US3265896A - Cold cathode neutron generator tube - Google Patents

Description

9, 1965 R. REDSTONE ETAL 3,265,896

COLD CATHODE NEUTRON GENERATOR TUBE Filed June 29, 1962 2 i. m E F F United States This invention relates to neutron generators of the type in which ions generated in a plasma in a cold cathode ion source, emerge as an ion beam accelerated in an acceleration space to a high energy to bombard a target containing an element capable of undergoing a nuclear reaction to produce neutrons, the ion source and the acceleration space being within a common sealed envelope at substantially the same pressure and it is concerned with pulsed generators which are designed to produce bursts of neutrons.

Neutron generators employing the DT reaction have already been proposed. In such generators a target containing one isotope is bombarded with high energy ions of the other isotope to undergo the reaction mev. For most efiicient creation of ions in the ion source it is desirable to have a high density of ionising material. Since the ionising material is normally in gaseous form it is therefore desirable to have a high gas pressure in the ion source.

The gas pressure in the acceleration space, on the other hand, should be as low as possible in order to minimise ion losses by collision with neutral gas molecules.

The incompatibility of these two requirements led to the first neutron generators being provided with a pump for maintaining a pressure differential between the ion source and the acceleration space. Neutron generators of this type were reliable and came to be widely used but they suffered from the disadvantages that they were large, expensive, practically non-portable and they required skilled and careful control.

A significant advance was made when Penning and Moubis in 1937 devised a neutron generator in which the ion source operated satisfactorily at the same pressure as the pressure in the acceleration space. The ion source was a modification of the ionisation gauge devised by Penning. In this source two parallel cathodes, one with a central hole to allow positive ions to issue, were held at the substantially same voltage and a plate with a small central hole was positioned between them and held at a positive voltage with respect to the cathodes so as to form an anode. If to this arrangement, there was applied a magnetic field normal to the cathode surfaces, Penning found that ionisation of a gas would occur at a gas pressure about 1000 times lower than was possible in a similar source without the magnetic field. The effect has been explained in terms of the movements of electrons under the combined electric and magnetic fields. The electrons are considered to move to and fro in the combined fields a large number of times before being captured by the anode, thereby raising the probability of producing ionisation by collision with gas molecules.

This early work has foreshadowed most of the more recent work on neutron generators.

A number of neutron generators have been developed using the ion source of the kind described above and have operated reliably.

They have, however, had features which can be considered to be disadvantageous from the point of view of the user.

" atent Firstly, they have necessitated elaborate means for supplying the required potentials to the various electrodes within the envelope. Since the envelope is composed of a suitable glass the mounting of the electrodes within the envelope has been difficult and laborious and care has had to be exercised during use or transport of the generators to avoid jolting the electrodes out of position.

Secondly, the yields of neutrons from the generators have been low.

At this point it would be appropriate to discuss the various ways of representing the neutron yields from neutron generators. The most commonly used method is to quote a rate in neutrons per second. This figure is meaningless until the mode of operation of the generator is known. Suppose for example, that each generator pulse has a duration of 10 sec. Suppose also that in each pulse the generator yields 10 neutrons. The generator thus has an output of 10 neutrons in 10* sec. and this corresponds to 10 n/sec. If the repetition rate of the pulse is 1 per sec. then the generator actually yields only 10 neutrons in each second. Great care should therefore be exercised when giving due weight to the quoted performances of neutron generators. In this specification the term neutron output will be used to indicate the actual number of neutrons generated in any specified time.

The factors affecting generator output are complex but most are sufiiciently well known to those skilled in the art not to need detailed definition here. Briefly the chief factors are limitations imposed by electrical breakdown, by the ion source, by energy dissipation in the generator and by heat dissipation at the target. The ion source limits the initial ion production and therefore determines the rate of the DT reaction, and excessive heat dissipated in the generator and in the target can cause structural breakdown and also drive off the target material and so hinder the reaction by another means. Other factors include the actual composition of the ion beam, the relative quantities of molecular ions and atomic ions, the loss of ions due to collisions in the accelerating space, and the etfect of secondary electrons released at the target under ion bombardment.

