  Galactic cosmic rays and solar protons ionize the present terrestrial
atmosphere, and the air showers are simulated by well-tested Monte-Carlo
simulations, such as PHITS code. We use the latest version of PHITS to evaluate
the possible ionization of protoplanetary disks by galactic cosmic rays (GCRs),
solar protons, and by supernova remnants. The attenuation length of GCR
ionization is updated as 118 g cm-2, which is approximately 20% larger than the
popular value. Hard and soft possible spectra of solar protons give comparable
and 20% smaller attenuation lengths compared with those from standard GCR
spectra, respectively, while the attenuation length is approximately 10% larger
for supernova remnants. Further, all of the attenuation lengths become 10%
larger in the compound gas of cosmic abundance, e.g. 128 g cm-2 for GCRs, which
can affect the minimum estimate of the size of dead zones in protoplanetary
disks when the incident flux is unusually high.
