Contains reports on four research projects

Adams, N. I., III

Arnold, R. D.

Bitter, F.

Davis, L., Jr.

Dickinson, W. C.

Eisinger, J. T.

Eshbach, J. R.

Feld, B. T.

Gokhale, B. V.

Hahn, T. M., Jr.

Harvey, George G.

Hill, Albert G.

Hillger, R. E.

Ingersoll, J. G.

Johnson, H. R.

King, G. W.

Kip, A. F.

Knight, G.

Lawrance, R. B.

Levinson, J.

Lew, Hin

Loomis, C. C.

Nagle, D. E.

Pohlman, W. B.

Strandberg, Malcom W. P.

Weiss, M. T.

Wimett, T. F.

Zabel, C. W.

Zacharias, J. R.

DSpace@MIT

V. RADIO-FREQUENCY SPECTROSCOPYA. MOLECULAR-BEAM RESEARCHProf. J. R. Zacharias Dr. J. LevinsonProf. B. T. Feld Dr. Hin LewDr. D. E. Nagle Dr. C. W. ZabelDr. L. Davis, Jr. J. T. EisingerDr. G. Knight T. M. Hahn, Jr.W. B. Pohlman1. Third Molecular-Beam ApparatusWork is continuing in the construction of this apparatus.Dr. Hin Lew2. Nuclear Spin and Hyperfine Structure of Cs1 3 5 and Cs 1 3 7A mixture of cesium isotopes produced by uranium fission was obtainedfrom the Oak Ridge National Laboratory. In the oven of a molecular-beamapparatus were placed .25 millicuries of this mixture to which 500 micro-grams of stable Cs 1 3 3 had been added. A molecular-beam magnetic resonanceexperiment was performed, using a mass spectrometer to sort out the iso-topes. Resonance transitions were observed for mass numbers 133, 135, and137. First results indicate that the nuclear spin of 135, like thatobserved for 133 and 137, is 7/2; that the sign of the nuclear magneticmoment is positive; and that the hyperfine structure Av = 9723.00 + 50megacycles. From Kusch, Millman and Rabi's value of 4133 = 2.558 nuclearmagnetons, the nuclear magnetic moment of Cs1 35 is (2.706) ± .5 per centnuclear magnetons (without diamagnetic correction). Dr. D. E, Nagle3. The Fourth Atomic-Beam ApparatusMachine work on the main vacuum system has been completed; the vacuumsystem has been set up, and pumps installed. The location of leads in theapparatus is now in progress.The construction of deflecting magnets is in progress, and machinework on the two mass spectrographs has been completed. The construction ofamplifiers, and other electronic circuits to be used with the apparatus hasbeen nearly completed. T. M. Hahn, Jr.4. An Electron-Bombardment IonizerThe problem of detection has restricted the choice of elements forstudy by the molecular- and atomic-beam magnetic resonance methods. Forexample, one would hesitate to investigate manganese using the current-26-~ I_ _I_ ^  _~I_ 1 L_~ILILX____* _-~-.-..- -- .I^1-^II*~.1.1I~-. .X.--I IX-. i .1~-^11- ----L --. - .-.-I ~i_-._i.ll- .^ ~~L---IIII~IIIIIIYII~~(V. RADIO-FREQUENCY SPECTROSCOPY)methods of detection, yet 2 5Mn5 5 would be an extremely interesting nucleusto investigate, since preliminary calculations indicate that the atom hasan electronic structure which offers the possibility of detecting anoctopole moment in the nucleus. Many other nuclei, too, cannot be attemp-ted for lack of a suitable method of detection.Therefore, a new method of detecting atomic and molecular beams isbeing investigated. The beam to be detected is bombarded by an electronbeam moving perpendicularly to it. Calculations indicate that a large pro-portion of the atoms could be ionized, and then continue to move in substan-tially the same direction. The ions are then accelerated electrostaticallyfor passage through a mass spectrograph, and finally counted by hittingthe first plate of an electron multiplier.A simple model of such a device has been built. In a test run it wasfound that excessive background exists, apparently due to ions formed whenthe electron beam strikes the anode. The model is now being modified soas not to admit ions formed at the anode into the mass spectrograph.Dr. J. LevinsonB. NUCLEAR MAGNETIC RESONANCEProf. F. Bitter W. DickinsonN. I. Adams, III T. Wimett1. Nuclear Magnetic Moments and Hyperfine Structureof the Rubidium IsotopesA recent determination (1) of the nuclear moments of the rubidiumisotopes indicates that, as was first suspected by Millman and Kusch (2),the observed ratio of the magnetic moments does not agree with the ratiocalculated from hyperfine structure,Rb 8 7Observed (1) 2.0327.