The lifetime of cosmic rays in the Milky Way

Understand

The most reliable method to estimate the residence time of cosmic rays in the Galaxy is based on the study of the suppression, due to decay, of the flux of unstable nuclei such as beryllium-10, that have lifetime of appropriate duration.

• The Cosmic Ray Isotope Spectrometer (CRIS) collaboration has measured the ratio between the fluxes of beryllium-10 and beryllium-9 in the energy range E_0 \simeq 70-145 MeV/nucleon, and has used the data to estimate an escape time tau_{ esc} = 15.0 +- 1.6 Myr.
• This widely quoted result has been obtained in the framework of a simple leaky-box model where the distributions of escape time and age for stable particles in the Galaxy are identical and have exponential form.
• In general, the escape time and age distributions do not coincide, they are not unique (because they depend on the injection or observation point), and do not have a simple exponential shape.