EP0340832A1 - Sealed, high flux neutron tube - Google Patents

Abstract

Device comprising a first part and a second part which are separated by the acceleration electrode (13) which forms a shield between the said parts and is integral with the earthed outer surround (15). <??>The said first and second parts of the tube contain respectively the ion source (9), brought to a positive potential of adjustable value, and the target (28), brought to a negative potential of likewise adjustable value, relative to the zero value of earth. <??>Application: neutron radiography. <IMAGE>

Description

The invention relates to a device for improving the life and reliability of a high flux sealed neutron tube containing a deuterium-tritium gas mixture and in which an ion source provides a high energy beam projected onto a target to produce a fusion reaction generating an emission of neutrons.

High flux sealed neutron tubes are used in fast, thermal, epithermal or cold neutron examination techniques.

The tubes currently available have an insufficient lifespan at the level of the emission necessary to obtain their full effectiveness in the various nuclear techniques: neutronography, analysis by activation, analysis by γ spectrometry of inelastic diffusions or radiative captures, neutron scattering. ..

The T (d, n) ⁴He reaction delivering 14 MeV neutrons is usually the most used due to its large cross-section for relatively low deutron energies but any other reaction considered adequate can be used.

However, whatever this reaction, the number of neutrons obtained per unit of charge passing through the beam is always increasing as the energy of the ions directed towards a thick target is itself increasing and this very beyond energies of the ions obtained in the sealed tubes currently available and supplied by a THT (very high voltage) rarely exceeding 200 kV, both for reasons of tube definition and of reliability of the THT generators and of the connection members.

Among the most important phenomena limiting the lifetime of the neutron tube, we must mention the irradiation defects of the target by the incident ions and the metallizations of the insulating walls of the tube.

These two phenomena being all the more important as the intensity of the beam is itself important, it would be advantageous to limit this parameter as much as possible and therefore, for a given neutron emission to use large acceleration voltages.

Unfortunately, unlike the case of vacuum tubes (X-rays for example), in practice in a sealed neutron tube of conventional design, it is not possible to increase the dimensions of the tube which would have the consequence, on the one hand , to lower the neutron yield and, on the other hand to cause the initiation of discharges in accordance with Paschen's law in the low pressure range.

Another risk of initiation of discharges in the gas results from the surface effect of the electrodes subjected to a high electric field. This effect is initiated by electric particles emitted from a part of the negative potential tube playing the role of cathode placed opposite another part of the positive potential tube and therefore behaving like an anode and which should not be confuse with parts of the tube having identical names such as for example the anode and the cathode of the ion source. These particles coming to strike other molecules of matter in the gas or on the electrodes can cause by secondary emission a certain amplification of the emission and thus arrive step by step to an electric current sufficiently important to establish a breakdown by rupture of qualities dielectric of the medium, either on the surface of the insulating parts of the tube, or through the gas space of the tube itself. In the cases of use of the reaction T (d, n) ⁴He already mentioned, the presence of tritium-emitter β⁻- further increases this type of risk, as well as the different ionizing radiation associated with the nuclear reaction (X, α, γ, n) or its consequences (radiation induced by neutron activation of the tube itself or its environment).

In vacuum tubes such as, for example, X-ray tubes, the resistance to breakdown on the surface of the insulators is markedly improved on the one hand by increasing the inter-electrode distances and by dividing the tube into two parts respectively constituting the anode and the cathode so as to reduce the potential by half in each part of the tube and on the other hand by giving the insulating parts a suitable inclination relative to the direction of the electric field (see for example the article entitled "Metal / ceramic X-ray tubes for non-destructive testing "by W.Harth et al. published in Philips Technical Review, vol.41, 1983/1984, N ° 1, pages 24-29).

In the case of neutron tubes, these are gas tubes whose content is at low pressures so that the product P × d of the pressure by the interelectrode distance is located on the left side of the Paschen curve. In this case, special discharge phenomena of the Townsend avalanche type can occur, which can be avoided by reducing the interelectrode distance, this means being limited, conversely, by the threshold of appearance of a strong cold emission. of electronic origin according to the law of Fowler-Nordheim (FN).

The values of the cold emission current density calculated by the Fowler-Nordheim formula show, according to the surface conditions of the electrodes, a high amplification coefficient of this current density for a given potential difference. As a result, a small voltage variation can produce a strong growth or a sharp decrease in the current depending on the direction of this variation. Qualitatively, there is such a high sensitivity of the current to the voltage for all the parasitic phenomena leading to the existence of a current between the electrodes.

Thus, beyond a certain voltage threshold, it becomes difficult to avoid ignition in the gas either by surface effect of the electrodes subjected to a high electric field, ie by collision of the ions with the molecules of gas if one increases the distance of isolation to reduce this same electric field.

