US3448314A - Neutron generators - Google Patents

Description

June 1939 J. E. BOUNDEN ETAL 3,443,314

NEUTRON GENERATORS Filed March 7, 1966 Sheet of 3 June 3, 1969 J. BQUNDEN EI'AL 3,448,314

NEUTRON GENERATORS Filed March 7, 1966 Sheet 2 of 3 June 3, 1969 J. E. BOUNDEN ET AL NEUTRON GENERATORS Sheet Filed March 7. 1966 United States Patent US. Cl. 31361 5 Claims ABSTRACT OF THE DISCLOSURE A neutron generator comprising a normally sealed off envelope, means to produce a plasma in gas within a portion of the envelope, a boundary electrode bounding said portion, from a further portion and having an aperture for the extraction of ions from the plasma, an extractor electrode and a target shield each having apertures in register with the boundary electrode aperture to allow passage of an ion beam and being successively spaced from the boundary electrode within said further portion, means to produce an axial magnetic field in the region of the boundary electrode aperture, and a target located behind the target shield.

This invention relates to neutron generators and relates particularly to generators comprising an envelope, normally sealed-off, in which deuterium and/ or tritium ions from an ion source are accelerated to strike a target containing deutariurn and/or tritium to produce neutrons by the D-D and/or D-T reactions, the gas pressures in the ion-source and accelerating portions of the envelope being equal. A generator of this type is described in US. Patent No. 3,344,299 issued Sept. 26, 1967, to I. E. Bounden.

The generator described in this patent comprises a tubular glass envelope, part of which forms the ion source and is encircled by an R. F. winding to produce a plasma therewithin. A frusto-conical extractor electrode having a central aperture projects into the plasma, a beam of ions being Withdrawn through the aperture and accelerated towards a target located at one end of the envelope. The target proper is shielded from the accelerating field by a shield electrode. An output of up to neutrons per second is obtainable in either continuous or pulsed operation.

To produce a larger output from this generator, the ion current withdrawn from the ion source through the aperture in the extractor electrode would have to be increased. In theory this could be done either by increasing the RP. power to increase the density of the plasma produced, or by increasing the diameter of the aperture. The first course is undesirable as likely to result in overheating of the glass envelope, whilst to the second there are two objections. Firstly the ion beam diameter is correspondingly increased, which means that the aperture diameter in the shield electrode has to be correspondingly increased; this is undesirable from the point of view of suppressing electrons emitted from the target region, as will be explained hereafter. Secondly, the increase in extractor aperture diameter would allow the accelerating field to penetrate farther into the ion source region, leading to the risk of long-path electrical breakdown between the shield and the interior of the ion source.

It has been found that the form of neutron generator provided by the present invention enables the ion beam current to be increased by approximately an order of magnitude, whilst maintaining the beam diameter at a value which does not necessitate an undesirable increase in the diameters of the electrode apertures.

According to the present invention a neutron generator comprises an envelope, means for producing a plasma in gas within a portion of said envelope, a boundary electrode bounding said portion from a further portion of said envelope and having an aperture for the extraction of ions from said plasma, an extractor electrode and a target shield successively spaced from said boundary electrode within said further portion and having apertures in register with the aperture therein to allow passage of an ion beam, means for producing an axial magnetic field in the region of the aperture in the boundary electrode, and a target located behind said target shield.

The plasma-producing means preferably comprises a RF winding encircling said portion of the envelope, and the magnetic field producing means a solenoid coil encircling said envelope between said RF winding and said boundary electrode.

Reference has been made to the suppression of secondary electrons emitted from the target. It is most important that back-streaming of these electrons, and of electrons formed by ionisation by the beam of the gas within the target shield, towards the ion source, should be reduced as much as possible because of the heat generated by this electron current. In the generator described in the aforementioned patent, this was effected by maintaining the target slightly positive with respect to the target shield, so that electrons emitted from the target were drawn back to the target. In the present generator the latter arrangement was found to be inadequate when the current reached only two or three times that in the earlier generator, the back-streaming current then becoming sufiicient to damage the tube. An alternative arrangement has been used to remove this limitation on increasing the ion current, allowing it to be increased about ten-fold.

Accordingly, the aperture in the target shield of the present generator is very preferably of channel-like form, and an open-ended tubular suppressor member is located Within said shield but electrically insulated from it, one end of said member encircling the target and the other terminating adjacent the target end of the channel in the target shield.

These and other features of the present invention will now be described, by way of example, with reference to the accompanying drawings, wherein:

FIGURE 1 is a sectional elevation of a neutron generator embodying the present invention.

FIGURE 2 shows the potential distribution between the extractor electrode and target of the embodiment of FIGURE 1.

FIGURE 3 is a sectional elevation of another embodiment of the invention.

