Geant4-DNA was used to calculate the direct DNA breaks due to the alpha and proton particles. For alpha particles with the energy of smaller than 1 MeV, the number of Single-Strand Breaks (SSBs), Double-Strand Breaks (DSBs) and the dose were found to increase. Beyond 1 MeV energy, SSB, DSB and Dose had a falling trend. In the case of DSBs, however, the alpha yields increased from 3.67 (GyGbp)−1 with the energy of 0.1 MeV to 6.43 (GyGbp)−1 at 1 MeV. Beyond 1 MeV energy, the yields decreased to 1.37 (GyGbp)−1 at 30 MeV. The ratio of alpha DSB yields to proton DSB yields increased from 0.1 MeV to 0.5 MeV, but, after 0.5 MeV, the trend changed to a decrease. The calculated results, demonstrated that the difference between alpha and proton SSB yields is not significant, while it is considerable for DSB yields especially with the energy of 2 MeV

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