CA1066819A - Cesium beam tube - Google Patents
Cesium beam tubeInfo
- Publication number
- CA1066819A CA1066819A CA318,221A CA318221A CA1066819A CA 1066819 A CA1066819 A CA 1066819A CA 318221 A CA318221 A CA 318221A CA 1066819 A CA1066819 A CA 1066819A
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- Canada
- Prior art keywords
- particles
- state selector
- transition section
- radio frequency
- tube
- Prior art date
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Abstract
A B S T R A C T
An atomic beam tube provides a single sealed envelope structure that serves both as vacuum envelope and as structural member to which the operative components are attached. The envelope is composed of a heavy and relatively rigid frame and a relatively thin and flexible cover sealed to the frame. The operative elements are separately assembled in independent sub-assembly units which are secured to the frame at a minimum of locations to provide fixed alignment and thermal isolation of the operative elements, and easy disassembly of the tube. The molecular beam tube apparatus comprises enclosure means, a source for providing a beam of molecular particles a first state selector for selecting a portion of the particles in the beam, a radio frequency transition section downstream from the first state selector for causing resonance transitions of some of the selected beam particles means for producing a weak generally homogeneous magnetic field in the radio frequency transition section, a second state selector downstream from said radio frequency transition section for selecting a further portion of the bean comprising these beam particles that have undergone resonance tran-sitions, and detecting means responsive to the particles in the further portion. The source and first state selector together comprise a first sub-assembly, the radio frequency transition section and the means for producing a weak field together comprise a second subassembly, and the second state selector and the detecting means together comprise a third subassembly. The enclosure means comprise a rigid base and a flexible cover, the frame and cover being sealed together to form a vacuum envelope. The three sub-assemblies are separately and removably secured to the frame.
An atomic beam tube provides a single sealed envelope structure that serves both as vacuum envelope and as structural member to which the operative components are attached. The envelope is composed of a heavy and relatively rigid frame and a relatively thin and flexible cover sealed to the frame. The operative elements are separately assembled in independent sub-assembly units which are secured to the frame at a minimum of locations to provide fixed alignment and thermal isolation of the operative elements, and easy disassembly of the tube. The molecular beam tube apparatus comprises enclosure means, a source for providing a beam of molecular particles a first state selector for selecting a portion of the particles in the beam, a radio frequency transition section downstream from the first state selector for causing resonance transitions of some of the selected beam particles means for producing a weak generally homogeneous magnetic field in the radio frequency transition section, a second state selector downstream from said radio frequency transition section for selecting a further portion of the bean comprising these beam particles that have undergone resonance tran-sitions, and detecting means responsive to the particles in the further portion. The source and first state selector together comprise a first sub-assembly, the radio frequency transition section and the means for producing a weak field together comprise a second subassembly, and the second state selector and the detecting means together comprise a third subassembly. The enclosure means comprise a rigid base and a flexible cover, the frame and cover being sealed together to form a vacuum envelope. The three sub-assemblies are separately and removably secured to the frame.
Description
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This invent;on relates, in general, to atomic beam apparatus, and, more particularly~ to atomic beam tubes which utili~e magnetic hyperfine re-sonance transitions.
Atomic beam tubes are the basic frequency determining elements in extremely stable frequency standards. Fundamen~ally, an atomic beam frequency standard detects a resonance within a hyperfine state of the atom to obtain a standard frequency. To utilize this resonance, atomic particles, such as cesium atoms, in a beam interact with electromagnetic radiation in such a manner that when the frequency of the applied electromagnetic radiation is at the resonance frequency associated with a change of state in the particular atoms, the atoms in selected atomic states are deflected into a suitable detector. The frequency o~ the applied radiation is modulated about the precise atomic resonance frequency to produce a signal from the detector cir-f cuitry suitable fo~ the servo control of a flywheel oscilla~or. Control cir-cuitry is thus employed to lock the center frequency of the applied radiation to the atomic resonance line.
~ When cesium atoms are employed in an atomic beam tube, the particular f resonance of interest is that of the transition between two hypf3rfine leveis resulting from the interaction between the nuclear magnetic dip~le and the spin magnetic dipole of the valence elefctron. Only ~Wffff stable confi~urationsof the cesium a~om exist in nature, in which the dipoles are either parallel or anti-parallel, corresponding to two allowed quantum states. Thus9 in the absencff of an external magnetic fieldJ there are two hyperfine energy levels, -:
each of which may be split by an external magnetic field into a number o~ :ZeeDan sublevels. -~;
The hyperfine resonance transition used in the a~omic beam tube of the present invention occurs between the (F_4, mp=0) and ~=3, mp=0) ; s~ates, where the first number F is related to ~he magnitude of the total angular mo~entum of the atom ~electronic plus nuclear~ while the second n~mber -:
30 m~ is related to the component of this total angular momentum which is in ~ ..........
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the direction of the applied external mac~netic field.
To cause a transition from one state to the other, an amount of energy E equal to the difference in energy of orientation must be either given to or taken from the atom. Since all cesium atoms are identical, E is the same for every atom. The frequency f of the electromagnetic energy re-quired to cause a change of state is given by the equation E=hf, where h is Planck's constant. For cesium, the magnitude of f is approximately 9,192.631770 megacycles.
A conventional cesium atomic beam apparatus provides a source from which cesium evaporates through a collimator which forms the vapor into a narrow beam and directs it through the beam tube.
This collimated beam of atoms is acted upon by a first state sel- -ecting magnet or "A" magnet, which provides a strongly inhomogeneous magnetic field. The direction of the force experienced by a cesium atom in such a field depends on the state of the atom. In ~his field, the energy states F=3 and P=4 are split up into sublevels. All of the atoms of the F=4 state, except those for which mp=-~, are deflected in one direction, and all other atoms are deflected in the other direction. In the apparatus of the present invention, the F=3 group ttogether with the atoms of the ~4, -4) sublevel) are retained in the beam, while ~he others are discarded. The undiscardecl atoms include those of the (3,0) sublevel.
Upon emergence from the A-~ield, those atoms enter a central region where they are subjected to a weak uniform C-field to assure the separation in energy of the mF = states from the nearby states for which m~ ~ 0. This small magnetic field also serves to establish the spatial orientation of the selected cesium ato~s and, therefore, the required direction of the microwave magnetic field.
While in this uniform weak field region, the cesium beam is subjected to an oscillating externally generated field of approximately the resonance frequency re~uired to cause transitions from the ~3,03 to the (4,03 sublevel.
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After l~aving ~his energy transfer region, the beam is acted on by a second state-selecting magnet, similar to the A-magnet, producing a strong inhomogeneous field. Here the atoms o~ all the F~3 groups ~and also those of the ~4, -4) sublevel) are discarded. The only undiscarded atoms are those of the ~4,0) sublevel, which exist at this point only because of the induced transition described above. These atoms are allowed to proceed toward a detector o~ any sui~able type, preferably of the hot-wire ionizer mass spec-tometer type.
The magnitude of the detector current, which is critically dependent upon the closeness to resonance of the i~pplied RF fre~uency, is used after suitable amplification to drive a servo system to control the frequency of the oscillator/multiplier which excites the RF cavity. `
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Cesium beam tubes as hi~her~o constructed have been expensive and ~.~
difficult to make. To provide a cesium beam tube suitable for use in the usual applications of atomic frequency standards, mechanical alignment of components is critical, and shifts in the alignment can destroy the functional ~i ~ frequency standard. The tube elements that have been descrîbed must be as~
,'! sembled and supported in place with a high degree of precision~ alignment ~ requirements relative to the beam deflection axis of the tube being approxl-.
mately .001" for effective tube operation. The precise alignment must be preserved under conditions of mechanical vibration and shock, and of a range of temperature ~ariations typical of practlcal applications of the tube.
Prior art tubes have employed complicated mounting means between the inner structural assembly of tube elements and either an inner or an outer vacuum-tigh~ envelope in an effort to meet the often-conflicting requirements of ri~idity against mechanical shock or vibration~ and flexibility to accomm~date to dif~erential expansion disturbance forces in the presence of thermal gradi- ~ -ents resulting ~rom bake out in tube processing and ambient tem~eratures in normal tube operation. A further limitation in prior art tubes is that these structure measures typically result in relatively large and heav~ tubes, ,: .
characteristics that are most ~mdesirable for certain important applications ; such as in air or space craEt.
Some prior art cesium tubes have been constructed using two separate envelopes. The first is an inner mounting chalmel to which the operative components are secured to provide mechanical stability and thermal isolation;
this inner envelope is suspended within an outer vacuum envelope. Since dif-ferential movement between the two envelopes must be allowed for, such a com-pound structure adds complexity to the manufacturing processO ~tis design also results in a relatively weak mechanical structure.
