bei48482_FM

70 Chapter Two

Tungsten

target

0 0.02 0.04 0.06 0.08 0.10

Relative intensity

2

4

6

8

10

Wavelength, nm

50 kV

40 kV

30 kV

20 kV

Figure 2.16X-ray spectra of tungsten at various accelerating potentials.

In modern x-ray tubes like these,

circulating oil carries heat away

from the target and releases it to

the outside air through a heat

exchanger. The use of x-rays as a

diagnostic tool in medicine is

based upon the different extents

to which different tissues absorb

them. Because of its calcium con-

tent, bone is much more opaque

to x-rays than muscle, which in

turn is more opaque than fat. To

enhance contrast, “meals” that con-

tain barium are given to patients to

better display their digestive sys-

tems, and other compounds may

be injected into the bloodstream to

enable the condition of blood ves-

sels to be studied.

Wavelength, nm

Relative intensity

0 0.02 0.04 0.06 0.08 0.10

2

4

6

8

10

12

Tungsten, 35 kV

Molybdenum,

35 kV

Figure 2.17X-ray spectra of tungsten and molybdenum at 35 kV accelerating potential.

side of the tube. The tube is evacuated to permit the electrons to get to the target

unimpeded.

As mentioned earlier, classical electromagnetic theory predicts bremsstrahlung when

electrons are accelerated, which accounts in general for the x-rays produced by an x-ray

tube. However, the agreement between theory and experiment is not satisfactory in cer-

tain important respects. Figures 2.16 and 2.17 show the x-ray spectra that result when

tungsten and molybdenum targets are bombarded by electrons at several different accel-

erating potentials. The curves exhibit two features electromagnetic theory cannot explain:

1 In the case of molybdenum, intensity peaks occur that indicate the enhanced pro-

duction of x-rays at certain wavelengths. These peaks occur at specific wavelengths for

each target material and originate in rearrangements of the electron structures of the

bei48482_ch02.qxd 2/7/02 10:52 PM Page 70