Directional Antineutrino Detection

We propose the first event-by-event directional antineutrino detector using inverse beta decay (IBD) interactions on hydrogen, with potential applications including monitoring for nuclear nonproliferation, spatially mapping geoneutrinos, characterizing the diffuse supernova neutrino background and searching for new physics in the neutrino sector. The detector consists of adjacent and separated target and capture scintillator planes. IBD events take place in the target layers, which are thin enough to allow the neutrons to escape without scattering elastically. The neutrons are detected in the thicker boron-loaded capture layers. The location of the IBD event and the momentum of the positron are determined by tracking the positron’s trajectory through the detector. Our design is a straightforward modification of existing antineutrino detectors; a prototype could be built with existing technology.

Figure 1

The detector consists of alternating layers of plastic scintillator, with the capture layers loaded with boron. IBD events take place in the thin target layers, and the positron subsequently deposits energy (purple boxes) within the target layer and travels to the adjacent thick capture layer, where it annihilates. The neutron propagates freely to the capture layer, where it diffuses and is captured on ${}^{10}\mathrm{B}$, depositing energy (yellow box), with a delayed coincidence from the positron annihilation.

Figure 2

We calculate $P({\theta }_{\text{error}}>\theta )$ for 4 MeV antineutrinos incident normal to the target plane for 0.5, 1, and 2 cm thick target layers, with ${\theta }_{\text{error}}$ the angular error in the reconstruction of ${\widehat{\mathbf{p}}}_{\nu }$: $\cos {\theta }_{\text{error}}={\widehat{\mathbf{p}}}_{\nu }·{\widehat{\mathbf{p}}}_{\nu }^{\mathrm{rec}}$. The antineutrino momentum is reconstructed using two methods. The first method only uses the neutron’s reconstructed momentum, ${\widehat{\mathbf{p}}}_{\nu }^{\mathrm{rec}}={\widehat{\mathbf{p}}}_{\mathbf{n}}$, while the second method uses the reconstructed neutron momentum and the positron momentum, which we assume is reconstructed exactly: ${\mathbf{p}}_{\nu }^{\mathrm{rec}}={\mathbf{p}}_{\mathbf{n}}+{\mathbf{p}}_{{\mathbf{e}}^{+}}$. A key tool for improving the angular resolution is timing. The left panel imposes a $<1\mu \mathrm{s}$ timing cut between the positron annihilation and the neutron capture, while the right panel uses a $<6\mu \mathrm{s}$ timing cut. The stricter timing cuts, however, result in a reduced fraction of events that are accepted by the analysis, as shown in the inset plot on the right panel. For reference, we also show the angular resolution and acceptance rate for a boron-loaded monolithic detector assuming perfect reconstruction of the neutron-capture location and the IBD event location.