US2919348A

US2919348A - Device for the separation of gas ions

Info

Publication number: US2919348A
Application number: US667491A
Authority: US
Inventors: Bierman Aron
Original assignee: Individual
Current assignee: Individual
Priority date: 1956-07-05
Filing date: 1957-06-24
Publication date: 1959-12-29
Anticipated expiration: 1976-12-29
Also published as: FR1178596A

Description

Dec. 29, 1959 A. BIERMAN DEVICE FOR THE SEPARATION OF GAS IONS Filed June 24, 1957 I 3 Sheets-Sheet 1 AMPLIFIER flkon/ INVENTOR.

Arr-mane Dec. 29, 1959 A. BIERMAN DEVICE FOR THE SEPARATION OF GAS IONS Filed June 24, 1957 3 Sheets-Sheet 2 A Bu k -44 INVENTOR.

Dec. 29, 1959 A. BIERMAN 2,919,343

DEVICE FOR THE SEPARATION OF GAS IONS Filed June 24, 1957 3 Sheets-Sheet 3 /?e ON .E/E'E HN INVENTOR.

United States This invention relates to a method and apparatus for the separation of gas ions and other particles of different mass.

A number of methods have already been suggested for the separation of gas ions by means of so-called mass spectrographs. Generally speaking, these belong to either of two main categories. In the methods of one of these the ions are separated by the different accelerations imparted to them, in accordance with their mass, by a magnetic field, or a combination of a magnetic field and an electrostatic field, through which the ions are made to pass. These mass-spectrographs require a very high mechanical precision as well as cumbersome and expensive magnets. The methods belonging to the other category use electric fields only and the particles, which have all the same initial energy and the same electric charge but different masses, attain dilferent velocities according to their different masses, and hence cover different distances in the same time.

Of the methods of the second category there may be mentioned in particular the one developed by Bennett (J. Appl. Phys. 21, 143, 1950). According to this method, a beam of ions is shot into an electrostatic field across several pairs of grids on which an alternating potential of high frequency is impressed. Among the different ions entering the space between each pair of the grids there will be some whose mass, and hence velocity is such that during the entire time of their stay in that space the direction of their flight coincides with that of the impressed alternating field. Of these ions some are said to be in resonance with the impressed high frequency by which they are accelerated, namely those whose passage between each pair of grids occurs during the same phase interval of the high-frequency potential. The.

other ions, which have a different mass, are not in resonance with the applied high'frequency and are. consequently not sufliciently accelerated by the action of the fields, or are even retarded.

This method has the drawback that all the ions have to have the same initial energy, for which purpose rather complicated apparatus has to be used. A further drawback is that ions of a certain mass are always accompanied by so-called ghost images or harmonics, i.e. by ions whose mass has a whole-number ratio to the mass of the ions under consideration, andwhose passage from one grid to the other requires a length of time that happens to have a whole-number ratio to the period of oscillation of the high-frequency potential, so that they are also in resonance with the latter. According to this method, the ghost images are reduced by an increase of the number of pairs of grids which, however, makes the apparatus still more complicated and expensive.

It is the object of the present invention to provide a simpler and more efiicient method and apparatus for the separation of gas ions of different mass, which may be used both for measuring the abundance of the ions and for their bodily separation for preparative purposes.

atent 2,919,348 Patented Dec. 29, 1959 The invention makes use of what is known as a potential well, i.e. a symmetrical electrostatic field whose potential is low in the centre and rises symmetrically towards opposite boundaries. The graphical representation in any plane normal to the electrodes, of the potential of such a field as a function of, for example, the distance from the median axis results in a well-like design, hence the term potential well.

If an ion is introduced with a certain initial energy into such a potential well, it starts performing harmonic, or very nearly harmonic, oscillations, just as a steel ball will do when dropped alongside the walls of a vessel whose smooth inner walls are parabolic.

The invention consists of a device for the separation of gas ions of dilferent mass, comprising means for building up a potential well, means for superimposing on the electric field of the potential well an alternating electric field of variable high frequency in one or more selected regions of the potential well, means for shooting ions into the potential well with a certain inclination towards the central axis of the device, whereby a zig-zag motion across the device is imparted to the ions and means for collecting and/ or measuring the abundance of those ions whose zig-zag motion is in resonance with said variable high frequency and which thus gather enough energy inside the device to enable them to reach the Zone of highest potential therein.

The ions that are not in resonance with the superimposed alternating frequency do not gather enough energy to reach the zone of highest potential, and they collect inside the device.

The invention is applicable not only to gas ions, but also to electrically charged minute particles of matter produced by an atomizing operation from the liquid or solid state. For .the sake of simplicity these are not specially mentioned in this specification and the appended claims, and they are deemed to be included in the term gas ions.

The potential constituting the outer boundary of the potential well will have to have the same sign as the ions to be separated, i.e. positive in the case of positive ions, and negative in the case of negative ions.

The alternating field can be impressed on different parts of the potential well. pressed on the high-potential boundaries of the Well, leaving thelow-potential regions unaffected. Alternatively, the alternating field can be impressed on the lowpotential regions of the well or, if desired, on its ascending regions. alt is also possible to impress several alternatlilng fields each on a diiferent region of the potential we A particular convenient case is the application of the high-frequency potential to the low-potential region of the well, and this will be described hereinafter.

. A potential well can be realized in a number of ways. For example, two hollow bodies, e.g. pipes of circular or angular cross-section, can be used. The diameter of the one has to be smaller than that of the other, but otherwise they'have to be of similar profile. The Walls of the pipe of smaller diameters are perforated and form a grid. These pipes are placed insulatedly one within the other and are connected to the two poles of 'a DC. source.

The simplest and most convenient way of producing a potential Well is to use two parallel plates between which two grids are disposed at equal distances from and in parallel to the plates. -1The plates are connected to the positive pole and the grids to the negative pole, of a DC. source. v

The invention is illustrated, by way of example only, in the accompanying drawings in which:

For example, it can be im- Fig. 1 is a diagrammatic illustration of the apparatus according to the invention;

Fig. 2 is a perspective view of a set of plates and grids for an apparatus according to the invention;

Fig. 3 is a plan view of another apparatus according to the invention, partly broken open; and

Fig. 4 is a perspective view of the inner parts of the apparatus according to Fig. 3 in a larger scale.

The apparatus according to Fig. 1 is adapted for the separation of positive ions and comprises two parallel plates, 1, 1', electrically interconnected through a resistor 2 which is tapped by a wire 3 connected to an amplifier 4. The latter is connected by a wire 5 to the positive pole of a high-potential D.C. source 6, and an indicator, for example, in the form of a milliammeter 4-, is connected in parallel with the amplifier 4, Placed between the plates 1, 1', each at the same distance from one of the plates and parallel to it, are two planar grids 7,7,

electrically interconnected through a resistor 8. The latter is tapped by a Wire 9 which leads to the negative pole of the high-potential source 6. Between the wires 5 and 9 a resistor 6 is connected in parallel to the DC. source 6. Also connected between the two grids 7, 7', and parallel to the resistor 8, is an oscillating circuit whose frequency can be varied arbitrarily and which includes the terminals 10.

At one end of plate 1 there is provided an ion source 11 with focussing means 12 and an accelerator 13, which latter is connected to wire 9 and hence to the negative pole of the DC. source 6, whereas the potentials for the ion source 11 and focusser 12 are drawn off from two different points of the resistor 6'. Thus there exists a drop of potential from the ion source 11 to the focusser 12 and from the latter to accelerator 13. The drop of potential, and consequently the acceleration imparted to the positive ions, is dependent on the magnitude of the DC. potential generated by the source 6 as well as on the location of the points at whichv the resistor 6' is tapped. It can, therefore, be adjusted at will by variation of the magnitude of the potential generated by the source 6, and/ or by the appropriate location of the points at which the resistor 6 is tapped. The ion source, focussing device and accelerator are so disposed that they can deliver into the space between the grids, a beam of gas ions in a direction other than normal to the grids and plates. Owing to this inclination, the ions receive a velocity component in the direction of the axis of the well. The electric energy required for the ion source is equally supplied by the DC. source and is a fraction of the total potential of the latter.

