98
6 Scintillation Counter
the radiation coming from the sample. The photomultiplier tube is connected to a
EHT unit (G) through a probe unit (F). The output of the photomultiplier tube is
connected through a probe unit to an amplifier (H ), a pulse height analyzer (I ), and
a scaler (J ).
6.5 Design of Photomultiplier Tube
There are different designs available for the photomultiplier tube. The design of a
tube given in Fig. 6.1 consists of an anode (K ), ten dynodes (L), and a photocathode
(M). Between the two dynodes a field spacing screen (N ) is used. The dynodes have
also varied designs. In Fig. 6.1, dynodes of a venetian blind type is shown. The design
and potential of the dynodes are made such that electrons can never be reflected in an
upward direction. The photocathode is usually made of cesium and antimony oxide,
as these materials can eject electrons when photon interacts with them. The entire
assembly, i.e., photocathode, dynode, and field spacing screen are enclosed in a
brown transparent glass and sealed under vacuum.
Each dynode is separately connected to a probe unit through which a high tension
voltage is applied in such a manner that the dynode next to the photocathode has the
lowest potential and the potential of other dynodes gradually increases till the last
dynode (next to the anode) has the highest potential. Normally, a photomultiplier
tube has about 11 dynodes.
The photocathode ejects approximately 2.5 electrons per photon incident on it
(i.e., when one photon reaches its plate). In this way, one photon produces approximately two thousand electrons and these are deposited at the anode, which produces
a negative potential pulse, similar to ionization counters. This pulse is amplified and
analyzed using a pulse height analyzer. Energy of electrons thus collected at the anode
and their number is proportional to the energy and the number of photons produced
by the interaction of the radiation with the scintillator, respectively. The energy of
photon corresponds to the energy of interacting radiation, while the population of
electrons of a particular energy corresponds to the number of interacting radiation.
Thus, by measuring the height and its corresponding number of negative pulses, we
get an idea about the energy and intensity of the interacting radiation.
In brief, the operation of scintillation counter depends on the following consecutive events:
1. Absorption of nuclear radiation from a radioactive sample (D) occurs with the
scintillator (NaI in this case) which results in excitation of the scintillator.
2. The de-excitation or ion re-combination produces a visible photon per radiation
absorbed by the scintillator. Photons of wavelength equivalent to the energy of
radiation are emitted. The magnitude of wavelength and its number depends upon
the energy and number of radiations which produced excitation per unit time by
the processes of luminescence.
6 Scintillation Counter
the radiation coming from the sample. The photomultiplier tube is connected to a
EHT unit (G) through a probe unit (F). The output of the photomultiplier tube is
connected through a probe unit to an amplifier (H ), a pulse height analyzer (I ), and
a scaler (J ).
6.5 Design of Photomultiplier Tube
There are different designs available for the photomultiplier tube. The design of a
tube given in Fig. 6.1 consists of an anode (K ), ten dynodes (L), and a photocathode
(M). Between the two dynodes a field spacing screen (N ) is used. The dynodes have
also varied designs. In Fig. 6.1, dynodes of a venetian blind type is shown. The design
and potential of the dynodes are made such that electrons can never be reflected in an
upward direction. The photocathode is usually made of cesium and antimony oxide,
as these materials can eject electrons when photon interacts with them. The entire
assembly, i.e., photocathode, dynode, and field spacing screen are enclosed in a
brown transparent glass and sealed under vacuum.
Each dynode is separately connected to a probe unit through which a high tension
voltage is applied in such a manner that the dynode next to the photocathode has the
lowest potential and the potential of other dynodes gradually increases till the last
dynode (next to the anode) has the highest potential. Normally, a photomultiplier
tube has about 11 dynodes.
The photocathode ejects approximately 2.5 electrons per photon incident on it
(i.e., when one photon reaches its plate). In this way, one photon produces approximately two thousand electrons and these are deposited at the anode, which produces
a negative potential pulse, similar to ionization counters. This pulse is amplified and
analyzed using a pulse height analyzer. Energy of electrons thus collected at the anode
and their number is proportional to the energy and the number of photons produced
by the interaction of the radiation with the scintillator, respectively. The energy of
photon corresponds to the energy of interacting radiation, while the population of
electrons of a particular energy corresponds to the number of interacting radiation.
Thus, by measuring the height and its corresponding number of negative pulses, we
get an idea about the energy and intensity of the interacting radiation.
In brief, the operation of scintillation counter depends on the following consecutive events:
1. Absorption of nuclear radiation from a radioactive sample (D) occurs with the
scintillator (NaI in this case) which results in excitation of the scintillator.
2. The de-excitation or ion re-combination produces a visible photon per radiation
absorbed by the scintillator. Photons of wavelength equivalent to the energy of
radiation are emitted. The magnitude of wavelength and its number depends upon
the energy and number of radiations which produced excitation per unit time by
the processes of luminescence.