Contains report on one research project.Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E)Sloan Fund for Basic Research (M. I. T. Grant 170)U. S. Navy (Office of Naval Research) under Contract N00014-67-A-0204-000

Kleppner, Daniel

Myint, M. T.

Walther, F. G.

Winkler, P. F., Jr.

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GENERAL PHYSICSI. ATOMIC RESONANCE AND SCATTERINGAcademic and Research StaffProf. D. Kleppner Dr. D. C. BurnhamDr. M. T. Myint Dr. D. E. PritchardGraduate StudentsG. N. Carter F. G. WaltherF. Y. Chu P. F. Winkler, Jr.A. MAGNETIC MOMENT OF THE PROTON1. IntroductionThe goall of this experiment is to obtain a measurement of the proton g-factor, interms of the electron g-factor, to a precision of one part in 108. The unique feature ofour experiment is that the proton is examined in atomic hydrogen, thereby minimizingtheoretical uncertainties resulting from diamagnetic effects, which have limited the pre-cision of previous determinations of this fundamental constant (in all of which protonswere observed in a molecular environment). Moreover, we employ a double-resonancetechnique whereby the electron and proton spin-flip transition frequencies are measuredsimultaneously. Thus systematic errors attributable to sample exchange and magneticfield fluctuations are eliminated.In the experiment an atomic hydrogen maser designed to operate in a magneticfield of 3500 Gauss is used. The maser is operated as an extremely narrow-bandamplifier at the frequency corresponding to the electron spin-flip transition, a, shownin Fig. I-1. The output level of the maser amplifier is changed when the proton spin-flip transition, b, is stimulated. This double-resonance technique enables precisedetermination of the two resonant frequencies. By using the Breit-Rabi formula, theg-factor ratio is calculated from these two frequencies and the well-known hyperfineseparation of ground-state hydrogen. 2The final precision of the g-factor ratio is limited by the precision to which the pro-ton frequency can be determined. Since the internal coupling field between the protonand electron in atomic hydrogen is 40 times stronger than the externally applied field,the proton frequency must be determined 40 times more accurately than the desiredprecision for the g-factor ratio.The permanent magnet used in the experiment is designed for very high homogeneityThis work was supported by the Joint Services Electronics Programs (U. S. Army,U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E), and in partby the U. S. Navy (Office of Naval Research) under Contract N00014-67-A-0204-0006,and the Sloan Fund for Basic Research (M. I. T. Grant 170).QPR No. 89(I. ATOMIC RESONANCE AND SCATTERING)w mi mih + +12 2(b) 0.64 GHz(G) 9.2 GHzBGHz %3500G2 2Fig. I-1. Energy levels of atomic hydrogen in an externalmagnetic field.-7(typically 3 X 10-7 over the volume of a maser storage bulb, 2 cm in diameter). Themeasured fractional linewidth of the electron transition is typically 2 x 10-8, and henceextremely fine thermal stabilization of the magnet, which has a temperature coefficientof 250 ppm/° C, is required. We have developed a temperature control system that willkeep the field stable to better than one part in 108 over a period of a few minutes, butduring an evening of data-taking a slow drift of one part in 107 is not uncommon. Thusit is desirable to have an electronic field regulation system to compensate for thermaldrift. This has been incorporated into our data-accumulation system.2. Automatic Data-Accumulation SystemThe past few months have been devoted primarily to designing and building a data-taking system to deal with the aforementioned problems. The basic functions of thesystem are twofold: to provide a feedback scheme to lock the magnetic field, and hencethe electron resonance frequency, at some known value; and to provide a means ofsweeping the proton line and printing out information in a computer-analyzable form.The field locking is accomplished by square-wave modulation of the field withQPR No. 89TO FIELDCORRECTION COILFig. 1-2. Data accumulation system.(I. ATOMIC RESONANCE AND SCATTERING)a small Helmholtz coil. If the peak-to-peak amplitude of this modulation is roughlyequal to the half-width of the resonance line, then the resulting signal output froma phase detector will be proportional to the deviation of the electron resonance fre-quency, v, from the applied frequency v . This DC output is amplified and fed to afield correction coil that nulls any small deviation of v from vo, thereby effectivelylocking the field to a preset value.When RF power at the proton resonance frequency is fed into the system, the steadyamplitude of the electron resonance signal is changed, because of a mixing of the uppertwo states shown in Fig. I-1. In order to observe this double resonance effect, RFpower to the proton is modulated off-on, and the electron signal is monitored with alock-in amplifier at the modulation frequency. This output, converted to DC, is readby a digital voltmeter and punched on paper tape. The entire system is illustrated inFig. 1- 2.The RF power modulation and field modulation are not carried out simultaneously,since the double resonance is observed through the effect of the proton resonance signalon the electron resonance signal. We use a measurement sequence (Fig. I-3) in whichwe alternately modulate the field and the RF power. During the field-modulationpart of the sequence, the necessary feedback current is determined. Holding thiscurrent constant for the short period during which the proton resonance is stimu-10 msec FIELD LOCKEDSDURINGFIELD READOUTMODULATION0I I I I III I IIIII I II I IONILEVEL OFFDATA TIME READOUT TIMEI1-10 sec I 2secRADIO FREQUENCYSTEPPED TONEW VALUEFig. 1-3. Timing sequence.QPR No. 89(I. ATOMIC RESONANCE AND SCATTERING)lated maintains the field at the preset value. One data point consists in repeatingthis sequence several times, after which the data are read out. Then the RF poweris automatically stepped to a new frequency, and another data point is taken. The totaltime for each point is a few seconds.The key element of the data-acquisition system is a master programmer, whichdrives the field modulation coil and the RF power gate. Each modulation is driven fora preset number of cycles of an internal audio-frequency clock. The signal is switchedby the programmer either to the phase detector, during the field-modulation part of thesequence, or to the lock-in amplifier, during the proton-modulation period. At the endof a data point, the programmer triggers the digital voltmeter, which immediately readsthe lock-in amplifier output. This signal level and the two frequency-settings arepunched out on tape.The RF power for stimulating the proton transition is obtained by mixing a 600-MHzsignal with the output of a 50-MHz digital frequency synthesizer. All frequency sourcesare phase-locked to a reference standard. The synthesizer is set by a versatile controlunit that is programmed to sweep the proton line. At the end of a data point the pro-grammer triggers this control unit, which in turn changes the synthesizer output to theproper value for the next data point.Both the programmer and the synthesizer control are rather complex digital logicdevices. Their design and construction was mercifully simplified by the extensive useof inexpensive RTL integrated circuits, several hundred of which were required.This system has only recently been completed, and we are now testing the systemand taking preliminary data. In the coming months we expect to take enough data toimprove significantly our knowledge of the proton magnetic moment.P. F. Winkler, Jr., F. G. Walther, M. T. Myint, D. KleppnerReferences1. The experimental goals were described more fully in Quarterly Progress ReportNo. 84, p. 27. For preliminary results see T. Myint, D. Kleppner, N. F. Ramsey,and H. G. Robinson, Phys. Rev. Letters 17, 405 (1966).2. S. B. Crampton, D. Kleppner, and N. F. Ramsey, Phys. Rev. Letters 11, 338 (1963).QPR No. 89 5

Atomic Resonance and Scattering

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Atomic Resonance and Scattering

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