Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Number of ions produced in air

  1. Jun 27, 2010 #1
    The average energy needed to produce an ion in air is 34eV, how many ions pair will be produced in air by a radiation of 1MeV, my teacher told me it is about 30,000 ions and electrons but can someone please prove it??:confused:
  2. jcsd
  3. Jun 27, 2010 #2
    Because there are a number of other processes also occurs like compton effect, pair production, so is it will be simple 10^6/34 or something else??
  4. Jun 27, 2010 #3
    The energy loss of charged particles in air is roughly 2 MeV per gram per cm2 (about 2500 eV per cm) at ~ 1 GeV, and increases at lower energies. See


    The Bethe-Bloch dE/dx energy loss equation, shown in Equation 27.3, describes the energy loss process. Using 2500 eV per cm, and 34 eV per ion, there are about 73 ions per cm at 1 GeV, increasing dramatically at lower energies (see Fig 27.2 for carbon). It is off the top of the graph at 1 MeV, so 30,000 ions per cm could be about right.

    For photons, the Compton cross section is dominant at 1 MeV. From Figure 27.16, a 1 MeV photon is absorbed in about 10 grams of air, or 7700 cm (or an average energy loss of 130 eV or 4 ions per cm in air).

    Bob S
    Last edited: Jun 27, 2010
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook