A container scanner is a very large transmission X-ray (or gamma-ray) system that produces a radiographic image of an entire shipping container, truck or rail wagon in a single pass. Because a loaded 20- or 40-foot container can contain tens of centimetres of steel-equivalent material, these systems use sources far more energetic than airport baggage scanners — typically a linear accelerator producing X-rays in the megavolt range. This page covers the equipment; the wider process of who screens what is described on cargo and freight screening.
How a container scanner works
The principle is the same as a baggage X-ray: a fan-shaped beam passes through the object and a line of detectors on the far side measures how much radiation gets through. Either the vehicle moves through a stationary beam, or the scanner moves along a parked vehicle. Each detector reading becomes one column of pixels, building a two-dimensional projection image of the whole load. Denser or thicker regions absorb more and appear darker.
What changes is scale. Baggage scanners use X-ray tubes at roughly 100–160 kV. Container scanners usually use electron linear accelerators (LINACs) at several MeV (commonly in the 3–9 MeV range), which can penetrate a few hundred millimetres of steel. Some older or lighter systems use radioactive sources (cobalt-60 or caesium-137), and some compact mobile units use betatrons.
Main system types
Fixed portal and gantry systems
Installed permanently at a port or border crossing, inside a shielded building or behind shielding walls. In a portal the truck drives or is towed through the beam; in a gantry system the truck parks and the gantry travels over it. Fixed systems generally offer the highest energy and image quality, and the largest throughput, but require significant civil works.
Mobile (truck-mounted) scanners
The source and a folding detector boom are mounted on a truck chassis. The unit can be driven to a terminal, deploy in a short time and scan parked containers or vehicles driving past. Mobile systems trade some penetration and image quality for flexibility; they are widely used by customs agencies that need to move capacity between sites. The original VACIS family (gamma-ray based, now part of Leidos) and many LINAC-based mobile systems from other manufacturers fall into this class.
Drive-through systems
Designed for high throughput at land borders: the driver stays in the cab and drives through. To protect the driver, these systems either skip the cab, switch the beam on only after the cab has passed, or use a lower-dose mode for the cab.
Rail scanners
Image entire trains as they pass at a controlled speed. Used at some rail border crossings.
Air cargo pallet and ULD scanners
Large-tunnel systems for palletised air freight and unit load devices. They work at lower energies than container scanners (often a few hundred kV) and may be dual-view or CT. See air cargo screening.
Image quality: what the specifications mean
| Parameter | What it tells you |
|---|---|
| Steel penetration | Thickness of steel behind which a test object can still be seen. Higher source energy gives more penetration. |
| Contrast sensitivity | Smallest difference in thickness (as a percentage) that can be seen behind a given amount of steel. |
| Wire / spatial resolution | Smallest wire or line pattern that is visible, which governs how small an object can be recognised. |
| Throughput | Vehicles or containers per hour. Drive-through systems are fastest; gantry and towed portals are slower. |
| Radiation exclusion zone | Size of the controlled area around the scanner; a key factor in where a mobile system can be deployed. |
Standardised test methods exist for these figures — for example ANSI N42.46 for the imaging performance of cargo and vehicle inspection systems, and IEC 62523 for testing high-energy (500 keV and above) cargo/vehicle radiographic inspection systems — so that claims from different manufacturers can be compared.
Dual-energy material discrimination at MeV energies
Many modern container scanners use dual-energy (interlaced) LINACs, for example alternating pulses at two energies. Comparing the attenuation at both energies gives a coarse estimate of effective atomic number, which is displayed as colour: organic, inorganic/light metal, and heavy metal classes. At megavolt energies the discrimination is cruder than in a baggage scanner and fails when the load is too thick, but it helps an analyst spot an organic mass hidden in a metal load (or vice versa). For the colour conventions used in baggage systems see X-ray image colours.
Reading a container image
- Start with the declaration. The image is only meaningful when compared with the manifest: commodity, quantity, packaging.
- Check the structure. Doors, corrugated walls, the floor, roof and front wall, and refrigeration machinery are classic places for false compartments.
- Look for inconsistency. Differences in density between pallets that should be identical, objects that do not fit the commodity, or voids where the load should be uniform.
- Use the tools. Contrast/brightness windowing, material colour, edge enhancement, and comparison with reference images of clean loads.
- Decide on follow-up. Clear, targeted physical inspection of a specific area, or full devanning.
More on the cognitive side is on X-ray image interpretation.
Manufacturers
Major suppliers of container and vehicle scanners include Smiths Detection, Rapiscan Systems (OSI Systems), Leidos, Nuctech, Astrophysics and several others. Product names and specifications change frequently, so procurement decisions should rely on current vendor data and independent test results rather than published summaries.
Radiation safety
MeV scanners are powerful radiation sources. Installations are designed with shielding, beam stops, interlocks, warning lights and an exclusion zone that keeps the public and staff below regulatory dose limits, and they are registered or licensed with the national radiation regulator. The design logic is explained on facility shielding and layout. The dose to the cargo itself is small and does not make goods radioactive.
Frequently asked questions
What are X-ray container scanners?
They are large transmission X-ray or gamma-ray systems that image an entire shipping container or truck in one pass, letting customs see the contents without opening the doors.
What energy do container scanners use?
Most use electron linear accelerators producing X-rays of a few MeV (commonly 3–9 MeV), much higher than the roughly 100–160 kV used in airport baggage scanners. Some use cobalt-60 or caesium-137 gamma sources.
What is the difference between a fixed and a mobile container scanner?
Fixed portals and gantries are permanent shielded installations with the best penetration and throughput. Mobile scanners are mounted on a truck and can be moved between sites, at some cost in image quality and penetration.
Can a container scanner see through a fully loaded 40-foot container?
Usually yes, but very dense cargo such as steel coils or stone can exceed the penetration of the system, leaving areas the analyst cannot interpret; those containers may then be physically examined.
Sources
- IEEE/ANSI N42.46 — Imaging performance of X-ray and gamma-ray systems for cargo and vehicle screening
- CBP — Non-Intrusive Inspection (NII) technology fact sheet
- DHS — Non-Intrusive Inspection Systems Program
- U.S. GAO-10-12 — 100 percent container scanning feasibility
Last reviewed on 2026-10-04.