EP0340126B1 - Parallax-free gas-filled x-ray detector - Google Patents

Description

The invention relates to detectors of ionizing radiation, in particular X-rays, and more particularly gaseous detectors, that is to say those for which the material absorbing the radiations to generate electrons is a gas (based on argon or xenon for example) see the first part of claim 1.

The invention also relates to the use of such a detector.

This type of detector is used for example to analyze samples of matter (metal alloys, proteins, crystal structures, biological macromolecules etc.) in order to determine the structure. The samples are placed in front of the detector and lit laterally (in general) by an X-ray source; they diffract the rays and return them to the detector and the role of the latter is to determine the angle of incidence at which it receives the X-rays, therefore the angle of diffraction by the sample. The measured diffraction angles provide indications of the structure of the sample material.

Known two-dimensional gas detectors have a structure which is generally represented in FIG. 1. They correspond for example to what is described in FIG. 1 of American patent US Pat. No. 4,595,834.

The detector comprises a sealed enclosure 10 containing the absorbent gas, and on the front face a sealed inlet window 12, transparent to X-rays. This window carries a transparent electrode 14 brought to a potential V1. Between the window 12 and the bottom of the enclosure 10 extends the space 16 called absorption and drift space, filled with gas (argon or xenon with polyatomic additives).

At the bottom of the enclosure, opposite the window 12 is placed an electron detector 18 called "location detector" because of its function which is to detect the presence and the position of a packet of electrons from the ionization of the enclosure gas. This detector 18 comprises an input electrode 19 transparent to the electrons brought to a potential V2 greater than V1 (for example 0V (volt) if V1 is at -4000V (volts) and the distance between the electrodes 14 and 19 is l '' order of 10 cm).

A sample of material 20, placed outside the enclosure, in front of the window 12 and at a certain distance from it, is lit laterally by a source 22 of X-rays.

By diffraction, photonic radiation 24 is re-emitted from the sample towards the absorbing gas with an angle of incidence that one seeks to know.

Upon entering the gas, a photon will be absorbed at a point in the enclosure and at this point it will emit an electron or a packet of electrons. The electric field in the absorption and drift space is created by the potential difference V2 - V1 so that the electrons drift, along the field lines, towards the detector 18 and their arrival position is detected. The field lines are straight lines perpendicular to the electrodes 14 and 19.

As can be seen in FIG. 1, depending on whether the incident photon is absorbed at a point A or a point B on its trajectory, the electron detector 18 will detect a position a or b for receiving an electron packet .

This means that, from the information on the position of reception of the electron packet, it is not possible to clearly ascertain the angle of incidence of the radiation 24.

There is a so-called parallax error due to the fact that the electric field which causes the electrons to drift is not oriented in the direction of the incident ray 24.

The object of the present invention is to produce a two-dimensional radiation detector without parallax error.

Partial solutions to this problem have already been proposed.

Some appear in US-A-4,595,834 already cited.

A theoretical solution is simple: it would consist in making a spherical enclosure with a spherical input electrode and an electron detector with localization also spherical and concentric with the input electrode, the sample being placed in the center of these elements. spherical. The electrons are then entrained in the direction of the incident radiation. There is no parallax error.

However, we do not know how to make a detector with a spherical location of sufficient dimensions because these detectors are of very delicate technology (they are generally made up of very fine wires which can be stretched in a plane but which cannot be bent).

Figure 2 of the aforementioned patent US-A-4,595,834 proposes to produce a radial electric field (that is to say spherical equipotentials) using a spherical input electrode, a spherical concentric auxiliary electrode, the space absorption and drift being delimited by these two electrodes, and a transfer space being provided between the spherical auxiliary electrode and the location detector which is planar.

The potential difference between the two electrodes creates a radial electric field and spherical equipotentials in the absorption space.

But the sample must necessarily remain in the center of the spheres.

In addition, the spherical electrodes are difficult to produce, especially the auxiliary electrode because it must be very transparent to the electrons since the electrons must reach the localization detector; it is therefore produced in the form of a grid of fine wires which is difficult to manufacture.

