National Standard of Gamma Radiation

Metrological Measurements of Dosimetric Quantities of Gamma Radiation

Metrological measurements of dosimetric quantities of gamma radiation are conducted in reference photon beams generated by radionuclide sources 137Cs^{137}Cs137Cs and 60Co^{60}Co60Co. The air kerma is determined by calculating the ionization charge (current) measured in an open-air ionization chamber, alongside measurements of temperature, pressure, and humidity at the chamber’s location. The determination is based either on the physical parameters of the ionization chamber and the air it contains or on the calibration factor of the ionization chamber. Additional correction factors are also applied in the calculations.

Calibration and Traceability of Dosimetric Quantities

The kerma rate scale is realized through standard measuring instruments and reference photon fields generated by radionuclide sources. These reference fields are calibrated at test reference points. The following scales are derived from the kerma rate scale in air:

• Ambient dose equivalent rate
• Personal dose equivalent rate
• Absorbed dose rate in water

To determine the kerma rate at a given time, corrections are applied, primarily for temperature, pressure, and radioactive decay. High therapeutic dose rates are measured using an ionization chamber serving as the primary standard. For low and medium dose rates, secondary ionization chambers are traceable to the primary standard maintained at the Slovak Institute of Metrology (SMÚ).

Composition of the Standard Equipment

• Two calibration gamma irradiators with eight 137Cs^{137}Cs137Cs sources (activities ranging from 200 MBq to 40 TBq).
• One calibration gamma irradiator with a 60Co^{60}Co60Co source (activity 140 TBq).
• Measurement benches with three-axis movement, enabling source-to-irradiator distance adjustment (0.5–8 m).
• Set of standard ionization chambers, including:
  • Primary carbon ionization chamber (1 cm³)
  • Air-equivalent ionization chambers (volumes ranging from 0.02 dm³ to 10 dm³)
  • Pressurized ionization chamber for natural background radiation measurements
• Standard electrometers:
  • KEITHLEY 6517A
  • KEITHLEY 6517B
  • PTW UNIDOS T10001

This national standard ensures high-precision dosimetric measurements in compliance with international metrological guidelines.

Documents for download

• Osvedčenie (certifikát) (pdf, 2.8 MB)