Contains reports on four research projects

Aubrey, B. B.

Bradley, L. C., III

Melissinos, A. C.

Stroke, H. H.

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IV. NUCLEAR MAGNETIC RESONANCE AND HYPERFINE STRUCTUREProf. F. BitterProf. L. C. Bradley IIIProf. J. S. WaughDr. P. L. SagalynDr. H. H. StrokeB. B. AubreyM. ýiftanF. W. DobbsR. W. FessendenH. R. HirschR. J. HullR. H. KohlerR. F. LaceyJ. H. LoehlinI. G. McWilliamsA. C. MelissinosS. T. Wray, Jr.A. PARAMAGNETIC RESONANCE IN ATOMS ALIGNED BY ELECTRON IMPACTObservation of resonances between the magnetic sublevels of the metastable 63p2state in the even isotopes of mercury was previously reported by us (1). The apparatusused in that experiment has been rebuilt for the purpose of scanning the hyperfine-structure levels of the 63 P2 configuration of Hg 1 9 9 and Hg201 in order to determinethe polarization of these levels and to observe transitions (AF = 0; AmF = ± 1) in theselevels. Polarization and resonances have now been observed in the even isotopes,Hg 1 9 8 and Hg 2 0 2 ; the F = 5/2 and F = 3/2 levels of Hg ;9 9  and in the F = 7/2 and F = 5/2201levels of HgWe are using essentially the same apparatus that is described in reference 1 exceptthat the performance of the various components has been greatly improved, and theHg 9 8 lamp has been placed in a magnetic field for the purpose of scanning the hyperfine-structure levels of the odd isotopes.2Z0g.0NmZERO ý- 0POLARIZAT IONcoZEROPOLARIZATIONZEROPOL ARIZ AT IONz•LLOO t-Z Q-Mm0 W-I00.5 GAUSS MAGNETICFIELD0.5 GAUSS MAGNETICFIELDow---Fig. IV-1. Recorder tracesof polarization of X5461 Aabsorption by (a) 63p 2 state198of Hg atoms, and (b) F = 3/2level of 6 32 state of Hg 9 9atoms, plotted against magneticfield.(b)(IV. NUCLEAR MAGNETIC RESONANCE)Figure IV-1 shows typical resonance curves at 50 mc in the F = 3/2 (gF = 1.8) levelof Hg 1 9 9 and in Hg198 (gj = 1.48). The curves were observed with a radiofrequencypower of 0. 5 watt and a vapor pressure of mercury of approximately 2 microns. Thehalfwidths of these curves were approximately 0.2 gauss and were found to dependappreciably on the rf power that was used.By using an rf power of 0.03 watt (the smallest power with which resonances couldbe observed) and a vapor pressure of approximately 2 microns, resonance curves with202a halfwidth of 0. 1 gauss were obtained in Hg 0 2  This corresponds to a lifetime of1. 5 X 10 sec of the atoms in the 63 P2 state in our discharge tube.At present, we are studying the effects of the discharge-tube current and mercuryvapor pressure on the lifetime of the 63P2 state. We are also determining the gF values199of the F = 5/2 and F = 3/2 levels in Hg , and the F = 7/2 and F = 5/2 levels201in HgB. B. Aubrey, L. C. Bradley IIIReferences1. B. B. Aubrey and L. C. Bradley III, Quarterly Progress Report, Reseach Labora-tory of Electronics, M.I.T., Oct. 15, 1957, p. 22.B. HYPERFINE STRUCTURE OF RADIOACTIVE Hg97 AND Hg 1 9 71. Preparation of Radioactive Mercury SamplesRadioactive Hg 1 9 7 and Hg 9 7 * have been prepared by this group for several years.The technique is now fully developed, and fair amounts of pure radioactive mercury(with practically no contamination) are being produced.A cyclotron bombardment for 12 hours at 40 4amp produces approximately 10 milli-197 197* 14 13curies of Hg and approximately 3.5 millicuries of Hg (about 10 and 10 atoms,respectively). The production of Hg 1 9 7 has been verified by the gamma-ray spectrumof the prepared samples that is shown in Fig. IV-2. The peaks (A-E) are associatedwith the following transitions:A - Escape peak from the 77 kev lineB - 77 kev gold X-ray from the K-capture and the transition to the ground state of197Au , which is converted by 0. 76 + . 10C - 138 key from the E 2 transition in mercury, converted by 0. 68 ± .05D - 195 key transition in gold, converted by 0. 