678 Chapter 12. Radiation Spectroscopy
and the energy absorbed in the detector. A good multi channel analyzer has a
very linear response in its full dynamic range. That is, the bins or channels of the
MCA are directly proportional to the energy absorbed in the detector. This implies
that if one plots the absorbed energy with respect to the MCA channel, the result
should be a straight line. Without this linearity it would become very difficult, if
not impossible, to calibrate the system.
In essence, calibration is a fairly straightforward process and can be performed
using one or more known radioactive sources. We saw in Fig.12.1.3 that the highest
peak corresponds to the energy of theγ-ray. Therefore one can determine the
bin interval of the MCA corresponding to this peak. Repeating the experiment
with another source would produce another bin-interval to energy relation. These
two points can be used to determine the calibration curve. Once could also use
other peaks in the spectrum to obtain more statistics and hence better calibration
parameters.
One might be tempted to calibrate the system using only one peak from a source.
This can be done provided the null points of the MCA bin and the energy scale
coincide. In general, this is not true due to the presence of an offset on the MCA
bin axis. That is, the zero energy does not correspond to the zero pulse height.
This offset is due purely to electronics and can be determined by using an external
pulser. However if one uses several known peaks in the spectrum and fits a straight
line to the data, the offset can be determined by extrapolating the line to the x-axis.
This point will become clearer when we discuss the calibration procedure for the
α-particle spectroscopy systems.
12.1.CX-raySpectroscopy........................
X-ray, due to its high penetration power, is an excellent material probe. But this is
not all, since spectroscopic techniques based on x-rays can be used to determine in-
tricate details about the material they pass through. With the advent of synchrotron
radiation sources, the field of x-ray spectroscopy has seen immense research and de-
velopment. In the following sections we will look at the most commonly used x-ray
spectroscopic techniques.
C.1 X-rayAbsorptionSpectroscopy................
We know that the absorption probability of x-rays depends on the type of mate-
rial they interact with. Hence if somehow we could determine the extent of the
absorption, we could determine the details about the material. This is the main
purpose of x-ray absorption spectroscopy. Specifically, one strives to determine the
x-ray absorption fine structure or XAFS of the material by measuring the photon
absorption at different x-ray wavelengths. The concept is fairly simple but since it
requires very intense source of x-rays, the experiment can only be done at specialized
laboratories. In fact, most XAFS measurements are done at synchrotron radiation
facilities. Synchrotron radiation was introduced in the first chapter as a highly in-
tense beam of x-rays. At all accelerators where charged particles are accelerated in
curved paths, synchrotron radiation is produced as a byproduct. Since the realiza-
tion of its potential as an excellent non-destructive tool for material research, it has
found more prominence in the scientific and industrial communities. So much that