Abstract: The practice of making broadly inclusive surveys, from time to time, of the status of our knowledge of the fundamental constants of physics and chemistry may be said to have started with a famous paper by Raymond T. Birge, of Berkeley, published in Reviews of Modem Physics in 1929. To Professor Birge, also, is due the credit for being the first, as far as I know, to apply the method of least squares in order to determine most probable values of three of the constants; e, the electronic charge m, the electron rest mass; and h, Planck's constant, using a highly overdetermined set of experimental data on functions of these three quantities. The fundamental constants of nature are so interrelated that a measurement affecting one affects them all. The author became interested when Millikan's oil‐drop value of the electron charge was different from the value given by x‐ray determination of crystal spacings. To assist in finding the true values, he invented a method for plotting various functions of the constants in a space of as many coordinates as there are constants. If all measurements are consistent, the plotted functions intersect in a point. When they do not intersect, one examines standard deviations, which correspond to thicknesses of surfaces, in an effort to find out what is wrong. In three decades, searches of this kind have reduced uncertainties in the constants from a fraction of a percent to, at most, tens of parts per million.

Publication: Physics Today Vol.: 18 No.: 10 ISSN: 0031-9228

ID: CaltechAUTHORS:20140621-114904849

]]>

Abstract: In session I 1 of the Berkeley meeting of December 30, 1960, S. D. Softky and R. K. Squire proposed a test for dispersive properties of space for electromagnetic radiation by detonating a nuclear explosive at a distance of 10^6 miles from the earth and noting the arrival times of different types of radiation at detectors above the atmosphere. The purpose of this note is to point out that Softky and Squire have overlooked the fact that a test for the dispersive properties they postulate already exists, covering perhaps not quite as extensive a range of the electromagnetic spectrum as they hope to cover (they claim a factor of 10^11) but nevertheless sufficient to render any such effect extremely unlikely over a range of frequencies of a factor of 5 X 10^9. I refer to a measurement performed in 1950 by means of the bent quartz crystal diffraction spectrometer [1] of the wavelength of the annihilation radiation generated in a block of copper by positrons from the nuclide 64Cu.

Publication: Proceedings of the National Academy of Sciences of the United States of America Vol.: 47 No.: 3 ISSN: 0027-8424

ID: CaltechAUTHORS:DUMpnas61

]]>

Abstract: Origin of the "X-Unit" and the Scale of X-Ray Wavelengths. - The wavelengths of X-ray emission lines measured relative to each other by the high precision methods of crystal diffraction are mostly known with a precision of 1 part in 10^(4) to 10^(5). By refined and very careful measurements with the 2-crystal spectrometer, an imprecision approaching and in some cases perhaps even smaller than a part in a million can be obtained.

Publication: Proceedings of the National Academy of Sciences of the United States of America Vol.: 45 No.: 7 ISSN: 0027-8424

ID: CaltechAUTHORS:DUMpnas59

]]>

Abstract: Book review of: Fundamental Constants of Physics. E. R. COHEN, K. M. CROWE, AND J. W. M. DuMOND. Pp. 287+ix. Interscience Monographs in Physics and Astronomy, Volume I. Interscience Publishers, Inc., New York, 1957.

Publication: American Journal of Physics Vol.: 27 No.: 1 ISSN: 0002-9505

ID: CaltechAUTHORS:20140624-081843163

]]>

Abstract: With the exception of certain early work with flat crystals by Rutherford & Andrade (1) and by Frilley (2), to date, crystal diffraction γ-ray spectroscopy has been accomplished almost entirely by means of transmission-type spectrometers using bent quartz crystal focusing. In this type of instrument, whose first conception was published in 1930 by DuMond & Kirkpatrlck (3), a thin cylindrical lamina of quartz of uniform thickness is bent so that its neutral plane, initially a right circular cylinder of radius 2R, assumes the form of a right circular cylinder of radius R. (By the "neutral plane" is meant that plane through the center of the thickness of the crystalline lamina which undergoes neither compression nor elongation when bending occurs.) The lamina is cut from the original unstressed crystal lattice so that the planes to be used in the selective reflection are made to converge after bending in such fashion that, if prolonged, they would all intersect in a common line distant 2R from the center of the lamina. Figure 1 illustrates the lamina, seen on edge, before and after bending, with lines indicating the direction of the atomic planes. These planes, after bending, converge in a line represented in projection by the point β. Confining ourselves to two dimensions, the normal projection of the neutral plane and the point β define the "focal circle," which is the locus of focal points R_ λ for radiations of different wavelength selectively reflected by the planes of the crystal in accord with Bragg’s law, nλ = 2d sin θ. In Figure 1 the thickness and the aperture angle, α, are greatly exaggerated. Cauchois has pointed out that the compression and dilation of the lattice constant, incident to bending, result in front-to-back focusing as well as side-to-side focusing, within the confines of Hooke’s law, so that the crystal behaves as though all selective reflection occurred on the neutral plane. It can also be easily shown that for selective reflection in transmission by planes normal to the lamina, the correction to Bragg’s law for crystalline refractive index diltering from unity vanishes exactly. This is explained in more detail in Reference 6, p. 241.

Publication: Annual Review of Nuclear Science Vol.: 8ISSN: 0066-4243

ID: CaltechAUTHORS:20140702-140059190

]]>

Abstract: It has been shown by one of the authors and his coworkers that it is possible to determine accurately the wavelengths of gamma rays with quantum energies up to 1.3 Mev by direct crystal diffraction methods using the Mark I 2-meter bent crystal spectrograph at the California Institute of Technology. They were able to record and to measure with a precision of one part in 10^3 the two gamma rays at 1.17 and 1.33 Mev which are emitted by a Co^(60) source.

Publication: Review of Scientific Instruments Vol.: 29 No.: 12 ISSN: 0034-6748

ID: CaltechAUTHORS:20140621-121044295

]]>

Abstract: Three years ago Dr. E. R. Cohen and I prepared and published our latest (1955) least-squares adjustment of all the most reliable data then available bearing on the universal constants of physics and chemistry. Since then new data and information have been accumulating so that a year or two from now the time may perhaps be propitious for us to prepare a new adjustment taking the newly-gained knowledge into account. At present it is too early to attempt such a re-evaluation since many of the investigations and re-determinations now under way are still far from completed. I shall be obliged, therefore, to content myself in this talk with a description of the sources of information upon which our 1955 evaluation was based, mentioning however, the weak points where these are now either well established as errors or at least considered to be under strong suspicion of systematic error. I shall also tell you a little of some of the new re-evaluations now under way.

Publication: IRE Transactions on Instrumentation Vol.: I-7 No.: 3 ISSN: 0096-2260

ID: CaltechAUTHORS:20170821-172631501

]]>

Abstract: Book review of: The Fundamental Constants of Physics. Vol. 1 of Interscience Monographs in Physics & Astronomy. By E. Richard Cohen, Kenneth M. Crowe, Jesse W. M. DuMond. 287 pp. Interscience Publishers, Inc., New York, 1957. Reviewed by H. Mendlowitz, National Bureau of Standards.

Publication: Physics Today Vol.: 11 No.: 9 ISSN: 0031-9228

ID: CaltechAUTHORS:20140624-100332618

]]>

Abstract: Nuclear levels in Tm169 excited by electron capture of Yb169, and levels in Lu175 excited by both beta decay of Yb175 and electron capture of Hf175 have been studied by using the curved-crystal gamma-ray spectrometer and the ring-focusing beta-ray spectrometer, as well as a semicircular beta-ray spectrometer for low energies. From the precision energies and the multipolarity determinations, the levels in Tm169 have the following energies in kev, and spin and parity assignments: A (ground state) (½+), B 8.42 (3/2+), C 118.20 (5/2+), D 138.95 (7/2+), E 316.19 (7/2+), F 379.31 (7/2-), G 472.91 (9/2-). Levels A, B, C, and D are members of a rotational band whose characteristic constants are given. Levels E and F are interpreted as particle excitations and level G as a rotational level based on the state F. The Lu175 excited states have the following energies in kev, spins, and parities: A (ground state) (7/2+), B 113.81 (9/2+), C 251.46 (11/2+), D 343.40 (5/2+), E 396.31 (9/2-), F 432.76 (7/2+), G 504.7 (1/2+). A, B, and C form a rotational band for which the characteristic constants are given. Some features of the levels and transition probabilities are discussed and compared with the unified model. A brief survey of second-order rotational energy constants and of intrinsic excitation levels is given.

Publication: Physical Review Vol.: 104 No.: 3 ISSN: 0031-899X

ID: CaltechAUTHORS:HATpr56

]]>

Abstract: A study has been made of the gamma radiation following the decay of Ir192. The energies, intensities, internal conversion coefficients and multipolarities of the gamma transitions have been determined. Energy level schemes for the daughter nuclei, Pt192 and Os192, are proposed. The spins and parities of most of the levels are given.

Publication: Physical Review Vol.: 100 No.: 5 ISSN: 0031-899X

ID: CaltechAUTHORS:BAGpr55

]]>

Abstract: This paper reviews the theory of low angle x‐ray diffraction as applied in the long wavelength region for the determination of particle sizes and shapes and other structural features in the submicroscopic size range and emphasizes the advantages to be gained by employing the longer x‐ray wavelengths (8 to 25 A). It also is intended to serve as an introduction to a description of an entirely new instrumental technique developed for this long wavelength range utilizing a diffraction apparatus consisting of a special gas‐filled x‐ray tube and a total reflection camera in which the primary radiation is simultaneously monochromatized and made to converge to a point focus. The sample for study is placed in the converging part of the primary beam and the diffraction patterns are formed around the point focus on a photographic film.

Publication: Journal of Applied Physics Vol.: 26 No.: 7 ISSN: 0021-8979

ID: CaltechAUTHORS:20140621-121757299

]]>

Abstract: The beta - decays of Ta182 and Ta183 into excited states of W182 and W183 have been studied using a curved crystal gamma-ray diffraction spectrometer and a homogeneous field, ring focusing beta-ray spectrometer. In each case de-excitation of the daughter nucleus gives rise to complex gamma-ray and conversion electron spectra. Energies and relative intensities of gamma rays and conversion lines arising from 27 transitions in W182 and 29 transitions in W183 are presented. Internal conversion coefficients and multipolarities have been deduced for most of the transitions and together with the gamma-ray energies form the basis of decay schemes proposed for both W182 and W183. The two decays are reported together because of the close experimental relationship which existed between them as a consequence of the method used for their production, namely, simultaneous production of Ta182 by single neutron capture and Ta183 by double neutron capture from stable Ta181. A corollary result is the value 1.3 x 10^4 barns for the thermal neutron cross section of Ta182. An interpretation of these results on W182 in terms of collective rotational motion has been given by A. Bohr and collaborators [Kgl. Danske Videnscab. Selskab, Mat.-fys. Medd. 29, No. 9 (1955)].

Publication: Physical Review Vol.: 97 No.: 4 ISSN: 0031-899X

ID: CaltechAUTHORS:MURpr55

]]>

Abstract: A method is described here for cutting and bending a single crystal for a point-focusing monochromator which does not produce a nondevelopable surface.

Publication: Review of Scientific Instruments Vol.: 25 No.: 12 ISSN: 0034-6748

ID: CaltechAUTHORS:20140621-162436551

]]>

Abstract: [No abstract].

Publication: Physical Review Vol.: 95 No.: 3 ISSN: 0031-899X

ID: CaltechAUTHORS:BOEpr54b

]]>

Abstract: Book review of: An Introduction to Scientific Research. E. BRIGHT WILSON, JR. Pp. 375+xiii, Figs. 53, McGraw-Hill Book Company, Inc., New York, 1952.

Publication: American Journal of Physics Vol.: 21 No.: 5 ISSN: 0002-9505

ID: CaltechAUTHORS:20140621-163604939

]]>

Abstract: The direct measurement of gamma‐rays only yields about half the picture in the study of nuclear energy levels. The other, and indeed to most physicists more familiar, half concerns the β‐rays, including in this term both the continuous β‐ray spectrum and the line spectrum by “conversion”, either internal or external. Ever since 1948, therefore, we have been much occupied with the design and construction along rather novel lines of a high precision helical focusing magnetic β‐ray spectrometer planned as a companion instrument to the crystal diffraction spectrometer as regards absolute precision and accuracy. We have only very recently completed this instrument and made the first tests on it which indicate that it will meet all our expectations both as to high absolute accuracy and high luminosity and sensitivity to weak sources. The beta‐ray spectrometer discussed in the following pages has recently been completed at the California Institute of Technology. It is shown in cross section on the cover.

