Liquid scintillation counting
Encyclopedia
Liquid scintillation counting is a standard laboratory method in the life-sciences for measuring radiation
from beta
-emitting nuclides. Scintillating
materials are also used in differently constructed "counters
" in many other fields.
Samples are dissolved or suspended in a "cocktail" containing an aromatic solvent (historically benzene
or toluene
, but more recently less hazardous solvents have come into favour) and small amounts of other additives known as fluors
, i.e., scintillants
or scintillator
s. Beta particles emitted from the sample transfer energy to the solvent molecules, which in turn transfer their energy to the fluors; the excited fluor molecules dissipate the energy by emitting light. In this way, each beta emission (ideally) results in a pulse of light. Scintillation cocktails often contain additives that shift the wavelength
of the emitted light to make it more easily detected.
The samples are placed in small transparent or translucent (often glass or plastic) vials that are loaded into an instrument known as a liquid scintillation counter. The counter has two photomultiplier
tubes connected in a coincidence circuit
. The coincidence circuit assures that genuine light pulses, which reach both photomultiplier tubes, are counted, while spurious pulses (due to line noise, for example), which would only affect one of the tubes, are ignored.
Counting efficiencies under ideal conditions range from about 30% for tritium
(a low-energy beta emitter) to nearly 100% for phosphorus-32
, a high-energy beta emitter. Some chemical compounds (notably chlorine
compounds) and highly colored samples can interfere with the counting process. This interference, known as "quenching", can be overcome through data correction or through careful sample preparation.
High-energy beta emitters such as P-32 can also be counted in a scintillation counter without the cocktail. This technique, known as Cherenkov counting, relies on the Cherenkov radiation
being detected directly by the photomultiplier tubes. Cherenkov counting in this experimental context is normally used for quick rough measurements, since it is more liable to variation caused by the geometry of the sample.
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
from beta
Beta particle
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
-emitting nuclides. Scintillating
Scintillator
A scintillator is a special material, which exhibits scintillation—the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light...
materials are also used in differently constructed "counters
Scintillation counter
A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic , or organic liquid that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube measures the light from the crystal...
" in many other fields.
Samples are dissolved or suspended in a "cocktail" containing an aromatic solvent (historically benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
or toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
, but more recently less hazardous solvents have come into favour) and small amounts of other additives known as fluors
Scintillator
A scintillator is a special material, which exhibits scintillation—the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light...
, i.e., scintillants
Scintillator
A scintillator is a special material, which exhibits scintillation—the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light...
or scintillator
Scintillator
A scintillator is a special material, which exhibits scintillation—the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light...
s. Beta particles emitted from the sample transfer energy to the solvent molecules, which in turn transfer their energy to the fluors; the excited fluor molecules dissipate the energy by emitting light. In this way, each beta emission (ideally) results in a pulse of light. Scintillation cocktails often contain additives that shift the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the emitted light to make it more easily detected.
The samples are placed in small transparent or translucent (often glass or plastic) vials that are loaded into an instrument known as a liquid scintillation counter. The counter has two photomultiplier
Photomultiplier
Photomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
tubes connected in a coincidence circuit
Coincidence circuit
In physics, a coincidence circuit is an electronic device with one output and two inputs. The output is activated only when signals are received within a time window accepted as at the same time and in parallel at both inputs...
. The coincidence circuit assures that genuine light pulses, which reach both photomultiplier tubes, are counted, while spurious pulses (due to line noise, for example), which would only affect one of the tubes, are ignored.
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
(a low-energy beta emitter) to nearly 100% for phosphorus-32
Isotopes of phosphorus
Although phosphorus has 24 isotopes from 24P to 47P, only one of these isotopes is stable 31P; as such, it is considered a monoisotopic element. The longest-lived radioactive isotopes are 33P with a half-life of 25.34 days and 32P with a half-life of 14.263 days. All other have half-lives under...
, a high-energy beta emitter. Some chemical compounds (notably chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
compounds) and highly colored samples can interfere with the counting process. This interference, known as "quenching", can be overcome through data correction or through careful sample preparation.
High-energy beta emitters such as P-32 can also be counted in a scintillation counter without the cocktail. This technique, known as Cherenkov counting, relies on the Cherenkov radiation
Cherenkov radiation
Cherenkov radiation is electromagnetic radiation emitted when a charged particle passes through a dielectric medium at a speed greater than the phase velocity of light in that medium...
being detected directly by the photomultiplier tubes. Cherenkov counting in this experimental context is normally used for quick rough measurements, since it is more liable to variation caused by the geometry of the sample.