Contains research objectives and reports on five research projects.Lincoln Laboratory, Purchase Order DDL-B222Department of the ArmyDepartment of the NavyDepartment of the Air Force under Contract AF19(122)-45

DiBartolo, B.

Hilar, E. P.

Ketudat, S.

King, J. G.

Searle, Campbell L.

Stroke, G. W.

Zacharias, J. R.

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VII. ATOMIC BEAMSProf. J. R. Zacharias Dr. R. F. C. Vessot D. S. Edmonds, Jr.Prof. J. G. King D. D. Babb E. P. HilarProf. C. L. Searle V. J. Bates G. W. StrokeDr. S. Ketudat H. H. Brown, Jr. R. WeissB. DiBartoloRESEARCH OBJECTIVESIn the Quarterly Progress Report of January 15, 1957, page 50, we wrote, "Thedistributions of electric charge and magnetism in an atomic nucleus are usuallydescribed in terms of multipole moments limited in number by the magnitude of thenuclear angular momentum. In this laboratory, atomic-beam techniques are used todetermine such electric and magnetic moments. In addition, information about theradial distribution of nuclear magnetism can be obtained in cases in which more thanone isotope is available. These techniques lend themselves to such precision that theywere used in this laboratory for the development of the most accurate atomic clock.In turn, these clocks are being used to make studies on the nature of time itself. Pre-cision apparatus is under construction to observe not only the dependence of atomictime on gravitational potential but also the epochal dependence of nuclear, gravita-tional, and atomic time. Similar studies are being made on the velocity of light." Thisis still a reasonable statement of our objectives. However, the past year has not beenvery fruitful for the group.Some experiments proved more complicated to construct than was originally hoped.One major effort of observation proved intractable and was finally abandoned.We pointed out earlier that we were going to try to eliminate one of the inher-ent weaknesses of atomic-beam frequency standards that use the Ramsey two-cavitymethod by using two antiparallel beams that pass simultaneously through the sameRamsey cavities. The notion was that if the two cavities are not exactly in phase,the average frequencies of the two beams would nonetheless average to the proper one.R. F. C. Vessot's apparatus for demonstrating this is on the point of working, but theneed for making two beams work simultaneously at a high precision makes the experi-ment difficult. The apparatus includes a device for selecting velocities in such a waythat the phases of the cavities can be set to equality with only one beam.An experiment for trying to observe atomic cesium under free fall for time intervalsof a second or more has been underway for several years. The major obstacles in thisexperiment were thought to be: (a) the general vacuum, and (b) scattering of atoms byother atoms of the beam, especially in the neighborhood of the oven. In the beam tube,which is 10 inches in diameter and 28 feet long, we can hold a pressure of 10-9 mm Hgwith the aid of a liquid-helium trap. We have tried two types of oven: one with canals,and the other an open boat of liquid cesium. In neither case do we see atoms focusedonto a detector. R. Weiss has gone further, to try to find slow atoms at the top of the28-foot tube. He finds slow atoms that live in the apparatus, in Maxwellian abundance,for almost one-fifth of a second. We seem to run out of signal-to-noise ratio for atomsthat are slower than that. We have therefore given up this experiment as being toodifficult, at least for us.Since we seek an absolute measurement of frequency that will stand the test of timeand be regarded as accurate within the limits of proposed error one hundred years fromnow, we still feel that it is necessary to try experiments of this sort. Such experimentsare still in the thinking stage.Results of the experiment for measuring the Stark effect on the hyperfine structureof cesium will appear as Technical Report 322, by R. D. Haun. An experiment fordiscovering any discrepancies in the application of the Breit-Rabi formula for atomicThis research was supported in part by Purchase Order DDL-B222 with LincolnLaboratory, which is supported by the Department of the Army, the Department of theNavy, and the Department of the Air Force under Contract AF19(122)-458 with M.I. T.