Minniti R, Chen-Mayer H, Seltzer S M, Huq M Saiful, Bryson L, Slowey T, Micka J A, DeWerd L A, Wells N, Hanson W F, Ibbott G S
National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8460, USA.
Med Phys. 2006 Apr;33(4):1074-7. doi: 10.1118/1.2184442.
This work reports the transfer of the primary standard for air kerma from the National Institute of Standards and Technology (NIST) to the secondary laboratories accredited by the American Association of Physics in Medicine (AAPM). This transfer, performed in August of 2003, was motivated by the recent revision of the NIST air-kerma standards for 60Co gamma-ray beams implemented on July 1, 2003. The revision involved a complete recharacterization of the two NIST therapy-level 60Co gamma-ray beam facilities, resulting in new values for the air-kerma rates disseminated by the NIST. Some of the experimental aspects of the determination of the new air-kerma rates are briefly summarized here; the theoretical aspects have been described in detail by Seltzer and Bergstrom ["Changes in the U.S. primary standards for the air-kerma from gamma-ray beams," J. Res. Natl. Inst. Stand. Technol. 108, 359-381 (2003)]. The standard was transferred to reference-class chambers submitted by each of the AAPM Accredited Dosimetry Calibration Laboratories (ADCLs). These secondary-standard instruments were then used to characterize the 60Co gamma-ray beams at the ADCLs. The values of the response (calibration coefficient) of the ADCL secondary-standard ionization chambers are reported and compared to values obtained prior to the change in the NIST air-kerma standards announced on July 1, 2003. The relative change is about 1.1% for all of these chambers, and this value agrees well with the expected change in chambers calibrated at the NIST or at any secondary-standard laboratory traceable to the new NIST standard.
本工作报道了空气比释动能 primary standard 从美国国家标准与技术研究院（NIST）转移至美国医学物理学会（AAPM）认可的二级实验室的情况。此次转移于2003年8月进行，其动因是NIST于2003年7月1日实施的60Coγ射线束空气比释动能标准的近期修订。该修订涉及对NIST的两个治疗级60Coγ射线束设施进行全面重新表征，从而产生了NIST发布的空气比释动能率的新值。这里简要总结了确定新空气比释动能率的一些实验方面；理论方面已由塞尔策和伯格斯特龙详细描述[《美国γ射线束空气比释动能 primary standard 的变化》，《国家标准与技术研究院研究杂志》108，359 - 381（2003）]。该标准被转移至由每个AAPM认可剂量学校准实验室（ADCL）提交的参考级电离室。然后使用这些二级标准仪器对ADCL的60Coγ射线束进行表征。报告了ADCL二级标准电离室响应（校准系数）的值，并将其与2003年7月1日宣布的NIST空气比释动能标准变更之前获得的值进行比较。所有这些电离室的相对变化约为1.1%，该值与在NIST或任何可追溯至新NIST标准的二级标准实验室校准的电离室的预期变化非常吻合。
