Radionuclides In Meteorites Research Articles

Temporal and Spatial Variations of Cosmogenic Radionuclide Production Rates in Chondrites During Their Passage Through the Inner Heliosphere

To study radiation environment in the interplanetary space, cosmogenic radionuclides in meteorites, the production rates of which are in direct proportionality to the intensity of cosmic rays, are used. The contents of cosmogenic radionuclides of different half-lives T1/2, measured in 42 stony meteorites (chondrites) having sequentially fallen onto the Earth during the period of 1959–2016, are analyzed. They are accumulated by the galactic cosmic rays (GCRs) along the orbits of the chondrites before their falls onto the Earth at some average heliocentric distances, depending on the size of the chondrite orbit and on T1/2 of the radionuclide. The comparison with the calculated production rates of radionuclides in the identical chondrites for isotropic irradiation by the GCRs at ~ 1 AU is demonstrated. The calculations are based on the stratospheric balloon monthly data on the GCR intensity [1] for the periods of accumulation of each radionuclide in each chondrite. The dependence of production rates of the radionuclides of different half-lives upon the GCR variations in the heliosphere is studied. The obtained long set of homogeneous data on cosmogenic radionuclide production rates in consecutively fallen chondrites provides the unique information on the space-time continuum of the cosmogenic radionuclide production rates and their variations over a long-time scale, which could be useful in the correlative analyses of processes in the inner heliosphere and, thus, in the forecast of radiation situation, which is important for the predicted manned flights.

Cosmogenic Radionuclides In Meteorites and Solar Modulation of Galactic Cosmic Rays In the Internal Heliosphere

Cosmogenic radionuclides with half-life periods T1/2 ranging from several days to a million years, produced in the nuclear reactions of galactic cosmic rays (GCR) with meteoritic matter, provide valuable information on the GCR intensity variations on a long time scale (∼1 million years) within meteorite orbits 2–4 AU from the Sun. Information on the variations of GCR gradients in the inner heliosphere was obtained by comparing the measured contents of 54Mn and 22Na in stone meteorites (chondrites) with known orbits at the time of their fall to Earth with the calculated production rates of these radionuclides in them using balloon intensity measurements of GCR (E > 100 MeV) in the stratosphere at appropriate times. Although individual gradient values show significant uncertainties, the important information is that cosmogenic radionuclides in chondrites predict low gradients (0–10% per 1 AU) for all periods of minimum solar activity in 1957–2013, according to direct measurements in interplanetary space. High gradients (50–100% per 1 AU) are predicted for periods of maximum solar activity, especially in 1992 and 2012 (up to ∼200% per 1 AU). Average values of gradients are (20 ± 10)% per 1 AU for modern solar cycles (according to the production rate of 22Na), similar to the average values for the last ∼1 million years (according to the production rate of 26Al), which indicates the constancy of the solar modulation of the GCR, at least for the last million years.

Constraining the stellar energetic particle flux in young solar-like stars

AbstractAnomalies in the abundance measurements of short lived radionuclides in meteorites indicate that the protosolar nebulae was irradiated by a large number of energetic particles (E≳ 10 MeV), often called solar cosmic rays. The particle flux of the contemporary Sun cannot explain these anomalies, but, similar to T Tauri stars, the young Sun was more active and probably produced enough high energy particles. However, the stellar particle (SP) flux of young stars is essentially unknown. We model the impact of high-energy ionization sources on the chemistry of the circumstellar environment (disks and envelopes). The model includes X-ray radiative transfer and makes use of particle transport models to calculate the individual molecular hydrogen ionization rates. We study the impact on the chemistry via the ionization tracers HCO+ and N2H+. We argue that spatially resolved observations of those molecules combined with detailed models allow for disentangling the contribution of the individual high-energy ionization sources and to put constraints on the SP flux in young stars.

