Physics and Engineering of Radiation Detection

674 Chapter 12. Radiation Spectroscopy

source reach the scintillator, where they get converted into light photons. To guide
the light photons towards the photomultiplier tube the scintillator is surrounded by a
light reflector assembly. The light photons are then absorbed by the photocathode of
the PMT, which emits photoelectrons. These photoelectrons get multiplied through
the dynode assembly and ultimately lead to a measurable pulse.

Scintillation

Detector

HV

PMT Amplifier MCA

Source

Shield

Figure 12.1.1: Block diagram of a typicalγ-ray spectroscopy setup

with a scintillation detector (such as NaI(Tl)) and a PMT.

In the above description we have not considered theγ-ray photons that hit the
shielding material. Since these photons can produce sizable effect on the output,
they must be taken into account. Fig.12.1.2 shows the interaction mechanisms in
the detector as well as in the surrounding shield. In chapter 2 we discussed different
ways in which a photon can interact with material. We saw that the three most
important interaction mechanisms are photoelectric effect, Compton scattering, and
pair production. Theγ-ray photons, as well as secondary photons, interact with
the detector material and the shield through all of these modes, provided they carry
enough energy.
The usual method adopted in spectroscopy is the pulse height analysis. This
technique is based on the fact that the height of the output pulse is proportional
to the energy deposited in detector’s active medium. Therefore if one plots the
number of pules obtained with respect to height of the pulses, it would correspond
to the energy spectrum of the deposited energy. Now, since the deposited energy
is directly related to the energy carried by the incident radiation, the spectrum
obtained actually corresponds to that of the spectrum of the incident radiation.
Pulse height analysis can be done using a single channel analyzer, though the process
in that case is fairly tedious and labor intensive. The best and the most commonly
used technique is the use of a multi channel analyzer or MCA. An MCA records
the number of pulses in eachpulse bin. The size of the pulse bin can normally be
defined by the user and is usually selected according to the particular constraints of
the experiment, such as required resolution and available time. The MCA spans its
full dynamic range in equal width pulse bins and generally displays the output on
a screen. The data can be saved, printed, or transferred. Modern MCAs can also
be directly interfaced to a computer for further analysis, display, and storage of the
data.
As shown in Fig.12.1.1, the photons from a radioactive source are emitted in all
directions. The ones moving directly to the scintillator deposit the most energy and
produce highest energy peak. TheFWHMof this peak determines the resolution