We present a comprehensive survey of B abundances in diffuse interstellar clouds from HST /STIS observations along 56 Galactic sight lines .
Our sample is the result of a complete search of archival STIS data for the B II \lambda 1362 resonance line , with each detection confirmed by the presence of absorption from other dominant ions at the same velocity .
The data probe a range of astrophysical environments including both high-density regions of massive star formation as well as low-density paths through the Galactic halo , allowing us to clearly define the trend of B depletion onto interstellar grains as a function of gas density .
Many extended sight lines exhibit complex absorption profiles that trace both local gas and gas associated with either the Sagittarius-Carina or Perseus spiral arm .
Our analysis indicates a higher B/O ratio in the inner Sagittarius-Carina spiral arm than in the vicinity of the Sun , which may suggest that B production in the current epoch is dominated by a secondary process .
The average gas-phase B abundance in the warm diffuse ISM [ log \epsilon ( B ) = 2.38 \pm 0.10 ] is consistent with the abundances determined for a variety of Galactic disk stars , but is depleted by 60 % relative to the solar system value .
Our survey also reveals sight lines with enhanced B abundances that potentially trace recent production of ^ { 11 } B either by cosmic-ray or neutrino-induced spallation .
Such sight lines will be key to discerning the relative importance of the two production routes for ^ { 11 } B synthesis .