Abstract:
The recently fallen Kunya-Urgench H5 chondrite was studied in various aspects including natural and induced (X-ray and gamma-ray) thermoluminescence, tracks of VH nuclei, and cosmogenic radionuclides with different half-lives. The experimental data, comparative analysis, and theoretical modeling were used to reconstruct the shock–thermal and radiation history of the chondrite, estimate its preatmospheric size and orbit dimensions, and characterize the radiation conditions in the heliosphere at the decline of the 22nd solar cycle. The results suggest that Kunya-Urgench belongs to shock stage S2, and that its material experienced only weak shock–thermal loading (shock pressure below 10 GPa and temperature <700°C). In a spherical model, the preatmospheric radius of the chondrite is R = 42–54 cm; shielding depth of the sample, 18±3 cm; preatmospheric weight, ~1.5–2.2 t; and degree of ablation, ~30–50%. However, a significant departure from the spherical form is possible for the chondrite. The Kunya-Urgench orbit is similar to that of the Pribram chondrite: aphelion q' ~ 4 AU, perihelion q ~ 1 AU, and average heliocentric distance from the Earth of about 3.3 AU. The average integral gradient of galactic cosmic rays along the Kunya-Urgench orbit in the years of the minimum of the 22nd solar cycle was –5.3 ± 5.4 % per astronomic unit (for a range of spectrum rigidity R> 0.5 GV). This estimate is in agreement with gradient values in the minimum phases of previous solar cycles determined from data on the St.-Severin, Innisfree, Torino, and Tahara chondrites and results of direct measurements in interplanetary space. This suggests a decrease in the modulation of galactic cosmic rays during quiet Sun years.
