  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. It is therefore
necessary to discuss the measurement of the beryllium ratio in a framework that
is more general and more realistic than the leaky-box model.
  In this work we compute the escape time and age distributions of cosmic rays
in the Galaxy in a model based on diffusion that is much more realistic than
the simple leaky-box, but that remains sufficiently simple to have exact
analytic solutions. Using the age distributions of the model to interpret the
measurements of the beryllium-10 suppression, one obtains a cosmic ray
residence time that is significantly longer (a factor 2 to 4 depending on the
extension of the cosmic ray halo) than the leaky-box estimate. This revised
residence time implies a proportional reduction of the power needed to generate
the galactic cosmic rays.