This is why the patent US-A-4,595,834 proposes purely and simply to suppress the auxiliary electrode by bringing the input electrode and the localization detector considerably closer to one another and to increase the pressure. some gas.

The parallax error is reduced by forcing the X-rays to be absorbed near the spherical input electrode where the field is approximately radial. This is obtained by using xenon under high pressure and limits the use of such a system to not too energetic X-rays and requires the use of a fairly thick spherical beryllium window. For reasons of pressure resistance, this window is necessarily of limited size.

Finally, it is necessary to point out another method proposed by G. Charpak in "Nuclear Instruments and Methods" 1982, N ° 201, pages 181-192, North-Holland Publishing Company, to produce a radial electric field without spherical electrode. It consists in replacing the spherical input electrode and the spherical auxiliary electrode of US Pat. No. 4,595,834 each with a set of planar electrodes brought to potentials different from each other, the potentials being calculated for each individual electrode so that the equipotentials throughout the absorption space are spherical and centered on the sample. This solution makes it possible to change the radius of curvature of the equipotentials and therefore the position of the sample relative to the detector input window by varying the potentials on the different conductors. But the realization of group of auxiliary electrodes located in the middle of the chamber is very complex (they must be transparent to the electrons) and an attempt at realization was only considered by the author to obtain cylindrical and non-spherical equipotentials.

To solve the problem in the case of spherical equipotentials, Charpak (FR-A-2363117) and Bolon et al. (IEEE Transactions on Nuclear Science, Vol. 5-26, Feb. 1979, PP 146-149) proposed to play on the potential of lateral electrodes but by maintaining at least one intermediate electrode.

The present invention proposes a new X-ray detector which makes it possible to avoid the drawbacks of the gaseous detectors of the prior art and in particular authorizes the placement of a sample at a variable distance from the entry window, while minimizing the 'parallax error, and simplifying manufacturing.

According to the invention, a gaseous detector of radiations emitted by a sample is proposed, comprising a closed enclosure containing an absorbent gas for the radiation, an entrance window transparent to the radiations to be detected, an absorption and drift space behind the entry window and, at the end of this space, a two-dimensional plane electron location detector to determine the coordinates of an electron arrival point generated by an impact of photons in the absorbent gas, the detector further comprising a group of input electrodes situated behind the input window and largely transparent to radiation; this detector further comprises a group of lateral electrodes surrounding the absorption and drift space, the individual input electrodes and the individual lateral electrodes being brought to potentials which are different from one another and which vary according to the position at which one wishes to place the sample relative to the entry window, the potentials chosen for each of the electrodes being such that the absorption and drift space is separated into two parts without the use of electrodes physically delimiting this separation , the equipotentials in the first part being spherical or quasi-spherical and centered on the position of the sample, and the equipotentials in the second part being continuously variable from a spherical shape, at the point of separation, to a planar shape in the immediate vicinity of the plane electron detector.

Thus, the disadvantage of having to manufacture and install a group of complex auxiliary electrodes is eliminated, in which each of the individual electrodes must be supplied separately and must be very transparent to the electrons.

Provision may be made for the first part of the absorption space (part with spherical equipotentials) to be as large as possible; thus, it will be possible to have an extended absorption zone without increasing the overall dimensions of the detector; this is all the easier when the sample is far from the input window (but then we can only detect a small range of angles of radiation incidence); when the sample is near the window we get a first part extending over 70 to 90% (percentage measured in the axis of the detector) of the distance between the entry window and the electron detector. By choosing a distance between the entrance window and the detector sufficient, for example 10 cm, almost all of the X-rays will be absorbed in the first part and this at a pressure equal to or slightly higher than atmospheric pressure.

A much simpler construction detector is thus obtained, presenting no parallax error, and making it possible to place the sample to be observed at a variable distance from the input window.

The lateral electrodes of the enclosure will preferably be formed on conical side walls laterally delimiting the absorption and drift space.

Preferably, the input electrodes are formed by screen printing on an insulating substrate and are separated from each other by a highly resistive substance allowing the flow of electric ionization charges which would risk accumulating between the electrodes.

