H. Instrumentation

3. Contamination Survey Instrumentation

This instrumentation (e.g., liquid scintillation counters, gamma counters) is required for counting contamination (wipe) survey samples. Since these surveys are semi-quantitative, the counting efficiency for this equipment must be determined at least annually. The RSO will contact the individual responsible for maintaining this requirement each year, requesting an updated efficiency check for the instrument. Note: Some of these instruments have internal sources in them. These sources should be removed by the vendor before an instrument is disposed.

Instrument counting efficiency

The appropriate method for determining counting efficiency is provided below:

A "standard" must be utilized or created to determine counting efficiency. Standards may either be vendor supplied or created by withdrawing an aliquot from a vial of known radioactive concentration. Depending upon the activity and concentration of the standard, it may be necessary to dilute the radioactive material to achieve microcurie (mCi) amounts.

For radionuclide standards with long half-lives (e.g., ¹⁴C - 5760 year half-life) correction for radioactive decay is not necessary. For radionuclide standards with shorter half-lives (e.g., ³H - 12.3 year half-life), it may be necessary to correct for radioactive decay. Decay correction is accomplished utilizing the following equation:

A₁ = A₀e^-lt

A₀ = Original activity (mCi)

A₁ = Decay-corrected activity (mCi)

l = 0.693 ) half life of the radionuclide (min^-1, days^-1, or yr^-1)

t = elapsed time between original activity date and present activity date (min., days, or yr.)

Note: The decay constant (l) and the elapsed time must be in corresponding units (e.g., if l is in days^-1 then the elapsed time must be in days).

Convert the known microcurie amount in the standard to disintegrations per minute (dpm) using the following conversion (some standards may already be converted to dpm):

dpm = (mCi) X (2.22 X 10⁶ dpm/mCi)

Place the standard in the counter and determine counts per minute (cpm).
Use the following equation to calculate efficiency:

efficiency = cpm ) dpm

If the counting efficiency is less than 0.10 (10%), the RSO should be consulted.

Note: This efficiency is specific for the radionuclide used as the "standard." If other radionuclides are being counted, the counting efficiency for those radionuclides may vary depending upon the number of particles, x-rays, and/or gamma rays given off per disintegration for that particular radionuclide. Additional correction factors may be necessary to determine the true efficiency for other radionuclides. Consult the RSO for assistance in determining counting efficiencies for radionuclides other than the standard.

If a liquid scintillation counter (LSC) is used to analyze wipe surveys, a ³H standard may be used to calculate the counting efficiency. Due to its low energy beta, ³H provides a conservative efficiency value. If a gamma counter is used to count wipe surveys, the efficiency must be calculated for each gamma emitter that may be used.

This information must be updated and forwarded to the RSO following maintenance/repair or when new instrumentation is purchased.

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This page last modified on Monday April 04, 2005
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