GB2190787A - Multiple sample radioactivity detector - Google Patents

Abstract

Detecting head assembly capable of detecting and/or scanning radioactive emissions from a sample. The assembly includes a multiplicity of drift chambers (21) below which one or a number of individual samples may be supported. A beta particle emitted from the sample ionises gas within chambers (21) and negative ions are directed, by the application of an electric field, towards anode wires (27) which cross with cathodes (28) above drift chambers (21). The proximity of the tops of drift chambers (21) with anode wires (27) serves to prevent radiation received by any one drift chamber (21) from being detected by any part of the electrode structure associated with another drift chamber (21). The assembly is useful in the field of medical diagnostics and biomedical research for providing information relating to radioactive samples.

Description

SPECIFICATION Multiple sample radioactivity detector This invention relates to a detecting head assembly capable of simultaneously detecting radioactive emissions (such as beta particles) from a multiplicity of sampling zones. The detecting head assembly may be used in apparatus for counting ionizing events due to beta particles emitted by a radiochromatogram or an electrophoretogram (hereinafter called "radiograms"), or by biological or other samples.

It may alternatively be used in apparatus where the detecting head assembly and an apertured mask are scanned over a sample to determine the spatial distribution of radioactivity in a radiogram.

A scanning type of detecting head assembly is known from our EP-A-0112645. It comprises electrode subassemblies which include an array of cathode strips crossing an array of anode wires to form a plurality of detector crossing points. The crossing points detect individual ionizing events due to the ionization of a gas or mixture of gases by beta particles emitted from radioactive materials in a radiogram. In order to improve spatial resolution, a mask having a plurality of radiation transmissive zones or apertures (corresponding in position and number with the anode/cathode crossing points) is supported between a sample holder and the electrode sub-assemblies.

The detecting electrode sub-assemblies and the mask are mounted in scanning means which are supported in juxtaposition to the sample holder so that the mask and associated detecting electrode sub-assemblies (crossing points) can be scanned across a sample. In a preferred arrangement, respective 'x' and 'y' co-ordinate stepper motors are used to provide a raster scan of a plurality of regions in the sample zone. The output signals from the detecting electrode assemblies are supplied to either conventional pulse counting means, or they are coded (with respect to the position co-ordinates of individual ionization events) and the coded signals are then supplied to a computer where they are processed in order to derive information relating to the distribution of radioactivity in the sample.

The apparatus described in EP-A-112645 is particularly useful in providing a high resolution "picture" of the spatial distribution of radioactivity in a radiogram and it was primarily developed for visualising the distribution of radioactivity in a sample. However, we have also developed a multiple sample, radioactivity detector assembly of relatively straightforward and robust construction which can be used, for example, in the field of medical diagnostics and biomedical research, to count beta particle emissions from radioactive materials taken from radiograms or in biological or other samples where only the radioactivity of the samples is required and not the distribution of radioactivity within a sample. The latter detector assembly is described in detail in our copending UK Application No: 8429500 (and PCT Application No: GB 85 00532).

Referring to Fig.1, which schematically illustrates the detector assembly according to our copending UK Application No: 8429500, a multiplicity of drift chambers 9, each containing gas, are defined by respective walls 10 (of an apertured plate 8) and by respective portions of an apertured mask 6. Apertures 7 in mask 6 enable the passage of ions therethrough and ionisation of the gas within the drift chambers occurs because of the radioactive emissions from a sample located in a respective zone 2 of a sample holder 1. Detecting electrodes include an array of anode wires 4 disposed transversely of an array of cathode strips 3 so as to define a plurality of crossing-points 5.A crossing-point 5 is disposed above each respective aperture 7 in the mask 6 and an electric field, created within the drift chambers 9, causes ions to pass through the apertures 7 for detection by the respective crossing-points 5. The anode wires 4 are located in respective channels defined by parallel and spaced isolating walls 11 supported by the mask 6. The top portions of walls 11 extend above the level of the anode wires 4 and a small gas communication gap 13 exists between the top of each wall and the 'ceiling' of the assembly (on which cathode strips 3 are supported).

