This type of scintillator material is the best one that an hobbist could afford. At the moment NaI(TI) crystals are easy to find on eBay from russian or ukrainian sellers. Take care that NaI(TI) are highly higroscopic and if they absorb too much water they turn color to yellow than brown and finaly their light output drop to unusable level. As the majority of the sintillation materials, they are used coupled with a photomultiplier tube for Gamma Spectroscopy.
Some pics of my crystal and test with UV. It’s light output is huge!!! Shine so bright with some little UV from an UV led.