AXIALLY SYMMETRIC MULTIPLE BEAM HIGH FLUX ATOMIC BEAM APPARATUS

Abstract: An axially symmetric multiple beam high flux atomic or molecular beam apparatus (16') in which state selection by means of multipole deflecting magnets (46') is employed. The source of the atoms (12), or molecules comprises a ring source (42) which enables high flux to be achieved, and permits a detector of simple construction to be employed on the beam axis, to provided high performance "flop-in" signal detection.

Description: AXIALLY SYMMETRIC MULTIPLE BEAM HIGH FLUX ATOMIC BEAM APPARATUS Technical Field This invention relates to an atomic resonator apparatus, and more particularly, to an improved ap¬ paratus in which the signal to noise ratio is ma¬ terially improved over the prior art. Background Art Atomic and molecular resonators are basic fre¬ quency determining elements in stable frequency stan¬ dards. They are widely used and both rely upon the predicted frequency of quantum state transitions to serve as a standard. Fundamentally, an atomic beam frequency stan¬ dard detects resonance transitions between specific energy states ofthe atom to obtain a standard fre- quency. To utilize this resonance, atomic particles, such as cesium atoms, in a beam interact with electro¬ magnetic radiation in such a manner that when the fre¬ quency 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 of the applied radiation is modulated about the precise atomic resonance frequency to pro¬ duce a signal from the detector circuitry suitable for the servo control of a flywheel oscillator. Con¬ trol circuitry is thus employed to lock the center fre¬ quency of the applied radiation to the atomic reson¬ ance line. In molecular resonators, such control circuitry locks onto the molecular resonance line. When cesium atoms are employed in an atomic beam tube, the particular resonance of interest is that of the transition between two hyperfine levels resulting from the interaction between the nuclear magnetic dipole and the spin magnetic dipole of the valence electron. Only two stable configurations of the cesium atom exist in nature, in which the dipoles are either parallel or anti-parallel, corresponding to two allowed quantum states. Thus, in the absence of an external magnetic field, there are two hyperfine energy levels, each of which may be split ' by an ex¬ ternal magnetic field into a number of Zee an sub- levels. 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 fre¬ quency f of the electromagnetic energy required 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 mega¬ hertz. A conventional cesium atomic beam apparatus includes a source from which cesium atoms are pro¬ duced. The source is an oven, which when operating at 90°C will emit a cloud of cesium atoms from a rectangular opening called the collimator, which gives the beam some degree of definition. The atoms emerge from the oven with thermal velocities of ^≠ 240 meters/sec. The beam is then directed through the beam tube. This collimated beam of atoms is acted upon by a first state selecting magnet or "A" magnet, which provides a strongly inhomogeneous magnetic field. The cesium atoms, which behave like miniature bar mag¬ nets, go through a quantum mechanical selection pro¬ cess that forces the atoms to take varying trajectories depending upon their energy level. The direction of the force experienced by the cesiumatom in such a field depends on the state of the atom. In this field, the energy states F=3 and F=4 are split up into sublevels. All of the atoms of the F=4 state. except those for which m J ,=-4 , are deflected in one direction, and all other atoms are deflected in the other direction. Upon emergence from the A-field, those atoms enter a central atomic interaction region. Atoms with certain desired energy levels are deflected into a "C" field region, where two fields act upon the atoms. A weak, < 100 milligauss, "C" field maintains energy level separation of the cesium atoms. The second field is a microwave R.F. field. The magnetic component of the microwave field induces transitions from one atomic energy state to another (such transi¬ tions may be between the (3.0) and the (4.0) sublevels). The transition can only occur if the applied micro- wave frequency is precisely the same frequency as the resonant frequency of the cesium atom. The reso¬ nant frequency is th.e precession of the cesium atom's valence electron, which is similar .to the pendulum of a clock swinging back and forth at the rate of 9,192,631,770 times per second. Atoms that have undergone a transition in the "C" field region (the magnetic moment is reversed or "flopped") are deflected into a detector (a pro¬ cess known as "flop-in" detection) . Those that are not flopped are deflected into the graphite getters, which are placed at critical areas throughout the tube to absorb stray cesium. The deflection is caused by the "B" magnet, which is similar to the "A" mag¬ net and is referred to as the "state analyzer." The "B" magnet is also known as a state selecting magnet. Detection of the flopped atoms is accomplished by means of a hot wire (T=905°C) , which ionizes the atoms. These atoms are then directed by an electric field through a mass spectrometer and finally to an electron multiplier. The electron multiplier boosts the relatively weak beam signal to a useful