Basic principle of liquid scintillation counter norfaizal

Follow-up Training Course (FTC) on
Environmental Radioactivity Monitoring (ERM)

Follow-up Training Course on Environmental Radioactivity Monitoring
Introduction
Liquid Scintillator
Quenching Effect
Sample Preparation
Measurement of Tritium
Q A

1947 - First and Kallman found that certain organic
chemicals emit fluorescence light when bombarded
by nuclear radiations
1953 - Hayes et. al. introduced radiolabeled biological
material into the scintillation solution
1953 - First commercial LSC manufactured by
Packard Instrument
Now - LSC, which is applicable to various types of
radiations, is the most sensitive and widely used
technique for measurement of radioactivity. It is
applied to environmental radioactivity monitoring,
for not only low energy β
β
β
β emitters such as 3H or 14C
but also for α
α
α
α or β
β
β
β-γ
γ
γ
γ emitters.

Liquid scintillation counter was originally
devised for the measurement of such low
energy β–emitter as 3H and 14C.
Variety of methods have been developed for
measurements of other nuclides.
Applied to various fields including the
industry and the environmental safety.

Aim - To measure the amount of activity
associated with individual radionuclides
The most sensitive and widely used technique
for the detection and quantification of
radioactivity
Applicable to all forms of decay emission
such as:
◦ alpha particle
◦ beta particle
◦ beta/gamma ray
◦ example: 3H, 14C, 22Na, 24Na, 32P, 32S, 35S, 45Ca

New generation LSC - classified as `low level’
instrument - because of their background reduction
features enable to quantify of much lower
activities for a range of radionuclides.
Example:
increased in counting sensitivity have extended the
effective age limit of radiocarbon dating from
50,000 years to 60,000 years.
Levels of 1 Bq/L of water can be detected for
environmental 3H.

Measurement of natural series radionuclides at natural
environmental level in a range of environmental sample
matrices.
- isotopes of radium (Ra), uranium (U), 210Pb, 222Rn, 231Pa
and 234Th.
Monitoring the environment around establishment
associated with the nuclear power industry for
anthropogenic radionuclides - principally beta emitters
without significant gamma emissions such as 3H, 14C, 35S,
55Fe, 85Kr and 89,90Sr.
Nuclear weapons decommissioning; measurement of gross
alpha activities in airborne particulate and surface wipes.
Radiocarbon dating.
Ground water / environmental 3H.

Advantages
No need of considering self- and external absorption of
radiations: low-energy beta-ray emitters can be
measured effectively.
4π
π
π
π geometry measurement: resulting in a high
counting efficiency.
Disadvantages
Quenching effect: radioactive material added in a scintillator
obstructs the light emission process of the scintillator, which is
called quenching effect.
Interference of chemiluminescence: unwillingly, other light
photon which may be produced in a sample interferes radiation
measurement.
Production of organic radioactive waste.

The energy of nuclear decay is proportional to light
intensity. The number of flashes of light (CPM) is
proportional to the number of disintegrations (DPM)

Scintillator is an energy transducer which
transforms radiation energy into light energy or
fluorescence or photons.
Solid scintillator - the energy transducer is
such a crystal as NaI
Liquid scintillator - the energy transducer
is the molecules of particular organic
compounds dissolved in a solution

Measuring the activity of radionuclides from the rate
of light photons emitted by a liquid sample
Field of application - Medicine, agriculture,
environmental, biological, tracer etc.
Tritium, 3H : Emax = 18.6 keV
Carbon 14, 14C: Emax = 156 keV
Phosphorus 32, 32P: Emax = 1710 keV

Consists mainly four components:
Solvent
Primary fluorescing solute (scintillator)
Secondary fluorescing solute (scintillator)
Surfactant

play a very important role in energy transfer process:
through the solvent, radiation energy is transferred to
fluorescing solute.

receives the excitation energy from solvent, and
emits fluorescence photon with λ
λ
λ
λ of about 360 nm.
(ca. the sensitivity of the photomultiplier tube of
about 420 nm, for changing the photons into
electric pulse).

has the maximum peak of emission spectrum at
420 nm - called as wavelength shifter.

surfactant is added to emulsify the sample into the
liquid scintillator. The surfactant has hydrophilic and
hydrophobic radical. The former is miscible with water,
and the latter is incorporated with aromatic
hydrocarbons.

When the luminescence process is interfered, the
photons generated in a sample is decreased
This phenomenon is called quenching effect.

