A portable nuclear gauge uses very small amounts of radioactive material, Cesium 137 and Americium 241, to produce just enough radiation to measure the density of soils and asphalt. They measure the compaction density of foundations under buildings and roadways as well as the compaction of newly laid asphalt. Without proper compaction our infrastructure, including building, bridge and roadway foundations, would frequently fail, resulting in increased construction costs, traffic delays and accidents.

The radioactive source material is bonded into a ceramic pellet that is the size of a small pebble. The sources are contained in laser-fused, double encapsulated steel housings that make them virtually impenetrable and indestructible. They must pass extensive testing under the inspection of the International Atomic Energy Agency (IAEA). The bonded material is not subject to leaking, oozing, melting or powdering. The steel canisters are further shielded inside the gauge’s steel source rod and surrounded by lead and tungsten bio-shields.

Although there have been catastrophic incidents where gauges have been demolished by tractor-trailers and heavy construction equipment, or melted down in fires, there has never been an instance where the damage has compromised the shielding to the point that contamination was released or caused harm to anyone.

Gauges do get stolen but typically the thief is after the vehicle that transports the gauge or they have mistaken the gauge for a conventional power tool. Gauges are typically recovered in short order.

How the Gauge Uses Radiation to Measure Density and Moisture

An 8 millicurie Cesium 137 source used to measure density resides at the tip of the gauge’s source rod. It is housed and surrounded by shielding inside the gauge. When in use the rod is lowered into the ground at a specified depth, but no deeper than 12 inches. Radiation emitted by the radioactive material passes through the soil and is either absorbed/stopped by the soil or makes its way to the detector tubes on the other side of the gauge. Density is determined by the amount of radiation that is able to pass through the soil.

A fixed 40 millicurie Americium 241 source used to measure moisture content resides inside the base of the gauge. The radiation from this source is emitted into the ground, determines hydrogen content (a key ingredient of moisture), and reflects back up to a detector tube in the gauge.

Users of portable nuclear gauges must receive initial and on-going safety and security training to maintain their certification for using these devices. Although the annual radiation exposure limit allowed when working around these gauge is 5000 millirem (5rem), in reality, a worker will typically receive far less than 100 millirem in a year. To put that amount in perspective, the average U.S. citizen receives about 360 millirem in natural background radiation during the same timeframe.

Portable Nuclear Gauges in the Age of Terrorism

The following excerpts from an article by the NRC discusses and describes a dirty bomb. There has never been a dirty bomb devised or exploded using sources from portable nuclear gauges, in fact, there has never been a radioactive dirty bomb of any kind ever exploded in the history of the planet.

Gauge licensees are well trained in operating and emergency procedures and do everything possible to reduce the number of thefts and damaged gauges. Strict adherence to safety and security regulations are used to meet this goal.

“Dirty Bombs”

In order to better inform the public on what a dirty bomb is and what terrorists might intend to try to accomplish in setting off such a weapon, the following information is provided. Given the scores of exercises–federal, state and local–being staged to assure that all emergency response organizations are properly equipped, trained and exercised to respond to terrorist chemical, biological or radiological attack, we believe members of the public, as well as news organizations, will value some concise, straightforward information.

Basically, the principal type of dirty bomb, or Radiological Dispersal Device (RDD), combines a conventional explosive, such as dynamite, with radioactive material. In most instances, the conventional explosive itself would have more immediate lethality than the radioactive material. At the levels created by most probable sources, not enough radiation would be present in a dirty bomb to kill people or cause severe illness. For example, most radioactive material employed in hospitals for diagnosis or treatment of cancer is sufficiently benign that about 100,000 patients a day are released with this material in their bodies.

However, certain other radioactive materials, dispersed in the air, could contaminate up to several city blocks, creating fear and possibly panic and requiring potentially costly cleanup. Prompt, accurate, non-emotional public information might prevent the panic sought by terrorists.

A second type of RDD might involve a powerful radioactive source hidden in a public place, such as a trash receptacle in a busy train or subway station, where people passing close to the source might get a significant dose of radiation.

A dirty bomb is in no way similar to a nuclear weapon. The presumed purpose of its use would be therefore not as a Weapon of Mass Destruction but rather as a Weapon of Mass Disruption.

Impact of a Dirty Bomb

The extent of local contamination would depend on a number of factors, including the size of the explosive, the amount and type of radioactive material used, and weather conditions. Prompt detectability of the kind of radioactive material employed would greatly assist local authorities in advising the community on protective measures, such as quickly leaving the immediate area, or going inside until being further advised. Subsequent decontamination of the affected area could involve considerable time and expense.

Sources of Radioactive Material

Radioactive materials are widely used at hospitals, research facilities, industrial and construction sites. These radioactive materials are used for such purposes as in diagnosing and treating illnesses, sterilizing equipment, and inspecting welding seams. For example, the Nuclear Regulatory Commission, together with 32 states which regulate radioactive material, have over 21,000 organizations licensed to use such materials. The vast majority of these sources are not useful for constructing an RDD.

Control of Radioactive Material

NRC and state regulations require licensees to secure radioactive material from theft and unauthorized access. These measures have been stiffened since the attacks of September 11, 2001. Licensees must promptly report lost or stolen material. Local authorities make a determined effort to find and retrieve such sources. Most reports of lost or stolen material involve small or short-lived radioactive sources not useful for an RDD.

Past experience suggests there has not been a pattern of collecting such sources for the purpose of assembling a dirty bomb. Only one high-risk radioactive source has not been recovered in the last five years in the United States. However, this source (Iridium-192) would no longer be considered a high-risk source because much of the radioactivity has decayed away since it was reported stolen in 1999. In fact, the combined total of all unrecovered sources over a 5-year time span would barely reach the threshold for one high-risk radioactive source. Unfortunately, the same cannot be said world-wide. The U.S. Government is working to strengthen controls on high-risk radioactive sources both at home and abroad.