US 3337728 A
Description (OCR text may contain errors)
llg- 22, 1957 L. FRIEDMAN ETAL 3337,?23
MASS SPECTROGRAPH ION SOURCE WHER EIN A PULSED ARC IS PRODUCED BY VIBRATING ONE ELECTRODE i 9, 1964 2 Sheets-5heet l Filed Oct;
iiiii NN` f www E Nl/- Aug. 22, 1967 2 Sheets-Sheet a L. FRIEDMAN ETAL MASS SPECTROGRAPH ION SOURCE WHEHEIN A PU ARC IS PRODUCED BY VIBRATING ONE ELECTR Filed OCt. 9, 1964 INVENTOR. LEWIS FRIEDMAN ADOLPH P. IRS/ United States Patent O MASS SPECTROGRAPH ION SOURCE WHEREIN A PULSED ARC IS PRODUCED BY VIBRATING ONE ELECTRODE Lewis Friedman, Patchogue, and Adolph P. Irsa, Plainview, N.Y., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Oct. 9, 1964, Ser. No. 402,980 5 Claims. (Cl. Z50-41.9)
ABSTRACT OF THE DISCLOSURE An ion source for a mass spectrograph having an anode, and a cathode shaped to conne a plasma producing arc so that the plasma that is generated in the arc has only a limited means of escape; the escape means being designed for eiiiciently extracting ions for the purpose of analyzing samples of limited size. A pointed anode is provided that is vibrated against the inside wall of a well-shaped cathode for extracting the ions in a narrow solid angle along the inside wall of the cathode.
This invention relates to mass spectrographs and in particular to novel method and apparatus for efficiently directing a beam of ions into a mass spectrograph.
In trace analysis with a mass spectrograph, particles from an ion source are focused in the spectrograph by electrostatic and magnetic elds. Since the masses of the ions of the different isotopes are different, the trajectories of the ions as they move under the iniluence of the forces exerted by the fields are slightly different so that the ions of different mass in the incident beam are separated to focus at different spatial positions in the spectrographs focal plane. If a photographic plate is placed near the focal plane, a mass spectrum will appear in which the distance along the plate corresponds to the mass of the separated ions and in which the blackness of the spectrum lines correspon-ds to the quantity of material kof each particular mass that is separated for recording on the plate.
The electrostatic and magnetic fields are combined to produce double focusing, that is, all the ions of the same mass entering the instrument within a small divergence angle and which have an energy different from the mean energy by a small amount will focus. It is thus apparent that the eiciency of such as instrument will depend upon the proper conduction of a maximum number of ions of the isotopes from the source into the instrument at a high density or small divergence angle beam, particularly when quantitative measurements with a small limit of detection down to a small fraction of a part per million are to be made or where low magnetic fields and low accelerating voltages are used.
Heretofore, the ion source has been a spark source since it has been applicable to a wide range of elements having wide extremes in chemical nature, eg. xenon and cesium. In this regard although these elements are adjacent on the periodic chart, the xenon is much more diicult to ionize than the cesium. The spark source has thus been particularly important in nuclear transformation determinations where uranium has been bombarded in a nuclear reactor or' an accelerator to produce known yields of Xenon and cesium. When this sample has been used in the spark ionization source the xenon has produced weaker lines than the cesium but the ratios of intensities have differed only by about a factor of two.
Although the spark ion source has been Widely used to introduce ions into spectrograph heretofore, these spark sources have produced ions having significantly wide bands of directional and velocity spreading, there has been no simple means for directing or confining the ions in a high density beam or a small solid divergence angle beam, the efficiency of the sources has been low, high radiation levels have been required in the ion source or large amounts of the isotope samples have been lost. These problems have been particularly troublesome in trace analysis work e.g. for nuclear transformation determinations, since the samples have often comprised small amounts of different isotopes that have been produced by the bombardment of targets in large and costly reactors and accelerators where many hundreds of expensive man hours have been required in many cases to plan, prepare and execute these bombardments or to produce even small amounts of the ldifferent isotopes.
It is thus an object of this invention to provide for the prevention or continual correction of the conditions described above and greatly to increase the efficiency of the spectrograph and its ion source;
As a further object, this invention involves the establishment of an ion source utilizing directional collimation;
-Still .another object is the provision for the deposition, re-ionization, and direction of ions for achieving an eflcient beam source for a spectrograph;
It is also an object of this invention eciently to utilize electrical and mechanical means for providing an incident beam source of ions for a mass spectrograph;
An additional object is to provide an efficient ion nozzle for an ion source for a mass spectrograph;
A still further object is to provide very low limit, spectrograph detections for nuclear transformation determinations.
By this invention, there is provided method and appar-atus for providing and collimating a beam of ions from a trace element sample for nuclear transformation determinations. The method and construction involved in this invention utilizes standard and well known techniques and apparatus and is highly flexible for a wide range of applications, isotope separations, resolutions, limits of detection, isotope samples, sample sources and spectrographs. More specifically, this invention involves the use of systems of electrodes which form and collimate the beam of ions to be separated. The systems of electrodes are arranged n one embodiment, with a vibrating electrode and a stationary electrode which contact each other in a confined area formed by said electrodes to provide and direct ions from a small spark source in a high density or narrow divergence angle beam and to provide deposition and re-ionization of said isotopes for direction in said beam. With the proper selection of electrodes, voltage, and positioning of the sparks, as hereinafter to be more particularly described, it is possible by this invention eiliciently to provide the desired beam.
The above and further novel features and objects of this invention will appear more fully from the following detailed description when the same is read in connection with the accompanying drawings. It is expressly understood, however, that the drawings are not intended as a definition of the invention but are for the purpose of illustration only.
In the drawings where like parts are marked alike:
FIG. 1 is a partial cross-sectional view of a mass spectrograph incorporating the source apparatus of this invention;
FIG. 2 is a graphic illustration of the energy spread of ions in the spectrograph lof FIG. 1;
FIG. 3 is a partial schematic representation of the principles of this invention;
FIG. 4 is a partial three directional View of the source apparatus of FIG. 1 embodying the principles illustrated in FIG. 4;
FIG. 5 is a partial cross-sectional View of the apparatus of FIG. 5.
Referring to FIG. 1, a mass spectrograph 11 is shown in which the method and apparatus of this invention are utilized. This spectrograph 11 employs the Mattauch- Herzog geometry, which utilizes the double focusing principle. This type spectrograph his high sensitivity for constituents having low concentrations, and in accordance with this invention, an incident high density beam is provided for determinations having a low limit of detection, e.g. down to 0.01 parts per million atomic particles. A more detailed description of this geometry is found in FIG. 24 in Chapter V, of Mass Spectroscopy 1958, Cambridge Press, by H. E. Duckworth.
Stop 13 has a predetermined, small entrance opening to receive a small cross-section ion beam 15 in a small angle 17 through exit slits in an ion source 19 along a straight axis 21. This stop 13 is in one end of an electrostatic sector 23, which has a positively charged side 25, a negatively charged side 27 and ends 28 and 29 forming entrance slit 13 and an exit slit 31 like in area with stop 13. Converging lines 32 and 33 drawn from these ends 28 and 29 form an angle Ni The beam 15 is bent in sector 23 along a first curved axis 37, travels along a straight axis 39 after exiting from Sector 23 and enters an annular magnet shunt 41 having beam monitor electrodes 43 and a small energy resolving opening 45 formed by the entrance to magnetic sector 47.
The paths of the charged particles are bent in magnetic sector 47 and focused thereby in focal plane 49, having between magnetic sector ends 51 and 53, a photographic plate mask. Converging lines 55 and 57 drawn from ends 51 and 53 form an angle 1r/ 2.
As is conventional, separate vacuum chambers are provided for the sectors of the analyzer 61, comprising the electric sector 23 having an entrance chamber 71 and magnetic sector 47 having a photographic system 73. To this end each sector has a liquid nitrogen cold trap and oil diffusion pump. Also, suitable valves and connecting pipes 81, 83 and 85 between the source chamber 86 and analyzer 61, and between the source chamber and source pumping system, permit the ion source to be vented and rough pumped without disturbing the analyzer vacuum. The differential pumping between the source slits 87 and 88 is accomplished by separate pumping systems to provide a very low pressure in the analyzer 61 during sparking. Slit 88 is at a first or ground potential and slit 87 is at a potential of between 15 to 20 kilovolts.