In the assembly shown in Fig.1, the limited size and the separation of the apertures 7 in mask 6, and the provision of the isolating walls 11 solve the problems of (a) ensuring that the radioactivity emanating from a particular sampling zone 2a is detected by only the respective electrode crossing-points 5a1 and 5a2, and (b) ensuring that there is no crosstalk with the electrode crossing-points sub1, 5b2 .... associated with an adjacent sampling zone 2b. For example, Fig. 1 shows a beta particle path 14 to illustrate how one of the walls 11 blocks the beta particle emitted from sampling zone 2a and thereby prevents it from being detected by the crossing-point 5b1 associated with an adjacent sampling zone 2b.

The present invention solves the abovementioned problems in a different way. It dispenses with the need for the apertured plate 6 (which limits the ion passageway by virtue of the fact that the apertures 7 are smaller than the extent of the drift chamber 9). It also dispenses with the need for isolating walls 11 (which are an added expense and which are troublesome to provide because they need to be very accurately aligned with (a) the solid portions of mask 6 to define the channels in which the anode wires 4 are located and (b) the anode wires). Not only does the present invention thereby provide the advantages of greater ease of manufacture and saving in ex pence, but it also provides improved performance.For example, the drift chamber structure in a c-:ecting head assembly according to the invention provides more detector gain than the assembly disclosed in our UK Application No: 8429560. Bt may also be advantageously used, in conjunction with the apertured (reso iueion, mask and scanning means disclosed in our EP-}t,-0112645 to greatly improve the sensitivity or detection efficiency of such a swanning head assembly.

Morse particularly, the present invention pro isdes a detector head assembly comprising a sample holder; wall means defining a multipli dnft :3f dri,t chambers for receiving radiation frnm respective zones of the sample holder and containing a gas which can be ionized by said radiation; a detecting electrode structure associated with one or more of said drift chambers and including anode means disposed with respect to cathode means to define a plurality of crossing-points, each crossing-point being provided for detecting ions in the respective drift chamber resulting from ionization of the gas therein; and means for applying potential to said sample holder, said wall means and said anode means, in order to create a field, within each of the drift chambers, to cause ions to move in the direction of the respective crossing-points, characterised in that said wall means extend between said sample holder and said electrode structure by an amount which is substantially sufficient to prevent the radiation received by any one drift chamber from being detected by any part of the electrode structure associated with another drift chamber, said wall means also defining unrestricted exits for the passage of ions to said electrode structure.

The latter-mentioned amount by which the wall means extend largely determines the screening effect between adjacent drift chambers, but the extent of the wall means may take account of other factors depending on the particular application of the detector head assembly.

For example, a detector head assembly according to one embodiment of the invention is users together with a resolution mask having a rn.'tiplicit" of apertures of the same size and shape le.g. as shown in scanning head assembly disclosed by EP-A-0112645). In such an embodiment, any given drift chamber effec tively has a floor portion defined by solid por- tions of the apertured resolution mask. In each such drift chamber, the floor portion of the drift camber may co-operate with the extent of the wall means of the drift chamber to provide, or to increase, the screening effect.

For example, the height of the wall means may be determined with regard to the size of the aperture used in the resolution mask.

Alternatively, a detecting head assembly according to another embodiment of the inven tier, can be used to determine the amount of radioactivity rather than the distribution of radioactivity within a sample and it does not employ an apertured resolution mask and scanning means. In such an embodiment, the height of the wall means of a given drift chamber substantially determines the screening effect.

The height of the wall means of the drift chamber may be determined with regard to the rate of energy loss of the radiation to be detected. For example, for radiation with a low rate of energy loss, which is therefore weakly ionising, it is advantageous to have long particle tracks inside the drift chambers and therefore long drift chambers (e.g. high walls). In a detecting head assembly according to the invention, since the anode means are not contained in channels bounded by isolating walls (11) and since the drift chambers are fully open to the detecting electrode structure without the intervention of an apertured plate (6), i.e. as in the case of the assembly shown in Fig.1, the drift chambers are effectively more extensive than those used in the assembly of our copending UK Application No: 8429540.Assemblies which embody the invention therefore have the advantage of increased sensitivity.

Where a gap is provided above the upper edges of the of the drift chamber wall means to accommodate the electrode structure (or proportional counter part) of the assembly and to enable free communication for gas contained in the drift chambers, the extent of this gap needs to be taken into account when determining e.g. the height of the walls of the drift chambers. The ratio of the extent of the drift chamber walls to the extent of the gap is preferably greater than 3:1 to enable acceptable packing the drift chambers (see below).