level. The mass spectrometer filters out spurious elements such as potassium, which are emitted from the hot wire itself. One of the ongoing difficulties in such atomic beam apparatus is the requirement to detect the sig¬ nal in presence of the noise background. One way of increasing the signal-to-noise (S/N) ratio is to at- tempt to increase the flux from the oven by increas¬ ing the oven temperature. This is not desirable, since the oven life is diminished, and the flux den¬ sity in the beam may reach levels where collisions between the atoms occur leading to scattering and other effects detrimental to the operation of the device. Another approach, is to narrow the line width of the beam. This can be done by lengthening the central atomic interaction region (the Ramsey cavity) , or by selecting atoms of the lowest velocity, but this approach decreases the flux, which results in lowering the S/N ratio. Conventionally, atomic beam tubes employed in the prior art employ dipole optics in the state-se¬ lector magnets. The cesium atoms move in all direc- tions under the influence of such optics, and the only useful beam is that in a single plane defined by such dipole. In an effort to increase the flux, a hexapole or multipole state selector was employed (See Becker References on Prior Art page) . One dif- ficulty with such multipole optic systems is that of detecting the beam. From the viewpoint of beam in¬ tensity, the optimal detector is a small spot on the ^4TI -6- beam center? but such a detector can only work for "flop-out" signals. The flop-out signal means that there is a high degree of background noise, and the detected signal is seen as a dip in the signal de- tected. Such "flop-out" detection has very poor S/N characteristics. In an effort to alleviate this problem, a "flop-in" annular detector had been pro¬ posed. (See Holloway Reference on Prior Art page) . It suffered from a number of mechanical difficulties, required a large area, and was never employed. Another disadvantage of prior art multipole atomic beam tubes involves placing the oven in close proximity to the state-selector "A" magnet. Such proximity is required for realizing the desired high efficiency, but in practice is difficult to achieve because of mechanical constraints. Further, for best focussing, the oven should produce a nearly point source of atoms, which is a severe restriction on the available atom flux be- cause of beam scattering effects. Further still, the prior art point source di¬ rects a beam along a straight line, which inter¬ cepts the detector unless measures are taken to block a large portion of the beam. Such direct line of sight between the source and detector detracts from good S/N performance, because atoms may pass to the detector irrespective of the actions of "A" and "B" magnets. An object of this invention is to provide an improved atomic beam tube which exhibits improved signal-to-noise characteristics. Another object of this invention is to pro¬ vide an improved atomic or molecular beam tube in which the detected flux of the beam is significantly increased, while the flux density in the beam is decreased. Still another object of this invention is to allow for optimum placement of a high flux oven with, respect to the "A" magnet, using practical mechanical means. Another object of this invention is to pro¬ vide a beam tube apparatus employing a detector of a simple mechanical construction. Still another object of this invention is to provide a detector which uses "flop-in" techniques in detecting the desired signal together with multi¬ pole state selecting magnets. Another object of this invention is to pro¬ vide a multiple beam or distributed beam high * flux source having complete rotational symmetry. This improved mechanical and geometric symmetry has ad¬ vantages with regard to frequency stability of the de¬ vice. Other objects, advantages and features of this invention will become more apparent from the following description. BUR]? Disclosure of the Invention In accordance with the principles of this in¬ vention, the above objects are accomplished by pro¬ viding an atomic or molecular beam apparatus for use with multipole state selectors including an oven pro¬ viding an axially symmetric multiple beam, high flux source in which an annular ring (or equivalent) oven is employed. The atomic beam produced by the annular ring features a high total flux while having a low flux density. A detector having a small active area, placed on the symmetry axis, provides "flop-in" de¬ tection. In this fashion high flux and high signal- to-noise operation can be -achieved, yet the detector can be of a simple construction, such as the prior art hot wire detector located along the center of the beam and constructed as a small spot. Brief Description of Drawings Fig. 1 is a perspective view of a prior art ces- sium beam tube. Fig. 2 is a diaqram of the atomic beam trajectory of Fig. 1. Fig. 3 is a diagram of prior art multipole tra¬ jectories. Fig. 4 is a diagram of the-atomic beam apparatus of this invention. Fig. 5 is an alternative embodiment of the detec¬ tor of the atomic beam apparatus of this invention. Fig. 6 is a sectional view of the oven of this invention. Fig. 7 is another embodiment of the oven of this invention. Best Mode For Carrying Out The invention Figs. 1 and 2 