Preliminary Monte Carlo simulations of linear accelerators in Time-of-Flight Compton Scatter imaging for cargo security

Abstract

The economic impact of illicit trade is in the trillions of dollars per year, with a proportion of this trade concealed within cargo containers. The interdiction of this trade relies upon efficient and effective external screening of cargo containers, typically using x rays. The present work introduces a technique of x-ray screening that aims to increase the efficiency and effectiveness of x-ray screening. Traditional X-ray screening of cargo containers is performed using high-energy (MV) transmission imaging or low-energy (kV) Compton scatter imaging to provide two-dimensional images. Two-dimensional images can contain complex, overlapping objects and require significant experience and time to interpret. Time-of-Flight information can be used in conjunction with Compton scatter imaging to provide information about the depth of each Compton scatter interaction, leading to three-dimensional images, reducing false positives and image analysis time. The expected Time-of-Flight from photons scattered back from a set of objects is well defined when the photons are produced with a delta-type (infinitely narrow) pulse duration, however, commercially available linear accelerators used for cargo screening typically have pulse widths of the order of 1 μs. In the present work, the possible use of linear accelerators for Time-of-Flight Compton scatter imaging is investigated using a mixture of analytic and Monte Carlo methods. Ideal data are obtained by convolving a number of wide x-ray pulses (up to 5 μs) with the expected Time-of-Flight from a set of objects using a delta-type pulse. Monte Carlo simulations, using Geant4, have been performed to generate x-ray spectra produced by a linear accelerator. The spectra are then used as the input for detailed Monte Carlo simulations of the Time-of-Flight of photons produced by a single linear accelerator pulse scattering back from a set of objects. Both ideal and Monte Carlo data suggest that Time-of-Flight information can be recovered from a wide linear accelerator pulse, provided that the leading and falling edge of the pulse are sharp. In addition, it has been found that using a linear accelerator leads to double the amount of Time-of-Flight information as both the leading and falling edge are utilised (unlike for a delta-type pulse).

Automated X-ray image analysis for cargo security: Critical review and future promise

“…For example, a 40 ft General Purpose cargo container has a volume of 67.6 m 3 [15] and is made out of steel, whereas hand luggage volume 1 is typically 0.063 m 3 and usually made out of fabric or plastics. The physical scale of cargo scanners makes it difficult to efficiently perform 3D Computed Tomography (CT) [8] but some multi-view systems do exist. Moreover, for cargo it is more difficult to extract material composition information due to the higher energies required for sufficient penetration to obtain good image contrast (Fig.…”

“…The use of cargo containers in global trade transactions continues to grow. From 2004 to 2014, and despite the 2008 global economic crisis, the number of Twenty-foot Equivalent Unit (TEU) container transactions more than doubled to reach almost 7×10 8 TEU per annum [77]. During this time, the US Container Security Initiative (CSI), proposed in the wake of the 9/11 terrorist attacks, has encouraged 100% screening of containers [69], and is being implemented by ports around the world [79].…”

Measuring and correcting wobble in large-scale transmission radiography

“…In some systems, mechanical instability (wobble) leads to effective loss of precision. Whilst large-scale X-ray Computed Tomography (CT) could alleviate the issues of wobble and shielding, such systems are not widely deployed because they are too expensive and inefficient to be competitive [2,27].…”