Biomedical Engineering Reference
In-Depth Information
Fig. 10.12
Examples of Geiger-Mueller and scintillation probes
with specifications. (Courtesy Fluke Biomedical.)
ing gas, the latter, because of its lower ionization potential, can transfer an elec-
tron to the counter gas, thereby neutralizing it. Positive ions of the quenching gas,
reaching the cathode wall, spend their energy in dissociating rather than producing
secondary electrons. A number of organic molecules (e.g., ethyl alcohol) are suit-
able for internal quenching. Since the molecules are consumed by the dissociation
process, organically quenched GM tubes have limited lifetimes (
∼
10
9
counts). Al-
ternatively, the halogens chlorine and bromine are used for quenching. Although
they dissociate, they later recombine. Halogen-quenched GM tubes are often pre-
ferred for extended use, although other factors limit their lifetimes.
10.2
Ionization in Semiconductors
Band Theory of Solids
Section 2.9 briefly described crystalline solids and the origin of the band structure
of their electronic energy levels. In addition to the forces that act on an electron
to produce the discrete bound states in an isolated atom, neighboring atoms in
the condensed phase can also affect its behavior. The influence is greatest on the
motion of the most loosely bound, valence electrons in the atoms and least on
the more tightly-bound, inner-shell electrons. As depicted schematically in Fig. 2.7,
with many atoms present, coalescing of the discrete states into the two bands of
allowed energies with a forbidden gap between them depends on the size of
R
0
,
the orderly spacing of atoms in the crystal. The figure indicates that bands are not
formed if
R
0
is very large. By the same token, the bands would overlap into a single
continuum with no forbidden gap if
R
0
is small.
