US3164718A - Ion pulse generator comprising deflector means to sweep an ion beam across an apertured member - Google Patents

Description

Jan. 5, 1965 R. F. KING 3,164,718

ION PULSE GENERATOR COMPRISING DEFLECTOR MEANS T0 swEEP AN ION BEAM ACROSS AN APERTURED MEMBER Filed April 11, 1962 2 Sheets-Sheet 1 FROM ION SOURCE 9 5-" I 9 4o I 32 I R;

I l I i i FREQUENCY I\5J:J I I I, FREQUENCY MULTIPLIER x I I II 4 MU,LTIPLIER //,/,Y/ I

I I I w m I I If I 2Q I I I 2 I l/ :1 E l I 8 6' i2: II I 20 r I I I I, /////I/ I! I II I III III I //5 I PHASE 38 I. PHASE SHIFT I 9 SHIFT \35 I II I I I I V 3 l 1 I 1 33 34 IsII r I n TO RF SWEEP TO RF SWEEP VOLTAGE 5/ I VOLTAGE T0 ACCELERATOR TUBE 1 I LATERAL SHIFT /42 D.C.VOLTAGE Fig. 2.

2I I 22 26 23 I ,24 27 25 ION BEAM ACCELERATOR TARGET DETECTOR SOURCE I PULSER TUBE I Fig. 1. INVENTOR. Rut/edge F. King ATTORNEY.

Jan. 5, 1965 R. F. KING 3,164,718

ION PULSE GENERATOR COMPRISING DEFLECTOR MEANS TO SWEEP AN ION BEAM ACROSS AN APERTURED MEMBER Filed April 11, 1962 2 Sheets-Sheet 2 INVHVTOR. v Rutledge F. King BY m, a W

ATTORNEY.

ted states The present invention relates to the art of generating a series of groups of energetic ions, and more especially to a novel apparatus for generating periodic short-duration bursts of such ions and at the same time eliminate undesired masses from the ion beam.

The present invention is an improvement over the ion beam pulser described in US. Patent No. 2,956,169, issued October 11, 1960. In this prior patent, the ion beam was swept from side to side so as to provide bursts of ions emanating through an exit aperture into an accelerator tube. In one modification of this patent, the beam sweep path was straight for one-half cycle so as to cross the exit aperture and semi-elliptical on the return path so as to miss the exit aperture. This was accomplished by using a square-wave signal on one set of deflection plates and a sine-wave signal on the other set of deflection plates.

However, it is not possible with the prior patented device to obtain ion bursts of shorter duration without at the same time increasing the repetition rate. In order to obtain ion bursts of shorter duration without changing the repetition rate, the present invention was conceived. Another shortcoming of the prior patented device is that it did not provide for any mass diiferentiation of the various ionic masses that are present in the incoming ion beam.

With a knowledge of the above-mentioned shortcomings of the prior device, it is a primary object of this invention to provide an ion beam pulser in which shorter-duration ion bursts can be produced without increasing the repetition rate.

It is another object of this invention to provide an ion beam pulser in which mass discrimination can be achieved while at the same time producing shorter-duration ion bursts without increasing the repetition rate.

These and other objects and advantages of this invention will become obvious upon a consideration of the following detailed specification and the accompanying drawings wherein:

FIG. 1 is a block diagram showing suitable apparatus for practicing this invention;

FIG. 2 illustrates a preferred form of a beam pulser interposed between a conventional ion source and a conventional accelerator tube in the present invention; and

FIGS. 3a, 3b, 3c and 3d are a series of sketches showing a Lissajous beam pattern and variations of the same, as used in the present invention.