± 0.001Observed (3) 2.038 ± 0.01Calculated (2) 2.0261 ± 0.0003The interaction between the optical electron and the nucleus is oftwo kinds - electrostatic and magnetic - and it is of interest to determinewhether the observed effect can be accounted for on the basis of either ofthese two forces. The effect of spreading the nuclear charge, usually-27-(V. RADIO-FREQUENCY SPECTROSCOPY)considered concentrated at a point at the origin, over a finite sphere ofradius Ro, is to reduce the electronic density Y)2 (o) at the nucleus. Thiseffect was shown by Rosenthal and Breit (4) to lead to a correction incalculating hyperfine structure of the order of zR /a for atoms having anuclear charge in the range of rubidium. The a is the radius of the firstBohr hydrogen orbit. While this ratio is 0.5 per cent for rubidium, andcan account for discrepancies in the hyperfine structure of the order ofthe observed discrepancy (0.3 per cent), it is very unlikely that this isthe explanation, as it would require an enormous difference in size ofRb8 5 and Rb . Besides the fact that all available evidence supports aslowly varying nuclear radius proportional to the cube root of the massnumber, we have specific evidence (5) that there is no observable isotopeshift in the spectrum of rubidium, and therefore no such very large differ-ence in nuclear size. Effects due to a magnetic structure inside thenucleus, first proposed by Kopfermann (6) in the days when such effectscould well be dismissed as negligible, and recently applied by Bohr (7) tothe proton-deuteron problem, also lead to a correction of the order ofzR /ao.  This is a much more reasonable explanation, since there is noobjection to assuming the wave function for the odd proton in rubidium tobe quite different for the two isotopes. In fact, this might be expectedon the basis of a vector model (8) of the nucleus, which would assign toRb 8 7 with I = 3/2, I + s = 1 + 1/2, and to Rb8 5 with I = 5/2, e- s =3- 2/2.The fact that effects of this order of magnitude have not been observedfor the gallium isotopes (9) or for the thallium isotopes (10) is notunreasonable since, in both cases, the two isotopes have the same nuclearspin and comparable nuclear moments, and it is therefore not unreasonableto assume that they also have comparable wave functions for the odd proton.Prof. F. Bitter2. Nuclear Magnetic Moments of Rb87 and Rb85An apparatus for a precision measurement of the g-factor ratio Rb87to Rb85 is being constructed. Other work is in temporary abeyance.N. I. Adams, III3. The Magnetic Moment of Be9The ratio of the frequency of the nuclear magnetic resonance of Be9to that of the proton has been measured at 7000 gauss. The seventhharmonic of the Be9 frequency was heterodyned with the fundamental of the-28-(V. RADIO-FREQUENCY SPECTROSCOPY)proton frequency in order to obtain higher precision in measuring thefrequency ratio. Both resonances were obtained from a single sample con-sisting of an aqueous solution of beryllium fluoride. The sample containerwas a cylindrical section 1/2 inches long and 1/2 inches in diameter. Thecoil for the 4.2-megacycle- (Be9 resonant frequency) bridge was wound aroundthe cylinder, and the coil for the 30-megacycle- (proton resonant frequency)bridge was wound at right angles over the first. This method assures thatthe external magnetic field seen by the two types of nuclei is the same.A