• la figure 1, déjà décrite, représente la structure générale d'un détecteur gazeux de type connu ;
• la figure 2 représente une coupe latérale schématique du détecteur selon l'invention ;
• la figure 3 représente une configuration schématique d'équipotentielle dans le détecteur selon l'invention ;
• la figure 4 représente une vue en plan des électrodes d'entrée ;
• la figure 5 représente une vue en coupe latérale agrandie des électrodes d'entrée et de leurs conducteurs d'alimentation ; et
• la figure 6 représente une réalisation de détecteur avec tube central pour l'analyse de la rétrodiffraction de l'échantillon.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the appended drawings in which:

• Figure 1, already described, shows the general structure of a gas detector of known type;
• Figure 2 shows a schematic side section of the detector according to the invention;
• FIG. 3 represents a schematic configuration of equipotential in the detector according to the invention;
• Figure 4 shows a plan view of the input electrodes;
• FIG. 5 represents an enlarged side section view of the input electrodes and their supply conductors; and
• FIG. 6 represents an embodiment of a detector with central tube for the analysis of the backscattering of the sample.

In Figure 2 we see the general structure of the detector according to the invention.

The detector comprises a sealed external enclosure 30 closed at the front by an inlet window 32 transparent to X-rays (or more generally to the radiation to be detected). The window is for example made of mylar or kapton (trademarks for polymer films) or beryllium.

The bottom of the enclosure 30 comprises, as in the prior art, a plane electron detector 34 which is a location detector, two-dimensional, for example a wire detector, with parallel plates or any other type of known gas detector.

At the rear of the input window 32 is placed a set of input electrodes which are in principle circular, concentric and all in the same plane, parallel to the plane of the electron detector. The fact that they are all in the same plan facilitates manufacturing but this is not an obligation. One can for example arrange them on a spherical surface. These input electrodes are symbolized by the reference 36; they are best seen in plan view in FIG. 4. The center of the circular input electrodes is located on the general axis 38 of the system, axis perpendicular to the electron detector 34 at its center.

The input electrodes 36 can be carried by a transparent X-ray support separate from the input window 32 or can be applied to the window, with the interposition of an insulating layer if the window is conductive.

The enclosure 30 is filled with gas absorbing the radiation to be detected: for example argon or xenon with one or more additives (hydrocarbon, CO₂, etc.) allowing the localization detector 34 to function properly and having good drift characteristics and the absence of excessive electronic recombination which would harm the collection of electrons.

In the enclosure, an absorption and drift space 40 is materially delimited, between the input electrodes 36 and the electron detector 34, by a side wall 42 of generally conical shape, having as axis the general axis 38 of the detector; this wall 42 surrounds the entire absorption and drift space in which electrons can be generated by incident radiation and then directed towards the electron detector 34.

The choice of a conical shape is the most practical choice and the most adapted to the aim pursued but it is not compulsory.

The conical side wall 42 need not be sealed; it only serves as a support for lateral electrodes 44 which surround the drift and absorption space 40.

The wall 42 may for example be a sheet based on glass fiber on which are deposited conductors constituting the electrodes 44, for example by screen printing or by printed circuit techniques.

The individual input electrodes 36 and the lateral individual electrodes 44 can be brought to potentials which are all different from each other, these potentials being able to vary depending on the distance at which the sample 20 to be observed will be placed relative to the electrodes d 'entry 36.

The lateral electrodes 44 are distributed over the entire length of the wall 42, between the small end of the cone (immediately adjacent to the plane of the input electrodes) and the large end of the cone (immediately adjacent to the plane of the electron detector).

The lateral electrodes are circular, centered on the axis 38 of the detector.

The number of electrodes 36 and 44 is a function of the desired precision on the electric field inside the absorption and drift space.

The individual potentials of the side electrodes are brought by conductors 46 external to the wall 42, through conductive passages arranged in the wall opposite each electrode. The outer conductors 46 are connected to connectors 48 through which the various potentials can be brought. The potentials can be generated by resistive dividing bridges, not shown, located outside the enclosure 30 and preset as required for desired sample distances, or by a more complex voltage generation system controlled by outside by the detector user.

