Scintillation Detector | A Brief Discussion
Nuclear
detectors are the special instruments that can detect nuclear radiation like alpha
particles, beta particles, gamma radiation and also determine their energy and
other such parameters of those particles. In the previous part [Part-1] we have
discussed the basic things about the nuclear detectors and some special type of
gas-filled nuclear detectors like Ionization chambers, Proportional Counters,
Geiger-Muller counter etc. They mainly work on the principle of ionization of
the gaseous medium due to the incoming ionizing radiation.
Now, there is
another very common type of nuclear detector which is called scintillation
detector. Here, we are going to know about the construction as well as the
principle and working mechanism of a scintillation detector.
A very simple
diagram of Scintillation detector is shown in the figure to represent the
basics of a scintillation detector.
As you can see
the in the figure above, scintillation detector mainly consists of two main
parts -the first is a scintillator material and the second is a photomultiplier
tube.
Basic Principle of Scintillation Detector:
An incident nuclear particle creates scintillation inside a scintillator material which basically leads to emission of photons. These photons go towards the photo cathode which leads to photoelectric effect i.e. the emission of photoelectrons. Then they enter into the Photomultiplier tube. This Photomultiplier tube basically increases the number of photo electrons to a significant value which can be easily observed by the electronic circuit associated with it. Now, the photoelectrons are directed toward an electrode (These electrodes are called dynodes) where they hit the surface and lead to ejection of a large number of secondary electrons. The electrons are directed towards another electrode again which is kept at a higher potential. Thus, kinetic energy of the electron and the number of the secondary electrons goes increasing. After hitting the surface of the electrode they again generate secondary electrons and then go to another electrode. In this repeated process the total number of electrons increases to a large number so that we can observe some kind of a current pulse associated with this nuclear particle. This is the basic working principle and mechanism of a scintillation detector.
Let us talk about each of the parts of these
detectors one by one in details.
Scintillator Medium:
what is a scintillator?
A scintillator
is a special kind of a material medium in which if a charged particle enters it absorbs the energy of the charged particle and emits light.
How does Scintillation happen?
When some external nuclear particle enters a
scintillator medium then the external nuclear particle collides or interacts with
the molecules of the material medium. Every time a collision or interaction
happens, it transfers energy to the material medium. Now, the electrons in a
material medium which are in the valence band absorb that energy and jump to
the conduction band. Again it returns to the ground state in a very short time.
Every time the electrons jump back from the conduction band to the valence band
it emits a particular photon. Every time this kind of de-excitation happens
particular photon is emitted.
So,
basically what happens is that the material medium absorbs the energy of the
incident particle and converts it into low energetic photons.
Thus,
the job of the scintillator is very simple when an external alpha particle or a
gamma radiation enters the material medium it basically converts the energy of
the external particle into low energetic photons. Greater the energy of the
incident radiation, the number of the photons created will be larger.
Different kinds of scintillation detectors use
different kind of scintillator medium. For example a scintillator made of
cesium iodide is used to detect protons and alpha particle while sodium iodide
is used to detect gamma radiation. Now all of these photons are focused into a
photo-cathode.
Photocathode:
The
photo cathode is simply a material that shows photoelectric effect. Whenever
some kind of incident photons falls into its surface then (If the energy of the
photons is sufficient enough) then electrons are ejected from the surface.
Some kind of a photo cathode material is
placed on top of a photomultiplier tube. When all of the light photons [Which
were created as an incident particle enters the scintillator ] are focused into
the photo cathode, they results in photoelectric effect and the emission of photo
electrons or primary photo electrons. Now these primary photo electrons enter
the photomultiplier tube.
Photomultiplier tube:
Photomultiplier
tube is the arrangement to increase the number of photo electrons emitted by
the photocathode before reaching the receiving plate.
Purpose of a photomultiplier tube:
The
purpose is very simple. It simply needs to increase the initial photo electrons
to a very high value so that it can lead to some sort of a significant current
pulse in the electronics circuit connected to this particular system.
It
does it in a very simple manner. There is a particular metrical construction
inside the photomultiplier tube. When the light photons come and hit the photo
cathode, it leads to the emission of photo electrons and these photo electrons
are directed towards a particular electrode.
Let,
this electrode is at plus 100 volt as you can see in the figure. All the curved
surfaces basically represent electrodes and they are called dynodes. There is a
particular reason why every single one of the electrodes is shaped like this.
Initially when the photoelectrons are emitted
by the photocathode, they are directed towards the first dynode due to the
electric field created by the potential difference. Every time the electrons
strike the surface of the dynode it leads to emission of secondary photo
electrons.
Thus, when the photo electrons hit the first dynode
they lead to the ejection of secondary photo electrons. Since, there is a
particular potential difference applied between two successive dynodes, there
is going to be an electric field working between them. So, the electrons will
be accelerated towards the second dynode itself.
Now, when the secondary photo electrons reach
the second dynode, the same thing happens again. Every single collision of
electron with dynodes results in the emission of a large number of secondary
photo electrons. So, here we will end up by getting a large number of photo
electrons which are now directed towards the third dynode itself because there
is the same successive potential difference applied between the two dynodes. They
again lead to the emission of more secondary electrons. Thus in every step
there is a cumulative increase in the total number of electrons which are again
directed towards the fourth the fifth dynodes and on and on.
Finally,
when the electrons reach the particular anode which is connected to some sort
of electronic system, then there is a huge number of electron there. Now, you
can easily realize that the construction of this entire setup is just meant to
increase a few number of electrons to a huge number of electrons to have a
significant current pulse associated with it.
The process of detection of the radiation:
The
height of the current pulse due to the electrons reached in the anode is
measured by some sort of an electronic setup. Again, the number of these
secondary photoelectrons is proportional to the number of primary
photoelectrons.
Then,
the size of the current pulse which is detected by some sort of an electronic
setup is basically dependent on the number of primary photo electrons.
The primary photo electrons are created
because of light photons. Every single light photon creates primary electron. So,
the primary electrons are basically dependent upon the number of light photons
and the number of light photons is dependent on the energy of the incident
nuclear particle.
So, in a way the size of the current pulse created here is dependent upon the energy of the incident nuclear particle. By studying the current pulse you can get an idea about the energy of the nuclear particle entered in this particular detector.
A
scintillation detector is very effective in telling us about the energy of the
particle as well as the intensity of incident particles.
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