US3408494A - Particle velocity sorter using an r.f. transverse electric space harmonic and a transverse bias field - Google Patents

Description

Oct. 29, 1968 P. M. LLEWELLYN 3,408,494 PARTICLE VELOCITY SORTER USING AN R.F'. TRANSVERSE ELECTRIC PACE HARMONIG AND A TRANSVERSE BIAS FIELD 3 Sheets-Sheet l 5 Filed April 19, 1966 1% 2h 5% 4%? PHASE SHIFT PER PERIOD x SYNCHRONOUS S1 (v,)|o-

. NON SYNCHRONOUS ION ZERO ELECTRIC 3 FIELD REGION 1 0.0.

1 i INVENTOR. n-ncxg j/ PETER M. LLEWELLYN Y w-yw ATTORNEY Oct. 29, 1968 PARTICLE VELO SPACE Filed April 19, 1966 P.- CITY SORTER M. LLEWELLYN USING AN R.F. TRANSVERSE ELECTRIC HARMONIC AND A TRANSVERSE BIAS FIELD FIG. 4

r 1,.\\ 42 1 46 lg 3 Sheets-Sheet 2 FIG. 5

INVENTOR.

PETER M. LLEWELLYN /a yzflan ATTORNEY Oct. 29, 1968 P. M- LLEWELLYN 3,408,494 PARTICLE VELOCITY SORTER USING AN R.F. TRANSVERSE ELECTRIC SPACE HARMONIC AND A TRANSVERSE BIAS FIELD Filed April 19, 1966 3 Sheets-Sheet 3 FIG. 8 55 52 SCAN 8 f VOLTAGE INVENTOR.

PETER M. LLEWELLYN BY My M ATTORNEY United States Patent Olfice 3,408,494 PARTICLE VELOCITY SORTER USING AN RF. TRANSVERSE ELECTRIC SPACE HARMONIC AND A TRANSVERSE BIAS FELD Peter M. Llewellyn, Menlo Park, -Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Apr. 19, 1966, Ser. No. 543,743 10 Claims. (Cl. 25041.9)

ABSTRACT OF THE DISCLOSURE A charged particle velocity sorter is disclosed. The sorter includes an ion source for forming and projecting a stream of ions over ,a predetermined beam path. A periodic wave supportive structure is disposed along the beam path for producing a spacially periodic radio frequency electric field transverse tothe beam path. The spacially periodic radio frequency field is selected to have an even numbered spacial harmonic component at the frequency of the radio frequency field for cumulative electronic interaction with charged particles of the beam to produce a net average transverse deflecting force tending to force the particles out of the beam which have a certain predetermined synchronous velocity. A DC. bias potential is also applied to the radio frequency wave supportive structure for producing a coextensive static electric field for applying a second transverse electric field to the charged particles of the beam. The static field is selected to have a magnitude to produce a net average transverse deflecting force on the synchronous particles which is equal and opposite to that produced by the synchronous radio frequency field such that charged particles having a certain predetermined synchronous velocity remain in the beam path and are detected at the end thereof, while nonsynchronous particles are removed from the beam. The velocity sorter is especially useful in mass spectrometers since with a given beam voltage only ions having a certain charge-to-mass ratio have the synchronous velocity to be detected. A scan of the beam voltage thus produces a scan of the charge-to-mass ratios of the ionized particles making up the beam.

The present invention relates in general to velocity sorters and, more particularly, to an improved velocity sorter using a synchronous phase transverse R.F. electric field for velocity sorting of charged particles and a transverse bias field for removing non-synchronous particles from the beam. Such an improved velocity sorter has all the advantages of size, weight and cost attributable to other types of pure electric sorters, while yielding mass resolution, when employed in a mass spectrometer, of about 500 and enhanced sensitivity as of 10 amps/torr.

Heretofore, periodic R.F. structures have been employed to obtain velocity sorting of an ion beam by producing synchronous interaction between the ion beam and the transverse radio frequency electric fields of the periodic structure. The transverse periodic structure was operated in the 1r mode, i.e., 180 of phase shift of the transverse R.F. field per section of the periodic structure. In this manner ions within the beam having synchronous velocities were undeflected by the transverse R.F. fields. Other ions having non-synchronous velocities would experience a net deflection and thus would be deflected out of the beam. The synchronous ions were then passed into either a magnetic or electrostatic deflector for mass analysis. This prior art mass spectrometer is described inPhysical Review, volume 40, page 429 et seq., published May 1, 1932.

