Physics and Engineering of Radiation Detection

3.1. Production of Electron-Ion Pairs 151

Table 3.1.1: Ionization potentialsIe, W-values, stopping powers (dE/dx), primary
ionization yieldnp, and total ionization yieldntof different gases at standard atmo-
spheric conditions for minimum ionizing particles (37) (ip stands for the number of
electron-ion pairs).

Gas Z Density Ie W dE/dx np nt

(× 10 −^4 g/cm^3 ) (eV) eV /pair (keV/cm) (ip/cm) (ip/cm)

H 2 2 0. 8 15. 4 37 0. 34 5. 2 9. 2

He 2 1. 6 24. 6 41 0. 32 5. 9 7. 8

N 2 14 11. 7 15. 5 35 1. 96 10 56

O 2 16 13. 3 1. 2 31 2. 26 22 73

Ne 10 8. 4 21. 6 36 1. 41 12 39

Ar 18 17. 8 15. 8 26 2. 44 29 94

Kr 36 34. 9 14. 0 24 4. 60 22 192

Xe 54 54. 9 12. 1 22 6. 76 44 307

CO 2 22 18. 6 13. 7 33 3. 01 34 91

CH 4 10 6. 7 10. 8 28 1. 48 46 53

To determine the number of total and primary charge pairs in a gas mixture, a
composition law of the form

nt =

∑

i

xi

(dE/dx)i

Wi

(3.1.4)

and np =

∑

i

xinp,i, (3.1.5)

can be used. Here the subscriptirefers to theithgas in the mixture andxiis the
fraction by volume of gasi.

Example:

Compute the total and primary number of charge pairs produced in a mixture

of 90%CO 2 and 10%CH 4.