(VII. ATOMIC BEAMS)cesium has been completed but the data have not been entirely evaluated. The resultsof this experiment will be reported later.J. G. King, J. R. ZachariasA. HYPERFINE STRUCTURE OF BROMINEIn our new laboratory we are constructing and re-erecting apparatus for the preci-sion measurement of the hyperfine structure of the stable bromine isotopes. We expectto obtain a precision that is a hundredfold greater that that attained by King and Jaccarinoin their early work on bromine (1). This will enable us to determine the octopole inter-action. We are also planning to measure Av and gj in the Pl/2 state.Klystrons that are phase-locked to harmonics of our standard frequency have beenprepared. The vacuum envelope (No. 2) has been overhauled and fitted with new pumptraps that should greatly reduce the amount of oil in the apparatus. As soon as avacuum is obtained, development of the detector mass spectrometer and the source ofatoms will begin.B. DiBartolo, S. Ketudat, J. G. King, J. R. ZachariasReferences1. J. G. King and V. Jaccarino, The hyperfine structure and nuclear moments of thestable bromine isotopes, Phys. Rev. 94, 1610 (1954).B. LINEAR DECELERATOR FOR MOLECULESThe linear decelerator is a device that is intended to slow down molecules in agiven state so that when they enter a microwave cavity they will undergo transitions ofreduced natural width. The apparatus that we have constructed during the past year anda half is designed to use the inversion line of NH 3 in the J=K=1 state, which occurs at23, 694.48 mec. This apparatus consists of a cold (100 ° K) directional NH 3 source; thedecelerator, which is an array (1 meter long) of parallel-plate capacitors and driftspaces of such lengths that the time of flight of the decelerated molecules through eachcapacitor and drift space is constant; and a cylindrical cavity operating in the TE0, 3, 12mode with appropriate input and output waveguides. The whole apparatus is operatedat a pressure of 10 - 8 mm Hg.The decelerator is intended to operate as follows: Molecules in the upper inversionstate J=K= 1, 1 m = 1 lose energy when they enter a region of large electric field in oneof the capacitors in the decelerator. If the field is greatly reduced while the moleculesare in the uniform field of the capacitor, they will not, when they leave, regain theenergy that they lost on entering. While the molecules are in the drift space the voltageacross the capacitor is raised and the molecules once again lose energy upon entering(VII. ATOMIC BEAMS)the high field. For reasonable fields (e.g., 105 volts/cm) the molecules lose kineticenergy at the rate of 10 K per stage. If the source temperature is 1000 K, moleculesof one-half of the most probable velocity will be brought to rest in the 25 stages of ourapparatus. These molecules, or rather their neighbors in the velocity spectrum, spenda long time in the cavity, where they contribute to the inducing rf field by undergoinginduced emission, and are thus detected.It is difficult to estimate the effective solid angle of the decelerator for the desiredmolecules. Calculations indicate that there is a net focusing effect for molecules inthe desired state. Molecules in the lower inversion state would also be decelerated onleaving the capacitor, but would be defocused. Phase stability is also of importanceand probably it will be necessary to use a triangular or sinusoidal voltage rather thanthe square wave that was technically easier to achieve with a conventional thyratronswitch arrangement that delivers a 15-kv square wave at 6 kc into the 0. 001-If capaci-tance of the decelerator unit. Some shaping of the square wave can, of course, be done.The microwave receiver uses type 2K50 as a local oscillator for a balanced detec-tor. The 2K50 is phase-locked to a harmonic of an S-band klystron which, in turn, isphase-locked to a harmonic of a 1-mc crystal, with various balanced modulators andmixers introduced to allow continuous tuning. A 200-kc i-f amplifier with a gain of120 db and a bandwidth of 20 cps is used to detect the signal from the cavity.All offset frequencies are derived from crystal oscillators. We are grateful toProfessor C. L. Searle and Mr. R. E. Lyon for the initial development of the micro-wave receiver.Tests of the completed system will soon be carried out, and further details will begiven in a later report.J. R. Zacharias, J. G. KingC. MEASUREMENT OF THE VELOCITY OF LIGHT1. Precision Cylindrical CavityWhat is probably the most precise cylindrical cavity in existence has now beencompleted to microinch tolerances by an optical process out of General Electric Com-pany well-annealed fused quartz by the Boston University Physical Research Labora-tory under the direction of George Random in consultation with our Research Laboratoryof Electronics group. The silver coating of this cavity to optical uniformity and goodelectric conductivity will also be undertaken by the Boston University laboratory.The basic mechanical elements of the apparatus, which will be used, first, forexperimenting with mechanical guidance schemes for moving the end plate parallel toitself at a reasonably uniform velocity with microinch precision, are being assembledin our Laboratory. The experimental results described in the Quarterly Progress(VII. ATOMIC BEAMS)Fig. VII-1. Fused quartz cavity with diameter-measuring device.Report of October 15, 1956, page 46, have determined the principle of having brassshoes guide a moving glass (or quartz) cylindrical (2-inch diameter) rod to avoid stick-slip effects. The shoes consist of two groups of three horizontal cylindrical (0. 