The problem with this prior art mass spectrometer was 3,408,494 Patented Oct. 29, 1968 that the velocity sorting was imperfect inasmuch as ion velocities which were synchronous with the higher odd space harmonics of the periodic structure, i.e., (2l1+1)1r phase shift per section, also passed without deflection. Furthermore, certain other non-synchronous ions were deflected very little and thus passed into the mass analyzer together with other peaks of unknown or unspecified origin.

In the present invention, the periodic radio frequency transverse field structure of the charged particle, i.e., ion or electron, velocity sorter is operated in an even numbered space harmonic, i.e., where there is an even number of, preferably 2, 1r phase shifts per section of the periodic structure at the synchronous particle velocity, whereby synchronous velocity particles see a net transverse deflecting R.F. electric field. In addition, a bias transverse electric field, preferably static, is applied to the particle beam of opposite phase to the net R.F. field and of an amplitude to produce a net displacement equal and opposite to that of the RF. displacement, whereby syn chronous particles are undeflected by the velocity sorter. However, non-synchronous particles are deflected out of the beam. In a preferred embodiment, the transverse static and RF. fields are applied over substantially coextensive periodic regions of the particle beam-field interaction region, whereby the synchronous particles are undeflected over any portion of their interaction region such that synchronous particle trajectories remain in the plane of symmetry of the periodic structure.

The principal object of the present invention is the provision of an improved electric field charged particle veloc ity sorting apparatus and improved mass spectrometers using same.

One feature of the present invention is the provision of an improved charged particle velocity sorter employing means for producing a periodic R.F. field transverse to the particle beam with a space harmonic wave of the RF. field synchronous with particles of a certain velocity to produce a net deflecting force on the synchronous particles and including means for producing a bias electric deflecting force on the particle beam of equal and opposite magnitude to that of the synchronous harmonic wave, whereby synchronous particles remain in the beam and non-synchronous particles are deflected out of the beam.

Another feature of the present invention is the same as the preceding wherein the bias and RE electric fields are applied to the beam over substantially coextensive periodic regions of the beam path.

Another feature of the present invention is the same as any one or more of the preceding wherein the bias field is derived, as by rectification, from the RF. field source, whereby the RF. and bias electric field intensities have like fluctuations to maintain balanced focusing conditions for synchronous particles.

Another feature of the present invention is the same as any one or more of the preceding wherein the synchronous particles are synchronized with the space harmonic of the R.F. field having 21r phase shift per period of the RP. field.

Another feature of the present invention is the same as any one or more of the preceding wherein the velocity sorter is employed in a mass spectrometer for mass analyzing an ion beam of nearly constant beam voltage and followed by an ion detector for detecting the ions sorted by the ion sorter, whereby the ions are separated according to their charge-to-mass ratio.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic diagram, partly in section, of an ion velocity sorter employing features of the present invention,

FIG. 2 is a plot of frequency versus phase shift in radians per period of the periodic ion sorter structure of FIG. 1 and showing several different ion velocities,

FIG. 3 is a composite line diagram of electric field strength for the various spacial harmonics of the ion sorter structure of FIG. 1 and also showing the ion trajectories for synchronous and non-synchronous ions,

FIG. 4 is a schematic diagram of a mass spectrometer employing the ion sorter of FIG. 1,

FIG. 5 is an enlarged sectional view of the structure of FIG. 4 taken along line 5-5 in the direction of the arrows,

FIG. 6 is a transverse sectional view of an alternative ion sorter structure to that of FIGS. 4 and 5,

FIG. 7 is a fragmentary sectional view of the structure of FIG. 6 taken along line 77 in the direction of the arrows,

FIG. 8 is a fragmentary sectional view of the structure of FIG. 6 taken along line 8-8 in the direction of the arrows, and

FIGS. 9 and 10 are a top and side elevational view, respectively, partly broken away, of a mass spectrometer employing features of the present invention.

