3 by 3 Sodium Iodide Absolute Efficiency Determination

Thallium doped sodium iodide (NaI-Tl) detectors are frequently used to measure the activity of samples exposed to ICF neutrons. The absolute detection efficiency for NaI-Tl detectors cannot be represented by a single quoted value. Detection efficiency is largely dependent on gamma ray energy, source location, activity, source and detector geometry and composition, all of which are quantities that are situational. A series of experiments were performed at the State University of New York at Geneseo to determine the absolute detection efficiency for both a single detector and two in-line coincidence detectors using 511 keV gamma rays. A low activity Na-22 gamma ray point source was placed flush against the detector faces of a pair of matched 3''x3'' cylindrical NaI-Tl detectors. The source was then moved relative to the detector face to determine geometric and off-axis effects on the detector's absolute efficiency. The manufacturers of the NaI-Tl detectors quote the absolute photo peak efficiency, at a distance of 10 cm from the detector face, to be 1.2{\%} for a low activity 511keV point source. Using this information the efficiency as a function of position was determined and compared to calculated values. Funded in part by a LLE contract through the DOE.