Semiempirical rates of formation of cosmic ray tracks in spherical objects exposed in space: Preatmospheric and postatmospheric depth profiles

Abstract

Summary. Observations of fossil tracks in meteorites and lunar rocks are used to estimate long‐term averaged production rates of tracks due to cosmic ray multicharged nuclei in common silicates in spherical meteorites. Results are presented graphically for track production rate (TPM, tracks/cm² m.y.) versus depth in meteorites with radii of 3–500 cm. Track production profiles are also presented for TPM values expected at the surface of a uniformly ablated meteorite as a function of ablated depth for ‘recovered,’ i.e., postatmospheric, radii values of 3–500 cm. In the case of meteorites having a ‘single’ exposure history their cosmic ray exposure age is given by measurements of cosmogenic gases, e.g., ³He and 22Ne. This procedure allows an estimation of TPM values at the surface of the meteorite from the observed track densities. The TPM curves presented herein then suffice to deduce the ablation during atmospheric transit of a spherical meteorite for cases where the assumption of a uniform burn is a reasonable approximation. Errors in the deduced extent of ablation due to uncertainties in the estimated TPM values and the size of the recovered meteorite are discussed.

Formation of accretionary dust mantles in the solar nebula: Evidence from preirradiated olivines in CM chondrites

“…Assuming a location just below the solar flare track production zone, i.e., in the uppermost millimeters of the regolith layer, the clast must have been irradiated for about 3 Ma by GCR. This value was obtained using the observed mean track density and the nomogram by Bhattacharya et al (1973). It is a minimum value since locations deeper within the regolith layer would increase the irradiation times significantly.…”

Itawa Bhopji (L3–5) chondrite regolith breccia: Fall, classification, and cosmogenic records

“…Track production rates within the meteorite were calculated using the exposure age of 19.6 Ma, estimated from the concentration of cosmogenic components in rare gases as discussed later. Shielding depths for each sample were calculated by comparing these values with the track production rates given by Bhattacharya et al (1973), after appropriate corrections for the low track recording sensitivity of olivines (factor of 2) was applied to the track density (Bhandari et al, 1972). The results (Table 3) show that the shielding depths are small, ranging between 1.5 and 3.6 em suggesting roughly symmetric all round ablation ofthe meteoroid.…”

Determination of the Specific Activities of the Cosmogenic Nuclides 10Be and 26Al in Meteorites from the Sahara by Means of Accelerator Mass Spectrometry

“…In Fig. 4 the semiempirical track densities of nuclear tracks in spherical bodies with symmetrical ablation according to Bhattacharya et al [27] based on the revised exposure age of St. Severin [28] are plotted versus the shielding depth of the sample for different meteoroid radii. The average nuclear track density was determined in olivine grains of Acfer 277, which were etched under Standard conditions [29], to 3.34 • W tracks/cm^.…”