Surface Gamma Radiation Detection

Description

At many Department of Energy (DOE) facilities, planning for decontamination and decommissioning requires the characterization of radiation fields inside and outside of structures. These structures (e.g., tanks, reactors, and glove boxes) often have very high levels of radiation. Because gamma radiation is more penetrating and travels further than alpha or beta radiation, and because most radionuclides produce some gamma radiation, gamma detection is the most common form of radiation detection.

The crucial component of any gamma-measuring device is the detector, which is a component that produces electrical signals as a result of the interactions of the gamma radiation. Commonly used technology employs hand-held survey instrumentation operated by radiological control technicians wearing anti-contamination coveralls with hoods and respirators. This method is cumbersome and costly, and it requires limited access to confined areas. Below is a description of some of the remote techniques that DOE has demonstrated and evaluated that address these problems.

• The 3-D GammaModeler™
The Three Dimensional (3-D) GammaModeler™ visual and gamma ray imaging system remotely surveys large areas for gamma-ray emissions and displays the results in 3-D representations of the radiation sources. The 3-D capability of the GammaModeler™allows the radiation environment inside an object to be determined. The system consists of four modules: a sensor head, a portable personal computer, a pan-and-tilt controller, and a 3-D workstation. The sensor head is controlled remotely by the computer. Remote operation and control of the sensor head minimizes operator exposure.

• RadScan 600 gamma-ray imaging system.
The RadScan 600 gamma-ray imaging system was developed by British Nuclear Fuels Ltd. (BNFL). The RadScan system characterizes contaminated sites containing high levels of surface radiation at a 12-inch distance. This system provides real-time data on the location and concentration levels of gamma radioactive material. The RadScan 600 employs spectroscopy to identify contamination and exposure level information, along with isotopic information for all surfaces surveyed, with a single detector. Since the inspection head is operated remotely, worker exposure and access constraints typically associated with traditional handheld survey instrumentation are minimized.

• In-Situ Gamma Spectroscopy with ISOCS (an In-Situ Object Counting System).
ISOCS is a complete In-Situ Object Counting System developed for use in a wide variety of measurement applications. Most radiological contamination situations do not result in uniform deposition of the offending material. Consequently, the selection of a small sample to send to the laboratory is a difficult and imprecise task. One solution is to take very large samples and average them over the entire object or area. The gamma radiation detector uses a high purity germanium crystal for high resolution and high efficiency. This gamma-ray spectroscopy system identifies radioactive isotopes and provides real-time assays of the radioactive contents of containers, surfaces, and samples. The system provides traditional spectra of counts as a function of gamma energy, which are then converted to radionuclide concentration using a software system. The entire system is mounted on a portable cart, which allows rotation of the detector about a horizontal axis. The ISOCS does not produce an image.

Limitations and Concerns

• GammaModeler™
  The deployment platform for the GammaModeler™ system must be robust and level. It can be moved, but it should not rotate or tilt during the measurement. Additional uncertainties caused by platform rotation or tilting could prevent 3-D rendering of the radioactive sources.

The dose estimates are based on calibrations of the instrument using isotopic sources. For a given dose estimate, knowledge of the isotope is assumed. However spectral modification often occurs due to intervening material, and the calibration values do not account for these changes. Future developments may provide a measure of this spectral modification and lead to better dose estimates.

• RadScan technology
  Due to the physical size and geometry of the RadScan technology, there is some difficulty maneuvering the unit in congested areas, and it is also somewhat difficult to obtain corner and wall measurements.

The demonstrated innovative system is not suitable for surveying areas with low levels of radiation, such as for release surveys.

• ISCOS
  Analysis results for in-situ surface soils are usually lower than the corresponding laboratory sample values. Samples should be weighed before and after drying so that an adjustment for soil moisture can be made.

Applicability

These technologies are used for the characterization and monitoring of surface radiation.

Technology Development Status

Each technology is in the advanced stages of development.

Web Links

http://www.osti.gov/bridge/servlets/purl/325789-gVwyNs/webviewable/

http://www.osti.gov/bridge/servlets/purl/774501-mcUqAD/webviewable/

http://www.osti.gov/bridge/servlets/purl/10106357-oCzHik/native/

http://www.osti.gov/bridge/servlets/purl/14046-eRy9Ko/webviewable/

http://www.inl.gov/technicalpublications/Documents/3491539.pdf

http://www.bnl.gov/isd/documents/43404.pdf

Other Resources and Demonstrations

See http://www.xrfcorp.com/technology/radiation_detection.html for a description of common radiation detection methods.

See http://www.ornl.gov/~webworks/cppr/y2004/rpt/121589.pdf for a description of radiation detection from fission.

See http://www.osti.gov/bridge/servlets/purl/949979-lLh9Ha/ for performance testing for Gamma measurements at Oak Ridge.