US2930917A

US2930917A - Ion sources

Info

Publication number: US2930917A
Application number: US716386A
Authority: US
Inventors: Nief Guy
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1957-02-23
Filing date: 1958-02-20
Publication date: 1960-03-29
Anticipated expiration: 1977-03-29
Also published as: GB877112A; LU35771A1; CH346621A; DE1071853B; BE564648A

Description

March 29, 1960 Filed Feb. 2o, 1958 NVENTOR @0r ./V/f

BMM; @6.4.4

ION SOURCES Guy Nief, Paris, France, assigner Vto Commissariat a lllnergie Atomique, Paris, France, a society of France The present invention relates to ion sources, and in particular to those intended to feed ions to a mass spectrometer in order to permit isotopic analysis of the element from which said ions have been obtained.

vThe chief object of this invention is to simplify the structure of such sources and to make their output more abundant and more representative of the true isotopic nature of the element that is ionized, in particular when the atomic number of this element is low (hydrogen, helium).

yThe ion source according to this invention comprises means forming a chamber spaced apart from any magnetic tield and containing, under low pressure, a gaseous substance to be ionized and three electrodes 1ocated in said chamber, said electrodes being, respectively, a iilament'ffor the emission of electrons, a plate and an accelerating grid' interposed between said filament andV said plate. Y

l According to the present invention, said filament is at a potential intermediate between that of said grid and that `of said plate, whereby the electrons issuing from said filament pass through said grid and then oscillate in the space between said grid and said plate where they form a kind of electron fog suitable for ionization of the particles of said substance by impact of the electrons thereon, means forming an electrostatic potential being provided to extract'the ions thus formed from said chamber.

In the ion sources known up to this time, `the ions of the element to be studied are generally produced by the action of a beam of electrons directed onto a sarnple of ,said element. The impact of the electrons against the particles (atoms or molecules) of this element gives rise to the desired ions. When the beam of ions thus produced is intended to feed a mass spectrometer in order to elect -the isotopic analysis of the ionized element, it is preferable not to use a magnetic field in order to fpcus the beam of'electrons because the presence of such a iield is a cause of complications. These complications are the greater as the relative difference between the masses of the isotopes to be separated is greater, that is to say as the atomicv number of the element that is considered is lower. This is due to the fact that the action of the magnetic iield on the different ions extracted from a sample of the element to be analyzed detiects the trajectories of these ions in a different manner according to the mass and, therefore, at the outlet of the source proper, the ion beam may have an isotopic composition diierent from that of the sample of the element the analysis of which is to be made. It will be readily understood that this involves a systematic error in the results supplied by the mass spectometer fed from such an ion source, this error being the greater as the magnetic iield of the source produces a greater dispersion of the isotope ions of one same element, that is to say as the atomic number of this element is lower.

. plate lare capable of passing through said holes or In order to obviate these drawbacks, it has already been suggested to cperatethe ion source without a magnetic field. 1n such asource,V for instance, the substance to be ionized is introduced in the gaseous state in a space where the pressure is low, said space containing a filament capable of emitting electrons, a grid at a potential which is positive with respect to ,thatl of the filament in order'to accelerate these electrons and to cause them to ionize by impact the particles Y of said substance, and a plate provided with a hole orY any suitable perforation, said plate being at a potential which is both positive with respect to that of the filament and slightly negative with respect to that of the grid, whereby the electrons are collected by this plate and someof the ions that have been formed, which drop in aV rain-like fashion on said plate, pass through said holes` or perforations to form the desired beam'ofions.

The outputV of such a source is limited by the fact that a great number of electrons are caught by the plate beforev giving rise to anion of the substance. Furthermore, among the ions that are formed, only those located opposite the holes or perforations of the perforations. i

In order to obviate these drawbacks, according to the present inventiomthe values of the potentials of the re# pective electrodes are such .that the potential of the,Y v

filament is negative with respect to the grid but positive with respect to the plate.

Thus, the electrons emitted by the tilament are pulled toward the ,grid through which they iirst pass ,to be subsequently Apushed back by the plate, the energy they have acquired between the filament fand the grid being not sutiicient to enable them to lreach the plate. These Velectrons therefore oscillate between the plate and the grid, forming an electron fog in the region close to the grid. It is in this region that ionization of the gaseous molecules takes place.

In order to extract the Vions. formed in said fog, use is made of a fourth electrode provided with suitable perforations and highly biased.

.The source of ions thusr obtained is particularly well adapted to the case of analysis by means of a mass spectrometer of elements having a low atomic number. v Concerning for instance ordinary gaseous hydrogen',` which is a mixture in variable proportions of moleculesk HHH+ containing three atoms of hydrogen, quickly de- Y creases when the kinetic energy of the ion increases. This is an important advantage of the source according to the present invention since, as the mass spectrometer is not capable of dierentiating the primary HD+ ions from the secondary HHH+ ions, these last mentioned ions, in so far as they were produced in conventional ion' sources, involved an uncertainty in the determination of the ratio of the ion currents due to the H2+ ions of mass 2 and HD+ ions of mass 3.