In some cases, the detecting head assembly may be a totally sealed or closed structure (e.g. where the entrances to the drift chambers are closed by radiation transmissive windows). The latter-mentioned gaps may still be provided in a closed structure to enable free communication and to accommodate the electrode structure (or proportional part) of the assembly. Where the structure is not totally sealed or closed, the detecting head assembly may be continually purged with a flow of gas (which escapes through minimal leakages).

A larger or more extensive gap facilitates assembly of the component parts during manufacture. For example, close tolerances need to be observed in positioning the anode means (e.g. anode wires) precisely, e.g. with respect to a preferred guard electrode (see below) and with respect to the cathode means.

In any event, the screening effect is substantially determined by the extent of the wall means which define the drift chambers.

Where an apertured resolution mask is used in conjunction with scanning means to deter mine the distribution of radioactivity in a sample, the size of the apertures in the mask can exert a limiting effect on the incidence of radiation on the electrode structure. This enables the above-mentioned gap to be made larger (thereby relaxing the demands on tolerance, as explained above). Thus, the size of the apertures in a resolution mask can be a factor to be taken into account in designing a scanning head assembly.

The drift chambers are preferably cylindrical and in this case they are largely defined by the wall which forms the cylindrical shape.

However, the drift chambers may have difference shapes.

The drift chambers, particularly those of cylindrical shape, are preferably arranged in either a square, or a hexagonal array. This advantageously increases the packing density of the drift chambers in the assembly, whilst preserving the screening effect of the drift chamber wall or walls (taking into account, where necessary, the size of the above-mentioned gap and/or transverse extent of apertures in a resolution mask, wherever these may be used, as explained above).

The cathode means may be in the form of planar elements (such as discs in the case of cylindrical drift chambers) which are located above respective drift chambers and which are electrically connected together in 'rows'. (With regard to the latter-mentioned 'rows', the anode means form 'columns' in a given scanning matrix-see detailed description below and EP-A-0112645). The cathode array may be produced by thin film/etching techniques to facilitate manufacture.

In the case of measuring the amount of radioactivity emitted by a sample (as opposed to scanning a sample to determine the distribution of radioactivity), any one drift chamber may serve a plurality of detector crossingpoints. However, when using an apertured mask and scanning means, respective detector crossing-points are normally associated with corresponding drift chambers.

Preferably, a guard electrode, which may be in the form of a conductive layer or sheet supported by, but insulated from the means defining the drift chambers, is located between the latter means and the detecting electrode structure. The guard electrode is normally maintained at a predetermined potential which is lower than that on the anode means and higher than the potential of the means defining the drift chambers, e.g. approximately 1/3 of the anode potential. The main purpose of such a guard electrode is to remove sharp discontinuities in the electric field which would otherwise occur at positions along the anodes where they cross over the edges of the drift chambers. (The sample holder is preferably maintained at a negative potential with respect to the drift chambers, which are preferably connected to earth or ground potential.These potentials together with those on the anodes and guard electrodes create the fields within the drift chambers which cause ions to move towards the electrode structures).

Preferred embodiments of the invention will now be described with reference to Figs 2-6 of the accompanying schematic drawings. In the drawings: Fig.1 is a section through apparatus in accordance with our copending UK Application No: 8429500 (as described above); Fig.2 is a sectioned, exploded and perspective view of part of a detector head assembly in accordance with an embodiment of the invention which includes a scanning mask; Figs 3 and 4 are respectively sectioned elevations and plan views (i.e. orthogonal to one another) through a similar part of the assembly shown in Fig.2; Fig.5 is a sectioned elevation, on an enlarged scale, of a part of the assembly as shown in Fig.3 (and indicating the path of maximum incidence of a beta particle on a detector electrode assembly;; Fig.6 is a sectioned, exploded and perspective view of a part of a detector head assembly according to another embodiment of the invention, which is used for counting radiation rather than scanning, where a single drift chamber serves a plurality of detector crossing-points.

A description will first be given of the embodiment shown in Figs 2-5 which is used in conjunction with an apertured mask and means (not shown) for causing relative scanning movement between a sample and the detector head assembly to determine the spatial distribution of radioactivity in each sample zone. A description Will then be given of the embodiment shown in Fig.6, which does not employ an apertured mask and scanning means, that can be used for counting radioactive emissions from the sampling zones. It will be understood that a 'sampling zone" may contain part of a comparatively large sample, such as a radiogram, or it may contain a specific and comparatively smaller sample, whereby a plurality of different samples may be analysed.