7E - 279 kev from the cascade in gold, converted by 0.4.197"The lack of any peak at 165 kev, corresponding to the M4 transition from Hg , whichis alarming at first sight, is justified because this line is known to be completely con-verted (1, 2).0.13C0.271100The gammaproduced byHg reaction.WAVELENGTH (MICRONS)spectrum of radioactive Hg 97 and Hgl 9 714-mev deuterons on gold by the Au(d, 2n)WAVELENGTH (MICRONS)4000 3000 2000 19000-MV-H WAVE NUMBERS (CM1 )15 mm G.E. QUARTZ AT 2.6 MICRONSTHE OH STRETCHING LINEMAY 22,1958WAVE NUMBERS (CM-1)Imm AMMERSIL QUARTZSQUARE TUBINGFig. IV-3. Infrared spectrometer curves of quartz samples.30,00025,00020,00015,00010,00050000Fig. IV-2.9 -4 W)l IP'__Au 197(IV. NUCLEAR MAGNETIC RESONANCE)The number under each peak gives its integrated intensity (total number of counts inthe peak). The assignments given above are also justified by the decay rates. As to thepurity of the samples, both spectroscopic and double-resonance data indicate the pres-ence of only very minute amounts of natural mercury.Another problem that was encountered in the preparation of the samples was the radi-ation damage to the cell. Whenever strong samples were prepared, a strong discolor-ation of the quartz appeared. Furthermore, when the cells were sealed at pressures of-410- 4 mm Hg, the vacuum deteriorated to 1-2 mm within a few hours. Very stronghydrogen lines were detected spectroscopically, and we suspect that hydrogen is knockedout of the quartz by beta and gamma radiation. Naturally, such pressures completelyquench the resonance radiation. We hope that this serious difficulty will be overcomeby use of quartz of the proper quality. As an example, the infrared spectra of samplesof General Electric Company and of Ammersil quartz are shown in Fig. IV-3. TheGeneral Electric quartz shows a very strong absorption band at 2.65p because of thepresence of OH.Thanks are due Professor R. C. Lord, Director of the Spectroscopy Laboratory,M.I.T., for the infrared spectra. All of the delicate glass blowing and handling ofradioactive material was done by Mr. A. J. Velluto, Research Laboratory of Electronics,M.I.T., whose ability has been an important factor in the successful preparation of oursamples.A. C. MelissinosReferences1. O. Huber, F. Humbel, H. Schneider, A. deShalit, and W. Ziinti, Helv. Phys. Acta24, 127 (1951).2. C. H. Braden, L. D. Wyly, and E. T. Patronis, Jr., Phys. Rev. 95, 758 (1954).2. Magnetic Dipole Moment and Isotope Shift of Radioactive Hgl 9 7Double-resonance signals in this isotope were observed for the first time a year agoand were reported in the Quarterly Progress Report of July 15, 1957, page 41. However,much stronger signals have recently been obtained with the use of a phase-sensitivedetector. The results are as follows.3 197For the magnetic dipole interaction constant of the 3P level of Hg from the-3 -1m = -1/2 - -3/2 transition, we obtain a = 513.0 0 2 X 10 cm ; from the-3 -1m = + 1/2 - + 3/2 transition, we obtain a = 513.8 ± 2 X 10 cm ; and we accept-3 -1a = 513.5 1 X 10 cmFor the zero-field location of the F = 3/2 level of Hgl 9 7 from the m = -1/2 scanning+7 -3 -1curve, we obtain 349. 8 X 10 cm ; from the m = + 1/2 scanning curve, we obtain-4SPLITTING FIELD20 mk -SCANNING FIELDHg197 F = 1 1 -HgPR FL m 2 2-+APRIL 18, 1958I I I I , I I , L475 500 525 550 575 600 625 650 675 700 750DIVISIONS SCANNING FIELDFig. IV-4. Superposition of double-resonanceconstruction of the F = 3/2, mF =signals to show the+1/2 scanning curve.TIC FIELD fp=8,860.9KCANT FREQUENCY 3053.2MCAPRIL 21, 1958_OCATION + 455 DIVI I I I I I I I650 600 550 500 450 400 350 300I I250 200 DIVISIONSFig. IV-5. Scanning curve for the F = 3/2, mF = -1/2 sublevel.7 GAUSS(IV. NUCLEAR MAGNETIC RESONANCE)AGNETIC FIELD fp = 10,151.6 KCESONANT FREQUENCY 3053.2 MCAPRIL 19, 1958 RUN.APRIL 21 RUN (NORMALIZED)LOCATION +607 DIV.I I I I I I 1 I750 700 650 600 550 500 450 400 DIVISIONSFig. IV-6. Scanning curve for the F = 3/2, mF = +1/2 sublevel.+4 -3 -1 -3 -1345. 27 X 10 cm ; and we accept 347. 5 ± 5 X 10 cm Hence the isotope shift197 198 -3 -1of the center of gravity of Hg from Hg is +90. 