PHYS 352 Radiation Detectors II: Scintillation Detectors

Scintillation • the physics definition of scintillation: the process by which ionization produced by charged particles excites a material causing light to be emitted during the de-excitation • one of the most common detection techniques for nuclear radiation and particles • earliest use by Crookes in 1903 – a ZnS-coated screen scintillates when struck by α particles • Curran and Baker in 1944 – coated a photomultiplier tube with ZnS producing the first scintillation counter that didn’t require the human eye • the scintillation process differs in different materials (e.g. inorganic crystals, organic liquids, noble gases and liquids, plastic scintillators); we’ll briefly examine each type…

More Definitions • when you excite a material (not thermally) and it subsequently gives off light, that is luminescence • how it’s excited determines the type of luminescence (e.g. photoluminescence, chemiluminescence, triboluminescence) • fluorescence is photoluminescence or scintillation (i.e. excitation produced by ionizing radiation) that has a fast decay time (ns to μs) • phosphorescence is the same, only with a much slower decay time (ms to seconds)

Stokes Shift • an important, general concept to keep in mind for all scintillators • emitted photons are at longer wavelengths (smaller energies) than the energy gap of the excitation – called the “Stokes shift” • the processes that produce the Stokes shift are different in different scintillating materials • this allows the scintillation light to propagate through the material • emitted photons can’t be self-absorbed by exciting the material again from Wikipedia

Characteristics of Different Scintillators • light yield: high efficiency for converting ionization energy to light output [photons/MeV] • emission spectrum: best if it overlaps with spectral response of light detector (e.g. PMT or photodiode have different spectral range of peak sensitivity) • decay time: how long it takes the excited states to de-excite and give off light • can be different for alphas and betas • because depends on ionization density • density and Z: determine response to γ, e− and other electromagnetic processes

Scintillation in Inorganic Crystals (e.g. sodium iodide, NaI crystals) • the scintillation mechanism requires the crystal band structure; you can’t dissolve NaI in water or melt these crystals and get scintillation • most are impurity activated • luminescence centres are associated with the activator sites in the lattice • interstitial, substitutional, excess atoms, defects

excitation

electron

hole

Doped versus Exciton Luminescence in Crystals • for doped crystals: the decay time primarily depends on the lifetime of the activator excited state • examples of doped crystals: NaI(Tl), CsI(Tl), CaF2(Eu)

• in crystals with exciton luminescence: e-h pairs stay somewhat bound to each other forming an exciton • the exciton moves together in the crystal • impurities or defects (w/o activator) → site for recombination • example of exciton luminescence: BGO (bismuth germanate Bi4Ge3O12)

BGO from Shanghai typical NaI(Tl) detector Institute of Ceramics in Queen’s undergraduate lab

CsI(Tl) from BaBar Roma group

Self-Activated Scintillating Crystals • chemically pure crystal has luminescence centres (probably interstitial) due to stoichiometric excess of one of the constituents • example: PbWO4 and CdWO4 • extra tungstate ions are the activator centres

PbWO4 crystals for the CMS ECAL at the LHC from Wikipedia

Comparison of Inorganic Crystals from Particle Data Group, Review of Particle Detectors

CaF2(Eu) 3.18 CdWO4 7.9

940 14000

435 475

1.47 2.3

50 40

no no

Comparison of Emission Spectra from Different Inorganic Crystals

light yield compared to NaI(Tl) from previous table is over the spectral response range of bi-alkali PMT  some crystals emit at longer wavelengths and are better matched to Si photodiode spectral response  e.g. CsI(Tl) with a photodiode would be 145% of NaI(Tl)

Organic Scintillators • the scintillation mechanism is determined by the chemistry and physics of the benzene ring • an organic scintillator will thus scintillate whether it’s in a crystal form, is a liquid, a gas, or imbedded in a polymer • all organic scintillators in use employ aromatic molecules (i.e. have a benzene ring) • chemical bonds in the benzene ring: • σ-bonds are in the plane, bond angle 120°, from sp2 hybridization • π-orbitals are out of the plane; they overlap and the π-electrons are completely delocalized

benzene from Wikipedia

Scintillation in Organic Molecules • after absorption of a photon or excitation by ionization, the molecule undergoes vibrational relaxation to S10 • the excited S10 state decays radiatively to vibrational sub-levels of the ground state; the S10 lifetime is ~ns • thus the fluorescence emission spectrum is roughly a “mirror image” of the absorption spectrum (same spacing) • emitted photons have less energy than S00-S10 – that’s the important Stokes shift

excited triplet state can’t decay to ground • no S2-S0 emission; internally de-excite in state (angular momentum selection rules) → results in delayed fluorescence and picoseconds (non-radiatively) phosphorescence

