# Number of ions produced in air

1. Jun 27, 2010

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??

2. Jun 27, 2010

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??

3. Jun 27, 2010

### Bob S

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

http://pdg.lbl.gov/2009/reviews/rpp2009-rev-passage-particles-matter.pdf

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