Scintillation Detectors

Question 1

What is the basic principle of scintillation detectors?

Answer 1

Ionising radiation is absorbed

This is converted to optical photons (scintillator)

Photodetector measures signal (PMT, photodiode)

Question 2

What are the different types of scintillators?

Answer 2

Organic:
low Z and ρ
Inexpensive
fast

Inorganic:
Range of Z and ρ
Expensive
Good light output

Question 3

What is the composition of organic scintillators?

Answer 3

Covalent bonds of carbon, one of the carbon bonds are stuck together (rigid bond) and the other part of bond is delocalised (pi bond)

Question 4

What does absorption of KE of nearby charged particle cause the organic scintillators to do?

Answer 4

Causes pi bonds to be in an excited state and raised to higher state, eventually collapse down via fluorescence

They are raised to S_1x, S_2x, S_3x etc

Question 5

What happens when an electron is absorbed in an organic scintillators?

Answer 5

Go to higher energy levels

They will de-excite through non-radiative relaxation and give vibrations in the molecule

They de-excite to S1 level instantaneously

Question 6

What are the electron pairs in the lowest energy state?

Answer 6

Electron pairs in bond are anti aligned so one is spin up and other is spin down

Question 7

What happens when the absorbed electron is in S1 state?

Answer 7

They can collapse to ground state quickly via fluoresence: optical photon is emitted

They can also collapse to ground state via phosphorescence:
the electrons in pi bond have same spin so one of them goes to a higher energy level and overall the energy state of the pair is lower and they transition to triplet state

Question 8

What is the function of the waveshifter?

Answer 8

To absorb primary scintillant and reradiates at longer wavelength

added to liquid and plastic

Question 9

What is the purpose of the waveshifter?

Answer 9

To more closely match emission to sensitivity of photodetector

Question 10

What are inorganic scintillators made of?

Answer 10

Crystalline atomic structure which is encapsulated in aluminium can with diffuse reflective inner surface so outer atmosphere cannot enter

Question 11

What do inorganic scintillators depend on?

Answer 11

Electronic band structure

Question 12

What is the conduction band?

Answer 12

Where electrons are allowed to move through lattice

Question 13

What is the forbidden band?

Answer 13

Where electrons are not permitted in pure crystal

Question 14

What is the valence band?

Answer 14

Where the electrons are bound to lattice sites

Question 15

What is the scintillation process?

Answer 15

Absorption → elections excited to conduction band

Scintillation → emission following relaxation (de-excited photon)

Question 16

What is added to counteract the energy lost in the band gap?

Answer 16

Add an activator: small amount of impurity which creates special sites in the lattice with modified band structure

It causes smaller transitions so optical photons are produced

Question 17

What is the scintillation process with the activator?

Answer 17

Ionising radiation energy absorbed: electron promoted to conduction band and leaves hole in valence band

Hole migrates to activator site: quickly ionises activator because ionisation energy is lower than normal lattice site

Free electron combines with ionised activator: Electron move through lattice in conduction band until it encounters an ionised activator

Electron relaxes through activator states: emits optical photon during deexcitation back to valence band

Question 18

What happens in inorganic scintillator when the electron is captured in activator excited state?

Answer 18

Electron cannot transition to ground state

Energy is required to move electron to an excited state with permitted route to ground

One source of energy is thermal and optical photon is release (phosphorescence)

Question 19

What happens during quenching?

Answer 19

Electron captured at an activator site which has non-radiative de-exciation pathway

No optical photon is produced so not useful