Measurement of the Isotopic Composition of Manganese, Iron, and Nickel in the Galactic Cosmic Rays

Abstract

Using the small collector of the Trek experiment, a nuclear track-etch detector deployed on board the Russian space station Mir, we have made a measurement of the isotopic composition of manganese, iron, and nickel in the Galactic cosmic rays (GCR) at energies between 150 and 400 A MeV. We find that the isotopic composition of iron and nickel in the GCR source is consistent with solar system values; in particular, [<SUP>60</SUP>Ni]/[<SUP>58</SUP>Ni] is 0.78+<SUP>-0.28</SUP><SUB>0.29</SUB> times the solar value, in contradiction to two previous results. The measurements were made in 1991-1992, during a maximum in solar activity and unusually strong solar modulation; we estimate that the analyzed GCR ions had kinetic energies between 550 and 1200 A MeV outside of the heliosphere. Based on our upper limit for the abundance of <SUP>56</SUP>Ni, we place a lower limit of ∼24 days on the time between nucleosynthesis and acceleration of GCR material; the lack of significant enhancements in <SUP>54</SUP>Fe or <SUP>60</SUP>Ni excludes the possibility of major contributions of neutron- rich supernova zones to the GCR source. We do not find a strong depletion of <SUP>54</SUP>Mn, indicating that this isotope does not completely decay during propagation. We also place upper limits on the abundances of <SUP>54</SUP>Fe, <SUP>57</SUP>Fe, <SUP>58</SUP>Fe, <SUP>62</SUP>Ni, and <SUP>64</SUP>Ni. Our limit on the abundance of <SUP>58</SUP>Fe is nearly a factor of 3 lower than previously reported upper limits.