INNM 2012 Field of View Measurements on a Collimated Transportable HPGe System Used in Search Systems

Mobile search systems are used to locate and identify radioactive material as the system is transported along a path. Both low and high resolution detectors are used in these systems. Low resolution detectors are often overwhelmed by high NORM background to the extent that the source signal is hidden. High resolution germanium detectors are more selective with easily identifiable peaks and the sensitivity can be improved further with background reduction. The detection efficiency is peaked in the forward direction, but to achieve the highest sensitivity possible the gamma ray flux from other directions must be reduced. In some circumstances the interfering radioactive background flux may be much higher than the flux from the source of interest. For example: a shipping container full of NORM may be used in an attempt to mask the presence of a concealed SNM source. It is therefore often desirable to examine only one area, object or container at a time to localize a source in a search operation. To do this, it is helpful to restrict the field of view of the instrument by shielding. For a mobile system, the shielding must be as small as possible to reduce the total weight of the system. The performance of a large diameter HPGe detector in a mobile housing was measured for all angles and for multiple energies to determine its effectiveness in reducing the count rate from material outside the desired field of view. Results show the need for the off-axis shielding to be increased to improve the detection of a particular nuclide source in an area where other significant radioactive materials are present.