Prog Med Phys.  2022 Mar;33(1):1-10. 10.14316/pmp.2022.33.1.1.

Effective Volume of the Korea Research Institute of Standards and Science Free Air Chamber L1 for Low-Energy X-Ray Measurement

• Yi CY
• Kim YH
• Jeong DY

• KMID: 2528099
• DOI: http://doi.org/10.14316/pmp.2022.33.1.1

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.
Methods
The mechanical dimensions were measured using a 3-dimensional coordinate mea suring machine (3-d CMM, Model UMM 500, Carl Zeiss). The diameter of the diaphragm was mea sured by a ring gauge calibrator (Model KRISS-DM1, KRISS). 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.
Results
The measured length of the collector was 15.8003±0.0014 mm with a 68% confidence level (k=1). The aperture diameter of the diaphragm was 10.0021±0.0002 mm (k=1). The mechanical measurement volume of the KRISS FAC L1 was 1.2415±0.0006 cm ³ (k=1). The elongated length of the collector due to the electric field distortion was 0.170±0.021 mm. Considering the elongated length, the effective measurement volume of the KRISS FAC L1 was 1.2548±0.0019 cm³ (k=1).
Conclusions
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 effective volume will replace the existing mechanically determined volume in establishing and maintaining the primary standard of the low-energy X-ray.

Keyword

Primary standard; Air kerma; Low-energy X-ray; Free air chamber; Effective volume

Figure

• 

  Fig. 1 Cross-sectional view of the KRISS FAC L1 for low-energy X-ray measurement. The front wall was composed of 10-mm-thick stainless steel. The diaphragm was supported by a tungsten holder. Front view (a) and side view (b). Note that the units are given in mm and the figures are not to scale. KRISS FAC, Korea Research Institute of Standards and Science free air chamber.

• 

  Fig. 2 Field line was drawn from the center of the air gap in the width of the collector electrode (a) and the field line drawn from the air gap in the length of the collector electrode (b). The X-ray beam axis is perpendicular to the figure plane (a), whereas the beam is incident from the left and exits right (b).

• 

  Fig. 3 Electric field line drawn along the height at the center of the air gap. The surface plane of the collector electrode was 0 mm-high, and the HV plate was 70 mm-high. The thin line indicates the air gap in the width of the collector, and the thick line indicates the air gap in the length of the collector. The ratio (∆w⁄∆l) in Eq. (6) was 2.06.

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  Fig. 4 Simulation geometry for calculating the energy deposition in the air slabs. An X-ray beam with a diameter of 10 mm was incident on the center of the front face of the air box. Fourteen air slabs with a thickness of 5-mm were piled from the bottom to the top of the air box and the energy deposited portion in each slab was calculated.

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  Fig. 5 Ratio of Ai, the sum of the cross-sectional areas of the paired 5-mm-thick slices in the elongated volume to A0 (=0.85 mm2) that of the cross-sectional areas of the paired 5-mm-thick slabs with the same length equal to the peak elongation of 0.085 mm. KRISS FAC, Korea Research Institute of Standards and Science free air chamber.

• 

  Fig. 6 Cross-sectional view of the collection volume, the elongated volume, and the effective collection volume.

Reference

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