The above objects have been accomplished in the pres ent invention by connecting a first source of high frequency to the first set of deflection plates in the beam pulser, by connecting a second source of high frequency, of some multiple of the first frequency, to the second set of deflection plates of the beam pulser, and by connecting a lateral D.C. shifting voltage to the first set of dell ction plates to achieve mass differentiation of the ion masses in the ion beam fed to the beam pulser. The use or" the different high-frequency sweep voltages across the deflection plates will produce a Lissajous-type sweep such that with the proper phase relationship between the sweep voltages and with the proper shift of the sweep with a D.C. voltage, a selected mass of ions can be withdrawn from the beam pulser once each cycle so as to produce the desired ion burst. The use of a Lissajous-type sweep permits the device of the present invention to produce a burst of ions assists with a duration shorter than that of the above prior patent while at the same time having a repetition rate comparable to that of the prior patent. The ions in each burst may then be accelerated by an accelerator tube to very high energies and directed against a target, containing the material to be investigated, placed in the high-energy end of the tube. Radiations from the target are analyzed or alternatively are directed against a second target, and the resulting radiations from the second target are analyzed. The analysis is conducted in the interval between incidence of the bursts of ions on the target and may utilize a scintillation spectrometer, or a detector connected to a pulseamplitude analyzer, for example.

Referring now to FIG. 1, suitable apparatus is shown for various research studies, such as the study of isotopes having a very short half-life. A continuous sup ly of ions is generated in an ion source 21, which may be of any suitable conventional type. Ions from the source are led through an aperture into the beampulser 22, where they are focussed into a narrow beam. The continuous ion stream is converted into a focussed beam of short-duration, periodic bursts of a selected mass in the beam pulscr, which is shown in detail in FIG. 2. An output aperture in the beam pulser admits ions of the selected mass periodically into a conventional lens 26, which focusses them into accelerator tube 23, where they are accelerated in the normal fashion to the high energies desired. After acceleration, the ions impinge upon target 24, which contains some element capable, when bombarded by the ions accelerated in the tube, of emitting radiations. These radiations are detected by a detector 25. By withdrawing only very short bursts of ions from the beam pulser for acceleration, only short bursts or groups of high-energy ions will arrive at the target, and there will be no beam in the neighborhood of the target during the periods between ion bursts. Therefore, there will be no interference by any continuous, higi -energy ion beam with the detector, and the radiations emitted by the target may be analyzed at the detector without error.

If ions are generated in the ion source, pulsed by the beam pulser, and accelerated by the accelerator tube, then used to bombard a target 24, and if the target is of such material that neutrons are given oil when bombarded by those ions, bursts of neutrons having a known time of departure from the target may be produced. Knowing the departure time, the energy of each neutron detected may be determined by measuring its time of flight over a predetermined course to the detector. In addition, neutrons from target 24 are caused to strike a secondary target 27, and the ernanations from the secondary target are analyzed. If desired, the secondary target 27 may be omitted.

FIG. 2 illustrates one suitable form of beam pulser in vertical cross section. Ions from the ion source are ad mitted along the axis of the pulser through a canal 11 in a flange 10 which may form the output electrode for a conventional ion source such as that described in Nucleonics, September 1951, page 20, or in US. Patent No. 2,975,277 to Von Ardenne, issued March 14, 1961. Flange It) is joined to an apertured, cylindrical cover section 9 forming the top part of a vacuumstype housing. Beneath the upper cover are disposed annular insulators 4, 5. An annular ring 8 and a lower axially apertured, cylindrical cover plate 6 complete the housing of the beam pulser. The unipotential or Einzel lens system comprises three cylindrical lens electrodes 1, 2, 3 disposed in vertical alignment about the axis of the beam pulscr. Electrodes 1, 3 are operated at the reference; potential of the accelerator, accelerator ground, while the negative voltage for the focussing lens 2 is supplied through the lead marked To Lens Voltage. The cylindrical electrodes 1, 2, 3 may be stainless steel, while the insulators 4, 5 may be Pyrex glass, for example. Voltage Patented Jan. 5, 1965 for the lens 2 is determined as in conventional design of unipotential or Einzel lenses. Electrodes l, 3 are provided with annular lips 37, 38 which separate the H36. fields on the deflection plates from the DC. fields of the lens. 'Ring-type rubber gaskets 12 are used between the various components 7 v Positioned Within the pulser and spaced approximately equally between the upper lens electrode 1 and the flange 10 are a pair of confronting flat, trapezoidal- shaped tantalum'deflection plates 13, 14 supported on metal mounting strips 31, 32 which are carried by a lavite'an nular insulating ring 15. The plates are mounted to diverge downwardly and symmetrically about the axis 90 degrees from their preferred position for clarity in the An oscillatory sweep voltage with a first frequency is connected by leads to the deflection plates 16, 17 through Kovar seals 19 which extend through the ring 8. The

