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Problems
Problems with counting changed particles Don’t go through matter very readily Means that they are easily shielded So if a particle need to go through any
barrier it will not do so effectively
Why LSC
No barrier that charged particles need to go through
Close contact between the isotope atoms and the solvent
Close contact between solvent and phosphor
PMTs Leads to high efficiencies and ability
to detect low energy particles
Energy deposition
Amount energy that is deposited per unit track length by radiation is dependant on Total energy imparted Speed Mass change
Charge
Small ions deposit lower amounts of energy
As Z increases the energy deposited per unit track length increases
High Z ions deposit lots of energy in short distance
Higher energy deposition greater response from the scintillator
Energy
absorption and re-emission, cocktails contain two basic components, the solvent and the phosphor(s).
The solvent carries out the bulk of the energy absorption.
Dissolved in the solvent, molecules of phosphor convert the absorbed energy into light
Solvent
Makes up to 90-99% of the total volume of scintillation fluid
Solvent collects energy of stopping particles
Aromatic hydrocarbons are best solventsRing structures in the molecule captures
energy from radiationEnergy passes among solvent molecules
until it hit a phosphor molecule
Phosphors
Makes up to 1-10% of volume of fluid Primary and secondary phosphors
Primary -convert capture energy to light Secondary- wavelength shifters used
originally to shift the wavelength of the primary phosphors so the they would interact with the PMT better. Even thought he tubes now are better they are still included as they increases efficiency
Photons
The number of photons created is proportional to the energy deposited in the solution Which is dependant on the length
traveled in the solution Which is dependant on the initial energy
with which the particle was emitted
Energy Path
• Beta decay creates free electron• E and solvent = energized solvent• energized solvent and flour=– energized flour and solvent
• energized flour creates light• Light and sec flour = energized sec
flour• energized sec flour creates light• Light enters PMT creates signal
Energy Path
• Efficiencies vary depending on – Isotope– Sample composition– Specific flours
• But usually low– Only about 4% of energy from particle is
converted to light
• But other parts of the LSC that helps with signal generation
Beta energies
• Max energy is determined by radionuclide
• Average energy is 1/3 that of max energy• So will get a variety of energies
deposited in the scintillation fluid• Each of these energies creates different
amounts of electrons• Each creates a different magnitude signal• Will get a variation of energies on the
spectrum
PhotoMultiplier Tubes
• Flours release the light which enters PMT• Each system has 2 PMTs– This cuts down on noise produced by random
light
• PMTs convert the light emitted by the flour into an electron which is sent to the first dynode
• Dynodes multiply the electrons as they pass through the PMT
• The Anode collects the multiplied number of electrons and generates a signal
Quenching
Quenching is anything that will reduce the energy transfer between the solvent and the flour
Can be Physical Chemical Color
Physical Quenching
• Physical quenching is easy to determine• Anything that will get in the way
physically from the particle moving through the fluid
• Anything that will get in the way of the light getting propagated through the fluid on its way to the PMTs
• Smear, or any debris in the sample• Can take into account if you count your
standard the same way
Chemical Quenching
Other chemicals in the sample may interfere with energy getting collected by the solvents
Chemical quenchers absorb the energy of the radiation before it is converted to photons
Reduce the number of photons that are generated but each charged particle
Color Quenching
Color quenchers absorb the light that is released by the flours
The number of the photons produced by the flour is not impaired the but number that gets to the PMTs are reduced
All three quenching reduces efficiency of the system
Interferences• Chemoluminescence- caused by the
chemical reactions between the sample and the scintillator fluid. Reactions creates an excited molecules that emits light
• This light then interferes with sample counting
• Usually chemoluminescence decreases in several minutes to several hours
• Can count sample twice in a time period and if counts have gone down dramatically , you may have had chemoluminescence
Interferences
• Static electricity – in dry environments static electricity can build up on the container
• If this static discharges in the instrument it will add a great error in counting
• Plastic vials and latex gloves increase the static
• Can eliminate or minimize by wiping down the vial with a moist cloth
Sample Preparation
Sample has to dissolve in fluid Water based samples need water
based fluid Organic based samples need organic
based fluids Have some that can accommodate
both types of samples
Sample Preparation Sample have the be prepared the
same way the standard is If you are counting a solid (smears)
then put smear in standard and count , will minimize errors
Anything that goes into your samples must be done to the standard to create similar bias
Sample Preperation
Ideally samples should be clear , pH neutral solution
Mix solvent and sample well Let bubbles settle Let sample stand for several minutes
to minimizes effects of chemoluminescence
Signal processing
As single exits the vial it will interact with the PMTs (usually a pair)
From the PWTs the signal will enter a preamplifier
Preamp to a coincidence counter Then to an amplifier Then to single channel analyzer
Signal processing
Can use discriminators to separate signals
Can set up windows (same as other detection systems) to collect signal only in a certain energy range
Since energy is dependant on the radionuclide , one can separate radionuclides by energy
Signal processing
Most often used radionuclides in medicine are H-3, C-14, P-32
Fortunately they have a great energy difference between their beta
H-3 18.6 keV max, ave 6.2 C-14156 keV max, ave 52 P-32 1710 kev max, ave 695
Uses
Medical Research Water sampling Ground water flow measurements Compliance Nuclear power plants Environmental sampling
Medical and Research
90% of all drugs are tested with the use of radionuclide tracers or additions
Testing of fluids from the body to see where and how effective drugs are
Can use LSC to determine doses to people from low energy emitting radionuclides
Ground water
• H-3 or C-14 tracers are used to determine direction and rate of ground water flow
• Used to movement of water through formation for oil production
• Add some T2O to ground water and them take sample s from a well down flow from area when radionulcide is detected then can determine how long it took water to travel from point A to point B
Nuclear power plants/Env Sampling One of the largest produced
radionulides in power plant is H-3 Need to determine if it is being
released off site Can see if it the low energy emitting
radionuclides are being biomagnified Very good for evaluating water
samples
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