[Physics Class Notes] on Scintillation Counter Pdf for Exam

The flash of light that is produced by a transparent material due to the passage of a subatomic particle (electron, ion, alpha particle, or photon) is called scintillation. The scintillation counter is a device that is used to detect radiation by means of a scintillation effect. It is also known as a scintillation detector. Scintillation is a major part of a scintillation detector. A scintillation detector usually consists of the following components:

• Scintillator: A scintillator is a device that emits light when a high-energy particle hits it. The energy of the emitted pulse of light is directly proportional to the particle that hits the scintillator. This makes it an efficient energy-dispersive radiation detector much used in spectroscopy. The generation of photons occurs in the scintillator as a response to the incident radiation.

• Photodetector: A photodetector converts light to an electrical signal in order to process the signal. A photomultiplier tube (PMT), a photodiode or a charged coupled device (CCD) is generally used as a photodetector.

 

Scintillation Counter How it Works

Let us try to understand the principle of the scintillation counter through the following points.

• When ionizing incident radiation enters the scintillator, it interacts with the material of the scintillator due to which the electrons enter an excited state.

• Charged particles follow the path of the particle itself.

• The energy of gamma radiation (uncharged) is converted to a high energy electron either through the photoelectric effect, Compton scattering, or pair-production effect.

• The excited atoms of the scintillator material gradually undergo de-excitation and emit photons in the visible range of light. This emission is directly proportional to the energy of the incident ionizing particle. The material shines or flows brightly due to fluorescence.

 

Three types of phosphors are used namely:

• Inorganic crystals,

• Plastic phosphors,

• Organic crystals.

• The pulse of light emitted by the scintillator hits the photocathode of the photomultiplier and releases at most one photoelectron for each photon.

• These electrons are accelerated through electrostatic means by applying a voltage potential and are targeted to hit the first dynode, by having enough energy to produce further electrons.

• These released electrons are called secondary electrons. They strike the second dynode, thereby releasing further electrons. This process occurs in a photomultiplier tube.

• Each subsequent impact on the dynode releases further electrons, and hence a current amplifying effect occurs on the dynodes. Each subsequent dynode is at a higher potential than the previous one, and so helps in enhancing the acceleration.

• Likewise, the primary signal is multiplied throughout 10 to 12 stages.

• At the final dynode, highly sufficient numbers of electrons are present to produce a pulse of high magnitude to develop amplification. This pulse carries information about the energy of the incident ionizing particle. The number of pulses per unit time gives the significance of the intensity of radiation.

 

Types of Scintillation Counter

There are basically two types of scintillators used in nuclear and particle physics. They are plastic or organic scintillators and crystalline or inorganic scintillators.

1. Organic Scintillators

Organic scintillators are organic materials that provide photons in the visible part of the spectrum after a charged particle is passed through it. The scintillation mechanism of organic material is different from that of inorganic material. The fluorescence or scintillation in organic materials is produced due to the transition of the energy levels of a single molecule. The fluorescence in organic materials can be observed independently in any of the physical states viz: vapor, liquid, and solid.

2. Inorganic Scintillators

Inorganic scintillators are crystals made in high-temperature furnaces. They include lithium iodide (LiI), cesium iodide (CsI), sodium iodide (NaI) and zinc sulfide (ZnS). NaI(TI) (thallium-doped sodium iodide) are highly used inorganic scintillation materials. The iodide present in sodium iodide provides the necessary stopping power (because it has a high Z = 53). The process of scintillation in inorganic materials is normally slower than that of organic materials. The inorganic scintillators have a very high efficiency to detect gamma rays and are also capable of handling high rates of counts.

 

Difference between Scintillation Counters and Geiger Counters

Now, you already know what scintillation counters are but the students must get an understanding of Geiger counters as well to understand major differences between the two. 

 

The Geiger counter is used to detect ionizing radiation. Essentially, it is also called GM Counter. GM stands for Geiger-Mueller. The GM Counter is named after Hans Geiger and Walther Mueller, who invented the Geiger-Mueller Tube in 1928. Along with scintillation counters, it is one of the most commonly used instruments to detect and measure radiation.

 

GM tube is a sensing element in a Geiger counter, which is filled with an inert (unreactive) gas like neon, helium or argon at low pressure. This gas is supplied with a high voltage. This gas becomes conductive by ionization from a particle or photon of incident radiation and hence, the Geiger counter can conduct an electric charge very briefly. 

 

But with the Townsend discharge or Townsend avalanche, ionization within the tube is significantly amplified, which makes it easy to measure the pulse to be detected. Then, this pulse is fed into the processing and display electronics. The display method used in the Geiger counter is the number of counts per time unit.

 

Hence, the main difference between Geiger counters and scintillation counters is the difference in the method used to detect and measure ionizing radiation.  Scintillation counters use the excitation effect of incident radiation on a scintillating material (materials that exhibit the property of luminescence upon the excitation effect of ionizing radiation) and detect the resulting light pulses with a photodetector to measure ionizing radiation. Whereas, Geiger counters use the ionizing effect on the gas within a GM tube to detect ionizing radiation.

 

The cost of scintillation counters is relatively higher compared to the Geiger counters which are
comparatively quite inexpensive. The size of Geiger counters is smaller than the scintillation counters. But the scintillation counters are considered more sensitive in detecting radiation compared to Geiger counters. 

 

This explains the higher accuracy of scintillation counters in comparison to Geiger counters. Scintillation counters have better quantum efficiency to detect and measure ionizing radiation and can also be used to determine the intensity and energy of incident radiation.  

 

Compared to Geiger counters, scintillation counters are better because scintillation counters are capable of not only detecting even the tiniest amount of radiation but also identifying the radioactive isotope in some cases.

 

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