This invention relates to molecular beam tube apparatus comprising enclosure means, a source for providing a beam of molecular particles a first state selector for selecting a portion of said particles in said beam a radio frequency transition section downstream from said first state selector for causing resonance transitions o some of said selected beam particles -means or producing a weak generally homogeneous magnetic field in the radio ~ frequency transition section a second state selec~or downstrec~t from said radio frequency transition section for selecting a further portion of said bean~t comprising those beam particles that have undergone said resonance transitions, and detecting means responsive to said particles in said further portion, said source and said first state selector together comprising a first subassembly, said radio frequency transition section and said means for producing a weak field together comprising a second subassemblyJ and said second state selector and said detecting means together comprising a third subassembly, said enclosure means comprising a rigid base and a flexible cover, said frame and cover bieng sealed together to for~ a vacuum envelope, ; said three subassemblies being separately and removably secured to said ~ frameO
- ~ The p~esent invention integrates the inneT assembly and the vacuum envelope into a single structure, thereby eliminating the need for support elements between the two. It further provides for a modular assembly in which ;6~
three subassembly units are assembled to the main structural member ~which ~ is also a portion of the vacuum envelope) by means of 10 machine screws, as :~ will be described. The invention also includes novel features providi.ng good thermal isolation, smaller and more efficient magnetic structures, smoother transition between strong and weak magnetic fields, and means to feed in RF
energy with less perturbation of the C-magnetic field than in prior art tubesO
These novel features make possible a tube, both more compatible with typical operating environments than conventional devices, and lighter in weight (9 lbso against the 16 lbs. of a typical prior-art tube~
The design of the present invention eliminates the need for expen-sive and complex internal support structures while providing a beam tube of simple modular design that maintains beam alignment and is highly resistant to external mechanical disturbances such as shock and vibrationO At the same time, the design of the present invention provides excellent thermal ~ isolation for the thermally sensitive componentsO
.. .! The atomic beam tube of the present invention provides a single :
structure that serves both as vacuum envelope and as structural member for the operative componentsO This envelope is composed of a heavy and relatively .. .... . .
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~ .-~ L0~;6~3~L9rigid frame and a relatively thin and flexible cover sealed to the fr~m~. The operative elements of the tube are secured to the frame; this provides fixed alignment of these elements. Th& flexible cover accommodates itself readily to externally caused mechanical distortions without transmi~ting them to the frQme or to the operative elements. The sealed unit acts as a vacuum envelope.
The operative elements of the tube are secured to thle heavy fram~ at a minimum of locations, and the connections have low thermal conductivity, in order to isolate the operative elements thenmally from the environment. For example3 the oven structure is secured to the frame through a connecting structure that is designed to provide a relatively long thermal path to the environment.
; It is industry practice to disasse~ble such tubes when they are ~, no longer operable (generally because the cesium getters are saturated) in order to salvage reuseable components. To disassemble prior art tubes has required extensiv0 machining which is both time-consuming and expensive, in-volving high labor costs. In the cesium beam tube of the present invention, the operative parts are provided in three main modular subassemblies, secured ~to the frame by a total of 10 screws, for quick and simple disassembly and reus0 o~ the modular portions.
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The operation of the cesium beam tube, as has been described, re-quires that the A and B magnets provids very strong fields tof the order of 10 kilogauss), while the C-~ield in the region between them must be re latively weak (of the order of .060 gauss) and as uniform as possible. Dis-continuities in the C-field are particularly likely to occur in the regions at which the beam enters and leaves the C-region, and can cause spontaneous transitions ~Majorana transitions) in the atomic beam which may distort the f;performance of the tube. The present invention provides a C-field winding of novel design that generates a C-field of superior uniformity at the beam apertures.
In general, i~ is desiTable to provide a cesium beam tube that is as compact~ light weight, and simple as posslble. The particular designs ~:
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of the A and B magnets in the pressnt invention realize such construction and are particularly adapted to the modular assembly previously described.
It is typical in the assembly and processing of molecular beam tubes to confine the source o the molecular beam material in a sealed ampoule during the bakeout ~nd exhaust part of the processing cycle, and as a final stage, while the tube is still being pumped, but after bakeout has been com-pl0ted, to open the ~mpoule. Any gases released in the opening process can then be pumped prior to the final sealing off of the tube.
A number of methods have been used in the prior ar~ for opening the ~:
ampoule. One such method is to provide means whereby a member of the ampoule is ruptured when electrical energy is applied to a heating coil to cause expansion in a member mechanically linked to a rupturing element. A more sophisticated prior art method is to discharge an external capacitor through electrical conducting paths into the tube, so arranged that a vaporizing arc is created at a member of the ampoule which is ruptured by the heat of the arc. Both of these methods require the inclusion in the beam tube of additional parts that are used only ~or this one operation; in particular, means must be provided to transmit electrical energy through the vacuum envelope, which complicates the construction of the tube.
2Q The pres~nt invention provides a novel ampoule structure and novel means for opening the ampoule that require no additional parts; i.n particular, no: additional electrical or mechanical fseds through the vacuum envelope are required.
Other objects, features, and advantages will appear from the follow-ing description of a preferred embodiment of the invention, taken to~ether with attached drawings thereof, in which:
Pig~ 1 is a schematic view of the principal beam-forming and detect-ing elements of the ~ube;
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Fig. 2 is a perspective view of the elements of Fig. l;
Pig. 3 is an exploded viev of the components of the oven and ampoule;
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Fig. 4 is a cross sec.tion of the ampoule;
Fig, 5 is a view of the assembled oven;
Fig, 6 is a view of the oven with reflector and support structure;
Fig. 7 is a Zeeman energy dia~ram for cesium 133 in the ground electronic sta-te; shol~ng the transition induced in the beam tube of the in-vention;
. Fig, 8 is a schematic view of the control circ~utry used with the cesium beam tube of the invention;
Fig, 9 is a perspective view of the first state selector magnet and '. 10 ion pump;
:~ Fig, 10 is an exploded perspective view of the first state selector magnet together with shielding and support structure;
Fig, 11 and 12 are longitudinal and cross sections respectively of .,, the first state selector and ion pump;
' Fig, 13 is a perspective view of the microwave structure and C-field ;~ eoil;
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Fig, 14 is a perspecti~e view of the C_field coil with portions ~i, broken away;
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Fig, 15 is a plan view o$ the unfolded C-field coil;
Fig, 16 is a cross section of the ass~mbled C~field coil at a beam aperture;
Fig, 17 is a detail of the conductors of the C_~ield coil at a beam :~, - . .
aperture; ~ .
Fig, 18 is an exploded view of the magnetic shield package and con-tents; . ;~ :~
Fig. 19 is a cross section of the outer en~elope and contents near ~ -he:cellter;
Fig. 20 is a perspectlvo vie~Of the B-ficld magnet and the detector;
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: Fig, 21 sllows the elements of Fig, 20 ~nth support st~lcture;
30 : Fig, 22 and 23 are a plan view ~nd a rear elevation YiÇW of t~le ~ 7 ~
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B-field magnet and the cletector;
Fig. 24 is an exploded view of the outer packaging and connections and ~he modular units; and Fig. 25 is a longitudinal view partly in sec~ion of the assembled units of Pig. 24.
General Referring to the drawings, and particularly to Figs. 1 and 2, the basic beam-forming and de~ecting elements of the cesium tube 11 of the in-vention are shown schematically and in perspective. A source of atomic par-ticles includes an oven 10 which evaporates liquid cesium and emits ~through a collimator~ a beam of neutral cesium ato~s which are statistically distri-buted between two stable energy states, as previously dçscribed. l`he first state selector or A magnet 12 splits these energy states into sublevels and selects the atoms in the F-3 states (together with those in the ~4,-4~ sub-level) and deflects the remaining atoms so that they no longer form part of the beam. The be~m of selected atoms then passes through the RF interaction section 14; in this region a weak homogsne~us magnetic field (C-field) is supplied by the winding 22. Microwave energy is supplied at the resonance frequency to induce transitions of some of the beam atoms from the t3,0) state to the (4,0~ state (Fig. 7). The beam atoms ln the ~4,03 state are then selected by the second state selector or B magnet 16, the atoms in the remaining states being deflected out of the beam. The cesium atoms selected by the B magnet strike the hot wire ionizer 20, and an electron is stripped from each cesium atom~ c~using the re-emission of cesium ions9 which are accelera~ed through a mass spectrometer 2~7 in~o the electron multiplier 18.
The electron mul~iplier provides an output current propor~ional to the number of atoms arriving at the hot wire 20, that is3 proportional to the number Gf atoms that have been raised to the second state in the mlcrowave cavity.
As shown in Fig. 8, the output of the atomic beam tube 11 is fed to control el0ctronics 260 which produce a suitable error output signal 261 ':
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which is applied to a crys-tal oscillator 262. The f`requency out-put of -the crystal oscillator (typically 5 megahertz) is controlled by the processed signal 261 from the cesium beam tube, and then multi-; plied in the frequency multiplier chain 264 and applied to -tube 11, at the precise resonance ~requency (typically 9192 mHz). Multiplier chain 264 and the controlled osailla-tor 262 from the microwave generator 266. The usable output signal is derived from controlled oscillator 262 at 268.
Summary of Modular Components The elements that have been described and shown in Fig. 8 are in general terms old and well-known in the art. The cesium tube of the invention provides -three modular subassemblies including a cesium ampoule and a first state selector magnet in combi.nation wi-th the ion pump, a second state selector magnet in combiination~wi-th bhe mass speatromeber,i and a G-field ~inding and microwave structure, all of novel design, as well as a novel outer ,., ' ', . ' ~ package for the entire tube.
