Origin of apparent Beam-1-side asymmetry in neon release

Ascertain the mechanism that causes more neon to be released on the incoming Beam 1 side near the z≈22 m NEG cartridge during heating, despite symmetric neon filling of the ATLAS beam pipe, and identify why the Beam 1 side exhibits greater neon emission.

Background

During the pressure bump tests, the analysis suggests a two-gas (hydrogen plus neon) hypothesis to account for observations near the 19 m and 22 m locations. The authors report that more neon appears to be released from the NEG cartridge on the incoming Beam 1 side near 22 m, which would produce an asymmetry in beam-induced backgrounds.

Given that the entire ATLAS beam pipe is filled with ultra-pure neon at atmospheric pressure during technical stops, the observed side-dependent neon release is counterintuitive. Understanding this asymmetry is necessary to correctly model gas distributions and resultant backgrounds, and to ensure reliable interpretation of pressure-bump-induced signals and future vacuum interventions.

References

Although this creates an asymmetry between the sides, it is unclear why more neon seems to be released from the NEG cartridge on the \beam{1} side, although the entire ATLAS beam pipe is filled with neon at atmospheric pressure for several months during the annual technical stop.

— Beam-induced backgrounds measured in the ATLAS detector during local gas injection into the LHC beam vacuum  (2405.05054 - Collaboration, 2024) in Section 6.3 (Possible neon contribution)