6.3 Principle of Scintillation Counter
97
number of radiations and energy of radiation which interact with phosphor, respectively. These processes of excitation and generation of photons are completed in few
milli-microseconds. Hence, unlike the ionization counter, there is no appreciable
dead time for the scintillation counter. The photons are then converted into negative
pulses, which later measured with the help of electronics devices, as described for
the proportional counter. The basic difference between the scintillation counter and
ionization counter is that the former relies upon production of photons from phosphor and the latter depends upon formation of ion-pairs from the enclosed gas in
the counter. Both the photons or ion-pairs are then counted by converting them into
electrical signals.
6.4 Components of a Scintillation Counter
The basic components of a scintillation counter are
1. A phosphor, which can produce visible photons per radiation by interacting with
it.
2. A photocathode, which converts photons into electrons.
3. A dynode, which produces approximately 2.5 electrons per electron interacting
with one dynode.
4. An electronic system which amplifies these electrons into an electrical pulse
so that the pulse height becomes proportional to the energy and its population
becomes equivalent to the intensity of the interacting radiation.
5. Amplification of these pulses and sorting them in similar fashion, as in the case
of proportional counter.
6. Finally recording these pulses.
A block diagram depicting the setup of a scintillation counter is shown in Fig. 6.1. The
system has a solid NaI(Tl) scintillator (B) (different scintillators can also be used)
kept over a photomultiplier tube (A). The contact between the scintillator phosphor
(B) and the photomultiplier base (A) is ensured by adding transparent silicon oil
which is visible to visible radiation. A bad contact or air gap may reflect the visible
photons away from the scintillator, preventing its interaction with the photocathode
(M), and leading to a loss in count. The photomultiplier tube is protected from
external visible light by either covering it and the phosphor with an aluminum cover
(C) or wrapping it and the scintillator with a black tape. Under no circumstances
should the photomultiplier tube be exposed to light when potential is applied to
the photomultiplier (i.e., when extra high tension (E.H.T.) voltage is applied to it).
This spoils the tube due to production of very high current. This is because the
photocathode or dynode is sensitive to visible light and it cannot differentiate between
light coming from the external source or phosphor.
The photomultiplier tube is also protected from external cosmic radiations by
covering the assembly (C) with a lead brick house, having a wall thickness of 2–
3
(E), because a scintillator cannot differentiate between cosmic radiations and
97
number of radiations and energy of radiation which interact with phosphor, respectively. These processes of excitation and generation of photons are completed in few
milli-microseconds. Hence, unlike the ionization counter, there is no appreciable
dead time for the scintillation counter. The photons are then converted into negative
pulses, which later measured with the help of electronics devices, as described for
the proportional counter. The basic difference between the scintillation counter and
ionization counter is that the former relies upon production of photons from phosphor and the latter depends upon formation of ion-pairs from the enclosed gas in
the counter. Both the photons or ion-pairs are then counted by converting them into
electrical signals.
6.4 Components of a Scintillation Counter
The basic components of a scintillation counter are
1. A phosphor, which can produce visible photons per radiation by interacting with
it.
2. A photocathode, which converts photons into electrons.
3. A dynode, which produces approximately 2.5 electrons per electron interacting
with one dynode.
4. An electronic system which amplifies these electrons into an electrical pulse
so that the pulse height becomes proportional to the energy and its population
becomes equivalent to the intensity of the interacting radiation.
5. Amplification of these pulses and sorting them in similar fashion, as in the case
of proportional counter.
6. Finally recording these pulses.
A block diagram depicting the setup of a scintillation counter is shown in Fig. 6.1. The
system has a solid NaI(Tl) scintillator (B) (different scintillators can also be used)
kept over a photomultiplier tube (A). The contact between the scintillator phosphor
(B) and the photomultiplier base (A) is ensured by adding transparent silicon oil
which is visible to visible radiation. A bad contact or air gap may reflect the visible
photons away from the scintillator, preventing its interaction with the photocathode
(M), and leading to a loss in count. The photomultiplier tube is protected from
external visible light by either covering it and the phosphor with an aluminum cover
(C) or wrapping it and the scintillator with a black tape. Under no circumstances
should the photomultiplier tube be exposed to light when potential is applied to
the photomultiplier (i.e., when extra high tension (E.H.T.) voltage is applied to it).
This spoils the tube due to production of very high current. This is because the
photocathode or dynode is sensitive to visible light and it cannot differentiate between
light coming from the external source or phosphor.
The photomultiplier tube is also protected from external cosmic radiations by
covering the assembly (C) with a lead brick house, having a wall thickness of 2–
3
(E), because a scintillator cannot differentiate between cosmic radiations and