One suitable double focusing spectrograph having a high resolution and a low limit of detection with the source 19 of this invention, is the model 21-110B Mass Spectrometer made by the Consolidated Electrodynamics Corporation, Pasadena, Calif. This spectrograph is particularly adapted for analysis of solids from a spark ion source having small round or rectangular exit slits 87 and 88 and is adapted to provide a mass spectrum covering a mass range of 36:1, thus permitting for example, recording from lithium through uranium, all the sample solids in the periodic table in a single photographic exposure in focal plane 49. A typical schematic comparison of an incoming ion energy spread (area under curve c) vs. the end energy spread (cross-hatched area) for this spectrograph is shown in FIG. 2.
Typical source samples 89 comprise trace elements from an elemental metal target, such as uranium that has been bombarded in a nuclear reactor, an accelerator or in other means to produce various nuclear transformations forming characteristic quantities of certain isotopes. After bombardment, the metal target is treated chemically to separate the trace elements, which are applied in solution to heated electrodes to evaporate the solution and adsorb the trace element solute on the electrodes.
In general, such trace element samples have been ionized in a vacuum by a spark, provided heretofore by a radio-frequency power supply that developed a peak voltage of about kilovolts 'between two large, wedgeshaped electrodes between which this supply caused sparks to jump. It has been necessary to pulse this supply to reduce the duty cycle of the spark because electrode heating has been a problem. Typically the electrodes were held in fixed vises conected to the source through vacuum tight connections 91 and 93 in the wall 95 of source chamber 86. The fixed position of these electrodes has required frequent and time consuming adjustments as well as high voltage supplies or highly radioactive samples 89. Also, these sources have not vaporized all the sample constituents non-preferentially, kept the bulk of the sample cool, or eiciently directed a large number of the constituents into the analyzer section 61. Moreover, it has been difficult or impossible to provide a simple high efficiency, high density, or small solid angle beam source with this or any of the other source systems known heretofore.
In order to explain how the method and apparatus of this invention accomplish the function of concentrating ions, such as the described metal ions, and directing them in a high density beam or in a small solid angle coinciding with the small entrance angle of the analyzer 61, reference is made to FIG. 3 wherein is illustrated a horizontal x--x axis coresponding to the axis of a first longitudinally extending first pointed electrode at right angles to a Vertical y-y axis. The 0-0 axis is at right angles to x-x and y-y represents the central axis of a cylindrical second electrode 113. The z-z axis represents an axis parallel to 0-0 on the inside wall of the electrode 113 formed by the intersection of the inside wall with a plane passing through the 0--0 axis. Point P is at the intersection of the x-x, y-y and z-z axes. It will be seen that a spark at point P Will produce a high pressure plasma which is ejected out the open end 145 of electrode 113 in a filled high density beam whose maximum divergence angle depends on the position of point P with relation to the open end 145.
It is also seen that the plasma will be produced and concentrated substantially instantaneously in a high pressure, high density plasma beam by a very low voltage source. This may be explained by the fact that increased plasma pressure facilitates the spark formation at low voltages. Moreover, this high pressure region will eject the ions at high energies along the P-Z axis while the potential ldifference between slits 87 and 88 extracts the ions and injects them into the analyzer 61. These energies are low enough to keep the ions from being imbedded too deeply in the electrodes or too -far from the spark so that they are lost or dificult to re-ionize. Thus some of the ions will be deposited for immediate spontaneous reionization and ejection in the beam along the P-Z axis or deposited on the inside of the electrode 113 and reionized and ejected in the beam by intermittent sparks between electrodes 111 and 113. Also, the energy spread of the ions produced by this low voltage source is low so that a majority of these ions are ejected in a well defined low energy spread, high intensity beam along the P-Z axis. Additionally, the low voltage prevents the undue heating of the spark electrodes while providing a localized hotter spark.
