Introduction into LS theory and practice
LIQUID SCINTILLATION
IRAD 2371
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 solvents
Ring structures in the molecule captures energy
from radiation
Energy 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-14
156 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