1. Scintillators

2. Scintillation Detector
Scintillation detectors are widely used to measure radiation.
The detectors rely on the emission of photons from excited states.
Counters
Calorimeters
An incident photon or particle ionizes the medium.
Ionized electrons slow down causing excitation.
Excited states immediately emit light.
Emitted photons strike a light-sensitive surface.
Electrons from the surface are amplified.
A pulse of electric current is measured.

3. Energy Collection
Counters need only note that some energy was collected.
For calorimetery the goal is to convert the incident energy to a proportional amount of light.
Losses from shower photons
Losses from fluorescence x-rays

4. Compton Peak
For incident photons, Compton scattering transfers energy to electrons.
This is an important effect for photon measurement below a few MeV.
The recoil energy:
Has a maximum at q = 180°:
For photons in keV:

5. Photon Statistics Typical Problem
Gamma rays at 450 keV are absorbed with 12% efficiency. Scintillator photons with average 2.8 eV produce photoelectrons 15% of the time.
What is the energy to produce a measurable photoelectron?
How does this compare to a gas detector (W-value)?
Answer
The total energy of scintillation is 450 x 0.12 = 54 keV.
5.4 x 104 / 2.8 = 1.93 x 104 photons produced
1.93 x 104 x 0.15 = 2900 photoelectrons produced
The equivalent W-value for the scintillator is:
450 keV/2900 = 155 eV/pe
W-value in gas = 30 eV/ip

6. Inorganic Scintillators
Fluorescence is known in many natural crystals.
UV light absorbed
Visible light emitted
Artificial scintillators can be made from many crystals.
Doping impurities added
Improve visible light emission

7. Band Structure
Impurities in the crystal provide energy levels in the band gap.
Charged particles excites electrons to states below the conduction band.
Deexcitation causes photon emission.
Crystal is transparent at photon frequency.
conduction band
impurity excited states hn
impurity ground state
valence band

8. Jablonski Diagram
Jablonski diagrams characterize the energy levels of the excited states.
Vibrational transitions are low frequency
Fluoresence and phosphoresence are visible and UV
Transistions are characterized by a peak wavelength lmax.

9. Time Lag Fluorescence typically involves three steps.
Excitation to higher energy state.
Loss of energy through change in vibrational state
Emission of fluorescent photon.
The time for 1/e of the atoms to remain excited is the characteristic time t.
10-12 s S1
10-15 s
10-7 s S0

10. Crystal Specs Common crystals are based on alkali halides
Thallium or sodium impurities
Fluorite (CaF2) is a natural mineral scintillator.
Bismuth germanate (BGO, Bi4Ge3O12) is popular in physics detectors.
Crystal t (ns) lmax(nm) output
NaI(Tl)
CsI(Tl)
CsI
ZnS(Ag)
CaF2(Eu)
BGO

11. Tracking Detector
Iarocci tubes used in tracking are arranged in layers.
Hits in cells are fit to a track.
Timing converted to distance from wire
Fit resolves left-right ambiguity

12. Organic Scintillators
A number of organic compounds fluoresce when molecules are excited.
The benchmark molecule is anthracene.
Compounds are measured in % anthracene to compare light output
absorption
emission
R. A. Fuh 1995

13. Pi-Bonds Carbon in molecules has one excited electron.
Ground state 1s22s22p2
Molecular 1s22s12p3
Hybrid p-orbitals are p-orbitals.
Overlapping p-orbitals form bonds
Appears in double bonds

14. Excited Rings p-bonds are most common in aromatic carbon rings.
Excited states radiate photons in the visible and UV spectra.
Fluorescence is the fast component
Phosphorescence is the slow component
At left: π-electronic energy levels of an organic molecule. S0 is the ground state. S1, S2, S3 are excited singlet states. T1, T2, T3 are excited triplet states. S00, S01, S10, S11 etc. are vibrational sublevels.

15. Plastics
Organic scintillators can be mixed with polystyrene to form a rigid plastic.
Easy to mold
Cheaper than crystals
Used as slabs or fibers

16. Transmission Quality
Scintillator is limited by the transmission efficiency.
It’s not clear
The attenuation length cannot be too long for the application.

17. Liquids
Organic scintillators can be mixed with mineral oil to form a liquid.
Circulate to minimize radiation damage
Fill large volume

18. Waveshifter
Photons from scintillators are not always well matched to photon detectors.
Peak output in UV-blue
Peak detection efficiency in green light.
Wavelength shifting fibers have dyes that can absorb UV and reemit green light.
Fibers can be bent to direct light to detectors.