Referring to FIGURES 4 and 5, in a practical arrangement for accomplishing the desired concentration and direction of the ions, first and second electrodes 111 and 113 provide a nozzle utilizing mechanical boundaries for collimating the ions. These electrodes connect in a circuit to a conventional constant power source 115 having a variable rheostat 117 lfor energizing the electrodes at a uniform constant level to produce sparks therebetween. Electrode 113, for example, is at ground potential and electrode 111 is at a higher potential. One electrode has a vibrationing means 118 for vibrating the electrodes into and out of contact to produce intermittent sparks. In this system the sparks are formed at a point P and the spark is extinguished as electrode 111 moves away from this point P by stretching the spark into a thin cross-section that cannot jump the gap formed Kby the movement of electrode 111 away from electrode 113.
Initially a suitable threaded feed means 119v biases electrode 111 into point contact with the inside of cylindrical electrode 113 at right angles to the z-z laxis of electrode 113. The arrangement and position of this contact point are so related to the arrangement and position of electrodes 111 and 113 and the source slits 87 and 88 and the direction stop 13 along the axis 21 of analyzer 61, that the maximum divergence angle of the plasma beam from this point corresponds with the maximum line of sight angle from point P through slits 87, 88 and stop 13 along the yP-Z axis, this axis coinciding vwith the laxis 21 of these slits. In `this regard threaded feed means 121 moves electrode 111 backward or forward` parallel with the axis z-z of elect-rode 113. Also, threaded feed means 123 biases electrode 113 in both directions parallel with axis z--z and threaded feed means 124 moves electrode 113 in and out at right angles to axis z-z of electrode 113. Windows, such as windows 125 and 125 provide visual viewing of the electrodes 111 and 113 and their sparks. This permits adjustment so that there is no -back spark `along axis z-z away from analyzer 61.
The first electrode 111 has -a longitudinally extending, uniform, cylindrical portion 126 and a uniformly tapered portion 127 forming a pointed end 128 on the electrode axis x-x at point P. This axis x-x of electrode 111 coincides with the mid-plane of uniform cylindrical electrode 113 passing through axes o o and z-z and the center of slot 130 in electrode 113. The contact point of these electrodes 111 and 113 thus-form aiirst confining zone providing confining forces in the x--x direction in a larger second confining zone formed by electrode 113 providing forces along the z-z axis equivalent to forces in the-y-y direction. The cross-sectional area of these forces is large `due to the large areas of the electrode surfaces relative to the small cross-section of the spark formed between the electrodes. This leads to the high eliiciency of these electrodes in directing ions therefrom in a high density or small solid angle beam. The taper of electrode 111, however, makes a small angle with its axis x-x extending from the inside surface 132 of electrode 113 past the opposite surfaces 133 and 135 of slot 130 so as to have aclearance from the sides 133 and 135 of slot 130 sufficiently small to prevent arcing across this slot 130 and large enough effectively to block the emergence of ions through slot 130.
It will be understood, from the above that the movement of electrode 111 toward surface 132 of electrode 113 causes a spark between tip 128 of electrode 111 and surface 132. This spark is small in cross-section and substantially corresponds to a point spark that ionizes the isotopes on tip 128 to provide a point ion source in nozzle 131 between the two electrodes. Thus this spark is effectively blocked in the irst small zone 143 between the electrodes from all the openings in electrode 113 except open end 14S thereof. It will be seen, for example, that tip 128 blocks the direct line of sight from point 128 through openings 183 and 185 in FIG. 5. Also, tip 128 `blocks the direct line of sight from point 128 through openings 191 and 193 forming the rest of slot 130 in FIG. 4, and leaves a line of sight opening from point 128 through open end 145 of electrode 113 whose maximum divergence angle is determined by the position of the tip 128 (point P) relative to the open end 145. Also, it will be understood that electrode 111 is locally heated much more than electrode 113 and as material is slowly eroded from the electrode 111 this electrode is biased toward inside surface 132 of electrode 113 continually to provide tion of electrode 111 along axis z-z of electrode 113 corresponding to axis 21 of the analyzer 61 automatically lre-ionizes and ejects these isotopes out end 145. Thus the nozzle 131 forms a first opening between electrodes 111 and 113 whose distance from the second opening 145 is adjustable.