In the embodiments of the invention illustrated by Figs 2-5, a thick electrically conductive plate 20 has an array of circular drilled holes 21 which are arranged in either rectangular, or preferably hexagonal patterns (see Fig.4) to improve the packing density. Each hole 21 forms a cylindrical drift chamber and contains a gas that can be ionized by radiation from a respective zone 22 of a sample 23 (see Fig.3). The sample may be an individual sample or it may be part of a larger sample such as a radiogram 23.The sample is supported by a conductive sample holder 24 which may be connected to a (preferably variable) source 25 of negative potential typicaliy of the order of--IOOOV. (Although the sample holder 24 is shown to be connected to a negative source of potential, it will be more convenient, for engineering purposes, to maintain the sample at earth potential in a scann ing head assembly). The conductive plate 20 is connected to earth or ground potential as shown in Fig.3. The potential differences between the sample 28, detector anode 27, guard electrode 33 and drift chamber walls 20 creates an electrical field within each of the drift chambers 21 which causes negative ions to move towards an electrode structure 26.

The electrode structure comprises anode wires 27 extending substantially transversely of cathode means 28 to form a plurality of detector crossing-points 29 in the proportional counter assembly. In this embodiment, one crossing-point is provided for each drift cham ber to detect the respective ionization events.

(However, in the embodiment of Fig.6, more than one crossing-point is provided for each drift chamber, as will be explained in more detail below). The electrode structure 26 is positioned in a gap x (see Fig.5) which enables the free communication of gas between the drift chambers 21.

An apertured resolution mask 30 is supported below plate 20. Mask 30 in this example has a plurality of rectangular apertures 31 corresponding with respective drift chambers. As disclosed in our EP-A-0112645, the mask 30 is interchangeable to provide another with e.g. differently sized apertures, and or apertures of different shapes (rectangular or round) depending on the application and on the required resolution. As shown in Fig.4, the crossing-point 29, the centres of the respective apertures 31 and the centres of respective drift chambers are aligned. As shown in Fig.2, the mask 30 is a composite of a very thin continuous layer or sheet 30a and a relatively thick apertured layer or sheet 30b.

Preferably, layer 30a is an extremely thin metailised layer deposited on a very thin insulating sheet 30b e.g. aluminised 5 micron Mylar. Layer or sheet 30a is supported on the sheet 30b and is preferably connected to a negative potential,if insulated from the walls of the drift chambers 21, although it is usually connected to the same potential as the walls of the drift chamber 21.

When the layer or sheet 30a is omitted it enables the unimpeded entry of radiation to the drift chambers (which is important when detecting radiation of lower penetrating power, e.g. in the case of Tritium). 8uch a structure may be referred to as "open". However, in the case of using a continuous layer or sheet 30a to form a "closed" structure, whilst the entry of low-penetrating radiation may be impeded, portions of layer or sheet 30a, which obscure the apertures in the layer or sheet 30b, prevent the escape of the gas from the detecting head assembly so that it can be made as a sealed structure.

Hence, a "closed" structure is best used where the penetrating power of the radiation to be detected is not impeded by the ob scured radiation transmitting zones and where a sealed assembly is preferred. (The 5 micron aluminated Mylar window mentioned above will enable the passage of most radiation, e.g.

other than that from Tritium).

In the case of an open structure, the drift chambers 21 can be flooded with a continuous stream of gas or mixture of gases (e.g.

90% argon, 10% methane) as described in our copending UK Application No: 8429540 and in our EP-A-0112645. The latter references also describe, in more detail, the general construction of detecting head assemblies Which may be adopted in the case of assemblies embodying the features of the present invention.

A 'closed' structure, provided with a vent to enable a minimal leakage of gas, may also be purged with a continuous stream of gas. Such a structure avoids the expense of special preparation of the assembly prior to filling it with gas and also of ensuring gas tight seals.

The upper surface of the drift chamber plate 20 supports an insulating layer or sheet 32 and a conductive layer or sheet 33. Layer or sheet 33 constitutes a 'guard electrode' which is maintained at a potential which is below that on the anode wires 27, e.g by means of resistor network 35, but which is above the potential on the drift chsmber plate 20. For example, the guard electrode 33 is preferably maintained at approximately 1/3 of the potential on the anode wires 27. The insulating layer 32 electrically isolates the guard electrode 33 from the conductive drift chamber plate 20. The function of the guard electrode is to remove the sharp discontinuities in the electrical field which would otherwise occur at positions along the anode wires 27 where they cross over the edges of the drift chambers or holes 21.This significantly improves the stability and uniformity of response of the 'proportional counter' part of the detecting head assembly. (The 'proportional counter' part is that part of the assembly containing the electrode structure 26).

