Answer to Question #9038 Submitted to "Ask the Experts"

I want to know if EPDs (electronic personal dosimeters) require calibration or not.

The short answer to your question is, "Yes, electronic personal dosimeters do require calibration." These devices may be initially calibrated by the manufacturer, but electronic components can degrade, electronic settings may change, and various physical effects can occur that may alter the performance of the dosimeters.

Many electronic dosimeters use a number of self-test procedures built into the devices to ensure that general operational requirements are being met, but such internal checks do not usually satisfy all the requirement for calibration. Some facilities use a self-contained irradiation system to carry out performance checks and/or limited calibration procedures to demonstrate consistency of response. An example of one such irradiation device is shown at Thermo Scientific. Such systems are often well suited to doing checks of devices to determine whether they are responding as expected, but they may not obviate the need for doing more complete calibrations with dosimeters mounted on appropriate phantoms.

We shall assume that the manufacturer has done sufficient testing to demonstrate that the dosimeters of interest meet required performance criteria for the measurement of dose from radiations of specific types and energies. Through appropriate calibration procedures, the manufacturer will determine calibration factors for a given dosimeter and will write these into firmware or software that operate to convert EPD responses to the proper doses or dose rates. The readers supplied by manufacturers may or may not have built-in capability for readjusting these calibration factors as necessary, and the manufacturer's recommendations should generally be followed.

For routine use applications, most electronic dosimeters will likely require some level of calibration at least once per year. This frequency is also generally consistent with many regulatory agencies' recommendations for instrument calibration. For the most common photon-sensitive devices, the procedure may involve evaluating responses to at least two different energy photon sources, sources such as ¹³⁷Cs and ²⁴¹Am being common. If the responses are within acceptable limits the dosimeters may be assumed to have remained in calibration. If responses are unacceptable, adjustments of the dosimeter calibration factors or other modifications may be necessary. This may require returning the dosimeters to the manufacturer for recalibration.

Specific calibration requirements vary among different user facilities and among different dosimeter types. You should work in conjunction with your calibration staff and the dosimeter manufacturer, taking account of specific requirements and recommendations of appropriate regulating agencies, to implement a satisfactory calibration program for the EPDs.

George Chabot, PhD, CHP