In operation, a small voltage is applied to electrodes 111 and 113 of about 10 volts for carbon electrodes 111 and 113. Soft metal electrodes, such as copper, however, require only 3 volts for sparking therebetween. Advantageously the source 115 is `a conventional 60 cycle source having a Voltage control, such as rheostat 117 and a 120 cycle/sec. vibrator 118 for moving electrode 111 into and out of contact with electrode 113 to produce intermittent sparks that ionize the sample 89 on the end 128 of electrode 111. The majority of these ions leave the nozzle 131 formed by these electrodes in the mid-plane between the electrodes 111 and 113 corresponding with the y-y taxis and the majority of the ions directed toward opening 145 are ejected through the opening in a high energy (20() to 5000 electron volts) high intensity, narrow angle beam whose maximum divergence angle from point P in the y-y direction is determined by the position of point P relative to opening 145. This position of point P is so located that the ejected beam from opening 145 substantially corresponds with the small solid angle of incidence through slits 87, 88 and 13 from point P.
Some isotope particles are deposited on the inside of electrode 113 near the spark between electrodes 111 and 113 and the vibration of electrode 111 back Iand forth a short distance from point P in the mid-plane of electrode 113 passing through axes o o, x-x and slot 130 has the advantageous effect of rapidly re-ionizing these particles and ejecting them from open-end 145 of electrode 113 in a high energy, high intensity, small, solid angle beam corresponding to the required small incident angle of analyzer 61. The isotope is thus removed from electrode 111 and efficiently introduced into analyzer 61 for quantitative and qualitative isotope determinations. The apparatus is then ready for the beginning of a new cycle with the same or a new electrode 111 having a newly adsorbed isotope sample thereon.
This new cycle is begun by :bringing the electrodes 111 and 113 in contact to produce sparks at the proper point to achieve the required small incident ion beam angle. In this regard, the electrodes 111 `and 113 are freely movable as described. In this regard, moving electrode 111 back away from analyzer 61 makes the ion solid angle smaller while moving this electrode toward analyzer 61 makes the ion solid angle larger. Movement of electrodes 113 toward land away from the analyzer has the opposite effect on the solid angle of the ion lbeam and suitable adjustment is made so that the beam O.D. just passes inside the edge of slits 87, 88 and 13.
In one actual example of this invention electrode 111 was heated to about the boiling point of water, and trace elements chemically separated from a sample containing 2 milligrams of U235, which had been bombarded in the Brookhaven Graphite Research Reactor, was applied to the tip of electrode 111 with an eye dropper, whereupon the solution was evaporated to leave the sample 89. Electrodes 111 and 113 were energized to 10 volts while electrode 111 was brought into contact with the inside wall of electrode 113 in the mid-plane through axes 0-0, x-x and slot in a vacuum of 10*6 tor-r. Means 119, 121, 123 and 124 operate by moving a bellows and bellows arms connected to the respective electrodes. Means 119 and 121, for example, move bellows 250 and bellows arm 251 connected to electrode 111. The end of vibrating means 118 butts up against the end of threaded feed means 121 thereby simply to vibrate elec-trode 111 in the same direction that means 121 moves the electrode i.e. in the mid-plane through axes -o, x-x and slot 130. By butting the vibrator 118 up against means 119, 123 or 124 the electrode vibration will correspond to the direction of movement of the electrodes by those means. One advantageous means 118 is a hand engraving tool. As is conventional with such tools, it operated on the vibrating reed or door-bell principle, and the vibration was readily felt by hand although not readily seen by the naked eye. This produced ionizing sparks which ejected Ce, Xe, Tb and other ions in a high density, narrow angle means which passed inside the edges of slits 87, 88 and 13. The potential difference `between round slits 87 and 88 is adjusted to between -20 kilovolts by means of adjustable source 301. By making the potential between slits 87 and 88 big enough and the sparks back far enough from open end 145, the beam can be extracted through slits 87 and 88 in a maximum divergence angle of as low as l". The actual dimensions of t-he electrodes in inches were- Electrode 111:
lOutside diameter tapered from .02 to .120
Tip length .300
Tip angle Tip -distance to slit 88 1A to 3A; for beam passing through slits 87 and 88 Vibration rate 120 cycles/sec.
