Scintillation Detectors Flashcards
What is the basic principle of scintillation detectors?
Ionising radiation is absorbed
This is converted to optical photons (scintillator)
Photodetector measures signal (PMT, photodiode)
What are the different types of scintillators?
Organic:
low Z and ρ
Inexpensive
fast
Inorganic:
Range of Z and ρ
Expensive
Good light output
What is the composition of organic scintillators?
Covalent bonds of carbon, one of the carbon bonds are stuck together (rigid bond) and the other part of bond is delocalised (pi bond)
What does absorption of KE of nearby charged particle cause the organic scintillators to do?
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
What happens when an electron is absorbed in an organic scintillators?
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
What are the electron pairs in the lowest energy state?
Electron pairs in bond are anti aligned so one is spin up and other is spin down
What happens when the absorbed electron is in S1 state?
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
What is the function of the waveshifter?
To absorb primary scintillant and reradiates at longer wavelength
added to liquid and plastic
What is the purpose of the waveshifter?
To more closely match emission to sensitivity of photodetector
What are inorganic scintillators made of?
Crystalline atomic structure which is encapsulated in aluminium can with diffuse reflective inner surface so outer atmosphere cannot enter
What do inorganic scintillators depend on?
Electronic band structure
What is the conduction band?
Where electrons are allowed to move through lattice
What is the forbidden band?
Where electrons are not permitted in pure crystal
What is the valence band?
Where the electrons are bound to lattice sites
What is the scintillation process?
Absorption → elections excited to conduction band
Scintillation → emission following relaxation (de-excited photon)
What is added to counteract the energy lost in the band gap?
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
What is the scintillation process with the activator?
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
What happens in inorganic scintillator when the electron is captured in activator excited state?
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)
What happens during quenching?
Electron captured at an activator site which has non-radiative de-exciation pathway
No optical photon is produced so not useful