Referring now to FIG. 1, there is shown an embodiment of the ion sorter of the present invention. The ion sorter 1 comprises a periodic radio frequency conductive structure arranged along an ion beam path 2 for electronic interact-ion between the radio frequency transverse electric fields of the ion sorter 1 and the ions of the beam passable therethrough. Although the R.F. periodic structure may have various forms, in one embodiment, as shown in FIG. 1, the structure comprises a pair of mutually opposed conductive comb like members 3 and 4 as of stainless steel disposed straddling the ion beam 2. The ion beam 2 is preferably of thin regtangular cross section or sheet form with the plane of the sheet beam defining the midplane between the combs 3 and 4. The teeth 5 and 6 of the comb members 3 and 4 are disposed in transverse registration. The comb members are electrically insulated from each other and excited with radio frequency energy derived from a radio frequency source 7 at a suitable operating frequency as of 2 to 4 mHz. In addition, a transverse bias electric field which is preferably static, but need only be essentially static during the passage of a synchronous ion, i.e., for 10- sec. is applied across the comb members 3 and 4. A static bias voltage is derived from a D.C. voltage supply 8 which is isolated from the radio frequency source 7 via isolating capacitor 9.

An ion collector electrode 11 is disposed at the terminal end of the beam path 2 for collecting ions which have passed through the ion sorter 1 without being deflected out of the beam path 2. A current meter 12 or other suitable current measuring device is connected in circuit with the collector electrode 11 for measuring the collected ion current. The ion beam source is not shown in detail and may have various forms depending upon the intended use of the ion sorter 1. A source 13 is schematically indicated by a repeller electrode 14 pushing the positive ions into the ion sorter 1 through a beam entrance aperture 16 in an accelerating electrode 17 with velocities determined by the beam voltage V supplied by a voltage supply connected between the accelerating electrode 17 and the repeller 14.

In a preferred embodiment of the ion sorter 1, the periodic R.F. structure is operated in the 21r space harmonic mode. More particularly, the 21r mode corresponds to a condition wherein there is 360 of phase shift in the R.F. transverse electric field per period of the periodic R.F. field as seen by an ion in the beam moving with a certain predetermined synchronous velocity V Ions having this predetermined velocity V will then see the same phase of the radio frequency field in successive 4. periodic transverse R.F. field regions. The ion velocity V period A, and driving radio frequency f are related by the following equation:

V fok Conditions specified by Equation 1 are plotted in the diagram of FIG. 2, wherein it is seen that for a velocity V; and a driving frequency of f there is 21r phase shift per period A of the periodic structure determined by the solid dot 18 defined by the intersection of lines V and f The period A of the periodic structure is the distance taken along the direction of the beam path between like positions, i.e., center points, in adjacent regions of transverse electric field. From the diagram of FIG. 2 it is seen that ions having velocities harmonically related such as 2V and subharmonically related such as %V and /2V have certain constant phase relations related by integral numbers of of phase shift per section. In particular, ions with velocities of 2V and %V experience 180 of phase shift per period of the R.F. field. Thus, for these ions the net R.F. deflection should average to zero over an even number of periods of the field. On the other hand, even number subharmonic ion velocities of V such as V /2, V /4, V 6, etc. have the possibiilty of experiencing a net R.F. transverse deflection out of the beam because the field is in the same phase in successive R.F. interaction regions. However, for these subharmonic velocities, as best seen in FIG. 3, unless the width of the interaction region W is reduced compared to the optimum for the 211' mode then the subsynchronous ion stays in each of the periodic interaction gaps for too long a period such that the deflection forces average out during the interaction time in each periodic interaction region. This is best seen in FIG. 3 where for the V 2 ion velocity (411- space harmonic mode) the electric field, at the driving frequency f averages to zero during the time it takes the ion to traverse the relatively large M2 interaction region. However, if the field interaction region W were reduced to A of A then the deflection would not average to zero. Thus for the other even subharmonic velocities they can be made to produce a net transverse deflection by reducing the W/)\ ratio to l/N where N is the number of the even number of 1r radians phase shift per period of the R.F. field at the particular subharrnonic ion velocity. In other words, even space harmonic interaction, i.e., deflection is enhanced for a particular harmonic by making the W/x ratio equal to 1/N. Thus for the preferred 21r mode of operation the W/)\ ratio is /2.

Actually, it may be desirable under certain conditions of sensitivity and resolution to operate the ion sorter on the fundamental Zrr mode of operation with W/)\ ratio less than 1/2 in order to obtain enhanced resolution of the 21r mode velocity ions, i.e., ions of velocity V While some subharmonic velocity ions may have enhanced deflection with these reduced W/A ratios, use of the maximum intensity DC. bias field should serve to discriminate against these ions. Reduction of the W/)\ ratio should help to reduce effects caused by phase shift of the transverse field in the time it takes the ion to traverse the transverse R.F. field.

