Background Reduction Using Collimators on a Portable HPGe Nuclide Identifier

The portable germanium detector based HHRIDs are generally intended to be operated in low to medium radiation fields with as much sensitivity as possible. However, in some circumstances the interfering radioactive background may be significant with respect to 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 searching operation. To do this, it is helpful to restrict the field of view of the instrument by collimation. Previous work showed the performance of two simple collimators (tungsten and steel) to be somewhat ineffective in reducing high energy (>1 MeV) gamma rays and that the tungsten fluorescence (59.3 keV) caused false positive misidentifications. A new graded-Z collimator was constructed of tungsten, tin and copper. The performance of this collimator was measured for all angles and for multiple energies to determine its effectiveness in reducing the count rate from material beyond the desired field of view. Results show the sensitivity for off-axis positions to be improved when locating a particular nuclide source in an area where other significant radioactive materials are present.