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Neutron stratigraphy in the lunar regolith

Citation

Russ, Guston Price, III (1974) Neutron stratigraphy in the lunar regolith. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-05292008-130623

Abstract

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Differences in the isotopic composition of Gd and Sm among lunar samples collected during the Apollo 11, 12, 14, 15, and 16 and Luna 16 missions have been measured by high precision mass spectrometry. These differences (up to 1.9% in [...]Gd/[...]Gd and 2.0% in [...]Sm/[...]Sm) can be attributed unambiguously to low energy neutron capture reactions and correspond to neutron fluences as high as 10[...] n/cm[...] (E < 0.18 eV). For the Apollo 16 samples, enrichments are reported for the [...]Gd/[...]Gd ratio which appear to be due to neutron capture by [...]Eu. The measured ratio of the number of neutrons captured per atom by [...]Sm to the number captured per atom by [...]Gd indicates that the low energy lunar neutron spectrum is in reasonable agreement with that calculated by Lingenfelter, Canfield, and Hampel but that the lunar spectrum is somewhat harder. A synthesis of all the available lunar neutron capture data for [...]Gd, [...]Gd, [...]Sm, [...]Eu, [...]Co, [...]Br, [...]Br, [...]Ba, [...]W, [...]U, [...]U, and [...]Ca indicates that only the [...]W and [...]U data are incompatible with the LCH neutron spectrum and flux.

Based on the LCH neutron flux, the neutron fluences, [...], determined for the lunar soils are all lower than would be expected for materials mixed in the upper few meters of the regolith for > 3 x 10[...] yr. In terms of a uniform mixing model the measured neutron fluences imply that the depth of the regolith is between 10 and 14 m at all of the Apollo sites studied. These are greater than the depths implied from cratering theory. The [...]Xe/[...] ratios for the soils indicate that they have been well mixed to a depth of > 200 g/cm[...]. Detailed fluence studies in the Apollo 12 double core show that these materials could not have been undisturbed for > 10[...] yr. The upper layers of the Apollo 15 and 16 drill stems also imply mixing on this time scale. However, the Apollo 15 drill stem has a smooth peak in the fluence at a depth of ~ 190 g/cm[...]. From the magnitude of this peak, it can be concluded that below a depth of ~ 75 g/cm[...] these materials have been undisturbed for at least 400 my. The fluence profile in the Apollo 16 drill stem is more complicated but indicates an area where material has been accumulating for several hundred million years.

A model is presented for the depth dependence of the ratio of [...]Xe produced by neutron capture on [...]Ba to [...]Xe produced by spallation reactions on Ba. Effective exposure depths are calculated for 24 rocks from Apollo 11 and 12. Detailed irradiation histories are presented for rocks 10017, 14310, and 15595. An erosion rate for Hadley Rille of ~1 cm/my is calculated from the neutron fluence of rock 15595.

From Gd and Sm isotopic measurements it is shown that the Norton County meteorite has been exposed to a neutron flux with a spectrum compatible with that predicted by the LCH calculations. The neutron fluence to which our sample of Norton County has been exposed is 1.2 x 10[...] n/cm[...]. No neutron produced effects were observed in the isotopic composition of Gd from a bulk sample of the Allende meteorite.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Burnett, Donald S. (advisor)
  • Wasserburg, Gerald J. (advisor)
Thesis Committee:
  • Unknown, Unknown
Defense Date:27 November 1973
Record Number:CaltechETD:etd-05292008-130623
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-05292008-130623
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2258
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:29 May 2008
Last Modified:26 Dec 2012 02:49

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