Physics and Engineering of Radiation Detection

694 Chapter 12. Radiation Spectroscopy

The shaped pulse is then input into a pulse height analysis system. Earlier in the
book we discussed one such instrument called the multichannel analyzer or MCA.
In brief, an MCA first digitizes the analog pulses and then increments the counts
of its channels according to the height of the pulse. The channels of an MCA, also
calledbins, span its whole dynamic range. These bins are divided into intervals or
bin widths. In general all the bin widths are of equal size but some modern MCAs
allow the user to manually set different bin widths as well. The bin width used in the
analysis and the overall MCA resolution set the limit on peak resolution capability
of the system. This concept is graphically depicted in Fig.12.2.2. As shown in the
figure, if the peaks are too close together, they can not be resolved and hence appear
as a single peak in the MCA spectrum.

Unresolvable Peaks

Bin Width

MCA Channel

C

oun

ts

Figure 12.2.2: Effect of bin width of an MCA on its energy

resolution.

Before the spectrum of an unknown source is obtained, the detection system must
be calibrated. This is usually done with the help of a pulser and a known radioactive
source. The commonly used sources for calibration are listed in Table.12.2.1. It
should be noted that except for americium, none of the sources listed here have an
energy spectrum with two or moreresolvablepeaks.
Calibration of the instrument is extremely important for absolute measurements.
We will therefore spend some time discussing how it is practically done. Suppose
we try to calibrate the setup using a source that emits mono-energetic
alpha-particles. The spectrum captured by the MCA will show a single peak oc-
cupying some of its channels. We can try to calibrate the whole dynamic range of
the MCA using the center of this peak. However since this method uses a single
point, it will be prone to large uncertainties. A better technique would be to use two
sources separately or their mixture. This would ensure that two calibration points
corresponding to the two peaks are clearly identifiable. An important point that
must be considered during calibration is the so calledzero point energy. In a perfect
system one would expect that the zeroth bin of the MCA would correspond to the
zero energy. That is, the straight line between the energy and the MCA channel
would cross the axes at the origin. However this in general is not true due to the