Airport Passenger Screening Using Backscatter X-Ray Machines Compliance with Standards (2015) / Chapter Skim
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4 Dosimetry for X-Ray Backscatter Advanced Imaging Technology
4 Dosimetry for X-Ray Backscatter Advanced Imaging Technology
Pages 40-47
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From page 40... ...
Quantification of radiation exposure is complicated because a wide range of ionizing radiation is found in nature, and the interactions that those radiation types have with matter are complex. The International Commission on Radiation Units and Measurements (ICRU)
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From page 41... ...
is air kerma.1 In the case of X rays, kerma is the kinetic energy transferred from photons to charged particles in a volume of 1 When a quantity to characterize energy lost by radiation, as opposed to energy deposited in mat ter, was first introduced, it was identified by the acronym KERMA, kinetic energy released in matter. More recently, kerma has been adopted as the name of the quantity.
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From page 42... ...
It measures the electric charge produced in a volume of air defined by two electrodes and a carefully collimated beam of photons. Because the radiation must be limited to a precisely defined beam, the free-air ion chamber cannot be used to measure kerma in a typical radiation environment.
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From page 43... ...
As a result, the absorbed dose, D, is defined as dε/dm, where dε is the element of energy absorbed from ionizing radiation by an element of mass dm.3 D and kerma have the same units but differ in that kerma describes the kinetic energy transferred to electrons without regard to their final destination, and D describes energy deposited without regard to origin. Thus, D does not include the energy escaping the volume but does include energy depositions within the volume from electrons initiated outside of the volume.
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From page 44... ...
Kerma and absorbed dose are numerically equivalent under the conditions of charged particle equilibrium. Effective dose,2 E, is the tissue weighted sum of equivalent doses in specific tissues and organs of the body, and is given by the expression sure to different organs depends on the attenuation of the beam at their locations, the most direct approach is to use a material phantom.
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From page 45... ...
, 2007. 3 The American National Standards Institute/Health Physics Society standard ANSI/HPS N43.17-2009, "Radiation Safety for Personnel Security Screening Systems Using X-Ray or Gamma Radiation," is available at the Health Physics Society website at http://hps.org/hpssc/ index.html.
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From page 46... ...
Of particular concern is the dose to the skin, because in most cases it is adjacent to air or in contact with another material and hence falls into this category. For a beam of a given intensity, the concentration of secondary electrons cre 4 The ANSI/HPS N43.17-2009 standard, "Radiation Safety for Personnel Security Screening Sys tems Using X-Ray or Gamma Radiation," is available at the Health Physics Society website at http:// hps.org/hpssc/index.html.
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From page 47... ...
Because most secondary electrons have much lower energies and therefore much shorter ranges, equilibrium between D and kerma is typically established well within 50 mm of the surface. Because the beam intensity decreases approximately exponentially with depth, diffusion causes D to be slightly different than the kerma, but this is usually a minor effect and can be ignored.
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