Physics and Engineering of Radiation Detection

224 Chapter 4. Liquid Filled Detectors

having an effective mass and unit positive charge. It is also able to move from one
site to another by a reciprocal movement of an electron.

Iliquid

Conduction Band

Valence Band

Forbidden Gap

Electron

Hole

Figure 4.1.2: Creation of an electron-hole pair. Hole

represents a vacancy created in the valence band cre-

ated by migration of an electron to the conduction

band. It has an effective positive charge.

The concept of hole is most exclusively used while treating movement of charges
in solids, in which case the molecules share the electrons through bonding with
neighboring molecules. The importance of such a treatment will become clear when
we discuss the semiconductor detectors later on in this book. The case of liquids
in a radiation detector is a little bit complicated as compared to gases and solids
since we must consider holes as well as ions. The reason is that, apart from the main
liquid molecules, there are other molecules in there too, which have their own energy
level structures. These molecules, having lower density, can in fact be ionized. Let
us suppose there is another moleculeXin the filling liquid composed of molecules
M. As the incident radiation passes through the liquid, it creates electron hole pairs
as described earlier. These pairs can become involved in the following reactions.

Recombination:The electron can fall back into the valence band and recom-

bine with the hole either at the site of origination or at a spatially separated

site. This would neutralize the moleculeMbut would leave it in an excited

state. The excess energy contained byM∗will be equal to at least the differ-

ence of energy between the conduction band’s lower edge and valence band’s

upper edge. M∗will eventually de-excite by emitting a photon, which can

either escape from the detector or create another electron ion pair at another

site. This process is graphically depicted in Fig.4.1.3.

Radiation Electron Photon

Incident Emitted

Hole

Charge Pair Creation Recombination and De−excitation

Figure 4.1.3: Creation of an electron hole pair and its subse-

quent recombination.

Charge Transfer:As stated earlier, the hole produced by the incoming radi-

ation can also transfer its charge to afreemolecule (see Fig.4.1.4).