Plutonium Biokinetics in Human Body A. Luciani - Kit-Bibliothek - FZK

The phoswich detector is based on two different scintillators coupled together and
mounted on a single photomultiplier tube (see Figure 2.3.1) [129]. It is used for detecting low
level radiation in presence of significant background. The scintillating detectors composing
the phoswich have different decay constants. In such way the shape of the output pulse from
the multiplier is dependent on the relative contribution of scintillation light from the two
scintillators. Differences in the pulse output are used to distinguish events that have occurred
in only one scintillator from those that occur in both. Typical scintillation crystal used for this
purpose are Sodium and Caesium iodide, both with Thallium as activator, NaI(Tl) and
CsI(Tl), because of their different decay constants (about 0.25 and 1.1 µs, respectively).
Lightly penetrating radiations can be stopped almost completely in the first scintillator, i.e.
NaI(Tl), giving a fast pulse as output because they will not reach the slow pulse generator
crystal, i.e. CsI(Tl). More penetrating radiations will reach both scintillators with a pulse
shape modified because of the different decay constants of the crystals. The two crystals in
the phoswich detector are commonly used for Compton interactions reduction purposes: a
Compton interaction delivers a signal in both detectors that can be discriminated by the a full
energy absorption event detected only in the first detector.
Optical coupling window
Entrance
window
NaI(Tl)
CsI(Tl)
Low noise
photomoltiplier
Figure 2.3.1 The basic elements of a phoswich detector.
Similarly to the case of a scintillation crystal, a semiconductor detector structure too
can be described with conduction and valence energy bands [128]. The bands are separated by
a band gap of forbidden energy states, the size of which determines whether the material is a
semiconductor or insulator. In the absence of thermal excitation both insulator and
semiconductor have a configuration in which the valence band is completely full and the
conduction band is empty. Therefore neither would show any electrical conductivity. In a
metal the high energy band is not completely occupied and therefore metals are always
characterized by high electrical conductivity. At any nonzero temperature thermal energy is
shared among the electrons of a semiconductor and therefore there is the possibility for a
valence electron to reach the conduction band. As a consequence of this process a vacancy
(hole) is created in the valence band. In a completely pure semiconductor all electrons and
vacancies will result from thermal excitation. This configuration is named intrinsic. If
impurities are present the properties of the material are mainly determined by them. Actually
controlled amounts of impurities are introduced by a process called doping, which increases
the conductivity of the material by orders of magnitude. Two types of doping can be
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Low background
housing