The connection system is the same for the input electrodes 36 but it has not been shown so as not to make Figure 2 heavy.

The potentials that must be applied to the different input electrodes 36 and to the different lateral electrodes 44 are calculated in the manner which will now be indicated: the explanation is given with reference to FIG. 3.

A distance D is chosen at which the sample 20 to be observed will be placed (distance between the sample and the plane of the input electrodes 36) and the sphere centered on the position of the sample and of radius D is called SPHD.

We choose a distance L corresponding to the radius of a virtual SPHL sphere centered on the position S of the sample, this SPHL sphere constituting an immaterial limit of separation between two regions A and B of the absorption and drift space 40 .

• la région A, située entre les électrodes d'entrée 36 et la sphère limite SPHL, soit soumise à un champ électrique radial centré sur le point S, c'est-à-dire que les équipotentielles y seront des sphères concentriques à la sphère SPHL; et
• la région B, située entre la sphère limite SPHL et le détecteur d'électrons plan 34, soit soumise à un champ électrique se déformant progressivement d'une direction radiale à une direction perpendiculaire au plan du détecteur d'électrons 34. Dans cette région B, les équipotentielles se déformeront pour passer d'une forme sensiblement sphérique à proximité immédiate de la sphère SPHL à une forme plane à proximité immédiate du détecteur 34.

The potentials to be applied to the electrodes 36 and 44 will be chosen so that:

• the region A, located between the input electrodes 36 and the limit sphere SPHL, is subjected to a radial electric field centered on the point S, that is to say that the equipotentials there will be spheres concentric with the sphere SPHL ; and
• region B, located between the limit sphere SPHL and the plane electron detector 34, is subjected to an electric field gradually deforming from a radial direction to a direction perpendicular to the plane of the electron detector 34. In this region B , the equipotentials will deform to pass from a substantially spherical shape in the immediate vicinity of the SPHL sphere to a planar shape in the immediate vicinity of the detector 34.

It should be noted that the radial electric field is produced not only by virtue of the lateral electrodes 44 situated inside the SPHL sphere, but also by virtue of an appropriate choice of the potentials of the lateral electrodes 44 situated outside the SPHL sphere; this remark is important because the absence of a material spherical auxiliary electrode at the location of the limiting auxiliary sphere SPHL or the absence of plane auxiliary electrodes between regions A and B to simulate a spherical electrode, requires doing also pay particular attention to potentials applied to the lateral electrodes 44 located outside the SPHL limit sphere. The spherical equipotentials in the vicinity of the limit sphere SPHL are in fact particularly sensitive to the proximity of the plane detector and they are not isolated by an electrostatic screen which until now constituted the auxiliary electrode or electrodes materially placed in the limit region between the regions A and B.

By considerations of simple electrostatic, one determines the equipotentials between two concentric conducting spheres centered on the point S, one being a starting sphere SPHD of radius D and the other the limiting sphere SPHL of radius L.

• d'une part les valeurs des potentiels sur toutes les sphères concentriques intermédiaires de la région A, la valeur du potentiel V(R) sur une sphère intermédiaire de rayon R étant : $$\text{V(R) = (VD - VL) x L x D/R (L - D) + (L x VL - D x VD)/(L-D)}$$
  
• d'autre part la valeur du champ électrique sur la sphère SPHL ; ce champ E est proportionnel à la différence de potentiel VL - VD et égal à $$\text{E = (VL - VD) x D/(L - D) x L}$$
  

For a potential VD imposed on the sphere SPHD and a potential VL imposed on the sphere SPHL, we obtain according to a first calculation:

• on the one hand, the values of the potentials on all the intermediate concentric spheres of region A, the value of the potential V (R) on an intermediate sphere of radius R being: $$\text{V (R) = (VD - VL) x L x D / R (L - D) + (L x VL - D x VD) / (LD)}$$
  
• on the other hand the value of the electric field on the SPHL sphere; this field E is proportional to the potential difference VL - VD and equal to $$\text{E = (VL - VD) x D / (L - D) x L}$$
  

In parallel to this, according to a second calculation, the equipotentials in region B are determined, by the method of electrical images, between a sphere SPHL brought to a constant potential VL and the plane of the detector 34, brought to a fixed potential VF; the electric field on the SPHL sphere is calculated as a function of VL and VF.

