A. Putze et al.: An MCMC technique to sample transport and source parameters of Galactic cosmic rays. II. correlated with V$_{a}$ , which is related to a smaller δ being obtained if more reacceleration is included. On the other hand, the positive correlation between δ and V$_{c}$ indicates that larger δ are expected for larger wind velocities. We show in Table 2 the most probable values of the transport parameters, as well as their uncertainties, corre- sponding to 68% confidence levels (CL) of the marginalised PDFs. The precision to which the parameters are obtained is excellent, ranging from a few % to 10% at most (for the slope of the diffusion coefficient δ in III). This corre- sponds to statistical uncertainties only. These uncertainties are of the order of, or smaller than systematics generated from uncertainties in the input ingredients (see details in Maurin et al. 2010). As found in previous studies (e.g., Lionetto et al. 2005), for pure diffusion/reacceleration models (II), the value of the diffusion slope δ found is low ( ≈ 0 . 23 here). When con- vection is included (I and III), δ is large ( ≈ 0 . 8 − 0 . 9). This scatter in δ was already observed in Jones et al. (2001), who also studied different classes of models. The origin of this scatter is consistent with the aforementioned correlations in the parameters (see also Maurin et al. 2010). The best-fit model parameters (which are not always the most probable ones) are given in Table 3, along with the minimal χ 2 value per degree of freedom, χ 2 $_{min}$/ d.o.f (last column). As found in previous analyses (Maurin et al. 2001, 2002), the DM with both reacceleration and convec- tion reproduces the B/C data more accurately than with- out: χ $^{2}$/d.o.f= 1 . 47 for III, 4.90 for II, and 11.6 for I. The B/C ratio associated with these optimal χ 2 values are dis- played with the data in Fig. 3. We note that the poor fit for II (compared to III) is explained by the departure of the model prediction from high-energy HEAO-3 data. For comparison purposes, we now focus on several datasets for the B/C data. Low-energy data points include ACE data, taken during the solar minimum period 1997-1998 (de Nolfo et al. 2006). Close to submission of this pa- per, another ACE analysis was published (George et al. 2009). The 1997-1998 data points were reanalysed and complemented with data taken during the solar maxi- mum period 2001-2003. The AMS-01 also provided B/C data covering almost the same range as the HEAO-3 data (Tomassetti & AMS-01 Collaboration 2009). Hence, for this section only, we attempt to analyse other B/C datasets that include these components: – F: HEAO-3+low+high energy [0 . 2 − 2 TeV/n], 31 data points; – A: HEAO-3 [0 . 8 − 40 GeV/n], 14 data points; – C: HEAO-3+low energy [0 . 3 − 0 . 5GeV/n], 22 data points; 6 4.2. Sensitivity to the choice of the B/C dataset Table 3. Best-fit model parameters for B/C data only ( L = 4 kpc). Fig. 2. From top to bottom: posterior PDFs of models I, II, and III using the B/C constraint (dataset F). The diagonals show th e 1D marginalised PDFs of the indicated parame- ters. O ff-diagonal plots show the 2D marginalised posterior PDFs f or the parameters in the same column and same line respect ively. The colour code corresponds to the regions of increas ing probability (from paler to darker shade), and the two con tours (smoothed) delimit regions containing, respec- tively, 6 8% and 95% (inner and outer contour) of the PDF. Model K best 0 × 10 2 δ best V best c V best a χ $^{2}$/d.o.f Data (kpc $^{2}$Myr $^{$_{−}$1}$) (km s $^{$_{−}$1}$) (km s $^{$_{−}$1}$) I-F 0.42 0.93 13.5 . . . 11.2 II-F 9.74 0.23 . . . 73.1 4.68 III-F 0.48 0.86 18.8 38.0 1.47