, . . -To provide the advantages o~ the modular assembly of the . ~ invention, as previously described, the oven 10 (witl~ cesium ampoule) and A~magnet 12 (with ion pump), shown separa-tely in the schematic ,, .
views of Figs. 1 and 2, are combined in an oven/A-magnet assembly module 240 (Fig. 24). The RF interaction region 14 and C-field, shown unenclosed in Figs. 1 and 2, are contained in magnetic shield package 179 (F`ig 24). The B-magnet 16, hot wire ionizer 20, mass spectrometer 207 and electron multiplier 18 are packaged together in a detector assembly module 244 (Fig. Z4)~ Referring to Figs.
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24 and 25, modules 240 and 244 and magnetic shield package 179 are `
essentially independent of one another and constitute the subassembly units within the outer package of the beam tube, and are assembled ~; 30 thereto by means of 10 screws, as will be described.
The details of each of these modular components are des~
cribed below.
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~66~g OvenJA-magnet module: oven and ampoule The structure of the novel oven-ampoule assembly 10 of the invention consti-tuting a source for providing a beam of ces:ium particles, is shown in de-tail in Figs. 3-6. The assembly 10 includes collimating means 42, not described, and oven means including a reservo:ir 29 contain-ing an ampoule 27. The ampoule 27 includes a -thin walled (0.015") generally cylindrical shell 30 and a -top 37 including a fill tube 38.
Top 37 and cylinder 30 -together form an enclosure.
The end of shell 30 opposite to top 37 provides an opening 49.
10 A cup shaped base 34 is sealed into shell opening 49 by an eutec-ti~
metal 32 designed to fail mechanically a-t a -temperature of approximately 600C. An example of such an eutectic metal is an alloy of 45% copper and 55% indium. A weak spring 35 is compressed between base 32 and top 37.
After the enclosure has been filled with liquid cesium, fill tube 38 is closed by pinching and heliarc welding.
A wire screen mesh 36 having high -thermal conductivity surrounds ampoule 27 within reservoir 29. The mesh 36 serves bo-th as a heat trans-fer element and as a retaining and suppor-t element for the ampoule.
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Ampoule 27 is supported within reservoir 29. A copper outer 20 cylinder 28 of reservoir 29 includes an annular recess 40 at its lower :~ .
i por-tion. A welding adaptor 39 having a lower flange 41 is brazed to , .
! recess 40 of outer cylinder 28. An ampoule support member 43 includes an `I inve~ted cup portion 44 and three spaced supports 45. Inverted cup portion l 44 of member 43 is heliarc welded at 46 (Fig. 4) to the inner surface 1 .
of welding adaptor flange 41 to seal -the lower end of reservoir 29.
, This creates an enclosed reservoir space 51 surrounding base 34 and communicating with mesh 36. Ampoule 27 is seated in support member 43 `'I ~ ,. . .
`,.,3 with ampoule base 34 within spaced supports45.
;i~ Two tantalum heaters 90 and 92, retained in a ceramic support `~330 structure 88, are inserted into coll1mator assembly 42 through quartz tubes 80 and 82. The ampoule is opened,after bakeout of the beam tube, by means of these heaters, which heat the ampoule to 600C, at which temperature the eu-tectic seal fails. The combination of the vapor ~I pressure of the cesium within ampoule :` :
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27 and the force of compressed weak spring 35 exerts a stress greater than ~he working stress of the metal of seal 32 and pushes base 34 out o shell 30~ thereby releasing the cesium in the ampoule. Weak spring 35 prevents the base from settling back into place, resealing the ampoule.
In later operation of the tube, tantalum heaters 90 and 92 are used to warm the entire oven assembly 10 to the operaLting temperature, typic-ally about 90C. At this temperature the liquid cesium in reservoir space 51 slowly vaporizes and diffuses fro~ the mesh 36 to collima~ing means 42.
Collimator 42 is functionally equivalent ~o a bundle of small tubes so oriented tha~ a dire~ted beam of cesium atoms emerges. Construction of collimating means is well known to ~he art, and will no~ be detailed here.
The oven support structure is designed to provide thermal isolation from outside the beam tube. Since the oven operates in a vacuum, there is no heat loss from convection; the major loss is by radiation, with some loss by conduction. The oven suppor~ struc~ure is therefore constructed of mater-ial of poor thermal conductivity such as stainless steel and includes ear portions 100 and 102 fo~ securing oven 10 to the A-magnet assembly9 as will be described. Additionally, 0.003" Kapton shims 99 between the ear portions o~ the support structure and the A-magnet assembly further discourage ~hermal conduction. A radiation shield 104 of highly polished aluminum surrounds the major portion of the o~en, and presents radiation heat loss from the oven.
An oven of the design described required less than two watts for operation.
Oven/A-ma~net module: A-magnet and ion p_mp Referring now to Figs. 9 through 12, a permanent magnet driver lll is shared by the first state selector magne~ tA magnet) 12 and the ion pump 110. The ion pump performs the well-known function of removing undesired gasses and maintaining tube va~uum during operation. Permanent magnet 111 is generally of a typical "C" shape, but with a novel reentrant inner surface shape that gives it the distinguishing capability of providing proper fields 0 or both selection and ion pumpingO The axis v~ magnet lll is parallel wi~h - .
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the beam.
"Dipole configuration" soft iron pole pieces 112 and 114, Of a well-known design, are secured in the gap of "C" shaped permanent magnet 111, and provide the inhomogeneous deflecting field of first state selector 12.
Reen-trant extensions 108 and 109 of permanent magnet 111 extend inwardly toward one another, and in conjunction with a second pair of short cylindrical pole pieces 116 and 118 provide the field for the ion pump 110, located between pieces 116 and 118. The ion pump is of any suitable design and is well known.
Permanent magnet 111 provides in effect two permanent magnet cir-cuits in parallel to drive both the "A" state selector 12 and the ion pump ~;
110. The magnetic driver is designed to provide approximately 10 1~ gauss in the state selector circuit while providing approxima~ely 1000 gauss for the ion pump. The compact arrangement of this combination permits the atomic .. . .
~ beam tube assembly to be smaller, lighter, and less expensive than those .... . .
i`,'! ~ hitherto constructed, and is also especially adapted to the modular design of the present beam tube apparatus.
A magnetic shield 132 covers approximately the upper half of the outer surface of magnet 111 and additionally on one end is interposed between Z 20 the magnet and the C-field/microwave structure module 179 ~Figure 24). Shield , 132 provides aperture 138 for the passage of the atomic beam from the A-magnet i' .,.
12 to module 179. The structure of shield 132 further provides field control ~}~ for the attenuation of the 10 K gauss deflecting field of the A-magnet down to the 0.060 gauss C-field in the RF transi~ion region 14.
; A mounting plate 128 is secured to the upstream side of permanent magnet 111, and provides brackets 134 and 136. Magnetic shield 132, stainless , . .
steel spacers 113, magnet 111, and another pair of s~ainless steel spacers 117 all are fastened together by a pair of machine screws 115 passing through clearance hole~ in each and threading into tapped holes in moun~irlg plate 128.
Oven 10 (Figure 6~ is secured by its support structure ear portions 1: . .
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110 and 102 to brackets 134 and 13G ~s thesc brackets are opCn in construc~
tion, rather than solid, thcy provide a relativcly :Long thcrmal path for the conduction of heat from the oven through the brackets to thc eventual point of contact ~th the outer framc of the beam tube. Shims 99 of 0 003" Kapton are interposed between ears 100 and 102 and brackets 134 and 136 and provide further thermal insulation.
Oven 10 and A_magnet 12 with ion pump 110 from theoven/A-magnet module 240 (Fig. 24).
C-field~icrowave ~tructure module ~, .
Referring again to Figs 1, 2 and 4, the C-field and RF (radio frequency) transition section 14, including magnetic shields to be described, are packaged together as a second module 179 As previously described in connection with Fig. 2, the cesium atoms that are selected by the A~magnet 12 form a beam that must next pass through RF transition section 14 In ~liS region a weak homogeneous magnetic field - (C-field~ of approximately 06 gauss directed transverse to the beam path is ~ ~ .
~ provided by a single_layer printed circuit solenoid 22 of novel design~ The .~ ' . ' .~ ' .
construction and mounting supports of this solenoid will be described b~ re-~erence to ~igs. 13 through 19.
Referring first to Fig 15, the csnductors of solenoid 22 are etched : , . . .
by well-kno~n printed circuit techniques from a thin copper ]ayer bonded to a base 152 of polyimidc material approximatel~ 0.002 inch thick. The general shape of the base material 152 and a pattern of eight ~mifoxml~r-spaced con-dRctors 150-1 through 150~8 is sho~m in Fig. 15. Eyelet holes 307 are pro-vided at each end of the conductors lSO This printed circuit solenoid pro_ vidcs thin, wide, and closely spaced conductors of very uniform cross sectiollAl , ~ :
area and constant conductivity.
