Arthur H. Compton

The Scattering of X Rays as Particles

The experimental evidence and the theoretical considerations that led to the discovery and interpretation of the modification of the wavelength of x rays as a result of scattering by electrons are reviewed, as is the controversy between Duane and the author that took place in 1923–24. The confirmatory evidence obtained by Bothe, Geiger, Simon, and Compton is summarized.

Recent Developments in Cosmic Rays

I N order to place the results of the recent studies of cosmic rays in appropriate perspective, let us recall very briefly their early history. It is well known how at the beginning of the present century C. T. R. Wilson,l and Elster and GeiteJ,2 showed that normal air is slightly ionized, and how McClennan and Burton3 at Toronto, and Rutherford and Cooke4 at Montreal showed by use of absorbing screens that a considerable part of this ionization is due to a penetrating radiation coming from outside the ionization chamber. This radiation was generally supposed to be the gamma-rays from radioactive materials known to occur in the ground and the air. About 1910, however, observations on high towers by WuIf6 and a series of balloon flights by Gockel6 showed that the intensity of these ionizing rays decreases less rapidly with altitude than could thus be explained. These striking results led to further balloon observations by Victor Hess7 • 8 and W. Kolh6rster,9 who established the fact that the penetrating radiation actually increases in intensity with increasing altitude, which, of course, it should not do if it emanted from the ground. From these studies Hess8 drew the bold conclusion in 1912 that a penetrating type of radiation enters our atmosphere from without, and .

A Measurement of the Polarization of Secondary X-Rays*

It was observed by Barkla that the intensity of X-rays scattered from the second radiator perpendicular to the theoretical plane of polarization was about 1/3 as great as the intensity of the radiation in the plane. Since then evidence has accumulated that secondary X-rays even from light elements are not of the same wave-length as the primary rays. If this difference in wave-length is due to fluorescent radiation, it should result in an incomplete polarization of the scattered rays. The object of this experiment was to test this point by measurement of the degree of polarization of the scattered rays.Heterogeneous X-rays of an average wave-length of about 0.25 A were scattered by paper, aluminium and sulphur. A geometric correction was made for the lack of complete polarization due to the solid angles subtended by the radiators, and the results were extrapolated to zero thickness of the radiators. The experiments indicate that, within a probable error of 1 or 2 per cent, the radiation is completely polarized. This precludes the possibility of there being any considerable amount of fluorescent radiation emitted from the radiator.

A PRECISION X‐RAY SPECTROMETER AND THE WAVE LENGTH OF Mo Kα1

An x‐ray spectrometer was designed with the first crystal mounted on an arm supported by the frame of the spectrometer, and with the second crystal mounted on the central table of the instrument, whose position is read from a precision circle. The ionization chamber (of 28 cc capacity, filled with krypton) is on an arm whose position is read by a second precision circle. The instrument was built by the Societe Genevoise de Physique.The first crystal was adjusted to throw the Kα1 line of molybdenum over the main axis of the spectrometer, and measurements were made with the second crystal in the (1, −1), (1, +1), (1, −4) and (1, +4) positions (Allisons notation). The reflection maxima from calcite (corrected to 18° C) occur at θ1=6°42′35.″5 and θ4=27°51′33.″0 with a probable error of 0.″25 due chiefly to errors in reading the circle. Using an apparent grating space for the first order of 3.02904A at 18° C, we get λ=707.830±.002 mA. Larsson obtained 707.831±.003 mA using Siegbahns photographic method. Comp...

An Improved Cosmic‐Ray Meter

An improved cosmic‐ray meter is described in detail. Directions for its use in the measurement of cosmic rays are given; and various errors and ways of minimizing them are discussed.

