Scintillators

Scintillators

When radiation interacts with certain types of materials, it produces flashes of light (scintillation)

Materials that respond this way are called scintillators.

These flashes can be collected and counted to obtain a measure of the radiation intensity.

Amount of flashes produced is proportional to the energy deposited in the crystal

Early detectors

1903 – Crookes invented a device called a spinthariscope used to see scintillations from alpha particle using zinc sulfide detector

1908- Regener used diamonds to count the scintillations of alpha particles

1944- Photomultiplier tube was invented

Characteristics

High efficiencyEfficiency should be linear over a wide

energy rangeTransparent Should be easily madeIndex of refraction should be close to glass

No material fits all of these criteria

F vs P

Flourensence- emission of visible radiation from a material. Prompt and delayed

Phosphoresence- emission of a longer wavelength light but at a much slower time interval

Good scintillator should convert most of the energy to prompt flouresence

Scintillators

Organic Anthracine, Napthaline, Stilbene Fast response but low efficiency Beta and neutron detection Can be solid or liquid

Inorganic NaI, CsI, ZnS, HgI, BGO Slower response but higher efficiency Higher density for gamma detection Usually solid

Organic

Pure crystals Anthracine highest efficiency of any organic Stilbene pulse shape discrimination

Fragile Hard to get in large sizes

Plastic Scintillators

Organic scintillators are dissolved in a solvent and can be polymerized

Can easily be made in large volumesInexpensiveHave to worry about self absorption

Liquid

Efficient for low energy beta particles and x rays

Can be in large volumesHigh efficienciesMore Later on Liquid Scintilation process

Toxic Benzene, Toluene, Xylene Non-toxic POP, POPOP, Ultima Gold

Other Organic scintillators

Thin Film Can be used as transmission detectors

Loaded Organic detectors Can add high Z material to increase efficiency of

energy conversion to light but lowers light transmission through material

Can add high neutron capture cross section material so can detect Neutrons through the proton recoil reaction

Inorganic

Valence band- bound electronsConduction band- electrons that can travel

within the crystalForbidden band- where electrons can not goElectrons jump from valence band to

conduction bandProbability of conduction band e- returning to

the valence band is small, so we add activators to the crystal

Band gap

Band gap is the energy difference between the valence band and the conduction band

In conductors the band gap is 0In insulators the band gap is largerIn semi-conductors the band gap is small

Activators

Are impurities that are added to the crystal to improve the probability of the e-returning to the valence band and hence releasing light in a wavelength we can detect

Impurities create energy states that in the forbidden zone of the original crystal giving the e- jumping off points

Inorganics

Sodium iodide crystals doped with thallium (NaI(Tl)) Most common scintillator generally employed for gamma and x-ray detection Can be made large Has excellent light production Very hydroscopic Linear response Very fragile

Inorganics

Cesium Iodide (CsI) with Tl or Na Less fragile than NaI Can be shaped Denser material Pulse shape discrimination properties can

differentiate between different type of radiation Good if need small efficient detector

Inorganics

Zinc sulfide doped with silver (ZnS(Ag)) ,

well suited for alpha and heavy ion detection Efficiency similar to NaI(Tl) Polycrystaline form limits size they use a large area but thin crystals for portable

survey instruments First type of radiation detector

Scintillators

Bismuth Germanate (BGO) Pure scintillator High density Not as fragile as NaI High efficiency Poor energy resolution

LaBr3(Ce)- Lanthanum Bromide High density Good resolution

Others BaF2 CaF2 CsF

Scintillator crystal

Must be clear with no defectsWhat would the effect on light propagation if

the crystal had a Crack Cloudiness Other than doped impurities

Photomultiplier Tube

Device that changes a small number of photons created in a scintillator (or other process) into a number of electrons that can easily be counted.

Glass enclosed, vacuum sealed components

Shock and vibration sensitiveMagnetic fields will effect PTMs

Photomultiplier Tube (PMT)

Photocathode- has the unique characteristic of producing electrons when photons strikes its surface (photoelectric effect)

Dynodes- When each electron from the photocathode hits the first dynode, several electrons are produced (multiplication), this sequence continues until the electron pulse is now millions of times larger then it was at the beginning of the tube

Photomultiplier Tube (PMT) cont

Anode- At this point the millions of electrons are collected by an anode at the end of the tube forming an electronic pulse.

Signal – multiplied pulse sent to other electronics for processing

Signal collected at the anode has been multiplied many times from when it entered the photocathode

Photomultiplier Tube (PMT)Photomultiplier Tube (PMT)

Incident Ionizing Radiation

Sodium-IodideCrystal

Photocathode

Optical Window

- PulseMeasuring

Device

Light Photon Photomultiplier Tube

Dynode Anode

PMT

Several configurations

Venetian blind

Box and grid

Linear structure

Circular grid

Types

Venetian blind- old , slow response time, not used much

Box and grid- old and slow but is good for large PMT

Circular grid and linear structure-faster response time