Tracer Techniques

Tracer Techniques

A radioactive tracer, or radioactive label, is a chemical compound in which one or more atoms have been replaced by a radioisotope so by virtue of its radioactive decay it can be used to explore the mechanism of chemical reactions by tracing the path that the radioisotope follows from reactants to products. Radiolabeling is thus the radioactive form of isotopic labeling.

Radioisotopes of hydrogen, carbon, phosphorus, sulphur, and iodine have been used extensively to trace the path of biochemical reactions. A radioactive tracer can also be used to track the distribution of a substance within a natural system such as a cell or tissue. Radioactive tracers are also used to determine the location of fractures created by hydraulic fracturing in natural gas production. Radioactive tracers form the basis of a variety of imaging systems, such as, PET scans, SPECT scans and technetium scans. Radiocarbon dating uses the naturally occurring carbon-14 isotope as an isotopic label that nature builds into any living thing.

TRACER-1

Nature of Radioactivity

An unstable nucleus will decompose spontaneously, or decay into a more stable configuration but will do so only in a few specific ways by emitting certain particles or certain forms of electromagnetic energy. Radioactive decay is a property of several naturally occurring elements as well as of artificially produced isotopes of the elements. The rate at which a radioactive element decays is expressed in terms of its half-life; i.e., the time required for one-half of any given quantity of the isotope to decay.

Radioactive decay

The decay of radioactive elements occurs at a fixed rate. The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material. For example, Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days.

Half life

The radioactive half-life for a given radioisotope is the time for half the radioactive nuclei in any sample to undergo radioactive decay. After two half-lives, there will be one fourth the original sample, after three half-lives one eight the original sample, and so forth.

GM counter

The Geiger–Müller counter, also called a Geiger counter, is an instrument used for measuring ionizing radiation. It detects radiation such as alpha particles, beta particles and gamma rays using the ionization produced in a Geiger–Müller tube, which gives its name to the instrument.

The original detection principle was discovered in 1908, but it was not until the development of the Geiger-Müller tube in 1928 that the Geiger-Müller counter became a popular instrument for use in such as radiation dosimetry, radiological protection, experimental physics and the nuclear industry. This was mainly due to its robust sensing element and relatively low cost, however there are limitations in measuring high radiation rates and in measuring the energy of incident radiation.

The Geiger counter consists of two main elements; the Geiger-Müller tube which detects the radiation, and the processing and display electronics. The Geiger-Müller tube is filled with an inert gas such as helium, neon, or argon at low pressure, which briefly conducts electrical charge when a particle or photon of incident radiation makes the gas conductive by ionization. The ionization current is greatly amplified within the tube by the Townsend avalanche effect to produce an easily measured detection pulse. This makes the GM counter relatively cheap to manufacture, as the subsequent electronic processing is greatly simplified.

The counts readout is normally used when alpha or beta particles are being detected. More complex to achieve is a display of radiation dose rate, displayed in a unit such as the sievert which is normally used for measuring gamma or X-ray dose rates. However a GM tube can detect the presence of radiation, but not its energy which also influences the radiation’s ionising effect.

The electronics also generates the relatively high voltage, typically 400–600 volts, that has to be applied to the Geiger-Müller tube to enable its operation.

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Limitations

There are two main limitations of the Geiger counter. Because the output pulse from a Geiger-Müller tube is always the same magnitude regardless of the energy of the incident radiation, the tube cannot differentiate between radiation types. A further limitation is the inability to measure high radiation rates due to the “dead time” of the tube. This is an insensitive period after each ionization of the gas during which any further incident radiation will not result in a count, and the indicated rate is therefore lower than actual. Typically the dead time will reduce indicated count rates above about 104 to 105 counts per second depending on the characteristic of the tube being used. Whilst some counters have circuitry which can compensate for this, for accurate measurements ion chamber instruments are preferred for high radiation rates.

Scintillation Counter

A scintillation counter is an instrument for detecting and measuring ionizing radiation. It consists of a scintillator which generates photons of light in response to incident radiation, a sensitive photomultiplier tube which converts the light to an electrical signal, and the necessary electronics to process the photomultiplier tube output.

Scintillation counters are widely used because they can be made inexpensively yet with good quantum efficiency and can measure both the intensity and the energy of incident radiation

Operation

 

animation of radiation scintillation counter

When a charged particle strikes the scintillator, its atoms are excited and photons are emitted. These are directed at the photomultiplier tube’s photocathode, which emits electrons by the photoelectric effect. These electrons are electrostatically accelerated and focused by an electrical potential so that they strike the first dynode of the tube. The impact of a single electron on the dynode releases a number of secondary electrons which are in turn accelerated to strike the second dynode. Each subsequent dynode impact releases further electrons, and so there is a current amplifying effect at each dynode stage. Each stage is at a higher potential than the previous to provide the accelerating field. The resultant output signal at the anode is in the form of a measurable pulse for each photon detected at the photocathode, and is passed to the processing electronics. The pulse carries information about the energy of the original incident radiation on the scintillator. Thus both intensity and energy of the radiation can be measured.

The scintillator must be in complete darkness so that visible light photons do not swamp the individual photon events caused by incident ionising radiation.

Detection materials

The scintillator consists of a transparent crystal, usually a phosphor, plastic (usually containing anthracene) or organic liquid that fluoresces when struck by ionizing radiation.

Cesium iodide (CsI) in crystalline form is used as the scintillator for the detection of protons and alpha particles. Sodium iodide (NaI) containing a small amount of thallium is used as a scintillator for the detection of gamma waves and Zinc Sulphide is widely used as a detector of alpha particles. Zinc Sulphide is the material Rutherford used to do his scattering experiment. Lithium iodide (LiI) is used as a neutron detector.

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Applications

Scintillation counters are used to measure radiation in a variety of applications including hand held radiation survey meters, personnel and environmental monitoring for Radioactive contamination, medical imaging, national and homeland security, nuclear plant safety, radon levels in water, and oil well logging.

Several products have been introduced in the market utilising scintillation counters for detection of potentially dangerous gamma-emitting materials during transport. These include scintillation counters designed for freight terminals, border security, ports, weigh bridge applications, scrap metal yards and contamination monitoring of nuclear waste. There are variants of scintillation counters mounted on pick-up trucks and helicopters for rapid response in case of a security situation due to dirty bombs or radioactive waste. Hand-held units are also commonly used.

 

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