UBC Theses and Dissertations
Inversion of cosmogenic nuclide data from iron meteorites Pearce, Steven James
The long accepted conclusion that the galactic cosmic ray flux has been "fairly" constant over the past billion years or so is based upon both weak inferences and self-inconsistent interpretations of the data. For example, the well known exposure age diserepency between the analyses based on ⁴⁰K and those of the shorter-lived radionuclides (i.e., ¹⁰Be, ²⁶Al, and ³⁶Cl) has yet to be properly reconciled. Recent work by Schaeffer et al. (1981) on space erosion rates for irons suggests that only a variation in the galactic cosmic ray flux yields the satisfactory explanation. Reformulating this problem within the framework of linear inverse theory (Backus and Gilbert, 1967, 1968, 1970) allows for an unprecedented perspective on this problem as compared with any previous analyses. In theory, it is found that smoothed estimates of the long term galactic cosmic ray prehistory can be formed with arbitrarily good resolution. The degree of smoothing is determined primarily by the relative exposure age differences among adjacent meteorite samples as well as their associated experimental uncertainties. Moreover, this is the only unique information available to this entire study. The near absense of a priori constraints promotes a simple objective philosophy of interpretation. Further, the lack of any independent exposure age determination presents an inherent non-linearity. But, it can be shown that a single model for both the cosmic ray flux and the exposure ages, mutually consistent with the observations and associated errors, can be derived under several fundamental criteria.
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