Detection of Fission Coincidences With Plastic Scintillators for the Characterization of Radioactive Waste Drums

This work addresses the use of plastic scintillators as an alternative to
³
He neutron detectors for radioactive waste drum characterization. The time response of scintillators is three orders of magnitude faster than that of gas proportional counters, and they offer similar neutron detection efficiency at lower cost. However, they are sensitive to gamma rays and the neutron-gamma pulse shape discrimination (PSD) is not possible with standard PVT scintillators. The proposed approach uses the neutron and gamma times of flight in triple coincidences recorded with
²⁵²
Cf, AmBe, and
⁶⁰
Co sources. A 2-D histogram of time delays between the second and first detected pulses, on the
$x$
-axis, and between the third and second pulses, on the
$y$
-axis, evidences a specific region of interest for spontaneous fission coincidences. MCNPX-PoliMi simulations are performed, which are in good agreement with previous experiments and allow for investigating the types of coincidences (
$\gamma \gamma \gamma $
, nnn,
$\gamma $
nn,
$\gamma \gamma \text{n}$
) and optimizing the rejection of neutron and gamma scattering crosstalk. The method was also experimentally tested with a 118-L mock-up drum filled with a metallic or an organic matrix, showing a correct estimation of the net fission signal up to an “alpha ratio” of 12 between fission and (
$\alpha $
,n) neutron emissions. Matrix and localization effects were also measured, showing a sensitivity (useful signal per gram of
²⁴⁰
Pu
_eq
) for a homogeneous distribution of plutonium in the iron matrix about three times larger than in the wood matrix, due to neutron slowing-down in the latter. This difference can be taken into account by the prior knowledge of the matrix characteristics or by neutron and gamma attenuation measurements. In contrast, in case of a point source with an unknown position in the matrix, the relative localization uncertainty is 26% for the metallic drum, and 41% for the organic one.