The printed circuit solenoid is assembled into a generally rectangu_ lar loop as shown particularly in Fig. 14, with the eyeleted ends of conduc-3~ tors 150 offset one conductor in xegistry so that the completed conductil~g ~C36~
path will form a one-layer spiral winding of equally spaced helical turns.
Electrical connection at each of the offset, but otherwise registered, ends of conductors 150 is made by soldering using indium washers (not shown) and secured by rivets 308 inserted through the eyelet holes. Electrical connec-tion to the solenoid is made by wire leads soldered to eyeletted pads 304 and 306 at the end of each of the outside turns.
The closed loop includes two end sections 140 and 142 that are transverse to the beam path and parallel to one another. Since the assembled solenoid winding must lie generally in the plane of the cesium beamg apertures 270 and 271 are provided in end sections 140 and 142 of such a size as to interrupt conductors 150-4 and 150-5.
Aperture 270 in base layer 152 has two opposed edges 144 (Figure 15) that interrupt the two adjacent inner strips 150-4 and 150~5 of continuous conductor 150, to provide four internal ends 122 of strips 150-4 and 150-5 adjacent the aperture edges. Ends 122 are eyeletted. To provide a continuous current path, it is necessary to bridge the aperture by connecting the inter-nal conductor ends. In addition, it is necessary to maintain uniformity of the C-field at the beam apertures insofar as is possible, to avoid field dis-continuities causing undesired transitions, as previvusly explained.
In the present invention, two patches 318 of printed circuit mater-ial similar to that described are provided to bridge the gaps and maintain unlformity of the C-field, each having an aperture 319. Two eyeletted conduct-ing~jumpers 166 and 168 are bonded to base layer 320, and angle around aper-ture 319. Referring particularly to Figures 14 and 17, a patch 318 is assembled to the winding by soldering to rivets 182 passing through the eye-lets of the jumpers and of internal ends 122. This construction maintains the continuous current path through the entire conductor 150 a-t the beam apertures.
Jumpere 166 and 168 lead the current around each aperture 270 and 271, effec-tively dou~ling the magneti~ing force at the edge of the apertures and tending to maintain a near uniform distribution of the C-field across the apertures.
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~6819 Tl s structure provides an exceedingly close approximation -to the ideal of a uniformly distribu-ted curren-t sheet.
Electrical :insula-tion around the solenoid is provided by polyimide strips 18~ and 186 (Fig. 1~) made -to t;he same shape as printed circui-t base 152, one being placed on ei-ther side of base piece 152.
Inner Magne-tic Shield I'acka~e .
The assembled C-field winding 22, comprising the -three layers and two patches as described, is mounted on the inner surface of inner magnetic shield 154 (Fig. 15~ and inner shield base pla-te 156 and is held in place by rivets passing through the shield material, the outer margins of the solenoid assembly of base material 152 and insulating strips 184 and 186, and aluminum pla-tes 282 of which representative ones are shown in Fig. 18. The assembly at the aperture locations 270 and 271 is made with aluminum plates 280 that provide apertures to register with apertures 270 and 271.
A flop coil 192 (Figs. 2 and 18) is mounted on one of the central aluminum plates 282 and supported from inner magnetic shield ;
l 154 so that it is coaxial to the beam axis. This coil is used in a !
, 20 manner well known to the prior art to introduce a 20 khz. electrical signal for the adjustment of the C-field solenoid current, and will not be described further.
The sides of inner magnetic shield 15~ (Fig. 18) 9 paralleling ;
the beam path, provide magnetic end caps for solenoid 22. The resulting field across the plane of solenoid 22 thereby approximates the classical uniform ~leld of an in~ tely long solenoid with flux lines normal to the cesium beam path. Inner magnet shield I54 in combination with spaced outer magnetic shieLd 157 effectively attenuates the strong , ~ magnetic ~ields produced by the A and B magnets and also shields the 1 30 RF transition region from external magnetic perturbations.
Microwave radiation -- : .
~ ~ Referring particularly to Fis. 1, 2 and 18, microwave radiation -~
A ~ ~ iS supplied within RF interaction section 14 by waveguide structure 190, which ~ . ,: .. .
68~g is of standard "Ramsey" type and well known in the art. It will not be described here.
In prior art atomic beam tubes, constructed with separate mechani-cal protective and vacuum isolation envelopes, differential motions between the two envelopes have made it necessary to provide Elex:ible connection means between the microwave structure and the exteLior of the -tube, capable of accommodating to such motions. Such flexible means requires a relatively large aperture, typically two inches in diameter, in the magnetic shield structure to accommodate the connection. Such a large aperture introduces perturbations in the magnetic C-field due to leakage effects, which must in turn ~e compensated for, for example by providing extra "baffling means" as in United States Patent No. 3,670,171 (Lacey et al) issued June 13, 1972.
In the present invention, the combination of mechanical support and ~ `
vacuum isolation envelope into a single structure eliminates such differential motions. The inlet arm of microwave structure lgO can therefore be intimately brazed to the lower surface of inner shield base plate 156~ This construction avoids the need for a large aperture through the magnetic shield; a relatively small aperture 194, about 1" x 1/2"~ is provided in base plate 156 (Figure 18). Such a small aperture introduces only relatively small perturbations in-eo the C-field, eliminating the need for "baffling" or other compensating structure, and this structure is therefore advantageous. ~ -Outer m gnetic shield package Referring particularly to Figures 18 and 19, inner magnetic shield package is contained within an outer magnetic shield 157 and outer base plate 159. Apertures 167 and 169 are provid~d for the cesium beam. The entire unit of outer and inner magnetic shield packages, ~ith -the contained R~ transition section, forms the C-field/micr~wave structure module 179 (Figure 24). ~:
Second state seIector ~ tector module .
~ Referring no~ to Figures 20-23, permanent magnets 19~ and 199, each - -generally ~of horseshoe form, are secured to a detector table 196, and lie in ~ ' ,:
110 6G~
a horizontal plane containing the beam axis. Magnets 198 and 199 are ; assembled to provide two gaps spaced about 180 apart, one gap being down-stream of RF transition section 14 on the beam axis and the other slightly ofEset therefrom and downstream of the first. Soft iron pole piecés 200 and 201, whose configurations are identical to those of the A-magnet pole pieces, - are provided in the first gap between permanent magnets 19~ and 199, on the beam axis. Pole pieces 200 and 201 are driven by magnets 198 and 199, and act as the second state selector (or B-magnet) 16. A second pole piece assembly 204 is provided in the second gap between permanent magnet pieces 1~8 and 199, slightly offset laterally from the beam axis and downstream from the first gap, pole piece assem~ly 204 is driven by permanent magnets 198 and 199 to function as a mass spectrometer 207. Thus the second state selector and the mass spectrometer are driven in series by a single pair of permanent magnet pieces 198 and 199. ~liS combination contributes to making the cesium beam tube of the present invention smaller and lighter than prior art atomic beam ~ tubes.
`, Detector table 196 is provided with three mounting tabs to which is secured a hot ~ire ionizer assembly 21 including hot wire 20. An electron .. . .
multiplier and shield assembly 18 is secured beneath detector table 196, and aperture 203 is provided in table 196, corresponding with an aperture 205 in the electron multiplier shield. The B-magnet 16, mass spectrometer 207, hot wire ionizer assembly 21 and electron ~ultiplier assembly 1~ together make up B-magnet/detector module 244 ~Figure 24).
The beam of cesium atoms that emerges from the RF transition sec-tion 14 (Figure 2~ contains certain atoms that have undergone a transition and other atoms to be discarded. The atoms selected by second state selector or ~-magnet 16 strike the hot wlre 20, which is of a standard type and will not be further described. Hot wire 20 strips an electron from each neutral cesium atom that serikes it, and re-emits a positively charged cesium ion. The cesium ions are then sorted by mass spectromPter 207 from ~ ,,.
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668~L~
impurities unavoidably emitted by hot wire 20 and are directed into electron multiplier 18~ which produces an amplified output proportional to the number of atoms incident upon the first dynode oE the multipl:Ler.
Outer package Referring particularly to Figures 24 and 25, the outer package of the atomic beam tube of the invention is a single vacuum tight envelope com-posed of a rigid base 210 (Figure 24), made of 1.8 inch thick stainless steel, and a relatively thin and flexible cover 212 made of 1 mm thick stainless ` steel. Base 210 provides the necessary ports with vacuum tight feed-through connections to power and RF sources, which are standard and will not be des-cribed in detail. The three main subassemblies or modules 179, 240 and 244, which have previously been described in detail, are secured to base 210.
In assembly, oven/A-magnet module 240 is secured to supports 222 and 224 on base 210 by two machine screws 400. Thus the path for heat con-:
duction from oven 10 to the exterior environment of the cesium tube extends through open brackets 134 and 136 and supports 222 and 224 to frame 210.
This structure provides a relatively long thermal path and aids in isolating `-oven 10 from the outside environment. ~
:
1 The C-field/microwave structure module 179 is secured to four posts 226 by four machine screws 228. B-magnet/detector module 244 is secured to brackets 234 and 236 by four machine screws 237. Detector table 196 and brac- ~ -kets 234 and 236 together provide a relatively long thermal path from ionizer 20 to the environment outside the beam tube.
