Radiation detection safety is hot. Don't get burned.

By Douglas Page

A flaw in the way radiation detection instruments are tested and calibrated was recently identified by researchers at the National Institute of Standards and Technology.

"The consequence of this is that radiation detection instruments can be incorrectly tested and calibrated," said Leticia Pibida, a physicist at NIST. Pibida documented her findings in a recent paper in the journal Health Physics (Health Phys 2008;94:126–33).

Government researchers recently identified a flaw in the way various radiation detection instruments are tested and calibrated. Solutions are offered in a recent paper."

Radiation detectors are indispensable tools for first responders working with radioactive sources. The instruments, however, particularly those used in homeland security, are useless if they are not well calibrated.

During instrument testing and calibration, technicians must ensure that the right radiation field is produced by the radioactive source. Details of how homeland security detectors are tested are documented in several standards available from the American National Standards Institute and the Institute of Electrical and Electronics Engineers, including IEEE N42.33 2006, American National Standard for Portable Radiation Detection Instrumentation for Homeland Security.

"It is critical that users know how to determine the radiation dose, known as air-kerma rate, delivered by radiation sources at a given distance from the source," Pibida said. Air-kerma is the current measure of the energy transferred to the air by gamma-rays emitted by a radioactive source, often used in place of the older roentgen unit, which is a measure of the ionization produced in air by incident radiation.

Radioactive source manufacturers, however, do not provide air-kerma rates, since these depend on several variable factors, including source-detector geometry. Instead, manufacturers provide the activity of the source, which is expressed either in units of bequerels or curies, and is a property inherent in the source.

"During instrument testing, it is the testers’ responsibility to determine the air-kerma rate produced by the source from the value of activity provided by the source manufacturer," Pibida said.

The steps involved in deriving the correct air-kerma rate values from the source activity include the use of calculated values of the so-called air-kerma rate constant, also known as the gamma-ray constant. "Unfortunately, there is a large variation in published calculated values of the air-kerma constant," Pibida said.

Pibida’s paper presents measurements that can be used to validate published calculated values. "We report experimentally measured values of the air-kerma rate for three radionuclides with well-defined source geometries," Pibida said.

The result of this work is that emergency responders can have greater confidence in the radiation detectors they carry when entering the hot zone of a radioactive incident.