Publication: Physics Today Vol.: 5 No.: 12 ISSN: 0031-9228

ID: CaltechAUTHORS:20140621-164718597

]]>

Abstract: For nearly six years there has been in existence at the California Institute of Technology a program of research work directed at increasing the precision of all measurements in nuclear spectroscopy and especially the precision with which nuclear energy levels may be determined. This trend toward higher precision of measurement is to be observed at present in nearly every area of nuclear physics (except possibly in the still new very high energy field) for it is generally realized that, while much primary qualitative information has been acquired, a real understanding and interpretation of the problems of nuclear structure and the internal mechanics of nuclei must wait for a sufficient fund of accurate and reliable quantitative numerical data to furnish a foundation for theory to build upon.

Publication: Physics Today Vol.: 5 No.: 11 ISSN: 0031-9228

ID: CaltechAUTHORS:20140621-164025966

]]>

Abstract: A point focusing x‐ray monochromator was designed and constructed for low angle scattering studies. The anastigmatic point focus is achieved by means of two cylindrically bent quartz crystals whose focal circles are mutually perpendicular. The beam, emanating from the copper target of an x‐ray tube, is reflected in succession, first from the crystal defining the horizontal focal circle, and second from the crystal defining the vertical focal circle following which it comes to a monochromatic point focus of wavelength 1.537A (CuKα_1). The sample to be studied is placed between the second crystal and the point focus, and the scattered beam is detected by means of a photographic film placed at the point focus, at right angles to the undeviated beam, the latter being suppressed by means of an absorber, or allowed to pass through a hole in the film. Mathematical analysis, in which a ray was traced through the two‐crystal system, revealed correctly the shape and size of the point focus, and the possibility of reducing the latter in size by stopping down the beam emerging from the target.

Publication: Journal of Applied Physics Vol.: 23 No.: 8 ISSN: 0021-8979

ID: CaltechAUTHORS:20140621-115706810

]]>

Abstract: We describe the results of experiments we have made, using the point focusing monochromator as the primary tool, to determine the particle size of latex spheres. The suitability of the instrument for this particular study is described and the experimental data obtained are tabulated, these data coming from our experiments with three physically distinct samples of Dow latex. We attempt to make a critical interpretation of the data by considering separately several possible space arrangements which the latex spheres might assume when the water, which is initially the suspending fluid, is evaporated. Corrections for the finite size and shape of the "point" focus are described. The absence of a significant difference in the mean particle sizes of the three samples considered is established. The data from all three samples is combined to yield a mean particle diameter under an external pressure of one atmosphere of 2687.5A with a statistical standard deviation of 1.2A and a fixed (systematic) error estimated to be not more than ±7A.

Publication: Journal of Applied Physics Vol.: 23 No.: 8 ISSN: 0021-8979

ID: CaltechAUTHORS:20140621-120410463

]]>

Abstract: As part of the work of the National Research Council Committee on Constants and Conversion Factors of Physics and at the request of its chairman, E. U. Condon, we have prepared a report,(1) dated December, 1950, giving a complete re-evaluation of the atomic constants in the light of the wealth of important new and very accurate data obtained since our previous study of January, 1948.

Publication: Physical Review Vol.: 82 No.: 4 ISSN: 0031-899X

ID: CaltechAUTHORS:DUMpr51

]]>

Abstract: Book review of: Crystals and X-Rays. KATHLEEN LONSDALE. Pp. 199, D. Van Nostrand Company, Inc., New York, 1949.

Publication: American Journal of Physics Vol.: 18 No.: 5 ISSN: 0002-9505

ID: CaltechAUTHORS:20140621-163056209

]]>

Abstract: Guinier has applied the principle of the reflection type curved crystal focusing x-ray monochromator to improve both intensity and angular resolution in low angle x-ray diffraction studies. In Guinier's method, a thin sample is placed in the convergent monochromatic x-ray beam and the low angle diffraction pattern is formed as a diffuse distribution symmetrically disposed on either side of the central primary line focused by the crystal.

Publication: Review of Scientific Instruments Vol.: 21 No.: 2 ISSN: 0034-6748

ID: CaltechAUTHORS:20140621-161721666

]]>

Abstract: Recent improvements in the two-meter focusing curved-crystal gamma-ray spectrometer are described which have extended its quantum energy range well above 1 Mev and have also yielded much better luminosity and resolving power than were obtained initially. The improved components are (1) the crystal holder whose aperture and resolving power have been nearly doubled and (2) the collimator the new model of which can now discriminate between the reflected and transmitted beams when these differ in direction by only 8 minutes of arc, a threefold improvement over our first model. Our plans for further possible improvements and the factors governing these are also discussed. Wavelengths of two gamma-rays emitted following β-decay of Co60 have been measured with this new equipment using a source of about 50 mc strength and found to have values of (9.308±0.005)×10-11 cm and (10.580±0.005)×10-11 cm corresponding to quantum energies of 1.3316±0.0010 Mev and 1.1715±0.0010 Mev, respectively. The lines appear to have equal intensities. The integrated reflection coefficient of the (310) planes of the curved-quartz crystal still appears to follow a λ2-dependence on wave-length down to 9 x.u. the shortest so far observed.

Publication: Physical Review Vol.: 76 No.: 12 ISSN: 0031-899X

ID: CaltechAUTHORS:LINpr49

]]>

Abstract: In a β‐ray spectrometer with axial homogeneous magnetic field, it is shown that optimum energy resolution and luminosity are obtained when the trajectories make an angle close to 45° with the field and that an annular resolving slit should be provided at a determined radial and axial location relative to the source. The combined effect of three independent sources of instrumental energy line width is analyzed for the optimum condition. Formulas are given for the optimum dimensions, the energy resolution and the luminosity.

Publication: Review of Scientific Instruments Vol.: 20 No.: 3 ISSN: 0034-6748

ID: CaltechAUTHORS:20140624-105800234

]]>

Abstract: A recent paper(1) describes a precision focusing curved crystal x-ray and gamma-ray spectrometer (2-meter focal length) constructed at the California Institute of Technology. This instrument has now been used to study the 0.41-Mev gamma-ray line from a 1-curie source of the artificial radioactive isotope of gold, Au198, of half-life 2.7 days.

Publication: Physical Review Vol.: 73 No.: 11 ISSN: 0031-899X

ID: CaltechAUTHORS:DUMpr48

]]>

Abstract: The present paper aims to deal only with a limited portion of the subject, the evaluation of the so-called atomic constants: e the electronic charge, m the electronic mass, and h Planck's constant of action together with certain auxiliary constants intimately associated with them. A number of useful physical constants which can be computed from the above data will also be evaluated.

