The BIPM standard for air kerma in  -radiation is a series of nominally identical parallel-plate cavity ionization chambers with a disc-shaped graphite collecting electrode at the centre of the measuring volume, and graphite walls. The nominal measuring volume is 6.8 cm3 and the normal polarizing potential is 80 V.
For Co-60 air-kerma comparisons, the NMI primary standard is normally transported to the BIPM for direct comparison with the BIPM standard, although when it is not desirable or practicable to transport the primary standard the comparison may be made indirectly using one or more transfer ionization chambers.
For Cs-137 air-kerma comparisons, the NMI primary standard is normally transported to the BIPM for direct comparison with the BIPM standard, although when it is not desirable or practicable to tranport the primary standard the comparisons may be made indirectly using one or more transfer ionization chambers. However, as the activity of the 137Cs source is only 1 TBq, a chamber volume of at least 1 cm3 is necessary.
NMI primary standard in the 60Co beam.
The BIPM primary standard is on the right. |
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Setting up the BIPM standard in the 137Cs beam |
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Notes:
• Kerma is the quantity that represents the kinetic energy transferred to charged particles by the uncharged particles per unit mass of the irradiated medium.
• The word kerma is effectively an acronym for kinetic energy released in material. It was coined by the ICRU in 1962 [ICRU Report 10a (1962), published as NBS Handbook 84] and in the SI has units of J/kg.
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Summary