One very important factor to which attention is directed in some detail is the danger of electrical breakdown in the acceleration space. At the low gas pressure (about 10" to 10- mm. mercury) used in the neutron generators it is necessary to keep the gap across acceleration space smaller than the mean free path of the electrons in order to avoid ionisation and breakdown. It is desirable to keep the gap well below this for another reason also, namely to keep the conditions well up the left hand side of the Paschen curve. Figures which have been proposed for the acceleration gap across which the ions are accelerated are in the range 1 to 2 cm.

Voltages used across the acceleration gap for the DT reaction are in the-range 60 to kv., since this range covers the optimum energy for the reactants. This voltage is much higher than the voltages used in the ion source, which are in the range 1 to 4 kv. and so the electrical stress on the gas in the acceleration space can be much greater than that in the ion source. It has up to now therefore been considered essential to ensure that the magnetic field in the ion source does not penetrate to any significant extent into the acceleration space.

As a result the magnetic field in the ion sources of the cold cathode type used up to now has been to some extent distorted in the region near the acceleration space and this has reduced the efiiciency of the ion source.

Breakdown in the acceleration space can arise from another source. In the ion source there is created, as mentioned earlier, a plasma during operation. The plasma can diffuse into the acceleration space through the hole in the cathode and can produce breakdown if it gets too near the electrode applying the high negative voltage for acceleration. The negative voltage repels the plasma, but if the target voltage is applied as a pulse in a time shorter than is required for the plasma to reach its equilibrium position, vacuum sparking can occur because of distortion in the electric field in the acceleration space.

Confinement of the plasma within the ion source has therefore been regarded as a highly important factor in neutron generators of the pulsed type and the cathode adjacent to the acceleration space has an aperture as small as possible, so as to give maximum confinement. This has, however, the elfect of concentrating the ion beam into the shape of a thin pencil. Although the beam broadens due to space charge effects and the electric field, it has up to now been necessary either to move the target away from the cathode of the ion source or to use auxiliary electrodes to broaden the beam. If a narrow concentrated beam were allowed to strike a target there would occur local overheating and the tritium or deuterium, which are the normal reactants, would be driven 01f. The neutron yield would therefore remain small.

Increasing the path length of the ion beam, however, necessitates the use of auxiliary electrodes and if the acceleration gap is made longer, it increases losses due to collision as discussed above.

The design of neutron generators can be seen, in view of the above discussion, to rest on a compromise between conflicting requirements, and the neutron output is a function of many variables. The neutron output of the neutron generator devised by Penning in 1937 was small, about 3x10 neutrons per second operating continuously and subsequent neutron generator designs have, of course, been able to raise this figure. It is found, however, that the neutron output per pulse of known pulsed cold cathode neutron generators of the type to which this invention relates have all been around to 10 A structural feature of neutron generators which has practical importance is the target. During the assembly into the generator, due to heat produced in the assembly processes by, for example, brazing or soldering, deterioration of the target film, be it integrated with the hydrogen isotopes or not, may occur. Furthermore during the exhausting and baking processes heating of the target film can also cause deterioration.

Furthermore during the operating of the generator under the high energy particle bombardment, heating either instantaneous or continuous may cause deterioration of the target due, for example, to evolution of the target gas, or fusing of the target film.

This invention provides a pulsed neutron generator which can give a greatly increased neutron output per pulse with a good working life.

In one form the invention provides a neutron generator which is of simple configuration, highly resistant to mechanical shock, and has an improved and simplified means of passing the required potentials to the various electrodes.

In another form the invention provides a neutron generator having a target assembly for neutron generators which facilitates removal of heat from the target area during assembly, any other heating processes necessary in manufacture or exhausting or in operating or any other condition or environment which may cause deterioration of the target. The improved heat dissipation from the target allows the apparatus to be run at a higher power level and thus enables the achieving of a higher pulse repetition frequency.

The target provides a simple means for irradiating materials thereby making the best use of the geometry of the neutron emission.

