Developing Standards for X-ray examination systems

Airline passengers know that X-ray examination systems are used at airports to examine their baggage. They will have seen their hand baggage being examined and may be aware that their hold baggage will have been examined as well. By international regulation all air cargo is examined. Because the threat of terrorist attack is ever present equipment is continually evolving to counteract potential acts by terrorists. But the test standards which are used to assess the performance of X-ray systems were created twenty years ago when border customs agents were searching solely for contraband. The esisting standards do not contain test objects which relate to the materials which may be used in Improvised Explosive Devices. Currently Creagh is involved in efforts to upgrade the existing standard ASTM F792-01 standard suitcase to redress this issue. It is essential the any airline standard has an excellent capability for materials discrimination (MD) The ancillary technique used at the passenger portal is an ion mobility spectrometer. Spectroscopy techniques such as this, and IR and Raman spectroscopy, are essentially surface techniques. They cannot test the contents of liquids, aerosols and gels (LAGS) contained in bottles and cans. And operators are not permitted by regulation to open any closed container. But X-ray techniques with MD capability can give images which show what is inside containers, and assess whether the contents are benign or dangerous. For air cargo systems no standard existed at the time when a major study on the detection of IEDs is air cargo was undertaken. During this project a prototype of a system was developed. This has been patented [1]. Standards for the testing of X-ray examination systems must contain objects which test: wire resolution, wire resolution behind an object, spatial resolution, penetration, contrast resolution, and the discrimination between materials (un-screened and screened by an object). The standards must be able to be used with X-ray sources of all types (constant potential, switched potential, LINAC and radio-isotope), with all possible transport systems of the object through the examination tunnel, and every means of photon detection. As well they have to be easy to use, providing a rapid evaluation of system performance with only one pass of the object through the tunnel for each source direction. Modern systems may contain as many as four X-ray sources. Materials can be characterized by their effective atomic number Zeff and density [2]. Zeff is derived by calculation and verified by testing in using data from the transmitted beam taken at different X-ray energies for the same density and thickness value of the test material. This paper will focus on the materials discrimination aspect of the testing process, in particular for LAGS testing at the passenger portal. We are members of an international committee being formed to create a new standard for this. As well we will demonstrate the ability of the Australian Image Quality Test Piece (AIQT) to give materials discrimination information. An existing standard (ANSI N42.46) cannot do this. And it takes 4 hours for a complete test. The AIQT produces all the information in one pass through the X-ray tunnel (6 minutes maximum). References [1] Creagh DC, Blagejovic, N. (2009). PCT/AU2009/000820. A new system for the evaluation of the performance of medium- and large- tunnel X-ray examination systems. [2] Smith JA, Martz HE, Kallman JS. (2014) Case for an improved effective-atomic-number for the electronic baggage screening program.LLNL-TR-520312. [3] Snyder M, Bell C. (2013). Test and evaluation report for the assessment of the Australian Image Quality Test for palletised cargo X-ray systems. DHS/ST/TSL-13/133