Calibrations, testing and irradiation of radiation meters

Radiation meters for radiotherapy are usually calibrated in terms of absorbed dose to water in ⁶⁰Co gamma radiation beam. Calibration can be performed either for the ionization chamber or, for the entire measurement chain, i.e. ionization chamber, electrometer and measurement cable. Information about the collecting voltages of the ionization chamber and the measurement settings of the electrometer (for example, high range and charge measurement) are required for the calibration. If needed, calibration can also be done in terms of air kerma for ⁶⁰Co gamma radiation.

Radiation meters used in x-ray diagnostics and interventional radiology are calibrated for standardized x-ray qualities. The customer can choose the calibration qualities based on, for example, the tube voltages. The applied radiation qualities depend strongly on the use of the meter.

Typical measurement quantities and radiation qualities:

• conventional ionization chambers: air kerma, RQR radiation qualities (40–150 kV)
• mammography meters: air kerma, MAM radiation qualities (25–35 kV)
• KAP meters: air kerma-area product, RQR radiation qualities (40−150 kV)
• CT chambers: air kerma-length product, RQT radiation qualities (100−150 kV)

All available x-ray qualities are shown in the table (.pdf).

A typical calibration of protection meter is performed with ¹³⁷Cs-gamma radiation in terms of ambient dose equivalent (H*(10)) and at different dose rates (typically three dose rates are used). The meters can also be calibrated with ⁶⁰Co and ²⁴¹Am gamma radiation sources, with ISO narrow-spectrum and ISO high air kerma rate (ISO 4037:2019) x-ray qualities as well as beta and neutron radiation sources. The radiation qualities used depend on the purpose of the meter.

Several different planar sources are in use for calibration of the surface contamination meters. The customer can choose from them the radionuclides used for calibration. The calibration quantity is the surface emission rate.

Available radiation qualities are shown in the table (.pdf).

For meters intended for self-acting protection measures (e.g. housing companies' civil defence shelters), an inspection of the operating condition is usually sufficient instead of calibration. Detailed information about the inspection can be found in the Radiation and Nuclear Safety Authority Regulation on Measurements of Ionizing Radiation (STUK S/7/2021). An inspection certificate is issued for the meter.

In licensed practices, meters must often be calibrated, and an inspection of the operating condition is not an adequate measure. In the inspection, the meter reading is compared to a reference value in different measurement ranges and the linearity of the meter and alarm functions are checked. The inspection is performed using ¹³⁷Cs gamma radiation.

Radon measuring instruments are used for integrated, continuous or spot measurements (for example, track-etch detectors, semiconductor detectors, ionization chambers and LUCAS chambers). The calibration of radon (²²²Rn) and thoron (²²⁰Rn) measuring instruments is performed by comparing the instrument reading with the known activity concentration of radon in the air (²²²Rn or ²²⁰Rn gas). Exposures of ²²²Rn of different lengths of time at the desired radon concentration (without calibration) can also be performed, for example, for  alpha-track detectors.

In irradiation, the target is irradiated to a certain predetermined dose, and an irradiation certificate is issued for this. Irradiation can be performed with ¹³⁷Cs, ⁶⁰Co and ²⁴¹Am gamma radiation sources and with ISO narrow-spectrum and ISO high air kerma rate (ISO4037:2019) x-ray qualities, as well as beta and neutron radiation sources. 

Calibrations and measurements can also be carried out at the place of use of radiation as agreed. Standardization tests of x-ray diagnostic equipment and irradiation of radiation meters, biological samples and electronic components are also carried out. These will be agreed separately with the customers.