US3450876A

US3450876A - Oven source for atomic beam tubes having a non-wettably coated gas passageway between the reservoir and the beam

Info

Publication number: US3450876A
Application number: US564117A
Authority: US
Inventors: Robert H Kern; Joseph H Holloway
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1966-07-11
Filing date: 1966-07-11
Publication date: 1969-06-17
Anticipated expiration: 1986-06-17

Description

June 11, 1969 R. H. KERN ETAL 3,450,876

OVEN SOURCE FOR ATOMIC BEAM TUBES HAVING A NON-WETTABLY COATED GAS PASSAGEWAY BETWEEN.THE 1 RESER IR AND THE BEAM Fil July 11. 1966 9 e r |8-L FREQUENCY MULTIPLIER I I I4) I5 I? l i g REFERENCE i CONTROL 1 OSCILLATOR MODULATOR cmcun INVENTORS ROBERT H KERN JOSEP H. H0 LOWAY ATT United States Patent and Joseph H. Holloway, Topsby mesne assignments, to Hew- Palo Alto, Calif., a corporation 6 Claims ABSTRACT OF THE DISCLOSURE An atomic beam tube including an oven source having portions of its interior coated with a dichlorodimethylsilane material which is resistant to wetting and chemical attack by certain atomic beam materials at their operating temperatures and pressures. The non-wettablc coating eliminates creepage and spillage of atomic beam material into the tube.

The present invention relates in general to oven beam sources for atomic beam tubes and, more particularly to an improved oven source wherein certain gas passageway defining members are coated with a material which is non-wettable by the atomic beam material, whereby undesired spillage of atomic beam material into the tube is prevented by preventing undesired creepage of the atomic beam material in the liquid phase within the oven due to capillary attraction and surface wetting effects. Such an improved oven source is especially useful in cesium atomic beam tubes employed as frequency standards, atomic clocks, and the like.

Heretofore, oven sources for atomic beam tubes have employed antispill devices interconnecting the reservoir of atomic material with the upstream end of the beam collimator. Moreover, the ovens have been designed with built-in temperatures gradients to prevent condensation of the vaporized atomic beam material in and around the beam collimator. In spite of these elaborate precautions, it has been observed that a certain amount of spillage of atomic beam material was occurring from the oven through the beam collimator and into the tube. The result of such spillage, aside from wasting the beam material and thus reducing the tubes operating life, was that it produced a large background signal as well as an altered resonance signal. In such a case, the increased signal may exceed the dynamic range of the frequency control circuitry of the frequency standard, thus, interrupting operation of the frequency standard. For many applications of frequency standards such interruptions of operation are intolerable.

It has been found that the atomic beam material such as, for example, cesium was wetting the surfaces of the collimator tubes or straws and also the tubular structure of the internal antispill network. As a result the atomic beam material, in the liquid phase, was creeping through the antispill tubulation into the region of the collimator and through the collimator into the tube. A search for suitable metals that would not be wet by the atomic beam material did not turn up any such suitable metals.

In the present invention, the antispill tubulation and the beam collimator are coated with a material that is resistant to attack by the atomic beam material under the operating conditions of temperature and pressure, is not wet by the atomic beam material, and which withstands the tube processing temperatures of 350 C. for 24 hours under vacuum of 10* torr or less. In one preferred embodiment of the present invention, (CH SiCl (Dry Film, a product of General Electric) is used as the coating material in ovens using cesium beam material. Coating materials useful with atomic beam materials other than cesium, such as sodium, potassium, and thallium, are Teflon and Dry Film. A particularly convenient combination of collimator and antispill material and coating material is oxidized stainless steel metal coated with (CH SiCl This coating material readily adheres by chemical bond oxides. Stainless steel is readily oxidized during the conventional copper brazing process wherein parts are heated to copper brazing temperature in a moist hydrogen atmosphere. It has been found that in ovens having the antispill tubulation and the beam collimator coated with such non-wettable materials, that spillage of atomic beam material into the tube is eliminated.

The principal object of the present invention is the provision of an improved atomic beam oven and atomic beam tubes using same.

One feature of the present invention is the provision of an atomic beam oven having gas passageway defining portions thereof coated with a material which is non-wettable and resistant to chemical attack by the atomic beam material, whereby spillage of atomic beam material from the oven into the tube is prevented in use.

Another feature of the present invention is the same as the preceding feature wherein the coated gas passageway defining portions include the beam collimator and/or the antispill tubulation.

Another feature of the present invention is the same as any one or more of the preceeding features wherein the coating material is selected from the class of alkylchlorosilanes, long chain saturated hydrocarbon, and Teflon.

Another feature of the present invention is the same as any one or more of the preceding features wherein the coating material is Dry Film, the atomic beam material is cesium and the material which is coated by the Dry Film is oxidized stainless steel.

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

FIG. 1 is a schematic diagram of an atomic frequency standard employing features of the present invention,

FIG. 2 is a longitudinal sectional view of an atomic beam oven source employing features of the present invention,

FIG. 2A is a detail of a portion of the structure of FIG. 2 delineated by line A-A, and

FIG. 3 is a view of a portion of the structure of FIG. 2 taken along line 33 in the direction of the arrows.