• d'une part, le champ électrique calculé sur la sphère SPHL à partir d'équipotentielles sphériques dans la région A, limitée par deux sphères à des potentiels VD et VL,
• d'autre part, le champ électrique calculé sur la sphère SPHL à partir des potentiels dans la région B, définis par des conditions aux limites qui sont le potentiel VL sur la sphère SPHL et le potentiel VF dans le plan du détecteur d'électrons 34.

VD and VF being fixed, we then seek the value of VL which makes it possible to make as identical as possible

• on the one hand, the electric field calculated on the SPHL sphere from spherical equipotentials in region A, limited by two spheres to potentials VD and VL,
• on the other hand, the electric field calculated on the SPHL sphere from the potentials in region B, defined by boundary conditions which are the potential VL on the sphere SPHL and the potential VF in the plane of the electron detector 34 .

As the electric field determined by the first calculation is constant over the entire SPHL sphere (proportional to VL-VD) and as the electric field determined by the second calculation is not constant over the entire SPHL sphere, the condition of identity indicated above is not strictly possible; but one can choose the value of VL for example so that the electric field at the intersection of the sphere SPHL and the axis 38 of the detector is identical in the two calculations.

For this value of VL we will obtain a good approximation for obtaining spherical or quasi-spherical equipotentials throughout the region A.

• à l'intersection entre les équipotentielles sphériques et le plan des électrodes d'entrée 36 (région A : premier calcul)
• à l'intersection entre les équipotentielles sphériques de toute la région A et les parois latérales 42 de l'espace d'absorption et de dérive (région A : premier calcul)
• à l'intersection entre les équipotentielles non sphériques de la région B et les parois latérales 42 au delà de la sphère limite SPHL (région B : deuxième calcul).

Having chosen the most appropriate value of VL, we return to equation (1) to determine the potentials in region A by the first calculation (boundary conditions imposed by two spheres) and in region B by the second calculation ( boundary conditions imposed by a sphere and a plane). We then determine the potentials:

• at the intersection between the spherical equipotentials and the plane of the input electrodes 36 (region A: first calculation)
• at the intersection between the spherical equipotentials of the entire region A and the side walls 42 of the absorption and drift space (region A: first calculation)
• at the intersection between the non-spherical equipotentials of region B and the side walls 42 beyond the limit sphere SPHL (region B: second calculation).

The intersections between the spherical equipotentials and the plane of the input electrodes are concentric circles and the input electrodes follow the course of some of these circles. We will assign to an input electrode 36 placed at a distance r from the sample the potential V (r) calculated by equation (1) for this distance, as a function of the values VD and VL chosen: $$\text{V (r) = (VD - VL) x L x D / r (LD) + (L x VL - D x VD) / (LD)}$$

Likewise, the intersections between the spherical equipotentials of region A and the conical side walls 42 are parallel circles centered on the axis 38; the lateral electrodes 44 follow the course of some of these circles and the potential V (r) obtained by equation (1) will be assigned to each electrode placed at the distance r from the sample.

Finally, the intersections between the equipotentials of region B and the side walls 42 are still circles (for reasons of symmetry); the lateral electrodes 44 of region B follow the course of some of these circles and are brought to potentials calculated by the method of electrostatic images (second calculation) as a function of the position of these circles.

FIG. 3 shows, in addition to the spherical equipotentials of region A, an intermediate equipotential EQB of region B, which is not a sphere centered on point S.

When the potentials thus determined are effectively applied to each of the input electrodes 36 and of the lateral electrodes 44 below and beyond the limit sphere SPHL, equipotentials are obtained which approach with good approximation of the equipotentials from which performed the calculation of potentials.

Even better results can be obtained by taking the potential values determined above for the electrodes 36 and 44 as a starting point for optimizing the equipotentials using a computer program for numerical resolution of the Laplace equation. running on a computer. The values of the potentials of the electrodes 36 and 44 are thus modified by iteration in order to make the equipotentials as close as possible to perfect spheres in region A.

