  The spin transverse relaxation time (T_2) of atoms is an important indicator
for precision measurement. Several methods have been proposed to characterize
the T_2 of atoms. In this paper, the T_2 of rubidium (Rb) atomic vapor in the
same cell was measured using four measuring methods, namely spin noise spectrum
signal fitting, improved free induction decay (FID) signal fitting,
w_m-broadening fitting, and magnetic resonance broadening fitting. Meanwhile,
the T_2 of five different types of Rb atomic vapor cells were measured and
characterized. A comparative analysis visualizes the characteristics of the
different measuring methods and the effects of buffer gas on T_2 of Rb. We
theoretically and experimentally analyzed the applicability of the different
methods, and then demonstrated that the improved FID signal fitting method
provides the most accurate measurement because of the clean environment in
which the measurements were taken. Furthermore, we demonstrated and
qualitatively analyzed the relationship between the atomic number density and
the T_2 of Rb. This work provides analytical insight in selecting atomic vapor
cells, and may shed light on the improvement of the sensitivity of atomic
magnetometers.