Referring now to FIG. 1 there is shown an atomic beam tube 1. The tube 1 includes a vacuum envelope 2 containing an oven type atomic beam source 3 disposed at one end of the envelope 2. The oven 3 will be more fully described below and serves to project a stream of collimated atomic particles, at thermal velocity, over a predetermined beam path 4. A first state selecting magnet assembly 5 deflects out of the beam the atomic beam particles of the undesired energy states. The selected energy state atoms then pass through a cavity resonator structure 6 containing split

field interaction regions

7 and 8. A weak magnetic field as of gauss is provided in the cavity region 6 by means of a C-field magnet 9 to separate the magnetic field dependent resonance lines of the atoms from the desired field independent resonance line. Microwave energy at the resonance frequency of the atoms is supplied to the cavity resonator 6 from a micro wave generator 11 to excite resonance of the atoms. A second energy state selecting magnet assembly .12 is disposed downstream of the resonance C-field region to defleet beam particles that have undergone resonance into a target detector 13 to produce a resonance output signal. A modulator 14 modulates the resonance conditions in the resonance region as by modulating the frequency of the applied microwave energy. This produces a modulation component on the output signal which may be phase detected in a control circuit 15 to control the carrier frequency of the applied microwave energy. The microwave generator 11 includes a reference oscillator 16 which provides an output at 17 at some convenient low frequency such as mHz. which output is locked to the resonance line of the atomic beam particles. Another output of the reference oscillator is used to provide the applied microwave energy by suitable multiplication in frequency multiplier 18.

Referring now to FIGS. 2 and 3 there is shown an oven structure of the present invention. The oven 3 includes a main block body portion 28 as of copper having a reservoir chamber 21 formed in the lower portion thereof and having a collimator chamber 24 formed in the upper half of the body 28. A collimator 23 is formed in one wall of the collimator chamber 24. The collimator 23 is formed by stacked layers of crinkled stainless steel foil as shown in the detail of FIG. 2A. Each layer of crinkled stainless steel foil is about 0.004" thick and 0.188" long with the wavelength for the crinkles being about 0.010" long. The crinkled foil is sandwiched between a pair of fiat stainless steel foils. The crinkled regions define small diameter tubes or straws which are 0.188" long through which the stream of atomic material such as, for example, cesium, thallium, rubidium, sodium or potassium etfuses. In a typical example, the cross section of the bundle of straws, which defines the size of the beam is 0.020" x 0.125". A Monel frame 29 holds the collimator 23 therein and is brazed into the copper body 28.

A stainless steel ampule 31 is contained in the reservoir chamber 21. A compression spring 32 holds the ampule 31 against an electrode 33. A pulse of current, when applied across the electrode 33 to ground, produces heating and vaporization of a thin portion 34 of the ampule 31 to open the ampule 31 and permit escape of the atomic material stored therein into the reservoir chamber 21. The sealed ampule 31 permits bakeout and evacuation of the over to 350 C. for 24 hours during processing of the tube 1 without loss of the atomic material which would otherwise be lost.

An antispill tube 35 as of stainless steel projects reentrantly into the reservoir 21. The reentrant length of the antispill tube 35 is selected such that the open inner end of the tube 35 is above the liquid level in the reservoir for all possible orientations of the oven 3. A pinch off protector cap 36, carried over the pinched off fill tube 37 of the ampule 31, serves as a cover for the open end of the antispill tube 35 to prevent splashing of atomic material into the end of the tube 35. A second stage antispill tube 38 as of stainless steel is coaxially disposed of the first tube 35. The inner end of the pinch off cap 36 also serves as a cap for the second antispill tube 38.

A constrictive tube 39 forms the constricted gas passageway 25 between the collimator chamber 24 and the reservoir 21. The constrictive tube 39 as of stainless steel, is mounted coaxially of and within the second stage of the antispill tubes, thus, forming a third stage of the antispill tubular network. In a typical example, the constrictive tube 39 is 0.025" inside diameter by 0.064 outside diameter and 0.200" long and provides a conductance of about 3.2)(- liters/second.

The collimator 23 and the reentrant antispill

tubulation comprising tubes

35, 38 and 39 define portions of the gas passageway for the atomic beam vapor and are all coated with a material which will not be wet by the liquid atomic beam material, which will resist chemical attack by such liquid, and preferably which will withstand the tube processing steps of bakeout at 350 C. for 24 hours while being pumped by a vacuum pump to 10- torr or less. By coating these parts with a non-wettable coating, the liquid beam material will not creep through these 'gas passageways to cause spillage of liquid beam material into the tube. The coating destroys any possible capillary tendency for the liquid beam material to be drawn into the antispill or collimator tubes.

