D. Lal

234Th/238U ratios in the ocean

The activity of the short-lived 234 Th (half-life = 24 days) is found to be in marked radioactive disequilibrium with respect to the concentration of its parent. 238 U in the top hundred meters of the sea water column; values above and below secular equilibrium exist. The mechanisms responsible for the observed distribution of 234 Th are discussed and their rates are evaluated.

Techniques for proper revelation and viewing of etch-tracks in meteoritic and terrestrial minerals

The revelation of fossil charged-particle tracks due to very heavy cosmic ray nuclei ( Z > 22) in meteoritic minerals and due to spontaneous fission fragments ( 238 U and 252 Cf) in terrestrial minerals was experimentally studied. Sodium hydroxide was found to be the ideal etchant for the entire group of plagioclase feldspars and several pyroxene species. Controlled etching at 120°–160°C reveals “proper” tracks in these minerals. The tracks formed are nearly cylindrical with sharph defined tips allowing reproducible and accurate track length measurements. A useful outcome of these experiments was the observation that controlled etching can fairly unambiguously help ascertain the position of a mineral species of a single given crystal in the solid solution series under consideration: the dispersion in etching-times for normal development of known terrestrial or meteoritic mineral species is small enough (in most cases) to distinguish neighbouring species. As an aid to easy and complete optical viewing of etch-tracks, we injected a concentrated blue dye solution into the track holes. This considerably increases the optical contrast. Colouring of etch tracks allows reliable measurements of track lengths and easy counting of track densities in the neighbourhood of 10 3 –10 4 /cm 2 , as often encountered for terrestrial minerals.

Characteristics of tracks of ions of 14⩽Z⩽36 in common rock silicates

Abstract This paper concerns the identification of heavy ion tracks in minerals by measurements of track-etch rates and total etchable track lengths. With beams of Si, Cl, Ti, Fe, Zn and Kr at energies up to 10.35 MeV/nucleon we have irradiated nine minerals commonly used to study fossil cosmic ray tracks in meteorites and lunar samples. From our measurements of etched track length as a function of residual range, we have determined response curves for various minerals as a function of ionization rate, using the expression previously derived by Price, Fleischer and Moak. These curves increase smoothly with ionization rate instead of rising abruptly at some critical value as was previously thought. We have shown that the track etch rate concept accounts qualitatively for total etchable track length distributions, but that the positions of the peaks of different elements in these histograms occur at shorter lengths for fossil tracks than for fresh tracks. Our annealing data indicate that, at maximum lunar surface temperatures, tracks in olivine, orthopyroxenes and feldspars may be significantly shortened whereas tracks in clinopyroxenes will not be affected. We cite additional evidence that gradual rearrangement of radiation damage at ambient temperature makes the properties of fresh tracks and of ancient tracks different. It is thus not surprising that the histograms of fresh and fossil tracks do not match perfectly.

Fission fragment tracks in apatite: recordable track lengths

Abstract This study is concerned with the measurements of total lengths of fission fragment tracks due to spontaneous fission of238U (fossil tracks) and neutron induced fission of235U (n-induced tracks) in several terrestrial apatites of geological age exceeding 0.4 billion years. The measurements of total etchable lengths are based on the revelation of confined tracks, TINTs and TINCLEs. The mean etchable length ofconfined n-induced tracks in fluor- and hydroxyl-apatites is measured to be 15.3 ± 1 μ; the longest tracks are about 18 μ in length. The length distribution ofconfined natural fossil tracks shows a wider but peaked distribution; the position of peak is shifted to lower lengths as compared to that forn-induced tracks. Thus measurable annealing effects are found in old apatites for the stored fission tracks. The measurements of TINTs and TINCLEs in apatites allow an estimation of the correction needed for annealing effects in fission-tracks ages.

