Contains research objectives and reports on two research projects.Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 36-039-AMC-03200(E

Badessa, R. S.

Billman, K. W.

Culter, L. S.

Golub, R.

Guttrich, G. L.

Johnston, W. D., Jr.

King, J. G.

Kukolich, S. G.

Searle, Campbell L.

Taylor, E. F.

Thornburg, C. O., Jr.

Yaffee, M. A.

Zacharias, J. R.

DSpace@MIT

GENERAL PHYSICSI. MOLECULAR BEAMSAcademic and Research StaffProf. J. R. Zacharias Prof. J. G. King R. S. BadessaProf. K. W. Billman Prof. C. L. Searle F. J. O'BrienProf. E. F. TaylorGraduate StudentsJ. F. Brenner W. D. Johnston, Jr. J. F. MartinR. Golub S. G. Kukolich C. O. Thornburg, Jr.G. L. Guttrich L. H. VeneklasenRESEARCH OBJECTIVESThree kinds of research are carried on in the Molecular Beams Group.1. High-precision studies of atomic and molecular radiofrequency spectra.2. Experiments directed toward establishing precise frequencies generated byatomic clocks of proved independence from external influence, and the intercomparisonof these frequencies.3. Experiments such as(a) measurement of the velocity of light in terms of atomic standards,(b) search for a charge carried by molecules, both by gas efflux and beamdeflection methods,(c) study of atomic beams from liquid helium,(d) an experiment on an aspect of continuous creation.These experiments are described in more detail below.J. R. Zacharias, J. G. King, C. L. Searle1. RF SpectraHyperfine structure of the 1-1, 2-2, 3-3, and 3-2 inversion transitions of N 14H 3has been measured with the two-cavity maser spectrometer. The Hamiltonian andcoupling system of Gordon 1 has been used to describe the spectra, and fits have beencarried out to determine the parameters in his theory. Small discrepancies of approxi-mately 0. 5 kc are present between theory and experiment.S. G. KukolichReferences1. J. R. Gordon, Phys. Rev. 99, 1253 (1955).This work is supported by the Joint Services Electronics Programs (U. S. Army,U. S. Navy, and U. S. Air Force) under Contract DA 36-039-AMC-03200(E).QPR No. 80(I. MOLECULAR BEAMS)2. Molecular ClocksWork on this program has been temporarily suspended while details of the NH 3inversion spectrum are investigated as mentioned above.S. G. Kukolich3. Atomic ClocksThe 10-ft Cs clock has been used with an electric field in an attempt to observe theelectric dipole moment of the Cs atom previously reported.2 Data obtained thus farestablish a limit of 0. 8 ± 8 X 10-20 electron-cm. Meanwhile, two duplicate 10-ft clocksare being constructed for use in the intercomparison program (see below).C. O. Thornburg, Jr.References1. T. G. H. Sanders and E. Lipworth, Phys. Rev. Letters 13, 718 (1964).4. Intercomparison of ClocksThe intercomparison experiment involving two commercial beam tubes coupled toelectronic equipment employing frequency impulse modulation has been completed.Detailed intercomparisons have been made by measuring the phase difference betweenthe clocks, and using a computer to calculate the phase spectral density and the mean-square frequency fluctuation as a function of averaging time. Final results indicate astability of 5 parts in 1012 for a 1000-second averaging time, a result quite consistentwith the measured signal-to-noise ratio for these particular beam tubes.The same method of intercomparison will now be used to measure the performanceof the new clocks described above.C. L. Searle, R. S. Badessa5. Decelerator for MoleculesThis apparatus makes use of nonuniform pulsed electric fields for slowing downmolecules to smaller velocities than have hitherto been observed. The construction ofthe apparatus is complete and a beam has been observed.R. Golub, G. Guttrich6. Velocity of LightBy investigating the resonances of an adjustable precision microwave cavity for bothlight and microwaves, one can, with suitable corrections, establish the ratio of the twowavelengths that are used; this can be interpreted either as the ratio of light in funda-mental constants or (through the usual standards) as the velocity of light. Preliminaryresults obtained by using a Nitrogen atmosphere have shown a reproducibility of 7 partsin 107. Multiple measurements will now be made in Helium.M. A. Yaffee, C. L. Searle, J. R. ZachariasQPR No. 80(I. MOLECULAR BEAMS)7. Neutrality of AtomsWe have performed gas efflux and atomic-beam deflection experiments, both designedto reveal a charge unbalance in supposedly neutral atoms. In both cases, we areattempting to analyze and understand in detail the large bulk of data obtained and todetermine whether certain persistent effects are of any fundamental interest. One of us(K. W. B.) has collaborated with C. G. Shull at the M. I. T. reactor in setting a newupper limit on the charge carried by the neutron.K. W. Billman, J. G. King8. Helium BeamsApparatus has been constructed and operated successfully to yield time-of-flightvelocity distributions for helium gas sources in the temperature range 3-4. 