US3058023A - Molecular beam source - Google Patents

Description

Oct. 9, 1962 .1. GEORGE 3,058,023

MOLECULAR BEAM SOURCE Filed March 9, 1960 INVENTOR. JAMES GEORGE ATTORNEYS United States 3,058,023 MOLECULAR BEAM SOURCE James George, Swampscott, Mass, assignor to National Company, Inc., Malden, Mass, a corporation of Massachusetts Filed Mar. 9, 1960, Ser. No. 13,909 9 Claims. (Cl. 313-231) This invention relates to a molecular beam source for supplying a molecular beam of cesium or the like from a liquid reservoir under high vacuum conditions. More particularly, it relates to a molecularbeam source which evaporates liquid cesium and forms the resulting vapor into a beam for use in a molecular beam frequency standard. The source is capable of operation in all orientations and under conditions of severe vibration and inertial forces with negligible leakage of the liquid from which the gaseous molecular beam is evolved.

The present invention may be used in various types of molecular beam apparatus. -It is of particular utility in a molecular beam frequency standard which utilizes the interaction of microwave energy of a particular wave length with a cesium beam in a frequency-determining arrangement. As used herein, the words molecule and molecular refer to both atoms of a single element and molecules 'of a compound; hence, the cesium beam is referred to as a molecular beam, although it is composed of cesium atoms. An instrument of this type is fully described in the copending application of Jerrold R. Zacharias, et al. for Molecular Beam Apparatus, Serial No. 693,104, filed October 22, 1957 and assigned to the assignee of the present application, now Patent No.

cules into a beam for passage through the beam tube.

iFrequency standards of the above type may be used in moving vehicles such as airplanes or missiles, where they are subject to severe inertial forces including substantial vibration; they also may be subject to complete change of orientation. Motion of this type has, in prior constructions, caused spillage of liquid cesium from the beam source. Since the only supply of heat for evaporation of the charge is generally intimately associated with the source, the cesium lost therefrom remains in liquid form or reverts to the solid state and is therefore of no further use in the frequency standard. The time during which the standard may be operated before being recharged with cesium is consequently materially reduced. The reliability of the apparatus is also adversely affected by the fact that the spilled cesium may lie in portions of the beam tube and associated components of the frequency standard in such manner as to cause serious malfunctions.

Accordingly, it is a principal object of my invention to provide an improved molecular beam source adapted to supply a beam of gaseous cesium or the like in 1a molecular beam instrument, particularly a molecular beam frequency standard.

A further object of the invention is to provide a reservoir designed for incorporation in a beam source of the character described which is adapted to retain a charge of liquid cesium therein under conditions of intense inertial forces and changes or orientation.

Another object of the present invention is to provide ice an improved beam source of the above character adapted to prevent leakage of liquid cesium into the frequency standard.

Since the frequency standard may be operated in moving vehicles such as airplanes or the like, the source should be small in size and capable of lightweight construction. Simplicity of design is also a feature of my invention, since it results in low cost fabrication.

Other objects and features of my invention will in part be obvious and will in part appear hereinafter.

My invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing in which:

FIGURE 1 is a sectional view of a molecular beam source made according to my invention, and

FIGURE 2 is a sectional view taken along line 2-2 of FIGURE 1 showing in detail a collimator structure which may be used in the source of FIGURE 1.

In general, a molecular beam source embodying the features of my invention includes a reservoir containing a charge of liquid cesium and a collimator which forms the gaseous molecules evaporated from the charge inito a beam. A barrier interposed between the reservoir and the collimator permits the passage of the gaseous molecules while preventing the escape of liquid from the reservoir. A heating element maintains the liquid cesium at a temperature which provides a gas pressure behind the collimator consonant with the desired flow rate from the source.

in accordance with my invention, the reservoir comprises a mass of packed fibers such as deoxidized steel wool, capable of being wet by the cesium charge. The charge is completely adsorbed by the wool. That is, the liquid cesium spreads out along the fibers to form a thin film thereon. This film, which may be only a few atoms thick, will ordinarily not be dislodged by even the severest vibrations, and therefore the cesium generally issues from the reservoir only in gaseous form through evaporation of the liquid charge.

