Measurements of neutron capture effects on Cd, Sm and Gd in lunar samples with implications for the neutron energy spectrum

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Abstract

The bombardment of the lunar surface by cosmic rays produces secondary neutrons, some of which are thermalized by the lunar soil and then interact strongly with the isotopes of Cd, Gd and Sm which have high neutron capture cross sections at thermal energies causing changes in their isotopic compositions. We have measured these changes in samples from the Apollo 14, 16 and 17 missions using thermal ionization mass spectrometry. Changes of 114Cd/113Cd of 0.3–0.5% in samples 60501,105, 65701,23 and 72161,73, of 0.4 and 0.8% in 158Gd/157Gd in samples 14163,848 and 60501,105 and of 0.8, 1.2 and 0.06% for 150Sm/149Sm in samples 14163,848 and 60501,105 and 74220,125 respectively, have been observed. This is the first time that neutron capture on Cd has been detected in lunar samples. Thermal neutrons are captured preferentially at resonance energies of 0.03 eV by 155Gd and 157Gd, at 0.09 eV by 149Sm and at 0.178 eV by 113Cd. A comparison of the changes in 114Cd/113Cd, 156Gd/155Gd, 158Gd/157Gd and 150Sm/149Sm due to neutron capture can therefore indicate the energy distribution of the neutrons. Previous work has compared changes in 158Gd/157Gd and 150Sm/149Sm, while this study extends the comparison to 114Cd/113Cd, where the resonance energy is significantly higher. This has enabled us to confirm evidence for a harder neutron energy spectrum in lunar soil than is predicted theoretically. This can be seen in significantly higher capture rates than predicted for 149Sm compared to 157Gd. We also present evidence of enhanced capture rates on 113Cd compared with 149Sm and 157Gd. The concentrations of Cd, Gd and Sm in nine lunar samples have also been measured by isotope dilution mass spectrometry with Cd being measured in lunar samples for the first time by this method. While Cd was found to be heterogeneously distributed in some samples, Gd and Sm showed good agreement with previous measurements, except for sample 14163,848 where it was significantly lower.

Introduction

Cosmic ray bombardment of the lunar surface produces a variety of secondary particles including neutrons that are thermalized in the lunar soil. These neutrons are preferentially captured by isotopes with high capture cross sections such as 113Cd, 149Sm, 155Gd and 157Gd (2.06×104, 4.01×104, 6.09×104 and 2.540×105 barns at resonance respectively: N. Holden, Brookhaven National Lab., USA, personal communication). These reactions have been measured for Sm and Gd in meteorites [1], [2], [11] and in lunar samples [3], [4], [5], [6], [7], [8], [9], [10], [12] but although evidence of neutron capture on Cd has been sought in meteorites [13] it has not been detected. No previous measurements have been made on Cd isotopes in lunar material.

Isotopes 113Cd, 149Sm and 155 and 157Gd have resonance energies for neutron capture at 0.178, 0.0973, 0.0268, and 0.0314 eV respectively. However, the energy spectrum of the thermal neutrons in the lunar surface is still not well known. Three decades ago Lingenfelter et al.[14] constructed a theoretical energy spectrum for neutrons in lunar soil in order to calculate relative changes in the isotopic composition of these and other isotopes. Although this work has provided a valuable basis for experimental studies, measurements of relative changes in the abundance of Gd and Sm isotopes suggest that the neutron energy spectrum in lunar soil is harder than expected from theoretical considerations [12].

Here we extend the earlier isotopic work on lunar samples to include measurements of neutron capture on 113Cd and in so doing explore a significantly higher energy region of the neutron spectrum. The work also provides concentration measurements of Cd in lunar rock and soils by isotope dilution mass spectrometry (IDMS), an accurate method that has not been used previously. Because earlier studies did not include some samples analyzed here, complementary measurements were also made of Gd and Sm isotopes in these samples.

Section snippets

Samples

Lunar samples were stored inside a clean-air laboratory in the aluminum capsules received from the NASA lunar sample curators. After weighing each sample, the material was immediately re-sealed.

Nine lunar soil samples were analyzed in this study, 10017,341 (vesicular basalt, high Ti mare basalt), 14163,848 (surface soil sample), 14310,615 (polymict rock melted on impact), 15041,188 (near surface soil sample), 15059,240 (interior chips from a regolith breccia containing mixed fragments),

Results and discussion

Cd, Gd and Sm concentrations in lunar samples measured by IDMS are listed in Table 1.

Conclusions

The concentrations of Cd, Gd and Sm in nine lunar samples have also been measured by IDMS with Cd being measured for the first time by this method. The measured Gd and Sm concentrations include values for sample 14163,848 which are significantly lower than the literature values.

Neutron capture effects in Cd, Gd and Sm have been measured in five lunar samples. Changes in the Cd isotopic ratios, reported for the first time in lunar material, reach ∼0.5% for 114Cd/113Cd in samples 60501,105,

Acknowledgements

The authors acknowledge the provision of nine lunar samples by NASA to J.R. De Laeter, the designated Principal Investigator for these studies, and the assistance of the NASA lunar sample curators. During this project helpful technical support and advice was received from staff and colleagues, in particular R. Loss, W. Chisholm, G. Burton, D. Nelson, I. Fletcher, C. Smith, R. Maas, and T. Margrain. The Curtin TIMS laboratory is supported by funds from the Australian Research Council. One of us

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    Present address: Land Operations Division, P.O. Box 1500, Salisbury 5108, Australia.

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