Cover 212 is welded to base 210 after the necessary connections have been made to the feed-through connectors. The tube is then evacuated under high temperature conditions.
This modular construction of the beam tube, with each module or subassembly indivldually secured at a minimum of polnts to the rigid frame of :. :the single ~nvelope structure, provides alignment and s~pport for the modules 30~ while simultaneously providing thermal isolation and mechanical protection ~ .
.~ -661 3~L~
o~ the components in -the modules from the outside environment. At the same -time, the relatively ~lexible cover accommodates to thermal and mechanical stresses induced by -the welding opera-tion; an outer structure entirely of the thicker material would not provide -this :~lexibili-ty, and alignment difficulties would result.
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This invent;on relates, in general, to atomic beam apparatus, and, more particularly~ to atomic beam tubes which utili~e magnetic hyperfine re-sonance transitions.
Atomic beam tubes are the basic frequency determining elements in extremely stable frequency standards. Fundamen~ally, an atomic beam frequency standard detects a resonance within a hyperfine state of the atom to obtain a standard frequency. To utilize this resonance, atomic particles, such as cesium atoms, in a beam interact with electromagnetic radiation in such a manner that when the frequency of the applied electromagnetic radiation is at the resonance frequency associated with a change of state in the particular atoms, the atoms in selected atomic states are deflected into a suitable detector. The frequency o~ the applied radiation is modulated about the precise atomic resonance frequency to produce a signal from the detector cir-f cuitry suitable fo~ the servo control of a flywheel oscilla~or. Control cir-cuitry is thus employed to lock the center frequency of the applied radiation to the atomic resonance line.
~ When cesium atoms are employed in an atomic beam tube, the particular f resonance of interest is that of the transition between two hypf3rfine leveis resulting from the interaction between the nuclear magnetic dip~le and the spin magnetic dipole of the valence elefctron. Only ~Wffff stable confi~urationsof the cesium a~om exist in nature, in which the dipoles are either parallel or anti-parallel, corresponding to two allowed quantum states. Thus9 in the absencff of an external magnetic fieldJ there are two hyperfine energy levels, -:
each of which may be split by an external magnetic field into a number o~ :ZeeDan sublevels. -~;
The hyperfine resonance transition used in the a~omic beam tube of the present invention occurs between the (F_4, mp=0) and ~=3, mp=0) ; s~ates, where the first number F is related to ~he magnitude of the total angular mo~entum of the atom ~electronic plus nuclear~ while the second n~mber -:
30 m~ is related to the component of this total angular momentum which is in ~ ..........
~6~8~L~
the direction of the applied external mac~netic field.
To cause a transition from one state to the other, an amount of energy E equal to the difference in energy of orientation must be either given to or taken from the atom. Since all cesium atoms are identical, E is the same for every atom. The frequency f of the electromagnetic energy re-quired to cause a change of state is given by the equation E=hf, where h is Planck's constant. For cesium, the magnitude of f is approximately 9,192.631770 megacycles.
A conventional cesium atomic beam apparatus provides a source from which cesium evaporates through a collimator which forms the vapor into a narrow beam and directs it through the beam tube.
This collimated beam of atoms is acted upon by a first state sel- -ecting magnet or "A" magnet, which provides a strongly inhomogeneous magnetic field. The direction of the force experienced by a cesium atom in such a field depends on the state of the atom. In ~his field, the energy states F=3 and P=4 are split up into sublevels. All of the atoms of the F=4 state, except those for which mp=-~, are deflected in one direction, and all other atoms are deflected in the other direction. In the apparatus of the present invention, the F=3 group ttogether with the atoms of the ~4, -4) sublevel) are retained in the beam, while ~he others are discarded. The undiscardecl atoms include those of the (3,0) sublevel.
Upon emergence from the A-~ield, those atoms enter a central region where they are subjected to a weak uniform C-field to assure the separation in energy of the mF = states from the nearby states for which m~ ~ 0. This small magnetic field also serves to establish the spatial orientation of the selected cesium ato~s and, therefore, the required direction of the microwave magnetic field.
While in this uniform weak field region, the cesium beam is subjected to an oscillating externally generated field of approximately the resonance frequency re~uired to cause transitions from the ~3,03 to the (4,03 sublevel.
~0~;~81~
After l~aving ~his energy transfer region, the beam is acted on by a second state-selecting magnet, similar to the A-magnet, producing a strong inhomogeneous field. Here the atoms o~ all the F~3 groups ~and also those of the ~4, -4) sublevel) are discarded. The only undiscarded atoms are those of the ~4,0) sublevel, which exist at this point only because of the induced transition described above. These atoms are allowed to proceed toward a detector o~ any sui~able type, preferably of the hot-wire ionizer mass spec-tometer type.
The magnitude of the detector current, which is critically dependent upon the closeness to resonance of the i~pplied RF fre~uency, is used after suitable amplification to drive a servo system to control the frequency of the oscillator/multiplier which excites the RF cavity. `
.
Cesium beam tubes as hi~her~o constructed have been expensive and ~.~
difficult to make. To provide a cesium beam tube suitable for use in the usual applications of atomic frequency standards, mechanical alignment of components is critical, and shifts in the alignment can destroy the functional ~i ~ frequency standard. The tube elements that have been descrîbed must be as~
,'! sembled and supported in place with a high degree of precision~ alignment ~ requirements relative to the beam deflection axis of the tube being approxl-.
mately .001" for effective tube operation. The precise alignment must be preserved under conditions of mechanical vibration and shock, and of a range of temperature ~ariations typical of practlcal applications of the tube.
Prior art tubes have employed complicated mounting means between the inner structural assembly of tube elements and either an inner or an outer vacuum-tigh~ envelope in an effort to meet the often-conflicting requirements of ri~idity against mechanical shock or vibration~ and flexibility to accomm~date to dif~erential expansion disturbance forces in the presence of thermal gradi- ~ -ents resulting ~rom bake out in tube processing and ambient tem~eratures in normal tube operation. A further limitation in prior art tubes is that these structure measures typically result in relatively large and heav~ tubes, ,: .
characteristics that are most ~mdesirable for certain important applications ; such as in air or space craEt.
Some prior art cesium tubes have been constructed using two separate envelopes. The first is an inner mounting chalmel to which the operative components are secured to provide mechanical stability and thermal isolation;
this inner envelope is suspended within an outer vacuum envelope. Since dif-ferential movement between the two envelopes must be allowed for, such a com-pound structure adds complexity to the manufacturing processO ~tis design also results in a relatively weak mechanical structure.
This invention relates to molecular beam tube apparatus comprising enclosure means, a source for providing a beam of molecular particles a first state selector for selecting a portion of said particles in said beam a radio frequency transition section downstream from said first state selector for causing resonance transitions o some of said selected beam particles -means or producing a weak generally homogeneous magnetic field in the radio ~ frequency transition section a second state selec~or downstrec~t from said radio frequency transition section for selecting a further portion of said bean~t comprising those beam particles that have undergone said resonance transitions, and detecting means responsive to said particles in said further portion, said source and said first state selector together comprising a first subassembly, said radio frequency transition section and said means for producing a weak field together comprising a second subassemblyJ and said second state selector and said detecting means together comprising a third subassembly, said enclosure means comprising a rigid base and a flexible cover, said frame and cover bieng sealed together to for~ a vacuum envelope, ; said three subassemblies being separately and removably secured to said ~ frameO
- ~ The p~esent invention integrates the inneT assembly and the vacuum envelope into a single structure, thereby eliminating the need for support elements between the two. It further provides for a modular assembly in which ;6~
three subassembly units are assembled to the main structural member ~which ~ is also a portion of the vacuum envelope) by means of 10 machine screws, as :~ will be described. The invention also includes novel features providi.ng good thermal isolation, smaller and more efficient magnetic structures, smoother transition between strong and weak magnetic fields, and means to feed in RF
energy with less perturbation of the C-magnetic field than in prior art tubesO
These novel features make possible a tube, both more compatible with typical operating environments than conventional devices, and lighter in weight (9 lbso against the 16 lbs. of a typical prior-art tube~
The design of the present invention eliminates the need for expen-sive and complex internal support structures while providing a beam tube of simple modular design that maintains beam alignment and is highly resistant to external mechanical disturbances such as shock and vibrationO At the same time, the design of the present invention provides excellent thermal ~ isolation for the thermally sensitive componentsO
.. .! The atomic beam tube of the present invention provides a single :
structure that serves both as vacuum envelope and as structural member for the operative componentsO This envelope is composed of a heavy and relatively .. .... . .
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~ .-~ L0~;6~3~L9rigid frame and a relatively thin and flexible cover sealed to the fr~m~. The operative elements of the tube are secured to the frame; this provides fixed alignment of these elements. Th& flexible cover accommodates itself readily to externally caused mechanical distortions without transmi~ting them to the frQme or to the operative elements. The sealed unit acts as a vacuum envelope.
The operative elements of the tube are secured to thle heavy fram~ at a minimum of locations, and the connections have low thermal conductivity, in order to isolate the operative elements thenmally from the environment. For example3 the oven structure is secured to the frame through a connecting structure that is designed to provide a relatively long thermal path to the environment.
