Contains reports on three research projects.Lincoln Laboratory (Purchase Order DDL B-00283)United States ArmyUnited States NavyUnited States Air Force (Contract AF19(604)-5200

Brown, H. H., Jr.

King, J. G.

Vessot, R. F. C.

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VIII. MOLECULAR BEAMSProf. J. R. Zacharias Dr. R. F. C. Vessot E. L. KamenProf. J. G. King H. H. Brown, Jr. F. J. O'BrienProf. C. L. Searle R. Golub R. WeissA. CHARGED ATOMSRenewed interest in the possibility that matter may not be electrically neutral, asconsidered by Bondi and Lyttleton, has led us to the design of an atomic-beam electricdeflection apparatus that is intended especially for setting an upper limit on the chargecarried by neutral atoms. The apparatus is of conventional design, except for the obvi-ous refinements required for measuring deflections of the beam that are of the order of-810 cm. Construction is now nearly completed.We have also repeated the experiment of Piccard and Kessler (1), in which gas isallowed to escape through a de-ionizer from a container connected to an electrometer.The present results are by no means conclusive, but it appears that the release of4 moles of the gas at a uniform rate over a period of 100 seconds causes less than6 X 1015 coulomb to flow in the electrometer. We conclude from a preliminary inter-pretation of the data that the charge carried by each helium atom is less than 2 X 1020of the charge on the electron. The situation with other gases is not clear, and a seriesof experiments must be performed to distinguish an effect caused by charge from manypossible artifacts. First, however, it is desirable to increase the sensitivity of theexperiment, and, fortunately, we have a factor of 10, at least, readily available.J. G. KingReferences1. A. Piccard and E. Kessler, Arch. Sci. Phys. et Nat. 7, 340 (1925).B. CESIUM MASERThe object of this experiment is still to observe the emission of coherent power fromthe hyperfine transition in cesium F = 4, mf = 0 - F = 3, mf = 0. A state-selectedbeam of atoms is focused into a cavity by means of a four-pole magnetic field. Thecavity has within it a buffer gas and is kept at liquid-helium temperature. Atoms thatenter the cavity will lose energy, because of the exchange of kinetic energy with thebuffer gas, and take a relatively long time to reach the cavity walls; thus the numberThis work was supported in part by Purchase Order DDL B-00283 with LincolnLaboratory, a center for research operated by Massachusetts Institute of Technologywith the joint support of the U.S. Army, Navy, and Air Force under Air ForceContract AF 19(604)-5200.(VIII. MOLECULAR BEAMS)of atoms within the cavity can be made large and their lifetime long. Furthermore, theQ of the cavity, resulting from its superconducting inner coating of lead, will be approx-imately 106. The combination of high Q and large numbers of atoms within the cavitywill result in sustained coherent oscillation on account of the cooperative, or ringing,effect of the atoms in the cavity as they decay to the lower energy state. In order tosustain this oscillation, the atoms are put into the so-called superradiant state by theapplication of a r/2 pulse as they pass through a prestimulating cavity. This gives theatoms phase coherence, and if they continue to be exposed to an inphase coherent field,the atoms will continue to decay to the ground state. Their energy, emitted into ahigh-Q cavity, provides the above-mentioned field, and a self-consistent calculation,such as that made by Bloom (1), demands the following requirements:(a) a lifetime of the atoms in the cavity of 1/10 sec;(b) number of atoms entering in the proper state, ~1010 per sec; and(c) a Q of 106 of the main cavity.The output power is approximately 10 - 1 5 watt, and, since the output spectrum shouldbe very narrow, that is, of the order of tens of cycles per second, this power will bedetectable with a narrow-band microwave receiver that has an ordinary noise figure.There are quite a few parameters in the experiment, and it is not surprising thatthe first try did not work. The apparatus that was described in the Quarterly ProgressReport of April 15, 1958 (pages 32-35) has been modified and, as the following descrip-tion will show, has a great deal more flexibility in operation.First, in order to determine the lifetime of the atom in the cavity, a