Directional Detection of Fast Neutron Beam Backgrounds at SuperKEKB*

SuperKEKB, the next generation upgrade to the KEKB accelerator facility in Tsukuba, Japan, employs a novel nano-beam scheme which focuses beams of colliding electrons and positrons to a vertical size of 50 nm at their interaction point (IP) to achieve a design luminosity of 8.0×10³⁵ cm^-2s^-1; nearly 40 times higher than the previous world record set by KEKB. Tighter squeezed beams and increased luminosities lead to pernicious beam-induced backgrounds that must be properly understood and mitigated to ensure both the success and longevity of the Belle II detector that SuperKEKB produces collisions for. In this talk, we present a detection of the fast neutron background during the so called "Phase 2" of beam commissioning at SuperKEKB; the first data collection period with the Belle II detector in its final position at the IP.

The fast neutron data used in this analysis was collected by a set of eight high resolution micro gas time projection chambers (μTPCs). These μTPCs reconstruct the detailed ionization distribution of individual nuclear recoils, allowing for unique directional capabilities such as measuring the directional distribution and energy spectra of neutron recoils.