The transverse bias electric field serves to provide a transverse deflection or bias force on the ions of the beam. The bias field strength is proportioned relative to the R.F. field strength of the desired 211' mode such that the average transverse deflection obtained by the 271' mode R.F. electric field is just oppositely balanced by the average transverse deflection force derived from the bias field. Since maximum net R.F. transverse deflection is obtained for ions having a velocity corresponding to the 21r mode, assuming the W/)\ ratio is /2, then, with the bias field adjusted to just balance this deflection, all other ions are focused out of the beam. This occurs because the net R.F. transverse deflection is always less for these other velocity ions than for the synchronous 21r mode ions.

The peak R.F. electric field intensity to obtain balanced v sorting out ions of a certain R.F. and D.C. deflection forces for the 21r mode, assuming both transverse fields are applied to the beam over coextensive periodic regions, is that value which produces a ratio of D.C. field intensity to peak R.F. field intensity of approximately 0.435. Aconvenient way of maintaining this ratio of electric field intensities, in practice, is to rectify a portion of the R.F. output of the R.F. source 7 and use this rectified and filtered energy to derive the applied transverse D.C. voltage.

The ion velocity sorter 1 may be used to advantage for predetermined velocity V and passing these ions to a detector or other utilization device, such as a magnetic mass analyzer. The more the number of periodic sections in the ion sorter 1 the higher the resolution. A typical ion sorter of the present invention included 40 periodic sections and gave a resolution of about 300 to 50% points on the output velocity spread. The ion sorter 1 of FIG. 1 may also be used to ad vantage as a charge-to-mass analyzer of a mass spectrometer since ions accelerated under the influence of a certain beam voltage attain velocities proportional to their chargeto-mass ratios according to the following equation:

where e is the charge on the ion, V is the beam voltage and m is the mass of the ion. The 211- ion velocity which is sorted by the ion sorter 1 is such that the mass of the ion is directly proportional to the beam voltage V. This being the case, a linear scan of V brings to a focus at the detector successive ions separated in mass units such that a linear scan by mass units of the ions is obtained. Moreover, assuming a fixed uncertainty in the beam voltage of, say 2 v., the mass unit resolution improves in a linear manner with increased mass units of the ions. Thus, the higher the number of mass units in the ion the higher the resolution obtained. In use, the ion sorter 1 is evacuated to a suitable operating pressure as of 10- torr.

Referring now to FIG. 4 there is shown a mass spectrometer using the ion sorter l of the present invention and previously described with regard to FIGS. 1-3. The ion sorter 1 is dimensioned for 211' mode operation at \=1 om., f =1 rnHz. and v-=l cm./sec. The ion source 13 includes a chamber 25 as of stainless steel into which gas to be analyzed is fed via a gas inlet, not shown. An electron beam 26 which is produced by a filamentary emitter 27 as projected across the chamber 25, for ionizing gas therein, and is collected on a beam collector electrode 30. The beam 26 passes through aligned apertures in the side walls 28 of the chamber 25 and is focused by a magnetic field H produced by a magnet, not shown. A repeller electrode 14 forms one end Wall of the ion source and is operated at a positive potential as of +20 volts relative to the side walls 28 and other opposed beam exit end 'wall 29 of the chamber 25. The beam exit wall 29 contains a beam defining exit slit 31 as of 0.5 mm. wide and 1 cm. in length. The filament 27 is operated to 100 volts relative to the beam exit and side walls which are operated at +50 volts relative to ground. The electron beam collector 30 is operated at volts relative to the side walls 28.

A first accelerating electrode 32 is operated at ground potential. An ion beam focus electrode 33 is operated at a small fraction of the potential of a beam voltage control electrode 34. The beam voltage control electrode 34 is operated at a scanned voltage which is variable from to 2000 volts relative to ground. The scanned voltage is obtained from a scan voltage supply 35 with the ion focus electrode voltage derived from an associated voltage divider network 36. The midplane of the ion sorter l, which is coextensive with the. plane of the strip or ribbon shaped ion beam 2, is operated at the same potential as the beam voltage control electrode 34. Likewise, a beam exit electrode 37 of the ion sorter 1 is operated slot width, as of Two the midplane of the beam and the 34.

voltage for the ion sorter 1 is obtained by rectifying, in rectifier 38, a portion of the R.F. output of the R.F. source 7. The rectified R.F. output is filtered in low pass filternetwork 39 and fed to a voltage divider network 41. The ends of the voltage divider 41 are connected across the insulated comb members 3 and 4 of the ion sorter 1 which are insulated by insulators 40. The centertap of the voltage divider 41 is connected to the ion beam voltage control electrode 34 and beam exit electrode 37.