leads are tied to lugs which are conductively connected to V the plates 16, 17 through strips 33, 44. This sweep voltage is also connected through phase shifts 35, 36 and frequency multipliers 4d, 41 to the upper deflection plates 13, 14 through Kovar seals 19 by suitable lead wires which are tied to lugs which are conductively connected to the plates 13, 14 through strips 31, 32. The frequency of the sweep voltage tothe plates 13, 14 is some selected multiple of the swcep'voltage frequency connected to plates 16, 17. The operable frequency ratio may be 3:1, 5:1, or 7:1, for example. The choiceof specific frequencies is'somewhat governed by the'desired pulse width and the repetition rate. It should be noted that a reversal of theseapplied sweep voltages would accomplish the same result. 1 V

A lateral shift DC. voltage 42 is also connected across the deflection plates to which the lower-frequency sweep voltage is connected, and as shown in FIG. 2 this DC. voltage is connected across the deflection plates 15,17.

The use of one frequency, f, on one set of deflection plates and a frequency of 3f on the other deflection plates located 90 from the first set will produce a Lissajoustype pattern such as that shown in FIG. 3a. If the phases are properly adjusted, the trace will double back on itself as shown in FIG. 3b. A slight change in phase then re sults in a pattern such as shown in FIG. 30. Referring specifically to FIG. 3c, if the ion beam is moved in a pattern symmetrical about the ion exit aperture (indicated by the dashed circle), the beam would not pass through the aperture. However, if the entire orb-it is shifted, the effective position of the aperture is as indicated by the complete circle; This shifting of the entire orbit is effected by the use of the lateral shift DC. voltage 42 connected to the deflection plates 16, 17. Thus, the beam will cross the aperture once each cycle so as to produce the desired ion burst. When the incoming ion beam from the ion sourcecontains ions of different masses, for example Hd, H and H5 the beam pulser will produce three similar Lissajous-type sweeps which are displaced each from the others depending upon the difference in mass because the time of flight through the pulser and lens is different for each mass. This is illustrated in FIG.

3d. Thus, mass differentiation can be accomplished by selecting the value of the lateral shift D.C. voltage 42 such that the'des'ired mass orbit will intersect'the ion exit aperture once each cycle so as to produce the desired ion burst.

A specific example of the operation of the beam pulser of FIG. 2 will now be given. The ion beam from. the ionsource,which may contain H Hf, and Hf, for example, is injected into the beam pulser in a manner such as set forth in the above-mentioned prior patent. A 3 mc. sinusoidal signal is applied to the upper deflection plates 13, i4, and a1 mc. sinusoidal signal is applied to the lower deflection plates 16, 17. These signals are maintained about 20 out of phase, for example, to create the desired ion orbits for the different masses in the injected ion beam. This phase'relationship is adjusted to optimize the output beam as to magnitude of the desired ion and elimination of the undesired ions. It should be noted that the ion energy and the length of the ion path through the pulser also influence the particular phase relationship of these signals applied to the "deflection plates. 1 J

A potential of about, 300 volts'D.C., for example, is applied across the deflection plates to which the 1 inc. signal is applied. This produces the desired lateral shifting required tocause the ion path of the desired mass to cross the ion exit aperture onch each cycle. This DC. potential is also adjustable in order to optimize the output ion pulses as to magnitude and mass differentiation. Adjustments of this DC. voltage permit the production of intense, essentially pure Ht, H or Hgr. Thus, it can be seen that the device of FIG. 2 can produce short-duration pulses of a selected mass depending upon adjustments of the voltages to the'beam" pulser. The pulses thus 7 produced are of shorter duration than those of the above prior patent and have a repetition rate comparable to that of the prior patent. higher sweep frequencies and producting a faster sweep.

