Investigations of cosmic-ray-produced nuclides in iron meteorites, 6. The Signer-Nier model and the history of the cosmic radiation

Abstract

About thirty years ago, a simple model was proposed by Martin and by Ebert and Wänke to describe the interaction of cosmic rays with meteoritic iron. The model was later used by Signer and Nier to calculate the spatial distribution of cosmogenic noble gas nuclides in spherical meteoroids. A rigorous mathematical evaluation of the model—known as the Signer-Nier model—is presented. The model is broadly compatible with data on noble gas nuclides and on 41K/ 40K exposure ages: (1)|In the framework of the model, the abundance ratio of any two of the cosmogenic nuclides 3He, 4He, 21Ne, and 38Ar is correlated with the ratio of any other pair of these nuclides. These ratios depend only on the effective irradiation hardness in the sample. Analysis of these nuclides in a single sample does not permit determination of radiation dosage, of size of the meteoroid, and of location of the sample within the meteoroid. The model-derived correlations agree with the corresponding experimental correlations except for samples from relatively large depths. (2)|The model makes it possible to derive a P 21- 4He 21Ne diagram predicting how 21Ne production rate P 21 and 4He/ 21Ne ratio depend on meteoroid size and sample depth. Possible combinations of values for P 21 and 4He/ 21Ne are restricted, they define the “allowed field” of the diagram. Each location in this field is associated with a distinct combination of values for meteoroid size and sample depth. The diagram is a tool to determine model-dependent values for sizes and depths. Values of P 21 and 4He/ 21Ne from previous noble gas analyses and from 41K/ 40K age determinations fall into or close to the allowed field. The model represents, therefore, a reasonable description of the cosmic ray interaction with iron meteoroids. Analysis of age-, size-, and depth-distributions indicates that space erosion was probably effective with a rate of the order of 1 cm per 100 Ma. An investigation of the compatibility of noble gas, potassium, and radionuclide data reveals the following: (1) the traditional age scale of the 41K/ 40K method is correct; (2) the intensity of the cosmic radiation was essentially constant in the course of the last 1–2 Ga; (3) the intensity exceeded its long-term average by 50% during the last few million years.