NaI(Tl) physical properties

NaI(Tl) is the most commonly used inorganic scintillator. It is very cost effective, has good stopping power and nowadays delivers quite good energy resolution. The material responds to γ-rays with a short rise time of less than 5 ns and an exponentially subsiding light pulse with a decay time of 230 ns at room temperature. The short decay time makes it suitable for pulse count rates in excess of 0.5 million counts per seconds (0.5 Mcps). And the eMorpho has been designed to support this speed.

In practice, many NaI(Tl) crystals exhibit a small-amplitude slow component lasting some 4 to 6 μs. For high precision applications this may limit the practical count rate to about 0.25 Mcps, which is still an impressive count rate.

The single one disadvantage of this material is its susceptibility to mechanical and thermal shock. Large single crystals, substantially bigger than 3-inches in any dimension, may cleave when subject to shock. When that happens, the crystal is rendered useless for spectroscopy applications. However, modern heat-fused polycrystalline material is less susceptible to this risk.

Characteristic	Value
Density	3.67 g/cm³, 2.12 oz/inch³

Atomic weight	149.894 g/mol
Molecular density	0.02448 mol/cm³, 1.475 10²² molecules/cm³
Refractive index	1.80

Features

Plus:	High stopping power Good energy resolution (<7.0% fwhm @ 662 keV) Low cost High speed
Minus:	Large crystals may cleave when subject to mechanical or thermal shock.

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NaI(Tl) Pulse Shape

Figure 1: The graph to the left shows the NaI(Tl) light pulse shape as measured using a 60 MHz, 10-bit eMorpho. The rise time appears slower than the true light pulse rise time of 5 ns because of the antialiasing filter integrated with the eMorpho. The 230 ns exponential decay time, however, is reproduced accurately.

Figure 2: This is a plot of the integrated charge of the electronic image of the light pulse vs. integration time. The obvious expectation is that collecting all the light, by integrating over all of the pulse, would yield the best energy resolution. In practice, integrating over the tail end of the pulse will be counter productive due to noise and ADC digitizing errors.

For NaI(Tl) we find that integrating over 1 μs produces good energy resolution, while integrating over 2 μs yields the best energy resolution.

Features

Pulse rise time:	5 ns for the light pulse 3 sampling clock periods for the electronic image, due to the antialiasing filter
Decay time:	230 ns at room temperature Faster at higher temperatures
Slow component:	Lasts 4 to 6 μs Carries about 5% of the charge
Light collection:	50% in 0.25 μs 90% in 1.0 μs

NaI(Tl) Non-proportionality

All inorganic scintillators are non-proportional. This means that the amount of scintillation light generated in response to energy deposited by a γ-ray is not strictly proportional to the deposited energy. Aside from photo-electron statistics, and especially at energy deposits beyond 1 MeV, this is a limiting factor for the energy resolution any scintillator can achieve.

One way to quantify this phenomenon is to compute the apparent brightness of the scintillator as a function of deposited energy. Here we measure brightness in units of PMT anode pulse charge vs deposited energy. We arbitrarily define the value at 662 keV as 100% and quantify the non-proportionality by how much the charge-to-energy ratio deviates from the pivot point at 662 keV.

Figure 3: Non-proportionality as measured in a 3-inch NaI(Tl) crystal. The general trend is that the NaI(Tl) crystal appears brighter at lower γ-ray energies and loses luster as the γ-ray energy increases. Note that this non-proportionality is a function of the γ-ray energy, not the total deposited energy. Cascade sums are not subject to scintillator non-proportionality.

⁶⁰Co is the perfect example for this phenomenon. Place a 1 μCi (37 kBq) ⁶⁰Co-source right against the front face of a 3-inch radiation sensor. The excited daughter of ⁶⁰Co emits an 1172 keV and a 1333 keV γ-ray in coincidence. When both are fully absorbed in the scintillator, the sum signal should precisely match the formula 1172 keV + 1333 keV = 2505 keV.

Note that the non-proportionality is mostly, but not exclusively a characteristic of the material. Especially at the low energy end the effect is more pronounced in small crystals due to geometry and light collection effects.

Summary

Non- prop.:	Light output per deposited energy decreases with γ-ray energy.
Cascades:	Cascade, ie coincidence, summing is not affected by this phenomenon.

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