Bitter, F.

Plotkin, H. H.

Richter, B.

Teviotdale, A.

Young, J. E. R.

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VI. MAGNET LABORATORY RESEARCHProf. F. Bitter B. Richter A. TeviotdaleN. I. Adams III P. L. Sagalyn T. F. WimettH. H. Plotkin J. R. Stanley J. E. R. YoungA. MAGNETO-OPTIC SCANNING1. Description of the MethodA new method has been proposed and tested for examining the hyperfine structureand the isotope shift of absorption lines. Its application to the ultraviolet mercury reso-nance line X = 2537 A.U. is particularly simple. A mercury arc lamp containing onlythe isotope of mass number 198 is placed in a magnetic field, and only the light emittedin the direction of the field is used in the experiment, as shown in Fig. VI- 1. The reso-nance line is broken up into a simple triplet by the magnetic field, and only the circu-larly polarized components having the normal frequency plus or minus the Larmorfrequency are emitted in the field direction. Either one of these can be cut out by aquarter-wave plate and an analyzer, so that we are left with a single line whose fre-quency can be adjusted over a considerable range by means of the applied field intensity.If this light is allowed to fall on a cell containing mercury vapor having any arbi-trary isotopic composition, absorption and re-emission of resonance radiation willtake place selectively when the frequency of the incident light is that of an absorptionline.2. Application to Natural MercuryThe results of a run, made with natural mercury in thermal equilibrium at roomtemperature, are shown in Fig. VI-2. The predicted position of peaks and the pre-dicted intensities of emission lines are shown. The positions agree qualitatively withthe observations. Further experiments are required to establish the positions of thePHOTO -TUBEFig. VI-1Diagram of the apparatus.21-CE(VI. MAGNET LABORATORY RESEARCH)204199A201a3 -I10 cmK (GAUSS)Fig. VI-2Natural Hg X 2536 resonance line.peaks more precisely. The small subsidiary peaks on the right side of the diagramare due to incomplete polarization of the incident light. The relative intensities arevery different from the predictions for emission lines because of quenching collisionswhich convert optical energy into thermal energy. If there were no such collisions,one would expect any one of these peaks to increase as a function of pressure at lowpressures and then to approach a saturation value at a pressure determined by the con-centration of the isotope responsible for the particular absorption line being studied.Because of inelastic quenching collisions, the intensity of the resonance line falls withincreasing pressure at higher pressures. Each of the five lines in Fig. VI-2 willtherefore have a maximum at some temperature, and this temperature will be differentfor the different lines.Experiments at reduced pressures and temperatures have shown that such behavioris actually observed.3. Experiments with Radioactive MercuryRadioactive isotopes of mercury were prepared by bombarding gold with 15 Mevdeuterons in the M.I.T. cyclotron. Several new lines were observed in subsequentmagneto-optic scanning experiments, indicating that the yield in a 12-hour run was morethan ample for work of this kind. Further work is required to identify the observedpeaks.F. Bitter, H. Plotkin, B. Richter,A. Teviotdale, J. E. R. Young-22-

Magnet Laboratory Research

Contains reports on two research projects

Sagalyn, P. L.

VII. MAGNET LABORATORY RESEARCHProf. F. Bitter H. H. Plotkin J. R. StanleyDr. P. L. Sagalyn B. Richter A. TeviotdaleN. I. Adams III J. E. R. YoungA. THE HYPERFINE STRUCTURE OF THE RESONANCE RADIATION OF MERCURYAfter our first successful observations almost a year ago, we experienced a varietyof difficulties which have only recently been completely overcome.The main points that must be watched are: preventing excessive gaseous impuritiesin the resonance lamp, avoiding the deposition of mercury near the windows of the reso-nance lamp, and providing for the cooling of the arc lamp if a broad or reversed lineis radiated. When proper precautions were taken regarding these points, good resultswere consistently obtained.An automatic recording device was developed by connecting the horizontal deflectingterminals of a dc oscilloscope to the terminals of a shunt through which the magnetcurrent passes, and connecting the vertical deflecting terminals to the output of the photo-multiplier. A curve obtained with a sample containing natural and radioactive mercuryis shown in Fig. VII-1.A first attempt to produce sufficient Hg 1 9 5 for magnetic scanning was unsuccessful.H. H. Plotkin, B. Richter, A. TeviotdaleB. QUADRUPOLE MOMENT OF Na 2 3We have computed the sign of the nuclear electric quadrupole moment to be expectedfor the sodium nucleus on the basis of the following assumptions: (a) zeroth-ordercentral-field approximation and (b) three protons in a (d 5 / 2 )3 configuration with totalresultant angular momentum equal to 3/2. The result was identically zero. This is-24 2in disagreement with our rather large experimental value of (+ 0. 12 + .07) x 1024 cm .P. L. SagalynFig. VII-1Automatically recorded curve showingthe hyperfine structure of a, sample ofmercury.-19-