8 ± 5 x 10 cmFigure IV-4 is a superposition of double-resonance signals obtained for varioussettings of the scanning field. It shows both the quality of the double-resonance signalsand the scanning curve that is the envelope of these signals.Figures IV-5 and IV-6 give the scanning curve for the m = -1/2 and m = +1/2 sub-197levels of Hg 19 7  These measurements were performed with 0.3 millicurie of radio-12 197active mercury; that is, with only 1012 atoms (approximate) of Hg 1These measurements conclude the present work on the 3p level of Hg 1 9 7  How-ever, a direct microwave resonance experiment at 23, 085 mc between the F = 1/2 andF = 3/2 levels now seems possible if sufficient power can be obtained.A. C. Melissinos197 197"3. Spectroscopic Study of Radioactive Hg and HgIn a program designed for gaining further insight into the static properties of nuclei,we have intensified the study of the shift in the spectral lines of atoms which resultsfrom a change in the nuclear charge distribution of a single atom (1). The possibility ofperforming such experiments is offered by studying the spectra of certain atoms in thenuclear ground state and in a reasonably long-lived isomeric level.(IV. NUCLEAR MAGNETIC RESONANCE)Theoretical work on this subject was initiated by Weiner (2). His treatment is basedessentially on the single-particle model and predicts large values (compared withaccuracies that can be obtained spectroscopically) for odd proton nuclei and small valuesfor odd neutrons. For the latter he ascribes any measurable effects to the electron-neutron interaction. We expect (3), however, that the main contribution to the isomericshift with odd neutrons will come from the nucleon-core coupling. In fact, we can studythis interaction in a simpler situation than in the isotope shift, in whicn different numbersof nucleons are involved. Isotopes of xenon, as well as of mercury, are being studied.A discharge tube containing approximately 3 millicuries of 65-hour radioactive Hg 1 9 7was prepared (approximately 4 X 1013 atoms). From one-third to one-fifth of the atoms197*were in the 25-hour isomeric state (Hg , I = 13/2), as evidenced by the gamma-rayspectrum (see Section IV-B 1).The tube was filled with argon at relatively high pressure (approximately 10 mm Hg)and excited with a 200-mc oscillator. It performed well, but after each 15-minuteperiod of operation it had to be flamed out because the mercury had cleaned up (wasdriven into the walls). As the mercury was gradually driven into the walls molecularbands appeared with increasing intensity. No evidence of natural-mercury contami-nation could be found.The tube was studied in emission with a Fabry-P'rot interferometer crossed witha fast spectrograph. Two etalons were used (spacers, 2.993 mm and 4.33 mm),and the following lines were analyzed:X4047 (6 6p 3 P 0 - 6s 7s 3 S )X4078 (6s 6p P1 - 6s 7s iS 0 )S 3 3X 4358 6 s 6 p P 1 - 6s 7s s 1)Exposures from 20 to 30 minutes were used.Echelle pictures were also taken, and the following lines were analyzed:X2537 6s52 1 0 - 6s 6p 3 1 )X 4047X 4358The exposures varied in duration from 30 minutes to 1 hour.197In all lines, the Hg components appeared strong and well resolved. The hyper-197 3 3fine splittings of Hg in the P 1 (770 mk) and in the S1 (1135 mk) are exactly as cal-culated from the paramagnetic resonance in the P 1 level (see Section IV-B 2).196 lThe Hg component was present in all of the lines at the correct position. ThisX = 2537NATURALRADIOACTIVE-Hg19 8Hg C D 4078BA 1961 1-654 -423(a)H 197Hg345 644A 2X= 4078SPACER 2.993 MM 1 6/7/58196 P•I I-282 -281 252 496(b)197 197*Fig. IV-7. (a) Echelle spectrogram of X2537A of radioactive Hg , Hg , 24 hoursafter bombardment. (b) Fabry-P6rot spectrogram of X4078A of radioactive197 197*Hg , Hg , Z4 hours after bombardment.ISoF= 3/2F=1/2537IsoI I . I i1TX = 4047NATURALRADIOACTIVEHg19 8SPACER 2.993MMFig. IV-8.F = 1/2A B6/7/58Echelle and Fabry-P4rot spectrograms of X 4047 A of radioactive197 197*Hg , Hg , 24 hours after bombardment.F = /23 P- -~--C~-r- L.I - - --OIF _m • qHgiv rNATURALRADIOACTIVE-Hg 19 8Ao1 I84701"I234bi438UC196Pc0 123 302] )~SPACER 2.993MMFig. IV 9. Echelle and Fabry-P97 197*Fig. IV 9. Echelle and Fabry-Perot spectrograms of X4358A of radioactive Hg Hg 24 hours after bombardment.X= 4358F = 3/1-F= 1/2A BC )3s,3 p!F= 3/2- I1/ 1770LLd D629 8977"6/7/58LrI -- I L-(IV. NUCLEAR MAGNETIC RESONANCE)was expected because the bombarding energy is above the 11-mev threshold of theAu(d, 3n)Hg reaction, and Hg196 is produced with Hg197 Furthermore, several weakcomponents appeared in all of the lines. They do not belong to natural mercury, to196 197 197"Hg , or to Hg ; they are attributed to the isomeric state of Hg . All of