In this drawing one end of a sealed tubular glass envelope 1 is encircled by an RF winding 2 for exciting a plasma in gas within the envelope. This portion of the envelope forms the ion source and is bounded by a boundary electrode 3 formed as a flat disc having a central aperture 4 and sealed to the envelope 1. Electrode 3 is made of molybdenum for good heat conduction. Between winding 2 and electrode 3 a solenoid coil 5 encircles the envelope to produce an axial magnetic field in the region of aperture 4 and so intensify the plasma adjacent thereto. This increases the ion current density extractable from the plasma boundary for a given RF exciting power. The surface of electrode 3 facing the plasma is thinly coated with vitreous enamel 6 to within about 0.005 inch of the edge of the aperture, leaving an enamel-free region 30, to shield the metal from the plasma in order to prevent sputtering and also to minimise recombination of hydrogen isotope atomic ions into molecular ions. Sputtering can also have a gas-absorbing effect which is clearly undesirable in a sealedoff tube having a limited gas content. Into aperture 4 is screwed an aluminum insert 34 having a lip which extends over region 30, aluminum having a much lower atomic recombination coeflicient than molybdenum or Nilo-K and also a much lower sputtering ratio (i.e. atoms emitted per impinging ion). The lip of insert 34 contacts the plasma, thus maintaining it at the potential of electrode 3 and also keying the plasma to the lip.

Spaced beyond boundary electrode 3 is a frustroconical extractor electrode 7 brazed to an annular ring 8 sealed to envelope 1 and having an aperture 9 into which is screwed an aluminum anti-sputtering insert 35 similar to insert 34. Spaced in turn from electrode 7 is a target shield 10 mounted on a metal tube 11 sealed to envelope 1 and having a channel-like aperture 12. Apertures 9 and 12 are seen to be in register with aperture 4.

Insulated from tube 11 by a tubular glass section 13 is a metal tube 14 on the end of which is a flange 15 supporting the target 16 which is of erbium evaporated onto a molybdenum pressing, a re-entrant cavity 29 being formed behind the target. The erbium layer is initially impregnated with deuterium which is converted in operation to an approximately 50/50 deuterium/tritium mixture by reason of the replenisher 19 being initially charged with a deuterium/tritium mixture having an excess of tritium suflicient to make the total gas content of the envelope (i.e. including both replenisher and target) an approximately 50/50 mixture. In operation the target is replenished in a known manner by the action of the mixed ion beam. Also mounted on flange 15 coaxially within shield 10 is a tubular suppression member 17, one end of which encircles target 16 and the other end of which terminates adjacent the target end of channel 12. In operation a suitable coolant, e.g. ICI Arcton 113 or Monsanto TAS 130, is circulated over the rear face of target 16 to remove the heat dissipated by the ion beam.

To the other end of the envelope 1 is sealed a metal tube 18 carrying an end-plate 21 on which are mounted the gas replenisher 19, a sealing-off tube 20, and a Pirani gauge (hidden behind tube similar in design to the corresponding components described in the aforementioned patent. Also fastened to plate 21, via support tube 22, is a copper disc 23 coated with evaporated aluminium to prevent sputtering by contact with the plasma.

Disc 23, which is cooled via aperture 24 in plate 21, acts as a stopper for back-streaming electrons, both those produced by ion bombardment of extractor electrode 9, and those from the region of target shield 10. Disc 22 is made of large diameter because the electron beam tends to be increased in cross-section by the defocussing effect of the diverging magnetic field produced by coil 5.

The drawing is approximately to scale, the external diameter of tubes 18, 11 and 14 being 2 inches. As atready mentioned, electrode 3 and target 16 are made of molybdenum. Tubes 22, 11, 18, and 14, flange 15, tube 17, shield 10, and the extractor assembly 7/8 are made of Nilo-K alloy. Envelope 1 is of Kodial glass.

Aluminium alloy guard rings 25 and 26 are clamped to disc 8 and tube 11 respectively to prevent high electric stresses arising where these components are sealed to envelope 1. The ion-source region of envelope 1 (as far as disc 8) including winding 2 and coil 5, are enclosed in a first methylmethacrylate jacket through which cooling oil is circulated, and the region between ring 25 and flange 15 in a second similar jacket containing high-grade insulating oil.

In operation the envelope 1 is filled with the deuterium/ tritium gas mixture supplied by replenisher 19, the gas pressure being maintained at approximately 15 X10- mm./Hg as measured by the Pirani gauge. A plasma is excited in this gas by applying R.F. power at 15 mc./s. to winding 2. Ions are extracted from the plasma through aperture 4 in boundary electrode 3 by a potential difference V of up to 5 kv. applied between electrode 3 and extractor electrode 7, the latter being at earth potential. The plasma takes up the potential of electrode 3 and of electron stopper 23, which are connected together externally, a curved plasma boundary or cap forming over the plasma side of the aperture 4. Coil 5 produces an axial magnetic field of about gauss in the region of aperture 4 to intensify the plasma, as already described.

