Contains reports on three research projects

Fohl, T.

Stroke, H. H.

Tomlinson, W. J., III

Walter, W. T.

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V. NUCLEAR MAGNETIC RESONANCE AND HYPERFINE STRUCTUREProf. F. Bitter W. D. Halverson S. R. MillerProf. J. S. Waugh E. R. Hegblom H. C. PraddaudeDr. L. C. Bradley III Ilana Levitan O. RediDr. H. H. Stroke F. A. Lidgeois C. J. Schuler, Jr.Dr. J. F. Waymouth J. D. Macomber R. W. SimonT. Fohl F. Mannis W. W. SmithR. L. Fork I. G. McWilliams W. J. TomlinsonJ. V. Gaven, Jr. P. G. Mennitt W. T. WalterA. HYPERFINE STRUCTURE AND ISOTOPE SHIFTS IN Hg 1 9 5 , Hg 1 9 5 , AND Hg 1 9 4Successful light sources of Hg 1 9 4 , Hg 9 5 ,and Hg 9 5  have been prepared asdescribed in Quarterly Progress Report No. 64 (page 40). Hyperfine-structure spectrawere obtained in the 2537 A, 4047 A, and 4358 A lines. We obtain the following prelimi-nary results:I(Hg 1 9 5 ) = 1/2I(Hg 1 9 5 :) = (13/2, 11/2)L(Hg 1 9 5 ) = 0. 54 ± 0. 02 nmFor I = 13/2:S(Hg 1 9 5 *) = -1. 05 ± 0. 02 nm195+ -24 2Q(Hg 1  ) = (1. 6±1. 0) x 10 cmIsotope shifts:Hg 1 9 4 - Hg 1 9 8 = 0. 302 ± 0. 018 cm-1Hg 1 9 5 -H198 = 0. 221 0. 020 cm-1Further analysis of recent data is in progress. This is expected to yield a definite result195 .'for the spin of Hg 95(which we expect to be that of the il3/2 neutron on the basis of theshell model) and more accurate values for the isotope and isomeric shifts.W. J. Tomlinson III, H. H. StrokeB. THE MAGNETIC MOMENT OF MERCURY 197 BY MEANS OF OPTICALPUMPINGThe use of optical pumping to orient nuclei and thereby enable ground-state nuclearmagnetic-resonance experiments to be performed in dilute vapors has been described inprevious reports, 1-3 particularly for mercury. The precision of the measurement of themagnetic moment of the 65-hour radioisotope Hg 19 7 has recently been improved by(V. NUCLEAR MAGNETIC RESONANCE)199making the following changes. (a) The cell containing Hg , which had been used beforeas a calibration standard was opened and refilled with both Hg197 and Hg 1 9 9 . This cellhad shown the largest amount of optical pumping of the seven cells tested, despite thefact that its walls had not been coated with G. E. Dri-Film. (b) The cell was placed in alarger splitting field (1200 gauss, instead of 450 gauss). (c) The stray magnetic field atthe position of the cell, which is influenced by the scanning field at the lamp, was reduced.We discovered that the major contribution to this leakage field was produced by currentin the bus bars on the ceiling of the Magnet Laboratory, which are used to supply thescanning magnet, a Bitter solenoid. Thirty-five feet of 1. 875-inch diameter "kicklesswelding cable" (2500-amp capacity) was purchased from Mackworth G. Rees, 1573 EastForest Avenue, Detroit, Michigan. This cable contains 6 conductors hexagonally spacedabout a fluted center. Three conductors are tied in parallel to carry current to thescanning magnet, while the other three in parallel form the return circuit. Hose connec-tions at either end of the cable allow water-cooling of the six conductors. The scanningmagnet has been connected to the bus bars in the motor generator room by means of thiskickless welding cable. This arrangement reduced the leakage field by more than a factorof 10, so that when the scanning magnet is changed from Hg 1 9 7 to Hg 1 9 9 optical-pumpinglight the leakage field changes by less than 0. 005 gauss.With these changes, resonances 25 cps wide were observed at 933 kc and 859 kc.197The measured ratio was = 1. 042479 (±15). Comparison of the measured ratio with199the ratio of Stager's values 4 of the interaction constants for the 3P l state, Al97/Al991. 043284 (±5), indicates a hyperfine-structure anomaly of 0. 772 (±16) parts in 1000.Using Cagnac's Hg199 result, 5 Y199/YH = 0. 1782706 (± 3), we obtain197= 0. 1858434 (± 27)YHandp197 = 0. 519014 (±10) 4Nwithout diamagnetic correction. In addition, comparing the Hg 1 9 9 resonance with theproton resonance, we obtain y199/YH= 0.178273 (±5), which is in agreement with Cagnac'sresult within our error limit. The errors quoted are three times the standard deviation.W. T. WalterReferences1. W. T. Walter, Nuclear orientation by optical pumping, Quarterly ProgressReport No. 57, Research Laboratory of Electronics, M. I. T., April 15, 1960, pp. 64-77.2. W. T. Walter, Nuclear orientation and magnetic moment of mercury 197,Quarterly Progress Report No. 62, Research Laboratory of Electronics, M. I. T.,July 15, 1961, pp. 105-108.