The barrier may also be of a porous fibrous material such as steel wool. However, the surfaces of the fibers should not be wetted by the cesium charge. The nonwetting characteristic may be obtained by forming chromium oxide on these surfaces. Thus, assuming that the material of the barrier is tightly packed so that the interstices between the fibers are sufliciently small, liquid cesium will be prevented from passing through the barrier in much the same manner as a liquid is prevented from passing through a capillary tube whose inner surface is not wetted by it. On the other hand, the passage of gaseous molecules is not affected by the non-wetting characteristics of the barrier, and therefore cesium vapors from the reservoir freely diffuse through the barrier toward the collimator.

It will be understood that while the apparatus to be described below in greater detail is specifically designed to be incorporated in a molecular beam frequency standard using an atomic resonance of the cesium atom as a reference frequency, the present invention is not limited to use with this particular element. Thus, my beam source may, in many cases, be used with frequency standards utilizing the characteristics of others of the alkaline metals or other materials having the proper molecular characteristics for use in molecular beam equipmerits.

As seen in FIGURE 1, my molecular beam source may be housed in a tube 10 closed at one end (the left end in FIGURE 1) by a cap 12 and provided with a flange 14 at the other end to facilitate attachment to the apparatus which is to use the beam. The tube 10 contains a cesium reservoir 16, a barrier 18 adjacent to the reservoir and a collimator generally indicated at 20 spaced from the barrier 18 by a chamber 22. A heating coil 24 is formed around the tube 10 over the collimator 20. The entire unit is encased in a heat insulation covering 26.

A tube 27, extending through the cap 12 and sealed at its outer end 27a, contains a glass ampoule 28. The ampoule is used to store the cesium charge prior to the time the beam source is placed in use. It also facilitates charging of the reservoir 16 under the high vacuum conditions which exist in the molecular beam system of which the source is a part. More specifically, after the source has been connected to a molecular beam tube and evacuated, the tube 26, which may be of copper, for example, is crushed without fracturing it, thereby breaking the ampoule 28 and releasing the contents thereof into the reservoir 16.

The reservoir 16 is a porous mass having a large internal surface area capable of being wetted by the cesium charge. Thus, when the charge is released from the ampoule 28, it rapidly spreads out along this surface to form a thin film thereon. I have found that stainless steel wool makes an excellent reservoir. By way of illustration, the wool may have a fiber size of 1.3 mils, with the fibers compressed to form a density of 8.3 grams per cubic centimeter. A volume of cubic centimeters will then hold 0.4 cc. liquid cesium with minimum loss of liquid in a severe vibrational environment.

In its normal condition, the surface of the stainless steel wool contains a number of oxides which are not wetted by cesium. Therefore, the material must be prepared for use by removing these oxides and other impurities having a similar effect. The first step is to pass the wall through a degreasing process to remove the grease therefrom. Next, it is heated in a hydrogen atmosphere to reduce the oxides. The atmosphere should have a low moisture content, e.g., a dew point of 60 F. The temperature should be over 1600 F. to insure reduction of the chromium oxides, which are the most 'diflicult to reduce.

The reservoir 16 should maintain firm contact with the inner surface of the tube 10 to prevent the liquid cesium from escaping along this surface. This requirement is consonant with the desirability of compressing the steel wool in order to increase the liquid holding capacity per unit volume, since the compression forces the wool, which is fairly elastic, into intimate engagement with the surface Tim. The compression is maintained by a retaining screen 30 fastened to a fiat ring 32 brazed to the tube 10 adjacent to a shoulder 34.

As pointed out above, the barrier 18 may also be of stainless steel wool. However, to prevent the passage of liquid cesium, it should not be wetted by this material. Accordingly, it is prepared in a manner calculated to give the opposite results from the treatment accorded the steel wool of the reservoir 16. More specifically, after degreasing, it is heated in a hydrogen atmosphere saturated with water. This insures rather complete oxidation of the chromium on the surfaces of the fibers, thereby giving them a non-wettable characteristic with a cesium charge.

