US2956169A - Ion pulse generation - Google Patents

Description

Oct. 11, 1960 V F. K'NG ET AL 2,956,169

ION PULSE GENERATION 4 Filed Dec. '7, 1956 o E J o T0 RF SWEEP TO RF SWEEP 4 v 7 VOLTAGE VOLTAGE TO ACCELERATOR TUBE F lg. 2.

I 26 23 I 24 I627 25 ION BEAM ACCELERATOR T R SOURCE I PULSER TUBE TTARGETTTDETEC O INVENTORS waawqw ATTORNEY U d States Patent ION PULSE GENERATION Rutledge F. King, Knoxville, and 'CharlesD. Moak, Oak.

Ridge, Tenn., and Vincent.E.: Parker, Baton Rouge,

La., assignors to the United States ofAmerica as represented by the United States Atomic Energy Commission Filed Dec. 7, 1956, Ser. No. 627,082

2 Claims. c1. 2so-,-s4.s

search utilizes beams of protons, neutrons, or the like to bombard a target containing the material to be investigated, detects any particles or rays emitted by the target, and analyzes the particles or rays as to type and energy. This technique is norm-ally practiced by generating a supply of ions, accelerating the ions to very high energies in an accelerator tube and then periodically deflecting the high-energy beam away from the target by voltages applied to plates adjacent the target, and detecting the radiations emitted by counters placed near the target. In practice, however, the above technique is not satisfactory for at least two reasons. Firstly, deflection of the high energy beam back and forth adjacent the target produces a high background radiation due to the striking of the beam against various materials within the apparatus around the target. That radiation may completely mask the short-duration radiation emitted by the target. Although substantial amounts of shielding have been provided and various compensation methods have been tried, the analyzing equipment cannot be completely shielded from the undesirable background radiation. Secondly, relatively large voltages, large deflectors, and complex and powerful equipment are required to deflect the very high energy particles near the target, greatly increasing the cost of laboratory experiments.

With knowledge of the excessive cost and complexity of the methods of the prior art and in an efiort to provide very short bursts of ions free from background or scattered radiation between bursts, applicants have as an object of their invention the generation of discrete groups of high energy particles unaccompanied by undesirable background radiation.

Another object of the invention is to provide means for producing bursts of charged particles of extremely short duration, and for directing those bursts onto a target.

Yet another object of the invention is to provide a method for generating discrete groups of particles in' bursts of extremely short duration with a minimum power requirement and without complex focussing equipment. These and other objects of the invention will be better understood from the following description of the inven-' tion, when read in connection with the appended drawings, wherein:

Fig. 1 is a block diagram showing suitable apparatus for practicing our invention, and

Fig. 2 illustrates a preferred form of a beam pulser interposed between a conventional ion source and a conventional accelerator tube in our novel apparatus.

According to our invention, ions are generated in an tube.

very high energies and are directed against a target placed ion source and formed into a stream. The stream is very rapidly deflected away from and back to its normal path along the axis of a cylindrical housing. The stream'is continually focussed by suitable means into a sharp, in-

termittent beam along the axis, and exits through an axial aperture into a lens which focusses it into 'an accelerator Theions in each burst are there accelerated to in the high-energy end of the tube. Radi'ations from the target are analyzed or alternatively are directed against a second target, and the resulting radiations from the sec- 1 ond target are analyzed. The analysis is conducted in the interval between incidence of the bursts ofions on the target, and may utilize a scintillation spectrometer, or a:

for various research studies, such as the study of isotopes having a very'short half life. A continuoussupply 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 beam pulser 22,-

where they are focussed into a narrow beam. The continuous ion stream is converted into a focussed beam of short duration, periodic bursts in the beam pulser, which is shown in detail in Fig. 2. An output aperture in the beam pulser admits ions 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, high 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 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 off when bombarded by those ions, bursts of ne'u-1 mitted along the axis of the pulser through canal 11 in a flange 10 which may form the output electrode for a conventional ion source such as that described in Nucleom'cs, September, 1951, page 20. Flange 10 is joined to an apertur'ed, cylindrical cover section 9 forming the top part of a vacuum type 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 for 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 ion beam. Electrodes 1, 3 are operated at the reference potential of-the accel erator, accelerator ground, while the negative voltage for the focussing lens 2 is supplied through the lead;

Patented ea. 11, 1960 markedfTdLens Voltage. 'Ihe cylindricalelectrod fi may be stainless steel, while the insulators may be pyrex glass, for example. Voltage' for the lens 2 is determined as in" conventional design ofunipoten-tial or Einzel lenses. Electrodes. 1, 3 are provided'with annular lips 37, 38 which separate the R.F. fields on the deflection plates from the DC. fields of the lens. Ring type rubber gase kets 12 are used between the various components.

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 IHOHIIF.

ing strips31, 32, which are carried by lavite insulating ring 15. The plates are mounted to diverge downwardly and symmetrically about the axis of the housing. The angle of divergence is not critical, but at least about 4 degrees is preferred to prevent ions from striking the plates when the deflecting voltage passes through zero. Similar deflection plates 16, 17 may be disposed adjacent the lower lens electrode 3, and converge at the same angles as above stated symmetrically toward the housing axis and toward aperture 7 in the lower cover. The plates 16, 17 are shown rotated 90 degrees from their preferred position for clarity in the drawing. While use of this second set of deflecting electrodes is preferred, it is not necessary for operation in a satisfactory manner. Oscillatory sweep voltages for the deflection electrodes are supplied through phase shift networks 35, 36, if desired, and Kovar seals 19, 19, which extend through the ring 8 and cover section 9. The lead wires may be tied to lugs which are conductively connected to the plates through strips 3134.

