Contains a report on a research project.Lincoln Laboratory (Purchase Order DDL-B222)United States Department of the ArmyUnited States Department of the NavyUnited States Department of the Air Force (Contract AF19(122)-458

Vessot, R. F. C.

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 H. H. Brown, Jr. G. W. StrokeDr. S. Ketudat B. DiBartolo R. WeissA. HELIUM-GAS-MODERATED ATOMIC-BEAM MASERThe linewidth of frequency standards that use stimulated emission or absorption ofradiation by atoms or molecules depends mainly upon the time interval during which theatom is available for observation in an unchanged state. In atomic-beam frequencystandards that are based on separation of atoms that have undergone transitions, thebandwidth can be decreased by lengthening the distance between the two rf sections.But in a maser, where the radiation itself is observed, rather than the change in themagnetic moment that is caused by resonance radiation, the particles must remainwithin the field of the radiation-detecting device over the whole observation time.Exchanges of energy between similar particles and with the walls of the vessel must beavoided.We plan to try the following scheme for lengthening the lifetime of a beam of state-selected atoms. A beam of atoms from an oven, separated according to their state bypassing through inhomogeneous magnetic or electric fields, is introduced into a regioncontaining a gas that has a spherically symmetrical ground state; this gas must be keptvery cold. Helium appears to be a good choice. The atoms collide elastically with thegas and reach thermal equilibrium, then eventually diffuse through the gas and hit a wallof the containing vessel. A further increase in lifetime might be obtained by the correctchoice of wall-coating material.The atom chosen for the present experiment is cesium 33 The transition chosenis the 9192. 631830 ± 10 mc/sec line from the F=4, mf=0 transition, which has no first-order dependence with the field and has a second-order dependence of 426 cycles/gauss .Cesium atoms emerge from an oven at the base of the apparatus and are physicallyseparated according to state by a four-pole magnetic lens that focuses only the F=4, mf=0hyperfine level. The atoms travel into the cavity and rattle about for approximately1 sec before striking a wall or otherwise changing their states. A mean life of 1 sec,which allows a linewidth of approximately 1 cps, should give a reading accuracy of one12part in 10 . Emission can be stimulated in one of two places, either directly at thecavity or before entry. If it is stimulated before entry, the advantage is that only poweremitted by the atoms is observed from the cavity. This can be important at low powerlevels.This 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)COPPERBAFFLESTIMULATIONAND OUTPUT-WAVEGUIOESSOLDERSEAL10-2 mm HgCOPPERBAFFLEWAVEGUIDETO MECHANICAL PUMP,HELIUM INPUTFOUR-POLEMAGNETVACUUMENVELOPEALIGININGSCREWSCESIUM,OVENFig. VII-1. The maser. (Not drawn to scale.)The apparatus, constructed of stainless-steel sheet (16-gauge), heli-arc welded,consists of two vacuum systems connected by a small hole. Figure VII-1 shows thearrangement. A resonant cavity, cooled to Z2 K by contact with a bottle of liquidhelium at low pressure, is kept filled with helium at 10 - mm Hg. To avoid excessiveradiation loss, this assembly is surrounded by a hollow cylindrical vessel containingliquid air. Leakage of helium from the cavity is kept pumped out to at least 10- mm Hgby a 4-inch oil diffusion pump. This low-vacuum region is separated from the high-vacuum region by a metal partition pierced by a small hole through which the beamenters. The oven and the state-selection magnet occupy the lower system, which oper-ates at 10 - 7 mm Hg or better.The oven is a monel cylinder with a single round hole (diameter, 0. 024 inch) col-limated to a half-angle of approximately 1'; it has a window on the bottom. The whole(VII. ATOMIC BEAMS)HELIUM LIGHT BULBOSCILLATING00' WAL - . /IFIELFOUR-POLE MAGNET0 TUNINGS - -- SCREWFig. VII-. Detail of the detection cavity. THIN DIAPHRAGMAND LEAD0PRESTIMULATING CAVITYSTAINLESS-WAVEGUIDEFOUR-POLE MAGNETFig. VII-2. Detail of the detection cavity. (Not drawn to scale.)assembly is mounted to the magnet by means of a bellows arrangement that facilitatesalignment. A small light bulb buried in the top of the cavity provides a light sourcefor visual alignment of the cavity and the oven with respect to the magnet before thecesium ampoule is broken; that is, while the system is cold.A four-pole magnet was chosen for separating the upper m f=0 state from theother 15 hyperfine states. It produces a radial field Bradial=-ar. Over the region0- 3000 gauss (roughly) the F=4, mf=0 state has a moment m that is proportional tothe field; that is, m=cB. Consequently the force, which is proportional to the productof the gradient and the moment, behaves as F radial=a cr and is directed towards thecenter for the atom in the upper state. This condition allows atoms of a given axialvelocity to be focused, and all atoms that have this velocity and are in the F=4, mf=0state that enter the lens will form an image of the oven aperture at the cavity entrance.The magnet, which is 29.4 cm long, has an acceptance half-angle of 10. Object andimage distances (measured to the entrance of the magnet, both 5 cm) give a magnification(VII. ATOMIC BEAMS)of unity for atoms having a velocity v mp2. 7 10 cm/sec.mpA numerical integration over the range of beam velocities 0. 9 to 1.2 vmp predictsthat at least 1.4 X 108 atoms per second will enter the cavity in the desired state.Radiation is detected in the usual manner by means of a rectangular cavity operatingin the 2, 1, 0 mode (see Fig. VII-2). This mode, which has an arbitrary length in onedirection, provides a large volume of in-phase magnetic field that can couple to the beamalong the central axis of the cavity. Since a lobe of scattered atoms with a length thatdepends upon gas pressure is expected, the cavity was made 6. 7 cm long with a squarecross section of 3. 65 cm. Power for stimulation and output is fed through special low-thermal conductivity waveguides made of 0. 001-inch stainless steel. Tuning is possibleby deforming one side of the cavity by means of a screw.The X-band receiver consists of a phase-locked klystron and a radar i-f strip havinga noise figure of approximately 2 and a bandwidth of 1. 5 mc/sec at 30 mc/sec. Thesignal from the i-f strip is heterodyned and fed into a 200-kc amplifier having a band-width of 20 cps. If the over-all noise figure is assumed to be 10, then the number ofparticles per second required to release their energy coherently to double the noise isabout 106. Further reduction in bandwidth will improve the sensitivity.The maser is in the process of construction.R. F. C. Vessot, J. R. Zacharias

Atomic Beams

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Atomic Beams

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