How Many Ions Are Produced in Air by 1 MeV Radiation?

[Ali Asadullah]

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??

 

[Ali Asadullah]

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??

 

[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