A preferred embodiment of the present invention will be hereinafter described with reference to the accompany' ing drawing, given merely by way of example and in which:

The only figure is a perspective diagrammatic view of an ion source according to this invention'.

vPatented Mgr. Y

3 On the drawing, 1 designates the filament, 2 the grid in the form of a conductor wound helically about {ilament 1, and 3 is the plate constituted by a semi-cylinder of revolution having its axis in coincidence with filament 1. Grid 2 being at the highest potential V2, filament 1 is at a potential V1 intermediate between the potential V2 of grid 2 and the potential V3 of plate 3. Al second plate 4, of at shape, at a potential V4 highly negative with respect to that of plate 3, creates the electrostatic field which accelerates the ions. Two flat elements S and 6 are at a potential (V5, V6) adjustable and close to that of plate 3. By adjustment of this potential to the desired value in accordance with the known laws of ion optics, it is possible to concentrate the beam and to focus the ions so that they ilow out through slit 7, provided in plate 4.

The gas-tight chamber which contains the gas to be ionized under low pressure is designated by the numeral 8. An interesting application of this ion source consists in isotopic analysis in a mass spectrometer of hydrogen containing a small amount of deuterium.

In a particular example of such an application, filament 1 is a tungsten wire of a diameter equal to 0.2 mm. and of a length equal to 30 mm., located at a distance of 2S mm. from plate 4. Grid 2 is a helix of a pitch equal to 2 mm., constituted by a nickel wire of a diameter of 0.15 mm. wound on a cylinder of 5 mm. diameter and extending over a length of 28 mm. Plate 3 is made of stainless steel and has a diameter of l5 mm., a length of 25 mm. and a thickness of 0.6 mm. Slit 7 is a rectangle one millimeter wide and ten millimeters long. The focussing side elements 5 and 6 are constituted by two rectangular plates of 20 x 25 mm., at a distance of 17 mm. from each other.

The potentials of the different electrodes are as follows:

V1=2.500 volts V2=2.550 volts V3=2.490 volts V4=0 V5 and V6=approximately 2.490 volts The stream of electrons from filament 1 to grid 2 is ten milliamperes.

With a hydrogen pressure of l*6 mm. of mercury in the source, the beam of ions of mass 2 received at the collecting element of the spectrometer reaches 10*g ampere and the energy dispersion of these ions is about thirty electron-volts. This is not a drawback since, in view of the fact that the masses to be separated are very different (masses 2 and 3), the separating power that is necessary is small and it is possible to admit a dispersion of potential energy of the ions of some tens of electron-volts without impairing the good separation of said ions on the collecting element.

'Ihe stream of HHH+ ions averages 10*14 ampere and is suiciently constant to permit a systematic correction, averaging 106 on the ratio D/ H.

Ionization taking place in a space where the electric 4 field is intensive, it is possible to use important electron currents without risk of ion beam dispersion phenomenons due to the space charge, and this is one of the advantages of the invention.

This source also gives good results on a mass spectrometer for detecting helium leakages.

It is of high sensitivity, since it supplies important ion currents, even with a :gas which is difficult to ionize under very low pressure.

Its construction is particularly simple.

In a general manner, while I have, in the above description, disclosed what I deem to be practical and ecient embodiments of my invention, it should be well understood that I do not wish to be limited thereto as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.

What I claim is:

1. An ion source which comprises means forming a chamber spaced apart from any magnetic field and containing, under low pressure, a gaseous substance to be ionized, at least three electrodes located in said chamber, said electrodes being, respectively, a ilament for the emission of electrons, a plate, and an accelerating grid interposed between said filament and said plate, said plate being at a potential lower than that of said filament and said filament being at a potential intermediate between that of said grid and that of said plate, whereby the electrons issuing from said filament pass through said grid and then oscillate in the space between said grid and said plate where they form a kind of electron fog suitable for ionization of the particles of said substance by impact of the electrons thereon, and means forming an electrostatic potential to extract the ions thus formed from said chamber.

2. An ion source according to claim 1 in which said lament is at least substantially rectilinear, said grid surrounding said filament, and said plate is in the form of a portion of a cylinder of revolution having its axis at least substantially in coincidence with said filament, said plate having its edges located along two generatrices of said cylinder located on opposed sides of said filament.

3. An ion source according to claim l in which said means to extract the ions from said chamber include a fourth electrode at a potential highly negative with respect to that of said plate, said fourth electrode being provided with a slit for the passage of the ions extracted -from said chamber, said ion source further including side plates disposed symmetrically on opposed sides of said slit `for focussing an ion beam toward said slit.

References Cited in the file of this patent UNITED STATES PATENTS 2,495,908 Rich Jan. 3l, 1950 2,499,320 Loevinger Feb. 28, 1950 2,772,364 Washburn Nov. 27, 1956

Claims

1. AN ION SOURCE WHICH COMPRISES MEANS FORMING A CHAMBER SPACED APART FROM ANY MAGNETIC FIELD AND CONTAINING, UNDER LOW PRESSURE, A GASEOUS SUBSTANCE TO BE IONIZED, AT LEAST THREE ELECTRODES LOCATED IN SAID CHAMBER, SAID ELECTRODES BEING, RESPECTIVELY, A FILAMENT FOR THE EMISSION OF ELECTRONS, A PLATE, AND AN ACCELERATING GRID INTERPOSED BETWEEN SAID FILAMENT AND SAID PLATE, SAID PLATE BEING AT A POTENTIAL LOWER THAN THAT OF SAID FILAMENT AND SAID FILAMENT BEING AT A POTENTIAL INTERMEDIATE BETWEEN THAT OF SAID GRID AND THAT OF SAID PLATE, WHEREBY THE ELECTRONS ISSUING FROM SAID FILAMENT PASS THROUGH SAID GRID AND THEN OSCILLATE IN THE SPACE BETWEEN SAID GRID AND SAID PLATE WHERE THEY FORM A KIND OF ELECTRON FOG SUITABLE