Pulse Formation in an Ionization Chamber

The formation and shape of the signal in an ionization chamber will be analyzed for a parallel-plate counter as shown in Fig. 5.1b. The analysis is similar for a cylindrical or a spherical chamber. 

Consider the ionization chamber shown in Fig. 5.6. The two parallel plates make a capacitor with capacitance C, and with the resistor R an RC circuit is formed. A constant voltage Fq is applied on the plates. The time-dependent 

Assume that one electron-ion pair has been formed at a distance Xq from the collecting plate (collector). The electron and the ion start moving in the electric field, and they acquire kinetic energy at the expense of the electrostatic energy stored in the capacitance of the chamber. If the charge moves a distance dx, conservation of energy requires that 

In general, the drift velocity is a function of the reduced field strength E/p, where p is the gas pressure in the chamber. 

The derivation up to this point is independent of the chamber geometry. To proceed further requires substitution of the value of the electric field from either Eq. 5.1, 5.2, or 5.3. For a plate-type ionization chamber the field is constant (Eq. 5.1), independent of x, and so is the drift velocity. Therefore, Eq. 

---