Publication: Reviews of Modern Physics Vol.: 20 No.: 1 ISSN: 0034-6861

ID: CaltechAUTHORS:20140731-184306534

]]>

Abstract: Description is given of a transmission‐type, curved‐crystal focusing spectrometer for short wave‐length x‐rays, and gamma‐rays having a dispersion of 1.186 x.u. per mm at short wave‐lengths. The spectrometer utilizes the (310) planes of quartz in a crystalline plate of dimensions 80×70×1.0 mm curved cylindrically to a radius of two meters. High luminosity is obtained since the useful aperture in the crystal holder has an area of 10 cm2 and subtends 0.00025 stereradians at the focus. It also affords high resolution since by photographic tests with x‐rays the curved plate has been shown to focus a specified x‐ray wave‐length to within 0.06 mm of the same position on the focal circle for all parts of its useful aperture and over the entire operating wave‐length range. The geometry of the mechanism permits absolute measurements with a precision screw of the sine of the Bragg angle on both sides of the reflecting planes, affording a wave‐length range which includes at longest wave‐lengths the K‐spectrum of silver and goes down to zero wave‐lengths. For short wave‐length gamma‐rays the source is placed at the focus. A multiple‐slit collimator of tapering die‐cast lead partitions spaced apart with tapering separators, is used at short wave‐lengths to transmit the monochromatic diffracted beam and absorb the directly transmitted and scattered heterogeneous beam. The present collimator limits the spectrum that can be studied to a shortest wave‐length of 7. x.u. corresponding to 1.75 Mev. The intensity of the diffracted beam is to be measured with a special multi‐cellular G. M. counting tube of high efficiency, provided with a number of thin lead partitions through which the beam passes successively. In photographic spectra made with this instrument the tungsten and also the silverKβ_1β_3 doublet is completely and clearly resolved. Reproductions of such photographic x‐ray spectra are shown in which the line breadths have substantially the natural breadth. Fluorescence spectra of silver have been made in 10‐minute exposures. A companion paper gives the all‐important precision technique of generating the curved cylindrical stainless steel clamping blocks for the crystal.

Publication: Review of Scientific Instruments Vol.: 18 No.: 9 ISSN: 0034-6748

ID: CaltechAUTHORS:20140624-103455929

]]>

Abstract: A method is here described for generating circular cylindrical surfaces of large radius of curvature on blocks of steel or other material with a close approach to optical precision utilizing an ordinary machine shop surface grinder. Convex and concave surfaces about 3×5 inches in dimensions with radii of curvature of 79 inches (two meters) have been ground by this method both on cast iron and on stainless‐steel blocks with a precision of about 0.0002 inches as regards surface imperfections. A very moderate amount of subsequent lapping sufficed to give surfaces of optical precision. The surfaces are used for clamping lamina of crystalline quartz for use in a curved crystal focusing gamma ray spectrometer. A companion paper describes the gamma ray spectrometer.

Publication: Review of Scientific Instruments Vol.: 18 No.: 9 ISSN: 0034-6748

ID: CaltechAUTHORS:20140624-101434316

]]>

Abstract: Experiments to ascertain the wave forms and laws of propagation and dissipation of ballistic shock waves to large distances (80 yards) from the bullet trajectory are described. Calibers 0.30, 0.50, 20 mm, and 40 mm were studied. In every case an N‐shaped wave profile was observed consisting of a sudden rise in pressure, the “head discontinuity,” followed by an approximately linear decline to a pressure about equally far below atmospheric and then a second sudden return, the “tail discontinuity,” to atmospheric pressure. The peak amplitudes of this disturbance are found to diminish about as the inverse 3/4 power of the miss‐distance (perpendicular distance from the trajectory) while the period T′ (measured between the discontinuous fronts) increases about as the 1/4 power of the miss‐distance for calibers 0.30, 0.50, and 20 mm. For 40‐mm shells the amplitude decays a little faster, about as the inverse 0.9 power of miss‐distance over the range studied. A theory taking account of the dissipation of the N‐wave energy into heat is developed to explain the observed behavior. A method of measuring absolute N‐wave amplitudes by observing the rate of change of period T′ with propagation is described. The theory leads to an absolute relationship at large distances between distance, amplitude, and period in which no arbitrary constants appear.

Publication: Journal of the Acoustical Society of America Vol.: 18 No.: 1 ISSN: 0001-4966

ID: CaltechAUTHORS:20140624-111542495

]]>

Abstract: The author has found great value in the use of solid models in freshman physics teaching to illustrate (1) the law of ideal gases, and (2) the van der Waals equation of state and the behaviors of vapors and liquids. Many freshman students have difficulty in visualizing the shape of the thermodynamic surfaces from, let us say, a blackboard sketch of a set of isotherms, and the better part of a precious lecture hour may be consumed in explaining, with many words, what would be almost obvious if one had a solid model in his hands. The author has found that more than half of a class of average intelligence, having learned that an ideal gas is described by the equation PV=NRT, will be sorely puzzled by the adiabatic equation for ideal gases, PVγ=const. It helps these puzzled students surprisingly little to be told that, since the first equation contains three variables, it is not inconsistent with the second. With many of them, however, the matter becomes clear when they are shown an ideal gas surface on which have been plotted isotherms, adiabatics, and so forth.

Publication: American Journal of Physics Vol.: 9 No.: 4 ISSN: 0002-9505

ID: CaltechAUTHORS:20140624-125116421

]]>

Abstract: Gold and copper were simultaneously deposited in vacuum by vaporization on a plate of glass, the copper being deposited at a uniform rate while the gold was deposited in one hundred stratified layers in the copper by alternately raising and lowering the temperature of the molybdenum vaporizing trough containing the boiling gold. The translucent deposit so formed had a total thickness of about 10,000A and hence an average interplanar distance of 100A. In an especially constructed constructed x-ray spectrograph selective diffraction of Mo K radiation from these stratified films was observed corresponding to the artificially imposed periodicity of the stratification and the intensity of this diffracted image relative to the direct beam was found to fall off with time so as to indicate a "half-life" for the stratified structure of about two days. This suggests a general method for the study of average rates of diffusion and the determination of diffusion coefficients of solids in solids by utilizing the decay of such stratified films. Simple theoretical considerations should, through the action of diffusion alone, rapidly and automatically lose the higher Fourier harmonics of its periodic density distribution function and retain the fundamental in such a way as to render the determination of the diffusion coefficient quite accurate. The observed behavior of the diffracted maxima seem to support these expectations as does also the absence of any intensity in higher orders than the first. This purification by diffusion probably takes place principally during the depositing process itself while the temperatures are still quite high. Formulae are derived relating the observed rate of decay of the diffracted intensity, the artificial "grating coefficient" of the strata, and the diffusion coefficient. The method seems especially promising for substances and temperatures where diffusion is so slow as to be otherwise quite unobservable because the diffusion time varies as the square of the distance over which atoms must migrate and in this method these distances are so many orders of magnitude smaller than in any other.