Non-wettable coating materials include dichlorodimethylsilane having the chemical formula (CH SiCl (Dry Film), Teflon, and long straight chain saturated hydrocarbons such as Parafiint, a trademarked composition sold by the Moore and Munger Co. of New York and having a chemical formula of the form CH (CH ),,CH where n ranges from 40 to 60. However, in a preferred embodiment, the coating material should withstand tube bakeout at 350 C. at 10- torr for 24 hours. Paraffins are unsuited for this since they break down at temperatures below 350 C. Also the coating material should be resistant to chemical attack by the beam material, such as Rb, Cs, K, Na and T1, under operating conditions of temperature and pressure. In the case of cesium and rubidium, Teflon is attacked by rubidium and cesium under operating temperatures of C. and pressure of 10 to l0 torr. However (CH SiCl (Dry Film) is a very satisfactory coating material for cesium and the other beam materials as it resists chemical attack by these materials and will withstand the bakeout process. Moreover, Dry Film is easily applied to selective regions of the oven 3 since it forms a chemical type bond to oxidized surfaces. This is especially convenient since stainless steel is oxidized whereas copper and certain other metals such as Monel are reduced by the conventional copper-silver entectic brazing process wherein parts are heated to brazing temperature in a hydrogen atmosphere moist with water vapor. Thus, by making the parts to be coated with Dry Film out of stainless steel and then brazing the assembly together the stainless steel parts are oxidized and the remaining parts are reduced or cleaned. Then the brazed assembly is heated to about C. and air moist with Water vapor is passed through the assembly for about 1 minute to moisten the surfaces to be coated with water. The air which is passed through the assembly is then first bubbled through Dry Film liquid to saturate the air with Dry Film. The Dry Film laden air is then passed into the assembly. The Dry Film, in the presence of the prior water coating, reacts with the oxidized Cr O constitutent of the stainless steel to form a chemical bond between the silicon atoms of the Dry Film and the oxygen atoms of the oxidized stainless steel. More particularly, the chlorine atoms of the Dry Film are replaced with the oxygen atoms of the stainless steel in the presence of the water. The assembly is then dryed and is ready to assemble in the tube for subsequent tube processing and use.

In a preferred embodiment of the oven of the present invention, the gals conductance of the antispill tubulation, including the constrictive tube 39, is less than 3 times the conductance of the collimator, whereby the vapor pressure at the upstream end of the collimator 23 is reduced by at least 25%, and preferably more compared to the vapor pressure inside the reservoir 21. With this arrangement, condensation of vaporized atomic beam material in the collimator chamber 24 is prevented even though the oven 3 is operated at one temperature throughout or even operated with the collimator region 24 slightly lower in temperature than the reservoir 21. The oven design having the constriction between the reservoir 21 and the collimator 23 is described and claimed in copending US. application Ser. No. 564,215 filed July 11, 1966, and assigned to the same assignee as the present invention.

Referring now to FIG. 3 the oven 3 includes a mounting face portion 42 having a pair of mounting holes 43 therein for receiving studs for mounting the oven to a vertical support structure, not shown. A pair of heater elements are contained in a second pair of bores 44 for heating the oven 3 to its operating temperature, as of 5 85 C.:10 C. for cesium. A temperature sensing thermistor assembly 45 is mounted to the flange 42 via a spring clip 46 which is held down by a pair of screws 47. The thermistor assembly 45 serves as a part of a thermal control circuit for holding the temperature of the oven constant as of :01" C.

The oven of FIGS. 3, 4 and 5 when employed in cesium atomic beam tubes has been successful in preventing condensation of cesium vapor in the collimator region and in preventing spillage of cesium into the tube.

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

What is claimed is:

1. An atomic beam oven for atomic beam tubes including means forming a reservoir for containing a supply of liquid atomic beam material a portion of which is to 'be vaporized in use to provide a source of atomic beam vapor material, means defining a gas passageway leading out of said reservoir through which the vaporized atomic beam material passes, and means providing a dichlorodimet-hylsilane coating on at least a portion of the interior surfaces of said gas passageway which is non- Wettable by and resistant to chemical attack in use by the liquid atomic beam material, whereby creepage of liquid atomic beam material through said gas passageway is prevented.

2. The apparatus of claim 1 wherein the atomic material is selected from the class consisting of cesium, thallium, sodium, rubidium, and potassium.

3. The apparatus of claim 1 wherein said coated gas passageway portions include a beam collimator structure.

4. The apparatus of claim 1 wherein said coated gas passageway portions include an antispill tubulation projecting into said reservoir means.

5. The apparatus of claim 1 wherein said coated gas passageway has a subsurface formed of stainless steel having an oxidized Cr O constituent, and said dichlorodimethylsilane coating material is chemically bound to the gas passageway by a bond between oxygen atoms of said oxidized Cr O constituent of the stainless steel and the silicon atoms of the coating material.

6. The apparatus of claim 1 including in combination, a collimator means defining the coated portion of said gas defining passageway for collimating the atomic vapor into a collimated beam, means disposed along the beam path for deflecting out of the beam certain atoms having certain energy states and retaining in the beam atoms having other energy states, means disposed along the beam path downstream of said energy selecting means for producing resonance of the beam particles, and means disposed at the terminal end of the beam path for detecting resonance of the beam particles to produce an output resonance signal.

References Cited UNITED STATES PATENTS WILLIAM F. LINDQUIST, Primary Examiner.