An attempt to analyse cosmic rays

The coincidence experiments of Bothe-Kolhorster, Rossi and others, considered in the light of various alternative interpretations, show the existence of penetrating, electrically charged particles which are either primary cosmic rays or are secondaries of primaries that are absorbed high in the atmosphere. The shower-producing radiation, which seems to consist of photons, must, in view of the marked latitude effect to which it is subject, be produced by some electrically charged primary rays, which are provisionally identified as electrons. The variation of cosmic-ray-intensity with latitude is shown to increase from about 16 per cent at sea-level to a ratio of 40-fold between 55° and 20° at very high altitudes. Extrapolation to the top of the atmosphere near the poles and the equator would indicate a ratio greater than 100: 1, which would imply that less than 1 per cent of the unfiltered primary cosmic rays are electrically neutral. In conjunction with the coincidence experiments, this means that the primary rays responsible for cosmic-ray effects are nearly all electrically charged particles. A method of analysing these electrically charged rays is described and applied to typical ionization data for different altitudes at various latitudes. The method consists in (1) calculating the effective minimum energy of various types of particles admitted through the earths field at different magnetic latitudes; (ii) determining the minimum ranges in air of the particles corresponding to these minimum energies; and (iii) analyzing the data relating the altitude with ionization and thus obtaining experimental range-minima that can be compared with the calculated values. The balloon experiments show two such range-minima, A and B, at higher and lower altitudes respectively. Their ranges progress with changing latitude approximately in the manner that the theory predicts. A third range-group C appears on analysis of the curve showing the latitude effect for sea-level. Using the best available information regarding magnetic latitudes and the relation between energy and range, these range-groups are identified respectively as alpha particles, electrons and protons. Comparison with directional experiments indicates that the electron group probably consists of about equal parts of positrons and negatrons. The possible errors involved in applying this analysis are of such magnitude as to leave the identification of the range-groups questionable. Comparison with other data is on the whole however confirmatory, except for the failure to find proton tracks in Wilson photographs of cosmic rays. For this a possible explanation is offered.

The Appearance of Atoms as Determined by X-Ray Scattering

Photographs are shown which are the images of atoms of helium, neon and argon as obtained by x-ray diffraction, magnified about 2×108 times. The images are obtained by photographing a rotating template whose shape is calculated by a mathematical transformation of our measured values of the x-rays scattered by the respective gases. This mathematical-mechanical procedure corresponds to the lens which forms the image when a microscope is used. The images formed by our procedure should be true representations of the electron distributions in the atom, except for the limited resolving power and certain minor aberrations. The photographs show the helium atom as a diffusely continuous region filled with electricity. In neon, the inner group of K electrons is clearly distinguishable from the L electron group. The resolving power is insufficient to distinguish the K and L groups of electrons in argon, but does separate these from the M electrons. The appearance of these atoms is in good accord with modern quantum theory of atomic structure.

A Poll of Scientists at Chicago, July 1945

In order to show how scientists felt about the employment and control of atomic energy prior to Hiroshima, the BULLETIN, Vol. 1, No. 10, published a report made to the War Department on June 11, 1945 by a Committee on Social and Political Implications, under the chairmanship of Dr. James Franck; a Memorandum on Atomic Bombs and the Postwar Position of the United States, prepared by Dr. Leo Szilard in March 1945, appeared in the BULLETIN, Vol. 3, No. 12. The poll taken at the Chicago Metallurgical Laboratory in July 1945, is here described by its sponsors for the first time; this account corrects a slightly inaccurate summary in the editorial that accompanied the Franck Report.

The Atomic Crusade and Its Social Implications

not overlooked the possibility of an atomic bomb. In fact, their progress in this direction was the spur that goaded us into its development. Their own wartime investigations had, however, convinced them that the construction of such a weapon could not actually be accomplished. In our own country the atomic bomb was one of the last major developments of war research to be undertaken. It was conceived as a practical undertaking only after other urgent war tasks had made heavy demands on the Nation’s major scientific talent. Thus it was the more remarkable that

Modern Physics and the Discovery of X-Rays

Among the most striking practical developments resulting from the discoveries of modern physics are radio and the atomic bomb. Both are fruits of the discovery of x-rays. In our effort to understand the nature of the world around us, prominent recent advances include knowledge of the arrangement of atoms in crystals and molecules, recognition of the several elemental particles of which atoms are built, the electron and the nucleus, the proton and the neutron, the positron, the mesotrons, and so on, and something of the way in which these particles combine to form atoms. We have learned in the theory of relativity the laws of motion of stars and atoms moving at very high speeds and more precise laws of gravitation. In the quantum theory we have greatly improved our understanding of the nature of light and x-rays and have learned how to describe the motions of atoms and the parts of atoms. All of these new findings stem from Rontgens discovery of x-rays fifty years ago, and in their development x-rays them...