Quest for Very Compact Antineutrino Detectors for Safeguarding Nuclear Reactors

Monitoring the status of a reactor and its fissile content in a continuous mode using antineutrinos is one of challenging topics for safeguarding. Detectors deployed thus far require volumes of several cubic metres. In our search for a very compact antineutrino detector we have investigated an indirect method for antineutrino detection. In a first test of this hypothesis de Meijer et al., we found an upper limit of δλ/λ = (-1 ± 1) x 10^-4 at a flux change of 2.5 x 10¹⁰ cm ^-2s^-1 at the 2 MW reactor at Delft, NL. We continued our search for effects on β⁺ -decay at the nuclear power plant Koeberg, South Africa. Here we report on two sets of measurements, one in 2011, another from December 2012 — February 2014. In the first experiment a LaBr3 detector was used. La has a natural radioactive isotope, decaying by either β^- or EC, hence the effect of reactor-status change could be measured during background measurements: no effect was observed. With a 22Na source an effect in the count rate was observed between reactor-OFF and reactor-ON which was not considered to be reliable, since an amplifier broke down and had to be replaced during the ramp-up of the reactor. In the recent measurement a 0.4 l NaI detector coupled via a PMT to a PMT-base MCA was used. After overcoming a number of technical problems a stable condition has been reached. Again an effect has been observed in the count rate during two reactor changes. Provided that this effect is only due to antineutrinos affecting β⁺ -decay, this result would correspond to a change in decay constant δλ/λ = (-0.52 ± 0.11) x 10^-4 at a flux change of 1.0 x 10¹³ cm ^-2s^-1. We are in the process to investigate instrumental effects as alternative explanations. (author)