Inorganic scintillator detectors

Inorganic scintillation counters are conventional detectors for gamma-ray radiation (above $\sim$10 keV) measurements. Detailed description of these instruments can be found e.g. in Knoll (1989), more exhaustively in Birks (1964), and e.g. in Giacconi & Gursky (1974) for their application in X-ray astronomy. The basic mechanism consists in measuring the scintillation light produced by a ionizing high energy photon (or alpha or beta particle) interacting with the scintillating material. The most commonly used inorganic scintillators are the activated alkali-alidi crystals NaI(Tl) and CsI(Na or Tl). Conventionally, the element the element used to activate the crystal is indicated between parenthesis. A gamma-ray photon arriving on the detector deposits all or part of its energy in the material (see section 2.1) in the form of kinetic energy of one or more electrons, depending on the type and number of interactions. These electrons are able to excite to the conduction band other electrons which can be captured by a trace impurity (the activator) and cause transitions leading to the emission of visible light. The role of the activator is to generate meta-states between the pure crystal valence and conduction bands, so that an electron excited to the conduction band can drop in one of this meta-states and de-excite from it to the valence band. This has the advantages of being a more efficient mechanism with respect to the normal de-excitation from crystal conduction band and to lead to the emission of visible light photons, because of the lower energy of meta-states with respect to the conduction band. The scintillation light pulse is then collected through a light pipe (typically a quartz pipe) to a Photomultiplier Tube (PMT), which finally converts it to an electric signal to be amplified and measured. Since in principle a linear relation exists between the energy released by the photon in the crystal and the intensity of the light produced, to get spectral information on detected photons the associated electronics is designed to perform Pulse Height Analysis (PHA): the amplified signal is fed to a multi-channel analyzer which associates it to the relevant channel, according to the signal amplitude.

The relevant factors that contribute to the detector performances are:

• crystal transparency to its own emitted radiation (typically in the UV range)
• the number of photons emitted (light yield, usually measured in photons/keV) for each detected event and its proportionality to the incident photon energy (linearity). The maximum efficiency in converting initial electron energy into photons is $\sim$13% for NaI(Tl). The NaI and CsI light yields are not exactly linear with energy, decreasing with increasing incident photon energy and are affected by temperature changes;
• decay time of the induced scintillation, determined by the decay time of the meta-state involved in the scintillation, limiting the detector temporal resolution;
• scintillation light collection efficiency, which depends on the geometry of the detector, on crystal coating and on the position in the detector in which the interaction with the incident photon takes place;
• crystal/PMT coupling; in particular the crystal refraction index should be as close as possible to that (1.5) of the glass for efficient coupling with the PMT;
• PMT quantum efficiency (typically 10-15%);
• PMT gain (typically from 10⁵ to 10⁸) and its stability. The potential needed to bias the PMT photocathode and the succeeding multiplier stage is provided by an High Voltage supply (HV) and is of the order of 1000 Volts. HV stability affects PMT gain stability.

The physical and mechanical properties of the material are also important. It should be manufacturable in sizes large enough to be of interest for practical detectors.

Although they have poorer energy resolution (see chapter 3) than gas proportional counters, the use of scintillators is preferable (unavoidable above 100 keV) because of their much higher detection efficiency (100% up to $\sim$100 keV for NaI and CsI), due to the high Z of their atoms and their density three orders of magnitudes greater than those of the gas in a typical proportional counter.

In Tab.1.2 the main characteristics of the NaI(Tl), CsI(Na) and CsI(Tl) scintillators, the most widely used in X and gamma-ray astronomy are reported.

 

NaI(Tl) CsI(Na) CsI(Tl)

Light output (% relative to NaI(Tl) 100 85 45

Wavelength of maximum emission (nm) 410 420 565

$$\mu$$ 0.23 0.63 1.0

Density (g/cm³) 3.67 4.51 4.51

Refraction index 1.85 1.84 1.80