Contains research objectives and reports on one research project

Bitter, F.

Bradley, L. C., III

Stroke, H. H.

Tomlinson, W. J., III

Waugh, J. S.

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VII. NUCLEAR MAGNETIC RESONANCE AND HYPERFINE STRUCTUREProf. F. Bitter E. R. Hegblom H. C. PraddaudeProf. J. S. Waugh D. J. Hegyi O. RediDr. L. C. Bradley III F. A. Li6geois C. J. Schuler, Jr.Dr. H. H. Stroke R. G. Little R. W. SimonDr. J. F. Waymouth J. D. Macomber W. W. SmithJ. C. Chapman F. Mannis M. J. StavnT. Fohl P. G. Mennitt W. J. Tomlinson IIIW. D. Halverson S. R. Miller W. T. WalterRESEARCH OBJECTIVESThere are four main objectives in the work of this group: first, the investigation ofmolecular and solid-state problems by nuclear magnetic-resonance techniques; second,the investigation of the structure of atomic nuclei, particularly of radioactive nuclei, bynuclear magnetic-resonance techniques and by the investigation of atomic hyperfine-structure and isotope shift; third, the development of atomic resonance techniques forthe study of resonance radiation; and fourth, the study of the earth' s magnetosphere.1. Nuclear Magnetic ResonanceOur molecular studies by means of nuclear resonance continue to emphasize: (a) thenature of phase transitions, particularly those involving order-disorder phenomena, insolids; (b) intermolecular forces, including their reflection in the transport propertiesof fluids; and (c) determination of the structures of simple molecules.J. S. Waugh2. Hyperfine StructureOptical pumping and nuclear resonance have been used successfully to achieve newresults. The results of our isotope-shift experiments have clarified the causes of the" odd-even staggering " effect observed in atomic spectra of heavy elements. Theseresults show that further systematic study of both isotope and nuclear isomer shiftsshould provide valuable information on nuclear interactions; in particular, on the polar-ization of nuclei by nucleons. The theoretical study of the "hyperfine- structure anomaly"and the formalism that we have developed has been used to test explicit assignmentsof nuclear wave functions. Atomic-beam absorption techniques are being devel-oped and used for precise spectroscopic measurements, as well as for detailed studiesof spectral-line profiles. Hyperfine-structure " level crossing" and double-resonanceexperiments have been carried out with both stable and radioactive atoms. Furtherwork with both types of experiments is expected to yield data on atomic collisions andradiation coherence. An experiment has been performed to test possible parity non-conservation in molecular interactions. This work is being extended to other tran-sitions, and to refining the upper limit on the mixing coefficient of a parity nonconservingstate.L. C. Bradley III, H. H. Stroke, F. Bitter3. Atomic Resonance SpectroscopyThe detailed analysis of light is becoming increasingly important in such varied fieldsas atomic and nuclear physics, communication, and astrophysics. In addition to resultsachieved by spectroscopic instruments that measure wavelengths, the use of magneticallytunable atomic oscillators for measuring frequency distributions offers great promise.In particular, the use of atomic beams in uniform variable magnetic fields promises tobecome a useful tool for studies of the shape of resonance lines transmitted throughradiating and absorbing media.Also, atoms may be considered as oscillators in which radiofrequencies and opticalQPR No. 68(VII. NUCLEAR MAGNETIC RESONANCE)frequencies are coupled whenever the simultaneous absorption of two such different fre-quencies involves a common level. Subtle line-narrowing effects resulting from theimprisonment of optical resonance radiation have been detected by using double-resonance techniques. Experimental investigations to determine the relative importance,on the one hand, of interatomic coupling caused by the radiation field, and on the otherhand, of the degree of coherence of the light in an absorbing cell should prove to be ofvalue in various investigations.Work in both of these areas was started some time ago in connection with hyperfine-structure investigations and will now be continued for its own sake.H. H. Stroke, L. C. Bradley III, F. Bitter4. The Earth' s MagnetosphereData obtained from satellites and space probes on the nature of the interplanetarymedium are reviving interest in laboratory experiments related to the conditions foundin space. We propose to set up models of the earth and its surrounding magnetic field,and to study, first of all, the diffusion of electrons and properties of trapped charges inthe surrounding Van Allen belts. The basic condition to be satisfied is that the ratio ofthe electron cyclotron radius " a" ,my 1 /2mVa eB BI / ehave the same ratio to, or should at least be adequately smaller than, the radius of thesolid model. If R is the radius of the model, and r the radial distance from the centerof the model to any point in space, we have, if the average intensity of magnetization ofthe spherical dipole is I, the surrounding field in the equatorial plane given by. I( 7r R 3)B o 347T 3rand therefore the required ratio a/R isa 8 -V (r)3-= 8 (j-R IR R22 2 6For the earth, we have a magnetic moment 8. 