As the air kerma is defined as the mean sum of the initial kinetic energies of all charged particles liberated in air by the incident photons divided by the mass of air, the volume of the FAC should be carefully evaluated when wishing to develop the primary standard air kerma ensuring traceability to the International System of Units. (2), the measurement volume of the FAC is an essential physical factor in conducting the absolute measurement of the air kerma using the FAC. Given that the air mass is usually determined by multiplying the dry air density by the measurement volume, as given in Eq. Where ρ air is the dry air density, V 0 is the measurement volume of air kerma, r 0 is the aperture diameter of the diaphragm, and l is the length of the FAC collector electrode. The air mass m air in the collection volume of FAC is given by the following: (2) m a i r = ρ a i r V 0 = ρ a i r π r 0 2 l Where K ˙ is the kerma rate, ( W air/ e) is the mean energy required to create an ion pair in dry air, I is the ionization current measured by the FAC, m air is the air mass in the collection volume of the FAC, (1- g) is the mean energy loss in air due to the bremsstrahlung of the secondary electrons generated by the incident X-ray beams, and k i is various correction factors introduced to correct for the limitations of the air kerma measurement using the FAC in view of the definition of air kerma. The air kerma from the X-ray beams in the reference plane is measured by the FAC using the following equation: (1) K ˙ = W a i r / e I / m a i r 1 − g ∏ i k i The structural information of the FAC is critical in realizing the unit of air kerma, and is obtained with sufficient accuracy by measuring it during the manufacturing and assembly stages of the FAC. We manufactured the FACs with our own designs, mainly because they were not commercially available. According to the NMI of Korea, we used two FACs to realize the primary standards of air kermas for low- and medium-energy X-ray beams, including the mammography X-ray beams specified in the International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC) reference documents. Most NMIs have two FACs: one for measuring the low-energy X-ray and the other for measuring the medium-energy X-ray.
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The air kerma standard of the X-ray beams is realized by the free air chamber (FAC), known as the “primary standard device” if it is used to establish the air kerma standard in the national metrology institute (NMI). The air kerma is the basic physical quantity for measuring the X-ray dose for radiation protection purposes in both medical and the industrial fields. Keywords : Primary standard, Air kerma, Low-energy X-ray, Free air chamber, Effective volume The effective volume will replace the existing mechanically determined volume in establishing and maintaining the primary standard of the low-energy X-ray. Considering the elongated length, the effective measurement volume of the KRISS FAC L1 was 1.2548☐.0019 cm 3 ( k =1).Ĭonclusions: The effective volume of the KRISS FAC L1 was determined from the mechanically measured value by adding the elongated volume due to the electric field distortion in the FAC. The elongated length of the collector due to the electric field distortion was 0.170☐.021 mm. The mechanical measurement volume of the KRISS FAC L1 was 1.2415☐.0006 cm 3 ( k =1).
The aperture diameter of the diaphragm was 10.0021☐.0002 mm ( k =1). Results: The measured length of the collector was 15.8003☐.0014 mm with a 68% confidence level ( k =1).
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The elongation of the collector length due to electric field distortion was determined from the capacitance measurement of the KRISS FAC considering the result of the finite element method (FEM) analysis using the code QuickField v6.4. The diameter of the diaphragm was measured by a ring gauge calibrator (Model KRISS-DM1, KRISS). Methods: The mechanical dimensions were measured using a 3-dimensional coordinate measuring machine (3-d CMM, Model UMM 500, Carl Zeiss). Abstract Purpose: To evaluate the effective volume of the Korea Research Institute of Standards and Science free air chamber (KRISS FAC) L1 used for the primary standard device of the low-energy X-ray air kerma.
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