DECK 8 — NaI(Tl) SCINTILLATION DETECTORS

What is a NaI(Tl) detector?

A NaI(Tl) detector is a scintillation detector that converts gamma energy into visible light through a thallium-doped sodium iodide crystal.

What is the role of thallium in NaI(Tl) crystals?

Thallium acts as an activator that creates energy levels enabling efficient light emission.

What is the scintillation process in NaI(Tl)?

The crystal absorbs radiation energy and re-emits it as visible photons.

Why does NaI(Tl) produce visible light during interaction?

Excited states in the thallium activator release energy as visible scintillation photons.

What is the purpose of the photomultiplier tube (PMT) in NaI detection?

The PMT converts scintillation light into electrons and amplifies the signal.

How does a PMT amplify signal?

The PMT uses a series of dynodes to multiply electrons by secondary emission.

Why do NaI(Tl) detectors have high gamma efficiency?

The high-Z and dense crystal increases the probability of gamma interactions.

Why do NaI(Tl) detectors have poor energy resolution?

Statistical fluctuations in light production and collection widen peaks.

Why is NaI(Tl) resolution worse than HPGe resolution?

Light production has large statistical spread, and the detector relies on PMT gain stability.

What determines the energy resolution of a NaI(Tl) detector?

Light yield, statistical fluctuations, PMT noise, and crystal quality determine resolution.

Why does NaI(Tl) exhibit a nonlinear light response at high energies?

Light yield does not scale perfectly with deposited energy due to activator saturation effects.

What spectral feature forms the Compton continuum in NaI(Tl)?

Photons that undergo Compton scattering and escape produce a continuum of pulse heights.

Why is the Compton continuum more pronounced in NaI(Tl) than in HPGe?

Lower resolution spreads Compton events over a broader energy range.

What causes the full-energy photopeak in NaI(Tl)?

The full-energy photopeak occurs when all gamma energy is absorbed through photoelectric interaction and subsequent cascades.

What creates backscatter peaks in NaI(Tl) spectra?

Photons scatter 180 degrees outside the detector and re-enter with reduced energy.

Why do NaI(Tl) detectors show escape peaks?

Pair production produces annihilation photons that may escape, reducing measured energy.

What energy corresponds to the single escape peak?

The single escape peak occurs at Eγ − 511 keV.

What energy corresponds to the double escape peak?

The double escape peak occurs at Eγ − 1022 keV.

Why does NaI(Tl) require optical coupling to the PMT?

Optical coupling ensures maximum transfer of scintillation light into the PMT photocathode.

Why must NaI(Tl) crystals be hermetically sealed?

NaI is hygroscopic and absorbs moisture, which destroys the crystal.

What happens to NaI(Tl) performance if moisture enters the crystal?

Light output decreases and resolution degrades.

Why is light collection uniformity important in NaI(Tl) detectors?

Non-uniform light collection leads to broadening of the energy peaks.

Why do NaI(Tl) detectors require reflective housing?

Reflective housing increases the number of scintillation photons reaching the PMT.

What causes afterglow in NaI(Tl) scintillators?

Some excited states decay slowly, releasing delayed scintillation photons.

Why can temperature changes affect NaI(Tl) performance?

Light yield and PMT gain vary with temperature.

Why is PMT high voltage important for NaI(Tl)?

Higher HV increases PMT gain and improves signal amplitude.

Why must PMT HV remain stable?

HV fluctuations change gain and shift energy calibration.

Why does peak position shift if PMT gain drifts?

Changes in gain alter pulse amplitude for the same deposited energy.

What defines intrinsic peak efficiency in NaI(Tl)?

The probability that gamma interactions in the crystal deposit full energy.

Why do larger NaI(Tl) crystals have higher full-energy peak efficiency?

Larger crystals increase the likelihood of capturing scattered photons.

What determines absolute detection efficiency in NaI(Tl)?

Crystal size, geometry, distance to source, and intrinsic efficiency.

Why is NaI(Tl) well suited for environmental gamma surveys?

High efficiency and moderate resolution allow rapid detection of gamma-emitting isotopes.

Why is NaI(Tl) not ideal for isotope identification requiring high precision?

Poor energy resolution makes closely spaced peaks hard to distinguish.