; It is industry practice to disasse~ble such tubes when they are ~, no longer operable (generally because the cesium getters are saturated) in order to salvage reuseable components. To disassemble prior art tubes has required extensiv0 machining which is both time-consuming and expensive, in-volving high labor costs. In the cesium beam tube of the present invention, the operative parts are provided in three main modular subassemblies, secured ~to the frame by a total of 10 screws, for quick and simple disassembly and reus0 o~ the modular portions.
~, .
The operation of the cesium beam tube, as has been described, re-quires that the A and B magnets provids very strong fields tof the order of 10 kilogauss), while the C-~ield in the region between them must be re latively weak (of the order of .060 gauss) and as uniform as possible. Dis-continuities in the C-field are particularly likely to occur in the regions at which the beam enters and leaves the C-region, and can cause spontaneous transitions ~Majorana transitions) in the atomic beam which may distort the f;performance of the tube. The present invention provides a C-field winding of novel design that generates a C-field of superior uniformity at the beam apertures.
In general, i~ is desiTable to provide a cesium beam tube that is as compact~ light weight, and simple as posslble. The particular designs ~:
.
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of the A and B magnets in the pressnt invention realize such construction and are particularly adapted to the modular assembly previously described.
It is typical in the assembly and processing of molecular beam tubes to confine the source o the molecular beam material in a sealed ampoule during the bakeout ~nd exhaust part of the processing cycle, and as a final stage, while the tube is still being pumped, but after bakeout has been com-pl0ted, to open the ~mpoule. Any gases released in the opening process can then be pumped prior to the final sealing off of the tube.
A number of methods have been used in the prior ar~ for opening the ~:
ampoule. One such method is to provide means whereby a member of the ampoule is ruptured when electrical energy is applied to a heating coil to cause expansion in a member mechanically linked to a rupturing element. A more sophisticated prior art method is to discharge an external capacitor through electrical conducting paths into the tube, so arranged that a vaporizing arc is created at a member of the ampoule which is ruptured by the heat of the arc. Both of these methods require the inclusion in the beam tube of additional parts that are used only ~or this one operation; in particular, means must be provided to transmit electrical energy through the vacuum envelope, which complicates the construction of the tube.
2Q The pres~nt invention provides a novel ampoule structure and novel means for opening the ampoule that require no additional parts; i.n particular, no: additional electrical or mechanical fseds through the vacuum envelope are required.
Other objects, features, and advantages will appear from the follow-ing description of a preferred embodiment of the invention, taken to~ether with attached drawings thereof, in which:
Pig~ 1 is a schematic view of the principal beam-forming and detect-ing elements of the ~ube;
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Fig. 2 is a perspective view of the elements of Fig. l;
Pig. 3 is an exploded viev of the components of the oven and ampoule;
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` ~668~
Fig. 4 is a cross sec.tion of the ampoule;
Fig, 5 is a view of the assembled oven;
Fig, 6 is a view of the oven with reflector and support structure;
Fig. 7 is a Zeeman energy dia~ram for cesium 133 in the ground electronic sta-te; shol~ng the transition induced in the beam tube of the in-vention;
. Fig, 8 is a schematic view of the control circ~utry used with the cesium beam tube of the invention;
Fig, 9 is a perspective view of the first state selector magnet and '. 10 ion pump;
:~ Fig, 10 is an exploded perspective view of the first state selector magnet together with shielding and support structure;
Fig, 11 and 12 are longitudinal and cross sections respectively of .,, the first state selector and ion pump;
' Fig, 13 is a perspective view of the microwave structure and C-field ;~ eoil;
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Fig, 14 is a perspecti~e view of the C_field coil with portions ~i, broken away;
: .
Fig, 15 is a plan view o$ the unfolded C-field coil;
Fig, 16 is a cross section of the ass~mbled C~field coil at a beam aperture;
Fig, 17 is a detail of the conductors of the C_~ield coil at a beam :~, - . .
aperture; ~ .
Fig, 18 is an exploded view of the magnetic shield package and con-tents; . ;~ :~
Fig. 19 is a cross section of the outer en~elope and contents near ~ -he:cellter;
Fig. 20 is a perspectlvo vie~Of the B-ficld magnet and the detector;
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: Fig, 21 sllows the elements of Fig, 20 ~nth support st~lcture;
30 : Fig, 22 and 23 are a plan view ~nd a rear elevation YiÇW of t~le ~ 7 ~
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~066~
B-field magnet and the cletector;
Fig. 24 is an exploded view of the outer packaging and connections and ~he modular units; and Fig. 25 is a longitudinal view partly in sec~ion of the assembled units of Pig. 24.
General Referring to the drawings, and particularly to Figs. 1 and 2, the basic beam-forming and de~ecting elements of the cesium tube 11 of the in-vention are shown schematically and in perspective. A source of atomic par-ticles includes an oven 10 which evaporates liquid cesium and emits ~through a collimator~ a beam of neutral cesium ato~s which are statistically distri-buted between two stable energy states, as previously dçscribed. l`he first state selector or A magnet 12 splits these energy states into sublevels and selects the atoms in the F-3 states (together with those in the ~4,-4~ sub-level) and deflects the remaining atoms so that they no longer form part of the beam. The be~m of selected atoms then passes through the RF interaction section 14; in this region a weak homogsne~us magnetic field (C-field) is supplied by the winding 22. Microwave energy is supplied at the resonance frequency to induce transitions of some of the beam atoms from the t3,0) state to the (4,0~ state (Fig. 7). The beam atoms ln the ~4,03 state are then selected by the second state selector or B magnet 16, the atoms in the remaining states being deflected out of the beam. The cesium atoms selected by the B magnet strike the hot wire ionizer 20, and an electron is stripped from each cesium atom~ c~using the re-emission of cesium ions9 which are accelera~ed through a mass spectrometer 2~7 in~o the electron multiplier 18.
The electron mul~iplier provides an output current propor~ional to the number of atoms arriving at the hot wire 20, that is3 proportional to the number Gf atoms that have been raised to the second state in the mlcrowave cavity.
As shown in Fig. 8, the output of the atomic beam tube 11 is fed to control el0ctronics 260 which produce a suitable error output signal 261 ':
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which is applied to a crys-tal oscillator 262. The f`requency out-put of -the crystal oscillator (typically 5 megahertz) is controlled by the processed signal 261 from the cesium beam tube, and then multi-; plied in the frequency multiplier chain 264 and applied to -tube 11, at the precise resonance ~requency (typically 9192 mHz). Multiplier chain 264 and the controlled osailla-tor 262 from the microwave generator 266. The usable output signal is derived from controlled oscillator 262 at 268.
Summary of Modular Components The elements that have been described and shown in Fig. 8 are in general terms old and well-known in the art. The cesium tube of the invention provides -three modular subassemblies including a cesium ampoule and a first state selector magnet in combi.nation wi-th the ion pump, a second state selector magnet in combiination~wi-th bhe mass speatromeber,i and a G-field ~inding and microwave structure, all of novel design, as well as a novel outer ,., ' ', . ' ~ package for the entire tube.
, . . -To provide the advantages o~ the modular assembly of the . ~ invention, as previously described, the oven 10 (witl~ cesium ampoule) and A~magnet 12 (with ion pump), shown separa-tely in the schematic ,, .
views of Figs. 1 and 2, are combined in an oven/A-magnet assembly module 240 (Fig. 24). The RF interaction region 14 and C-field, shown unenclosed in Figs. 1 and 2, are contained in magnetic shield package 179 (F`ig 24). The B-magnet 16, hot wire ionizer 20, mass spectrometer 207 and electron multiplier 18 are packaged together in a detector assembly module 244 (Fig. Z4)~ Referring to Figs.
~: :
24 and 25, modules 240 and 244 and magnetic shield package 179 are `
essentially independent of one another and constitute the subassembly units within the outer package of the beam tube, and are assembled ~; 30 thereto by means of 10 screws, as will be described.
The details of each of these modular components are des~
cribed below.
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~66~g OvenJA-magnet module: oven and ampoule The structure of the novel oven-ampoule assembly 10 of the invention consti-tuting a source for providing a beam of ces:ium particles, is shown in de-tail in Figs. 3-6. The assembly 10 includes collimating means 42, not described, and oven means including a reservo:ir 29 contain-ing an ampoule 27. The ampoule 27 includes a -thin walled (0.015") generally cylindrical shell 30 and a -top 37 including a fill tube 38.
Top 37 and cylinder 30 -together form an enclosure.
The end of shell 30 opposite to top 37 provides an opening 49.
10 A cup shaped base 34 is sealed into shell opening 49 by an eutec-ti~
metal 32 designed to fail mechanically a-t a -temperature of approximately 600C. An example of such an eutectic metal is an alloy of 45% copper and 55% indium. A weak spring 35 is compressed between base 32 and top 37.
After the enclosure has been filled with liquid cesium, fill tube 38 is closed by pinching and heliarc welding.
A wire screen mesh 36 having high -thermal conductivity surrounds ampoule 27 within reservoir 29. The mesh 36 serves bo-th as a heat trans-fer element and as a retaining and suppor-t element for the ampoule.
