Study of Boron Coated Straws and mixed (10B/3He) detectors for passive neutron measurements of radioactive waste drums

In the framework of the radioactive waste drums characterization, 90% of plutonium neutron measurement devices are composed of
³
He detectors which show excellent thermal neutron efficiency due to the large cross section of the
³
He (n, p)
³
H reaction (5330 barns) and low sensitivity to gamma rays. However, the demand exceeds worldwide production and the cost of the
³
He sensors has greatly increased during the last two decades. The main objective of this work is to compare commercially available alternatives such as Boron Coated Straw (BCS) proposed by Proportional Technologies and mixed boron coated / 1 bar
³
He manufactured by Reuter Stokes. The main characteristics of these counters for passive neutron coincidence counting, such as detection efficiency and die-away time, have been compared to those obtained with classical 4 bar
³
He counters in the same experimental conditions. Additionally, a panel composed of seven BCS detectors dedicated to the localization of neutron interactions has been tested, in view to reduce uncertainties associated to plutonium position inside radioactive waste drums. Measurements with BCS or mixed detectors show smaller efficiencies and longer die-away times with respect to reference
³
He detectors. Therefore, these last remain a good trade-off for neutron coincidence measurements of radioactive waste drums. However, BCS imaging panels open interesting prospects to reduce the uncertainty associated to plutonium localization.