NBSIR 86-3469 ledinkal Activities 1986 Center for Basic Standatcls NBSIR 86-3469 liechnical Activities 1986 Center for Basic Standards Peter L. M. Heydemann U.S. DEPARTMENT OF COMMERCE National Bureau of Standards National Measurement Laboratory Center for Basic Standards Gaithersburg, MD 20899 October 1 986 U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary National Bureau of Standards, Ernest Ambler, Director :j. -. I I * 4*^ - * '^'^'i?4‘ ^'v -Iv.'; I y - ^ ' / . v#>! V. -’ ^ Vf v f i/ ^ c: i (' ,5: '<: ..'-;:/^r':f'-'-ii:..' ,u ' ’* ; -.j-vt' ' nA ' %: ' ‘. ;. 4 d»*"' ' ' > . : >V : / V. X i;, ' ' ’ ' - . ,r <5^- - ^'i’ - "i; • 'II ~'ifi~ i; T iHn mnin^iiwiiiii r unTmiinn ,.-.aiit:.-'i*JUWiii ' J(d^ *',' v'V.'-' o-n.' '-• '•'i.H<',(3 r-V-O.r-fi^ A .- " ' ',:, . I- 'ftt'- 'JjUtl . ',X- ' .....5’ >.. '-K • . •. ,' V ,. 'tf. < , , '•7 t! ,.. «•• :/: =v- !* ABSTRACT This report summarizes the research and technical activities of the Center for Basic Standards during the Fiscal Year 1986. These activities include work in the areas of electricity, temperature and pressure, mass and length, time and frequency, quantum metrology, and quantum physics. Keywords: Astrophysics; atomic and molecular physics; chemical physics; electrical standards; fundamental constants; gravity measurements; laser physics; length standards; mass standards; pressure and vacuum standards; temperature standards; time and frequency standards; X-ray and gamma-ray wavelength standards. 1 INTRODUCTION This report is a summary of the technical activities of the NBS Center for Basic Standards (CBS) for the period October 1, 1985 to September 30, 1986. The Center is one of the four centers and operating units in the National Measurement Laboratory. The summary of activities is organized in six sections, one for the technical activities of the Quantum Metrology Group, and one each for the five divisions of the Center. Each division or group tells its own story in its own way. In general, there is an overview followed by a series of short reports on current projects. Then the publications, invited, talks, committee participation and professional interactions during the year are listed. More information about particular work may be desired. To obtain this, the reader should address the individual scientists or their division, c/o Center for Basic Standards, B1 60 Physics Building, National Bureau of Standards, Gaithersburg, Maryland 20899. 11 TABLE OF CONTENTS Abstract i Introduction ii Quantum Metrology Group 1 Electricity Division 14 Temperature and Pressure Division 80 Length and Mass Division 146 Time and Frequency Division 157 Quantum Physics Division 207 iii V A' ' 4> : - {' ' '^W'^^T|frg}i " >. '; .i-v‘ "'! 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Two persistent themes remain: of more obvious relevance to CBS we have several fundamental measurement exercises, each of which has required serious improvement in metrological technique and all of which bear strongly on issues of basic scientific moment. The second, less metrologically demanding exercise, concerns the physics of inner-shell processes where need for clarification of fundamental mechanisms has led us to extensive involvement with synchrotron radiation facilities and the National Synchrotron Light Source (NSLS) at Brookhaven, in particular. Although practiced in the organizational locale of CBS, rationalization of this work needs to look toward molecular and condensed matter science and applications include even surface studies. For convenience and variety, this year's review is organized geographically, focussing in order on Gaithersburg, Grenoble and Brookhaven. (Although the West-German connection for high Z, few electron systems remains important, it was, aside from planning and analysis, dormant.) As described below, there have been notable successes at these three sites but none appears ripe for closing at this time. TECHNICAL ACTIVITIES Gaithersburg Activities : As mentioned in last year's panel book, our most serious and urgent problem has been the large discrepancy between our (1974-76) measurements of a Si lattice parameter and the subsequent (1979-82) results from PTE. Over the past three years, we established two major efforts to deal with this important question. Firstly, the entire x- ray/optical interferometer (XROI) experiment was re-constructed with major improvements including: use of a frequency-agile, offset-locked slave laser to drive the system through each x-ray interference profile studied; provisions of a unique and sensitive 4-beam path curvature interferometer for on-line trajectory characterization; full computer controlled automation; and, an advanced non-contacting, servoed vibration isolation system. Secondly, an entirely new lattice spacing comparator (the delta-d machine) was built so the question of sample- to-sample variability could be addressed with a precision well in excess of the targeted accuracy range. 1 r The re-constituted XROI experiment was working at the time of the last panel meeting but had not achieved definitive results or suggested a resolution of the NBS-PTB discrepancy. The A-d machine had been assembled but was not yet operational. It is gratifying to report major successes in both areas. In the case of the XROI experiment, through the on-line path monitoring capability, a serious systematic error was uncovered which arose from a combination of pitch error larger than indicated by x-ray fringe contrast and an Abbe' offset differing slightly from zero owing largely to x-ray beam non- uniformity. Our efforts to follow this trail began last winter and required installation of a considerably up-graded table suspension before reliable control could be obtained. Subsequent data runs in March, June and September showed good concordance (better than 0.1 ppm) and give numerical values which substantially remove the NBS-PTB discrepancy. Actually small differences remain but these are well within the range of generally expected sample-to-sample variability. It is just for this state of affairs that the A-d machine was designed. Following initial testing, the spectrometer was moved to a passive vibration isolated surface plate in the basement. The instrument was surrounded with a curtain-type enclosure and the crystals were thermally shielded. An electrical umbilical cord was installed from the spectrometer to the electronics rack. Two x-ray tubes are used to permit simultaneous recording of left and right nondispersive curves. Operation of both tubes with a single x-ray power supply and two constant potential units was achieved. Computer software to automatically control the spectrometer and permit round- the-clock comparisons of four crystal samples was developed. The spectrometer uses a long transfer crystal (=10 cm long x 1 cm high) on a rotating axis whose angular position is measured with a polarization sensitive Michelson interferometer. The crystals whose lattice spacings are being compared reside on a motorized slide and are automatically moved in and out of the x-ray beam. These crystals are approximately 1 cm x 1 cm and are equipped with piezo electric tippers which permit establishing parallelism of the planes of the transfer and test crystals. The long transfer crystal and four test crystals were oriented, cut, and etched from several silicon samples. Non-dispersive double crystal reflections from thin samples (=0.5 mm) of nearly equal thickness (<10 u) show narrow fine structure on the normal x-ray profile. This fine structure which is about 0.02 of the FWHM makes the comparison of profile positions much more accurate. Great care was exercised during the crystal preparation to insure that crystal thicknesses were within the required tolerances. First profiles were recorded in the spring and after some modifications to improve both performance and convenience profiles containing symmetric distinct fine structure were obtained. As of September (when this report is being written) it appears to live up to all of our expectations regarding sensitivity and stability. With the 2 clear delineation of expected hyperfine detail one easily sees lattice constant differences (Ad's) near 0.05 ppm with the expectation of accuracies at or below 0.01 ppm after more careful analysis. There are evidently two approaches to analyzing data, one depending on fitting model profiles, the other simply uses a shifting algorithm to compute the correlation integral, the location of whose maximum yields Ad in a simple, model-independent way. Theoretical profiles have been obtained from calculations using dynamical diffraction theory. At the present time (September) we have in hand samples of monocrystalline silicon from the PTB and from NRLM in Japan. Near the close of the month's running currently underway (assuming no unforseen problems), we plan