(V. NUCLEAR MAGNETIC RESONANCE)3. W. T. Walter, Mercury 197-199 hyperfine-structure anomaly, QuarterlyProgress Report No. 63, Research Laboratory of Electronics, M. I. T. , October 15,1961, pp. 82-83.4. C. V. Stager, Hyperfine Structure of Hg 1 9 7 and Hg 1 9 9 , Ph. D. Thesis, Depart-ment of Physics, M. I. T., 1960.5. B. Cagnac, Ann. phys. 6, 467 (1961).C. D-C DISCHARGE IN THE LARGE SPHERICAL LAMPIn Quarterly Progress Report No. 64 (pages 40-42), a spherical lamp that is pro-vided with movable anode, cathode, and Langmuir probes was described. It was statedthat by operating the tube with the anode and cathode close together, a point source ofions and electrons could be approximated. Since then, measurements have been madewhich indicate that the discharge has the expected form.In our model of the cathode region we consider that secondary electrons in the cath-ode glow form a dense, spherically symmetric cloud about the cathode spot. These,together with positive ions, diffuse outward because of the density gradient. For a cer-tain distance the electron diffusion current is sufficient to carry the tube current, andthere is no accelerating electric field. In fact, the field is negative over part of theregion because of the ambipolar field. In this region (Faraday dark space) the electronsare relatively cool and do not excite the atoms.When the diffusion current density becomes less than that required to maintain thetube current, the electric field becomes accelerating and the electrons are heated. Thesehotter electrons excite and ionize atoms, and the glow of the positive column is observed.The factor determining the length of the dark space is the diffusion rate of the elec-trons. The diffusion rate 1 is given by..V 2F = -D Vn where D e (1)e e 3vmwhere D is the diffusion coefficient; V, the electron velocity; and n, the electron den-esity. As Vn diminishes, the diffusion current must eventually become inadequate, anda positive column forms. The distance from the cathode at which the column formsdepends on the inverse of the pressure through the collision frequency, vm. If thereare walls nearby, as in a cylindrical lamp, wall recombination will provide an additionalloss of carriers, and the dark space will be still smaller.When the anode is near the cathode, and the walls far away, the diffusion currentwill be symmetric about the cathode, and the current density will be inversely propor-tional to the square of the radial distance from the cathode. If the anode is close enoughto intercept enough current to carry the tube current, no accelerating field develops and(V. NUCLEAR MAGNETIC RESONANCE)2625 -w 24-23 -0, 22-mI 21 K200 1 2 3 4 5 6 7 8 9 10 II 12 13 14 15ELECTRODE SEPARATION (CM)Fig. V-1. Voltage drop across tube as a function of electrode separation.(Current, 2 amps; argon pressure, 2 mm Hg; mercury vaporat room temperature; anode in dark space until separation is"15 cm.)the Faraday dark space envelops the anode.The characteristics of a discharge operating in this mode are that all ionization andheating of electrons occurs in the immediate region of the cathode, and the ions andelectrons diffuse radially. The only electric field that is external to the cathode regionis the ambipolar field, and it is also radially directed.These features seem to be found in our discharge. As we showed in the previousreport, the distance between the cathode and the edge of the positive column increaseswith decreasing pressures. The dark space is also larger than that usually found incylindrical tubes at these pressures. A typical curve of tube voltage as a function ofelectrode separation is shown in Fig. V- 1. We note that the voltage drop decreaseswith increasing distance over a portion of the curve. This indicates that a negativeelectric field exists.Probe measurements, taken with the pulse-heated apparatus described in QuarterlyProgress Report No. 62 (pages 110-115), indicate that the electron temperature anddensity are spherically symmetric about the cathode, to at least 4 cm from the cathodespot. The only variation is in the immediate region of the cathode support. Similarly,the potential seems to be a function of the radius only. The electron temperature variesfrom ~6000 0 K near the cathode to ~35000K near the wall. The electron density variesfrom - 5 X 101 1 /cm 3 near the cathode to - 5 X 109 /cm 3 near the wall. The potentialnear the wall is ~2 volts lower than that near the cathode.For ions and electrons diffusing outward from a central source of constant supply,the flow obeys the equationdiv F = 0 (2)(V. NUCLEAR MAGNETIC RESONANCE)when there is no recombination in the volume of the tube. For spherical symmetry,zF o (3)2rwhere Z is the rate of production inside the cathode region.However, from Eqs. 1 and 2, we get Laplace's equationSF = 7 (Dn) = 0 (4)where D is the ambipolar diffusion coefficient. This has a solution of the formr2= -+ Cr0 = Dn.Using the boundary conditions 0 = 0 at r = rwall and 0 = 0o at r = rsource, we geto =o r -r0 o ( rw -rrsor -rw sro sC -1 r -rw srwrVariations from this law will indicate the presence of recombination in the volume ofthe tube.Also, the fact that the electron temperature is a function of radius should allow aninvestigation of energy transfer in the electron gas.T. Fohland

Nuclear Magnetic Resonance and Hyperfine Structure

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Nuclear Magnetic Resonance and Hyperfine Structure

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