It will be noted, with regard to the lateral electrodes 44 located in region B, that satisfactory results can be obtained in practice even if one is satisfied with applying to them potentials varying linearly with the distance between the sphere SPHL and the electrode 34. In this case we get rid of the aforementioned second calculation but we can always carry out an iterative optimization.

The distance D at which the sample to be observed can be changed, and this results in a new preferential distribution of potentials to be assigned to the input electrodes 36 and to the side electrodes 44. It is therefore possible to move the position of the sample while retaining spherical equipotentials, centered on the sample, in most of the absorption and drift space 40.

If the sample is not too close to the input window, it is possible to obtain practically spherical equipotentials in a region A which can extend up to approximately 90% of the distance between the input electrodes and the electron detector, distance measured along axis 38 of the detector.

If the sample is very close, the extension of region A can go down to 70% of this distance.

FIG. 4 represents the configuration of the input electrodes 36. These are concentric conductive circular tracks. They are produced in this example by screen printing of a conductive paste of carbon (carbon having the advantage of being fairly transparent to X-rays) on an insulating support.

The individual electrodes are supplied by conductors located on the other side of the support. The support is then pierced with holes 50 filled with conductive paste and the supply conductors 52 are electrically connected to these holes. The supply conductors can be screen printed on the other side of the insulating support. They must be as transparent as possible to the radiation to be detected.

FIG. 5 represents the configuration of the input conductors seen in cross section perpendicular to the plane of the input window, through only one of the conductive passages 50 and along the supply conductor 52 which is connected to this hole. The insulating support is designated by the reference 54.

Preferably, a highly resistive paste 56 is deposited between the circular conductive tracks constituting the electrodes 36 intended to evacuate towards the electrodes 36 the electric charges (ions) which are liable to accumulate at the interface between the insulating substrate 54 and the gas. of the enclosure. These charges come from the ionization of the gas and disturb the shape of the equipotentials towards the input of the detector if they remain stored on the insulating substrate. Here we propose to evacuate them by this resistive deposit between the tracks. The resistance can be a few megohms between two adjacent tracks separated by a few millimeters. Obviously, it must not lead to excessive current consumption and care must be taken to ensure that the neighboring tracks can be brought to potentials differing by several tens of volts or even more. The highly resistive paste can be a paste based on carbon in small proportion in an insulating resin.

It could also be envisaged that the conductive electrodes 36 are deposited directly (by screen printing for example) on a resistive substrate (highly resistive) and not insulating; the same result would be achieved with regard to the removal of troublesome loads.

• 1° il n'y a pas le problème de transparence aux rayons X ;
• 2° le problème des charges électriques à évacuer est moins crucial ; la pâte résistive 56 est utile mais moins nécessaire.

For the lateral electrodes 44, the constitution may be the same as that of the input electrodes but

• 1 ° there is no problem of transparency to X-rays;
• 2 ° the problem of electrical charges to be removed is less crucial; resistive paste 56 is useful but less necessary.

The side electrodes 44 can be deposited by screen printing on a flexible insulating sheet constituting the side wall 42; this flexible sheet is then rolled up in the form of a truncated cone. The electrodes can also be produced in flexible printed circuit or else by stacking circular electrodes spaced by insulating shims. The connections with the supply conductors will however always be outside the space 40 so as not to disturb the electric field on the inside of the side wall 42.

To complete this description, FIG. 6 shows a slightly different constitution of the detector, in which an attempt is made to analyze the rear X-ray diffraction, by a sample of material.

This assumes that the source and the detector are placed on the same side of the sample.

Provision has therefore been made for the detector to be traversed in its center by a pierced axial tube 60 through which an X-ray emission beam can pass, directed towards the sample 20. The rays re-emitted towards the rear by the sample are captured and analyzed by the detector.

To implement the invention, it must then be considered that the walls of the tube 60 are also side walls of the absorption and drift space 42, and that they also carry individual side electrodes 44; these electrodes are brought to potentials which are calculated in the same way than the others both in the upper region and in the lower region of the enclosure.