In typical operation, ions are generated in the source and then accelerated by conventional probe extraction in a stream downward through the canal 11 by application of a potential of 1.5- kilovolts positive to accelerator ground to the probe electrode of the ion source, while the flange contacting the source is at that ground potential. The ions are directed toward aperture 7, which admits them to a conventional lens to focus them into a suitable accelerator. When ion pulses are desired, an R.F. sweep or oscillatory voltage is applied between electrodes 13, 14 to deflect the beam Within the pulser first to one side and then to the other. No significant difference results when the sweep voltage is also applied to electrodes 16, 17, or when those electrodes are grounded or omitted. In any event, a burst of ions will emerge from the aperture 7 only twice per cycle; that is, while the deflection potentials are passing through approximately zero. With a sufficiently rapid frequency of de flection and a sufliciently high deflection potential, only an extremely short burst of ions will be passed through the aperture '7. For example, a 500 kilocycle signal of 5000 volts and a sinusoidal waveform may be utilized to produce a pulse of protons having a width at half maximum of 6.6 millimicrosec. A square waveform gives better results, but results in higher power losses.

In many experiments, an interval, such as 1 microsecond, should be allowed for the radiations from the target to reach the detector and be analyzed. That interval which determines the burst repetition rate, may be so long that the bursts themselves are not sufliciently short in duration or sharply defined. Yet application of higher frequencies would cause ion bursts to reach the target during the analysis. It has been found that if the lower deflection plate assembly is rotated 90 degrees about the axis of the unit from the position shown and if the de flection potentials are applied in the proper phase relation, the ion beam will move in a somewhat elliptical path so that it is aligned with the aperture 7 only once instead of twice per cycle. The desired burst rate may then be achieved by doubling the frequency of the deflection voltage applied. For the same voltage magnitude, the use of this double frequency produces a much faster rise time in the deflection voltages, and thus results n m h rper, .hetter efinedbur ts q t usr ast u 9t. shorter periods. The voltages are applied to the upper and lower deflection plates about degrees out of phase, to prevent alternate bursts from the upper plates from leaving via aperture 7. To so deflect alternate bursts away from aperture'7; the voltage applied to the lower plates may-,be ahalf-wave rectified -A.C. or a single square wave per cycle. The phase shifters 35, 36 may be adjusted to change the, phase angle tocompensate; for the effect of ion transittimebetween the sets of plates.

Other equivalent methods and apparatus for generating short well-defined bursts of high energy particles will suggest themselves to those skilled in the art from the teachings of 'oui invention, It-is apparent that we have provided for the first time, a.meth od of generating very short bursts of particles having very high energies uncontaminated by radiation background normally attendent upon such operations, and a; method which is cheaper and simpler since it obviates the necessity for very high dcflection voltages andlarge expensive equipment.

Having described our invention, what is claimed as novel is:

1. In an accelerator for directing high energy ions against a target, said accelerator including a source of ions, an accelerator tube through which said ions are directed to said target, a lenstofocus ions from said source into said accelerator tube, and means for developing high voltages for accelerating said ions, the improvement which comprises means for forming and introducing sequentially into said accelerator tube discrete groups of ions comprising: a housing disposed between said ion source and said lens and provided with two end walls, one of said walls being provided with an axial passageway communicating with said ion source to receive an ion stream and the other of said walls being provided with an axial passageway communicating with saidlens, first and second deflecting plates disposed within said housing in confronting relationship adjacent the aperture of said one wall and oriented to diverge away from said; aperture, ion focussing means disposed between said deflecting plates and the other of said walls and concentrically about the axis of said housing, third and fourth confronting deflecting plates disposed between said focussing;

means and said other wall and oriented to converge toward said other wall, said first and second deflecting plates being oriented on opposite sides of said beam, said third and fourth deflecting plates being oriented, respectively,-

90 from said first and said second plates around the axis of said beam, a source of focu ssing voltage coupled to said focussing means, and a source of an oscillatory voltage coupled to said deflecting plates to move said ion stream back and forth across said other wall aperture to allow ions to enter said accelerator tube through said lens.

2. A device for generating periodic, short duration groups of highly energetic particles comprising means for generating continuously relatively low energy ions in an ion source, a housing provided with an entrance aperture and an exit aperture, means for forming said ions into a continuous stream directed along a straight path and through said entrance aperture of said housing, means disposed in said housing for focussing said stream into a relatively narrow beam, means disposed in said housing for deflecting said stream away from said path in first and second directions prior to said focussing, means for deflecting a portion of said beam prior to its arrival at said exit aperture in said housing in third and fourth directions displaced substantially 90 from said first and.

said second directions, respectively, subsequent to focussing, means for removing a portion of said beam along said path periodically through said exit aperture as the beam crosses said path, means for accelerating those particles which are removed to relatively high energies, and means for directing said high energy particles upon a, primary target.

(References 011 following page) References Cited in the file of this patent UNITED STATES PATENTS Davisson Oct. 8, 1940 Kallmann et a1. June 23, 1942 Schade July 16, 1946 5 Washburn Dec. 31, 1946 Law et a1. Feb. 12, 1952 6 Berry Oct. 5, 1954 Tinlot et a1. Oct. 30, 1956 Adler Jan. 14, 1958 Pollock Jan. 6, 1959 FOREIGN PATENTS Great Britain Feb. 23, 1955

Images