Figure 14 illustrates a simple ionization circuit consisting of two parallel plates of metal with an
air space between them. The plates are connected to a battery which is connected in series with
a highly sensitive ammeter.
Figure 14 Simple Ionization Circuit
If a radioactive source that is an emitter of beta particles is placed near the detector, the beta
particles will pass between the plates and strike atoms in the air. With sufficient energy, the beta
particle causes an electron to be ejected from an atom in air. A single beta particle may eject
40 to 50 electrons for each centimeter of path length traveled. The electrons ejected by the beta
particle often have enough energy to eject more electrons from other atoms in air. The total
number of electrons produced is dependent on the energy of the beta particle and the gas between
the plates of the ionization chamber.
In general, a 1 MeV beta particle will eject approximately 50 electrons per centimeter of travel,
while a 0.05 MeV beta particle will eject approximately 300 electrons. The lower energy beta
ejects more electrons because it has more collisions. Each electron produced by the beta particle,
while traveling through air, will produce thousands of electrons. A current of 1 micro-ampere
consists of about 1012 electrons per second.
If 1 volt is applied to the plates of the ionization chamber shown in Figure 14, some of the free
electrons will be attracted to the positive plate of the detector. This attraction is not strong
because 1 volt does not create a strong electric field between the two plates. The free electrons
will tend to drift toward the positive plate, causing a current to flow, which is indicated on the
ammeter. Not all of the free electrons will make it to the positive plate because the positively
charged atoms that resulted when an electron was ejected may recapture other free electrons.
Therefore, the ammeter will register only a fraction of the number of free electrons between the