illustrate a conventional prior art beam tube apparatus, A cesium oven 10 emits a cloud of cesium atoms 12 which pass through an "A" magnet first state selector 14. The state selector 14 operates on the atomic beam so that the atoms take varying trajections depending on their energy level. The thus altered beam then passes into an atomic interaction region 16, the interaction region being formed by an enclosed Ramsey cavity 18, the cavity being enclosed in a shielded housing 20 having an inlet window 22 and an exit window 24. In the Ramsey ca¬ vity, the atoms are affected by two magnetic -fields as described above. Some of the atoms undergo a transition in the "C" field region 26 in which the maqnetic moment is reversed or "flopped". Those flopped atoms then pass through a second state se¬ lector B magnet 28. Detection of the flopped atoms is accomplished by a hot wire 30 located along the beam center 32. These atoms are then directed by an electric field through a mass spectrometer (not shown) and finally to an electron multiplier 34. The electron multipler boosts the relatively weak beam signal to a useful level. The diagram of the atomic beam trajectory of Fig. 2 illustrates the magnetic moment of atoms in the atomic beam 34, 36 and 38, showing the effects of state reversal. Fig. 3 is a diagram of normal hexapole or quad- rupole focussed trajectories in prior art multipole state selector beam apparatus. Atoms from oven 10 are acted upon by state selector magnets 46 and 48. Some background signal is prevented from reaching de¬ tector 50 by beam stop 53, Ramsey cavity 18 and in¬ teraction region 16 are so described above. Fig, 4 is a diagram of the atomic beam apparatus of this invention in which the same numerals are used to designate the same parts of the prior art. A ring source oven 42 is shown from which an annular ring 44 of atoms are emitted. The annular ring of atoms pass through first multipole state selector 46'. Thereafter, the atomic beam passes throuqh the atomic interaction region 16' , which operates in a man¬ ner similar to atomic interaction region 16 of Fig. 1. Thereafter the beam passes through analyzer magnet "B" 48' which deflects some of the atoms toward de- tector 50 ' . Detector 50' is located along the beam axis 52 and may be a small spot detector, such as a hot wire ionizer or a "hot ribbon". Those atoms 55 are those to be detected, and since the action of "A" and "B" magnets, together with beam stop 56, (see Fig. 6) , prevent a direct path from oven (source) to detector, a "flop-in" detection is achieved, be¬ cause only when ions are detected is there a siσnal of interest. By employing the high flux density oven in conjunction with the multipole state se- lectors, their focussing permits the "flop-in" de¬ tection scheme to be employed. Additionally, a sim¬ ple construction for the detector is available, even thouqh multipole state selectors are employed. Fig. 5 is another embodiment of the detection of this invention in which the plane of the hot wire ionizer ribbon is parallel to the symmetry axis of the beam. Both sides of the ribbon 57 are utilized to detect atoms or molecules by surface ionization. A clear path for collecting the resultant ions in the next stage of the electronic detection apparatus is also afforded. Figs. 6 and 7 are sectional views of alternative embodiments for the ring source oven 42. In one em¬ bodiment the annular ring is formed by an annular space 54 formed between two concentric copper cylinders 56 and 58 located at the output part of the oven. The center cylinder 56 is a plug and prevents a line of sight between the source and the detector. Fig. 7 shows a front view of the output part of the oven in which the annular space 54 provides collimation through holes, channels, or other means 60 to. define the geometry of the atomic beam. The annular space may be conical as shown or cylindircal. The advantages of the present invention with re¬ gard to significantly enhancing the signal performance of a cesium beam tube represent an order of magnitude improvement in such beam tubes. These tubes are widely employed in frequency and time standards, and this invention represents a major functional improvement for such device. Althouσh multipole state selecting magnets are generally descriptive of a class of such focussing or detecting magnets. pre¬ ferred embodiments include quadrupole and hexapole. Other advantages, some of which have been des¬ cribed above include the physical separation of the oven from the "A" state selector magnets at the same time allowing high flux density to be achieved with the present oven. Such physical separation allows for thermal isolation of the oven from the "A" state selector. The detector distance may be more easily adjusted away from the "B" state selector to enhance the ion collection and mass spectographic processes. Further, there is complete axial symmetry with the present invention, which reduces overall system sen¬ sitivity and enhances ease and accuracy of alignment. This invention has been described with respect to the embodiments illustrated, but other variations ' will be clear to those of skill in the art. For ex¬ ample, the claims refer to an atomic beam apparatus comprising a concentrated source of atoms. Such a reference impliedly includes a concentrated source of molecules.

Claims: Claims 1. An atomic beam apparatus comprising a concen¬ trated source of atoms, a first multiple pole magnetic state selector, said source of atoms passing through and -being acted on by said multiple pole magnetic state selector, an atomic interaction region having an in¬ put port and an output port, said atoms after having been acted on by said first multiple pole magnetic state selector then passing through said atomic inter- action region during which selected atoms of said beam are acted upon to move from one atomic energy state to another, said atomic beam emerging from said atomic interaction region, a second multiple pole magnetic state selector through which said emerging atomic beam passes and a detector for detecting those atoms which have undergone a change of atomic energy state, an improvement comprising said source of atoms being formed by an oven emitting a cloud of atoms, said oven comprising a ring formed at the output of said oven through which said cloud of atoms pass to provide a ring source of said atoms being directed toward said first multiple pole state selector. 2. An atomic beam apparatus as claimed in Claim 1, wherein said detector comprises a detector located on the beam center line. -3. An atomic beam apparatus as claimed in Claim 2, wherein said detector is a small spot detector. 4. An atomic beam apparatus as claimed in Claims 1/ 2 or 3 wherein said ring source is formed by an an- nular shaped ring having its center along the axis of the beam center line, said annular shaped ring being formed bv a pair of concentric cylinders having an an¬ nular space therebetween. O H 5. An atomic beam apparatus as claimed in Claim 4, wherein said annular space is conically shaped. 6. An atomic beam apparatus as claimed in Claim 4, wherein said annular space is cylindrically shaped, 7. An atomic beam apparatus as claimed in Claims 1 or 2, wherein said ring source comprises an annular ar¬ ray of apertures, said annular array having its center along the axis of the beam center line. 8. An atomic beam apparatus as claimed in Claims 1 or 2, wherein said ring source comprises an annular ar¬ ray of channels, said annular array of channels hav¬ ing its center line along the axis of the beam center line. 9. An atomic beam apparatus as claimed in Claims 1, 2 or 3, wherein said detector has its detecting surface perpendicular to the center line of the atomic beam. 10. An atomic beam apparatus as claimed in Claims 1, 2 or 3, wherein said detector has detecting sur¬ faces parallel to the center line of the atomic beam. 11. An atomic beam apparatus as claimed in Claim 2, wherein said detector comprises a hot wire detec¬ tor. 12. An atomic beam apparatus as claimed in Claim - 2, wherein said detector comprises a hot ribbon de- tector. 13. An atomic beam apparatus as claimed in Claim 1, wherein said oven is thermally isolated from said first multiple pole magnetic state selector. 14. An atomic beam apparatus as claimed in Claim 1, wherein the components of said atomic beam ap¬ paratus are rotationally symmetrical about the cen¬ ter line of the beam. 15. An atomic beam apparatus as claimed in Claim 1, wherein said first state selector comprises a hexa- pole arrangement. 16. An atomic beam apparatus as claimed in Claims 1 or 15, wherein said second state selector comprises a hexapole arrangement, 17. An atomic beam apparatus as claimed in Claim 1, wherein said first state selector comprises a quadrupole arrangement. 18. An atomic beam apparatus as claimed in Claims 1 or 17, wherein said second state selector comprises a quadrupole arrangement. 19. An atomic beam apparatus as claimed in Claim.1, wherein said detector position is adjustable with respect to said second state selector. 20 " . An atomic beam apparatus as claimed in Claim 1, wherein said oven position is adjustable with res- pect to the first state selector. 21. An atomic beam apparatus comprising a con¬ centrated source of atoms, a first multiple pole magnetic state selector, said source of atoms passing through and being acted on by said multiple pole magnetic state selector, an atomic interaction re¬ gion having an input port and an output port, said atoms after having been acted on by said first mul¬ tiple pole magnetic state selector then passing through said atomic interaction region during which selected atoms of said beam are acted upon to move from one atomic energy state to another energy state, said atomic beam emerging from said atomic interaction re¬ gion, a second multiple magnetic state selector through which said emerging atomic beam passes and a detector for detecting those atoms which have under¬ gone a change of atomic energy state, an improvement comprising said detector being located on the beam center line to perform "flop-in" signal detection. PI 22. An atomic beam apparatus as claimed in Claim 21, further comprising a high flux oven for providing said concentrated source of atoms. 23. An atomic beam apparatus as claimed in Claim 22, wherein said oven provides a rinq source of said atoms. 24. An atomic beam apparatus as claimed in Claim 23, wherein said ring source comprises a multi-channel ring source of said atoms. 25. An atomic beam apparatus as claimed in Claim 21, wherein said ring source comprises distributed ar¬ ray of source channels having angled aiming directions. 26. An atomic beam apparatus as claimed ' in Claim 25, wherein said distributed array of channels forms a multi-channel source. 27. An atomic beam apparatus as claimed in Claim 25, wherein said angled aiming direction forms a coni¬ cal shape.

Inventors: EMMONS DONALD A, EMMONS, DONALD, A.
Applicants: FREQUENCY & TIME SYSTEMS, FREQUENCY AND TIME SYSTEMS, INC.
Application year: 1982
Main classification: H01S1
IPC codes: H01S1, H03L7, H05H3