Constraining Galactic cosmic-ray parameters with Z<=2 nuclei
  The secondary-to-primary B/C ratio is widely used to study Galactic
cosmic-ray propagation processes. The 2H/4He and 3He/4He ratios probe a
different Z/A regime, therefore testing the `universality' of propagation. We
revisit the constraints on diffusion-model parameters set by the quartet (1H,
2H, 3He, 4He), using the most recent data as well as updated formulae for the
inelastic and production cross-sections. The analysis relies on the USINE
propagation package and a Markov Chain Monte Carlo technique to estimate the
probability density functions of the parameters. Simulated data are also used
to validate analysis strategies. The fragmentation of CNO cosmic rays (resp.
NeMgSiFe) on the ISM during their propagation contributes to 20% (resp. 20%) of
the 2H and 15% (resp. 10%) of the 3He flux at high energy. The C to Fe elements
are also responsible for up to 10% of the 4He flux measured at 1 GeV/n. The
analysis of 3He/4He (and to a less extent 2H/4He) data shows that the transport
parameters are consistent with those from the B/C analysis: the diffusion model
with delta~0.7 (diffusion slope), Vc~20 km/s (galactic wind), Va~40 km/s
(reacceleration) is favoured, but the combination delta~0.2, Vc~0, and Va~80
km/s is a close second. The confidence intervals on the parameters show that
the constraints set by the quartet data are competitive with those brought by
the B/C data. These constraints are tighter when adding the 3He (or 2H) flux
measurements, and the tightest when further adding the He flux. For the latter,
the analysis of simulated and real data show an increased sensitivity to
biases. Using secondary-to-primary ratio along with a loose prior on the source
parameters is recommended to get the most robust constraints on the transport
parameters.