straightforwardmolecular-beam technique was adopted. We employed an A magnet (the focuser) and aB magnet (another four-pole magnet state selector), between which the C region con-taining the cavity was located. The cavity now has two apertures so that the beam maypass through. Atoms that go through the apparatus are detected by means of a hot plat-inum ribbon ionizer, a mass spectrometer, and an electron multiplier. The lifetimeof the atom is inferred from the linewidth of the resonance curve. Both the lifetimeand the frequency shift are of interest, and several gases will be tried at various tem-peratures and pressures in order to obtain pressure-shift data at low temperatures.Once the lifetime of the state and the frequency of the radiation have been determined,the beam is used to determine the level of prestimulation by applying power to the stim-ulator cavity, after detuning the main cavity. The proper level, giving the greatest rateof decay, is determined by adjusting the power so that one-half of the atoms make thetransition to the lower state. The main cavity is then retuned, and there should be afurther reduction in the number of atoms to the detector, since the atoms give powerto the cavity and go to the ground state.Figure VIII-1 shows the apparatus as it now exists. The can consists of three cham-bers; the upper and lower operate at high vacuum (10 - 7 mm Hg, or better), and ther WINDOW2 - SOLDER1 - STAGE | TOGELECTRON STROUGHMULTIPLIERMOVABLE C, IONSIONIZER MASS"" '" IMASSS-SPECTROMETERBUFFER GASINLETLIOUIN- HELIUM ==z LI QUID-HELIUM BREATHERINLET >0ONE OF 3FOUR - POLE LEVELLING" "-" "LEVELLINGPERMANENT SCREWSMAGNET0\:o 0 . I - SOLDERANNULAR TROUGHLIQUID- f.., × \\ SEALSNITROGEN , -TRAP I- METAL\\\SHIELDSTO VACUUM PUMPSz AT 10- 4 mmHgi -f50 L/SECONE OF 3CAVITYPOSITIONING LIQUID toCOPPERSCREWS HELIUM DOUBLE-THERMALBAFFLESPRESTIMULATORBEAM FLAGWAVEGUIDE TO VACUUM PUMPSMOVABLE LIGHT BULB AT 10 7 mmHgMAGNET0L/SECSOFT-IRON WINDINGSSHIELD(LVACUUMCE ENVELOPE-8500zrI MAGNETNDINGSw 0-f- CESIUM OVENAMPOULE IN CRUSHABLE COPPER TUBEGLASS- WOOL INSULATIONFig. VIII-1. Cesium maser.85(VIII. MOLECULAR BEAMS)-4middle one at 10 mm Hg, or better. The pumping speeds are such that these pres--2sures are possible with the cavity pressure at 10-2 mm Hg at 4. 2K.At the present time, a cesium beam is being run with no buffer gas, and work is inprogress to optimize the "flop-to-background-ratio." The linewidth is around 2 kc,which implies a beam velocity of approximately 2.5 X 104 cm/sec for the focused atoms.R. F. C. VessotReferences1. S. Bloom, J. Appl. Phys. 27, 785 (1956).C. PRECISION MEASUREMENT OF THE HYPERFINE-STRUCTURE CONSTANTSOF THE STABLE BROMINE ISOTOPESBefore the vacuum envelope of the atomic beam apparatus was last closed, twohot-wire detectors were installed: one was tungsten; and the other, thoriated tungsten.Since the thoriated wire would not follow 10-cps modulation of the beam, the plaintungsten wire was used with a 10-cps lock-in amplifier. With this arrangement, low-frequency (AM=±, AF=0) transitions were observed, and also one high-frequency(AM=0, AF=±1) transition. The Ramsey pattern of the high-frequency transition wasnot clean, and could have been caused by atoms of approximately the same velocity. Atthis point, the bromine background became so bad that the signal-to-noise ratio wasalmost two to one.To discriminate against the bromine background, the hot-wire detector was put ina box. The mass-spectrometer magnet, an ionization gauge, heaters, and a liquid-nitrogen trap are also in the box. It has a crinkly foil entrance to admit the beam. Thecrinkly foil will not let atoms that are more than approximately 5" off the beam axisinto the box. The box also has a hole that can be covered or uncovered by a door. Thehole looks out to another liquid-nitrogen trap. When a beam is being detected, the doorwill be shut. To get rid of the bromine that accumulates in the box, the door will beopened, and the heaters turned up.A thoriated tungsten wire that should give a better signal-to-noise ratio than theplain tungsten wire has been installed. We may, or may not, modulate the beam. Ifthe beam is modulated, the modulation frequency will be 1 cps, or less.H. H. Brown, Jr.

Molecular Beams

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Molecular Beams

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