Subatomic Physics

4.3. Semiconductor Detectors 59
known. This fact is expressed through the variance of the mean, givenby
σ 2 m =
�N 2
i=1 (n − ni)
N(N − 1) =
σ2 . (4.8)
(N − 1)
The measured quantity with its standard deviation σm is usually quoted as
4.3 Semiconductor Detectors
result = n ± σm. (4.9)
Scintillation counters started a revolution in the detection of nuclear radiations, and
they reigned unchallenged from 1944 to the late 1950s. They are still essential for
many experiments, but in many areas they have been replaced by semiconductor
detectors. Before discussing these, we compare in Fig. 4.7 a complex gamma-ray
spectrum as seen by a semiconductor and by a scintillation detector. The superior
energy resolution of the solid-state counter is obvious. How is it achieved? In the
scintillation counter, the efficiencies in Eq. (4.1) reduce the number of photoelectrons
counted; it is difficult to imagine how each of the efficiency factors in Eq. (4.1) could
be improved to about 1. A different approach is therefore needed and the solidstate
(semiconductor) detector offers one. The idea underlying the semiconductor
counter is old and it is used in ionization chambers: A charged particle with kinetic
energy Ee moving through a gas or a solid produces ion pairs, and the number of
Figure 4.7: Complex gamma-ray spectrum, due to gross fission products, observed by a germanium
detector (upper curve) and a scintillation detector (lower curve). [From F.S. Goulding and Y.
Stone, Science 170, 280 (1970). Copyright 1970 by the American Association for the Advancement
of Science.]