Vibration distance (of the order of a few thousandths of an inch) Electrode 113:
Outside diameter .120 Inside diameter .070 Length .700 Slot width .035 Slot length .115 Wall thickness .050
The ability to detect Ce isotopes was shown in an actual example of this invention where the total sample was 8 l013 Ce atoms and `Ce-136 corresponding to 2X 1011 -atoms was detectable.
In another embodiment, the electrode 111 comprising a solid conducting material to be tested for trace constituents. In one example, this material is uranium metal, but any conducting solid could be used.
The source in this invention has the advantage of efficiently providing a high intensity, high energy, small solid angle beam of ions and iinds utility for directing ions into a high resolution, double focusing spectrograph for `qualitative and quantitative isotope determinations. Actual tests, for example, have shown the source of this invention to have very high eiciency in providing low detection limits for tra-ces of Xe and rare earths, such as Ce, Tb, etc. Moreover, the source and nozzle of this invention are simple, inexpensive and durable and permit the use of a simple, conventional low voltage source while preventing overheating of the source electrodes and providing ion beam with a relatively low energy spread. Additionally, the system of this invention permits low radiation sources, whereas the samples required heretofore had radiation levels over a curie. Thus this invention decreases the health hazards to the operators while increasing the utility of spectrographs for small amounts of sample material, e.g. from reactor and accelerator bombardment, and high sensitivity for quickly determining nuclear transformation reactions.
What is claimed is:
1. In an ion source for a mass spectrograph having an ion extraction means at one end thereof, the improvement comprising, a first and solid pointed anode having a first axis, a slitted, hollow, cylindrical cathode forming an inside Wall around a uniform diameter well having a second axis at right 'angles to said first axis, said cathode forming a slit in said inside wall parallel with said second axis for inserting said anode from one side of said well to the other side thereof transverse to said second axis, means for energizing said anode and cathode at low voltage for producing ions from solid material in an arc between said pointed anode and the inside wall of said cathode at the point of intersection of said first axis and said wall opposite to said slit, and means for vibrating said anode back and forth in a first plane passing through said slit and intersecting said second axis and the inside wall of said cathode, the direction of said vibration being at right angles to said second axis whereby said arc is pulsed at right angles to said second axis to provide a pulsed jet of ions that is shaped by said anode and cathode to limit the escape thereof in a small solid angle into said ion extraction means along a third axis corresponding to the intersection of said first plane and the inside wall of said cathode in a direction at right angles to the directions of said vibration of said anode and the pulsing arc produced thereby.
2. The invention of claim 1 in which the point of said anode is tapered through said slit to a pointed tip adjacent the inside wall of the cathode to provide a small cross-section arc and to prevent arcing in the cathode slit while blocking the emergence of ions therefrom.
3. The invention of claim 1 in which said vibration is at a rate of about cycles/ second over a small distance to produce a high density jet of ions having a relatively high energy in a narrow velocity band.
4. The invention of claim 4 having means for moving said anode in said first plane relative to said cathode during said vibration thereof for maintaining said pulsing and for providing for re-ionization of material deposited on said cathode from said jet.
5. The invention of claim 1 having means for moving said anode in said first plane relative to said ion extraction means during said vibration for providing a jet of ions having an adjustable solid angle relative to said ion extraction means.
References Cited UNITED STATES PATENTS 2,724,056 l1/l955 Slepian Z50-41.9 2,736,809 2/1956 Bacon Z50-41.9 2,848,620 8/1958 Backus Z50-41.9
RALPH G. NILSON, Primary Examiner.
W. F. LINDQUIST, Assistant Examiner.