total of ten determinations of the frequency ratio was made. Thefollowing result was obtained:O(Be9) = 0.1405187 t 0.000002u(H )All of the ten values fall well within the above limits which weredetermined entirely by the uncertainty in locating the exact centers ofthe resonance curves. W. Dickinson, T. Wimett4. The Nuclear Resonance of Li6A search was conducted for the nuclear magnetic resonance of Li6 ina Li 6 enriched solution of Li 2S04 obtained from the Atomic Energy Commis-sion. The resonance was found after adding a small amount of paramagneticion to shorten the thermal relaxation time of the nuclei. Work is nowin progress to make a determination of the resonance frequency ratio ofLi 7 to Li 6 . W. Dickinson5. Deuteron-Proton Moment RatioA measurement of the ratio of the deuteron resonance absorptionfrequency to that of the proton has been made at 6900 gauss in the iron-core magnet. The sample used contained both nuclei in a mixture of two-thirds heavy water and one-third tap water with two drops of glycerineadded to enhance the proton resonance. The two r-f coils were wound onthe single sample as was done in the Be 9 experiment. Frequencies employedwere 4.51 Mc for deuterons, 29.4 Mc for protons, and the ratio was calcu-lated from measurements of the deuteron frequency and the beat note obtainedby mixing the thirteenth harmonic of the deuteron frequency with the secondharmonic of the proton frequency. Two runs were made yielding thirteendeterminations from which the following result was obtained:d = 0.15350585 ± 0.00000085p-29-(V. RADIO-FREQUENCY SPECTROSCOPY)The thirteen determinations fall within the given limits of error.This value is in agreement with a recent measurement by Bloch andcoworkers. It does not agree, however, with two other similar measurements,one by Bitter, Alpert, Nagle and Poss (16) and one by Siegbahn and Lind-strom (18), although these latter two agree with each other within limitsof error.Work is in progress to further improve the precision of this measure-ment by means of Helmholtz coils constructed for the purpose and tocarefully investigate the above discrepancies. T. Wimett6. Nuclear Magnetic Moment ValuesThe following is a list of all the nuclear magnetic moments which todate have been measured by the high precision nuclear induction and/ormagnetic resonance absorption methods. In cases where the value of amoment has been determined by more than one investigator the value listedand its uncertainty have been weighted according to standard statisticalmethods.The third column of the table lists the moments in units of themagnetic moment of the proton vp. The nuclear magneton unit was not usedfor two reasons. First, in order to retain the high precision in themeasured resonance frequency ratios, the value of the proton moment innuclear magnetons would have to be known to equally high precision. Atpresent this is not possible because of the relatively large probableerrors associated with the physical constants used in conversion. Second,the nuclear magneton is essentially an artificial unit of magnetic moment,having no such physical significance as the Bohr magneton in the electroniccase.The uncertainty in each value in column 3 is given by the two sub-script figures following the value. They indicate the uncertainty in thelast two significant figures; thus 0.7630 70 = 0.7630 ± 0.0070.The fourth column gives literature references in order of decreasingprecision of the results obtained.The fifth column lists the diamagnetic correction as can best becalculated using the Hartree atom model plus a direct calculation usingthe electronic wave function in the case of hydrogen. H' is the smallinduced magnetic field seen by the nucleus in addition to the externallyapplied field H. In order