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Neutron capture effects on samarium, europium, and gadolinium in Apollo 15 deep drill‐core samples
Author(s) -
HIDAKA Hiroshi,
EBIHARA Mitsuru,
YONEDA Shigekazu
Publication year - 2000
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2000.tb01438.x
Subject(s) - gadolinium , samarium , neutron , neutron capture , isotope , radiochemistry , europium , materials science , analytical chemistry (journal) , chemistry , mineralogy , nuclear physics , physics , metallurgy , inorganic chemistry , optoelectronics , chromatography , luminescence
— The isotopic compositions of Sm and Gd in seven lunar samples from the Apollo 15 deep drill core were determined to discuss the effects of neutron capture near the lunar surface. Large isotopic deviations of 150 Sm/ 149 Sm, 156 Gd/ 155 Gd, and 158 Gd/ 157 Gd derived from neutron capture effects were observed in all samples. Although neutron capture products in lunar samples were investigated extensively in the 1970s, our precise isotopic measurements resulted in several new findings. The neutron fluence in the Apollo 15 drill core is a function of depth with a symmetric peak at 190 g/cm 2 depth from the surface, confirming the results of earlier investigations. Neutron fluence values calculated from the isotopic shifts by comparison to artificially irradiated standard reagents were (5.16–7.49) × 10 16 n/cm 2 . These values are 1.3 to 1.4x larger than those previously reported. Variations of ε Sm /ε Gd with depth are interpreted as being due to variations in the neutron energy spectrum. Here ε Sm and ε Gd are defined as in previous studies of lunar neutron stratigraphy. Our data suggest that the neutron is more thermalized at the lower layers than it is at the upper layers. In addition to large isotopic shifts for 149 Sm, 150 Sm, 155 Gd, 156 Gd, 157 Gd, and 158 Gd, isotopic enrichments of 152 Gd and 154 Gd derived from neutron capture for 151 Eu and 153 Eu, respectively, were also observed in all samples.