The connections to bring the potentials to the different electrodes along the tube are made with the same constraints as above, and it is also recommended to provide a resistive substance between the electrodes peripheral to the tube.

Claims (10)

1. A gas detector for radiations emitted by a sample (20), comprising a closed chamber (30) containing a gas absorbing the radiation, an input window (32) transparent to the radiations to be detected, an absorption and drift space (40) behind the input window and, at the extremity of this space, a plane two-dimensional detector for the localization of electrons (34) for determining the coordinates of an arrival point of electrons generated by a photon impact in the absorbing gas, the detector further comprising a set of input electrodes (36) placed behind the input window and highly radiation-transparent,
      characterized in that it further comprises a set of lateral electrodes (44) surrounding the absorption and drift space, the individual input electrodes (36) and the individual lateral electrodes (44) being set to voltages different the ones from the others and variable as a function of the position where it is desirable to place the sample with respect to the input window, the voltages determined for each of the electrodes being such that the absorption and drift space is shared into two parts without using electrodes physically delimiting this separation, the equipotentials in the first part being spheric or quasi-spheric and centered on the position of the sample, and the equipotentials in the second part being continuously variable from a spheric shape, at the place of the separation, to a plane shape at close proximity of the plane electron detector.
2. A gas detector according to claim 1, characterized in that the lateral electrodes (44) are distributed on the whole distance separating the input electrodes (36) from the electron detector (34).
3. A gas detector according to claim 1 or 2, characterized in that the first part (A) of the absorption and drift space extends over a distance of about 70 to 90% of the distance between the input electrodes (36) and the electron detector (34), said distance being measured along the axis of the detector.
4. Utilization of a gas detector according to any of claims 1 to 3, characterized in that the voltage values of the different input electrodes and of the different lateral electrodes result from a calculation carried out in the following manner:

   a) determining the equipotentials between a sphere having a radius corresponding to the distance (L) between the sample and the first of the two parts of the absorption and drift space set to a voltage VL and a concentric sphere having a radius corresponding to the distance (D) between the sample and the input window set to a voltage VD,

   b) setting the potential of the input electrodes (36) and lateral electrodes (44) placed in the first part as a function of said determination, and

   c) setting the potential of the electrodes placed in the second part by means of linear interpolation.
5. Utilization of a gas detector according to any of claims 1 to 3, characterized in that the voltage values of the different input electrodes and different lateral electrodes result from a calculation carried out in the following manner:

   a) determining the equipotentials between a sphere having a radius corresponding to the distance (L) between the sample and the first of the two parts of the absorption and drift space set to a voltage VL and a concentric sphere, having a radius corresponding to the distance (D) between the sample and the input window set to a voltage VD,

   b) determining the equipotentials between the sphere set to a voltage VL and a plane set to a voltage VF, and

   c) determining the resulting potentials at the places where the different electrodes are positioned, the voltage values assigned to the different electrodes being those resulting voltages.
6. Utilization of a gas detector according to claim 5, characterized in that the voltage values of the different electrodes are the ones resulting from the additional calculation consisting in choosing the voltage VL in such a way that the electric field, at a point of the sphere set to a voltage VL, has the same value in the calculation carried out at step a) and in the calculation carried out at step b).
7. Utilization of a gas detector according to any of claims 4 to 6, characterized in that the voltages at the input and lateral electrodes are optimized by an iterative calculation carried out by a computer.
8. A detector according to any of claims 1 to 3, characterized in that a highly resistive substance (56) is disposed between the input electrodes (36) for avoiding the storage of electric charges between two adjacent electrodes.
9. A detector according to any of claims 1 to 3 and 8, characterized in that the lateral electrodes (44) are formed on a conic wall (42) delimiting the absorption and drift space.
10. A detector according to any of claims 1 to 3, 8 and 9, characterized in that said detector is provided with an axial tube (60) crossing it along its center for permitting the lighting of a sample and observing the rear diffraction, lateral electrodes (44) being also distributed along the tube wall in the absorption and drift space.

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