Scintillation Crystals

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Scintillator Properties

A large number of different scintillation crystals exist for a variety of applications. Some important characteristics of scintillators are:

Afterglow

To detect fast changes in transmitted intensity of X-ray beams, as e.g. in CT scanners or luggage X-ray detectors, crystals are required exhibiting extremely low afterglow. Afterglow is defined as the fraction of scintillation light still present for a certain time after the X-ray excitation stops. Afterglow originates from the presence of millisecond to even hour long decay time components. Afterglow in most halide scintillation crystals can be as high as a few percent after 3 ms. The long duration afterglow in e.g. CsI(Tl) can be a problem for many applications. Afterglow in halides is believed to be intrinsic and correlated to certain lattice defects. BGO and Cadmium Tungstate (CdWO4) crystals are examples of low afterglow scintillation materials.

Neutron Detection

Neutrons do not produce ionization directly in scintillation crystals, but can be detected through their interaction with the nuclei of a suitable element. In a 6 LiI(Eu) scintillation crystal for example, neutrons interact with 6Li nuclei to produce an alpha particle and a triton (tritium nucleus), which both produce scintillation light that can be detected. For this purpose, also enriched 6Li containing glasses can be used, doped with Ce as activator.

Radiation Damage

Radiation damage is defined as the change in scintillation characteristics caused by prolonged exposure to intense radiation. This damage manifests itself by a decrease of the optical transmission of a crystal which causes a decrease in pulse height and deterioration of the energy resolution of the detector. Radiation damage other than radio-activation is usually partially reversible; i.e. the absorption bands disappear slowly in time. In general, doped alkali halide scintillators such as NaI(Tl) and CsI(Tl) are rather susceptible to radiation damage. All known scintillation materials show more or less damage when exposed to large radiation doses. The effects usually can only be observed clearly in thick (> 5 cm) crystals. A material is usually called radiation hard if no measurable effects occur at a dose of 10.000 Gray. Examples of radiation hard materials are CdWO4 and GSO.

Emission Spectra of Scintillation Crystals

Each scintillation material has a characteristic emission spectrum. The shape of this emission spectrum is sometimes dependent on the type of excitation (photons / particles). This emission spectrum is of importance when choosing the optimum readout device (PMT /photodiode) and the required window material. Fig. 3.1 and 3.2 show the emission spectrum of some common scintillation materials.

Temperature influence

Temperature Influence on the Scintillation Response

The light output (number of photons per MeV gamma) of most scintillators is a function of temperature. This is caused by the fact that in scintillation crystals, radiative transitions, responsible for the production of scintillation light. compete with non-radiative transitions (no light production). In most scintillation crystals, the light output is quenched (decreased) at higher temperatures. An example of the contrary is the fast component of BaF2 of which the emission intensity is essentially temperature independent..

The scintillation process usually involves as well production, transport and quenching centers. Competition between these three processes each behaving differently with temperature, causes a complex temperature dependence of the scintillation light output. Fig. 3.3 shows the temperature dependence of some common scintillation crystals.

For applications such as oil well logging and space research, where it is very difficult to control the temperature, this dependence should be taken into account. The doped scintillators NaI(Tl), CsI(Tl) and CsI(Na) show a distinct maximum in intensity whereas many undoped scintillators such as BGO show an increase in intensity with decreasing temperature.

From Fig. 3.3 it can be seen that at above 120 oC, NaI(Tl) has the highest light output followed by CsI(Na) and CsI(Tl). For lower temperatures CsI(Na) is a good alternative to NaI(Tl) because it has better mechanical characteristics

Which Scintillator For Your Application?

Which Scintillator For Your Application?

When we observe Table 3.1 carefully, it is clear that none of presently known scintillation crystals possesses all the above mentioned (ideal) characteristics such as high density, fast decay etc. The choice of a certain scintillation crystal in a radiation detector depends strongly on the application. Questions such as :

  • What is the energy of the radiation to measure ?
  • What is the expected count rate ?
  • What are the experimental conditions (temperature, shock)?

are very important in this respect to determine the optimum choice.

Table 3.2 presents an overview of the most commonly used scintillation materials with some of their specific applications.

MATERIAL

IMPORTANT PROPERTIES

MAJOR APPLICATIONS

NaI(Tl) Very high light output, good energy resolution General scintillation counting, health physics, environmental monitoring, high temperature use
CsI(Tl) Noon-hygroscopic, rugged, long wavelength emission Particle and high energy physics, general radiation detection, photodiode readout, phoswiches
CsI(Na) High light output, rugged Geophysical, general radiation detection
CsI(undoped) Fast, non-hygroscopic, radiation hard, low light output Physics (calorimetry)
CaF2(Eu) Low Z, high light outut

b detection, a, b phoswiches

6LiI(Eu) High neutron cross-section, high light output Thermal neutron detection and spectroscopy
6Li – glass High neutron cross-section, non-hygroscopic Thermal neutron detection
BaF2 Ultra-fast sub-ns UV emission Positron life time studies, physics research, fast timing
YAP(Ce) High light output, low Z, fast MHz X-ray spectroscopy, synchrotron physics
GSO(Ce) High density and Z, fast, radiation hard Physics research
BGO High density and Z Particle physics, geophysical research, PET, anti-Compton spectrometers
CdWO4 Very high density, low afterglow, radiation hard DC measurement of X-rays (high intensity), readout with photodiodes, Computerized Tomography (CT)
Plastics Fast, low density and Z, high light output Particle detection, beta detection

Medical probe with fiber optics light guide and built-in HV supply