The Cosmic-Ray Radioactive Nuclide ³⁶Cl and Its Propagation in the Galaxy

Abstract

Radioactive ³⁶Cl has been resolved from ³⁵Cl and ³⁷Cl for the first time in the cosmic radiation with measurements obtained from the high-resolution High-Energy Telescope (HET) carried on the NASA-ESA Ulysses spacecraft. Within the framework of a homogeneous propagation model wherein all ³⁶Cl is of spallation origin, the average density of the interstellar gas ρ, through which all cosmic-ray nuclei propagate, was determined. Heliospheric modulation was included in the analysis. The abundance ratio ³⁶Cl/Cl=5.2+/-1.8% at an average energy of 238 MeV u^-1 yields ρ=0.28^+0.12_-0.10 atoms cm^-3. Although ³⁶Cl has a relatively short half-life (τ_1/2=3.01×10⁵ yr) compared with the approximately 20 Myr confinement or escape time T_esc for cosmic rays in the Galaxy derived from ¹⁰Be or ²⁶Al analysis, it is shown that the confinement time based on ³⁶Cl analysis is 18⁺¹⁰_-6 Myr. Radioactive ¹⁰Be, ²⁶Al, and ³⁶Cl are all produced by nuclear interactions during propagation from Galactic sources to the observer. However, ¹⁰Be, ²⁶Al, and ³⁶Cl are each predominantly products of different primary cosmic-ray nuclei. ⁵⁴Mn is the fourth radioactive isotope measured by the Ulysses HET. Unfortunately, its decay half-life to ⁵⁴Fe is difficult to determine. Recent measurements yielded τ_1/2=6.3×10⁵ yr, leading to ρ=0.40^+0.23_-0.15 atoms cm^-3 with a confinement time T_esc=11 Myr.