The target assembly is also easily replaceable.

It follows furthermore that it is possible in this invention to preload the target with tritium or deuterium or mixtures of these gases and retain them during all subsequent manufacturing processes by cooling the target thereby simplifying the exhaust process.

It has been found that by allowing the magnetic field to extend beyond the ion source thereby ensuring that the magnetic field lines remain substantially parallel at the extremities of the ion source, and by ensuring that the field lines are as straight as possible within the ion source, a large ion source can be used and a surprisingly intense ion beam can be produced having a high proportion of atomic ions, as shown by the high neutron outputs of the generators.

It has also been found that with this ion source a small aperture can be used to define the ion beam width thereby assisting maintenance of a good magnetic field configuration and the confinement of the discharge plasma within the ion source, and yet the large target does not suffer from localised overheating.

An embodiment of the invention is illustrated by way of example in the drawing accompanying the Provisional Specification in which FIGURE 1 is a sectional elevation along AA and FIGURE 2 is a third angle projection.

In the drawings an anode 1 of non-magnetic material comprises a cylinder containing stress-relieving slots 2, and it is attached to a rigid ring 3 to which are fused two glass tubular portions 4 and 5. Outer edge 6 of ring 3 is clear of the glass and provides good electrical contact for voltage supply means (not shown). An extractor cathode 7 is formed as a disc of ferromagnetic material fused to glass tubular portions 5 and 8. Outer edge 9 of extractor cathode 7 extends clear of the glass and provides good electrical contact for voltage supply means not shown. Holes 10 in extractor cathode 7 comprise a ring of small circular holes about a larger central circular hole. A skirt 11 of metal is fused to glass tubular portion 8 and has a shaped edge 12 to which is brazed a target 13 consisting of a nickelbase supporting a tritiated or deuterated titanium film. Other materials such as for example, molybdenum may be used for target 13 and other hydrogen occluding materials, for example zirconium, may be used for the target film.

Glass tubular portion 4 has fixed to it a metal skirt 14 similar to skirt 11, and having a dished edge 15, and to this is brazed a support dome 16. Dome 16 carries an apertured cathode cap 17 and a baffle 18 having a central aperture 19. A cathode plate 20 is brazed to dome 16 and to metal tubes 21 and 22, there being a third tube not visible. To tube 21 is fused a hollow glass angle arm 23 containing a pressure gauge of the conductivity type. It consists of a filament 24 having supply leads 25 and a support .rod 26. The filament can be connected to form one limb of a Wheatstone bridge and the amplified out-of-balance current used to control the temperature of a gas replenisher (described and claimed in our United Kingdom Patent No. 850,950) enclosed within a hollow glass angle arm 27 fused to tube 22. The gas replenisher consists of a precision bore glass tube portion 28 of increased radius relative to angle arm 27. Against a step 29 is pressed a mica washer 30 supporting one end of a nickel tube 31.

A second mica washer 32 supports the other end of tube 31 and a sleeve 33 acts as a strut between the two washers. Pinch wires 34 urge washer 32 against the sleeve and a heater filament 35 is connected across the pinch wires and passes down the interior of tube 31. Pinch wires 34 are attached to leads 36.

A third stem 37 acts as a pumping tube. In the operation of the tube a small current was applied to the heater in the gas replenisher to drive off deuterium until the pressure inside the anode was 10 to 20 microns. A single layer magnetic field coil (F) having turns was placed round the envelope to produce, when pulsed, a field of 500 gauss or more in the space between the cathode and the extractor cathode, the field penetrating the anode which is made of a low conductivity non magnetic alloy of copper and nickel, and also penetrating deeply into the gap between cathode 10 and target 13. A pulse voltage of about 1 kilovolt amplitude and about 13 microseconds duration was applied between the anode and the cathodes which were grounded and a current of 40 amps to the magnetic field coil. At the same time a pulse of microsecond duration and amplitude of up to 150 kilovo-lts was applied to the acceleration gap between the extractor cathode and the target.

A burst of at least 10 neutrons was obtained.