Publication: Journal of Applied Physics Vol.: 11 No.: 5 ISSN: 0021-8979

ID: CaltechAUTHORS:20140621-165917384

]]>

Abstract: In the transmission type curved crystal focusing x-ray spectrograph, a crystalline lamina is imprisoned between the convex and concave surfaces of two rigid metal plates cut in the shape of arcs of right circular cylinders of appropriate radius to impose the correct curvature on the lamina. Recently I have sought a method of cutting such circular cylindrical arcs of very large radius of curvature (say 2 meters) without the delay and expense involved in the use of a large vertical boring mill of four meters diameter. The method which I describe here might also prove of use in the production of circular cylindrical arcs of metal for lapping cylindrical lenses.

Publication: Review of Scientific Instruments Vol.: 9 No.: 10 ISSN: 0034-6748

ID: CaltechAUTHORS:20140624-081334983

]]>

Abstract: During the summer of 1937 it was my pleasure and privilege to visit on many occasions the "Palace of Discovery" of the Paris Exposition. This vast, splendid and instructive exhibit of scientific principles and progress was essentially the idea of that "grand old man" of French science, Jean Perrin. The details were admirably worked out with the wholehearted cooperation of many of the most important French scientific workers who gave freely of their time and energy.

Publication: Journal of Applied Physics Vol.: 9 No.: 5 ISSN: 0021-8979

ID: CaltechAUTHORS:20140623-134516545

]]>

Abstract: This report describes in detail with photographs and cross‐sectional views an x-ray tube with a gyrating water‐cooled target which the authors have operated continuously for periods of hours at 287,000 volts and 105 milliamperes. The theoretical predictions of DuMond, Watson and Hicks as to the ratio of safe load in moving and fixed targets seems to be roughly fulfilled by these results. The computed power output in the form of x-rays alone is some 3/4 kilowatt. The report further describes in detail the power supply equipment consisting of a synchronous motor driven 40 kw motor generator set giving 150 cycle current both half-waves of which, after transformation to high voltage, are rectified by two thermionic valves (filament emission 2 amperes maximum) of our own design and construction here fully described. The powerful electrical filter system to reduce the ripple to a low value and some technical problems arising from the large power rating of the equipment as well as the facilities now almost completed for stabilizing and measuring precisely the high voltage supplied to the x‐ray tube are discussed. At the outset a brief indication of the program of research work projected for this set is given.

Publication: Review of Scientific Instruments Vol.: 8 No.: 8 ISSN: 0034-6748

ID: CaltechAUTHORS:20140624-075853098

]]>

Abstract: This article describes the design and construction of a two crystal x‐ray spectrometer for the study of radiation of wave‐length from (but not including) the vacuum region to about 100 X.U. Versatility coupled with precision of angular measurement is obtained by the use of worm gears, the spacing of whose teeth has been corrected by a relatively cheap but effective lapping method herein described.

Publication: Review of Scientific Instruments Vol.: 8 No.: 4 ISSN: 0034-6748

ID: CaltechAUTHORS:20140623-153755590

]]>

Abstract: The McLeod, Pirani, Knudsen and ionization gauges have been studied as to their suitability in the operation of large metal vacuum systems with organic vapor pumps now coming into extensive use in modern physics. Advantages and disadvantages of each type based on our experience are critically discussed. The superiority of a gauge on the Knudsen principle for this work is stressed and we present a new design and a simpler mode of operation of such a gauge together with a full discussion of its construction, calibration and characteristics.

Publication: Review of Scientific Instruments Vol.: 6 No.: 11 ISSN: 0034-6748

ID: CaltechAUTHORS:20140623-142056291

]]>

Abstract: The term "vacuum plumbing" is intended to express briefly the mass of technique which has recently been developed by which vacua from 10^(-4) to 10^(-6) mm Hg or better are obtained in systems mainly of metal and of large volume. The large high voltage x-ray tubes and ion accelerating tubes of C. C. Lauritsen at the California Institute of Technology, E. O. Lawrence at Berkeley and Merle Tuve at the Bureau of Terrestrial Magnetism are three examples of such systems.

Publication: Review of Scientific Instruments Vol.: 6 No.: 9 ISSN: 0034-6748

ID: CaltechAUTHORS:20140623-150018912

]]>

Abstract: We use the adjective "mobile" to describe a focal spot in an x‐ray tube which moves cyclically in a closed path relative to the target surface on which it is generated quite without reference to whether the spot be fixed or in motion with respect to the tube. Alex Müller and also A. Bouwers have made calculations of the thermal effects in such spots energized for very short fractions of one cycle of motion. This paper treats the case where the focal spot is energized continuously so that a "steady" state of thermal oscillation is set up. The solutions obtained refer only to conditions after this steady state of oscillation is reached. Only the ideal case of a spot of uniform intensity with sharp boundaries is treated. For simplicity the flow of heat from front target surface to cooling water is assumed one dimensional and normal to the front surface over an everywhere uniform thickness d. Curves are plotted showing the ratio of temperature rise in moving spots to that in fixed spots of the same size as a function of r the size of the spot relative to the length of path it describes and of θ a variable depending on the speed of rotation, the thickness d and the thermal constants of the target material. Curves of the ratio of permissible energy input for moving and fixed spots, respectively, as a function of the aforementioned variables are also shown. The paper gives a solution reduced to figures and curves of the heat flow equation for certain boundary conditions which to the author's knowledge has never before been obtained and therefore has some interest beyond its immediate application to x‐ray tubes. Certain mathematical difficulties of practical interest are also overcome in a way which may be helpful in other problems of similar nature. For the reader uninterested in mathematics the results are independently discussed in a separate section. Approximate methods are given for applying the results of the paper to targets consisting of two materials such as tungsten and copper.

