DOSE CALIBRATORS

The current NRC regulations no longer mention the accuracy, constancy, linearity, and geometry tests. According to 10 CFR Part 20 Section 35.60, a licensee shall calibrate the radioisotope dose calibrator in accordance with nationally recognized standards or the manufacturer’s instructions. The bottom line: current manufacturers’ recommendations are identical to the old NRC regulations so our requirements for the accuracy, constancy, linearity, and geometry tests are annually, daily, quarterly, and at installation, respectively.

Deviation from standard or expected values for all dose calibrator tests must be within ± 10%. If Deviation >10%, then obligation is to record value, note repair or recalibration of instrument, retest, and record new values. In addition to the above steps, every dose must be corrected mathematically until the instrument is repaired. There is NO LONGER a reporting requirement

The accuracy test is performed at installation and annually. It is designed to show that the calibrator is giving correct readings throughout the entire energy scale that we are likely to encounter. Low, medium, and high energy standards (usually Co-57, Ba-133 or Cs-137, and Co-60, respectively), are measured in the dose calibrator using appropriate settings. Standard and measured values are compared.                                                                                                                                                                                                                                          

This accuracy test is a pass:

The constancy test is performed at installation and daily. It measures instrument precision and is designed to show that a long-lived source, usually 30 y Cs-137, yields reproducible readings on a daily basis on all isotope settings we are likely to use. The Cs-137 source is placed in the dose calibrator. Activity is then measured on the Cs-137 setting and all other settings used on a daily basis. Values are recorded in the dose calibrator logbook and are compared with recent values to determine if instrument is maintaining constancy on a daily basis.

This constancy test is a pass:                                       

The linearity test, performed at installation and quarterly, is designed to prove that the dose calibrator readout is linear for sources varying from the mCi range through the mCi range. A high activity Tc-99m source (50-300 mCi) is measured at T₀ and at predetermined time intervals up to 72 hours. Expected and actual measurements are compared (and may be analyzed graphically) to determine if the instrument is linear throughout the activity range we are likely to encounter.

This linearity Test is a Pass:

The Geometry Test, performed at installation, is designed to show that correct readings can be obtained regardless of the sample size or geometry. 0.5 ml of Tc-99m in a 10 ml syringe (activity 25 mCi) is measured in the dose calibrator and the value obtained is recorded. The activity is then diluted with sterile water to 1 ml, 2 ml, 3 ml, 4 ml, etc. At each of these points a reading is taken and the value recorded. Data are then evaluated to determine the effect of sample geometry on the dose calibrator reading. If instrument is geometry-dependent, it may be necessary to routinely correct readings obtained when using calibrator.

This Geometry Test is a Pass: