Info

Log hv-Log hv-

(c) Global view

Log hv-Log hv-

Figure 10.6. Absorption of photons in matter. (a) Sketch of cross section vs. photon energy for photoelectric absorption by hydrogen, near the K edge at hv = 13.6eV. (b) A similar sketch for a heavier element. (c) Sketch showing global view of the several processes that can lead to photon absorption. The vertical scale is nominally linear, but the relative amplitudes of the several processes are not to scale. The abscissa is the photon energy in units of the rest mass of an electron, mec2 = 0.5 MeV. The Thomson/Compton scattering and pair-production cross sections are sufficiently small so these processes are generally not important in the interstellar medium. Thus x rays and gamma rays can usually traverse the Galaxy without significant absorption. [(c) is adapted in part from E. Fermi, etal., Nuclear Physics, University of Chicago Press, 1950, p. 50]

Figure 10.6. Absorption of photons in matter. (a) Sketch of cross section vs. photon energy for photoelectric absorption by hydrogen, near the K edge at hv = 13.6eV. (b) A similar sketch for a heavier element. (c) Sketch showing global view of the several processes that can lead to photon absorption. The vertical scale is nominally linear, but the relative amplitudes of the several processes are not to scale. The abscissa is the photon energy in units of the rest mass of an electron, mec2 = 0.5 MeV. The Thomson/Compton scattering and pair-production cross sections are sufficiently small so these processes are generally not important in the interstellar medium. Thus x rays and gamma rays can usually traverse the Galaxy without significant absorption. [(c) is adapted in part from E. Fermi, etal., Nuclear Physics, University of Chicago Press, 1950, p. 50]

10.5 Photoelectric absorption in the interstellar medium Table 10.2. Solar-system abundances'1

Element ( Z )

Abundance by number

Element (Z)

Abundance by number

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