Determination of Absolute Activities and Neutron Fluence Rates Using a Coincidence Method

Activation studies are an important tool for nuclear decommissioning. By activating material samples in controlled conditions in a known neutron fluence rate, the expected activity in decommissioning can be estimated. This work focuses on determining the fluence rate of a moderated (α, n) neutron source, a key parameter for performing quantitative activity experiments. The fluence rate is determined by an absolute measurement of the accumulated activity of neutron-activated reference samples with a well-known composition using a βγ-coincidence setup. This method requires both single β- and γ- detection, as well as coincidence data. A main advantage of this technique is that the resulting measurement is mostly independent of individual detector efficiencies. The current setup uses a combination of two detectors, consisting of one γ- and one β-detector, paired with a multi-channel data acquisition system, simultaneously recording hits in the β- and γ-channels including their timestamps. Coincidences are extracted in the offline analysis from the stored data. Having singles and coincident data in one dataset reduces the impact of certain corrections, e.g. dead time. Additionally, Monte Carlo techniques were implemented to assess γ-interactions in the γ-detector and to account for the fraction of fake coincidences. Samples, including aluminum ( 27 Al), gold ( 197 Au), Vanadium ( 51 V), sodium chloride (NaCl), and manganese ( 55 Mn) were activated. Calculated thermal fluence rates from weakly attenuating samples agree within uncertainties, with an average fluence rate of Ψ 0 = (2.58 ± 0.09) × 10 5 cm -2 s -1 .