A secondary electron suppressor electrode 42, operated at 5-50 volts negative with respect to ground potential, is disposed between the ion collector electrode 11 and the exit electrode 37. The ion collector electrode 11 is connected to ground via a very high impedance resistor 43 as of 10 9 forming the input impedance of an electrometer amplifier 44. The output of the electrometer 44 is fed to a recorder 45 for recording as function of the scan voltage derived from the scanned beam voltage source 35 to obtain an output mass spectrum.

Referring now to FIGS. 6-8 there is shown an alternative ion sorter 1. More particularly, the periodic R.F. and D.C. electric fields are produced by a pair of slotted plates 51 as of 0.010" thick stainless steel which are disposed between a second pair of plates 52. The D.C. and R.F. fields are applied across the outer plates 52, as schematically indicated in FIG. 8. The slotted plates 51 each include an array of transverse slots 53 as of wide and long in the direction transverse to the direction of the ion beam 2. The slots 52 are spaced apart by a width equal to the slot width. The slots in the plates 51 are disposed in transverse registration with each other. The slotted platese 51 are operated at the same electrical potential which is intermediate the potential of the outer plates 52. The operating potential for the slotted plates 51 is derived by connecting the plates 51 to the centerta-p of the voltage divider network 41. The slotted plates are also operated at the same potential as the beam voltage control electrode 34 and beam exit electrode 37. The R.F. and D.C. potentials applied across the outer plates 52 leak through the slots 53 into the region of the beam 2 to produce the periodic transverse R.F. and D.C. field components.

A pair of rectangular cross section conducting rods 54 as of stainless steel hold the inner plates 51 in parallelism and spaced apart by a distance approximately equal to the pairs of stepped insulators 55 as of boron nitride hold the outer plates 52 in parallel spaced relation to the inner plates 51. The spacing between the outer plates 52 and the inner plates 51 is small as of 0.060". The ion sorter structure 1 of FIGS. 6-8 is supported within a tubular vacuum envelope 56 as of stainless steel from a pair of axially directed stainless steel rods 57 as of diameter and 50 cm. in length. A plurality of split centrally apertured blocks 58 as of stainless steel are clamped to the rods 57 at suitable intervals along their length. Metallic support arms 59 as of stainless steel interconnect the split blocks 58 and insulating transverse cross arm members 61 as of boron nitride. The cross arms 61 support the ion sorter electrodes by being screwed to the outer edge of one of the central spacing rods 54.

Referring now to FIGS. 9 and 10 there is shown the ion sorter 1 of FIGS. 6-8 as mounted in the vacuum envelope 56 of a mass spectrometer. The envelope 56 is flanged at both ends to receive flanged end caps 62 and 63. End cap 62 contains the gas inlet system of pipes and valves including electrical connectors, not shown, for leaking gas to be analyzed into the source 13 and feeding through the various electrical wires. The other end cap 63 contains the ion beam collector electrode 11 and its associated wiring and insulators. A flange 64 at one end of the central tubular envelope section 56 carries the axially directed support rods 57 from one of their ends at the same potential as beam control electrode The transverse D.C.

as by welding. A closed rectangular yoke permanent magnet assembly 65 is located over the ion source 13 to produce the electron beam focusing magnetic field and is carried from the support rods 57.

In a mass spectrometer of the type described with regard to FIG. 4, using the ion sorter 1 of FIGS. 6-8, and supported as shown in FIGS. 9 and 10, a mass unit resolution 300 was obtained between the peaks of mass units 91 and 92 of toluene. The parameters of this spectrometer were: f =3.85 mHz., beam voltage V=692 volts, dispersion=7.6 volts/mass unit, transverse DC. bias voltage :38 volts, transverse RF. voltage=35 volts R.M.S., detected ion current approximately equal to 10" amps., spacing between slotted plates 51 of slots were and long, and spacing between plates 51 and 52 of 0.060", with the sorter being 40 periods A in length.

