Gamma Energy Lab

Question 1

Why are our scintillator crystals doped with thallium?

Answer 1

• to control the wavelength of the emitted scintillations (specifically to 3 eV blue light wavelengths)
• undoped NaI would emit a wavelength that the crystal is not transparent to

Question 2

What is between the scintillator crystal and photocathode layer?

Why?

Answer 2

light coupling liquid

**- **because without it the two materials would not connect directly and the air between them would cause total internal reflection of the scintillations

Question 3

How can the number of secondary electrons be controlled in a PMT?

And how can this number be calculated?

Answer 3

via increasing or decreasing the anode voltage

• total # of secondary e^-s is the number of secondary e^-​s per dynode to the power of the number of dynodes

Question 4

What is the process of scintillation creation in an NaI (Tl) crystal?

Answer 4

1. a primary electron with high E_kin is formed via photoeffect or Compton scatter
2. the primary electron slows down as it collides with other atoms of the crystal and loses energy
3. these collisions excited secondary electrons of the crystal material from the valence to the conduction band
4. excited secondary electrons relax (through the Tl-doped energy levels) to create 3 eV blue scintillations