Publication: Review of Scientific Instruments Vol.: 6 No.: 7 ISSN: 0034-6748

ID: CaltechAUTHORS:20140623-144531863

]]>

Abstract: The author's theory of the broadening of the Compton line as a Doppler effect of electron velocities is briefly reviewed and it is pointed out that only the component velocity along a direction which nearly bisects the angle between primary and scattered x-ray beams should be effective in broadening the line. Crystalline Ceylon graphite possesses properties which lend hope to the belief that a class of weakly bound or structure electrons in this crystal might have momenta restricted uniquely to parallelism with one plane in the crystal: the (0001) plane. A composite scatterer was built up out of blocks consisting of the crystal flakes all orientated with their (0001) planes in mutual parallelism and the blocks in turn were so orientated that the normals to these planes bisected the angle formed by the primary and scattered x-ray beams. If the electron momenta are orientated parallel to the plane of the graphite flakes one should expect the contribution of such electrons to the shifted scattering to give a sharp line or peak superposed on the broader structure caused by the remaining isotropically distributed momenta. Details of the experimental set-up are described and the spectrum obtained from scattering by the Ceylon graphite scatterer is compared with the spectrum from an isotropic Acheson graphite scatterer. The breadth and structure of the shifted line proves to be quite identical in the two cases and the conclusion is drawn that if a class of electrons having selectively orientated momenta exists in the crystal-line graphite it constitutes less than 5 percent of the total number. The bearing of this result on related questions is discussed.

Publication: Physical Review Vol.: 40 No.: 2 ISSN: 0031-899X

ID: CaltechAUTHORS:DUMpr32b

]]>

Abstract: According to a theory of the author first published in Phys. Rev. 33, 643 (1929), the considerable natural spectral breadth of the Compton modified line was ascribed to the initial momenta of electrons in the scattering body and as one consequence a mathematical expression was derived for the behavior of this breadth as a function of the primary wavelength and scattering angle (formula 23, page 657, above article).

Publication: Physical Review Vol.: 39 No.: 1 ISSN: 0031-899X

ID: CaltechAUTHORS:DUNpr32a

]]>

Abstract: The present report is a continuation of the work reported previously. The main purpose of the research is to test the correctness of the assumption that the initial velocities of electrons in the scattering body cause the observed breadth of the Compton shifted line. In the previous paper the natural breadths of the Compton line were observed for different angles of scattering and the breadth was found to obey the functional dependence on scattering angle predicted by DuMond on the assumption that electron velocities cause the breadth. According to DuMond's theory the breadth should also be nearly proportional to the primary wave-length. This point is tested in the present paper. Exposures of about one thousand hours each were made with characteristic K radiation from molybdenum, silver and tungsten target tubes. In each case the radiation was scattered from a graphite scatterer at a very well defined large scattering angle of 156°, the multicrystal spectrograph being used to analyze the scattered radiation. Reproductions of the photographic spectrograms are shown and also microphotometer curves of the spectra. The breadth of the Compton line is found to diminish with shorter primary wave-lengths in complete accord with the predictions of DuMond's theory. Unless some other cause can be found to explain the observed behavior of the breadth the results of this paper and the above mentioned previous paper constitute a complete vindication of this theory and of its basic assumption that the initial velocities of the electrons in the scatterer cause the breadth of the Compton shifted line. If this is correct, then these experiments constitute direct experimental evidence of the dynamic nature of atoms. The Compton shifted line can indeed be thought of as broadened by the Doppler effect of the motion of the scattering electrons. A simplified form of the theory of modified scattering by initially moving electrons is presented to supplement and clarify the more elaborate and exact theory of the previous paper. We call attention to the fact that the theory and experimental results are in no way discordant with wave mechanics or the uncertainty principle but we believe that to translate the present exposition into the language of wave mechanics would tend to obscure rather than clarify the picture. The results of this work are so clearly defined that the reality of the much narrower Compton lines obtained recently by several investigators seems, in our opinion, to be very doubtful.

Publication: Physical Review Vol.: 38 No.: 6 ISSN: 0031-899X

ID: CaltechAUTHORS:DUMpr31b

]]>

Abstract: Duane(1) has recently reported the observation of a new K-series line or band situated between the familiar gamma line and the critical limit of the K-series (position of the K absorption discontinuity.) This band is of great interest because in all probability it can be ascribed to transitions to the K level of electrons which in their initial state are dissociated from particular atoms and from a group resembling that predicted by Sommerfeld based on the Fermi statistics.

Publication: Physical Review Vol.: 38 No.: 4 ISSN: 0031-899X

ID: CaltechAUTHORS:DUMpr31a

]]>

Abstract: Three experimenters using the double crystal spectrometer have recently reported very much narrower structure for the Compton modified line than the structure observed photographically by DuMond and Kirkpatrick with the multicrystal spectrograph whose resolution was entirely adequate to reveal such a narrow structure. We have repeated the experiment using a double crystal spectrometer similar to theirs. The results obtained are in satisfactory agreement with the previous observations of Compton line breadth by DuMond and Kirkpatrick with the multicrystal spectrograph and in disagreement with the experimenters who used double crystal spectrometers. The conclusions drawn are: (1) The Compton modified line due to Mo Kα radiation scattered by graphite at angles of 165° ± 10° and examined with the double crystal spectrometer has a breadth of from 21 to 22 XU at half-maximum height; (2) Under the above conditions there is no appreciable separation of the Kα doublet in the shifted position; (3) No fine structure exists in the scattered spectrum of intensity greater than one-fifth the modified line intensity; (4) The ratio of modified to unmodified line intensity is estimated at (7.3:1); (5) There appears to be no essential difference between the results obtained with the single crystal, multicrystal and double crystal spectrometers in the study of this problem.

