Measuring the Galactic Cosmic Ray Flux with the LISA Pathfinder Radiation Monitor

TL;DR

This paper reports on measurements of galactic cosmic ray flux by a radiation monitor on LISA Pathfinder, revealing solar cycle variations and modulations, and estimating the cosmic ray energy spectrum using in situ data and modeling.

Contribution

It introduces a novel in situ radiation monitor for LISA Pathfinder and demonstrates its capability to characterize cosmic ray flux and energy spectrum variations during the mission.

Findings

Observed a 40% increase in cosmic ray count rate over the mission.

Detected 10% flux modulations linked to solar activity and heliospheric structures.

Estimated the galactic cosmic ray energy spectrum using measurements and GEANT4 modeling.

Abstract

Test mass charging caused by cosmic rays will be a significant source of acceleration noise for space-based gravitational wave detectors like LISA. Operating between December 2015 and July 2017, the technology demonstration mission LISA Pathfinder included a bespoke monitor to help characterise the relationship between test mass charging and the local radiation environment. The radiation monitor made in situ measurements of the cosmic ray flux while also providing information about its energy spectrum. We describe the monitor and present measurements which show a gradual 40% increase in count rate coinciding with the declining phase of the solar cycle. Modulations of up to 10% were also observed with periods of 13 and 26 days that are associated with co-rotating interaction regions and heliospheric current sheet crossings. These variations in the flux above the monitor detection threshold (approximately 70 MeV) are shown to be coherent with measurements made by the IREM monitor on-board the Earth orbiting INTEGRAL spacecraft. Finally we use the measured deposited energy spectra, in combination with a GEANT4 model, to estimate the galactic cosmic ray differential energy spectrum over the course of the mission.