VII. MAGNET LABORATORY RESEARCHProf. F. Bitter B. Richter J. R. StanleyN. I. Adams III Dr. P. L. Sagalyn A. TeviotdaleH. H. Plotkin J. E. R. YoungA. THE HYPERFINE STRUCTURE OF THE RESONANCE RADIATION OF MERCURY1. Natural MercuryA new apparatus for magnetic scanning, described in the Quarterly Progress Report,April 15, 1953, has been completed and is working satisfactorily. The preliminaryresults for the line shapes and intensities in natural mercury are complicated and con-fusing. In the experiments the resonance cell was kept at approximately 70 0 C, whilethe temperature of the tail of the resonance lamp was varied from -30'C to 100°C.There is apparently some form of hysteresis present, and the critical factors have notyet been determined.F. Bitter, H. H. Plotkin, B. Richter, A. Teviotdale, J. E. R. Young2. Radioactive MercurySeveral runs were unsuccessful because of the presence of impurities. A run was197made in which the Hg was caught on a pure gold surface, the cell pumped out, andthe mercury then driven from the gold to the cell. This technique made possible theprovisional identification of two of the previously reported peaks. On the basis of thisresult and additional assumptions, provisional values were obtained for the nuclearmagnetic dipole moments and the electric quadrupole moments of Hg 1 9 7 in both of itsisomeric states. A check of these results is being attempted by observations on thehyperfine structure of other levels which are described below.F. Bitter, H. H. Plotkin, B. Richter, A. Teviotdale, J. E. R. Young3. Emission Spectrum of Hg1 9 7Preliminary observations indicate that we can make enough of this isotope to photo-graph its emission spectrum, using a high-frequency electrodeless discharge and anechelle available in the Spectroscopy Laboratory. We hope to check the conclusionsreached by magnetic scanning of the 3P1 level by supplementary observations on otherexcited levels.F. Bitter, H. H. Plotkin, B. Richter, A. Teviotdale, J. E. R. YoungB. THE SPONTANEOUS MAGNETIZATION AND SPECIFIC HEAT OF IRONThe electronic contribution to the specific heat of a ferromagnetic is linear withtemperature at low temperatures but increases more rapidly with temperature at highertemperatures, rising abruptly to a second-order discontinuity at the Curie point. This-32-(VII. MAGNET LABORATORY RESEARCH)behavior is attributed partly to the kinetic energy of the free or quasi-free electrons,which accounts for the low-temperature linear term, and the "quasi-magnetic" orexchange energy EM of the electrons, which accounts for the abrupt rise to the Curiepoint. Stoner (1, 2) gives expressions for the specific heat and the spontaneous magnet-ization in terms of certain interaction parameters for a Fermi-Dirac assembly of elec-trons, assuming a band of standard form, but agreement with experiment is not good.It has recently been suggested (3) that the cause of this disagreement is the assumptionthat the exchange energy depends only on the square of the magnetization and that higher-order terms should be included. We have obtained fair agreement for nickel (4) andpoorer agreement for iron (5), using an energy expression of the formEM = k (1 + A ) (1)where 0' is an interaction parameter, t is the relative magnetization, and A is a smallpositive constant. The indications are that a more general expression would give betteragreement. Independent evidence at low temperatures (5) indicates further that thedensity of states for iron is constant over the relevant energy range, and that a moreappropriate value of A in this region would be negative.Computations to test these indications were made by first finding (with a band ofrectangular form (6)) the form of E M to obtain agreement with the spontaneous magnet-ization (7) and then using this to calculate the specific heat. The parameters involvedare n = kO'/E , where E is the Fermi energy, and the number of "holes" in the d-band,N. Though in detail the agreement is poor, there is general quantitative agreement asto order of magnitude and shape, and this may be regarded as satisfactory. Potter's(6) results are published in the form of a curve, and interpolation is inaccurate; theprocess of numerical differentiation to obtain the specific heat adds further to the inac-curacy. It is thought that the results give some support to the treatment given elsewherefor iron (5). The effective 0' shows a maximum at ,; w 0. 75 or T/0 - 0. 8. It is possiblethat this maximum is related to the maximum in the dilatation of iron (8) and the sharpchange of slope in the elastic moduli (9) about this temperature.A. TeviotdaleReferences1. E. C. Stoner: Proc. Roy. Soc. (London) A 165, 372, 19382. E. C. Stoner: Proc. Roy. Soc. (London) A 169, 339, 19393. E. P. Wohlfarth: Phil. Mag. 40, 703, 1095, 19494. K. L. Hunt: Proc. Roy. Soc. A 216, 103, 195333-(VII. MAGNET LABORATORY RESEARCH)References (continued)5. A. Teviotdale: Doctoral Thesis, Department of Physics, University of Leeds,England, 19526. E. P. Wohlfarth: Phil. Mag. 42, 374, 19517. H. H. Potter: Proc. Roy. Soc. (London) A 146, 362, 19348. H. Esser, H. Eusterbrock: Arch. Eisenhuttenwesen 14, 341, 19419. K. Iokib6, S. Sakai: Sci. Rep. Tohoku University 10, 1, 1921-34-

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