Radiation Physics

In the past few years there has been an increase in the deployment of x-ray screening systems at security checkpoints around the U.S.  These systems are designed to detect potential threats that may be present in cargo containers and vehicles that pass through country borders.  

X rays are used for imaging the contents of vehicles and cargo. Because of the need to penetrate through the thickness of the wall of containers and vehicles in addition to the objects contained within, these systems rely on the use of penetrating radiation such as high energy megavoltage x rays with peak voltages of at least 5 MV.

As with any radiation device, users need to know the radiation doses involved and potential exposures to people.  There are currently no national standards for megavoltage x-ray beams. As a result there are no explicit protocols on how to calibrate radiation measuring instruments to determine the air kerma delivered by these x-ray units. The Radiation Interactions and Dosimetry Group at NIST have initiated an effort to develop a protocol to calibrate instruments for high energy megavoltage x-ray beams.  As a first step in this direction, a prototype cavity chamber has been developed that will allow the direct realization of air kerma from megavoltage x-ray beams.  The instrument was designed and constructed at NIST and assembly was completed in the spring of 2010.  Further investigations are underway for realizing the quantity air kerma and providing traceable measurements to users of x-ray based security screening systems.