The Process of Scintillation, Types of Scintillators

3.2.1.1 The Process of Scintillation, Types of Scintillators

The gamma photons arriving at the detector generate high-energy free electrons by photoelectric effect or Compton scattering, which excite electrons until they give off their energy. The relaxation of the excited electrons is the process that produces visible light signals. Parallel to this, several other processes occur with a lower probability, which appear as noise in the signal. Examples include Bremsstrahlung, the emission of Auger electrons, electron-electron collisions or the relaxation of energy in nonradiative transitions. The electrons excited in the basic process get into so-called luminescence centres, from where they relax and emit light, hence the name.

172

Types of scintillators:

Fundamentally there are two types of scintillators based on whether the existence of luminescence centres is the property of a certain material (intrinsic scintillators), or if the luminescent centres appear due to the addition of some sort of contamination (extrinsic scintillators).

An example of the inorganic intrinsic scintillators is the so-called BGO: Bi4Ge3O12 – Bi3+.

Extrinsic (or contaminated) scintillators include:

 NaI:Tl-Tl+: very widespread due to the fact that they can be produced easily and at a low price

 CsI:Na-Na+

 Ce3+: its advantage is the fact that transition (relaxation) happens very fast, many photons are emitted, so its energy resolution is fairly good

The shape of a light pulse in the scintillator depends mainly on the mobility of charge carriers within the particular material.

173

If the stopping power is high, the so-called absorption length is shorter, thus shorter crystals can be produced, which allows for a more accurate position determination, and the production of the crystal is cheaper as well. Absorption length is the distance through which intensity drops to 1/e of its initial value.

It is also important to know whether the photoelectric effect or the Compton effect occurs with higher probability in the given crystal, since, as mentioned previously, the photoelectric effect is needed to cause scintillation, the Compton scattered photons appear as noise.

Light yield

The light yield is the number of visible photons emitted per unit of gamma energy in the particular scintillation crystal. This factor fundamentally affects the signal-to-noise ratio and the energy resolution of the detector. In a scintillation flash a few thousand to a few ten thousand photons occur on average.

Energy resolution

The energy resolution is practically the widening of the photopeak. The widening is dependent on the material of the scintillator and it is highly affected by the light yield (the goodness of the statistics), the nonlinearity (the trueness of the assumption that two times as high gamma energy produces two times as many photons) and the inhomogeneity of the scintillator. It is important to know that the energy resolution, which is given on the data sheet, should be considered as a value that implies the effect of electronic signal processing.

Emission spectrum

The emission spectrum is the wavelength of the scintillation photons emitted in the visible range, which is between 350-550 nm on average. It has to be in a range that is transparent for optical materials because the light conductor, the window of the PMT (photomultiplier tube) and other components of the detector are made of such materials. Fortunately PMTs are available for a very wide range of the spectrum, so this requirement does not reduce the group of scintillators significantly.

174 Fall time

The light pulse produced during scintillations falls exponentially with time. The shorter the fall time of the scintillator is, the better. Fall time is defined as the time interval during which 63% of the energy of the pulse is emitted. Fall time ranges from 10 ns to a few microseconds.

Rise time

In essence rise time is a less important property of the scintillator, it is much more significant in TOF (time of flight)-PETs. This value is characteristic of the rise time (the time interval during which the amplitude rises from 10% of the maximum to 90%) of the light pulse following the absorption of the gamma photon.

Hygroscopicity

A considerable amount of scintillation materials can absorb water vapour present in air, which can have damaging effects. The most typical symptom of the damage is the fact that the crystal turns yellow, thus conditions can become unfavourable as far as light emission is concerned. Consequently, it is very important to protect scintillation crystals from humidity.

This is a much easier task in case of larger crystals (e.g. in NaI gamma cameras) than in scintillators that consist of many small crystal needles (e.g. used in PET). In case of pixelated crystals the fill factor of the crystal matrix also deteriorates considerably owing to the fact that every single crystal has to be protected from humidity.

Background radiation

Some of the scintillators have background radiation as well, which appears as noise in the signal of the detectors. Considering this, it would seem reasonable to try to avoid using materials that have background radiation as scintillator materials; however, there are materials that improve other scintillation properties to such a considerable extent that despite the fact that they have background radiation they are worth applying as scintillation crystals.

An example is lutetium (Lu) that emits beta radiation (thus increasing noise and dead time), still, it is used in LSO and LYSO crystals.

Manufacturing, mechanical properties, refractive index

Single crystals are usually grown from melt, the melting point of the base material of the particular crystal highly affects the price. The maximum size of the crystal that can be grown is important to know. It is also essential to examine how mechanically resistant the scintillator is, how it responds to gamma rays, how easily workable it is and what kind of thermal expansion properties it has (it is important for easy installation). The refractive index of the crystal is significant because of the optical coupling, the reflection of the photons emitted in the visible range off the surface of the crystal has to be as low as possible.

175 Technical aspects of SPECT and PET scintillators