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Secondary Standard for Personal X-ray Dosimetry

PTB has developed a new secondary standard for the operational quantity «personal dose equivalent», Hp(10), in use in X-ray radiation protection. The standard resolves several problems concerning the calibration of whole-body personal dose meters with low-energy photon fields.

PTB’s new secondary standard for the operational quantity «personal dose equivalent» Hp(10) comprises an ionisation volume inside a polymethyl metacrylate (PMMA) slab (30 cm x 30 cm x 15 cm). A step cylinder in the centre of the front surface leads to an almost constant energy and angle dependence with respect to Hp(10).

In order to evaluate the risk of ionising radiation to human bodies in a more specific way rather than measuring the dose on the surface of a person’s body, the quantity «personal dose equivalent» Hp(10) has been introduced by international institutions. The quantity Hp(10) is defined in 10 mm depth in a person’s body in order to take into account, e. g., the radiation exposure of deep-lying organs. This quantity has to be transferred into national legislation of the countries of the European Community in 2000. For calibration purposes, the quantity is defined in a slab phantom, and is determined from the «old» quantity air kerma (kinetic energy released in material, here: released in air) by means of conversion coefficients. For reference radiation fields, these coefficients are tabulated in the international standard ISO 40 37-3.

A problem not solved in this standard is the calibration in photon fields with a mean energy lower than about 25 keV. In this range of photon energies, the conversion coefficients depend strongly on the spectral distribution of the photon radiation, which is influenced by both the air density and the characteristics of the X-ray facility used to generate the reference photon field. Small spectral differences may lead to conversion coefficients for nominally the same X-ray field which differ by up to several ten per cent.

PTB has developed a new secondary standard for photon radiation which measures the true value of the personal dose equivalent Hp(10) on a slab phantom directly. This procedure avoids the use of conversion coefficients. The standard is optimised to provide a nearly constant response with respect to Hp(10) for photon energies from about 10 keV to 1400 keV and for angles of incidence from 0° to 75°. The secondary standard is commercially available.

Contact at PTB:

Phone: +49-531-592-0