Chemical quenching is the quenching which occurs before
scintillation photons are emitted from the solute. This is
caused by the interference of the energy transfer process
between solvent and solute.
Color quenching is the quenching which occurs after the
scintillation photons are emitted. This is due to the
absorption of photons by colored material in a sample.

Reduction of counting efficiency
due to quenching effect

① Chemical quenching
It takes place before the solute emits fluorescence;
impurity is responsible for this phenomenon.
② Color quenching
It takes place after the solute emits fluorescence, caused
by the substance with absorption spectrum overlapping
the emission spectrum of the solute.
③ Oxygen quenching
A kind of chemical quenching caused by oxygen dissolved
in liquid scintillator.
④ Concentration quenching
It is caused by the solute of very high concentration (self-
quenching or self-absorption).

① Cosmic rays
secondary electrons produced by collisions of cosmic ray with the
material around the detective part of a LSC.
② Natural radioactivity
40K contained in glass vials, 222Rn, 220Rn and their daughters are
present in air in laboratories.
③ Chance coincidence counting
To suppress pulses except those from signal, a method of
coincidence counting has been adopted; however, complete
removal of noise is difficult even with coincidence circuit, and
noise can be counted as the “chance coincidence counting” leaking
from the coincidence circuit.
④ Cross-talk
two PMTs are located face-to-face with an angle of 180 degree,
the light generated in one of the two PMTs can be sensed by the
other.

In sample preparation, 10 – 15 ml of emulsion
scintillation is added in a counting vial, and then
1 – 10 ml of the sample to be measured is added
in it, and shaked.
To obtain reliable counting data, it is essential to
disperse homogeneously the activity sample into
a liquid scintillator, and to prepare a transparent
sample.
The sample thus prepared is measured with a
liquid scintillation counter.

the activity of a sample is calculated from the counting
rate (cpm) obtained from the counter:
the counting efficiency varies complicatedly with the
quenching condition of the sample, it is necessary to
determine the counting efficiency for each sample to
calculate the activity

There are four methods for determining the
counting efficiency:
1. Internal standard method - accuracy
2. Sample spectrum method - quench curve
3. External standard method - quench curve
4. Efficiency tracing method
In these methods, external standard method
is generally used.

CAUTION
1. Radioactive radionuclide must be the same as sample
2. Activity added should be greater than sample activity
3. Internal standard DPM accurate, known
4. Internal standard must not affect quenching of sample

a. Ten standards all with 100,000 DPM
(stock solution 120mL at 100,000 DPM/
10mL)
b. Add 10 mL to each vial
c. Add increasing amount of quench agent to
each sample - such as nitromethane 0 -50 µ
µ
µ
µL
d. Determine the CPM and QIP (Quench
Indicating Parameter) for each standard
and plot data

How Are DPM Calculated for Unknowns?
1. Count sample obtain CPM, e.g., 36,000 CPM
2. Determine isotope - H-3
3. Determine spectral index of sample (SIS)
- 12.0 %eff = 48%
⇒
⇒
⇒
⇒ DPM unknown = 36,000/0.48

tSIE - transformed spectral index of external standard (i.e. : Ba-133)

1. Independent of vial size - 4, 7, 20 mL
2. Independent of vial material - glass, plastic
3. Independent of quenching agent - color /
chemical
4. Independent of sample volume

Determine Radioactivity of Sample
Assumptions:
1. Homogeneous sample
2. 4π
π
π
π counting geometry

to achieve the precision and accuracy for the
measurement
optimizing the sample counting efficiency
Important properties:
i. Homogeneous and single-phase sample condition so
as to ensure that the radionuclide is in solution and
contacts with the scintillator
ii. Clear translucent sample condition free of
quenching and chemiluminescence

Scintillator based on toluene or xylene solvent for 222Rn analysis
- This scintillator consists of a primary solute (PPO or butyl-
PBD, 4-8g/l), a secondary solute (bis-MSB, 1 g/l) and solvent
(toluene or xylene), and does not contain surfactant due to high
solubility of 222Rn gas in these solvents.
Emulsion scintillator based on di-isopropylnaphthalene
- the solvent is nontoxic, nonflammable and biodegradable, it
has been widely used in a conventional emulsion scintillator.
Extractive scintillator
- consists of liquid-liquid extractant and liquid scintillator, and
has been developed for the alpha-ray spectrometry of
actinides. This allows extraction of the nuclide of interest from
an aqueous sample directly into the scintillator.

