Engineering topics

Kohl, C. P.

Publications and source records attributed to Kohl, C. P..

14 records

NASA NTRS1996

Cosmogenic Production of Be-7 and Be-10 in Water Targets

We have measured Be-10(t(sub 1/2) = 1.5 x 10(exp 6) years) and Be-7 (t(sub 1/2) = 53.28 days) concentrations in water targets exposed for 1 to 2 years at Echo Lake, Colorado (elevation = 3246 m) and at La Jolla, California (140 m). Neutron monitor data were used to normalize the measured concentrations in order to calculate production rates equivalent to the cosmic ray flux averaged over four solar cycles (43 years). The Be-7 production rates thus obtained correspond to 6.03 +/- 0.07 x 10(exp -6) atom/g.O/s at Echo Lake and 5.06 +/- 0.20 x 10(exp -7) atom/g.O/ s at La Jolla. The Be-10 production rates correspond to 3.14 +/- 0.18 x 10(exp -6) atom/g.O/s at Echo Lake and 2.68 +/- 0.47 x 10(exp -7) atom/g.O/s at La Jolla. When compared with Be-10 production rates determined in Be-10-saturated rocks from the Antarctic and with theoretical calculations based on meteorite and lunar sample data, we find that the million-year average production rate is about 14 - 17% greater than the present production rate averaged over the last four solar cycles. Comparison with production rates determined by measuring glacially polished rocks from the Sierra Nevada in California indicates that average production (based on a revised 13,000-year deglaciation age and a geographic latitude correction) is a about 11% greater than the average over the last four solar cycles. The measured Be-10/Be-7 production ratio in oxygen is 0.52 +/- 0.03 at Echo Lake and 0.55 +/- 0.07 at La Jolla.

Nishiizumi, K.↗

NASA NTRS1993

Measurements of cosmogenic nuclides in lunar rock 64455

Eleven samples were ground from the glass coated surface of lunar rock 64455,82 with an average depth resolution of 50 microns and were measured for Be-10, Al-26, and Cl-36 using AMS (accelerator mass spectrometry). Results show no evidence of SCR (solar cosmic ray) effects. The flat cosmogenic nuclide profiles and activity levels are consistent with a 2 My exposure history for the rock and a sample location on the bottom of the rock. These AMS measurements are some of the most precise ever obtained for these three nuclides. This precision and the demonstrated fine depth resolution will enable us to conduct a number of detailed studies of depth effects in lunar and meteoritic samples, including investigating SCR effects in the surface exposed top of the glass coating of 64455 and possibly in the underlying rock.

Arnold, J. R.↗

NASA NTRS1992

Chemical isolation of quartz for measurement of in-situ-produced cosmogenic nuclides

An isolation method relying totally on chemical steps was developed to separate large quantities (10-200 g) of clean mono-minerallic quartz samples from a variety of terrestrial rocks and soils for the purpose of measuring Be-10 (t1/2 = 1.5 Myr) and Al-26 (t1/2 = 0.705 Myr) produced by cosmic rays in situ in the quartz phase. The procedure consists of grinding the sample, heating it in HCl, and treating it with a series of leaches using a dilute HF/HNO3 mixture in a heated ultrasonic tank. The purified quartz was also used for the measurements of in situ cosmic-ray-produced Ne-21 and C-14 (t1/2 = 5730 yr). The method is applicable to any problem requiring purified quartz on a large scale.

Kohl, C. P.↗

NASA NTRS1991

In situ Be-10-Al-26 exposure ages at Meteor Crater, Arizona

A new method of dating the surface exposure of rocks from in situ production of Be-10 and Al-26 has been applied to determine the age of Meteor Crater, Arizona. A lower bound on the crater age of 49,200 + or - 1,700 years has been obtained by this method.

Nishiizumi, K.↗

NASA NTRS1991

Cosmic ray produced Be-10 and Al-26 in Antarctic rocks - Exposure and erosion history

Cosmic-ray produced Be-10 and Al-26 were measured in purified quartz fractions of selected rock samples from Antarctic mountains. From these data, mean erosion rates were calculated for the limiting case of steady-state surface exposure to cosmic rays, and minimum exposure ages, for the limiting case of no erosion. Calculated mean erosion rates are very low, on the order of a few times 0.00001 cm/yr; the sampling is believed to be sufficient to generalize this result to exposed bedrock in Antarctica. In favorable cases it is possible to distinguish between the limiting cases; steady-state erosion seems a better description in such cases. Most samaples, including some taken a few meters above the present ice level, seem to have been exposed for millions of years, without major episodes of burial or abrasion by ice.

Nishiizumi, K.↗

NASA NTRS1989

Cosmic ray production rates of Be-10 and Al-26 in quartz from glacially polished rocks

The concentrations of Be-10 and Al-26 in quartz crystals extracted from glacially polished granitic surfaces from the Sierra Nevada range are studied. These surfaces are identified with the glacial advance during the Tioga period about 11,000 yr ago. The measurements yield the most accurate estimates to date for the absolute production rates of three nuclides in SiO2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43.8-44.6 N and altitudes 2.1-3.6 km.

Nishiizumi, K.↗

NASA NTRS1989

Measurements of long-lived cosmogenic nuclides in returned comet nucleus samples

Measurements of long lived cosmic ray produced radionuclides have given much information on the histories and rates of surface evolution for meteorites, the Moon and the Earth. These nuclides can be equally useful in studying cometary histories and post nebular processing of cometary surfaces. The concentration of these nuclides depends on the orbit of the comet (cosmic ray intensity changes with distance from the sun), the depth of the sampling site in the comet surface, and the rate of continuous evolution of the surface (erosion rate of surface materials). If the orbital parameters and the sampling depth are known, production rates of cosmogenic nuclides can be fairly accurately calculated by theoretical models normalized to measurement on lunar surface materials and meteoritic samples. Due to the continuous evaporation of surface materials, it is expected that the long lived radioactivities will be undersaturated. Accurate measurements of the degree of undersaturation in nuclides of different half-lives allows for the determination of the rate of surface material loss over the last few million years.

