Isotope measurements of the trans-iron elements Cu (Z=29) and Zn (Z=30) in the galactic cosmic rays have been made for the first time by the Cosmic Ray Isotope Spectrometer (CRIS) instrument on ACE. These isotopes are clearly resolved despite their scarcity. Cu and Zn are ~1500 times less abundant than Fe (Z=26) and 1% of Ni (Z=28) in the cosmic rays. These elements provide direct information about the source composition of galactic cosmic rays since they are largely uncontaminated by secondary fragments during transport through the Galaxy. All elements with Z > 30 total only ~40% of the abundance of either Cu or Zn and so cannot produce significant amounts of Cu or Zn secondaries.
The figure shows a nuclear charge histogram from CRIS. Elements are well-separated with no contamination from neighboring species. Because the energy deposited in the silicon detectors depends on both the nuclear charge and the atomic mass, the charge histogram contains subpeaks corresponding to distinct isotopes. The two stable isotopes of Cu (A=63,65), and two isotopes of Zn (A=64,66) are easily separated. The Zn peak also includes heavier isotopes for which identifiable subpeaks should appear with additional data collection. The Cu and Zn isotopes are comparable in abundance to 64Ni, another rare isotope whose abundance is also being measured for the first time in CRIS.
The inset box shows the relative abundances of these isotopes in the solar system as taken from the tabulation of Anders and Grevesse. The striking difference between the observed Zn/Cu ratio and the solar system value reflects the well-known first ionization potential (FIP) fractionation in the galactic cosmic rays. This effect amounts to the observation that elements with first ionization potential higher than ~10 eV are depleted relative to those with lower FIP when compared to solar system abundances based on solar photospheric and meteoritic data. A similar bias has been observed in the composition of the solar corona, solar wind, and solar energetic particles.
The data shown here were collected over a period of 15 months since ACE was launched in August 1997. Additional data may make possible isotope measurements of other elements with Z > 30. CRIS and the ACE spacecraft are in excellent health and should continue to accumulate galactic cosmic ray data during solar quiet periods for at least five years.
Contributed by Jeff George of Caltech and Mark Wiedenbeck of JPL.
See The CRIS Home Page for more information about the CRIS instrument.
Last modified 21 December 1998,
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