A Balloon Measurement of the Isotopic Composition of Galactic Cosmic Ray Iron

Citation

Grove, Jon Eric (1989) A Balloon Measurement of the Isotopic Composition of Galactic Cosmic Ray Iron. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3cr8-jv31. https://resolver.caltech.edu/CaltechETD:etd-05312007-121425

Abstract

We have measured the isotopic composition of galactic cosmic ray iron in the energy interval ~1550-2200 MeV/nucleon using a balloon-borne mass spectrometer. The instrument was flown from Palestine, Texas, in May 1984 for >35 hours at an atmospheric depth of ~6 g/cm². Masses were derived by the Cerenkov-Energy technique. The Cerenkov counter employed a silica aerogel radiator with index of refraction n = 1.1. Particle energies were measured in a stack of NaI(Tl) scintillators, which also provided particle trajectories. The calibration of the detectors is discussed, along with the algorithms we have used to calculate velocities, energies, and masses. The limitations of aerogels as Cerenkov radiators, particularly the stability of their light yield, are considered. A detailed discussion of the sources of mass uncertainty is presented, including an analytic model of the contribution from fluctuations in the Cerenkov yield from knock-on electrons. The achieved mass resolution is ~0.65 amu, which is consistent with the theoretical estimate. We report an ⁵⁴Fe/⁵⁶Fe abundance ratio of 0.14^+0.18_-0.11 and an 84% confidence upper limit of ⁵⁸Fe/⁵⁶Fe ≤ 0.07 at the top of the atmosphere. Combining our data with those of previous measurements of the composition of iron at lower energies, and using a model of the galactic propagation, we derive cosmic-ray source abundance ratios of ⁵⁴Fe/⁵⁶Fe = 0.064^+0.032_-0.027 and ⁵⁸Fe/⁵⁶Fe ≤ 0.062. These values are consistent with the composition of solar-system iron and place restrictions on the conditions under which cosmic-ray iron is synthesized.

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