The object of the invention is to provide a neutron tube device supplied at voltages much higher than 200 kV and allowing, with satisfactory maintained reliability, the increase in the lifetime mentioned above.

The device of the invention is remarkable in that said neutron tube comprises a first part and a second part separated by means of an acceleration electrode forming a screen between said parts, said first part containing the ion source carried at a positive potential of adjustable value and said second part containing the target brought to a negative potential of value also adjustable with respect to the zero value of the potential of said acceleration electrode grounded by the external envelope of the tube of which it is united.

Thus for the same level of neutron emission, the intensity of the ion beam is reduced by the possibility of doubling the potential difference between source and target without increasing the risks of initiation in the deuterium-tritium mixture by collision of the ions with the gas molecules, because the material separation of said neutron tube into two parts by means of said screen keeps unchanged the travel distances of the ions in each of said parts. It is remarkable that this arrangement allows a significant reduction in the critical value of the product P × d along the electric field lines joining the electrodes.

During the process of forming cold emission currents in the neutron tube, said external envelope and said ion source respectively constitute the cathode and the anode of said first part of the tube on the one hand, said target and said external envelope constitute respectively the cathode and the anode of said second part of the tube on the other hand. Said cold emission currents thus developed in each of said parts of the tube by surface effect of the facing electrodes are assigned a high reduction factor of up to 10⁶ depending on the nature and the surface condition of said electrodes, because the potential difference required for the acceleration of the ion beam is distributed in halves between said first and second parts of the tube.

This distribution of the overall potential difference of the tube can be asymmetrical between the two parts of the tube - either because of the applied potentials, or because of the geometric distances separating the electrodes - which gives the interesting possibility of varying the spaces of acceleration between the separating electrode and the ion source on the one hand and between this same electrode and the target on the other hand, so as to better control the focusing of the ion beam in order to improve the lifetime of the tube .

The following description with reference to the accompanying drawings, all given by way of non-limiting example, will make it clear how the invention can be implemented.

Figures 1 and 2 show respectively in longitudinal section, a first and a second variant of neutron tubes according to the prior art.

Figures 3 and 4 respectively show the same longitudinal section of a first and a second variant of neutron tubes according to the invention.

In the first variant of the known model shown in FIG. 1, an envelope 1 contains a gaseous mixture of deuterium and tritium coming from a reservoir 2. This mixture is ionized in the ion source 3 brought to ground potential. An ion beam 4 is extracted therefrom by the acceleration electrode 5 secured to the target 6 and brought to the negative potential of very high voltage (-THT).

The wall part 7 opposite the acceleration space is necessarily made of an insulating material. The path metallic sprays from the ion source delimits the zone 8 of this part of the wall exposed to metallization, which constitutes the major drawback of this first variant.

In the second known model variant shown in FIG. 2, the ion source 9 is brought to a positive very high voltage potential + THT via the cable 10, the end of which is surrounded by the insulating sleeves 11 and 12 between which is provided a space intended to allow the circulation of an insulating cooling fluid. The acceleration electrode 13 cooled at 14 by a liquid circuit is brought to ground potential which allows it to be made integral with the metal wall 15. This arrangement which avoids metal spraying on the insulating parts of the tube constitutes the nearest prior art.

The gaseous mixture of deuterium and tritium is supplied via a pressure regulator 16. The gas pressure is controlled using an ionization pressure gauge 17.

The ion source 9 of the Penning type in the example described (but which could be of a different type without harming the invention) comprises an anode 18 to which the potential + THT is applied, two cathodes 19 and 20 brought to a same negative potential of the order of 5 kV relative to the anode 18 and a permanent magnet 21 creating an axial magnetic field and the magnetic circuit of which is closed by the ferromagnetic socket 22 which envelops the ion source 9.

The ion beam 23 extracted from the ion source passes through the suppressor electrode 24 and strikes the target 25 cooled at 26 by a circulation of a liquid. A similar neutron generator is described in more detail in French Patent No. 2,438,153.