20 K. Inves-tigations are proceeding to lower temperatures, and a comparison of the gas data withvelocity distributions of atoms evaporating from a liquid source is planned for the future.W. D. Johnston, Jr.9. Mercury Boiling ExperimentIn this experiment we seek to place an upper limit on the appearance of Hydrogen inMercury. The apparatus is essentially a Mercury still with a scanning mass spectrom-eter. The object of the experiment is to test a remotely conceivable density-dependentcontinuous creation hypothesis. A new small apparatus containing less Hydrogen is beingdeveloped.E. F. Taylor, J. G. KingA. LOW-TEMPERATURE HELIUM BEAM EXPERIMENTTime-of-flight velocity distributions from a helium gas source have been obtainedfor source temperatures of 3-4. 2 0 K. A signal-to-noise ratio of 10: 1 at the peak of thedistribution has been obtained, with a chopper admittance time of 0. 75 msec and adetector resolution time of 0. 25 msec. The distribution curves retain significance downto one-third of the most probable velocity.The distributions are in qualitative agreement with expectations based on the assump-tions of a Maxwell-Boltzmann distribution in the source and an effective detector sizeinversely proportional to the kinetic energy of the beam particles. Quantitative com-parisons will be made after the results of a computer calculation of the theoreticalcurves, which is now in progress, are obtained.Data at lower temperatures are now being taken with the gas source. The ultimategoal is a comparison of the gas source data with data derived from a beam formeddirectly from evaporating liquid helium. Only a minor modification to the presentapparatus is required to permit observation of the evaporating liquid source; this willbe carried out soon.W. D. Johnston, Jr.QPR No. 80(I. MOLECULAR BEAMS)B. SOME ASPECTS OF THE THEORY AND MEASUREMENT OFFREQUENCY FLUCTUATIONS IN FREQUENCY STANDARDS[This report summarizes a paper that is to appear in the February 1966 issueof Proc. IEEE.]Precision quartz oscillators have three main sources of noise which contribute tofrequency fluctuations: thermal noise in the oscillator, additive noise contributed byauxiliary circuitry such as AGC, and fluctuations in the quartz frequency, as well as inthe reactive elements associated with the crystal, leading to an f-I type of power spec-tral density in frequency fluctuations. Masers are influenced by the first two types ofnoise, and probably by the third.The influences of these sources of noise on frequency fluctuation versus averagingtime measurements is discussed. The f- 1 spectral density leads to results that dependon the length of time over which the measurements are made. An analysis of the effectsof finite observation time is given.The characteristics of both passive and active atomic standards obtained by using aservo-controlled oscillator are discussed. The choice of servo time constant influencesthe frequency fluctuations observed as a function of averaging time; it should be chosenfor best performance with a given quartz oscillator and atomic reference.The conventional methods of handling random signals, that is, variances, autocor-relation, and spectral densities, are applied to the special case of frequency and phasefluctuations in oscillators, in order to obtain meaningful criteria for specifying oscil-lator frequency stability. The interrelations between these specifications are developedin the course of the paper.L. S. Cutler, C. L. Searle[Mr. Leonard S. Cutler is with the Physics Research Division of the Hewlett-PackardCompany, Palo Alto, California.]QPR No. 80

Molecular Beams

http://dspace.mit.edu/bitstream/1721.1/54157/1/RLE_QPR_080_I.pdf

Molecular Beams

Abstract

Similar works

Full text

Available Versions

DSpace@MIT