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Ampoule 27 is supported within reservoir 29. A copper outer 20 cylinder 28 of reservoir 29 includes an annular recess 40 at its lower :~ .
i por-tion. A welding adaptor 39 having a lower flange 41 is brazed to , .
! recess 40 of outer cylinder 28. An ampoule support member 43 includes an `I inve~ted cup portion 44 and three spaced supports 45. Inverted cup portion l 44 of member 43 is heliarc welded at 46 (Fig. 4) to the inner surface 1 .
of welding adaptor flange 41 to seal -the lower end of reservoir 29.
, This creates an enclosed reservoir space 51 surrounding base 34 and communicating with mesh 36. Ampoule 27 is seated in support member 43 `'I ~ ,. . .
`,.,3 with ampoule base 34 within spaced supports45.
;i~ Two tantalum heaters 90 and 92, retained in a ceramic support `~330 structure 88, are inserted into coll1mator assembly 42 through quartz tubes 80 and 82. The ampoule is opened,after bakeout of the beam tube, by means of these heaters, which heat the ampoule to 600C, at which temperature the eu-tectic seal fails. The combination of the vapor ~I pressure of the cesium within ampoule :` :
661!3~
27 and the force of compressed weak spring 35 exerts a stress greater than ~he working stress of the metal of seal 32 and pushes base 34 out o shell 30~ thereby releasing the cesium in the ampoule. Weak spring 35 prevents the base from settling back into place, resealing the ampoule.
In later operation of the tube, tantalum heaters 90 and 92 are used to warm the entire oven assembly 10 to the operaLting temperature, typic-ally about 90C. At this temperature the liquid cesium in reservoir space 51 slowly vaporizes and diffuses fro~ the mesh 36 to collima~ing means 42.
Collimator 42 is functionally equivalent ~o a bundle of small tubes so oriented tha~ a dire~ted beam of cesium atoms emerges. Construction of collimating means is well known to ~he art, and will no~ be detailed here.
The oven support structure is designed to provide thermal isolation from outside the beam tube. Since the oven operates in a vacuum, there is no heat loss from convection; the major loss is by radiation, with some loss by conduction. The oven suppor~ struc~ure is therefore constructed of mater-ial of poor thermal conductivity such as stainless steel and includes ear portions 100 and 102 fo~ securing oven 10 to the A-magnet assembly9 as will be described. Additionally, 0.003" Kapton shims 99 between the ear portions o~ the support structure and the A-magnet assembly further discourage ~hermal conduction. A radiation shield 104 of highly polished aluminum surrounds the major portion of the o~en, and presents radiation heat loss from the oven.
An oven of the design described required less than two watts for operation.
Oven/A-ma~net module: A-magnet and ion p_mp Referring now to Figs. 9 through 12, a permanent magnet driver lll is shared by the first state selector magne~ tA magnet) 12 and the ion pump 110. The ion pump performs the well-known function of removing undesired gasses and maintaining tube va~uum during operation. Permanent magnet 111 is generally of a typical "C" shape, but with a novel reentrant inner surface shape that gives it the distinguishing capability of providing proper fields 0 or both selection and ion pumpingO The axis v~ magnet lll is parallel wi~h - .
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the beam.
"Dipole configuration" soft iron pole pieces 112 and 114, Of a well-known design, are secured in the gap of "C" shaped permanent magnet 111, and provide the inhomogeneous deflecting field of first state selector 12.
Reen-trant extensions 108 and 109 of permanent magnet 111 extend inwardly toward one another, and in conjunction with a second pair of short cylindrical pole pieces 116 and 118 provide the field for the ion pump 110, located between pieces 116 and 118. The ion pump is of any suitable design and is well known.
Permanent magnet 111 provides in effect two permanent magnet cir-cuits in parallel to drive both the "A" state selector 12 and the ion pump ~;
110. The magnetic driver is designed to provide approximately 10 1~ gauss in the state selector circuit while providing approxima~ely 1000 gauss for the ion pump. The compact arrangement of this combination permits the atomic .. . .
~ beam tube assembly to be smaller, lighter, and less expensive than those .... . .
i`,'! ~ hitherto constructed, and is also especially adapted to the modular design of the present beam tube apparatus.
A magnetic shield 132 covers approximately the upper half of the outer surface of magnet 111 and additionally on one end is interposed between Z 20 the magnet and the C-field/microwave structure module 179 ~Figure 24). Shield , 132 provides aperture 138 for the passage of the atomic beam from the A-magnet i' .,.
12 to module 179. The structure of shield 132 further provides field control ~}~ for the attenuation of the 10 K gauss deflecting field of the A-magnet down to the 0.060 gauss C-field in the RF transi~ion region 14.
; A mounting plate 128 is secured to the upstream side of permanent magnet 111, and provides brackets 134 and 136. Magnetic shield 132, stainless , . .
steel spacers 113, magnet 111, and another pair of s~ainless steel spacers 117 all are fastened together by a pair of machine screws 115 passing through clearance hole~ in each and threading into tapped holes in moun~irlg plate 128.
Oven 10 (Figure 6~ is secured by its support structure ear portions 1: . .
~066~
~=
110 and 102 to brackets 134 and 13G ~s thesc brackets are opCn in construc~
tion, rather than solid, thcy provide a relativcly :Long thcrmal path for the conduction of heat from the oven through the brackets to thc eventual point of contact ~th the outer framc of the beam tube. Shims 99 of 0 003" Kapton are interposed between ears 100 and 102 and brackets 134 and 136 and provide further thermal insulation.
Oven 10 and A_magnet 12 with ion pump 110 from theoven/A-magnet module 240 (Fig. 24).
C-field~icrowave ~tructure module ~, .
Referring again to Figs 1, 2 and 4, the C-field and RF (radio frequency) transition section 14, including magnetic shields to be described, are packaged together as a second module 179 As previously described in connection with Fig. 2, the cesium atoms that are selected by the A~magnet 12 form a beam that must next pass through RF transition section 14 In ~liS region a weak homogeneous magnetic field - (C-field~ of approximately 06 gauss directed transverse to the beam path is ~ ~ .
~ provided by a single_layer printed circuit solenoid 22 of novel design~ The .~ ' . ' .~ ' .
construction and mounting supports of this solenoid will be described b~ re-~erence to ~igs. 13 through 19.
Referring first to Fig 15, the csnductors of solenoid 22 are etched : , . . .
by well-kno~n printed circuit techniques from a thin copper ]ayer bonded to a base 152 of polyimidc material approximatel~ 0.002 inch thick. The general shape of the base material 152 and a pattern of eight ~mifoxml~r-spaced con-dRctors 150-1 through 150~8 is sho~m in Fig. 15. Eyelet holes 307 are pro-vided at each end of the conductors lSO This printed circuit solenoid pro_ vidcs thin, wide, and closely spaced conductors of very uniform cross sectiollAl , ~ :
area and constant conductivity.
The printed circuit solenoid is assembled into a generally rectangu_ lar loop as shown particularly in Fig. 14, with the eyeleted ends of conduc-3~ tors 150 offset one conductor in xegistry so that the completed conductil~g ~C36~
path will form a one-layer spiral winding of equally spaced helical turns.
Electrical connection at each of the offset, but otherwise registered, ends of conductors 150 is made by soldering using indium washers (not shown) and secured by rivets 308 inserted through the eyelet holes. Electrical connec-tion to the solenoid is made by wire leads soldered to eyeletted pads 304 and 306 at the end of each of the outside turns.
The closed loop includes two end sections 140 and 142 that are transverse to the beam path and parallel to one another. Since the assembled solenoid winding must lie generally in the plane of the cesium beamg apertures 270 and 271 are provided in end sections 140 and 142 of such a size as to interrupt conductors 150-4 and 150-5.
Aperture 270 in base layer 152 has two opposed edges 144 (Figure 15) that interrupt the two adjacent inner strips 150-4 and 150~5 of continuous conductor 150, to provide four internal ends 122 of strips 150-4 and 150-5 adjacent the aperture edges. Ends 122 are eyeletted. To provide a continuous current path, it is necessary to bridge the aperture by connecting the inter-nal conductor ends. In addition, it is necessary to maintain uniformity of the C-field at the beam apertures insofar as is possible, to avoid field dis-continuities causing undesired transitions, as previvusly explained.
In the present invention, two patches 318 of printed circuit mater-ial similar to that described are provided to bridge the gaps and maintain unlformity of the C-field, each having an aperture 319. Two eyeletted conduct-ing~jumpers 166 and 168 are bonded to base layer 320, and angle around aper-ture 319. Referring particularly to Figures 14 and 17, a patch 318 is assembled to the winding by soldering to rivets 182 passing through the eye-lets of the jumpers and of internal ends 122. This construction maintains the continuous current path through the entire conductor 150 a-t the beam apertures.
Jumpere 166 and 168 lead the current around each aperture 270 and 271, effec-tively dou~ling the magneti~ing force at the edge of the apertures and tending to maintain a near uniform distribution of the C-field across the apertures.
.:
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~6819 Tl s structure provides an exceedingly close approximation -to the ideal of a uniformly distribu-ted curren-t sheet.
