2023MNRAS.521.3150L

Damped Lyman-α absorber (DLA), or HI 21cm absorber (H21A), is an important probe to model-independently measure the acceleration of spectroscopic velocity (v_S) via the Sandage-Loeb effect. Confined by the shortage of DLAs and background radio sources (BRSs) with adequate information, the detectable amount of DLAs is ambiguous in the bulk of previous work. After differing the acceleration of scale factor ($\ddot{a}$) from the first-order time derivative of spectroscopic velocity ($\dot{v}_\mathrm{S}$), we make a statistical investigation of the amount of potential DLAs in the most of this paper. Using kernel density estimation to depict general redshift distributions of BRSs, observed DLAs and a DLA detection rate with different limitations (1.4 GHz flux, HI column density, and spin temperature), we provide fitted multiGaussian expressions of the three components and their 1σ regions by bootstrap, with a proportional constant of H21As in detected DLAs, leading to the measurable number predictions of H21As for FAST, ASKAP, and SKA1-Mid in HI absorption blind survey. In our most optimistic condition (F_1.4 GHz_ > 10 mJy, N_HI > 2 × 10²⁰ cm^–2, and T_S> 500 K), the FAST, AKSAP, and SKA1-Mid would probe about 80, 500, and 600 H21As, respectively.