Although the preferred embodiment of the present invention employs a transverse DC. bias electric field, this field need only remain essentially static during the time of flight of an ion through the ion sorter 1. This time is on the order of 10- seconds. Thus, the :bias field could be supplied by an audio frequency field, preferably a square wave. Phase sensitive detection of the amplified collected ion current could be used for detection of the sorted ions. Such a system would have less efficiency than an ion sorter 1 using a static DC. bias field.

Also the velocity sorter 1 of the present invention has been described as it would be operated for sorting of positive ions. The apparatus is equally applicable to sorting of electrons or negative ions. For negative ions and electrons, the aforedescribed accelerating negative potentials would be changed to positive potentials and vice versa. The transverse electric deflection fields need not be changed. For electrons, the velocity sorter 1 would be employed to produce a monoenergetic electron beam. Such a beam of electrons would be useful in an electron impact spectrometer where the energy absorbed from an electron upon impact with a gaseous molecule is to be determined.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A charged particle velocity sorter including, means for forming and projecting a stream of charged particles over a predetermined beam path, means disposed along the beam path for producing a spacially periodic radio frequency electric field transverse to the beam path, said periodic radio frequency field having an even numbered spacial harmonic component at the frequency of the radio frequency field for cumulative electronic interaction with charged particles of the beam to produce a net average transverse deflecting force on particles having a certain velocity synchronous with the phase velocity of one of the even number space harmonics of the radio frequency field, means coextensively disposed along the beam, with at least a portion of said transverse radio frequency field producing means for applying a second transverse electric field of a frequency different than the frequency of the radio frequency field to the charged particles of the beam, the second transverse electric field having a direction and magnitude to produce a net average transverse deflecting force on the synchronous charged particles which is equal and opposite to that produced by the radio frequency field, whereby the charged particles having the certain predetermined synchronous velocity pass through the velocity sorter without experiencing a net average defiection whereas charged particles with non-synchronous velocities are deflected out of the beam.

2. The apparatus of claim 1 wherein said second transverse electric field is essentially static during the transit time of a synchronous charged particle through the velocity sorter apparatus.

3. The apparatus of claim 1 wherein said first and second transverse electric field producing means superimpose their respective electric fields upon the beam over substantially coextensive periodic regions along the beam path.

4. The apparatus of claim 1 wherein said second transverse electric field producing means derives its electric field from said radio frequency transverse electric field producing means, whereby the ratio of intensities of the first and second fields is precisely controlled.

5. The apparatus of claim 1, wherein said charged particle beam producing means includes an ion accelerator structure for projecting and accelerating an ion beam with essentially a uniform beam voltage within the cross sectional area of the beam through the transverse electric fields, whereby the synchronous velocity ions emerging from said transverse electric field applying means have essentially only one charge to mass ratio.

6. The apparatus of claim 5, including means for scanning the intensity of the uniform ion beam voltage, and means for detecting the synchronous velocity ions emerging from said transverse electric field applying means, whereby a mass spectrum output of the ion beam is obtained.

7. The apparatus of claim 1 wherein said means for producing the spacially periodic radio frequency electric field comprises a pair of metallic comb shaped members disposed straddling the beam path with the free ends of the finger portions of said opposed comb members being disposed in transverse registration taken across the beam, and means for electrically insulating one of said comb members from the other.

8. The apparatus of claim 1 wherein said means for producing the spacially periodic radio frequency electric field comprises a first pair of spaced parallel conductive plate members, means for electrically insulating one of said first plate members from the other, and a second pair of spaced parallel plate members disposed in between said first pair of plates and being parallel thereto, said second pair of plates each including an array of elongated slots with the elongation of the slots extending transversely of the beam path and the slot array extending along the beam path, said slotted plates being disposed on opposite sides of the beam path with the slots in each array being disposed in transverse registration taken across the beam path.

9. The apparatus of claim 1 wherein said means for producing the spacially periodic radio frequency field comprises a spacially periodic metallic structure with the period of the structure being dimensioned to produce 211' radians of phase shift between adjacent periodic radio frequency field regions in the time it takes a synchronous velocity charged particle to be sorted by the sorter to travel one period of the periodic field.

10. The apparatus of claim 9 wherein said metallic periodic structure includes an array of spaced metal portions taken in the direction of the beam, and wherein said spaced metal portions have a width taken in the direction of the beam which is essentially equal to one half of a period of said periodic structure.

References Cited UNITED STATES PATENTS 2,818,507 12/1957 Britten 250-419 WILLIAM F. LINDQUIST, Primary Examiner.

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