The cocktail is a major determining
factor of the quality of the data
obtainable from LSC. Criteria in
selecting a cocktail:
sample compatibility
counting efficiency
cost
convenience
safety

sample compatibility
- the best performance is obtained when the analyte is entirely dissolved in the
cocktail (homogeneous phase)
- samples in heterogeneous phase (precipitate, separate liquid phase) yield lower
counting efficiency
- check always the sample loading capacity of the cocktail, I.e. the amount of
sample that may be incorporated in given cocktail
counting efficiency
- the best cocktail is the one allowing higher detection efficiency and higher
resistance to quenching
cost
- some economy may be made preparing the cocktail in the lab, but quality may
be lower than in commercially available cocktails
convenience
- the use of an universal cocktail may represent an economy and reduces risk of
mistakes in sample preparation
safety
- Fire hazard: solvents are flammable; check the flash point
- Health hazard: solvent vapors are toxic; especially toluene. Excessive exposure to
vapors may cause headache, nausea etc.

Provide adequate ventilation in areas
where solvents are used and stored
use dispensing devices to transfer
cocktail to vials and to limit solvent
evaporation
Volume of cocktail to use
10 or 15 ml per vial is, in general
sufficient (sample load)
standardize and keep constant during
one experiment

The vial is the container for the analyze
and the scintillation cocktail. It permits
light transfer from the liquid scintillator
cocktail to PMT.
Economic glass vials:
- soda-lime (flint) glass
- non-permeable by chemicals
(solvent)
- optical clarity adequate
- background counts; adequate for use
with radiotracers

Low -Background glass vials:
- low potassium borosilicate
glass
- low radioactivity background
- better optical quality
- adequate for low radioactivity
(environmental research)
- expensive
Special vials:
- teflon, quartz
Polyethylene vials:
- high density polyethylene
- very low radioactivity
background
- very low cost
- high transmission of light
although they are opaque to the
eye
Vial closure:
must be tight to prevent evaporation of solvent and analytes
- screw cap, snap-cap or plug cap
- urea-formaldehyde with or without Al foil liner

90Sr and 89Sr are fission products, so their main sources in
environment are atmospheric nuclear weapon testing and
releases from the nuclear fuel cycle. In general, 90Sr is in
equilibrium with its 90Y daughter. Water, milk, soil,
vegetation and urine are typical sample to be analyzed.
The analysis involves the sample pretreatment to bring the
sample into suitable form, radiochemical separation, and
radiation measurement. The most popular separation methods
involve the use of ion exchange chromatography, liquid-liquid
extraction, and extraction chromatography.

Anthropogenic tritium is from atmospheric weapons
testing and nuclear fuel cycle
Weapons testing from 1954 to 1963
Natural levels are now back to the levels of pre-
atmospheric bomb tests
The present day activity in precipitation is
approximately 2 Bq/L
3H, Emax = 18.6 keV, half-life 12.32 y

Direct addition - mix with cocktail and measure (sample 10 mL)
◦ less labor intensive
◦ distillation or purification by Eichrom 3H column is needed to
remove impurities from a low activity sample
Electrolytic enrichment (starting volume 100-300 mL)
◦ enrichment system required, no commercially made systems
readily available
◦ time consuming
Benzene synthesis (C6H6 contains 3 times as much 3H as H2O)
◦ synthesis apparatus required, no commercially made
synthesizers readily available
◦ labor intensive, time consuming
◦ carcinogenic end product

typical 3H eff typical bkg detection limit
Direct counting 25 % 1.0 CPM 2.5 Bq/L
Benzene synthesis 60 % 1.2 CPM 0.37 Bq/L
Enrichment 25 % 1.0 CPM 0.13 Bq/L
Direct counting and enrichment calculations are made for 10 mL water and 500 min counting time
20 mL benzene is equivalent to 30 mL water with 100 % yield.
Numbers are typical for Quantulus

Samples measured first as they are
Very small volume of known activity standard
material added and recounted
Efficiency verified for each sample and activity
calculated
Advantage:
◦ Based on raw data, no quench curves needed
◦ Works on any counter (performance is an issue)
Disadvantages:
◦ Destroys samples, recounting not possible

Cocktails for aqueous 3H samples
Ultima Gold LLT, high capacity, acceptance of
mineral acids
Ultima Gold XR, high capacity, acceptance of
mineral acids
OptiPhase HiSafe 3, multipurpose cocktail,
lower water capacity than Ultima Gold’s
Cocktails are based on di-isopropyl-
naphthalene solvent, which has very low
vapor pressure and high flash point (148°C)

Basic principle of liquid scintillation counter norfaizal

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