Sr. Health Physicist
When you mention radioactive material, many people automatically think reactor or medical facility. They’re not aware that radiation could be used right outside their door. One example — portable gauges containing sealed sources of radioactive materials are often used during construction or repair of roads.
When paving a new road, construction crews need to know the amount of moisture in a roadbed and the density of the bed so the new road will last for many years. Portable gauges use small quantities of radioactive materials in sealed sources to make these measurements without damaging the roadbed.
To ensure the roadbed is dense enough, a sealed source emitting gamma radiation is lowered from the portable gauge into a small hole drilled into the road bed, at a specific distance from the gauge. Inside the gauge, a detector measures the amount of radiation that travels through the soil from the bottom of the drilled hole. The denser the soil, the more gamma radiation will be absorbed by the roadbed material and not reach the detector. A computer program in the gauge calculates the density of the roadbed based on the amount of radiation that reaches the detector.
Similarly, a sealed source containing a small amount of radioactive material that emits neutrons can be used to determine the amount of moisture in a roadbed. Neutrons are absorbed by water in the roadbed material, and scattered by mineral and other solid materials. The neutron source remains inside the portable gauge, and a shutter is opened to allow the neutrons to be emitted against the road bed surface. A detector inside the portable gauge detects neutrons that are scattered back from the road bed surface. This is why such source-detector combinations are referred to as “backscatter” devices.
In this case, the more moisture in the roadbed, the more neutrons are absorbed and fewer neutrons are available to be scattered back into the detector. A computer program in the gauge calculates the amount of moisture in the roadbed material from the number of neutrons detected by backscatter.
Although there are other ways to obtain this information, they may take more time, or require more invasive methods to obtain samples for analysis.
The radiation from these sources is not detectable even a few feet away, as long as the devices are used as designed. Portable gauges must be used by workers trained in radiation safety and use of the devices. And, workers using the sources are required to keep the gauges with them at all times, unless they are locked or secured as required by regulation.
NRC and Agreement State inspectors periodically inspect the companies that are licensed to operate portable gauges to ensure they’re being used safely.