With regard to the electrode structure 26 (Fig.3) the cathode means 28 is provided in the form of planar, generally disc-shaped elements 28a which are electrically interconnected in 'rows' by small strips 28b (Figs 2 and 4). Thus, the cathodes in each 'row' are commonly connected together. The 'row' of cathodes is not linear in the case of hexagonal arrays of drift chambers, because the discs 28 and the interconnecting strips form respective 'zig-zags' (Fig.4) which transversely cross the anode wires 27. The cathodes may be provided by thin film/ etching techniques to facilitate manufacture. The cathode discs 28a and the interconnecting strips 28b are mounted on an insulating plate 34, i.e. which may be the substrate of an etched circuit board.

When the gas in the drift chambers 21 is ionized by radioactive emissions from the sample (or samples), negative ions are captured by the anode wires 27, at respective crossing-points 29, and a resulting signal is developed on both the respective anode wire 27 and cathode 28. These signals can be processed by conventional means to provide a count at the respective position co-ordinates of the detector crossing-points. Suitable processing and counting circuitry is disclosed in more detail in EP-A-0112645. Details are also given in EP-A-0112645 of suitable scanning means (not shown in the drawings) to cause relative movement between the detecting head assembly and a sample, such as a radiogram, supported by sample holder 24.Therefore, details of the signal processing and scanning means (as they apply to the detecting head assembly described with reference to Figs 25) need not be given here.

As noted above, and as shown in more detail in Fig.4, a hexagonal array of drift chambers 21 is preferred to improve the packing density within the detector head assembly. This means that the maximum number of drift chambers per unit sample area can be obtained thus maximising sensitivity. In the case of hexagonal arrays of drift chambers, the connecting strips 28b which interconnect the cathode discs 28a (Fig.2) form a set of zig-zag conductors running at right angles to one another and transversely to the anode wires 27.

Referring to Fig.5, which is an enlarged cross-sectional eievation through part of the assembly (detector electrode space), the proportional counter part of the assembly occupies the gap or space x between the guard electrode 33 and the insulating substrate 34 of the cathode array 28. Fig.5 also shows radiation, in the form of a beta particle, originating from a sample zone 22a (which corresponds with the cross-sectional area of an aperture 31a in the scanning mask 30). It can be seen that the wall 20a acts as a barrier to prevent radiation, within drift chamber 21a, from entering the proportional counter part of the assembly at a position which would result in a signal from electrode structure 26b associated with drift chamber 21b.In the illustrated example, the elimination of cross-talk is effective provided that the track of the beta particle, from aperture 31a of drift chamber 21a, does not cross the mid-line of the wall 20a between drift chambers 21a,21b. If the drift chambers 21 are to be packed as closely as possible to give a high detection efficiency, then the extent x must be very much smaller than the extent y of the wall means defining the drift chambers 21. (In Fig.5, the "wall means" defining the drift chambers includes the thickness of the insulating sheet 33 and the guard electrode sheet 27 although these will be minimal compared with the height of the wall 20). Typically, y is of the order of 0.5 to 1.0cm and xis of the order of 0.15 to 0.30cm.The relatively long track length y shown in Fig.5 is used to advantage by creating electrical fields in the drift chambers 21 so that any negative ions, created by the passage of radiation (e.g. beta particles), are caused to drift into the respective proportional counter region where they are detected. Because of the long track length, more ions are produced than in the case with the detecting head assembly described in Patent No: EP-A-0112645 and the detection of weakly ionizing radiations, such as those from phosphorous 32, is improved. As explained above, the field in the drift chambers is created by applying appropriate potentials to the guard electrode 33, the drift chamber plate 20 and the sample holder 24 or metalised layer on sheet 30a, and anode wires 27.

In some cases the width of the upper part of the drift chamber walls 20 may need to be taken into account (with respect to wall height) when designing the structure. The minimum wall width will depend, to some extent on the shape and distribution (i.e. packing pattern) of the drift chambers. However, the wall width will normally be a minimal factor in view of the substantial screening effect of the drift chamber walls.

