Precision energy measurement of the 166 keV γ ray in 139Ce decay

An intrinsic Ge (LEPS-type) detector was used to measure the energy of the 166 keV γ ray in 139Ce decay against the γ rays of 182Ta. Reference values for 182Ta.were taken from the recent list of γ ray energy standards as updated by Helmer and van der Leun. Based on 18 runs taken under a variety of conditions, the average value for this 139La γ ray is 165.8575(11) keV. This is mid-way between two earlier results, but about six times as accurate. Some advantages of 139Ce as both a γ ray and internal-conversion electron calibration source are pointed out.