The interstices between the fibers in the barrier 18 should be small enough to prevent liquid flow, which can occur in spite of the non-wettable characteristics, if the spaces between the fibers are sufiiciently large. With a fiber size of 1.3 mils, the wool may be compacted to a density of 8.3 grams per cubic centimeter to provide an effective impediment to liquid flow under the vibration al conditions likely to be encountered in operation. The compression required for the desired density is provided by a second retaining screen 36 which also serves to fix the barrier 18 in its proper position. The screen 36 is 4 afiixed to a ring 38 brazed to .the tube 10 adjacent to a shoulder 40.

As seen in FIGURES l and 2, the collimator 20 is disposed between and clamped in position by a pair of supporting members 42 and 44. The members 42 and 44 conform to the innersurface of the tube 10 and are secured against a shoulder 46 within the tube by a retaining ring 48 brazed in place. In its preferable form, the collimator comprises alternate fiat strips 50 and corrugated strips 52 of nickel foil (FIGURE 2). The cesium atoms entering the collimator 20 from the chamber 22 issue from the opposite end of the collimator as a narrow, well-defined beam, as required in a molecular beam frequency standard.

The liquid retention characteristics of the molecular beam source are further enhanced by the location and operation of the heating coil 24. The temperature of the coil 24 is controlled by a thermostatic switch schematically indicated at 54 (FIGURE 1) which is set to maintain the temperature of the reservoir 16 at 'C., a level high enough to supply sufficient cesium vapor at the output of the source. The temperature of the collimator 20, which is within the coil 24, will then be somewhat hotter, say C., than the reservoir 16. Thus, the coolest location which the cesium may contact is within the reservoir 16, and the temperature progressively increases as one moves from the reservoir to the right through the barrier 18 and the chamber 22 to the collimator 20.

Should some of the liquid cesium escape from the reservoir 16, it will arrive at a warmer portion of the molecular beam source; there it will tend to evaporate faster than the liquid within the reservoir. Furthermore, gases contacting the escaped liquid will tend to condense at a slower rate than those in contact with the charge contained in the reservoir 16. Consequently, there is a tendency for the escaped cesium to migrate back into the region of lowest temperaturethe reservoir 16. Thus, over a substantial period of time, there will be essentially no net flow of liquid from the reservoir 16, even under the severest of environment conditions.

The insulating cover 26 serves to insulate the tube 10 and the parts contained therein from the effect of the ambient temperature and thereby maintain the desired temperature gradient within the source.

The molecular beam apparatus of which the beam source is a component is generally internally evacuated to a hard vacuum, preferably 10* mm. mercury or better. Therefore, the source must be effectively sealed from the atmosphere. This may conveniently be accomplished by brazing the cap 12 to the tube 10, and, accordingly, the material of which these parts are made should be susceptible of joining by suitable brazing techniques. It should also be relatively inert, particularly with respect to cesium, and it should emit little or no contaminating material into the apparatus. Oxygen-free, high-conductivity copper meets this requirement, and, additionally, it facilitates conduction of heat from the heating coil 24 to the reservoir '16. The members 42 and 44 should also be of copper to facilitate conduction of heat inwardly to the collimator 20.

Copper is wetted by cesium, and therefore liquid cesium introduced to the reservoir 16, coming in contact with the surface 10a of the tube 10, will tend to spread out along this surface. To prevent the liquid on the surface 10a from spreading beyond the limits of the reservoir 16, I prefer to fabricate the ring 32 of a material such as nickel (grade A or better), which is not wetted by cesium. The ring 32, which is brazed in place, thus prevents surface flow of cesium to the right (FIGURE 1) along the inner surface of the tube 10. For the same reason, the ring 38 should be of a material not wetted by cesium. The screens 30 and 36 may also be fabricated from nickel.