to keep experimental values separate from theo-retical calculations the diamagnetic correction has not been applied to themoments in column 3. It can easily be done by multiplying by the factor:-30-~__()1( ~___1 1  ^ ~ 1_11 _ _I~(V. RADIO-FREQUENCY SPECTROSCOPY)S- (H'/H)potonH NucleusTABLE-31-(V. RADIO-FREQUENCY SPECTROSCOPY)C. PARAMAGNETIC RELAXATIONProf. A. F. KipR. D. Arnold1. Resonance AbsorptionA doctoral thesis entitled "Paramagnetic Resonance Absorption inTwo Cupric Salts" has been written and submitted.2. RelaxationOur first attempt to observe spin-spin relaxation by noting theabsorption of electromagnetic energy in a paramagnetic salt, with and with-out an applied d-c magnetic field, produced an effect too small forquantitative measurement.The presence of the sample salt in the resonant cavity in which theabsorption takes place changed the resonant frequency of the cavity from480 Mc to 350 Mc. A new, higher-frequency cavity has been constructed,the design similar to that described in the previous progress report butwith smaller dimensions.It may be necessary to go to lower temperatures (the first run was atroom temperature) to obtain adequate sensitivity.3. Low-Temperature ResonanceSome new experiments are being planned in conjunction with the low-temperature group. We plan to investigate resonances in a number of para-magnetic salts at liquid He2 temperatures. Low temperatures will greatlydecrease spin-lattice relaxation broadening and may also show up otherchanges in absorption spectra. We hope to investigate both copper sulfateand cesium titanous alum single crystals.D. MICROWAVE SPECTROSCOPYProf. M. W. P. Strandberg J. G. IngersollProf. G. G. Harvey H. R. JohnsonProf. A. G. Hill G. W. KingJ. R. Eshbach R. B. LawranceB. V. Gokhale C. C. LoomisR. E. Hillger M. T. Weiss1. Germane MonochlorideWork has been discontinued on this molecule, since work elsewhere on-32-i _~( ^~I____~__L~I IX~_L1IIIC_ __I~~I_ _ ~ LIW~~^- ~--~-. 11LI -iLX~.-. -~- .I~iY --lll^~..-~-dIIIY*- ~WILLLL~-LIY IL i)i-~I~Ltil~~yl_(V. RADIO-FREQUENCY SPECTROSCOPY)the same molecule is being successfully carried out. Work on telluriumin an appropriate molecule will probably replace this project.R. E. Hillger2. KeteneWork on ketene, H2C2 0 has proceeded well since the last progress report.The three strong lines of the rotation transition J = 1 to J = 2, occurringat .75 cm wavelength, have been found and their frequencies measured. Withthe previosly measured transition J = 0 to J = 1 occurring at 1.5 cm, we havebeen able to obtain accurate values for two of the reciprocal principal mo-ments of inertia b and c. Then, using two rather inaccurate methods, weobtained a rough estimate of the value of the deviation 5, (5 = (b-c)/(a-c))from a symmetric rotor; this appears to be roughly .001 - .005. Satellitelines, possibly from vibration interaction, were not found on the 1-2 transi-tion, probably because of unfavorable statistics and the use of the lesssensitive K/2 spectroscope on straight absorption.We plan to try to obtain the structure of ketene completely by measur-ing the 0-1 and 1-2 transitions of deutero-ketene. Furthermore we hope togain enough information to interpret the satellite lines of the 0-1 lineby investigating the 2-3 line with the K/2 spectroscope, and by investiga-ting the relative intensities of the components of the 0-1 transition atroom and dry ice temperatures. Finally, we may look for some of the AK = 0,AJ = 0 transitions of high J rotational levels.H. R. Johnson3. FormaldehydeSeveral AJ = 0 lines of ordinary formaldehyde H2012016 have beennewly observed in the comparatively low-frequency microwave region 8,800to 17,000 Mc. Stark effect measurements have been made on appropriatelines with J equal to 2,7,8, and 16. It is expected