At relatively low values of V part of the ion beam impinges on extractor electrode 7, but as V is increased (keeping the plasma density constant) the entire beam is focused through aperture 9 as the plasma boundary or cap is pushed back by the adjacent electric field pro duced by V The initial ion bombardment of electrode 7 produces secondary electrons which are accelerated back into the ion source through aperture 4 but are stopped by disc 23 as described.

The ion beam leaving extractor electrode 7 is further accelerated by the main accelerating field produced by the potential dilference V of -120 kv. applied between electrode 7 and target shield 10, and impinges on target 16. Target 16 and suppressor'member 17 are maintained at a potential V of about +400 v. with respect to the target shield 10. FIGURE 2 shows by means of equipotential lines the potential distribution with electrodes 7, 10 and 17 held at 0, -100 kv. and 99.6 kv. respectively, plotted by the electrolytic tank technique. It will be seen that the arrangement provides a relatively equipotential drift space 27 extending from the target (not shown in FIGURE 2) to near the end of member 17, from where the potential rises sharply to a peak potential of about 99.85 kv. in the region of point 28 before falling steadily towards 0 v. This peak or hump of about 250 v. around point 28 acts as a trap for electrons formed within the shield 10 either by ion bombardment of the target or by ionisation of the gas molecules by the beam ions, and inhibits their escape into the main accelerating field.

The use of a comparatively narrow channel 12 in shield 10, as compared with the simple apertures 4 and 9 in the other two electrodes, has the advantage that the penetration of the main accelerating field into the shield is reduced. This in turn allows an electron-trapping hump of given height to be produced with a lower bias voltage V than would otherwise be the case, which is advantageous since the bias voltage has to be superimposed on the main accelerating potential of 100 kv., and hence should be as simple to generate as possible. It also has the eifect of locating the hump 28 closer to extractor electrode 7 than would otherwise be the case, which reduces the length of ion beam path between electrode 7 and the hump, in which no trapping takes place. The use of too high a value of V may also tend to establish an undesirable discharge between the suppressor tube and the target shield, which may cause sputtering of the latter.

As has already been mentioned, the relatively narrow, high-density, ion beam produced in the present generator allows aperture 9 to be kept small, which reduces the penetration of the main accelerating field beyond electrode 7 into the ion-source region and thus reduces the risk of long-path breakdown. (It may be noted that generators of the present type operate in that region of the Paschen curve, Where, for constant pressure, the breakdown voltage decreases as the gap length increases.) The narrow beam furthermore makes it possible to use the comparatively long narrow channel in the target shield which, as discussed above, facilitates electron trapping.

Another advantage of the narrow ion beam is that its smaller angle of divergence on entering the target shield allows the target to be located relatively far back from the shield aperture without the beam cross-section ex ceeding the target diameter when the beam impinges on the target. This means that the neutrons are produced at a location more remote from the main accelerating region of the tube and its associated high-grade insulation, which greatly improves access to the target. Comparison with the generator described in U.S. Patent No. 3,344,299 shows that in the present generator the target structure is much less re-entrant, which makes it correspondingly easier to irradiate samples (e.g. for activation analysis) in the high-flux region immediately behind the target surface 16. As the re-entrant cavity also has to carry the target coolant pipes, this is an important factor, especially if a compressed-air operated conveyor for rapid transfer of samples is to be included. In the embodiment of the invention shown in FIGURE 3 the re-enter form of target is replaced by a target of which the surface is flush with the end of the tube, as hereinafter described.

A further advantage of the form of ion source used in the present generator is that it enables the neutron output to be easily and accurately controlled by varying the value of V rather than by varying the RF power supply to the plasma-exciting winding, as in the generator described in the aforementioned patent. The latter system is rather dependent on gas pressure fluctuations, which are difiicult to control precisely. Ease of output control is particularly advantageous in applications requiring a modulated neutron output, e.g. in nuclear reactor experiments.

A yield of 9X10 neutrons per second has been obtained with the above-described embodiment, filled solely with deuterium, under the following opearting conditions:

Target voltage (V kv 120 RF power dissipated in plasma (approx) w 380 Target shield current (I ma 6.0 Target current (I ma 5.0 Total tube current (I ma 11 Suppressor bias (V v 440 Magnetic field (approx) gauss 100 Extractor voltage (V kv 3.3 Extractor current (I ma 11 Interception current (I ma The value of the target current I quoted above is not equal to the ion beam current striking the target, which is estimated to be at least 8 ma. under the above conditions, because the target/ suppressor assembly also collects electrons resulting from ionisation of the gas by the high energy ion beam.

The above neutron output is increased approximately 100-fold by filling the tube with a 50/50 deuterium/tritium mixture, instead of with deuterium only.

