bei48482_FM

are only loosely bound to their parent atoms. Other electrons, however, are very tightly

bound and when struck by a photon, the entire atom recoils instead of the single elec-

tron. In this event the value of mto use in Eq. (2.21) is that of the entire atom, which

is tens of thousands of times greater than that of an electron, and the resulting Comp-

ton shift is accordingly so small as to be undetectable.

2.8 PAIR PRODUCTION

Energy into matter

As we have seen, in a collision a photon can give an electron all of its energy (the pho-

toelectric effect) or only part (the Compton effect). It is also possible for a photon to

materialize into an electron and a positron, which is a positively charged electron. In

this process, called pair production,electromagnetic energy is converted into matter.

Particle Properties of Waves 79

Relative intensity

Wavelength

φ = 0°

Relative intensity

Wavelength

φ = 90°

∆λ

Relative intensity

Wavelength

φ = 45°

∆λ

Relative intensity

Wavelength

φ = 135°

∆λ

Source of Collimators

monochromatic

x-rays Path of

spectrometer

Scattered

x-ray

Unscattered

x-ray

φ

X-ray spectrometer

Figure 2.23Experimental demonstration of the Compton effect.

Figure 2.24Experimental confirmation of Compton scattering. The greater the scattering angle, the greater the wavelength

change, in accord with Eq. (2.21).

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