Publication: Physical Review Vol.: 37 No.: 11 ISSN: 0031-899X

ID: CaltechAUTHORS:HOYpr31

]]>

Abstract: The main purpose of this research was to test the correctness of the assumption that the initial velocities of electrons in the scattering body cause the observed breadth of the Compton shifted line. The test consists in observing the natural breadths of the Compton line for different scattering angles and primary wave-lengths and comparing these with the functional dependence of breadth on scattering angle and primary wave-length deduced theoretically on the basis of the assumption under test. It is shown in this paper that if electron velocities are the cause of the breadth then this breadth should increase with the scattering angle according to the approximate formula Δλ=Kcos1/2θ where Δλ is the breadth, θ the scattering angle and K a constant depending on the primary wave-length and the scattering substance. For the same scattering angle and substance the breadth should be proportional to the primary wave-length. The experimental test was made with the multicrystal spectrograph of fifty units herein briefly described. Three scattering angles were used, 63½°, 90°, 156°, the inhomogeneity in each case being less than one degree. The spurious breadth due to this inhomogeneity is negligible compared to the observed breadths. Three very clear cut spectrograms are reproduced representing MoK radiation scattered from graphite together with microphotometer curves taken from them. The increase of shifted line breadth with scattering angle is clearly visible and compares favorably with the theoretical prediction. The increase of line breadth with increasing primary wave-length comparing the breadths of shifted α1 and shifted β1 lines seems just detectable. The unshifted lines are very sharp, the α doublet being clearly resolved. Incidentally the cause of the heavy background so frequently observed on Compton effect spectrograms has been found to be non-selective scattering at the crystals and a great reduction of background and improvement in contrast has been effected by the use of baffles to diminish this effect. The observed shift of the Compton line supports Compton's formula δλ=(h/mc)(1-cosθ) where h/mc=24.2X.U. The shifted line breadths are greater than those reported by Gingrich and Bearden and possibly Ross but seem to be in general accord with previous breadths obtained at this laboratory and with those of H. M. Sharp and of F. L. Nutting. The reason for these discrepancies is unknown. The possibility of double or higher multiplicities of scattering is being investigated. (See note added in proof at the end of this article.)

Publication: Physical Review Vol.: 37 No.: 2 ISSN: 0031-899X

ID: CaltechAUTHORS:DUMpr31

]]>

Abstract: The double crystal x-ray spectrometer is distinctly different in principle from the single crystal spectrometer and calls therefore for a completely new design rather than an adaptation of single spectrometer design. One such new design is described in this paper. Emphasis is laid on the fact that the wave-length selected by the process of two successive crystal reflections in any prescribed orders depends solely on the dihedral angle between the crystals. The advantages of rotating both crystals in equal and opposite directions with respect to a plane through their axes of rotation are pointed out and a spectrometer designed to accomplish this is described. In this spectrometer the turning of a single shaft drives the two crystals, the spectrometer as a whole and the x-ray tube, at the proper angular rates about the proper centers to insure that the x-ray beam will at all times remain centered on the crystal faces and the window of the stationary ion chamber. It is pointed out that spurious fluctuations may be introduced in spectral curves by the x-ray beam migrating across small steps in the cleavage surface reflecting the radiation. Description of design covers design of spectrometer and turning mechanisms, the tube housing, lead shields, the detecting system consisting of an ion chamber with internal grid of special design to minimize natural leak and a Hoffmann vacuum electrometer connected to it through a short evacuated shield. Description of operating technique covers optical method of accurately orienting the crystal faces, methods of aligning the tube, precautions to eliminate background.

Publication: Physical Review Vol.: 36 No.: 12 ISSN: 0031-899X

ID: CaltechAUTHORS:DUMpr30d

]]>

Abstract: In the experimental study of the spectral distribution of x-radiation scattered by light elements it has always, up to the present, been assumed that multiple scattering could be neglected. Recent improvements in experimental technique however make it possible to suppose that multiple (especially double) scattering may now be detectable from large scattering bodies. Multiple scattering may (1) affect the breadth of the modified line, (2) change the structure of the modified line, (3) distort the background in such a way as to render measurements of shift unreliable. It is therefore valuable to analyze the effect of multiple scattering in case some of the recent mutually discordant experimental results may be explained and harmonized in this way. Assuming initially monochromatic radiation, scattering of any degree of multiplicity contributes a spectral band whose wave-length limits are here determined and discussed. The Breit, Compton, Jauncey formula taking polarization into account is adopted for the purposes of calculation of modified intensity. The results are thus fairly accurate for hard radiation scattered from very light elements. For softer radiation modified scattering at small scattering angles is greatly reduced below the values given by the Breit, Compton, Jauncey formula and unmodified scattering appears. The effect of this on the results of this paper is discussed in a qualitative way. The case of double scattering from a spherical scatterer is computed in complete detail and a formula for the ratio of double to single scattering is derived. Curves of the spectral distributions due to double scattering are shown. The dependence of the total doubly scattered intensity on the primary scattering angle is plotted. The natural width of the modified line is neglected throughout these calculations. Absorption in the scatterer is also neglected. The ratio of double to single scattering for a spherical scatterer observed under any given angle is proportional to the radius of the scatterer (neglecting absorption) and is given by Doubly scattered energy /Singly scattered energy =9/32σrR(θ) where σ is the linear scattering coefficient for the material of the scattering sphere, r the radius of the sphere, θ the angle under which single scattering occurs. R(θ) never differs greatly from 2.5. Triple scattering is negligible in comparison to single scattering. Twice modified doubly scattered radiation may contribute a faint asymmetric line or edge at the shifted position Δλ=2(h/mc)(1+cos1/2θ) For hard radiation when the Thompson formula (with Breit correction) applies with fair accuracy to modified scattering alone twice modified doubly scattered radiation contributes a spectral band of breadth 4(h/mc) cos1/2θ Once modified doubly scattered radiation may cause a slight broadening of the Compton line except in regions near θ=0 and θ=180°.

Publication: Physical Review Vol.: 36 No.: 12 ISSN: 0031-899X

ID: CaltechAUTHORS:DUMpr30e

]]>

Abstract: With a double crystal spectrometer of special design here briefly described the following questions were investigated with a view to determining the origin of the satellite Kα3 of copper. (1) Dependence of intensity of satellite Kα3 of copper on voltage with constant current. (2) Dependence of intensity of parent line Kα1 of copper on voltage with constant current. (3) Ratio of intensities of satellite to parent. (4) Dependence of satellite intensity on current at constant voltage. The conclusions are (1) that the satellite is excited at a voltage differing from the excitation voltage of the parent line by too small an amount to be measured with certainty (less than 200 volts). (2) That the ratio of intensities of α1 to α3 as estimated from the areas of the spectral line structures is about 1:120. (3) That the intensity of the satellite Kα3 of copper is strictly proportional to the current at constant voltage. These facts seem to invalidate the Wentzel-Druyvesteyn "spark line" hypothesis as an explanation of Kα3 of copper. Richtmyer's "double jump" hypothesis remains tenable. In addition a doublet structure (separation of components about 2 X.U.) in Kα3 of copper was observed (in accord with the doublet structure of this satellite in elements of lower atomic number).