In FIGURE 3, which illustrates another embodiment of the invention, the second and third digits of the reference numerals refer to portions which are correspondingly numbered in FIGURE 1. The portion of the generator to the left of ring 108 is the same as in FIGURE 1, but the remainder of the tube is modified as shown.

It has been found that there exists a tendency, when operating under high-voltage conditions, for electrical breakdown to occur between the target shield (FIG- URE 1) and the extractor electrode 7 along the inner surface 37 of the glass envelope 1 extending between ring 8 and tube 11. There is reason to believe that such breakdown may be initiated by irradiation of the. glass surface either by charged particles originating from the ion beam, or by electromagnetic radiation (e.g. ultraviolet light from the ion source) or by X-rays due to highenergy electrons striking the extractor or the back-stop and ion-source walls. Another possible cause is deposition on the glass of any material sputtered by the beam from the outer region of shield 10 surrounding the end of channel 12.

Whatever the cause, it is found that this tendency to breakdown is reduced by a configuration of the extractor and shield electrodes which serves to screen the glass surface from such influences. In FIGURE 3 the shield includes a cylindrical portion 131 which projects within a corresponding cylindlical portion 132 of electrode 107 so that there is a substantial overlap. The baflle eflect resulting from this configuration prevents the portion 137 of the glass surface 101 extending between these two electrodes from seeing the ion beam, or the apertures in electrodes 107 and 103 (and hence the plasma in the ion source) or the end of channel 112.

To :preserve the electron-suppression and ion-optic characteristics of FIGURE 1 with the extractor/shield configuration of FIGURE 3, the end of suppressor tube 117 adjacent channel 112 has a conical termination 136. This has the effect of keeping the suppression hump approximately the same distance from the extractor as in FIGURE 1 and thus minimising the unsuppressed length of ion beam. A conical-ended suppressor tube has the eflect of reducing the height of the hump (for a given V but this eifect is countered by the increased effective length of channel 112, and the net efiect is a hump of suflicient height for eifective suppression with V =400 v.

The tendency to breakdown between electrodes 110 and 107 is further reduced by two other modifications. Firstly the right-hand end of portion 132, facing the shield 110, has an increased radius of curvature, as compared with FIGURE 1, to prevent the formation of local high electric fields. Secondly, portion 132 is stepped at 133 to increase the clearance between the extractor and the envelope, which reduces the potential gradient along the inner surface 137 of the envelope. A similar step may be provided on the outer surface of the shield adjacent the envelope.

Mention has already been made of the advantage of reducing the degree of re-entry of the target, to facilitate the irradiation of samples. In the embodiment of FIG- URE 3 the target 116 is seen to be a flat disc located beyond flange 115.

What we claim is:

1. A neutron generator comprising an envelope, means for producing a plasma in gas within a portion of said envelope, a boundary electrode bounding said portion from a further portion of said envelope and having an aperture for the extraction of ions from said plasma], an extractor electrode and a target shield successively spaced from said boundary electrode within said further portion and having apertures in register with the aperture therein to allow passage of an ion beam, means for producing an axial magnetic field in the region of the ape!- ture in the boundary electrode, and a target located beh nd said target shield.

2. A neutron generator as claimed in claim 1 wherein said plasma-producing means comprises an RF winding encircling said portion of the envelope and said magnetic field producing means comprises a solenoid coil encircling said envelope between said RF winding and said boundary electrode.

3. A neutron generator as claimed in claim 1 wherein the aperture in the target shield of the generator is of channel-like form, and an open-ended tubular suppressor member is located within said shield but electrically insulated from it, one end of said member encircling the target and the other terminating adjacent the target end of the chnanel in the target shield.

4. A neutron generator as claimed in claim 1 wherein the aperture in at least said boundary electrode is provided with an insert made of a metal having a low sputtering ratio and a low atomic recombination coefficient.

5. A neutron generator as claimed in claim 1 wherein said extractor electrode comprises a hollow cylindrical portion within which extends a cylindrical portion of said target shield in concentric overlapping relationship, said relationship being adapted to prevent the portion of said envelope extending between the extractor electrode and the target shield from being in direct view of said ion 15 beam and of the apertures in said electrode and shield.

References Cited UNITED STATES PATENTS 3,112,401 11/1963 Dorsten et a1. 313-61 X 3,302,026 1/1967 Mallon et a1. 313-61 X 3,344,299 9/1967 Bounden 250-845 X OTHER REFERENCES Activation Analysis: New Generators and Techniques Make It Routine by Meinke et al., Nucleonics, vol. 20,

10 No. 3, March 1962, pages 60 to 65.

JAMES W. LAWRENCE, Primary Examiner.

R. F. HOSSFELD, Assistant Examiner.

US. Cl. X.R. 250-845

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