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What X-Rays Can Tell Us about AtomsAuthor(s): Samuel K. AllisonSource: The Scientific Monthly, Vol. 38, No. 6 (Jun., 1934), pp. 571-573Published by: American Association for the Advancement of ScienceStable URL: http://www.jstor.org/stable/15499 .

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SCIENCE SERVICE RADIO TALKS 571

WHAT X-RAYS CAN TELL US ABOUT ATOMS By Dr. SAMUEL K. ALLISON

DEPARTMENT OF PHYSICS, UNIVERSITY OF CHICAGO

X-RAYS were discovered in 1895 by the German physicist, Wilhelm R6ntgen, then working at the University of Wiirz- burg. The importance of the discovery to the medical profession was at once recognized. In a few weeks after Ront- gen's announcement the new rays had been used in the diagnosis of bone frac- tures and in the location of foreign bodies in wounds. To-day it is difficult to imagine surgery and dentistry being practised without the use of the information readily obtained from x-rays.

It is not of these applications to the medical profession, which are familiar to every one, that I wish to speak in this brief talk. I shall, however, try to give some idea of the use of x-rays as a tool in the investigation of the fundamental problems confronting the physicist and the chemist in their studies of the ulti- mate nature of matter. It is probably not popularly realized that the advances in the sister sciences, physics and chem- istry, achieved by the use of x-rays have been fully as great as the resultant improvements in medical and surgical practise.

I shall try to give a brief explanation of the extraordinary importance of x- rays as a scientific tool. In our studies of the structure of matter, we are inter- ested in atoms, and in the electrons which make up atoms. Atoms are extremely minute things, and we know definitely that they are so small -as to be forever beyond the possibility of direct vision, even when aided by the most powerful microscope. This statement is not a criticism of the skill of the instrument makers who construct microscopes; the fundamental reason for the invisibility of atoms lies much deeper than a ques- tion of instrument design. It lies in the fact that we see by means of light waves,

and the light waves to which our eyes re- spond with the sensation of sight have wave-lengths a thousand times greater than the diameters of the atoms which we wish to see.

I shall attempt to illustrate the diffi- culty,by means of an analogy. Let us imagine a town on the seacoast with a not too well-protected harbor into which ocean swells are advancing on. a calm day. As the waves strike piers, wharves or ships at anchor, reflected waves will be set up which interfere with the advancing waves and with each other, so that the surface of the water near these large bodies is crisscrossed by smaller waves and has especially high humps of Water where two wave-crests overlap, and especially deep troughs where one wave-trough coincides with another. The pattern produced on the water in such a case is called a diffraction pattern. Now it is conceivable that a physicist, seeking to enter the harbor in a fog which cut off vision of the shore, could locate the posi- tion of large objects by observing the diffraction pattern and thus studying the w,aves reflected from them. I must here remark, however, that in any actual case the services of a certified pilot would be infinitely preferable, because while the physicist was making the elaborate calculations necessary, the ship he was steer- ing would be likely to crash into the ob- ject he wished to avoid.

The point of the illustration is that, although the problem of locating large- sized objects in the harbor by means of the reflected waves is not insoluble, it would be quite impossible to locate the barnacles on a ship or the grains of sand on a beach by such a study. The wave- length, or crest-to-crest distance of the advancing waves, is far too large to reveal such minute details. In an analo-



572 TIHE SCIENTIFIC MONTHLY

gous, way an attempt to study the positions of atoms in solids, or the shapes of atoms, by means of visible light is bound to fail, due to the great disparity in the w7ave-length of the light and the dimen- sions of the atoms. Here is where the x-rays come to our aid. They are simi- lar in nature to ligh-t waves, but their wave-length is a thousand times shorter. In studying the diffraction patterns produced when x-ray waves iinpinge on matter, the physicist has ample time to make his calculations, since, in contrast to the situation in the preceding fanciful illustration, his life is not in danger, and he can actually locate the positions of the atoms, each of which reflects, or in a more technical language, scatters secon- dary x-ray waves.

As a result of this success, the advent of x-rays eventually caused an upheaval in the venerable science of crystallography. It is unfortunate that the non- technicali person las little or no contact with crystals in the sense that the scientist understands the word. In popular speech the word crystal suggests isome- thing transparent, especially a transparent jewel, but in its technical sense transparency is not at all an essential attribute of a crystal. To the scientist a large crystal is a solid which has been formed in such a way that it can exhibit its natural shape, which for all substances is one bounded by flat surfaces and edges. The materials with which we come in contact in everyday life, with the exception of glass, can usually be shown under the microscope to be ag- glomerations of enormous numbers of minute crystals, wedged together. Snow- flakes, if examined closely, exhibit the most beautiful regular patterns and are bounded by planes and, edges, consti- tuting examples of crystals.

