tokens: are identical to the selection in Adriani et al. ( 2013b ), and we refer to that paper for more details: 1. Event quality selections: We have selected events that do not produce secondary particles by requiring a single track fitted within the spectrometer fiducial acceptance and a maximum of one paddle hit in the two top scintillators of the ToF system. The analysis procedure was similar to previous work on high-energy proton and helium fluxes (Adriani et al. 2011 ). 2. Galactic particle selection: Galactic events were selected by imposing that the lower edge of the rigidity bin to which the event belongs exceeds the critical rigidity, , defined as 1.3 times the cutoff rigidity computed in the Störmer vertical approximation (Shea et al 1987 ) as , where L is the McIlwain L-shell parameter obtained by using the Resurs-DK1 orbital information and the IGRF magnetic field model (MacMillan Maus 2005 ). 3. Charge selection: The charge identification uses the ionization measurements provided by the silicon sensors of the magnetic spectrometer. Depending on the number of hit sensors, there can be up to 12 dE / dx measurements. The arithmetic mean of those measurements is shown in Figure 2 as a function of the rigidity. The actual selection of Z = 1 or Z = 2 particles is depicted by the solid lines. A similar figure has already been shown in Adriani et al. ( 2013b ) but was kept in this paper to help the reader. Figure 2. Ionization loss ( dE / dx in MIP, energy loss of minimum ionizing particles) in the silicon detectors of the tracking system as a function of reconstructed rigidity. The black lines represent the selection for Z = 1 or Z = 2 nuclei. 3.2. Isotope Separation in the <sans-serif><italic>PAMELA</italic></sans-serif> Instrument In each sample of Z = 1 and Z = 2 particles, an isotopic separation at fixed rigidity is possible since the mass of each particle follows the relationwhere R is the magnetic rigidity, is the particle charge, and γ stands for the Lorentz factor. The particle velocity β can be provided either directly from the timing measurement of the ToF system, or indirectly from the energy loss in the calorimeter, which follows β via the Bethe-Bloch formula: (neglecting logarithmic terms). 3.2.1. Isotope Separation Using ToF versus Rigidity For the ToF analysis, we can use directly the β provided by the timing measurement. In Figure 3 we show β versus the particle rigidity for Z = 1 and Z = 2 data. The black lines in the figure represent the expectations for each isotope. A similar figure has already been shown in Adriani et al. ( 2013b ) but was kept in this paper for an easier comparison with the alternative identification method using the calorimeter. Figure 3. β versus rigidity for Z = 1 (top) and Z = 2 (bottom) particles. The black lines were calculated for each isotope using Equation ( 1 ). Since the particle mass is calculated using Equation ( 1 ), misidentified 4 He wrongly reconstructed as Z = 1 particles could result
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