25-inchdiameter) brass rods, two rods in each group forming a "vee" and the third pressingthe vertical quartz rod against them by gentle spring action. The quartz rod and themoving end plate, which will be interferometrically servo-controlled in rotation withrespect to the rod so that they are normal to the cylinder axis, have not yet beenoptically manufactured.Figure VII-1 shows a photograph of the finished fused quartz cavity with themechanical-interferometric curvature -measuring device that was especially designedfor the testing of this cylinder. The cylinder is 11.5 inches long, with an inner diameterof 6. 350 inches and an outer diameter of 9. 25 inches. The uniformity of the inner diam-eter (with respect to both ellipticity and axial-diameter variations) was found frommeasurement to be within +1. 5 X 10-6 inch, and the highly polished inner surface hasconsiderably less than one part in 10 of its area covered by residual bubbles. Theelectromagnetic diameter of the silver-plated cavity will be determined to a goodapproximation by microwave and interferometric guide wavelength measurements attwo frequencies in a simple, exactly known ratio. We are planning to determine theabsolute diameter more accurately by comparing the mechanical diameter values withan adjustable Fabry-Perot interferometer system by means of a null-reading device,such as a two-outlet (or possibly three-outlet) air gauge mounted on the moving end-plate system. When the air-gauge detectors read "zero" in both the cavity and theII i(VII. ATOMIC BEAMS)Fig. VII-Z. Fused quartz cavity in process of being ground.Fabry-Perot interferometer, the diameter-measuring problem has been reduced to adetermination of the spacing of the two Fabry-Perot plates, which can easily be achievedby classical "inspection" methods in terms of known light wavelengths.Figure VII-2 shows the cavity in the process of being ground on an apparatus con-structed for this purpose, in which the cylinder is rotated about its axis and the grindingtool moved back and forth axially. The noteworthy feature of this procedure is that,with the two movements originating at independently driven motors, the almost perfectcircularity has been achieved with comparative ease, while the axial uniformity indiameter had to be carefully watched by varying the stroke length and the axial centeringof the tool in both the grinding and polishing processes.J. R. Zacharias, G. W. StrokeD. STABILIZATION IN KLYSTRON PHASE-LOCKED OSCILLATOR LOOPSFurther theoretical and experimental work on the problem of phase-locking S-, X-,and K-band klystrons to 5-mec crystal oscillators (1, 2) has revealed several previouslyneglected sources of phase shift which prove to be of considerable importance in thedesign of stable feedback loops. A block diagram of a representative klystron stabili-zation loop is shown in Fig. VII-3. The klystron has a transfer function of the formK/s, with K varying from 106 radians/volt for an S-band klystron to 107 radians/voltfor a K-band tube. The essential problem in stabilizing such a system is to limit allother phase shifts in the feedback loop to less than a total of 90* until I K/sI becomes~3. L~- I __(VII. ATOMIC BEAMS)I F PHASE CORRECTIVE KLYSTRONAMPLIFIER DETECTOR NETWORKFig. VII-3. Block diagram of klystron stabilization loop.less than unity. The three principal sources of phase shift, other than the 900 shiftimplied by the 1/s characteristic of the klystron, are tuned circuits in the phase detec-tor, including any tuned element used to feed in the reference signal, the detection pro-cess in the phase detector, the repeller input capacitance of the klystron, and the shuntcapacitance in the i-f amplifier.1. Phase DetectorAny tuned circuit in the phase detector will cause phase shift in the device. A tunedcircuit used to obtain a balanced reference signal to drive the detector would not, atfirst, appear to influence the input-signal phase, but the capacitance to ground of sucha tuned circuit does introduce substantial phase shift unless the tuned circuit is of verylow Q.The "sampled-data" phase shift that is inherent in the detection process of the phasedetector was a major source of phase shift in early designs. The time delay involvedin any rectification-and-holding process, such as a peak detector, is inversely propor-tional to the carrier frequency. If "box-car" holding is assumed for the sake of sim-plicity, the phase shift corresponding to this time delay will bemodulating frequency4 = X 360 02 X carrier