We present a comprehensive survey of boron abundances in diffuse interstellar clouds from observations made with the Space Telescope Imaging Spectrograph ( STIS ) of the Hubble Space Telescope .
Our sample of 56 Galactic sight lines 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 O I \lambda 1355 , Cu II \lambda 1358 , and Ga II \lambda 1414 ( when available ) at the same velocity .
Five previous measurements of interstellar B II from Goddard High Resolution Spectrograph observations are incorporated in our analysis , yielding a combined sample that more than quadruples the number of sight lines with significant boron detections .
Our survey also constitutes the first extensive analysis of interstellar gallium from STIS spectra and expands on previously published results for oxygen and copper .
The observations probe both high and low-density diffuse environments , allowing the density-dependent effects of interstellar depletion to be clearly identified in the gas-phase abundance data for each element .
In the case of boron , the increase in relative depletion with line-of-sight density amounts to an abundance difference of 0.8 dex between the warm and cold phases of the diffuse interstellar medium .
The abundance of boron in warm , low-density gas is found to be B/H = ( 2.4 \pm 0.6 ) \times 10 ^ { -10 } , which represents a depletion of 60 % relative to the meteoritic boron abundance .
Beyond the effects of depletion , our survey reveals sight lines with enhanced boron abundances that potentially trace the recent production of ^ { 11 } B , resulting from spallation reactions involving either cosmic rays or neutrinos .
Future observations will help to disentangle the relative contributions from the two spallation channels for ^ { 11 } B synthesis .