  Simple and efficient "\lambda-method" and "\lambda/2-method" (\lambda is the
resonant wavelength of laser radiation) based on nanometric-thickness cell
filled with rubidium are implemented to study the splitting of hyperfine
transitions of 85Rb and 87Rb D_1 line in an external magnetic field in the
range of B = 0.5 - 0.7 T. It is experimentally demonstrated from 20 (12) Zeeman
transitions allowed at low B-field in 85Rb (87Rb) spectra in the case of
\sigma+ polarized laser radiation, only 6 (4) remain at B > 0.5 T, caused by
decoupling of the total electronic momentum J and the nuclear spin momentum I
(hyperfine Paschen-Back regime). The expressions derived in the frame of
completely uncoupled basis (J, m_J ; I, m_I) describe very well the
experimental results for 85Rb transitions at $B > 0.6 T (that is a
manifestation of hyperfine Paschen-Back regime). A remarkable result is that
the calculations based on the eigenstates of coupled (F, m_F) basis, which
adequately describe the system for low magnetic field, also predict reduction
of number of transition components from 20 to 6 for 85Rb, and from 12 to 4 for
87Rb spectrum at B > 0.5 T. Also, the Zeeman transitions frequency shift,
frequency interval between the components and their slope versus $B$ are in
agreement with the experiment.
