Development of novel plastic scintillators based on polyvinyltoluene for the hybrid j-pet/mr tomograph

3. Scintillators and scintillation mechanism 
Ionising radiation interacting with matter excites its molecules. When they return to 
the ground state, photons of the visible or near to visible light spectrum range are 
produced. This phenomenon is called scintillation. A material in which conversion of 
excitation energy into light is highly efficient is called scintillator. According to [30] [31], 
scintillator used in the radiation detectors should be characterized by several properties: 

transparent at the wavelength of emitted scintillation light 

high efficiency of light production 

short light pulses to exclude delayed light emission 

the amount of light should be proportional to the energy deposited in the material 

chemical and mechanical stability

not prone to radiation damage. It was estimated that one plastic scintillator with 
dimensions of 0.7 cm x 1.9 cm x 50 cm during one year of utilization in PET 
scanners would receive a radiation dose about 0.1 kGy. According to article [32], 
this is more than one order of magnitude less than the dose causing noticable 
radiation damage in plastic scintillators.
Nowadays, scintillation detectors are one of the most popular detectors of radiation. 
Scintillators are common gamma ray, X-ray, charged and neutral particles detectors. They 
are utilized in many fields of science and industry. They are the most common detectors 
used in experiments, regarding the fundamental research in particle and nuclear physics, 
e.g. to detect particles formed during the process of artificial fusion of atomic nuclei. They 
are also used as detectors of cosmic rays and in the detectors systems installed at the Large 
Hadron Collider at the European Centre for Nuclear Research (CERN) in Geneva. 
Scintillators are widely used in astrophysics to observe emerging stars, the searches for 

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mineral resources and to ensure security at the airports. One can use them on minefields to 
help in locating the explosive materials without endangering human life. 
Scintillators are divided into two groups: inorganic and organic. Physics of 
scintillation mechanism as well as their properties and applications are different. The main 
difference is that organic scintillators predominantly consist of low atomic number (Z) 
elements, like carbon and hydrogen, and they have relatively long attenuation length. 
Inorganic scintillators contains large fraction of elements with high Z (e.g. an effective 
atomic number of LYSO crystal is equal to 66 u [14]) and attenuation length in that type of 
scintillators is short.
The vast majority of inorganic scintillators are crystals. The mechanism of 
scintillation is based on electron-hole pairs production in the valence and conduction band 
during interaction with incident radiation. Light output of inorganic scintillators can be 
higher (see Table 2) in comparison to organic ones. However they are expensive and the 
process of crystal growth is difficult to carry out [33].
Organic scintillators are built of chemical substances including phenyl rings. They 
are found in three types: crystalline, like anthracene or stilbene, liquid, when the 
scintillator is dissolved in solvent e.g. xylene or toluene and plastic scintillators. 
Crystalline organic scintillators are expensive and vulnerable. Liquid ones are toxic and its 
utilization is inconvenient. Because of the volatility, they need to be stored in special 
containers. 
The mechanism of luminescence in organic and inorganic crystals differs 
significantly, what is determined by their intrinsic structure. In organic crystals, molecules 
are weakly bounded in comparison to inorganic compounds. In such loose arrangement 
energetic levels are not disturbed by the environment [34].
This thesis concerns plastic scintillators. The scintillation mechanism of organic 
scintillators will be exemplified for plastic scintillators in the following chapter.

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