Neutron multiplicity measurements on 2201 waste drums containing Pu in the range 0.1-1 g ²⁴⁰Pu_eff with the Time Interval Analysis method

Measurement results are presented for the assay of plutonium in 220 1 waste drums containing Pu-masses in the range 0.1-1 g ²⁴⁰Pu_eff obtained with the Time Interval Analysis (TIA) method. TIA is a neutron multiplicity method based on the concept of one- and two-dimensional Rossi-alpha distributions. The main source of measurement bias in neutron multiplicity measurements at low count-rates is the impredictable variation of the high-multiplicity neutron background of spallation neutrons induced by cosmic rays. The TIA-method was therefore equipped with a special background filter, which is designed and optimized to reduce the influence of these spallation neutrons by rejecting the high-multiplicity events. The measurement results, obtained with the background correction filter outlined in this paper, prove the repeatability and validity of the TIA-method and show that multiplicity counting with the TIA-technique is applicable for masses as low as 0.1 g ²⁴⁰Pu_eff even at a detection efficiency of 12%.