Dosimetry System Calibration - Overview

The practice of dosimetry involves estimating absorbed dose from a quantifiable change in a ‘dosimeter’. A 'dosimeter' is made from a material that has a unique measurable property that responds in a predictable way when exposed to a source of ionizing radiation. Calibration is the process of relating a dose traceable to a national standard to the change in the measurable property of a dosimeter.

It is important to be aware that one does not calibrate dosimeters but rather one calibrates a dosimetry system for use with a specific batch of dosimeters. The process of calibrating dosimeters is actually called ‘dosimetry system calibration’ because the process calibrates the system as a whole and not just the routine dosimeters that are only a part of the system. A Dosimetry System is defined in ASTM E2628 and is comprised of the dosimeters, measurement instrumentation and their associated reference standards, and procedures for the system’s use (this would include the calibration curve fitting and the software or other method used to routinely estimate a dose value). Since each of the components of the system can affect the accuracy of the measurement, the system must be calibrated as a whole. If components of the measurement system are changed, the user must assess any impact on the system’s measurements before implementation and determine any impacts on the system calibration.

Dosimeter batch calibration, also known as ‘dosimetry system calibration’, is necessary to establish a mathematical relationship between the dosimeter response and dose (Gy or kGy).

Calibration of a routine dosimetry system must follow ASTM 51261 Standard Practice for Calibration of Routine Dosimetry Systems for Radiation Processing. See NPL Report CIRM 29 “Guidelines for the Calibration of Dosimeters for use in Radiation Processing” for guidance. The dosimetry system must be calibrated to achieve dose values traceable to an international standard (per ASTM 52628 Standard Practice for Dosimetry in Radiation Processing).

A dosimetry system calibration that is suitable for the conditions of use (provides dose measurements for a specific radiation facility from a specific measurement system) must be demonstrated to be valid under those conditions of use, or even better, actually conducted under the conditions of use. It also must be traceable to national or international standards, meaning that an unbroken chain of comparisons is linking the national standard with the dose measurement at the facility. The dose values used in calibration must be traceable, and this can be obtained if the doses are certified by an accredited laboratory.