Thus, I have described an improved molecular beam source adapted to provide a beam of gaseous molecules evolved from a liquid charge. The charge is contained in a reservoir having a large surface area of material wetted by the charge, and, accordingly, the liquid spreads out along the reservoir surfaces to form a thin film there on which generally adheres even when exposed to the severest inertial effects. In its preferable form, the reservoir consists of a wool or other fibrous material which permits free flow of the gaseous molecules while retaining the liquid. The retention characteristics of the material are aided by the fact that, should some of the liquid contained in the reservoir be dislodged, it faces a tortuous labyrinthine path to escape from the reservoir, making it extremely likely that it will contact and adhere to some of the many surfaces encountered.

Following the reservoir is a barrier, which again may be of wool, though of a material not wetted by the liquid charge. While permitting gas to flow through it, the barrier, whose interstices are considerably restricted, serves an effective impediment to the flow of liquid. Accordingly, there is a minimum probability that any of the liquid charge will escape from the molecular beam source.

It will be apparent that other porous or spongy masses than wool may be used in constructing the reservoir and barrier. For example, an aggregate of small particles may be sintered to form a spongy block having a large internal surface and providing many of the desirable char acteristics set forth above.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efl'iciently attained and, since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

I claim:

1. A molecular beam source adapted to form a beam of gaseous molecules evaporated from a liquid charge, said source including a housing having an exit aperture, a reservoir containing said liquid charge and disposed within said housing, said reservoir comprising an inert porous mass of liquid adsorbent elements, said elements being elastically compressed by said housing, and said mass being wettable by said charge, whereby said charge spreads out over the internal surfaces of said elements to form a thin fil-m thereon.

2. The combination defined in claim 1 including a collimator disposed in said aperture to collimate the gaseous molecules evaporated from said charge and issuing from said housing.

3. The combination defined in claim 2 including a heating element adapted to raise the temperature of said charge to facilitate evaporation thereof.

4. The combination defined in claim 1 including a liquid barrier interposed between said reservoir and said aperture, said barrier providing a plurality of capillary passageways between said reservoir and said aperture, the surfaces of said passageways being of a material not Wettable by said liquid charge.

5. A molecular beam source adapted to form a beam of gaseous cesium molecules evaporated from a charge of liquid cesium, said source including a housing having an exit aperture, a reservoir for said liquid charge disposed within said housing, said reservoir comprising a mass of deoxidized steel wool, a charge of liquid cesium in the form of films on the fibers of said wool, and a collimator disposed to form a beam of the gaseous molecules evaporated from said charge and issuing from said aperture.

6. The combination defined in claim 5 including a barrier substantially impervious to said liquid cesium disposed between said reservoir and said collimator, said barrier comprising a mass of steel wool having a material on the surfaces of the fibers thereof which is not wettable by liquid cesium.

7. The combination defined in claim 6 including a. heating element disposed around said collimator and adapted to raise the temperature of said charge to facilitate evaporation thereof.

8. A molecular beam source adapted to form a beam of gaseous molecules evaporated from a liquid charge, said source including a housing having an exit aperture, a liquid charge, a reservoir for said charge disposed within said housing, said reservoir being a sponge whose interior surfaces are of a material wettable by said charge, a collimator disposed to form a beam of gaseous molecules evaporated from said charge and issuing from said aperture, a barrier disposed in said housing between said reservoir and said collimator, said barrier being a porous mass whose inner surfaces are not wettable by said charge, said barrier having a plurality of interstices providing passageways therethrough, said interstices being of capillary dimensions, thereby to substantially impede the passage of the liquid of said charge from said reservoir to said collimator, and a heater adapted to raise the temperature of said charge to facilitate evaporation thereof.

9. The combination defined in claim 8 in which said heater is disposed adjacent to said collimator, whereby said collimator has the highest temperature in said source and the temperature of said source decreases between said collimator and said reservoir.

References Cited in the file of this patent UNITED STATES PATENTS

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