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Everything you wanted to know about Liquid Scintillation Counting but were
afraid to ask:
LIQUID
SCINTILLATION
COUNTING
Liquid
scintillation
counting
is an analytical technique which is defined by the incorporation of the
radiolabeled analyte into uniform distribution with a
liquid chemical medium capable of converting the kinetic
energy of nuclear emissions into light energy. Although the
liquid
sc intillation counter is a sophisticated laboratory
counting system used the quantify the
activity of particulate emitting (ß and a) radioactive samples, it can also
detect the auger electrons emitted from Cr 51 and I
125 samples.
LIQUID
SCINTILLATION
PRINCIPLES
Figure
1 provides a graphic illustration of the way the emitted radiation interacts
with the cocktail (a mixture of a solvent and a solute) leading to a count
being recorded by the system.
Step 1.
Beta particle is emitted in a radioactive decay. To assure efficient
transfer of energy between the beta particle and
the solution, the solution is a solvent for the sample material.
Step 2.
In the relatively dense liquid, the beta
particle travels only short distances before all
of its kinetic energy is dissipated. Typically a beta particle will take
only a few nanoseconds to dissipate all its
kinetic energy. The energy is absorbed by the medium in 3 forms: heat,
ionization and excitation. Some of the beta energy is absorbed by solvent
molecules making them excited (not ionized).
Step 3.
Energy of the excited solvent is emitted as UV light and the solvent
molecule returns to ground state. The excited
solvent molecules can transfer energy to each other and to the solute
(Figure 2).  The solute is a fluor. An excited solvent molecule which passes
its energy to a solute molecule disturbs the orbital electron cloud of the
solute raising it to a state of excitation. As the excited orbital electrons
of the solute molecule return to the ground state, a radiation results, in
this case a photon of UV light. The UV light is absorbed by fluor molecules
which emit blue light flashes upon return to ground state. Nuclear decay
events produce approximately 10 photons per keV of energy. The energy is
dissipated in a period of time on the order of 5 nanoseconds. The total
number of photons from the excited fluor molecules constitutes the
scintillation. The intensity of the light
is proportional to the beta particle's initial energy.
Step 4.
Blue light flashes hit the photo cathode of the photo multiplier tube (PMT).
Electrons (proportional in number the blue light pulses) are ejected
producing an electrical pulse that is proportional to the number of blue
light photons. A LSC normally has two PMT's. The amplitude of the PMT pulse
depends on the location of the event within the vial. An event producing 100
photons will be represented by a larger pulse if the event is closer to the
PMT than if the event is more remote. The signal from each PMT is fed into
a circuit which produces an output only if the 2 signals occur
together, that is within the resolving time of the circuit, approximately 20
nanoseconds (coincidence circuit). By summing the amplitude of the pulses
from each PMT, an output is obtained which is proportional to the total
intensity of the scintillation. This
analog pulse rises to its maximum amplitude and
falls to zero.
Step 5.
The amplitude of the electrical pulse is converted into a digital value and
the digital value, which represents the beta
particle energy, passes into the analyzer where it is compared to digital
values for each of the LSC's channels. Each channel is the address of a
memory slot in a multi-channel analyzer which consists of many storage slots
or channels concerting the energy range from 0-2000 keV.
Step 6.
The number of pulses in each channel is printed out or displayed on a
CRT. In this manner, the sample is analyzed and
the spectrum can be plotted to provide information
about the energy of the radiation or the amount of radioactive material
dissolved in the cocktail.
LSC
LINGO (TERMINOLOGY)
Chemi-
Random single photon events which are generated as a result of the
luminescence
chemical
interaction of the sample components. Except at high rates, most
chemiluminescence events are excluded by the coincidence circuit.
Chemical
A reduction in the scintillation
intensity seen by the photomultiplier tubes
Quenching
due to materials present in the scintillation
solution that interfere with the processes leading
to the production of light. The result is fewer photons
per keV of beta particle energy and usually a reduction in
counting
efficiency.
Cocktail
The scintillation fluid; a mixture of 3
chemicals (solvent, emulsifier, and fluor) which
produces light flashes when it absorbs the energy of
particulate radioactive decay.
Compton
Elastic scattering of photons (x/?-rays) by electrons. In each such process
Scattering
the electron gains energy and recoils and the photon loses energy. This is
one of the three ways photons lose energy upon interacting with
matter, and is the usual method with photons of
intermediate energy and materials of low atomic
number. Named for Arthur H. Compton, the American
physicist who discovered it in 1923.
