.

As it is known, electron-capture detectors of the present type are heated in such a way that they work at strictly controlled temperatures. For this purpose an electronically actuated device 70 is used to control the temperature via a heating jacket 72 surrounding said block 30 and transmitting heat to the electrodes; in particular it transmits heat to electrode 18 by conduction in the contact zone between the latter and plate 38.

It will thus be seen that the present invention provides a detecting apparatus which reduces the need of involving the radioactive chamber during the cleaning process. In particular impurity deposits occur substantially on the non-radioactive electrode and which may therefore be treated and handled as well as removed from the apparatus with substantially reduced danger of contamination to the operator. Also the non-radioactive electrode can be removed for cleaning in a particularly simple and rapid manner.

Claims (6)

1. An electron capture detector comprising an electrode chamber, the side walls of which constitute a first electrode supporting a radioactive source and an end wall of which supports a second electrode, means for feeding a current of gas through said chamber, and means for detecting electron flow variations between said two electrodes as a result of electron capture by electron capturing substances in said gas as it passes through said chamber, wherein said second electrode is removably mounted in a seating provided in said end wall and is removable therefrom from outside said chamber.

2. A detector according to claim 1, wherein said end wall comprises a metal plate separated from said side walls by an insulating member and having said seating formed therein, said second elecrode being removably mounted in said plate and projecting therefrom through a passageway in said insulating member into said chamber, and being removable from said seating from the side of said plate remote from said chamber.

3. A detector according to claim 2, wherein said second electrode comprises a screw-threaded ele mentthreadedly engaging in a screw-threaded seating provided in said plate, said element having a passageway therethrough for the outflow of the current of gas from the chamber, said insulating member and metal plate providing a gas-tight seal around the second electrode at said end of the chamber.

4. A detector according to claim 3, wherein said passageway through the second electrode comprises an axial bore through the threaded member aligned with the gas inlet to the chamber.

5. A detector according to any one of the preceding claims having a heating jacket positioned about said chamber.

6. A detector according to claim 1 substantially as hereinbefore described with reference to the accompanying drawings.