This generator could generate 1O ,usec pulses of 5 10 neutrons with good repeatability and without apparent deterioration or excessive dissipation of heat.

We claim:

1. A pulsed neutron generator of the type specified in which the ion source comprises two parallel plate cathode members, each member being a thin fiat disc penetrable by a magnetic field and disposed normal to a common axis, one of the cathode members having a small axial aperture defining the initial width of the ion beam, an anode member positioned between the said cathode members, the said anode having an axial aperture defining the width of the plasma in the ion source, and in which generator the target member is located on the said axis next to and a small distance from the cathode member having the said aperture to form therewith a single stage acceleration gap, and means tor maintaining a magnetic field in the ion source, the field lines of the said magnetic field over the entire distance between the said cathode members being substantially parallel to the said axis.

2. A pulsed neutron generator as claimed in claim 1 in which the area of the small aperture in the said cathode is not more than one tenth of the area of the anode axial aperture.

3. A pulsed neutron generator as claimed in claim 1 in which the anode has a rigid radially flared portion penetrating the envelope so as to provide external electrical contact for the anode and mechanical support for the envelope.

4. A pulsed neutron generator as claimed in claim 1 in 6 which the said cathode having the small axial aperture has a rigid radially flared portion penetrating the envelope so as to provide external electrical contact for the said cathode and mechanical support for the envelope.

5. A pulsed neutron generator as claimed in claim 4 in which the said cathodes, anode and target are held in space-relationship by three glass co-axial tubes to which they are fused, the glass tubes forming the envelope, and the target member and one cathode member closing the ends of the envelope.

6. A pulsed neutron generator as claimed in claim 1 in which the target member comprises a cup having its base oriented towards the dielectric envelope and its lip sealed to the edge of a skirt fused to one end of the envelope.

'7. A pulsed neutron generator as claimed in claim 1 in which the anode member is composed of non-magnetic material of low electrical conductivity.

8. A pulsed neutron generator as claimed in claim 1 in which means are provided to maintain a gas pressure of hydrogen isotopes in the range about 10 to about 20 microns.

References Cited by the Examiner UNITED STATES PATENTS 2,826,708 3/1958 Foster 250-41.9 2,951,945 9/1960 Goodman 25084 2,967,245 1/1961 SoloWay 25084.5 2,994,777 8/1961 Tittle 25084.5 3,082,326 3/1963 Arnold 25084.5

FOREIGN PATENTS 724,441 2/ 1955 Great Britain.

5 RALPH G. NILSON, Prima'ry Examiner.

JAMES \V. LAWRENCE, Examiner.

Claims (1)

1. A PULSED NEUTRON GENERATOR FOR THE TYPE SPECIFIED IN WHICH THE ION SOURCE COMPRISES TWO PARALLEL PLATE CATHODE MEMBERS, EACH MEMBER BEING A THIN FLAT DISC PENETRABLE BY A MAGNETIC FIELD AND DISPOSED NORMAL TO A COMMON AXIS, ONE OF THE CATHODE MEMBERS HAVING A SMALL AXIAL APERTURE DEFINING HE INITIAL WIDTH OF THE ION BEAM IN ANODE MEMBER POSITIONED BETWEEN THE SAID CATHODE MEMBERS, THE SAID ABODE HAVING AN AXIAL APERTURE DEFINING THE WIDTH OF THE PLASMA IN THE ION SOURCE, AND IN WHICH GENERATOR THE TARGET MEMBER IS LOCATED ON THE SAID AXIS NEXT TO AND A SMALL DISTANCE FROM THE CATHODE MEMBER HAVING THE SAID APERTURE TO FORM THEREWITH A SINGLE STAGE ACCELERATION GAP, AND MEANS FOR MAINTAINING A MAGNETIC FIELD IN THE ION SOURCE, THE FIELD LINES OF THE SAID MAGNETIC FIELD OVER THE ENTIRE DISTANCE BETWEN THE SAID CATHODE MEMBERS BEING SUBSTANTIALLY PARALLEL TO THE SAID AXIS.

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