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

Abstract

Gaseous time projection chambers (TPCs) with high readout segmentation are capable of reconstructing detailed 3D ionization distributions of nuclear recoils resulting from elastic neutron scattering. Using a system of six compact 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 neutron backgrounds in the tunnel regions surrounding the Belle II detector at the SuperKEKB e + e − collider. With the use of 3D recoil tracking, we show that these TPCs are capable of maintaining nearly 100% nuclear recoil purity to reconstructed ionization energies (E reco ) as low as 5 keV ee . Using a large sample of Monte-Carlo (MC)-simulated 4 He, 12 C, and 16 O recoil tracks, we find consistency between predicted and measured recoil energy spectra in five of the six TPCs, providing useful validation of the neutron production mechanisms modeled in simulation. Restricting this sample to 4 He recoil tracks with E reco > 40 keV ee , we further demonstrate axial angular resolutions within 8 • and we introduce a procedure that under suitable conditions, correctly assigns the vector direction to 91% of these simulated 4 He recoils. Applying this procedure to assign vector directions to measured 4 He recoil tracks, we observe consistency between the angular distributions of observed and simulated recoils in five of the six TPCs, providing first experimental evidence of localized neutron "hotspots" on either side of the Belle II detector. Observed rates of nuclear recoils in these TPCs suggest that simulation overestimates the neutron flux from these hotspots. Despite this, simulation predicts these hotspots to produce the majority of neutron backgrounds in the accelerator tunnel at SuperKEKB's design luminosity of 6 × 10 35 cm −2 s −1 , making them important regions to continue to monitor.

Measurements of Beam Backgrounds in SuperKEKB Phase 2

“…Detailed descriptions of gas delivery, high voltage operation, and the data acquisition system in Phase 2 can be found elsewhere [15]. The added detector coverage in φ allows for a measurement of spatial distributions of fast neutron recoils that compliments the fast neutron-recoil angular distribution measurements previously performed in Phase 1 [16].…”