At the top of one of the grids 7' a plate 14 is provided for collecting undesired ions, as will be explained below.

This apparatus works as follows:

Let it be assumed that the accelerator 13 emits into the evacuated interior of the apparatus a beam consisting of a mixture of positively charged ions of different masses, and the

plates

1, 1 are charged positively. In the absence of a super-imposed high-frequency, all the ions, owing to their initial energy, will proceed towards the plate 1'. Their velocities are, however, difierent owing to their different masses. The path of the ions isindicated by the arrow. Their velocities diminish more and more as they approach the plate 1, and the ions are finally repelled by the electric field existing between the grid 7 and plate 1. This repulsion deflects the ions obliquely back towards the plate 1 where they are again similarly deflected. This Zig-zag movement in sweeps continues until the ions eventually hit plate 14. The time elapsing between consecutive deflections depends on the velocity of each ion, i.e. on its mass. This time will hereinafter be referred to as sweeping time.

Suppose now that-the grids are connected to a highfrequency oscillating circuit whose oscillating potential is super-imposed on the two grids. As stated above, the frequency of the oscillating circuit is variable. The circuit can, therefore, be so tuned that its frequency is in resonance with the zig-zag movement of a selected kind of ion of the beam. This means that each cycle of the superimposed frequency substantially corresponds to the oscillating period of the ion, i.e. to the aggregate time of two consecutive sweeps back and forth across the apparatus. Accordingly, when an ion of the selected kind crosses the space between the grids for the first time during a phase interval of the superimposed oscillating frequency which gives rise to the acceleration of the ion, this ion will be accelerated also during all its other passages between the grids. Its energy is thereby increased so that the ion approaches the one plate 1 (or 1') slightly more than it did the opposite plate 1' (or 1), at the end of the preceding sweep, as a consequence of which this ion, in contrast to the ions not in resonance with the superimposed oscillating frequency, gradually builds up sufiicient energy to hit one of the plates 1 or 1'. In the drawing, the path of the selected kind of ion is indicated by a full line whereas the paths of some other ions are indicated in dashed lines.

It has been found that although the selected ions con tinually build up energy, their sweeping times are substantially not altered from reversal to reversal. On the one hand, the time of passage between the two grids is shortened from sweep to sweep owing to the increasing velocity of the ions, but on the other hand, the time they dwell in the spaces between each grid and the coordinated plate 1 increases from sweep to sweep owing to the lengthening of their paths within these spaces. The net result is that the sweeping time remains constant within very narrow limits irrespective of the variations of the energy of the particles. Similarly the sweeping time is not substantially affected by the energy-spread of ions emitted by the source of ions. This is particularly true if the apparatus is so designed, and the conditions are so chosen, that during the particular sweep when the ionvelocity component in the direction perpendicular to the grids has attained the geometrical mean between the initial and end values, the ions spend substantially equal times in their passage from grid to grid and in the two spaces between the grids and the coordinated plates.

At the place where the ions reach the plate 1 and/ or 1 the latter can be provided with a window and a cage for collecting'the selected ions. Instead, the ions may be made to impinge on the plate 1 or 1' whereby the charge of the latter is increased. This increase can be measured with the aid of the amplifier 4.

All the undesired ions move at such velocities that their sweeping times are not in resonance with the oscillations of the oscillatory circuit. Consequently, they do not build up suflicient energy to reach either plate 1 or 1', and their path is eventually interrupted by the plate 14.