Crystallography is the study of these objects and, due to their great regularity of shape, scientists have long suspected that the atoms of which they are made

are not arranged in haphazard fashion, but on the contrary are placed in some very regular array. By the study of the diffraction patterns produced when x- rays strike crystals, the expectations of the crystallographers have been con- firmed, and what was previously a guess has been reduced to tabulated numerical values of interatomic distances and de- sigins. Thus the positions of atoms in crystals of ordinary table salt is known with great precision. Chemists have long known that table salt contains only two kinds of atoms, those of the elements sodium and chlorine. From the use of x-rays wve know that these are arranged in rows and columns throughout space in such a way that in the three principal directions sodium and chlorine atoms al- ternate.

This is a very simple type of crystal, and others, much more complicated, have been unraveled. As the wvork goes on, scientists are learning more and more about the problem of chemical combina- tion; the relative positions of atoms in solids giving important clues as to the nature of the chemical forces which hold them together.

X-rays can be produced whose wave- length is short enough for the study of the positions of electrons inside atoms. Each electron scatters a wave which in- terferes with waves scattered by other electrons in the same atom, producing a diffraction pattern the study of which will reveal the electronic structure of the atom.

I shall now mention quite a different way in which we can get information about atoms from x-rays. In the present case, the study of the beam comilng from an x-ray tube tells us something about the atoms in the target of the tube. In the production of x-rays, high speed electrons strike against a metal block in the tube called the target. Professor A. H. Compton has given Ius the following instructive analogy. He compares the process to the impact of machine gun



SCIENCE SERVICE RADIO TALKS

bullets on: a steel wall. The impactinig bullets are the electrons, and the noise produced by thle imnpacts corresponds to the x-ravs. It is true that most of the x-rays produced in a tube have this noise- like or irreoular character, but a careful analysis of the radiation discloses that amuid the irreglularity there stand out a few simaple waves. It is as if the bullets in a fewr cases penetrated the steel wall ancd struck a musical oonog or bell, caus- ingo it to emit a pure tone, which is al- most but not quite drowned out byA the general din. These the physicist de- scribes as characteristic wave-lengths, superimposed on the general background.

It is found 'that these characteristic wave-lenogths depend on the kind of atomu of wlhich the target is built; copper, iron, aluminumu, atcd sxo forth, havino different ancd typical wave-lengaths'. These arise fromn vibrations of the electrons deeply embedded in the atomic structure, and from the wave-length we deduice the forces with whieh these electrons are bouncd.

WTheui these characteristic w ave- le.ngths have been determined for a large number of different elemuelnts, we find that the elements, arranged in the order of their wave-lenoths, fall into the samue order as they do when they a-re listed according to their atonmic weiohts. This reoularity; convinces us that the diectronic structure of atoms is not built in a randoim ma.nner, but proceeds ac- co,rdinog to a dcefinite and inflexible plan,

at. least as far as the inner regions are conzcerned.

These iniier regions in atomis like those of gold and lead we find to be exactly alike., the only differenee beinog that, in the ease of lead, the inner electrons are somnewhat more tightly bound than are those of gold. How then shall we ae- eonnt for the obvious differences inl properties between the two substances; the yellowness of gold and the grayness of lead, the melting of lead at a much lower teemperature than gold, and so, on?

These properties we ascribe. to the ar- rangement of the very outLermost elee- trons in the atomn, which do not, give us. information as to their positions tlhrough the medium of x-rays. It is these super- ficial electrons which control the properties of matter, with the exception of weight, with which we comiie into contact in everyday life. It is these which cause oxygen to support life, nitrogen to allow it to stifle, and chlorine to deliber- ately destroy it.

Thus the study of the characteristic x-rays, bas, enabled us to define regions of the atom toq which various properties are assigned. In their interior, from which the characteristic x-rays come, we detect a stabilized region which persists uinehanged as the atoin undergoes various chemieal colmbinations. I hope that in these few mnoments at my disposal I have succeeded in giving you a glimpse, at. least, of what x-rays can tell the scientist about atons.



Download - What X-Rays Can Tell Us about Atoms

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