The cobalt-57 or cadmium-109 isotopes give off gamma and X rays at all times. Since the intensity of this emission decreases over time (‘decay rate’), the radiation source in the XRF analyzer must be replaced periodically so that the instrument remains capable of detecting lead-based paint in a sample in a reasonable amount of time.
Each radioactive element decays at a specific rate. Decay refers to the spontaneous transformation of a radioactive element into another element, which may or may not be radioactive (e.g., cobalt-57 over time converts to an isotope of iron). The term half-life refers to the time it takes half of the material to decay into another radionuclide or element.
The amount of time that it takes for an XRF analyzer to determine if lead is present is related to the number of source atoms that decay and emit gamma and X rays during sampling. After one half-life, the length of time it takes to obtain a proper reading doubles. Today’s XRF analyzers normally correct automatically for source decay by extending the length of time of a reading.
The half-life for cobalt-57 is approximately 9 months and the cadmium-109’s half-life is approximately 15 months. For example, a reading that takes five second initially will take ten seconds nine months after a cobalt-57 source is installed by the manufacturer into an XRF analyzer. Therefore, the older the source, the longer it will take to conduct a lead-based paint inspection using an XRF analyzer.
Some instruments have been programmed so that if the source is more than a prescribed age, the instrument will not take readings. This forces the owner to get a new source. The cost of resourcing and XRF varies by manufacturer and source strength, but is approximately $1500 to $4000 per source, which includes factory calibration.
You are advised to contact the manufacturer before shipping the instrument for re-sourcing to ensure that a source is available. Because the sources decay whether in an instrument or not, it is not cost effective for a manufacturer to have sources sitting on the shelf waiting for a buyer. Likewise, an instrument owner will be paying for source decay whether or not the instrument is being use for lead-based paint inspections.
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