How do radiation detectors work? And how does Homeland Security use them at the borders to prevent nuclear materials from being used in weapons of mass destruction?

After 9/11, the need to keep unauthorized radioactive materials from crossing U.S. borders became an important part of the new homeland defense program. Terrorists could unleash either nuclear bombs fueled by enriched uranium, or “dirty bombs,” which are conventional explosives containing small amounts of radioactive material that could be spread around the bomb site.

Today's more reliable and accurate radiation detectors are helping prevent terrorists from transporting nuclear materials across the border.

What is Radiation?

Radiation is energy traveling through space. Not all radiation is harmful (think of the sun radiating heat). And not all radioactive materials can be used by terrorists for nuclear weapons.

In fact, many of the newer radiation detection portals at shipyards and border crossings have been set off by false alarms due to the low-level, naturally occurring radiation in:

• Kitty litter
• Bananas
• Earthenware dishes
• Ceramic toilets.

Other legitimate radioactive materials, such as those found in medical equipment, industrial gauges, or spent fuel rods from nuclear power plants, may show up in international shipments. At the same time, terrorists could hide enriched uranium or plutonium for nuclear weapons inside a shipment of these “innocent” materials.

Different Types of Radiation

Radiation comes in 2 forms: highly energized particles, or energy waves. Different types of radiation include:

• Alpha radiation--atomic particles containing two neutrons and two protons. Alpha radiation is the weakest type of radiation, and can be stopped by a piece of paper.
• Beta radiation--a stream of high-energy electrons with a negative charge. This type of radiation can be stopped by wood or aluminum.
• Gamma rays--pure electromagnetic waves, or photons. These penetrate further, and can only be stopped by a thick shield of water, earth or hardened steel.
• Neutrons--atomic particles given off during radioactive decay of an element such as uranium. As they speed away, they knock more neutrons off of other atoms, eventually creating a chain reaction that releases other types of radiation.

How Radiation Detectors Work

The three main types of detectors include:

• Gas-filled tube detectors, such as Geiger counters, which were invented in the early 1900s. These detectors use small metal tubes filled with gas and containing a wire down the center. The wire and the tube are connected across an electrical circuit. When gas molecules inside the tube get hit by radiation, they create an electrical current through the wire to a readout.
• Scintillation Crystal detectors, made from materials that fluoresce, or give off a spark of light, when hit by a photon. The light sparks can be amplified and read out electrically.
• Solid-state semiconductor detectors. This is the same type of technology used to make integrated circuits, such as computer chips. When photons from a gamma ray hit the surface of the semiconductor, they create free electrons which can be detected as an electrical current.

Radiation Detector False Alarms

Radiation detectors at U.S. borders look like large pairs of panels arranged on either side of the train, car or truck, as it passes through. When any radiation is detected, an alarm will sound. Then, the vehicle or container must be diverted until the source is found. Every diversion costs time and money, which slows down commerce and adds to the costs of the goods being shipped (Customs Today, April/May 2007).

In addition, there’s the problem of shielding. This happens when an illicit nuclear source is hidden inside a metal container, or shielded behind the truck’s metal body.

Tuning the Detectors

To counteract these problems, engineers are trying to refine the criteria used to set off a detector alarm. They are creating more sophisticated mathematical formulas to accurately predict when a true threat is on-board and applying electronic signal processing to improve the detectors’ sensitivity to low-level radiation.

By using several different detectors, optimizing each for different types of radiation, engineers can more accurately predict the source of the radiation—whether from naturally occurring materials, industrial scrap, or fuel for a terrorist's nuclear weapon.

For More Information

Click here to learn more about how U.S. scientists and engineers developed the first nuclear weapons during World War II. Click here to find out about the many non-defense uses of nuclear technology. Click here for more information on nuclear power plants.

For clear photographic illustrations of how gas-filled (Geiger) detectors and scintillation detectors work, see the Harvard University Operations Services Environmental Health & Safety article on Survey Meters.

Click here for a short history of radiation detectors, by the XRF Corporation.