scintillation detectors

ionizing rad deposits

energy into scintillator

radidation particle interacting in scintillator results in

one scintillation (flash) of light and, therefore, one p

the greater the energy transferred to scintillator, the greater…

the number of excited electrons, and the greater the number of photons of light emitted

decay time is time required for

emission of 63% of light photons produced in scintillation

shorter the decay time, the better (mostly)

ideal scintillator: probability of radiation interacting w scintillator should be

high

ideal scintillator: a large portion of absorbed rad energy should be

emitted as light

ideal scintillator: linear proportionality…

should exist bw energy deposited and light produced

ideal scintillator: emission of light should occur…

as fast as possible

ideal scintillator: scintillator emission spectrum should…

overlap the photomultiplier tube absorption spectrum

ideal scintillator: material should be ___ to photons emitted by scintillator

transparent

ideal scintillator: absorbs all energy of…

every photon entering detector volume

scintillating material might be

inorganic, organic

scintillating material can be what of three things?

gas, liquid, solid

inorganic scintillators: splitting energy levels

greater number of atoms (heavier element) the greater the splitting of energy levels

(band formation) when many atoms combine to form a solid…

valence shell has split so many times that it forms a continuum of energies, i.e., a band

valence band

highest energy band occupied by electrons

band gap

electrons cannot possess energies here; unless there are impurities or defects

insulator

valence band full; electrons immobile and material doesn’t conduct electricity

conductor

valence band not full; electrons mobile and material conducts electricity

semiconductor

valence band is full; electrons are immobile and material does not conduct electricity

difference bw insulator and semiconductor

the width of the band gap

insulator band gap is

greater than 5 electron volts

semiconductor band gap is

less than 5 electron volts (one eV is typical)

light emitted by insulating material wo an activator is not…

detected by photomultiplier tube as efficiently as light of a longer wavelength (lower energy)

to reduce energy of emitted light…

an activator impurity is added to the crystal

an activator is an impurity such as…

thallium

sodium iodide scintillation detectors

detection of gamma rays/xrays

gamma spectroscopy

exposure rate measurements (uR meter)

NaI crystal activated w

with thallium

NaI has extremely high…

light yield; ca. 12% of energy deposited by gamma ray or xray is emitted as light

NaI has long

decay time (230 ns), limits effectiveness at high count rates

organic scintillators made of

aromatic hydrocarbons

organic crystals: only two have been used to any extent, which are

stilbene, anthracene

plastic scintillators (organic)

fluor (3% vol) is mixed w plastic solvent (97%) that is then polymerized

most commonly employed plastics in plastic scintillators

polystyrene

polyvinyl toluene

advantages of plastic scintillators

manufactured in any size/shape, rugged, light, nonhygroscopic, flat energy response

disadvantages of plastic scintillators

not suitable for gamma/xray, low atomic number means more likely for compton

not super detailed data on energy

scintillators must be protected from

ambient light, sometimes specific types of radiation

if scintillator is hygroscopic, must be housed in

hermetically sealed container

when scintillator and PMT cannot be in physical contact due to hermeticallly sealed,

housing must have transparent window to transmit emitted light

well detector

used to maximize counting efficiency for small samples;

well detectors limited to

small samples; therefore sample count rate might still be low

optical coupling

window of scintillator housing is direct contact w photocathode of PMT

optical coupling grease or gel almost always applied

photomultiplier tube converts

each flash of light from scintillation crystal to electronic pulse

usually round in cross section but might be square if multiple tubes packed close together

PMTs are

fragile, affected by magnetic fields

photocathode

thin metallic deposit inside window of PMT; usually flat and circular

absorption of light photons knocks electrons off photocathode

20-30% light photons incident on

photocathode, resulting in freeing electrons

dynodes

attract and accerlerate electrons

scintillators are essentially

the photon detectors

if PMT in magnetic field

change the paths of electrons, particularly those near photocathode end of the tube

liquid scintillation

used to measure activity (mostly beta/alpha) in liquid samples

very high sample efficiencies for counting (nearly 100%)

liquid scintillation: wipe samples placed in empty vial and

filled with “cocktail” fluid

this converts KE of particles into visible light that is detected by PMT of counter

the higher the energy of the nuclide…

the further it travels in liquid → more light produced

zinc sulfide (ZnS (Ag))

primary application in alpha detection; only available as very fine powder