Various types of instrumentation may be used to detect the presence and/or amount of radioactive material. Methods used range from direct measurement to the counting of samples. Three types of monitoring equipment and their required uses are listed below:
- Constant Area Monitors
- required for areas where continuous radiation levels may exceed 5 mR/hr
- should be calibrated annually by the RSO and be in good working order
- Portable Survey Meters
The PH must have a portable survey meter in his/her possession if the permit experimental limits exceed those listed below. It is sometimes permissible for more than one PH to share a common instrument; however, assurances must be made to the RSO that such an arrangement is practical and will not result in failure to perform adequate surveys.
Type of Radiation Energy Example Experimental Limit Requiring a Meter Type of Detector Beta Emitter < 50 keV 3H NA NA Beta Emitter 50 keV to 300 keV 14C, 35S,33P 1 mCi or more Pancake or End Window GM, or Beta Scintillator Beta Emitter > 300 keV 36Cl, 32P Any Pancake or End Window GM, or Beta Scintillator X/Gamma Emitter ≤ 100 keV 125I 1 mCi or more ∼2 mm NaI X/Gamma Emitter > 100 keV 111In,51Cr, 22Na Any ∼2.5 cm NaI, or End or side window GM
Consultation with the RSO is advisable prior to the purchase of any survey instrument. All survey instruments are calibrated by the RSO initially, annually, and after repair. To assure such calibrations are performed, each PH should notify the RSO when new survey instruments are purchased or when existing instruments are repaired. The PH is responsible for all repairs and replacing dead batteries.
- 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)
- 1 = 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 (1) and the elapsed time must be in corresponding units (e.g., if 1 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.