This invention has beendescribed by way of illustration rather than limitation and it should be apparent that this invention is equally applicable in fields other than those described;

What is claimed is: g

1. In a device for generating'periodic, short-duration groups of highly energetic particles comprising means for generating continuously relatively low energy ions of different masses in an ion source, a housing provided with an entrance aperture an an exit aperture, means for forming said ions into a continuous stream directed along a straight path and through said entrance aperture, means disposed in said housing for focussing said stream into; a relatively narrow beam, first deflecting means disposed in said housing for deflecting said stream away from'said path prior to said focussing, second deflecting means displaced from said first deflecting means for deflecting a portion of said beam prior to its arrival at said exit aperture in said housing subsequent to focussing, and a source of focussing voltage coupled to said focussing means, the improvement comprising a first source of (iscillatory voltage with a first frequency f coupled to said second deflecting means, a second source of oscillatory voltage with a second frequency of a selected multiple of said first frequency and being coupled to said first deflecting means and being adjustable in phase with respect to said first source, said first and second sources of oscillatory voltage deflecting said beam to produce a separate Lissajous-type beam sweep Within said housing for each of the ion masses provided by said ion source, an adjustable source of DC. voltage coupled to said second deflecting means to provide a selected lateral shift to said. beam sweeps such that only the beam of a selected mass crosses said exit aperture once during each cycle of the sweep of the selected mass, and means for removing and accelerating. the ions of said selected mass from said exit aperture as the beam periodically crosses said exit aperture and directing said selected ions upon a primarytarget.

2. The device set forth in claim 1, wherein said first frequency f is about 1 mo, said second frequency is about 3 mc., and said second source being about 20 out of phase with said first source.

This is'accomplished by using References Cited by the Examiner UNITED STATES PATENTS s OTHER REFERENCES Cathode-Ray Oscillograph, Type TMU-122-B operat- Beny ing instructions and service notes, RCA Manufacturing i; 328 229 (30., Inc., Camden, N.I., publication number IB-23339, Charton 5 pages 5 to 9, received in Division 54, September 7, 1935.

Hofiman 315-8.5

King at a} 250 84.5 RALPH G- NILSON, Primary Examiner.

Claims (1)

1. IN A DEVICE FOR GENERATING PERIODIC, SHORT-DURATION GROUPS OF HIGHLY ENERGETIC PARTICLES COMPRISING MEANS FOR GENERATING CONTINUOUSLY RELATIVELY LOW ENERGY IONS OF DIFFERENT MASSES IN AN ION SOURCE, A HOUSING PROVIDED WITH AN ENTRANCE APERTURE AN AN EXIT APERTURE, MEANS FOR FORMING SAID IONS INTO A CONTINUOUS STREAM DIRECTED ALONG A STRAIGHT PATH AND THROUGH SAID ENTRANCE APERTURE, MEANS DISPOSED IN SAID HOUSING FOR FOCUSING SAID STREAM INTO A RELATIVELY NARROW BEAM, FIRST DEFLECTING MEANS DISPOSED IN SAID HOUSING FOR DEFLECTING SAID STREAM AWAY FROM SAID PATH PRIOR TO SAID FOCUSSING, SECOND DEFLECTING MEANS DISPLACED 90* FROM SAID FIRST DEFLECTING MEANS FOR DEFLECTING A PORTION OF SAID BEAM PRIOR TO ITS ARRIVAL AT SAID EXIT APERTURE IN SAID HOUSING SUBSEQUENT TO FOCUSSING, AND A SOURCE OF FOCUSSING VOLTAGE COUPLED TO SAID FOCUSSING MEANS, THE IMPROVEMENT COMPRISING A FIRST SOURCE OF OSCILLATORY VOLTAGE WITH A FIRST FREQUENCY F COUPLED TO SAID SECOND DEFLECTING MEANS, A SECOND SOURCE OF OSCILLATORY VOLTAGE WITH A SECOND FREQUENCY OF A SELECTED MULTIPLE OF SAID FIRST FREQUENCY AND BEING COUPLED TO SAID FIRST DEFLECTING MEANS AND BEING ADJUSTABLE IN PHASE WITH RESPECT TO SAID FIRST SOURCE, SAID FIRST AND SECOND SOURCES OF OSCILLATORY VOLTAGE DEFLECTING SAID BEAM TO PRODUCE A SEPARATE LISSAJOUS-TYPE BEAM SWEEP WITHIN SAID HOUSING FOR

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