511 keV Background

The 511 keV background can be split into five components:

• the continuum background under the 511 keV line.
• 511 keV photons from passive material, due to decays of unstable nuclei in the materials. These are nuclei hit by protons and neutrons produced by interactions between cosmic ray particles and material from the detectors, shield or cryostat. The unstable nuclei decay through decay. The annihilation of the positron leads to the emission of two 511 keV photons in opposite directions. If one is absorbed by the detector and the other escapes, a 511 keV background event is produced.
• 511 keV photons, originating from interactions of cosmic rays with passive spacecraft materials, that are not rejected by the BGO shield (shield leakage).
• 511 keV photons originating from cosmic ray interactions with the mask material. The main source is pair creation by cosmic ray proton interactions with W nuclei. This component is significantly reduced with the Plastic Scintillator.
• BGO shield blocking time component: 511 keV photons produced by decays in the BGO shield when the ACS electronics is blocked by a large energy deposit and the veto is on.

All these components were reproduced using Monte Carlo simulations. The resulting line strengths for the 511 keV line with and without the PSAC are given in Table .

Table: 511 keV background line strength with and without PSAC. Calculations are for 5 cm BGO shield, 80 keV shield threshold, without PSD. Fluxes in cts cm s .

component	with PSAC	without PSAC
continuum
passive material
shield leakage
mask
BGO shield
blocking time
Total