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Gaseous time projection chambers (TPCs) with high readout segmentation arecapable of reconstructing detailed 3D ionization distributions of nuclearrecoils resulting from elastic neutron scattering. Using a system of sixcompact TPCs with pixel ASIC readout, filled with a 70:30 mixture of He:CO$_2$gas, we analyze the first directional measurements of beam-induced neutronbackgrounds in the tunnel regions surrounding the Belle II detector at theSuperKEKB $e^+e^-$ collider. With the use of 3D recoil tracking, we show thatthese TPCs are capable of maintaining nearly $100\%$ nuclear recoil purity toreconstructed ionization energies ($E_\text{reco}$) as low as 5$\mathrm{keV_\text{ee}}$. Using a large sample of simulated $^4$He, $^{12}$C,and $^{16}$O recoil tracks, we find consistency between predicted and measuredrecoil energy spectra in five of the six TPCs, providing useful validation ofthe neutron production mechanisms modeled in simulation. Restricting thissample to $^4$He recoil tracks with $E_\text{reco}>40$$\mathrm{keV_\text{ee}}$, we demonstrate axial angular resolutions within$8^{\circ}$ and we introduce a procedure that under suitable conditions,correctly assigns the vector direction to $91\%$ of simulated $^4$He recoils.Applying this procedure to assign vector directions to measured $^4$He recoiltracks, we observe consistency between the angular distributions of observedand simulated recoils, providing first experimental evidence of localizedneutron "hotspots" in the accelerator tunnel. Observed rates of nuclear recoilsin these TPCs suggest that simulation overestimates the neutron flux from thesehotspots.

Application of recoil-imaging time projection chambers to directional neutron background measurements in the SuperKEKB accelerator tunnel