Electrical :insula-tion around the solenoid is provided by polyimide strips 18~ and 186 (Fig. 1~) made -to t;he same shape as printed circui-t base 152, one being placed on ei-ther side of base piece 152.
Inner Magne-tic Shield I'acka~e .
The assembled C-field winding 22, comprising the -three layers and two patches as described, is mounted on the inner surface of inner magnetic shield 154 (Fig. 15~ and inner shield base pla-te 156 and is held in place by rivets passing through the shield material, the outer margins of the solenoid assembly of base material 152 and insulating strips 184 and 186, and aluminum pla-tes 282 of which representative ones are shown in Fig. 18. The assembly at the aperture locations 270 and 271 is made with aluminum plates 280 that provide apertures to register with apertures 270 and 271.
A flop coil 192 (Figs. 2 and 18) is mounted on one of the central aluminum plates 282 and supported from inner magnetic shield ;
l 154 so that it is coaxial to the beam axis. This coil is used in a !
, 20 manner well known to the prior art to introduce a 20 khz. electrical signal for the adjustment of the C-field solenoid current, and will not be described further.
The sides of inner magnetic shield 15~ (Fig. 18) 9 paralleling ;
the beam path, provide magnetic end caps for solenoid 22. The resulting field across the plane of solenoid 22 thereby approximates the classical uniform ~leld of an in~ tely long solenoid with flux lines normal to the cesium beam path. Inner magnet shield I54 in combination with spaced outer magnetic shieLd 157 effectively attenuates the strong , ~ magnetic ~ields produced by the A and B magnets and also shields the 1 30 RF transition region from external magnetic perturbations.
Microwave radiation -- : .
~ ~ Referring particularly to Fis. 1, 2 and 18, microwave radiation -~
A ~ ~ iS supplied within RF interaction section 14 by waveguide structure 190, which ~ . ,: .. .
68~g is of standard "Ramsey" type and well known in the art. It will not be described here.
In prior art atomic beam tubes, constructed with separate mechani-cal protective and vacuum isolation envelopes, differential motions between the two envelopes have made it necessary to provide Elex:ible connection means between the microwave structure and the exteLior of the -tube, capable of accommodating to such motions. Such flexible means requires a relatively large aperture, typically two inches in diameter, in the magnetic shield structure to accommodate the connection. Such a large aperture introduces perturbations in the magnetic C-field due to leakage effects, which must in turn ~e compensated for, for example by providing extra "baffling means" as in United States Patent No. 3,670,171 (Lacey et al) issued June 13, 1972.
In the present invention, the combination of mechanical support and ~ `
vacuum isolation envelope into a single structure eliminates such differential motions. The inlet arm of microwave structure lgO can therefore be intimately brazed to the lower surface of inner shield base plate 156~ This construction avoids the need for a large aperture through the magnetic shield; a relatively small aperture 194, about 1" x 1/2"~ is provided in base plate 156 (Figure 18). Such a small aperture introduces only relatively small perturbations in-eo the C-field, eliminating the need for "baffling" or other compensating structure, and this structure is therefore advantageous. ~ -Outer m gnetic shield package Referring particularly to Figures 18 and 19, inner magnetic shield package is contained within an outer magnetic shield 157 and outer base plate 159. Apertures 167 and 169 are provid~d for the cesium beam. The entire unit of outer and inner magnetic shield packages, ~ith -the contained R~ transition section, forms the C-field/micr~wave structure module 179 (Figure 24). ~:
Second state seIector ~ tector module .
~ Referring no~ to Figures 20-23, permanent magnets 19~ and 199, each - -generally ~of horseshoe form, are secured to a detector table 196, and lie in ~ ' ,:
110 6G~
a horizontal plane containing the beam axis. Magnets 198 and 199 are ; assembled to provide two gaps spaced about 180 apart, one gap being down-stream of RF transition section 14 on the beam axis and the other slightly ofEset therefrom and downstream of the first. Soft iron pole piecés 200 and 201, whose configurations are identical to those of the A-magnet pole pieces, - are provided in the first gap between permanent magnets 19~ and 199, on the beam axis. Pole pieces 200 and 201 are driven by magnets 198 and 199, and act as the second state selector (or B-magnet) 16. A second pole piece assembly 204 is provided in the second gap between permanent magnet pieces 1~8 and 199, slightly offset laterally from the beam axis and downstream from the first gap, pole piece assem~ly 204 is driven by permanent magnets 198 and 199 to function as a mass spectrometer 207. Thus the second state selector and the mass spectrometer are driven in series by a single pair of permanent magnet pieces 198 and 199. ~liS combination contributes to making the cesium beam tube of the present invention smaller and lighter than prior art atomic beam ~ tubes.
`, Detector table 196 is provided with three mounting tabs to which is secured a hot ~ire ionizer assembly 21 including hot wire 20. An electron .. . .
multiplier and shield assembly 18 is secured beneath detector table 196, and aperture 203 is provided in table 196, corresponding with an aperture 205 in the electron multiplier shield. The B-magnet 16, mass spectrometer 207, hot wire ionizer assembly 21 and electron ~ultiplier assembly 1~ together make up B-magnet/detector module 244 ~Figure 24).
The beam of cesium atoms that emerges from the RF transition sec-tion 14 (Figure 2~ contains certain atoms that have undergone a transition and other atoms to be discarded. The atoms selected by second state selector or ~-magnet 16 strike the hot wlre 20, which is of a standard type and will not be further described. Hot wire 20 strips an electron from each neutral cesium atom that serikes it, and re-emits a positively charged cesium ion. The cesium ions are then sorted by mass spectromPter 207 from ~ ,,.
: : .
668~L~
impurities unavoidably emitted by hot wire 20 and are directed into electron multiplier 18~ which produces an amplified output proportional to the number of atoms incident upon the first dynode oE the multipl:Ler.
Outer package Referring particularly to Figures 24 and 25, the outer package of the atomic beam tube of the invention is a single vacuum tight envelope com-posed of a rigid base 210 (Figure 24), made of 1.8 inch thick stainless steel, and a relatively thin and flexible cover 212 made of 1 mm thick stainless ` steel. Base 210 provides the necessary ports with vacuum tight feed-through connections to power and RF sources, which are standard and will not be des-cribed in detail. The three main subassemblies or modules 179, 240 and 244, which have previously been described in detail, are secured to base 210.
In assembly, oven/A-magnet module 240 is secured to supports 222 and 224 on base 210 by two machine screws 400. Thus the path for heat con-:
duction from oven 10 to the exterior environment of the cesium tube extends through open brackets 134 and 136 and supports 222 and 224 to frame 210.
This structure provides a relatively long thermal path and aids in isolating `-oven 10 from the outside environment. ~
:
1 The C-field/microwave structure module 179 is secured to four posts 226 by four machine screws 228. B-magnet/detector module 244 is secured to brackets 234 and 236 by four machine screws 237. Detector table 196 and brac- ~ -kets 234 and 236 together provide a relatively long thermal path from ionizer 20 to the environment outside the beam tube.
Cover 212 is welded to base 210 after the necessary connections have been made to the feed-through connectors. The tube is then evacuated under high temperature conditions.
This modular construction of the beam tube, with each module or subassembly indivldually secured at a minimum of polnts to the rigid frame of :. :the single ~nvelope structure, provides alignment and s~pport for the modules 30~ while simultaneously providing thermal isolation and mechanical protection ~ .
.~ -661 3~L~
o~ the components in -the modules from the outside environment. At the same -time, the relatively ~lexible cover accommodates to thermal and mechanical stresses induced by -the welding opera-tion; an outer structure entirely of the thicker material would not provide -this :~lexibili-ty, and alignment difficulties would result.
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Claims
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Molecular beam tube apparatus comprising enclosure means, a source for providing a beam of molecular particles a first state selector for selecting a portion of said particles in said beam a radio frequency transition section downstream from said first state selector for causing resonance transitions of some of said selected beam particles means for producing a weak generally homogeneous magnetic field in the radio frequency transition section a second state selector downstream from said radio frequency transition section for selecting a further portion of said beam comprising those beam particles that have undergone said resonance tran-sitions, and detecting means responsive to said particles in said further portion, said source and said first state selector together comprising a first subassembly, said radio frequency transition section and said means for producing a weak field together comprising a second subassembly, and said second state selector and said detecting means together comprising a third subassembly, said enclosure means comprising a rigid base and a flexible cover, said frame and cover being sealed together to form a vacuum envelope, said three subassemblies being separately and removably secured to said frame.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA318,221A CA1066819A (en) | 1974-10-09 | 1978-12-19 | Cesium beam tube |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/513,289 US3967115A (en) | 1974-10-09 | 1974-10-09 | Atomic beam tube |
| CA237,259A CA1056957A (en) | 1974-10-09 | 1975-10-08 | Cesium beam tube |
| CA318,221A CA1066819A (en) | 1974-10-09 | 1978-12-19 | Cesium beam tube |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1066819A true CA1066819A (en) | 1979-11-20 |
Family
ID=27164146
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA318,221A Expired CA1066819A (en) | 1974-10-09 | 1978-12-19 | Cesium beam tube |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1066819A (en) |
-
1978
- 1978-12-19 CA CA318,221A patent/CA1066819A/en not_active Expired
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