that within theimmediate future a total of fifteen or more lines will have been measured;twelve are already known from our work and that of Bragg and Sharbaugh (29).For each K_1 value several lines of different J have been measured, thusmaking it possible to det'ermine quite well the form and magnitude ofcorrections for centrifugal distortion and zero point vibration. Thisshould, when completed, provide a useful check on current semi-rigid-rotortheory.In order to determine the molecular structure accurately and to checkthe correction theory, the isotopic molecule using C13 will be used.(V. RADIO-FREQUENCY SPECTROSCOPY)The chemistry of this appears well in hand and work with the isotopicmolecule will start as soon as the measurements on ordinary formaldehydeare complete.A paper "Microwave Spectrum of Formaldehyde" was presented before thePhysical Society's anniversary meeting in Cambridge, June 18, 1949.R. B. Lawrance4. OxygenThe fine structure of oxygen at low pressures, the order of a milli-meter of mercury, has not been detected as yet. The program is continuingwith the Zeemann detection system cleaned up. So many variables, e.g.,pressure, Zeemann field strength, and modulation breadth, must be investi-gated that the progress will be slow until the first line is resolved andthe system optimized by means of it.5. ApparatusWork continues on a slow sweep system and the system to work athigher than room temperatures.References(1) F. Bitter, Phys. Rev. 75, 1326 (1949).(2) S. Millman and P. Kusch, Phys. Rev. 58, 438 (1940).(3) P. Kusch and S. Millman, Phys. Rev. 56, 527 (1939).(4) J. E. Rosenthal and G. Breit, Phys. Rev. 41, 459 (1932).(5) H. Kopfermann and H. Kruger, Zeits. f. Physik 103, 48J (1936).(6) H. Kopfermann, Kernmomente, Akad. Verlagsgesellschaft, M.B.H.p. 17, (1940)(7) A. Bohr, Phys. Rev. 73, 1109 (1948).(8) See, for example, Brookhaven National Laboratory publication 1-5,by H. H. Goldsmith and D. R. In§lis, on "Spins, Magnetic Moments,and Electric Quadrupole Moments , dated October 1, 1948.(9) R. V. Pound, Phys. Rev. 73, 1112 (1948).(10) H. Poss, Phys. Rev. 75, 600 (1949).(11) F. Bloch, D. Nicodemus, and H. H. Staub, Phys. Rev. 74, 1025 (1948).(12) W. R. Arnold and A. Roberts, Phys. Rev. 71, 878 (1947).(13) H. A. Thomas, R. L. Driscoll and J. A. Hipple, Phys. Rev. 75, 902 (1949).(14) H. Taub and P. Kusch, Phys. Rev. 75, 1481 (1949).(15) T. F. Wimett, R. L. E. Progress Report p. 29, July 15th, 1949.(16) F. Bitter, N. Alpert, D. Nagle and H. Poss, Phys. Rev. 72, 1271 (1947).(17) F. Bloch, E. Levinthal and M. Packard, Phys. Rev. 72, 1125 (1947).-34-_~_ _ _I^_.___ ~I~^_( ___~ll_ I_ 1 ~_ 111(__1_~1_1111__^  - ~ ~ --~_~-llii I^l n-ll(-LI__II__ 1-. _ - _-._~11.1_- _ --- -_. --.--1- -r~- __. _._ -~-Lirm----~I- IPII(V. RADIO-FREQUENCY SPECTROSCOPY)(18) Kai Siegbahn and Gunnar Lindstr6m, Arkiv F6r Fysik, Band 1 Nr 6,p 193.(19) A. Roberts, Phys. Rev. 72, 979 (1947).(20) F. Bloch, A. C. Graves, M. E. Packard and R. W. Spence, Phys. Rev. 71,551 (1947).(21) H. L. Anderson and A. Novick, Phys. Rev. 71, 372 (1947).(22) H. L. Anderson and A. Novick, Phys. Rev. 73, 919 (1948).(23) J. R. Zimmerman and D. Williams, Bull. Amer. Phys. Soc. p 7,April 28, 1949.(24) W. C. Dickinson and T. F. Wimett, Phys. Rev. 75, 1769 (1949).(25) R. V. Pound, Phys. Rev. 73, 523 (1948).(26) R. V. Pound, Phys. Rev. 72, 1273 (1947).(27) W. Chambers and D. Williams, Bull. Amer. Phys. Soc. p 16,June 16, 1949.(28) W. G. Proctor, Phys. Rev. 75, 522 (1949).(29) J. K. Bragg and A. H. Sharbaugh, Phys. Rev. 75, 1774 (1949).(30) W. Dickinson, Nuclear Moment of La 1 3 9 , in manuscript.-35-

Radio-Frequency Spectroscopy

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Radio-Frequency Spectroscopy

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