Spallation cross sections for long-lived radionucleides in iron and light nuclei

The cross sections for the formation of the long-lived radionuclides / sup 10/Be, /sup 32/Si, /sup 40/K, /sup 42/Ar, and /sup 44/Ti/ and a few shortlived nucleides in Fe by 730-Mev protons and of /sup 10/Be in C, N, and O by 220-Mev protons were measured. These data were obtained for interpretation of the observed results of cosmic ray induced radioactivity in meteorites and on the earth. Over a wide range of mass numbers, the measured cross sections in iron for various radioactive nucleides, especially those close to the nuclear stability line, can be expressed fairly accurately by a semi-empirical relation first discussed by Rudstam. Using this relation, reasonable values for the half- lives of the radioactive nucleides /sup 32/Si and /sup 44/Ti, for which only crude estimates were available, were calculated. (auth)

Silicon, radium, thorium, and lead in seawater: In-situ extraction by synthetic fibre

Abstract Acrilan fibre loaded with ferric hydroxide concentrates Si and trace elements from tons of sea water. This synthetic fibre is cheap, relatively blank free and easily processed. 228 /Ra 226 Ra activity ratios were measured by this technique in the Indian Ocean and the Mediterranean; the results are consistent with those made directly on seawater. Data are also reported on fibre enrichment factors relative to seawater for Si, Ra, Th, and Pb; these factors range from 200 to 10 000.

Characteristics of fission tracks in zircon: Applications to Geochronology and Cosmology

We describe a simple method for nearly isotropic revelation of fission tracks in zircon. The etchant is an equivolume mixture of 48% HF and 98% H2SO4; etching is carried out under pressure at 150° – 180°C. For fossil track densities above ∼ 5 × 106, the etching time is strongly anticorrelated with track density. The total etchable fission fragment track length (2 fragments) is 11 ± 0.5 μm. The length distribution is sharply peaked; the standard deviation in different samples is ± (0.7 – 0.8) μm. Thermal annealing studies indicate a lower activation energy for track fading than was previously repoted using a phosphoric acid etchant. A practical chemical dissolution method is described for quantitatively recovering zircon crystals from rock samples; this method should find application in fission track dating of even zircon poor rocks.

On the Energy Spectrum of Iron-Group Nuclei as Deduced From Fossil-Track Studies in Meteoritic Minerals

We present here experimental data on the depth variation of fossil-track densities in the hypersthene and oligoclase crystals sampled from several cores taken from the least ablated regions of the meteorite Saint-Severin. The implications of these results (and of those in the Patwar meteorite) to the extent of atmospheric/preatmospheric ablation, and to the time-averaged flux and energy spectrum of cosmic-ray iron-group nuclei during the last 10–50 × 106 years, are discussed.

Phosphorus isotopes P32 and P33 in rain water

Abstract In 1955, Marquez and Costa 1) detected Phosphorus 32 (half life ≈ 14 days) in rain water. They attributed its presence to the spallation of atmospheric Argon nuclei by cosmic rays. We have detected, in rain water, the second short lived radio-isotope of Phosphorus, P 33 (half life ≈ 25 days). Approximate concentrations (atoms m1 −1 of rain water) of the two isotopes have been obtained; both are present in comparable amounts. The observed concentrations agree in order of magnitude with those reported by Marquez and Costa for P 32 . They agree also with a rough estimate based on the number of disintegrations produced in the atmosphere by cosmic radiation and the estimated cross section for this reaction. A simultaneous determination of the concentrations in rain water of these chemically identical radioactive nucleides, may prove valuable in meteorological studies. It may be useful for tracing the large scale motion of air masses.

Radiochemical measurements of 10Be and 7Be formation cross sections in oxygen by 135 and 550 MeV protons

Abstract Radiochemical measurements of the activities of 10Be and 7Be in oxygen due to the bombardment of water samples by protons of energies 135 and 550 MeV are reported. The ratios σ( 10 Be ) σ( 7 Be ) of the cross sections in oxygen are found to be 0.19 ± 0.01 and 0.32 ± 0.02 for 135 and 550 MeV protons, respectively, based on t 1 2 ( 10 Be) = 2.7 × 10 6 y and t 1 2 ( 7 Be) = 53.6 d . Using the best estimate of the excitation function of 7Be in oxygen, the formation cross sections for 10Bein oxygen are estimated to be 1.0 ± 0.06 and 2.9 ± 0.2 mb for protons of energies 135 and 550 MeV respectively. Cross sections for 10Be in oxygen, averaged over the energy spectrum of cosmic rays, and based on the observations of its activity in meteorites and deep-sea sediments are discussed. The implication of the present measurements for the use of the 10Be isotope as a “clock” to estimate the “age” of cosmic rays is briefly noted.