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      The Metrology of a Rastered Spot of X Rays used in Security Screening

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          Abstract

          In recent times, ionizing radiation has been used around the world to screen persons for non-medical purposes, namely to detect bulk explosives or other contraband hidden on the body including materials not registered by metal detectors. In contrast to conventional transmission or projection imaging, backscatter and forward-scatter systems employ a “flying spot” of x rays and large-area detectors. A small spot is rastered across an individual and the Compton scatter signal collected by these detectors is quickly integrated and assigned to a pixel value in an image corresponding to the transient location of the small flying spot. These systems have been controversial due in part to possible radiation health risks, and lack of independent and accurate measurements of radiation exposures to the subjects, bystanders, and operators of such systems. In this paper we will outline the techniques and instrumentation used at the National Institute of Standards and Technology (NIST) to accurately determine the incident air kerma from a swept beam of x rays. We discuss in detail the response of a large-area free-air ionization chamber under the unusual temporal and spatial radiation fields delivered by commercial scanning systems and report typical values for air kerma levels as well as estimates of air kerma rates.

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          Most cited references 18

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          International Organization for Standardization

          (2008)
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            The recombination correction for an ionisation chamber exposed to pulsed radiation in a 'swept beam' technique. I. Theory.

             J. Boag (1982)
            The established theory for the collection efficiency of an ionisation chamber irradiated by a series of uniform short pulses is extended to the more complex case of irradiation in a pulsed and electromagnetically swept electron beam. The radial variation of exposure rate is assumed to be Gaussian with scale constant equal to alpha, and the pulses are assumed to be uniformly distributed over the area swept out. A formula for the collection efficiency is derived and the influence of varying some of the parameters is studied. The collection efficiency, phi 1, at voltage V1 can be calculated if the two dosemeter readings, Q1 and Q2, at two widely different chamber voltages, V1 and V2, are known. phi 1 depends primarily upon the ratios Q1/Q2 and V1/V2 and to a much lesser extent upon the geometry of the chamber or the ratio of Gaussian scale constant to a typical linear dimension of the field scanned. Charts for calculating phi 1 are provided.
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              XCOM: photon cross sections database

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                Author and article information

                Contributors
                Journal
                J Res Natl Inst Stand Technol
                J Res Natl Inst Stand Technol
                JRES
                Journal of Research of the National Institute of Standards and Technology
                [Gaithersburg, MD] : U.S. Dept. of Commerce, National Institute of Standards and Technology
                1044-677X
                2165-7254
                2014
                06 November 2014
                : 119
                : 540-553
                Affiliations
                National Institute of Standards and Technology, Gaithersburg, MD 20899
                Article
                jres.119.021
                10.6028/jres.119.021
                4487281

                The Journal of Research of the National Institute of Standards and Technology is a publication of the U.S. Government. The papers are in the public domain and are not subject to copyright in the United States. Articles from J Res may contain photographs or illustrations copyrighted by other commercial organizations or individuals that may not be used without obtaining prior approval from the holder of the copyright.

                Funding
                Funded by: National Institute of Standards and Technology
                Award ID: 9999-NIST
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