Scintillation Counter Principle Construction and Working
What is Scintillation
The scintillation counter is a device used for detecting and measuring the energy of radiation.
The Scintillation counter in its simplest form was first introduced by Rutherford and his co-worker while studying the luminance excited in ZnS by Alpha particles. A screen coated with zinc sulfide or barium Platinocyanide or calcium tungstate when exposed to Alpha particles produces scintillations which were counted by a low power microscope.
The instrument so devised was called Spintheriscope. The process of counting scintillations is a tedious process. The eye restricts the count to about 100 per minute.
The invention of the photomultiplier tubes and a better understanding of the luminescent properties of organic and inorganic substances have removed this drawback and the scintillation counter is now widely used in studying nuclear radiations.
Principle of Scintillation Counter
A simple scintillation counter was first introduced by Karan and Barca in 1994. The pulses produced are detected in conventional electronic circuits after suitable amplification. Pulses produced by Alpha particles were detected by ZnS, phosphor with an efficiency of 100%.
Later Kallman (1947) extended its application β and γ ray detection by using Anthracene and Naphthalene transparent crystals as fluorescent media. Hofstadter discovered that Nal had better efficiency and larger intensity for γ ray counting work.
Construction of Scintillation Counter
The complete scintillation counter consists of three basic parts:
- The scintillating material or phosphor produces a tiny light flash when a charged particle strikes it.
- The photomultiplier tube detects the light flash and produces an electric pulse.
- Amplifiers and electronic circuits record and count the electrical pulses from the photomultiplier tube.
The job of the microscope in a simple scintillator is replaced by a photomultiplier tube. This tube has many electrons card dynodes to which progressively higher potentials are applied as shown in the figure.
The photoelectrons are accelerated in the electrostatic field between the cathode and the first dynode, which is at a positive potential with respect to the cathode. The accelerated electrons impart enough energy to electrons in the dynode to eject some of them.
There may be as many as 10 secondary electrons for each electron that strikes the dynode. This process of multiplication goes on till the last dynode gets an Avalanche of electrons which are finally collected by the anode.
The output current or pulse at the anode may be more than a million times greater than the current originally emitted from the cathode.
Working of Scintillation Counter
The block diagram of the scintillation Counter is shown in the figure. S is a source that emits ionizing radiations to produce short-duration light flashes in the phosphor placed in front of the photocathode of a photomultiplier tube.
The process of multiplication takes place to produce an Avalanche of electrons which are finally collected by the anode. A large pulse of several tens of millivolts is produced at the output.
Preamplifier amplifies these Signals and then they are fed to the discriminator whose function is to remove low energy pulses and then they are counted in the scalar. Power to the various stages is supplied by the stabilized power supply.
Producing of a scintillation flash by the incoming ionizing particles and subsequent generation of an electrical pulse in a photomultiplier are divided into five distinct events.
- The incident radiation is first absorbed in the phosphor material and its atoms or molecules are excited.
- The excited atoms or molecules of the fluorescent material of the phosphor decay and produce light flash of short duration.
- The emitted photons are transmitted to the photocathode of the photomultiplier.
- Photoelectrons are produced due to absorption of light photons.
- Electron multiplication takes place very quickly and all these operations take place with in about 10-8 seconds.
The electrical pulses produced by photomultiplier tube are proportional to the energy of incident photons. Thus scintillation counter detects radiation as well as measure the energy of radiation.
A typical γ ray spectrum obtained with Cs137 source is shown in figure.
We know that γ-photons of rays interact with matter mainly in three ways:
- Photoelectric effect.
- Compton effect.
- Pair production ( production of positron-electron pair).
Photoelectric effect and Compton effect are most important for γ rays having energy up to 2 MeV. However, the Photoelectric effect is actually utilized because when γ ray incident on a material, photoelectron is emitted.
The energy of the photoelectron is equal to the energy of the absorbed γ ray. In the Photoelectric effect, γ ray loses all its energy to the electron. Therefore, γ rays of the same energy produced photoelectrons of the same energy in a scintillating crystal. The electrical pulse produced in a photomultiplier tube is proportional to the energy of incident γ rays.
A scintillation counter coupled with a multi channel analyser is known as γ ray spectrometer. This spectrometre is calibrated using γ rays of known energy. The width of the full energy peak at half height is called full width at half maximum (FWHM).
The energy resolution of spectrometer is defined as the ratio of FWHM to the energy of γ rays corresponding to the full energy peak.
That it, energy resolution of spectrometer = Δ E / Eγ
Typically, Δ E / Eγ =20% at Eγ = 100 k eV.
When γ rays energies are very close to each other, scintillation counter is not able to separate them. in In such cases, semi conductor counter is used.
Types of Scintillation Counter Used
- Sodium Iodide.
- Zinc Sulphide.
- Anthracene and Stilbene.
- Plastic and Liquid Scintillators.
Sodium Iodide (Thallium Activated)
This is the most commonly used scintillator in the study of γ rays. In a comparison of GM counter, the efficiency of γ-ray detection is very large. It has one drawback, it is is hygroscopic and therefore has to be sealed in an aluminum can with reflecting or diffusing walls.
It is extensively used for the detection of those particles which have short ranges. It cannot be used in thick layers because it rapidly becomes opaque to its own radiation.
This is not hygroscopic and is therefore preferred over sodium iodide.
Anthracene and Stilbene
These are organic Phosphors which have a faster decay time then the inorganic Phosphors. For heavy particles, these have very poor efficiency. These are useful for the detection of β-particle. Anthracene gives highest yield of photons about 15 for each 1000 eV.
Plastic and Liquid Scintillators
In these scintillators, the energy of excitation is transferred from the solvent to the solute. This then re-emits radiation in a wavelength range for which the solvent is transparent. These are used in Counter telescopes which are generally used in high energy physics.
For counting heavy charged particles in the presence of γ-radiation, Xenon is used which emits radiation in the ultraviolet region.
The high efficiency of detection, short resolving time, linearity in response in a wide range of the energy of incident radiation are some of the advantages of the scintillation Counter which make this instrument superior to the conventional G.M. counters.
The most outstanding feature of the scintillation counter over the proportional counter is its extremely short duration pulses and higher resolution.
Applications of Scintillation Counter
- It is most efficient for γ-ray counting.
- With its large size and highly transparent phosphor, it displays very high efficiency.
- As the pulse height is proportional to the energy of the incident radiation, it is used for the investigation of the energy distribution of nuclear radiations.
- It is capable of a fast counting rate because the dead time and resolving time are of the order of 10-19 sec. as against 10-5 sec. in the G.M. counter.