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Neutral Helium multiplets, HeI*3189,3889,10830 are very useful diagnostics tothe geometry and physical conditions of the absorbing gas in quasars. So faronly a handful of HeI* detections have been reported. Using a newly developedmethod, we detected HeI*3889 absorption line in 101 sources of a well-definedsample of 285 MgII BAL quasars selected from the SDSS DR5. This has increasedthe number of HeI* BAL quasars by more than one order of magnitude. We furtherdetected HeI*3189 in 50% (52/101) quasars in the sample. The detection fractionof HeI* BALs in MgII BAL quasars is about 35% as a whole, and increasesdramatically with increasing spectral signal-to-noise ratios, from 18% at S/N<= 10 to 93% at S/N >= 35. This suggests that HeI* BALs could be detected inmost MgII LoBAL quasars, provided spectra S/N is high enough. Such asurprisingly high HeI* BAL fraction is actually predicted from photo-ionizationcalculations based on a simple BAL model. The result indicates that HeI*absorption lines can be used to search for BAL quasars at low-z, which cannotbe identified by ground-based optical spectroscopic survey with commonly seenUV absorption lines. Using HeI*3889, we discovered 19 BAL quasars at z<0.3 fromavailable SDSS spectral database. The fraction of HeI* BAL quasars is similarto that of LoBAL objects.

A Comprehensive Study of Broad Absorption Line Quasars: I. Prevalence of HeI* Absorption Line Multiplets in Low-Ionization Objects