I S D V S T R I d L A S D ELI'GIlVEERIA*G CHE-INSTKY
March. 1 9 2 i
339
Some Technical Uses of X-Rays' By Ancel St. John 505
FIFTHAvB., SEWYORK,X . I-.
RECENT article in THISJOURSAL~ has called attenVisible light results from agitation of the electrons formirig tion to certain aspects of the current use of x-rays the outer portion of the atom; x-rays from the inner portion. for the solution of industrial problems. The uses The agitation of the inner electrons is usually produced by therein mentioned by no means exhaust the possibilities. the impact of free electrons moving with speed enough to These may in fact be said to be legion. Many of them have penetrate to the interior of the atom. The x-rays produced already been reduced to actualities, many others are nom in in this manner have wave lengths peculiar to the atom and process of reduction, and the keen-minded ex6cutive who is are called the characteristic x-rays of the element. I n wondering if x-rays 1iai.e anything to offer toward the solution addition, the stoppage of the free electrons also sets up a of his problems is constantly presenting more Not all these series of waves, known as the general or independent x-rays, problems are adapted to treatment by x-ray methods, the of various wave lengths exceeding- a minimum determined by the speed of the impinging t r e a t m e n t of others may electrons. turn out t o be unprofitable, I n this country the x-rays but there will always be A picture is presented of the nature and origin of used for technical purposes enough for which the new x-rays, ways they can be used for technical purposes, are usually produced in the tool is suited to warrant and some jobs which have been handled successfully. Coolidge type of x-ray tube, serious consideration of its Since the absorption of x-rays in material traversed which is a highly evacuated, use wherever important independs upon numbers and kinds of atoms present, thin-walled glass bulb conformation cannot be disradioscopic examination discloses information containing a spiral filament and closed readily by less novel cerning the distribution of the material as a whole. a heavy metal block or tarmethods. I n o r d e r t h a t The advantages and limitations of fluoroscopic and of get, usually of tungsten or those whose interest and photographic inspection are indicated. molybdenum. The supply curiosity as to the possible Since the scattering of x-rays by materials depends of electrons is obtained by application of this tool to upon the arrangement of the atoms, diffraction analyheating the filament as in a their own problems may sis yields information concerning the way the atoms radio tube, and the speed is have a clearer vision, it has are arranged in the ultimate particles of the material, imparted by impressing the seemed appropriate for one the size of those particles, and their arrangement in voltage of a high-tension m-ho has used i t for many the mass. Light is thus thrown on the fundamental transformer between the years and who has had some causes of the changes in properties resulting from filament and target. To share in the development of thermal, mechanical, or chemical treatment. prevent the flow of electrons current methods to set forth A few typical diffraction patterns are exhibited and in the wrong direction rea few facts as to the characsome important achievements of this method are quires the use of auxiliaries ter of x-rays, the way they enumerated. such as a rotating high-tenwork, what they have alsion switch or a rectifying re a d y accomplished. and t u b e u n l e s s the target is what they - may - be expected t o do, drawing the examples principally from his own ex- kept cool enough for the x-ray tube to act as its own rectifier, perience.
A
Theoretical Basis
The background of the picture is our present conception of the structure of atoms. According to this, a n atom consists of electrically charged particles. A positive center called the iiucleus is surrounded by a number of negative particles called electrons. These are believed to move in orbits forming a sort of miniature solar system about the nucleus. The mass and charge of an electron are the same no matter with what atom it may be associated. The mass and charge of the nucleus and the number of the associated electrons are nearly proportional to the atomic weight, actually to the atomic number. I n addition to their normal motions about the nucleus, the electrons vibrate in other ways if the atom is sufficiently agitated, and in so doing may set up electromagnetic or "light" waves. The force between the nucleus and an electron decreases with increasing distance between them but increases with the charge on the nucleus. It may be shown that as this force increases the work required to agitate an electron and the frequency of the resulting vibration both increase, much as a short, stiff spring requires more energy to set it in motion and vibrates more rapid11 than one that ic long and flexible R e c c i x e d October 7 1026 K Ilriier T H I SJ O U R Y A L , 18 5;;
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Detection of X-Radiation
X-rays are only slightly visible to the human eye and produce very serious physiological consequences when they fall upon any part of the body frequently or for a long time. Hence some other method than direct vision must be used for detecting them. When they traverse a substance part of their energy is absorbed, part is scattered, and the rest passes through. Some of the absorbed energy may be given off again as longer x-rays, known as secondary x-rays, or sometimes as visible or fluorescent light. In some cases part of the absorbed energy may cause physical or chemical changes, such as ionizing the gas between the plates of a condenser or activating a photographic emulsion. The intensity of these effects depends upon the intensity of the x-rays producing them and may be used as a means of detecting and recording the radiation. The amount of the absorption and scattering depends upon the wave length of the rays, upon the number and arrangement of the electrons in the absorbing atoms, and upon the number of atoms of each kind encountered, but is practically independent of the way the atoms are arranged. I n other words, it depends somewhat upon the atomic weight of the target but principally upon the voltage acro-:. the tulle. and the conipo~ition,density, and thick-
of tlic &orher. lii general, for a giveii voltage thc ab~ ~ r p t i oincream ii with the atomic iveiglit CIS tlic absorber; x i i given absoriier it decreases dly with tlic rolt,age. \ V h m :in nppmpriate wlt.:qe is use( s possible tLJ determine the distribut,inn of abnonnnl mnterial, s i i c l i as sand or cavities iii stfwl castings, niciallic incliisiiiris in electrical insulators, ims
tlir equipiiiciit, iroiii one place to aiiotlicr. A similar outfit has been witliilrawii froin service i u one plare, taken doirn, lr:i~isportedseveral miles, set up again, and used i n rout.ine iiiqicction on c,miincrcial work within twaty-four Iiours. Routine Inspection of Materials
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