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Particle Properties of Waves 71

target atoms after having been disturbed by the bombarding electrons. This phenom-

enon will be discussed in Sec. 7.9; the important thing to note at this point is the pres-

ence of x-rays of specific wavelengths, a decidedly nonclassical effect, in addition to a

continuous x-ray spectrum.

2 The x-rays produced at a given accelerating potential V vary in wavelength, but none

has a wavelength shorter than a certain value min. Increasing Vdecreases min. At a

particular V, minis the samefor both the tungsten and molybdenum targets. Duane

and Hunt found experimentally that minis inversely proportional to V; their precise

relationship is

X-ray production min Vm (2.12)

The second observation fits in with the quantum theory of radiation. Most of the

electrons that strike the target undergo numerous glancing collisions, with their energy

going simply into heat. (This is why the targets in x-ray tubes are made from high-

melting-point metals such as tungsten, and a means of cooling the target is usually em-

ployed.) A few electrons, though, lose most or all of their energy in single collisions

with target atoms. This is the energy that becomes x-rays.

X-rays production, then, except for the peaks mentioned in observation 1 above,

represents an inverse photoelectric effect. Instead of photon energy being transformed

into electron KE, electron KE is being transformed into photon energy. A short wave-

length means a high frequency, and a high frequency means a high photon energy h.

1.24 10 ^6



V

In a CT (computerized tomography) scanner, a series of x-ray exposures of a patient

taken from different directions are combined by a computer to give cross-sectional

images of the parts of the body being examined. In effect, the tissue is sliced up by the

computer on the basis of the x-ray exposures, and any desired slice can be displayed.

This technique enables an abnormality to be detected and its exact location established,

which might be impossible to do from an ordinary x-ray picture. (The word tomogra-

phy comes from tomos, Greek for “cut.”)

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