The three photon interaction processes trade dominance at characteristic energies…

PHYSICS · Multi-part · Fermi University Physics · Nuclear Physics · Radiation interactions, detection and dosimetry · Radiation interactions, detection and dosimetry

The Nucleus Resolved, Ch Radiation in matter, computed: Bethe, Klein-Nishina and what they leave out

Problem

The three photon interaction processes trade dominance at characteristic energies that scale differently with the absorber's atomic number

Z

: the photoelectric effect falls steeply with energy (roughly

Z^{4\text{ to }5}/E^{3.5}

), the Compton cross section per atom grows roughly as

Z

and falls slowly with energy, and pair production grows as

Z^2\ln E

above its threshold. In water the photoelectric/Compton crossover is at about

30\,\mathrm{keV}

and the Compton/pair crossover is at about

24\,\mathrm{MeV}

. In lead the corresponding crossovers are at about

0.5\,\mathrm{MeV}

and

5\,\mathrm{MeV}

. ((a)) State the dominant process for a

20\,\mathrm{keV}

photon in water. ((b)) State the dominant process for a

20\,\mathrm{keV}

photon in lead. ((c)) State the dominant process for a

6\,\mathrm{MeV}

photon in water. ((d)) State the dominant process for a

6\,\mathrm{MeV}

photon in lead. Justify each answer by comparing the photon energy with the crossover energies quoted for that absorber.

Related theory

Nuclear Physics

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