frequency2. KlystronThe klystron introduces a fixed, 900 phase shift, and a variable phase shift becauseof repeller input capacitance. For an S-band klystron, this capacitance can be aslarge as 100 tjif. There is also a possibility that some reactive component will beintroduced because of the interaction of the repeller and the electron stream inside thetube. This effect has not been observed, probably because it occurs at a much higherfrequency than the frequencies that have been considered here.3. I-F AmplifierThe slope of the phase characteristic of the i-f amplifier is a direct measure of theamount of phase shift imparted to a phase-modulated wave passing through the amplifier.Since an RLC circuit has twice the phase-characteristic slope of an RC circuit, for a(VII. ATOMIC BEAMS)given RC product, it is desirable, whenever it is possible, to use a video i-f amplifierrather than a tuned amplifier.4. Design CriteriaThe following design criteria are suggested by the preceding discussion:1. Use an untuned (video) i-f amplifier with either a broadband input transformeror a vacuum-tube inverter circuit to convert the balanced i-f input signals to a single-ended output.2. Use a low-impedance phase detector to minimize klystron input capacitanceeffects.3. Choose an i-f frequency that is as high as possible to minimize "sample-data"phase shift.Obviously, these criteria are not necessarily compatible. Specifically, some com-promise must be made in the selection of the i-f amplifier center frequency in accord-ance with items 1 and 3.5. Experimental VerificationAn S-band klystron stabilization loop that was designed in accordance with two ofthe criteria has proved to be very successful. The specific departure from the designwas the use of a 3. 7-mec tuned i-f amplifier. In effect, increased phase shift in thei-f amplifier was traded for decreased sampled-data phase shift in the detector. Theperformance of the loop was outstanding in that the klystron tube could be struck witha screwdriver without loss of lock. Notwithstanding, we feel that system performancecan be improved by striking a different balance between criteria 1 and 3, namely, bythe use of a 1-mc untuned i-f amplifier. Such a system is now being constructed.C. L. SearleReferences1. C. L. Searle and D. D. McRae, Stabilization of klystrons by phase locking to low-frequency quartz crystal oscillators, Quarterly Progress Report, Research Labor-atory of Electronics, M.I.T., April 15, 1956, pp. 37-41.2. R. E. Lyon, Frequency stabilization of K-band klystrons, Quarterly ProgressReport, Research Laboratory of Electronics, M.I.T., Jan. 15, 1957, pp. 50-51.E. RESONANCE DETECTOR: VELOCITY OF LIGHT EXPERIMENT1. System DesignA system has been constructed to detect cavity resonance. This system uses theamplitude variation of the cavity's output signal, but provisions have been made for(VII. ATOMIC BEAMS)phase comparison of input and output ifPSEUDO 92 KMC DAVITY IVECLOCK CAVITY ... MOTOR the accuracy requirements demand it.PHASE- The cavity that is being used is the inter-LOCKEDKLYSTRONS mediate cavity that was constructed ayear and a half ago. It is mounted on aOSCILLATOR BFO lathe bed with one end plate on the toolAUDIO carriage. The carriage is driven withTONEa reciprocating motion by a small motorRECORDER DETECTOR through a distance of 0. 030 inch, whichtakes the cavity through one of its reso-Fig. VII-4. Resonance detection system. nances. The microwaves are passedthrough two small holes in the fixed plate.Two phase-locked klystrons are used in the system shown in Fig. VII-4, one tosimulate the final frequency source, an atomic clock, and the other as a local oscillator.The output signal is mixed with the local-oscillator signal, and the difference signal of3.5 mc is amplified by a communication receiver. Since the output signal is of lowfrequency, the beat frequency oscillator of the receiver is used, and the modulatedtone is rectified by a peak detector. This provides a signal that is proportional to theoutput amplitude of the cavity even though it has a de component. The peak detectorvoltage is recorded on a pen recorder.2. ResultsPreliminary results are shown in Fig. VII-5. A succession of cavity resonancesis plotted, and the recorder sensitivity is increased by factors of 10 and 100. The7 - - I . ....-LiFig. VII-5. Signal-to-noise measurement for resonance detector.signal-to-noise ratio, as determined from these curves, is 300 to 1. This ratio canbe improved by a factor of 2 or 3 by eliminating the obvious dissymetry in the base(VII. ATOMIC BEAMS)line, which is caused by signal leakage around the cavity.The pen recorder does not have the accuracy that is required for this measurement.It will be replaced by better detection equipment.E. P. Hilar

Atomic Beams

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