By virtue of this project, a primary standard for China and a verification scheme to measure air ...

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Establishment and Research of Primary Standards In (10-60) kV X-Rays

 

Summary:

By virtue of this project, a primary standard for China and a verification scheme to measure air kerma in (10-60) kV x-ray are established. It is the first time we have set up a national primary standard. It has solved many difficult problems in ionizing chamber traceability, semi conductor detectors and any other x-ray measurement using low energy x-rays. The measure work done through a newly designed low energy vacuum chamber reveals a combined relative uncertainty of 0.25%. In order to gain international acknowledgment, the lab has participated in key comparison for air kerma in low energy x-rays in Asia Pacific Metrology Programme (APMP). The primary standard has shown a good agreement with APMP reference value.

 

Description:

Among all unit names derived by SI (international System of Units), Gray (Gy) is the most important in x-rays dosimetry. The relevant quantity air kerma is widely used in x-ray diagnosis, radiotherapy and industry application. Absorbed dose and dose equivalent could be obtained by air kerma and transfer coefficient. The air kerma measurement in (10-60) kV x-rays is a key comparison content in CCRI and RMOs. Establishing a primary standard to produce and disseminate air kerma in low energy x-rays is absolutely important in the field of ionizing radiation. This project resolved the problem of air kerma tracing in low energy x-rays.

   A low energy free -air ionizing chamber was designed to measure the absolute value of air kerma according to its definition. This chamber is a parallel-plate type. An X-ray beam passes through a defining aperture, whose size is accurately known, then enters the collecting area. A secondary electrons released by primary x-ray lose all their energy after being ionized. All electrical charges were collected by a collecting electrode. There is a high EMF between high voltage electrode and colleting electrode. A guard bar system, to which graded EMFs are applied, together with guard electrodes, ensures the field lines to be parallel to each other and perpendicular to the surface of the electrodes. The values of several correction factors including recombination, air attenuation, field distortion, front penetration, high voltage polarity, scattered photon and electron loss were obtained by experiment and simulation. The uncertainties of these values were both evaluated by A and B type analysis.

  Besides the air-free chamber, an x-rays facility was also home-designed and established. It can produce x-rays whose beam qualities are recommend by CCRI for comparison and national or international standards, such as ISO 4037, IEC61267. The automatic filtration system is convenient to change one beam quality from the other by rotating the wheel. A developed diaphragm and a shuttle are to control beam size in reference plane and x-ray exposure time, respectively. A precision 5-meter linear positioning track and multifunction carriage were designed to navigate calibrated chambers or to measure length. The half value layer and radiation filed characteristic were measured and evaluated. The x-ray spectra of different beam codes is measured and researched by a new high purity x-ray spectrometer.

   The relative combined uncertainty of developed primary standard reaches 0.28% and was confirmed by APMP.RI(I)-K2 comparison. This primary standard was authorized by government and began to disseminate air kerma for secondary laboratory and customers in 2012. The verification work organized by Technical Committee for air kerma on (10-60) kV x-ray is concluded. Its publication is due this year.

 X-ray radiation facilities, filtration system, positioning track and carriage