SCINTILLATION DETECTORS AND PM TUBES
General Characteristics
Introduction Luminescence Light emission without heat generation Scintillation Luminescence by radiation Scintillation detector Radiation detector using scintillation mechanism of radiation
Scintillator Absorbs radiation energy and then emits visible light Photocathode Absorbs light and then emits photo-electron Material: Cs-Sb, Ag-Mn PM (photomultiplier) tube Electron from photocathode is amplified
Scintillator Materials which absorb radiation energy and then emit visible light State: (1) solid, (2) liquid, and (3) gas Divided into (1) organic and (2) inorganic materials Light output is generally expressed relatively by light output of anthracene (100%) Mechanism of light emission Mechanisms of light emission are a little different between organ and inorganic scintillator Commonly speaking, excited electron by radiation when the excited electron goes down to the ground state it emits light
Scintillation detector Detect radiations by lights from scintillator Measure radiation energy by light outputs Properties of an ideal scintillator High scintillation efficiency for converting charge-particle kinetic energy into detectable light This conversion should be linear (radiation energy vs. light output) Minimum self-absorption of the emitted light Fluorescence should exhibit a fast decay time Material should be subject to manufacture in large sizes. Index of refraction should be that of glass (~1.5) to permit optical coupling with the PM tube.
2 General Classes of Scintillation Detectors 1. Organic crystals Low light output Fast response time can be used for higher dose rate, higher flux Used in β spectroscopy and fast neutron detection (low z) (eg) Anthracene, stilbene, plastic scintillator, liquid scintillator 2. Inorganic crystals Good light output and linearity Slow response time Used in γ-spectroscopy (high z) (eg) NaI(Tl), CsI(Tl), ZnS(Ag), LiI(Eu), BGO
Organic Scintillators
Scintillation Process Characteristics of scintillation process of organic scintillators No activator (c.f., inorganic scintillators need activator) By excitation of energy levels of an molecule (cf, inorganic energy state of crystal lattice ) Scintillation process Absorption of kinetic energy from a charged particle excited ( ) De-excitation to S 1 electron state ( ) through radiationless internal conversion Excess vibration energy state (S 11, S 12 ) is not thermal equilibrium with its neighbors quickly loses the vibration energy. Scintillation light is emitted in transition from S 10 to any of ground state (S 0x )
Stokes shift Emission light energy < absorbed radiation energy Downward arrow energy < minimum energy required for excitation (little overlap between optical absorption and emission spectra) consequently little self-absorption of the fluorescence.
Characteristics of Organic Scintillators Characteristics of organic scintillators Low light output Fast response time can be used for higher dose rate, higher flux Used in β spectroscopy and fast neutron detection (low z) Light output is expressed relatively by light output of anthracene (100%)
Crystal scintillator Anthracene, Stilbene Difficult to make a big crystal Anthracene Organic scintillator used for longest time Highest light output among organic scintillators Difficult to make a big one and easy to crumble Very short response time Mainly used for β spectroscopy (low Z, if high Z, β is back scattered a lot) Fast neutron detection (low z)
Plastic and liquid scintillator It is difficult to make a big crystal organic scintillator Therefore, organic scintillation materials are used as a detector by polymerizing the material (plastic scintillator) or by dissolving the material into solvent (liquid scintillator) (The plastic and liquid scintillators are known as model name rather than scintillation materials) Plastic scintillator Easy to process material any size and any shape Thin plastic scintillator: for charged particles Thick plastic scintillator: for cosmic ray or γ Fast neutron by recoil proton due to high concentration of hydrogen
Liquid scintillator (Used by dissolving scintillation material with solvent) Can be used by mixing sample (detector + sample) Therefore, very good for alpha and low-energy beta (H-3, C-14) detection by preventing radiation absorption by window or air No good for γ due to low z material (H, C) Compton is dominant rather than PE effect (from energy spectroscopy aspect) Quenching (i.e., light absorption) depending on sample type
Organic scintillators for gamma spectroscopy Main components are carbon and hydrogen For photon, Compton scattering rather than photoelectric effect is dominant No good for gamma spectroscopy To increase, high-z materials (e.g., Sn) may be added (called loaded scintillator)
Inorganic Scintillators
Scintillation Mechanism Scintillation mechanism Scintillation mechanism depends on energy state of crystal lattice of material (cf., organic molecule energy level) Therefore, if dissolved, organic scintillator works, but inorganic one don t work.
Pure Crystal vs. Crystal with activator Activator To enhance the probability of visible photon emission during the deexcited process, small amount of an impurity (~ 0.1%) are commonly added to inorganic scintillators called activator (eg) NaI (Tl) Tl = activator
Competitive processes Phosphorescence ( afterglow ) Need additional energy increment to transit ground state Radiationless transitions ( quenching ) Transit to ground state without visible photon (electron capture created, excited state) Optical coupling to PM tube
Characteristics of Typical Inorganic Scintillators NaI(Tl) Most important scintillator Thallium added @~0.1 % by mass NaI(Tl) is hygroscopic and must be sealed in air-tight containers ( can not used for β radiation) Excellent light yield and linearity for electrons Good for γ and x (due to high Z of I) Widely used for γ -spectroscopy Phosphorescence contributes ~9 % of the overall light yield and thus limits the use of NaI(Tl) to low to moderate counting rates CsI(Tl) and CsI(Na) Higher interaction cross section for γ per unit weight compared with NaI(Tl) No hydroscopic property But, late pulse generation no good for high count rate. Applications in space measurements (PSD of particle types: Pulse shape discriminator)
LiI(Eu) Measurement of low-energy neutrons through the 6 Li(n, α) by using Li-6 concentrated material Bi 4 Ge 3 O 12 (or BGO) High density and large Z (Z (Bi) = 83 versus Z (I) = 53) Good for γ Light output only 10-20% that of NaI(Tl) Therefore, no good energy resolution No hygroscopic property, strong, no late pulse generation, no activator ZnS(Ag) Polycrystalline powder ( cannot get large size crystal latice) No hygroscopic property Used as thin screens in particle detectors as part of contamination survey meters α detect
Signal Multiplication and PM Tube
Signal Multiplication Signal multiplication Without the amplification provided by the PM tube, a scintillator is useless as a radiation detector. Electrons from photocathode is multiplied by PM tube (10 5 10 7 times) PM (photomultiplier) tube The PM tube is essentially a fast amplifier (amplification time: ~1 µs) Total gain: 10 5 10 7 Composed of a series of dynodes (10-14 dynodes) Electrons produced in the PM tube de are directed from one dynode to the next by an electric field established by applying a successively increasing positive high voltage to each dynode. Typical voltage difference between dynodes: 80 120 V Phototube voltage = total voltage applied
Efficiencies Related to Scintillator Material and Its Geometry Photon production efficiency Scintillation efficiency Light collection efficiency
Efficiencies Related to the PM Tube and Its Geometry Quantum efficiency Dynode quantum efficiency Dynode emission efficiency
Phoswich Detectors
Phoswich Detectors Phoswich (Phosphor sandwich) detector Detectors consisting of two different scintillators coupled together and mounted on a single photomultiplier tube. Scintillator types are chosen to have a widely different scintillation decay times. Consequently, pulse-shape discrimination can be used to distinguish events occurring in either detector. Examples: NaI(Tl) and CsI(Tl) Phoswich CaF 2 (Eu) and NaI(Tl) Phoswich
NaI(Tl) and CsI(Tl) Phoswich Applications: Detection of environmental levels of plutonium, americium, and uranium. (α)
CaF 2 (Eu) and NaI(Tl) Phoswich Applications: Determination of gross alpha and gross beta activity in low-level sample