Publication: Physical Review Vol.: 36 No.: 5 ISSN: 0031-899X

ID: CaltechAUTHORS:DUMpr30b

]]>

Abstract: Richtmyer(1) has recently proposed the hypothesis that many of the x-ray satellite lines may be due to double transitions in which two electron transitions cooperate to emit one quantum. I should like to call attention to a number of known facts about x-ray satellites which are very satisfactorily explained on this hypothesis several of which facts have not yet been mentioned by Richtmyer.

Publication: Physical Review Vol.: 36 No.: 5 ISSN: 0031-899X

ID: CaltechAUTHORS:DUMpr30c

]]>

Abstract: Professors Ross and Clark of Stanford, using the ingenious balanced filter method of Ross, have investigated the shifted line in the Compton effect for antimony, Kα1 and Kα2, lines scattered from beryllium. In this method the scattering angle is varied so as to vary the shift of the modified line.

Publication: Physical Review Vol.: 36 No.: 1 ISSN: 0031-899X

ID: CaltechAUTHORS:DUMpr30a

]]>

Abstract: The "Parallel Plate Comparator" is best adapted for measuring the separation of two parallel lines (eg. spectral lines) whose structures are practically identical and which are on the same negative and only a few millimeters apart. It consists of two plane parallel plates of glass attached to a mechanism such that they can be set at equal and opposite angles with the plane of the negative. The two equal and opposite angles between the plates and the negative must lie in a plane normal to the lines whose separation is to be measured. The lines are viewed through the inclined glass plates with a long focus microscope having a sufficiently large field to include both plates and both lines. The lines are brought into apparent coincidence in the field of the microscope by varying the obliquity of the glass plates to the line of sight. This obliquity is then a measure of the separation of the lines. If the lines are of identical structure and intensity, even though they are not symmetrical in structure, much greater precision is possible by bringing them into apparent coincidence than by attempting to set a crosshair first on one line and and then on the other. (The lines should be photographed or blocked off so that the upper half of one and the lower half of the other only is visible. The slightest fault in the apparent coincidence of the ends of the two half lines is then glaringly evident through the microscope. When coincidence is obtained, the two half lines give a good photometric match across their juncture and appear as one. In a properly designed instrument, the crack separating the two plane parallel glass plates is invisible because it is not in the focal plan of the objective.)

Publication: Review of Scientific Instruments Vol.: 1 No.: 2 ISSN: 0034-6748

ID: CaltechAUTHORS:20140623-140038596

]]>

Abstract: The need for improvements in scattered x-ray spectroscopic technique along the lines of increased intensity and contrast is discussed and a new instrument composed of fifty small units, each a Seeman Spectrograph in itself cooperating to form a single spectrogram, is described in detail. The technique of adjusting the instrument is also described.

Publication: Review of Scientific Instruments Vol.: 1 No.: 2 ISSN: 0034-6748

ID: CaltechAUTHORS:20140623-141533773

]]>

Abstract: A tube especially designed for the study of the Compton effect at large angles of scattering.—The structure of the Compton line obtained with this tube for a scattering angle of nearly 180° with a metallic beryllium scatterer is shown. Interpretation of the structure of the Compton line.—The diffuse structure of the Compton line is here attributed to a broadening caused by the velocity distribution of the scattering electrons in the solid scatterer analagous to a Doppler broadening and a relation between line structure and velocity distribution is derived. The observed line structure from the beryllium scatterer is compared with theoretical structures computed on several alternative assumptions as to electron velocity distribution. It is assumed: —1. That electrons in the solid scattering substance have the velocity distribution required by a wave-mechanical atom model for a free atom of that substance far removed from neighbors. 2. That electrons may be divided into two classes, one class the metallic or conductive electrons in the state of a degenerate electron gas subject to the Pauli Exclusion Principle and having the velocity distribution derived by Sommerfeld, and the other class as in the 1st assumption unperturbed by the neighboring atoms. 3. That electrons may be divided into two classes as before but that those forming an electron gas have the classical velocity distribution required by the Maxwell-Boltzmann equipartition law. 4. That electrons have the velocity distribution required by the older Bohr-Sommerfeld atom model with point electrons executing Kepler orbits and as in the first assumption unperturbed by neighboring atoms. The distribution of electron velocities in metals.—The results strongly contradict the classical distribution of electron velocities in solid bodies predicted by the rigid interpretation of the Maxwell-Boltzmann equipartition principle. They are also in contradiction with the older Bohr-Sommerfeld atom model. The results are in accord with the wave-mechanical atom model and constitute favorable evidence for the Sommerfeld distribution of metallic electron velocities and for the degenerate gas state.

Publication: Physical Review Vol.: 33 No.: 5 ISSN: 0031-899X

ID: CaltechAUTHORS:DUMpr29a

]]>

Abstract: According to Wentzel's(1) wave mechanical theory of modified x-ray scattering and also according to Jauncey's (2) classical-quantum theory of the same effect (though not explicitly so stated by Jauncey), the modified scattered x-radiation is regarded as due to many Smekal transitions (3) in each of which the initial state of the electron is a discrete negative energy level and the final state is one of the continuum of positive energy levels. In both of these theories the shifted "line" is predicted as a diffuse band, the diffuseness being due to the momenta of bound electrons in the dynamic atom model. Indeed, the natural breadth of the Compton modified line can be regarded as a Doppler broadening for x-rays scattered by moving electrons in much the same way as the temperature broadening is regarded in the case of optical spectra. The broadening of the Compton line is the most direct evidence for a dynamic atom model yet found. The structure of the line can be interpreted in such a way as to give the probability distribution of electron momenta in atoms. The breadth for circular orbits should be proportional to the effective atomic number of the scatterer.

Publication: Proceedings of the National Academy of Sciences of the United States of America Vol.: 14 No.: 11 ISSN: 0027-8424

ID: CaltechAUTHORS:DUMpnas28

]]>

Abstract: The need for a device to shorten numerical work with complex quantities is pointed out and a description is given of a slide rule in two dimensions devised to fill this need.

Publication: Journal of the A.I.E.E. Vol.: 44 No.: 2 ISSN: 0095-9804

ID: CaltechAUTHORS:20170906-162911641

]]>