thesecomponents vanished after three days when exposures of as long as 1 hour were taken,except for the +817-mk component in the 4047 line, which persisted but was extremelyweak.The complete analysis of the two etalon spectrograms, in conjunction with theechelle pictures, uncovered the structure of the four lines, as given in Figs. IV-7a,IV-7b, IV-8, and IV-9.197The energy-level assignments of Hg are also given in these figures. However,197"an attempt to make energy-level assignments for Hg did not seem feasible. Forexample, the intensities of the two components in X2537 A indicated a positive moment197"for Hg ; but the two unassigned components in X4047 indicated - by their position -a negative moment, unless an extremely large isomeric shift in the 3S1 level exists.Moreover, the energy-level scheme for the P 1 level cannot be constructed consistentlyfrom X4078 and X2537, both of which lead to it.Therefore we have decided to postpone further analysis of the energy-level scheme197"of Hg (and of the isomeric isotope shift) until more data are collected on the struc-ture of the lines.A. C. Melissinos, L. C. Bradley III, H. H. StrokeReferences1. F. Bitter, S. P. Davis, B. Richter, and J. E. R. Young, Optical studies of radio-active mercury, Phys. Rev. 96, 1531 (1954).2. R. Weiner, Nuovo cimento 10, 1587 (1956).3. L. Wilets, private communication to H. H. Stroke.C. INTENSITY OF ULTRAVIOLET RADIATION OF A MICROWAVE-EXCITEDMERCURY PLASMA IN A MAGNETIC FIELDIn the study of the hyperfine structure of the 2537 A mercury line, electrodelessdischarges in magnetic fields are used. It has been known that the intensity of theemitted ultraviolet radiation varies with the magnetic field; but the variation is notmonotonic, and consistently, the curve for an increasing field differs from the curve fora decreasing field.In this experiment (our data are shown in Fig. IV-10), the intensity was measuredevery 500 gauss up to 10, 000 gauss. "Steps" appear to occur at 2000 gauss and 6000gauss, while at 8000 gauss saturation seems to have been reached. The continuous curvet30zSn 252t· 20c10,000 8000 6000 4000 2000 0 2000 4000 6000 8000 0,000GAUSSFig. IV-10. Intensity of XZ537A from Hg 1 9 8 plasma excited at 3000 mcin a magnetic field.31SE2E2'a;21I IAMPERES0 200 400 600 830 1000 1200 1400 1600 1800 2000Fig. IV-11. Calibration of 4-inch Bitter solenoid.41(IV. NUCLEAR MAGNETIC RESONANCE)gives the results for increasing fields; the dotted curve, for decreasing fields. Notethat the sharp increase at 6000 gauss has a corresponding decrease on the dotted curveat 5000 gauss. The curve is drawn for both directions of the magnetic field in order toshow the reproducibility of the results.The source was an electrodeless discharge of Hg 1 9 8 with 1 mm of argon, excited by10-cm microwaves. Although the power delivered to the magnetron was constant, thepower delivered to the load was not monitored. We did, however, measure the temper-ature of the vycor envelope of the source in order to gain some information about thevapor pressure of the mercury. We found that the temperature of the external side ofthe envelope varied in exact proportion to the intensity variations that are shown inFig. IV-10.A. C. MelissinosD. VARIATIONS IN THE CALIBRATION OF HIGH-CURRENT SOLENOIDSSince high-current "Bitter" solenoids (1) are now in extensive use, it should bereported that their calibration - field versus current - is not constant. As we expected,it depends upon the clogging of the cooling ducts in the solenoid because temperature-distribution variations change the current distribution in the copper plates.In Fig. IV-11 calibration curves for the 4-inch solenoid used by our group are givenfor November 1953, March 1956, February 1957, and, after the cooling system hadbeen cleaned, for May 1958. It is also worth mentioning that the most homogeneousspot of the solenoid was shifted 1 inch between the last two calibrations.A. C. MelissinosReferences1. Magnet No. 4, described by F. Bitter, Rev. Sci. Instr. 10, 373 (1939).

Nuclear Magnetic Resonance and Hyperfine Structure

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Nuclear Magnetic Resonance and Hyperfine Structure

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