Introduction into LS theory and practice

Agenda

Energy deposition Overview of the LSC process Theory of operation Quenching Machine Uses

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

What LSC is used to count Low energy betas Low energy x rays Alphas

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

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

Spectrum

Different energies

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

Questions