Physics and Engineering of Radiation Detection

698 Chapter 12. Radiation Spectroscopy

HereCαrepresents the area under theαpeak,Tis the measurement time,dis the
perpendicular distance between the source and the detector, andris the radius of
the detector. The second term on the right side of this equation corrects for the
solid angle subtended by the source at the detector. This is necessary since we know
that a radioactive source emits particles in all directions with equal probability.
A point worth mentioning here is that determination of the area under theα-
peak requires subtraction of the background. The reason is that the peak is actually
embedded on top of the background activity (see Fig.??). The simplest procedure is
to discard the area below a line stretched between two baseline points of the peak.
The baseline points at the two ends can be determined by taking averages of a few
points on the two tails of the peak. Other methods also exist but require fitting
the peak with a Gaussian function and then determining the end points using some
criterion based on the properties of the function. Such methods, however, do not
offer much improvement over the simple method mentioned above and are therefore
not generally used.

Energy

Intensity

Figure 12.2.5: Typicalαspectrum

of a source. The area under the peak

is proportional to the source activity.

12.2.BElectronSpectroscopy

Just likeα-particle spectroscopy is used for nuclear structure analysis, electron spec-
troscopy can be used for characterization of atomic structure. The experimental
setups and procedures for electron spectroscopy with radionuclides are similar to
theα-particle spectroscopy we have already discussed.
There are other kinds of electron spectroscopy as well. In fact, we have already
discussed two such techniques, namely XPS or x-ray photoelectron spectroscopy and
AES or Auger electron spectroscopy. We saw that the spectra of the electrons, which
are emitted as a result of photoelectric absorption of x-ray photons, can reveal the
intricate details of the atomic orbitals and the bonding between atoms. That is
why XPS is sometimes referred to aselectron spectroscopy for chemical analysisor
ESCA. In Auger electron spectroscopy, instead of photoelectrons, Auger electrons
are used for spectroscopic purposes.
ESCA and Auger electron spectroscopy are very useful techniques to analyze the
surfaces of solids. The reason is that if such electrons are produced deep inside the
solids, they can not escape to the surface and be detected. In most materials the