Nishiizumi, K.↗

NASA NTRS1979

The extent of lunar regolith mixing

The activity of solar cosmic-ray-produced Mn-53 measured as a function of depth in the upper 100 g/sq cm of lunar cores 60009-60010 and 12025-12028 is discussed. Analyses of samples from the Apollo 15 and 16 drill stems together with authors' previously published results (1974, 1976), and the Battelle Na-22 and Al-26 data, indicate that in three of the four cases studied the regolith was measurably disturbed within the last 10 m.y. Activities measured in the uppermost 2 g/sq cm indicate frequent mixing within this depth range. The Monte Carlo gardening model of Arnold (1975) was used to derive profiles for the gardened moon-wide average of Mn-53 and Al-26 as a function of depth. The Mn-53 and Al-26 experimental results agreed with theoretical predictions, but the calculated depths of disturbance appeared too low.

Nishiizumi, K.↗

NASA NTRS1978

The cosmic ray record in the San Juan Capistrano meteorite

Measurements of cosmic ray track densities, rare gas concentrations, and radioisotope activities on the same sample of the San Juan Capistrano (SJC) meteorite (March 15, 1973) were combined in order to obtain a consistent picture of the irradiation history of the meteorite. The latter was recovered in two fragments weighing 50.5 and 5.5 g, respectively, which were then sampled according to the objectives set forward. The preatmospheric size of SJC and the depth occupied by the sample in the preatmospheric body were determined within narrow limits. In addition, the study compared estimates of the cosmic ray exposure age as determined by various techniques and evaluated the amount of ablation undergone when SJC penetrated the earth's atmosphere.

Finkel, R. C.↗

NASA NTRS1978

Evidence for the constancy of the solar cosmic ray flux over the past ten million years - Mn-53 and Al-26 measurements

Mn-53 and Al-26 activities have been measured in fourteen samples carefully ground from the upper 1.5 cm of rock 68815 to discern whether the magnitude and shape of the solar cosmic ray (SCR) flux in the last 2 m.y. has differed from the recent SCR flux and that averaged over the last 10 m.y. Results yield no evidence that the SCR flux has varied among these time intervals. Assuming the appropriate erosion rates, the measured profiles of rock 68815 (with a 2 m.y. surface exposure) and of rocks 12002 and 14321 (saturated with respect to Mn-53 and Al-26) can all be fit with the SCR parameters of R sub zero = 100 MV and J = 70 protons/sq cm/sec (4 pi, energy greater than 10 MeV). The measured profiles of 68815 and 14310 are incompatible with the short surface exposure age (less than 3 m.y.) proposed for 14310 on the basis of track studies. Activity vs. depth profiles for three different faces of 68815, show surface activities nearly independent of sample inclination.

Kohl, C. P.↗

NASA NTRS1976

Mn-53 in the Apollo 15 and 16 drill stems - Evidence for surface mixing

The activity of cosmic ray produced Mn-53 has been measured in a series of samples from the upper 10 cm of the Apollo 15 and 16 drill stems. The activity profiles for both cores indicate disturbance to depths of about 3 g/sq cm within the last 6 m.y. In at least one case (Apollo 16) the soil has been gardened to at least 14 g/sq cm within the last 10 m.y., and material from the upper less than 2 g/sq cm has been buried to 14 g/sq cm by this gardening. The results for the Apollo 15 core are compatible with a wide variety of possible histories including loss or gain of material.

Nishiizumi, K.↗

NASA NTRS1974

Depth profiles of Mn-53 in lunar rocks and soils

Results of measurements of cosmic-ray-produced Mn-53 taken down the length of the Apollo 16 deep drill core are presented. They indicate that the lunar regolith has been unmixed, on a meter scale, for the past 5 million years at the location of this core. The data are in agreement with earlier Mn-53 measurements on the Apollo 15 drill core. Mn-53 activity profiles in 14310, 12002, and 14321 are compared to each other; all three rocks have probably been on the lunar surface long enough to saturate their solar cosmic-ray-produced Mn-53 (half-life = 3.7 m.y.) activity.

Imamura, M.↗

NASA NTRS1973

Co-60 in lunar samples.

Results obtained for Co-60 in lunar soils and rocks of different depths are discussed. These results are compared to a production calculation based on the work of Lingenfelter et al. (1972). The measurement of Co-60 activities can give information about the present-day equilibrium neutron flux in the lunar regolith. The most important quantity which can be derived from these data is the absolute present-day lunar neutron production rate.

Wahlen, M.↗

NASA NTRS1972

Cosmogenic nuclides in football-sized rocks.

The activity of long- and short-lived isotopes in a series of samples from a vertical column through the center of rock 14321 was measured. Rock 14321 is a 9 kg fragmental rock whose orientation was photographically documented on the lunar surface. Also investigated was a sample from the lower portion of rock 14310, where, in order to study target effects, two different density fractions (mineral separates) were analyzed. A few nuclides in a sample from the comprehensive fines 14259 were measured. This material has been collected largely from the top centimeter of the lunar soil. The study of the deep samples of 14321 and 14310 provided values for the activity of isotopes at points where only effects produced by galactic cosmic rays are significant.

Wahlen, M.↗