Stellar energetic particle ionization in protoplanetary disks around T Tauri stars

Anomalies in the abundance measurements of short lived radionuclides in meteorites indicate that the protosolar nebulae was irradiated by a high amount of energetic particles (E$\gtrsim$10 MeV). The particle flux of the contemporary Sun cannot explain these anomalies. However, similar to T Tauri stars the young Sun was more active and probably produced enough high energy particles to explain those anomalies. We want to study the interaction of stellar energetic particles with the gas component of the disk and identify possible observational tracers of this interaction. We use a 2D radiation thermo-chemical protoplanetary disk code to model a disk representative for T Tauri stars. We use a particle energy distribution derived from solar flare observations and an enhanced stellar particle flux proposed for T Tauri stars. For this particle spectrum we calculate the stellar particle ionization rate throughout the disk with an accurate particle transport model. We study the impact of stellar particles for models with varying X-ray and cosmic-ray ionization rates. We find that stellar particle ionization has a significant impact on the abundances of the common disk ionization tracers HCO$^+$ and N$_2$H$^+$, especially in models with low cosmic-ray ionization rates. In contrast to cosmic rays and X-rays, stellar particles cannot reach the midplane of the disk. Therefore molecular ions residing in the disk surface layers are more affected by stellar particle ionization than molecular ions tracing the cold layers/midplane of the disk. Spatially resolved observations of molecular ions tracing different vertical layers of the disk allow to disentangle the contribution of stellar particle ionization from other competing ionization sources. Modeling such observations with a model like the one presented here allows to constrain the stellar particle flux in disks around T Tauri stars.

Patterns of cosmogenic radionuclide production rates in the heliosphere and problems of solar modulation on a long time scale

The results of long-term studies of cosmogenic radionuclide production rates along the orbits of 39 chondrites that fell to the Earth between 1959 and 2013 are presented. They constitute a long series of homogeneous data, a statistical smoothing of which demonstrate some general patterns of the distribution and variations of Galactic cosmic rays (GCRs) with energy >100 MeV in the inner (<5 AU) heliosphere. A correlation analysis of the production rates of radionuclides with different half-lives suggests that the solar GCR modulation mechanism is constant over at least ~1 million years. The role of stochastic factors in the polarity reversal of the general solar magnetic field for the phases of maximum solar activity has been revealed. The subtle sensitivity of the 54Mn production rate in the Chelyabinsk chondrite to the short-term closure of the heliosphere for positively charged particles over 14 months between June 2012 and July 2013 is used as an example to show the high resolution of the method of using cosmogenic radionuclides in meteorites as natural GCR detectors.

Solar system genealogy revealed by extinct short-lived radionuclides in meteorites

Little is known about the stellar environment and the genealogy of our solar system. Short-lived radionuclides (SLRs, mean lifetime shorter than 100 Myr) that were present in the solar protoplanetary disk 4.56 Gyr ago could potentially provide insight into that key aspect of our history, were their origin understood. Previous models failed to provide a reasonable explanation of the abundance of two key SLRs, 26Al (mean lifetime = 1.1 Myr) and 60Fe (mean lifetime = 3.7 Myr), at the birth of the solar system by requiring unlikely astrophysical conditions. Our aim is to propose a coherent and generic solution based on the most recent understanding of star-forming mechanisms. Iron-60 in the nascent solar system is shown to have been produced by a diversity of supernovae belonging to a first generation of stars in a giant molecular cloud. Aluminum-26 is delivered into a dense collected shell by a single massive star wind belonging to a second star generation. The Sun formed in the collected shell as part of a third stellar generation. Aluminum-26 yields used in our calculation are based on new rotating stellar models in which 26Al is present in stellar winds during the star main sequence rather than during the Wolf-Rayet phase alone. Our scenario eventually constrains the time sequence of the formation of the two stellar generations that just preceded the solar system formation, along with the number of stars born in these two generations. (Continued).

