Radiation Detection Instrument Calibrations

Performing Calibrations Instrument Sensitivity What is Calibration? Register an Instrument

Instruments used to detect radioactive material must be calibrated at least annually, and often more frequently. This includes all portable rate meters (otherwise known as 'Geiger Counters') and dose-rate meters (or Ion Chambers). Liquid scintillation counters and Gamma counters are not subject to the same rules, but must be normalized on a regular basis. Without these procedures the data produced by the instruments can not be trusted. While the requirement for calibration is critical, where the calibration takes place is not.

The UW Radiation Safety Office (RSO) operates an instrument calibration facility (call 206.543.6328 or e-mail: radsaf@u.washington.edu for more information). Costs of meter calibration at the UW facility are comparable to other calibration facilities. Some advantages to using the UW calibration facility are:

• Shortened turn-around time.
• Avoidance of shipping charges for on-campus users.

It is not required that the RSO calibrate your instruments. Calibrations may be performed by any qualified agency or by the instrument owner, provided that it can be demonstrated that the calibration is performed correctly.

To insure compliance with state regulations:

• All survey instruments must be calibrated at least annually.
• A record of the results and the method of calibration must be kept on file in the lab. Records shall be kept for a minimum of 3 years.
• The records must be made available to the RSO or the Washington State Department of Health upon request.

If the Radiation Safety Office does not calibrate your instruments, Washington state law and the UW's Radioactive Materials License requires that they be provided with copies of your calibration records. Call 206.543.6328 or e-mail: radsaf@u.washington.edu for more information.

Instrument sensitivity is frequently raised as an issue when instruments are calibrated. Your instrument should be sensitive enough to detect the following radiation levels:

1. 10 nCi at a distance of 1 cm from the surface for Group I radionuclides.
2. 1 nCi at a distance of 1 cm from the surface for Group II radionuclides.
3. 1 nCi at a distance of 1 cm from the surface for Group III and above radionuclides.

(See the Radionuclide Hazard Groups Table for a list of radionuclides classified into Hazard Groups.)

Radiation detection instruments detect radioactive emissions by various processes. Without going into too much detail, the fact is that they are capable of detecting only a percentage of the emissions that they may encounter. For example, if a source of radiation is emitting 100,000 particles in one minute, the detection instrument might only see .05%, or 50 particles in that minute. The efficiency varies from instrument to instrument and from radionuclide to radionuclide; .05% is a fair generalization for portable rate meters. Liquid scintillation counters produce efficiencies of between 50% and 90% for the radionuclides for which they are designed.

Since the instruments see only a portion of the available detectable particles it is important to be able to infer the correct number from the observed number. Using the above example, you might infer that a reading of 50 observed counts in one minute means that there were really about 100,000 counts.

What calibration does is measure the instrument's response to a known number of counts. The first part involves hooking the instrument to a pulser, a device which sends a discrete number of electronic pulses to the instrument. The instrument's response is measured at several different levels (i.e. 100, 1000, 10,000 pulses) and compared to make sure that the observed counts are within 10% of the known counts. If so, the instrument is considered to be in calibration. The instrument is then exposed to different radioactive sources of known activity to determine efficiencies for each.

Ion chambers are calibrated differently since it is not the number of counts observed by them that is important but their dose response to a known radiation dose. The calibration is achieved by exposing the instrument to the radiation produced by a large source of known activity at several precisely measured distances. The ion chamber will produce a reading at each distance which is compared to the dose that actually is being given off by the source at each of those distances. If the response is within 10% at every distance, then the instrument is considered to be in calibration. If not, adjustments are made until the instrument reads in the correct range.