Breakdown phenomena can occur in the enclosure of a gas tube under the effect of high voltage applied between the electrodes and whose initiation process in the case of the neutron tube of Figure 2 is as follows. The envelope of the ion source 9 constituted by the magnetic circuit 22 is at a high positive potential relative to that of the envelope 15 of the tube brought to the zero potential of the mass. The envelope 22 of the ion source will therefore play the role of an anode and the envelope 15 of the neutron tube will play the role of a cathode at the level of which a macroscopic electric field develops. The micro-asperities presented on the surface of this cathode are capable, according to their geometry, of microscopically amplifying the value of this field; there is then the possibility of cold emission of electrons. This electronic current also causes ionization of the molecules of the gas contained in the tube. This results in an avalanche effect which risks leading to an accidental short-circuit, that is to say a breakdown between electrodes.

avec E = βE₀
E = champ électrique microscopique en V/cm
E₀ = champ électrique macroscopique en V/cm
β = facteur d'amplification dépendant de la géométrie des microaspérités.
W = énergie nécessaire en eV à un électron pour s'échap­per de la surface du solide (Travail de sortie). Cet­te quantité dépend principalement de la nature du ma­tériau constituant l'électrode ou des impuretés de surface.
J = densité de courant d'émission froide en A/cm².The simplified Fowler-Nordheim formula makes it possible to assess the density of the cold emission current. This formula is as follows (in a vacuum, therefore without taking into account the possible amplification due to the presence of the gas):

with E = βE₀
E = microscopic electric field in V / cm
E₀ = macroscopic electric field in V / cm
β = amplification factor depending on the geometry of the microasperities.
W = energy required in eV for an electron to escape from the surface of the solid (Work of exit). This quantity depends mainly on the nature of the material constituting the electrode or of the surface impurities.
J = cold emission current density in A / cm².

The amplification factor β can be estimated from curves according to the shape of the end of the microspheres (spherical, ellipsoidal) and their height h above the surface of the electrode. β ≈ 10² for a ratio h / r = 10², r being the radius of a microasperity whose end is spherical in shape.

The cold emission current density J is given as a function of the microscopic field E for different values of the output work W varying from 1.6 to 5 eV.

For electrodes carrying alkali metal impurities on the surface, the output work is 2.5 eV. The macroscopic electric field is of the order of 210⁵ V / cm in the usual neutron tubes. If we accept an amplification factor of 10² caused by the existence of micro-roughness we find a cold emission current density of the order of 4 10³µA / µm². For a macroscopic electric field of 10⁵V / cm, that is to say reduced by half, the density of the cold emission current becomes approximately 3.10⁻³µA / µm², that is to say that it is reduced in a ratio close to 10⁶. This considerable reduction practically eliminates the risks of original F-N breakdown between electrodes and thus ensures good reliability of the tube.

We also know that improving the life of a neutron tube by reducing the intensity of the ion beam requires increasing the potential difference applied between source and target, which greatly increases the risk of breakdowns beyond a THT close to 200 kV. If the isolation distances are increased to reduce the electric field, this will result in a greater probability of ignition in the gas by collision of the ions with the molecules of said gas.

The device of the invention provides the best possible compromise between the lifetime and reliability of a neutron tube by making it possible to increase the acceleration voltage of the ion beam while maintaining the electric field values between acceptable values. the tube electrodes.

Figure 3 shows the diagram of a first variant of this device which is presented as two parts similar to the part of the tube of Figure 2 between the accelerator electrode 13 and the THT supply cable 10. One of these parts always contains the ion source 18, 19, 20, 21 inside the envelope 15 while the other part contains the suppressor electrode 27 and the target 28 inside the envelope 15 ′. These two parts are joined by their face having the acceleration electrode 13 which is common to them and therefore arranged symmetrically with respect to the median plane of this electrode.

In this figure the elements of the first part of the tube identical to those of Figure 2 are indicated by the same reference numbers. The elements of the second part of the tube having a character of symmetry with respect to those of said first part are indicated by the same reference number assigned to the sign ′: thus 10 and 10 ′ for the cable, ... 22 and 22 ′ For the ferromagnetic socket. In this version, the pressure regulator 16 and the ionization manometer 17 are carried over to the end of this second part of the tube comprising the target.

The arrangement in FIG. 2 allows the tube to be fed by means of a single positive polarity, ie + V.

The arrangement of FIG. 3 allows the use of a generator with two polarities + V transmitted to the ion source by the cable 10 and - V transmitted to the target by the cable 10 ′. These two polarities are referenced with respect to the mass to which the accelerator electrode 13 is attached, integral with the outer envelopes 15 and 15 ′.

Thus the electric fields at the cathode 15 of the first part of the tube on the one hand and at the cathode 22 ′ of the second part of the tube on the other hand are maintained at values compatible with acceptable reliability, so that the potential difference regulating the acceleration is equal to 2V in order to increase the service life of the tube by reduction of the target current, as already mentioned above.

Such a mode of supply of the neutron tube making it possible to double the difference in the potential of acceleration of the ion beam thus offers the possibility of compensating for the reduction in the neutron emission which would have resulted only from the reduction of the target current.

