Basic Properties of Scintillators

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Stopping Power

One of the fundamental properties of scintillators is the so-called stopping power, which is meant to provide information about the efficiency with which a crystal ‘stops’, absorbs photons. The higher atomic number and density a scintillator has, the better its stopping power is.

Atomic number dependence of the photoelectric fraction

 
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. Time dependence proceeding of the scintillation light impulse phenomenon is presented on the figure below.

Scintillation light impulse (from the Saint-gobain manufacturer site)

 
Hygroscopicity

A considerable amount of scintillation materials /e.g. NaI(Tl), LaBr(Ce)/ can absorb water vapour present in air, which can have damaging effects -> hygroscopic phenomenon. 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 size. 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.

SPECT és PET szcintillátorok technikai szempontjai

*Some dedicated SPECT system (McSPECT detector system /see chapter 3.3.1/, [12], [13], [14], [15]) uses modular structure. More and more dedicated SPECT systems may use for future either modular structure or pixellized crystal. Till nowaday majority of the general purpose Large Field Of View (LFOV) multi-detector based SPECT systems are made 1 large crystal / detector.