  Decoupling of total electronic and nuclear spin moments of Cs atoms in
external magnetic field for the case of atomic $D_1$ line, leading to onset of
the hyperfine Paschen-Back regime has been studied theoretically and
experimentally. Selective reflection of laser radiation from an interface of
dielectric window and atomic vapor confined in a nanocell with 300 nm gap
thickness was implemented for the experimental studies. The real time
derivative of selective reflection signal with a frequency position coinciding
with atomic transitions was used in measurements, providing $\sim$ 40 MHz
spectral resolution and linearity of signal response in respect to transition
probability. Behavior of 28 individual Zeeman transitions in a wide range of
longitudinal magnetic field (0 - 6 kG) has been tracked under excitation of Cs
vapor by a low-intensity $\sigma^+$- polarized cw laser radiation. For $B\ge
6~$kG, only 8 transitions with nearly equal probabilities and the same
frequency slope remained in the spectrum, which is a manifestation of the
hyperfine Paschen-Back regime. The obtained experimental results are consistent
with numerical modeling. Due to small divergence of selective reflection
signal, as well as sub-wavelength thickness and sub-Doppler spectral linewidth
inherent to nanocell, the employed technique can be used for distant remote
sensing of magnetic field with high spatial and $B$-field resolution.