CPM
Counts
per minute. This is the number of light flashes or counts the LSC
registered per minute. The number of decays produced by the
radioactivity is usually more than the number of
counts registered.
Discriminator
An electronic circuit which distinguishes signal pulses according to their
pulse height or voltage. It is often used to exclude noise or
background radiation counts.
DPM
Disintegration per minute. The sample's activity in units of nuclear decays
per minute.
Efficiency
The ratio, CPM/DPM, of measured counts to the number of decays which
occurred during the measurement time.
Fluor
A chemical component of the liquid
scintillation cocktail that absorbs the
UV light emitted by the solvent and emits a flash of blue light.
Fluorescence
The emission of light
resulting from the absorption of incident radiation and
persisting only as long as the stimulating radiation is continued.
Luminescence
A general term applied to the emission of light by causes other than high
temperature.
Optical
A reduction in the scintillation
intensity seen by the photomultiplier tubes
Quenching
due to absorption of the scintillation
light either by materials present in
scintillation solution or deposited on the particle energy and usually a
reduction in
counting efficiency.
PMT
The Photo-Multiplier Tube is an electron tube that detects the blue light
flashes from the fluor and converts them into an electrical pulse.
Phosphor
A luminescent substance or material capable of emitting light when
stimulated by radiation.
Photo-
Delayed and persistent emission of single photons of light following
luminescence
activation by radiation
such as ultraviolet.
Pulse
Electrical signal of the PMT; its size is proportional to the radiation
energy absorbed by the cocktail.
Quenching
Anything which interferes with the conversion of decay energy emitted
from the sample vial into blue light photons. This usually results in
reduction in counting efficiency.
QIP
The Quenching Index Parameter is a value that indicates the sample's level
of quenching. Another parameter that describes the amount of
quenching present is the transformed Spectral
Index of External Standard (tSIE) or "H" number.
Secondary
Material in the scintillation cocktail
which absorbs the emitted light of the
Scintillator
primary scintillator and remits it at a longer wavelength, nearer the
maximum spectral sensitivity of the photomultiplier tubes. It is
added to improve the
counting efficiency of the sample.
Solvent
A chemical component of the liquid
scintillation cocktail that dissolves the
sample, absorbs excitation energy and emits UV light which is
absorbed by the fluors.
LSC
EXTERNAL SETTINGS
LSC's come in a variety of shapes and types and manufacturers may use
different terminology, however, the following
basic external controls are commonly found on most systems.
Gain
A control used to adjust
the height of the signal received by the detecting system. The gain control
for newer LSC's is often automatically set for the particular
radionuclide selected.
LLD
The lower level discriminator setting is used to discriminate against (i.e.,
not count) betas with energy below that setting.
This setting is also used to decrease system noise
which often occurs in the region below 3 keV.
ULD
The upper level discriminator setting is used to discriminate against any
beta energy higher than that setting.
A particular LSC may have other external controls depending on the
counter type and model. Read the instruments
operating manual to gain familiarity with the controls and operating
characteristics.
QUENCH
Quench is a reduction in system efficiency as a result of energy loss in the
liquid
scintillation solution. Because of quench, the energy spectrum
detected from the radionuclide appears to shift toward a lower energy.The
three major types of quench encountered are photon, chemical, and optical
quench.
Photon
quenching occurs with the incomplete transfer of beta
particle energy to solvent molecules.
Chemical,
sometimes called impurity, quenching causes energy
losses in the transfer from solvent to solute.
Optical
or color quenching
causes the attenuation of photons produced in solute.
Effect of Quenching on an Energy Spectrum:
Chemical quenching absorbs beta energy before it is converted to photons
while color quenching results from the passage of photons through the
medium. Color quenching depends on the color of the interfering chemical and
path length that the photon must travel . In a chemically quenched sample,
all energy radiations appear to be equally affected whereas, for a colored
sample, events that take place close to one PMT will give rise to a large
pulse and a smaller pulse in the other PMT. By summation, the pulses are
added so the resultant pulse height may be as large as from unquenched, only
the # of events will be significantly reduced. Thus, at equal quench levels,
the pulse height of colored samples are spread over a wider range than
chemical quench samples. Chemical vs Color Quench...Because
quench affects the efficiency of sample detection, quench could have a
significant impact on your LSC results. To better understand the importance
of quench on your work note these three different quench curves and the
resulting efficiencies. These quench standards were counted on a Packard
1900 LSC. On a different system it is likely that the quench numbers and
resultant efficiencies will be a little different, but not the general
effects of quench. The Packard allows the user to select keV regions of
interest. For this demonstration the three channels selected were: Channel
A, 0.0 - 18.7 keV, Channel B 18.6 – 156 keV, and Channel C 0.0 - 2000 keV.
