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Reeves, J. H.

Publications and source records attributed to Reeves, J. H..

8 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Induced radioactivity of LDEF materials and structural components

We present an overview of the Long Duration Exposure Facility (LDEF) induced activation measurements. The LDEF, which was gravity-gradient stabilized, was exposed to the low Earth orbit (LEO) radiation environment over a 5.8 year period. Retrieved activation samples and structural components from the spacecraft were analyzed with low and ultra-low background HPGe gamma spectrometry at several national facilities. This allowed a very sensitive measurement of long-lived radionuclides produced by proton- and neutron-induced reactions in the time-dependent, non-isotropic LEO environment. A summary of major findings from this study is given that consists of directionally dependent activation, depth profiles, thermal neutron activation, and surface beryllium-7 deposition from the upper atmosphere. We also describe a database of these measurements that has been prepared for use in testing radiation environmental models and spacecraft design.

NASA Discipline Radiation Health↗
NASA NTRS1987

Al-26 survey of Antarctic meteorites

An Al-26 survey by nondestructive gamma ray spectrometry was conducted as part of the preliminary examination effort for Antarctic meteorites. A total of 220 samples were studied. The majority of the samples were from the Allan Hills area; however, samples from several other sites, including the Yamato Mountains, were studied. Compared to worldwide falls and finds, the Antarctic group shows a clear trend toward lower Al-26 levels. At least 10 percent of the samples studied were clearly undersaturated in Al-26. The frequency of undersaturated samples suggests that terrestrial ages of several hundred thousand years are common among the Antarctic meteorite collection. The absence of samples with extremely low Al-26 implies that the upper limit for terrestrial ages in the Antarctic is on the order of 1 million years.

Evans, J. C.↗
NASA NTRS1986

Cosmogenic radionuclides and noble gases in the Wethersfield (1982) chondrite

The Wethersfield (1982) chondrite was assayed for a suite of cosmogenic radionuclides shortly after fall. Data are reported for Be-7, Na-22, All-26, Sc-46, V-48, Cr-51, Mn-54, Co-56, Co-57, and Co-60. A comparison is made with predicted results based on a scaling to the Deep River Neutron Monitor. Noble gases were also assayed in a subsample. The cosmic-ray-exposure age is estimated to be 45 Myr.

Evans, J. C.↗
NASA NTRS1982

Studies of lunar regolith dynamics using measurements of cosmogenic radionuclides in lunar rocks, soils and cores

The present investigation is concerned with two types of measurements. The first deals with the use of Al-26 to define regolith gardening processes which have occurred in the upper portion of the lunar cores that were collected both by the deep drilling and drive tube operations. The second portion of the study involves the use of Mn-53 and Al-26 concentrations to determine the surface exposure histories of individual rocks over the past ten million years. It is found that during the past two million years, 15011 has had a history of simple gardening with an accumulation rate of 2 cm/m.y. The Apollo 15 Station 2 boulder appears to have been in place for about five million years. The 'shadowed' soil (75237) shows a shielding effect of about 40%. The Apollo 16 Station 9 boulder appears to have been in place for about four million years and thus is apparently not related to the South Ray Crater event.

Fruchter, J. S.↗
NASA NTRS1980

Recent depositional history of Apollo 16 and 17 cores

Measurements of Al-26 and Mn-53 in the Apollo 17 drive tube 76001 show a clear integral excess of activity above expected production down to a depth of at least 9 gm/sq cm. This excess is interpreted as a continuous accretion of highly irradiated surface material slowly transported downslope by impact or other processes at a buildup rate of 1.5 cm/m.y. This drive tube, 76001, was collected near a change in slope on the North Massif. Integral excesses have also been observed in other Apollo 16 and 17 cores. Evidence in the Al-26 and Mn-53 record for continuous accretion is shown for Apollo 16 cores 60007 and 60010 and the Apollo 16 deep drill string 70009-70001. This evaluation shows that accretion at specific sites, either through continuous downslope movement or regolith material on the moon, or simply through small impact redistribution, is common on both small and large scales.

Evans, J. C.↗
NASA NTRS1979

History of the Apollo 17 deep drill string during the past few million years

Measurements of the cosmogenic radionuclides 22-Na, 26-Al and 53-Mn in the Apollo 17 deep drill string 70002-70009 have shown that this core sample was taken in an area of considerable recent surface activity. Measurements of 22-Na indicate that the core had not been significantly altered during its collection and subsequent handling, while measurements of the 26-Al and 53-Mn showed a very substantial excess in these isotopes in the top 25 cm and a depletion below about 50 cm. The analysis of the data indicates that a crater excavation occurred to a depth of 50 to 60 cm at the site of the deep drill string no longer that 0.5 m.y. ago, and probably 0.3 m.y. ago. Since that time, the crater has filled to a depth of about 25 cm with near-surface material which contains high concentrations of 26-Al and 53-Mn. This recent activity at the site of the Apollo 17 drill string places firm time constraints on the models developed by other investigators which describe the history of this site.

Fruchter, J. S.↗