2X10 amp-meter , R = 6.4X10 meters,and an average intensity of magnetization I = 76 amp/meter. For the outer edge of theVan Allen belt near the surface of the magnetosphere,R) = 10V = 5x104 electron volts;and consequently2a 80. 71X10 3 1. -3=8X10 = 1.2X10 .R 84.9X10Two spherical dipoles have been designed by Bruce Montgomery of the National Mag-net Laboratory. The first is a superconducting coil with an iron core having a mag-netic moment 820 amp-m2 and a diameter of 2" . The second is a water-cooled solenoidwith an iron core having a magnetic moment 14, 000 amp-m2 and a diameter of 7",QPR No. 68(VII. NUCLEAR MAGNETIC RESONANCE)requiring 500 kw for its operation. The values of a/R for these dipoles at points distant10 R from their centers is 3-4 X 10 - 2 N V. These would seem to be adequate models fora convenient range of electron energies. The field of these dipoles at a distance 10 R,or 10" and 35" , respectively, would be 20-50 gauss. It would be quite feasible toimmerse them in very large coils that produce fields of this order of magnitude to sim-ulate the interplanetary field bounding that of the earth, if it should become desirablefor other lines of investigation.A further requirement of such a model would be that the pressure be made low enoughso that electronic mean-free paths would be at least of the order of meters. This indi-cates operation at pressures below 1 micron, or with an atom density below 1013 percubic centimeter. Insofar as pumping or the production of localized discharges to gen-erate ions and electrons are concerned, this offers no difficulty.The minimum electron densities required for convenient observations by means ofLangmuir probes are of the order of 109 per cubic centimeter. This is in a range thatwould appear to be of interest and practically achievable, at least locally in a large tank.The electronics of the problem is perhaps its most critical aspect. Other approachesfor work at very low densities would seem to be desirable.This program marks the evolution of work on mercury-rare gas discharges, whichhas been going on for the past two years. The theory of Langmuir-probe measurementsin a magnetic field has been developed by J. F. Waymouth. 1 Observations have beenmade by T. Fohl on the diffusion and thermal conductivity of electrons in a large spher-ical container with a localized discharge at the center. Magnets for the study of cylin-drical plasmas in cusped or uniform longitudinal fields, and in quadrupole or uniformtransverse fields are available. It is hoped that these investigations will lead to actualmagnetosphere model experiments in the course of the coming year.F. Bitter, H. H. StrokeReferences1. J. F. Waymouth, Perturbation of a Plasma by a Probe, Technical Report 406,Research Laboratory of Electronics, M. I. T., December 26, 1962.A. HYPERFINE STRUCTURE AND ISOTOPE SHIFTS IN NEUTRON-DEFICIENTMERCURY ISOTOPESProduction techniques are now under study for the purposes of obtaining Hg 1 9 3 andHg193 . free of Hg 1 9 5 and Hg 1 95* to permit a verification of the preliminary assignment,I(Hg 1 9 3 ) = 13/2 and 4.(Hg 1 9 3 * )  .(Hgl 9 5*), and to obtain the Hg 1 9 3 spectrum.1 Theseresults refer solely to the 2537 A resonance line.The spectrum of Hg 1 9 4 has been observed over a period of almost one year, in thehope that by comparing its intensity with that of stable Hg ,9 6 which was producedsimultaneously in the bombardment, we may be able to determine its halflife. At presentwe are investigating the influence of self-absorption and cleanup on such a determination.We have also obtained a corrected value for the Hg194-Hgl96 isotope shift in the2537 A line (0. 280+ 0. 015) cm-1. The result is that y(195) and y(1 9 5") are even morenearly equal to unity. 2W. J. Tomlinson III, H. H. StrokeQPR No. 68(VII. NUCLEAR MAGNETIC RESONANCE)References1. W. J. Tomlinson III and H. H. Stroke, Hyperfine structure and isotope shiftsin Hgl 9 5 , Hgl 9 5 , Hg 9 4 , and Hg 1 9 3 , Quarterly Progress Report No. 66, ResearchLaboratory of Electronics, M. I. T., July 15, 1962, pp. 18-19.2. W. J. Tomlinson III and H. H. Stroke, Phys. Rev. Letters 8, 436 (1962).QPR No. 68

Nuclear Magnetic Resonance and Hyperfine Structure

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Nuclear Magnetic Resonance and Hyperfine Structure

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