PHYS 352 Radiation Detectors II: Scintillation Detectorsphys352/lect19.pdf · photoluminescence, chemiluminescence, triboluminescence) ... •the scintillation mechanism requires

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

electron

hole

exci

tatio

n

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)

CsI(Tl) from BaBarRoma group

typical NaI(Tl) detectorin Queen’s undergraduate lab

BGO from Shanghai Institute of Ceramics

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 LHCfrom Wikipedia

Comparison of Inorganic Crystalsfrom Particle DataGroup, Review ofParticle Detectors

CaF2(Eu) 3.18 940 435 1.47 50 noCdWO4 7.9 14000 475 2.3 40 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

• no S2-S0 emission; internally de-excite in picoseconds (non-radiatively)

excited triplet state can’t decay to ground state (angular momentum selection rules)→ results in delayed fluorescence and phosphorescence

Absorption and Emission Spectra

• mirror images of each other

• individual vibrational states are thermally broadened and smear together

Excitation Fluorescence and Phosphorescence in Organic Scintillator 5.

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

• 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

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

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

4-16

General Properties of Scintillation Detectors

• scintillator (inorganic crystal, plastic, liquid) must be optically coupled 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 PMT might not be required

• acrylic light guides are sometimes used to couple scintillator to photomultiplier tube – work by total internal reflection

SCINTILLATION DETECTORS particle energy converted to visible light in a scintillating material light sensed by photomultiplier tube and converted into electrical pulse

light

dynodes

vacuum

PHOTOMULTIPLIER SCINTILLATOR

electrons

e-electrical pulse anode

- ray

Glass envelope Reflector

Ideal Scintillator Properties 1. Convert energy deposited into light (visible & near UV) with high

efficiency 2. Conversion should be linear (light energy absorbed) 3. Scintillator medium should be transparent to its own emissions 4. Decay time of induced luminescence should be short 5. Good optical quality - large detectors 6. Index of refraction near that of glas (~1.5) for good optical coupling

to photomultiplier compromises usually required

Practical Scintillators

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

• 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

PMMA first claddingfluorinated PMMA second cladding

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.

• 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

Ar2*