Nuclear Engineering

Scintillation Detectors

In the previous lectures we have seen two different detection media, i.e. gas and semiconductor. Although these two are two different media, the detection mechanism in these two has lot of similarities. When energetic particles enters inside in these media , electron-positive ion pairs (in gas detectors) or electron-hole pairs (in semiconductor detectors) are generated, which eventually creates a signal through a proper arrangement of electrodes. Now the question arises:
Can we design detectors different from gas and semiconductor detectors?


In this lecture we will discuss another type of detector where detection media are organic /inorganic materials and work in a different manner as compared to the above two detectors. These materials are being routinely used as nuclear detectors, due to their unique property, like scintillation. Scintillation is a mechanism by which photons are emitted when an external perturbation (like energetic particles) are incident on a scintillating material. This material should convert the kinetic energy of the incident particle with detectable light, with a short decay time and a linear conversion. The meaning of linear conversion is that the light yield should be proportional to the deposited energy. Let us first try to understand the meaning of linearity.


The linearity property of a scintillator means the proportionality of the light yield with the deposited energy over a range as wide a range as possible, which is mathematically represented as

L = ΔE ————— (m2.40)
where L is the fluroscence light emitted and ΔE is the energy deposited in the system. However, this linear relation does not strictly hold well in practice, and the response of scintillators is not only a complex function of energy of radiation but also the type of radiation and specific ionization. A semi-empirical model was first put forward by Birks⁵, which is based on the assumption that a high ionization density along the track of the particle leads to quenching from damaged molecules and a lowering of the scintillation efficiency. In this model, the light output per unit length, dL/dx, is related to the specific ionization by

where, η is the scintillation efficiency, kB is the parameter relating the density of ionization centres to dE/dx and can be used as fitting parameter of this equation. In the case when dE/dx is small (e.g. electron irradiation), Birk’s formula then predicts

or, the incremental light output per unit energy is

The total light output in this case is

is linearly related to the particle energy E. On the other hand, for an alpha particle, dE/dx is very large, so that the saturation occurs along the track and the Birk’s formula becomes

Organic crystals like anthracene (C₄H₁₀), trans-stilbene (C₁₄H₁₂), naphthalene (C₁₀H₈) etc. , scintillation light arises from transition made by the free valance electrons of the molecules. Electrons occupy the molecular orbitals of these materials and transition due to external energy causes the emission of light. They are generally faster but yield lesser light. For detailed emission mechanism you may look into other texts.^1,3
In case of inorganic crystals like sodium iodide (NaI), Bismuth Germanate or BGO (Bi₄Ge₃O₁₂) etc. the scintillation process depends on the band structure. Due to the impact of energetic particle an electron can be excited to conduction band thus creating a free electron and hole. Otherwise, it can create an exciton, by exciting an electron to a band (exciton band) located just below the conduction band. The exciton band is a bound state of electron-hole pair. Unlike an excitation in a single atom or molecule, an exciton can in general move through the solid like a particle. Excitons transport energy, not charge or mass. After creation by external perturbation, the exciton moves through the crystal; and finally the electron and hole recombine, resulting in the emission of photons.
Now I am going to explain only basic operation of a scintillator detector assembly with the help of following schematic diagram (Fig. m2.21).


FIGURE m2.21 Schematic diagram of a scintillator detector assembly. A PMT is shown in the inset.


General Characteristics of Scintillation Detectors


A scintillation detection system consists of a thin window followed by the scintillator, which is optically coupled to a photomultiplier tube (PMT) either directly or via a light guide housed in a same assembly. The PMT converts the light falling on it into photoelectrons, which eventually gets further dynodes as shown in Fig. m2.21. Finally the electrons are collected at anode form a signal using a suitable electronic circuit , giving rise to a current. The entire structure is properly shielded to protect from any unwanted radiation coming from any other source and also light loss through the boundaries. The design of the detection system is based on two important points – light collection and transport. It is important to collect as many of the emitted photons as possible and to efficiently couple them to the PMT. The main problem encountered in this detector is the loss due to transmission through the scintillator boundaries. Light emitted at any given point of the scintillator travels in all directions and only a fraction of it reaches to photocathode of PMT. Rest of it travels towards the scintillator boundaries from where a fraction of it reflects back and other part gets transmitted depending on the angle of incident. This is schematically shown in Fig. m2.22. This loss reduces the efficiency and energy resolution of the detector.


FIGURE m2.22 Light transmission in a typical scintillator

In order to improve light collection the boundary materials of high reflectivity are chosen. The most common materials used for this purpose are MgO, TiO₂, Al etc. These are usually form of a powder or as a white paint. It has been shown that Al and MgO retain relatively high reflectivity down to low wavelengths, TiO₂ drops off sharply at ^~400 nm where it becomes a poor reflector. Apart from this, the maximization of internal reflection is another important task. It is known that light impinging at an angle greater than Brewester angle, θ_B where


with n_med being the refractive index of the surrounding medium and n _scint that of the scintillator, total internal reflection occurs. The medium surrounding the scintillator should therefore have low refractive index value to minimize q B. For this reason, a layer of air should be maintained between the reflector and the scintillator. Next stage is the coupling between the scintillator and the PM, which has to be made in such a way so that maximum light transmits. The optical contact between scintillator and PM window is made through such a material whose index of refraction is as close as possible to both. In case of organic scintillator, silicon grease or oil is chosen, however, for inorganic material finding an appropriate material is difficult. Apart from this direct coupling, in many cases coupling is done through a light guide (or light pipe). Light guide works in the principle of internal reflection, that is the light entering from one end is ‘guided’ along the pipe by internal reflections back and forth between the interior wall.


The next stage of the scintillation detection system is photomultiplier tube (PMT), an electron tube device that converts light into a measurable electric current. The basic components of a PMT are schematically represented in Fig. 3.20. It consists of a cathode made of photosensitive material, followed by an electron collection system, an electron multiplier section, and finally an anode from where the final signal is to be taken. All parts are usually housed in an evacuated glass tube. The phtocathode converts incident light into a current of electrons by the photoelectric effect. Efficiency of photoelectric conversion varies strongly with the frequency of the incident light and the structure of the material. The overall spectral response is expressed by the quantum efficiency, which is defined as the ratio of the umber of photoelectrons released to the number of incident photons on cathode. An equivalent quantity is the radiant cathode sensitivity, which is defined as





where I _k is the photoelectric emission current from the cathode and P(λ) is the incident radiant power. After emission from the photocathode, electrons in the PM are focused to the electron-multiplier system by an electron-optical input system. This focusing is accomplished through the application of an electric field in a suitable configuration.


The design of the electron-optical input system is based on the requirements of efficient collection of electrons i.e. as many electrons as possible must reach the electron-multiplier section regardless of the point of origin on the cathode. The electron-multiplier system amplifies the weak photocurrent by using a series of secondary electrodes or dynodes. The dynodes work in the principle of secondary electron emission, which is like photoelectric effect where photon is replaced by electron. The materials used for such purpose should have high secondary emission factor i.e. the average number of secondary electrons emitted per primary electron. Materials in common use today are Ag-Mg, Cu-Be, and Cs-Sb. Photomultiplier’s generally have a wide range of possible working voltages extending over a 1000 V or more for an optimized gain and pulse height of the output signal.

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Scintillation Detectors

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