Absorption and Emission Spectra • mirror images of each other • individual vibrational states are thermally broadened and smear together

Types of Organic Scintillator

single crystal anthracene

liquid scintillator

plastic scintillator

Comparison: Organic versus Inorganic Scintillator •

137Cs

spectrum in NaI crystal

same in plastic scintillator

Figure 4.13: Cs-137 spectrum taken with 2m X-Plank.

• the photopeak is not seen with plastic scintillator; that’s the Compton edge • organic scintillators: low density; just C and H; photoelectric effect goes as Z4

General Properties of Scintillation Detectors

Figure 4.14: Na-22 spectrum taken with 2m X-Plank.

• scintillator (inorganic crystal, plastic, liquid) must be optically4-16coupled to the light detector • very frequently a photomultiplier tube is used though photodiodes (or avalanche photodiodes) have been used also as the light sensor • when there is a lot of scintillation light, the high gain, high sensitivity of a SCINTILLATION DETECTORS PMT might not be required x particle energy converted to visible light in a scintillating material

• acrylic light guides are sometimes used to couple scintillator to x light sensed by photomultiplier tube and converted into electrical photomultiplier tube – work by total internal reflection pulse SCINTILLATOR J - ray

PHOTOMULTIPLIER dynodes

elight

electrons

vacuum Reflector

Glass envelope

anode

electrical pulse

Energy Resolution of Scintillation Detectors • Poisson statistics of the number of scintillation photons emitted per MeV energy deposited • NaI is about 25 eV per photon (40,000 photons/MeV) • plastic scintillator is about 100 eV per photon (10,000 photons/MeV) • Fano factor for scintillators F = 1 • unlike gas detectors and semiconductor detectors (we will study those next) where F < 1 and the resolution is better than sqrt(N) • hand-waving reason: the ionization energy and excitation energy produced by the interactions degrade almost continuously down to the main excited state via coupling to vibrational states (in the molecule or to phonons in the crystal lattice) so it is quite a good approximation that there is a single independent statistical quantity that determines the average energy required to be deposited to get one scintillation photon emitted

When to Use Inorganic? When to use Organic? • for spectroscopy, use inorganic (or better still semiconductor) • NaI has high density, high Z, good photopeak • for timing applications, use organic • most common inorganic scintillating crystals have longer decay times than organic scintillators which have scintillation lifetimes of ~few ns • e.g. for cosmic ray detector, use sheets of plastic or tanks of liquid to cover a large area cheaply (compared to many crystals) since you might not care about energy resolution since a cosmic ray crossing your detector deposits a known amount of energy but rather care when the cosmic ray hit your detector • or use time-of-flight spectrometry to determine a particles mass/ momentum/energy • for neutron detection, having C and H is favourable hence use an organic

Scintillating Fibres • the fibre core is • glass with activator

PMMA first cladding fluorinated PMMA second cladding

• plastic scintillator • can take a bundle of scintillating fibres and observe which fibre lights up when radiation interacts or a particle traverses the bundle

polystyrene core fibres from Saint-Gobain Crystals

glass fibers from Pacific Northwest National Lab

Scintillation in Noble Gases/Liquids • scintillation mechanism is again different • noble gases/liquids are monatomic but excited atoms can form dimers (excited dimer or excimer) • e.g.

Ar2*

• the excited dimer is either in a singlet or triplet state • singlet state is fast (6 ns for argon) • triplet state is slow (1.6 μs for argon) • it decays by photon emission with photon energy less than what’s needed to excite the monomer – Stokes shift • hence, transparent to its own scintillation light • high light yield: e.g. 40,000 photons/MeV for argon, 50,000 photons/MeV for xenon