(tSIE). A maximum efficiency of approximately 48% is achieved with a quench
parameter (tSIE) of 518. The minimum efficiency of 0.33% is obtained with a
quench of 17.9. Thus a quench of 45 or below would result in essentially
background counts (efficiency ~ 3%).
Quench is important.
You
must understand the impact of quench and how the system you are using
represents it if you want to obtain viable results. Quench calibration
delimits the valid ranges for quantifying a sample. Samples with quench
numbers outside the calibration range will raise a flag which
indicates the value is out of range. The conversion to dpm will be made, but
will be an extrapolation from the highest/lowest recorded
quench value.
CHEMILUMINESCENCE/PHOTOLUMINESCENCE
Luminescence is a single photon event and is registered as a count due to
the probability of having coincidence events at high luminescence activity.
Although LSC's employ a coincidence circuit, luminescence events stimulate
each PMT within the resolving time of the coincidence circuits.
Chemiluminescence
is the
production of light as a result of a chemical reaction between
components of the scintillation
sample in the absence of radioactive material. This most typically occurs
when samples of alkaline pH and/or samples containing peroxides are mixed
with emulsifier-type scintillation
cocktails, when alkaline tissue solubilizers are added to emulsifier type
scintillation cocktails, or when
oxidizing agents are present in the sample. Reactions are usually
exothermic and result in the production of a large number of single
photons.
Photoluminescence
results in the excitation of the cocktail and/or vial by UV light (e.g.,
exposure to sunlight or UV lights). Chemiluminescence has a relatively slow
decay time (from 0.5 hr to > 1 day depending on the temperature) while
photoluminescence decays more rapidly (usually < 0.1 hr).
The
luminescence spectrum has a pulse height distribution which overlaps the 3H
spectrum. The maximum pulse height corresponds to approximately 6 keV and
the spectrum is (chemical) quench independent. The equivalent of a few keV
of beta particle energy, the maximum number of events will occur between 0
and 2 keV and remain there independent of quenching. Contrary to popular
belief, cooling the luminescent scintillation
samples reduces the photon intensity to low
levels, but interference is still present and provides false indication of
luminescence control.
STATIC ELECTRICITY
Static
electricity on liquid
scintillation vials is a single photon
event with pulse height limited to about 10 keV. Many items used in the
liquid
scintillation counter environment are conducive to the
development of static charges. In general, glass vials have less problems
with static than plastic vials; small vials in adapters are particularly
prone to static charge build up. Most systems offer an option which employs
a static charge device or and electrostatic controller.
SAMPLE VOLUME/DUAL PHASE SAMPLES
As the
sample volume decreases, light output falls on less efficient areas of the
PMT, so energy detection becomes less efficient with low volumes. When 2
phases are present, each phase will have its own
counting efficiency.
CERENKOV COUNTING
Some
beta emitting isotopes can be analyzed on an LSC without using any cocktail.
The liturature of several manufacturer's discusses
counting high energy (Emax > 800 keV)
beta emitters without cocktail or with only a little water, using a
technique called Cerenkov counting. When
high energy beta particles travel faster than the speed of light relative to
the medium they are traversing (e.g., water, etc.) Cerenkov radiation (i.e.,
light) is produced. Cerenkov radiation is the blue light that you see when
you look into a reactor pool. Cerenkov radiation allow some beta emitting
radionuclides to be analyzed with a liquid
scintillation counter without
using any cocktail. For Cerenkov radiation to be
produced, the beta particle must exceed a minimum threshold energy.
So, now you know! :)
By the way, we also carry
Beckman benchtop liquid
scintillation systems (LS 3801, LS 5801, LS 6000,& LS 6500 models)
(or
Packard TriCarb Systems) to fit every budget and need....from simple wipe tests
and surveys to sophisticated dual-label assays. From lowest possible
cost to state-of-the art-current model, a GMI solution will still save you $$
thousands over buying new. We look forward to your call
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