What is the role of Thallium in NaI scintillators?

[prochatz]

Hello.

I was wondering what's the purpose of using Thallium (Tl) in Iodine Sodium (NaI) scintillators.

 

[Ed Aboud]

Sodium iodide (NaI) in its pure
state can detect gamma rays only at very low temperatures. By adding a small
amount of Thallium metal to the crystal (about 0.01%), the crystal can detect
gamma rays or nuclear particles at much higher temperatures.