It has been found that the ions separated by the method'according to the invention are free fro'm the ghost images referred to above. This can possibly be explained by the fact that when a mass m is in resonance with the high-frequency, the mass m =(2Z+l) m (Z being an integer) will equally be in resonance, but the energy built up by the latter mass is only the (2Z-I-l)th part of the energy built up by the first mass. This energy does not suffice for the ions having the masses corresponding to the above formula to reach plate 1 or plate 1' in spite of the fact that their sweeping time is equally in resonance with'the high frequency.

Where two or more kinds of ion of different mass are not only to be separated, but all of them are to be collected or detected, the frequency of the superimposed oscillations has to be varied during the operation so that it is in resonance first with the one kind of ion and then with the other, or with the several others in succession.

Ina'n advantageous embodiment of the invention the

plates

1, 1 are made concave and face each other symmetrically with their concavities. Such an arrangement is diagrammatically illustrated in Fig. 2. The somewhat concave plates 1, 1' are connected to the positive pole of the D.C. source and thus form the positive upper boundaries of the potential well. Between the

plates

1, 1 and at equal distance from them there are disposed the grids 7, 7. The electrical connections and the remaining parts of the arrangement of the apparatus, which have not been shown in Fig. 2 will be similar to those illustrated in Fig. l, or in Figs. 3 and 4 which are described below.

The advantage of this embodiment lies in the fact that the force of the electrostatic field formed between plates 1 and grid 7 and plate 1' and grid 7, respectively, has components directed towards the horizontal median plane of the device, whereby the ions are focussed to a certain degree towards that plane and their random scattering is prevented or minimized.

Figs. 3 and 4 illustrate in greater detail a device according to the invention designed only for the determination of the abundance of ions in gaseous mixtures. This device comprises a tube 1, e.g. of glass, composed of a main part 2, an oblique side arm 3 and a tubular connection 4 for a hose leading to a vacuum pump. Both the arm 3 and the connection 4 can be sealed hermetically. The main part 2 of the tube 1 houses the two plates 5 and 5' which are U-shaped and whose

rims

6, 6 face each other across the inner space of the tube. The ends of the plates are interconnected by transverse members 7, S of insulating material, to which they are screwed.

In parallel to plates 5, 5' and at a certain distance from them there are disposed grids 9 and 9' which are constituted each by a set of parallel wires. One end of these wires is anchored in the member 8. The opposite ends of the grid wires 9 are fixed to a plate 10 of conducting material mounted on member 7, whereas the corresponding ends of the grid wires 9 extend without contact through the plate 10. The latter is a collector for the not-selected ions and corresponds to the plate 14 of Fig. 1.

The plates 5, 5' are provided at one end with

terminals

11, 11, and at the opposite end with terminals 12, 12'. Either of these pairs of terminals can serve for connection to the positive pole of a D.C. source, as in Fig. 1. Similarly the

grids

9, 9 are provided at one end with

terminals

13, 13, and at the opposite end with terminals 14, 14'. Either pair of these terminals may serve for connection to the negative pole of the D.C. source and also for connection to the high-frequency A.C. potential.

Through plate 5' there extends a sleeve 15 which supports an ion-producing unit 16, e.g. a gas discharge tube producing positive rays and being associated with focus sing and accelerating devices. Such units are known and need, therefore, not be described in detail. Unit 16 is provided with an inlet 17 for the introduction of the gas which is to be ionized. The sleeve 15 may be made, for example, from metal and be welded to plate 5. It will then be necessary to interpose a layer of insulating material (not shown) between the sleeve and unit 16.

In operation the device has first to be evacuated through the connection 4, then the gas mixture to be ionized is introduced through inlet 17. If desired, the interior of the device may be swept several times with the gas or gas mixture under investigation before the actual beginning of the operation for the removal of all foreign molecules. The degree of vacuum has to be chosen so that collisions between the ions and not-ionized molecules are avoided as far as possible.