The device of the invention has an additional advantage from the point of view of reliability by the fact that the reduction in the target current is obtained by a correlative reduction in the current of the ion source by means of a reduction in the operating pressure.

This same device also makes it possible to reduce sprays originating from the ion source, as well as those resulting from parasitic ionizations on the path of the beam.

Furthermore, the accelerating electrode 13 also plays the role of a "screen" between the ion source and the target, which appreciably reduces the possible paths of the ions in the gas and therefore further limits the risks of breakdown in the prospect of even greater reliability.

The symmetrical feeding mode of the neutron tube offers another interesting possibility which is to be able to vary the acceleration spaces between the two parts of the tube and thus to achieve an ion optic making it possible to improve the adjustment of the focusing of the beam. . This amounts to reacting to the electric field values in each part of the tube.

Thus in the first part of the tube, it is the envelope 1 which is cathode. This envelope constituting the outer wall of the tube has a high radius of curvature and an electric field E₁ is developed between this envelope and the envelope 11 of the ion source playing the role of anode.

In the second part of the tube, it is the envelope 11 ′ of the target which is cathode. This envelope has a radius of curvature smaller than that of the wall because it is inside the tube and an electric field E₂ is developed between this envelope and the external envelope 1 ′ of the tube playing the role of anode.

If the voltage supply of the ion source and the target is symmetrical, there is the inequality E₂> E₁ due to the difference in the radii of curvature at the level of the two electrodes playing the role of cathode in each part of the neutron tube.

To obtain an equivalent functioning of each part of the tube, it is necessary to rebalance the electric fields (E₂ = E₁) by readjusting the value of the THT applied on the side of the target.

A second variant of the device of the invention shown schematically in Figure 4 defines the geometry of the insulating walls of the neutron tube so as to minimize the effect of "flash-over" along said walls. This effect is manifested by successive secondary emissions which develop on the surface of the insulator from the impact of a particle coming to strike this surface. This results in a damaging surface effect for the insulator which can be counteracted by tilting the insulating surfaces at an angle to the electric field so that rebounding does not occur. The geometry of the insulators can be different depending on the polarity.

In FIG. 4, the second part of the neutron tube containing the target is identical to that of FIG. 3. In the first part of the tube containing the source, the content of the ferromagnetic socket 11 is also identical to that of FIG. 3.

On the other hand, the insulating sleeves 12 ′ and 12˝ which correspond in the active areas of the tube have their surfaces inclined at a certain angle relative to the direction of the ionic flow indicated by arrow 29.

The sleeve 11˝ of the cable 10˝ supplying the THT anode has been designed to adapt to this arrangement.

Claims (6)

1. Device for improving the lifetime and reliability of a high flux sealed neutron tube containing a deuterium-tritium gas mixture and in which the ion source provides a high energy beam projected onto a target for producing therein a fusion reaction leading to an emission of neutrons, characterized in that said neutron tube comprises a first part and a second part separated by means of an acceleration electrode forming a screen between said parts, said first part containing the ion source brought to a positive potential of adjustable value and said second part containing the target brought to a negative potential of value also adjustable relative to the zero value of the potential of said acceleration electrode grounded by the envelope external of the tube of which it is integral. 2. Device according to claim 1, characterized in that the possibility of doubling the potential difference between source and target causes, while keeping the neutron emission level constant, the reduction of the intensity of the ion beam and by consequently increasing the life of the tube without increasing the risks of priming in the deuterium-tritium mixture by collision of the ions with the gas molecules, due to the fact that the separation of said neutron tube into two parts via said screen decreases the travel distances of the ions in each of said parts. 3. Device according to claim 1, characterized in that said external envelope and said ion source respectively constituting the cathode and the anode of said first part of the tube, said target and said external envelope constituting respectively the cathode and the anode of said second part of the tube, the cold emission currents developed by the surface effect of the facing electrodes in each of said parts of the tube, are assigned a factor of high reduction favorable to reliability, owing to the fact that the potential difference required for the acceleration of the ion beam is distributed in halves between said first and second parts of the tube. 4. Device according to one of claims 1 to 3, characterized in that the electric fields in said first and second parts of the neutron tube are distributed asymmetrically either because of the applied potentials, or because of the geometric distances separating the electrodes, when adjusting the acceleration spaces so as to better control the focusing of the ion beam and / or the cold emission currents in each of said parts. 5. Device according to one of claims 1 to 4, characterized in that said first and second parts of the neutron tube are arranged symmetrically relative to the median plane passing through said acceleration electrode forming a screen. 6. Device according to one of claims 1 to 4, characterized in that the surfaces of the insulating walls which correspond in each of said first and second parts of the neutron tube have an inclination in the same direction relative to the direction of the beam of ions.

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