bei48482_FM

Photon

Positron

• Electron

+

Nucleus

Figure 2.25In the process of pair production, a photon of sufficient energy materializes into an elec-

tron and a positron.

Bubble-chamber photograph of electron-positron pair formation. A magnetic field perpendicular to

the page caused the electron and positron to move in opposite curved paths, which are spirals be-

cause the particles lost energy as they moved through the chamber. In a bubble chamber, a liquid

(here, hydrogen) is heated above its normal boiling point under a pressure great enough to keep it

liquid. The pressure is then released, and bubbles form around any ions present in the resulting un-

stable superheated liquid. A charged particle moving through the liquid at this time leaves a track of

bubbles that can be photographed.

No conservation principles are violated when an electron-positron pair is created

near an atomic nucleus (Fig. 2.25). The sum of the charges of the electron (q e)

and of the positron (qe) is zero, as is the charge of the photon; the total energy,

including rest energy, of the electron and positron equals the photon energy; and lin-

ear momentum is conserved with the help of the nucleus, which carries away enough

photon momentum for the process to occur. Because of its relatively enormous mass,

the nucleus absorbs only a negligible fraction of the photon energy. (Energy and lin-

ear momentum could not both be conserved if pair production were to occur in empty

space, so it does not occur there.)

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