The following two examples will illustrate the invention:

Example 1 In order to separate and collect deuterium (m=2) from particles of other masses one proceeds as follows:

A beam consisting of a mixture of ions of deuterium and the other kinds in question is shot into the apparatus with an aperture angle of 14 and a mean inclination of 17 with regard to a plane normal to plate 1 of Fig. l in the vicinity of the accelerator. The length of the grids is 6.1 cm., the distance between each grid and the corresponding near plate 1 cm., and the distance between the grids 2.8 cm. The D.C. potential applied to the plates is 1000 volts, the D.C. potential at the grids is 0 volt and the initial energy of the ions entering the apparatus is 500 electron volts. The grids are connected with an oscillating circuit whose frequency is 2.3 mc./sec. with a potential peak value of 200 volts. After 3-8 sweeps of the ions through the apparatus,'the exact value being dependent on the inclination at which the particular ion is shot into the apparatus, the deuterium ions build up enough energy to reach the plate 1 or 1' where they can be either collected or measured. In this manner pure deuterium is isolated.

Example 2 For the separation of particles of mass 20 from particles of any other possible masses the arrangement is as follows:

Length of grids 37 cm.; peak value of superimposed alternating potential 22.5 volts; frequency of superimposed alternating potential 0.7 mc./sec.; aperture 7; mean inclination 13 /2". The remaining conditions are the same as in Example 1.

It is found that after 27-45 sweeps, the exact value being dependent on the inclination at which the particular ion was shot into the apparatus, the particles of mass 20 are obtained in a pure state.

I claim:

1. A device for the separation of gas ions comprising two electrode plates facing each other with their longitudinal axes in parallel, one pair of grids placed between said electrode plates at equal distances from the latter, a source of D.C. current, said plates being connected to one pole of said D.C. source and said grids being connected to another pole of said D.C. source thus forming a potential well, means for superimposing on the electric field of said potential well an alternating electric field of variable high frequency in at least one selected region of the potential well, means for shooting into said potential well ions with a certain inclination towards a central axis of the device midway between said parallel longitudinal axes of the plates, thereby imparting to the ions a zig-zag motion across the device, and means for separately receiving those ions whose zig-zag motion is in resonance with said variable high frequency.

2. A device for the separation of gas ions as in claim 1; wherein both said electrode plates and said grids are planar and parallel to each other.

3. A device for the separation of gas ions as in claim 1; wherein said electrode plates are concave and disposed symmetrically in relation to a median plane extending through said central axis perpendicular to the grids, said plates having their concavities facing each other.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. A DEVICE FOR THE SEPARATION OF GAS IONS COMPRISING TWO ELECTRODE PLATES FACING EACH OTHER WITH THEIR LONGITUDINAL AXES IN PARALLEL, ONE PAIR OF GRIDS PLACED BETWEEN SAID ELECTRODE PLATES AT EQUAL DISTANCE FROM THE LATTER, A SOURCE OF D.C. SOURCE AND SAID GRIDS BEING CONONE POLE OF SAID D.D. SOURCE AND SAID GRIBS BEING CONNECTED TO ANOTHER POLE OF SAID D.C. SOURCE THUS FORMING A POTENTIAL WELL, MEANS FOR SUPERIMPOSING ON THE ELECTRIC FIELD OF SAID POTENTIAL WELL AN ALTERNATING ELECTRIC FIELD OF VARIABLE HIGH FREQUENCY IN AT LEAS ONE SELECTED REGION OF THE POTENTIAL WELL, MEANS FOR SHOOTING INTO SAID POTENTIAL WELL IONS WITH A CERTAIN INCLINATION TOWARDS A CENTRAL AXIS OF THE DEVICE MIDWAY BETWEEN SAID PARALLEL LONGITUDINAL AXES OF THE PLATES, THEREBY IMPARTING TO THE IONS OF ZIG-ZAG MOTION ACROSS THE DEVICE, AND MEANS FOR SEPARATELY RECEIVING THOSE IONS WHOSE ZIG-ZAG MOTION IS IN RESONANCE WITH SAID VARIABLE HIGH FREQWUENCY.