The use of an array of holes as drift chambers, the walls of which prevent cross-talk between adjacent regions provides a means of placing what are in effect walls in all directions relative to the anode wires. This allows a closer packing of detector elements and consequently a greater overall detection efficiency than with the assembly shown in our Patent EP-A-0112645 and also our copending UK Application No: 8429540.

Where a high resolution image is not required and it is only the amount of radiation from a series of discrete samples or sample zones which needs to be measured, it is not necessary to use a scanning mask 30. For example, individual samples can be placed on individual planchettes and each planchette placed under a hole or drift chamber (see our copending UK Application No: 8429540). The properties of the drift chambers embodying the present invention result in a high detection efficiency and relative insensitivity to sample position and distribution. This is because all particles entering the drift chambers 21 from any direction or position will produce ions, all or part of which will be caused to drift into the proportional counter part of the assembly to be detected.Where planchettes are used, the samples should be as thin as possible to reduce self-attenuation of radiation in the sample. It is therefore advantageous when measuring planchettes to have a large sample area so that they can be spread out at thinly as possible. With large planchettes the drift chambers 21 should ideally be correspondingly large, as shown in Figure 6, where a large hole 21 is placed over a large area planchette 36 containing a sample 23. lonisation will be detected at the small array of crossing-points 29 associated with this hole. An array of such large holes 21, corresponding to discrete sets of crossing-points 29, allows for the simultaneous counting of a number of planchettes 36. Associated conventional signal processing means are used to allocate counts to appropriate planchettes.

The structure of the embodiment shown in Fig.6, which does not require a scanning mask 30, is otherwise generally similar to the embodiment described with reference to Figs 25. It will also be noted that the drift chamber walls provide unrestricted entrances for the passage of radiation from the sample 23.

The assemblies described above are but examples of how the invention may be applied in practice and various other modifications and changes may be made to suit individual requirements without departing from the scope of the invention.

Claims (10)

1. A detector head assembly comprising a sample holder; wall means defining a multiplicity of drift chambers for receiving radiation from respective zones of the sample holder and containing a gas which can be ionized by said radiation; a detecting electrode structure associated with one or more of said drift chambers and including anode means disposed with respect to cathode means to define a plurality of crossing-points, each crossing-point being provided for detecting ions in the respective drift chamber resulting from ionization of the gas therein; and means for applying potentials to said sample holder, said wall means and said anode means, in order to create a field, within each of the drift chambers, to cause ions to move in a direction of the respective crossing-points, characterised in that said wall means extend between said sample holder and said electrode structure by an amount which is substantially sufficient to prevent the radiation received by any one drift chamber from being detected by any part of the electrode structure associated with another drift chamber, said wall means also defining unrestricted exits for the passage of ions to said electrode structure.

2. An assembly according to claim 1 incorporating a mask having a plurality of radiation transmissive zones or apertures therein for limiting the entry of radiation into the respective drift chambers, the extent of said wall means taking into account the extent of the radiation transmissive zones or apertures in said mask with regard to preventing the radiation received by any one drift chamber from being detected by any part of the electrode structure associated with another drift chamber.

3. An assembly according to claim 1 wherein said wall means also define unrestricted entrances for the passage of radiation into the respective drift chambers.

4. An assembly according to claim 3 wherein each drift chamber is served by a plurality of said crossing-points.

5. An assembly according to any one of the preceding claims wherein a gap is provided between upper edges of said wall means and the electrode structure, the extent of said wall means taking into account the extent of said gap.

6. An assembly according to any one of the preceding claims wherein the drift chambers are cylindrical.

7. An assembly according to claim 6 wherein said cathode means are provided in the form of planar, generally disc-shaped elements which are electrically interconnected, by strips, in substantially respective rows.

8. An assembly according to any one of the preceding claims wherein said drift chambers are arranged in either square, or hexagonal arrays to improve the packing density of the drift chambers within the assembly.

9. An assembly according to any one of the preceding claims wherein a guard electrode is provided between said electrode structure and said wall means, said guard electrode being maintained at a potential which is lower than that on the anode means and higher than the potential of said wall means in order to remove discontinuities in said electric field which would otherwise occur at positions along said anode means where said anode means cross over the edges of the drift chambers.

10. A detecting head assembly substantially as herein described with reference to either Figs 2-5, or Fig.6 of the accompanying drawings.