National Standard for Dosimetric Quantities of X-ray Radiation

Metrological Measurements of Dosimetric Quantities of X-ray Radiation

Metrological measurements of dosimetric quantities of X-ray radiation are performed in reference X-ray beams generated by an X-ray tube. X-ray radiation consists of an energy spectrum of electromagnetic radiation ranging from approximately 10 keV to 400 keV. By adjusting different filtration settings and the voltage generated by the X-ray tube, it is possible to shape the energy spectrum of X-ray radiation to the desired energy level.

The primary dosimetric quantities include:

• Air kerma (Ka) – determined by calculating the ionization charge (current) in an ionization chamber and measuring temperature, pressure, and humidity at the chamber location.
• Absorbed dose in water – essential for medical applications.

The determination of these quantities relies on either:

1. The physical parameters of the ionization chamber and the air contained within it.
2. The calibration factor of the ionization chamber.

The calculation process also incorporates various correction factors.

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Significance of Air Kerma in X-ray Radiation

Air kerma (Ka) for X-ray radiation is a fundamental dosimetric quantity that plays a key role in ensuring the quality and accuracy of all dosimetric measurements. It is directly linked to:

• Ambient dose equivalent (H)*
• Personal dose equivalent (Hp(d))
• Dose rate values

These quantities are calculated based on international recommendations, standards, and procedures.

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Measurement Corrections

To determine the dose rate at a given time, corrections are applied, primarily for:

• Temperature
• Pressure
• Radioactive decay

Air kerma is realized using free-in-air measurements with:

• Ambient dosimeters (freestanding instruments)
• Personal dosimeters (placed on a phantom simulating the human body)

The air kerma rate is linked to secondary ionization chamber standards that are further traced to international metrology institutes.

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Composition of the National Standard for X-ray Radiation

The National Standard for X-ray Radiation consists of:

• Set of standard ionization chambers for measuring the air kerma rate.
• Transmission monitoring chamber PTW type 786.
• Standard ionization current meters (KEITHLEY 6517A).
• Reference X-ray beams generated in an MG324 Philips irradiator with a Philips MCN 321 X-ray tube.
• Measurement bench with three-axis movement, allowing a maximum distance of 5 meters from the irradiator.
• Three sets of collimating diaphragms.
• Goodfellow energy filters for producing X-ray beams of selected narrow spectrum qualities (ISO 4037).
• Gammex RMI 115A filters for determining half-value layers.
• X-123 CdTe spectrometer for X-ray spectrum measurements.
• ISO PMMA and water phantoms.
• Atmospheric pressure, temperature, and humidity sensors (ALMENO) for monitoring laboratory conditions.

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Basic Metrological Properties of the Standard for Narrow Spectra (40 – 300 keV)

Quantity	Symbol	Range	Expanded Uncertainty (Uc, k=2) [%]
Air kerma in X-ray radiation	Ka	5 × 10⁻⁴ Gy ÷ 5 × 10⁻¹ Gy	1.6%
Air kerma rate in X-ray radiation	K̇a	7.5 × 10⁻⁴ Gy·h⁻¹ ÷ 2.5 × 10⁻¹ Gy·h⁻¹	1.6%
Ambient dose equivalent	H*	1 × 10⁻³ Sv ÷ 5 × 10⁻¹ Sv	2.6%
Ambient dose equivalent rate	Ḣ*	1 × 10⁻³ Sv·h⁻¹ ÷ 4 × 10⁻¹ Sv·h⁻¹	2.6%
Personal dose equivalent	Hp(d)	5 × 10⁻³ Sv ÷ 5 × 10⁻¹ Sv	2.6%
Personal dose equivalent rate	Ḣp(d)	1 × 10⁻³ Sv·h⁻¹ ÷ 4 × 10⁻¹ Sv·h⁻¹	2.6%

 

Documents for download

• Osvedčenie (certifikát) (pdf, 1.1 MB)