Taking recent observations for the differential radial gradients and energy spectra of quiet time energetic particles of different energies and masses and over a large heliocentric radial distance from 0.3 to 18 AU into account, we show that the transport equation for these particles (Fokker-Planck equation) reduces to a first-order differential equation for the radial diffusion coefficient, which then can be solved analytically. For energetic helium and oxygen nuclei we then discuss the radial dependence of their diffusion coefficient.