US3248587A - Electron bombardment ion source - Google Patents

Description

April 26, 1966 KlKUO SANO ETAL 3,248,587

ELECTRON BOMBARDMENT ION SOURCE Filed April 7, 1964 FIG FIG.2

l 3,248,587 ELECTRGN BUMEARDMENT IQN SUURCE Iiiltuo Sane and Hitoshi Tsuyama, Kitatama-gun, Tokyoto, .lapan, assignors to Kahushiki Keisha Hitachi Selsa= inisho, Tokyoto, Japan, a joint-stock company of Japan Fiied Apr. '7, I964, Ser. N 357,879 Claims priority, application Japan, Apr. 18, 1963, SEE/28,262 3 Claims. (Cl. 313-85) This invention relates to improvements in ion sources of the electron bombardment type.

It is a general object of the invention to overcome certain difficulties encountered in the prior art and to improve the collimation of electron beams used in ionization.

The nature, principle, and details of the invention, as well as specific objects thereof, will be best understood by reference to the following description, taken in conjunction with the accompanying drawing in which like parts are indicated by like references characters, and in which:

FIGURE 1 is a sectional view showing the essential parts of one example of an electron bombardment ion source;

FIGURE 2 is a perspective view showing the essential parts of one example of an ion source of known type; and

FIGURE 3 is a perspective view showing the essential parts of a preferred embodiment of the ion source according to the invention.

in mass spectrometers for gas analysis, ion sources of the electron bombardment type in which ionization is accomplished by bombarding gas molecules with electrons are widely used. The ionization region in one example of this type of ion source is shown in FIGURE 1. An elec tron beam e emitted from a ribbon or tape filament 2, focused by a grid 3, and accelerated by an anode 4- is caused to collide with gas molecules g introduced into an ionization chamber 1, thereby to accomplish ionization of the gas molecules g and cause an ion current to be generated. An electron collector 5 is provided at the end of the ionization chamber 1 opposite to the filament 2.

In this case, in order to generate a large quantity of ions, a filament 2 in the form of a tape is used, but in the case wherein the collimation of the electron current is poor, the electron current passing through the entrance slit for electrons becomes small, and, moreover, an energy spread in the generated ions occurs, whereby a lowering of the accuracy of mass analysis occurs. For this reason, the collimation of a stable electron current is required.

Heretofore, for this purpose, a grid electrode 13 as shown in FIGURE 2, which is a perspective view in the direction indicated by A in FIGURE 1, has been used. A tape filament 12 provided at its ends with filament anchors 6 and e, is interposed centrally between the end edges of the flanges of a grid electrode 13, which is made of a metal plate formed into a configuration with a channel cross section having rectangular projections 31 at the central parts of the end edges of its flanges. When a current is passed through the filament 12 to cause it to emit an electron beam, and negative and positive potentials are applied respectively to the grid electrode 13 and the anode 4, an electron current is generated, on which there acts an electric field collimating electrons in the x direction, but in the y direction there acts an electric field which diffuses electrons. For this reason, collimation in only one direction is possible. Consequently, the re quired conditions of constricting the electron current as much as possible and of causing as large a quantity of electrons as possible to pass through the entrance slit 4 for electrons are not fulfilled, and, accordingly, the current passing through the entrance slit 4 is small for the same magnitude of thermionic emission. Moreover, since the collimation is poor, the electron current reaches a wide area, thereby soiling the internal surface of the ion source chamber.

States Patent 0 It is a specific object of the present invention to overcome the above-described difficulties and to improve the collimation of electron beams used for ionization.

The nature and details of the invention will be more clearly apparent from the following description of a preferred embodiment of the electron bombardment ion source according to the invention as shown in FIGURE 3, which shows an electron gun suitable for use in the said embodiment and which is a perspective view similar to that in FIGURE 2. The electron gun shown in FIGURE 3 differs from that shown in FIGURE 2 in that cutout recesses are provided in the central parts of the projections 31 formed on the end edges of the flanges of the grid electrode 23. As a result, each of the projections 31 has substantially prominent projections 32 and 33 at its two ends.

As the grid electrode, its construction may be such that it is provided with mutually opposite sides between which the filament is disposed and it will be obvious that the cutout recesses may be provided on the end edges of the flanges of a grid electrode which has no projections.

By the above-described construction of the grid, collimation is effected not only in the x direction but also in the y direction as shown in FIGURE 3. Consequently, the electron beam is subjected to collimation from four directions, and collimation is substantially improved. As a result, the current used for ionization increases and, since dilfusion of the electron current is prevented, the soiling due to impingement of electrons on the source chamber wall is reduced. Moreover, the energy spread of the ions can be reduced. Therefore, all of these favorable results are advantageous for increasing the accuracy of mass spectrometry.

While the above-described embodiment of the invention has been described with respect to its use for a mass analyzer, it will be apparent the ion source of the present invention is effectively applicable to many other kinds of apparatuses and instruments.

It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that it is intended to cover all changes and modifications of the example of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. In an ion source of the type wherein ionization is accomplished by means of an electron beam of sheet-like configuration emitted from an elongated filament, a pair of grid electrode plates having their flat surfaces parallel to each other and to the longitudinal axis of said filament interposed therebetween, each of said electrode plates lying in a plane parallel to the direction of the electron beam emission and having at the central part of its edge in the direction of the electron beam emission projections with a cutout recess therein, the resulting construction making possible tridimensional control of collimation of said electron beam.

2. The ion source as defined in claim 1, wherein said filament is a wire.

3. The ion source as defined in claim 1, wherein said filament is a tape.

References Cited by the Examiner UNITED STATES PATENTS 2,820,169 1/1958 Morris 313- X 3,062,984- 1l/1962' Hofker 313-69 X GEORG N. WESTBY, Primary Examiner.

ROBERT SEGAL, Examiner.

S. D. SCHLOSSER, Assistant Examiner.

Claims (1)

1. IN AN ION SOURCE OF THE TYPE WHEREIN IONIZATION IS ACCOMPLISHED BY MEANS OF AN ELECTRON BEAM OF SHEET-LIKE CONFIGURATION EMITTED FROM AN ELONGATED FILAMENT, A PAIR OF GRID ELECTRODE PLATES HAVING THEIR FLAT SURFACES PARALLEL TO EACH OTHER AND TO THE LONGITUDINAL AXIS OF SAID FILAMENT INTERPOSED THEREBETWEEN, EACH OF SAID ELECTRODE PLATES LYING IN A PLANE PARALLEL TO THE DIRECTION OF THE ELECTRON BEAM EMISSION AND HAVING AT THE CENTRAL PART OF ITS EDGE IN THE DIRECTION OF THE ELECTRON BEAM EMISSION PROJECTIONS WITH A CUTOUT RECESS THEREIN, THE RESULTING CONSTRUCTION MAKING POSSIBLE TRIDIMENSIONAL CONTROL OF COLLIMATION OF SAID ELECTRON BEAM.

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