Laudatio for Dr. Richard M. Lindstrom

Dr. Richard M. Lindstrom was born in Ashland, Wisconsin, USA. He received his B.S. (Honors) from the University of Wisconsin, Madison, in 1963, and Ph.D. in 1970 from the University of California, San Diego. His Ph.D. dissertation ‘‘Radionuclides in Meteorites and in the Lunar Surface’’ was one of the first theses to be based on the Apollo 11 lunar samples. He then went to the Tata Institute of Fundamental Research in Bombay as a postdoctoral fellow, and a year later moved to Brookhaven National Laboratory. Dr. Lindstrom joined the National Bureau of Standards (NBS, now NIST) in 1972 to develop a lowbackground counting laboratory. In 1974 he moved to the Inorganic Chemical Metrology Group, concentrating on accurate methods of elemental analysis using a variety of nuclear techniques. Dr. Lindstrom pioneered the development of promptgamma activation analysis (PGAA) and its application to the elemental characterization of substances and certification of Standard Reference Materials (SRMs). He also led a team of specialists to develop the world’s first cold-neutron PGAA system at NIST, and participated in establishing PGAA facilities in Japan and Hungary. With other scientists, Dr. Lindstrom used PGAA to detect neutron standing waves for the first time, and also measured the 200microbarn thermal neutron cross section of Pb, with implications for nucleosynthesis and nuclear models. Dr. Lindstrom has contributed significantly to understanding of several fundamental aspects of gamma-ray spectrometry, namely dead time and pileup effects, lossfree counting methods, and methods for determining of peak areas. He developed an interactive computer program called SUM, originally written to address the incorrect calculation of peak area uncertainty in a commercial gamma spectrometry packages, but which also provides improved accuracy in cases where a photopeak is very large, very small, or non-Gaussian. Dr. Lindstrom has contributed to improved nuclear data, and thus improved accuracy of NAA measurements. He provided a more precise value for half-life of As and several other nuclides. As a direct result of his work, the nuclear data evaluation community has changed its algorithms for combining published values. He was one of the first to identify every feature in the background of a gamma-ray detector. He operates a state-of-the-art lowbackground counting facility, and has recently installed a low-background detector more than 500 m underground. S. Glover (&) University of Cincinnati, Cincinnati, USA e-mail: seg3@cdc.gov

Experimental set-up and optimization of a gamma-ray spectrometer for measurement of cosmogenic radionuclides in meteorites

We have developed a highly efficient and selective gamma-ray spectrometer with extremely low background for activity measurement of gamma emitting cosmogenic radionuclides in meteorites. This spectrometer can operate in specific modes to match decay scheme of a particular radionuclide and is specially suited for measurement of positron emitters. The system consists of a hyperpure Ge detector (3 kg, 147% relative efficiency), operating in coincidence with an umbrella of NaI(Tl) scintillator (90 kg) in order to achieve low background. The system is tuned such that strong interference due to naturally occurring uranium daughters, e.g. 214Bi present in the meteorites and in the laboratory environment, is minimized. It enables us to measure 44Ti ( T 1 / 2 = 59.2 y ) which is ideal for studying centennial scale variations of cosmic ray flux in the interplanetary space with good reliability. The specific configuration of the coincidence system and electronics are described here.

Recent advances in studies of meteorites using cosmogenic radionuclides

The use of accelerator mass spectrometry (AMS) to measure cosmogenic radionuclides in meteorites has greatly enabled studies of cosmic-ray exposure histories. The high sensitivity of AMS has allowed measurements of samples that are very small or have very low activities. AMS measurements have much better accuracy and precision than usually possible with decay counting. AMS allows the routine measurements of many long-lived radionuclides, including some with half-lives not regularly measured previously or some made by neutron-capture reactions (such as 36Cl, 41Ca and 59Ni). These advancements have enabled many meteorite studies that previously were not often done, such as terrestrial ages. All aspects of a meteorite's cosmic-ray exposure history, its ages and geometries, now can be better studied.

Cosmogenic radionuclides in meteorites including recently fallen ones –Constraints on the exposure history of chondrites–

Cosmogenic radionuclides in meteorites including recently fallen ones –Constraints on the exposure history of chondrites–

Some selected comments on cosmic radioactivities

Radionuclides with half-lives ranging from some years to billions of years presumably synthesized outside the solar system are now recorded in ‘live’ or ‘fossil’ form in various types of materials, like meteorites or the galactic cosmic rays. They bring specific astrophysical messages, the deciphering of which is briefly reviewed here, with special emphasis on the contribution of Jerry Wasserburg. First, the virtues of the long-lived (half-lives t 1/2 close to, or in excess of 10 9 yr) radionuclides as galactic chronometers are discussed in the light of recent observational and theoretical works. It is concluded that the trans-actinide clocks based on the solar system abundances of 232Th, 235U, and 238U, or on the 232Th surface content of some old stars are still unable to meaningfully complement galactic age estimates derived from other independent astrophysical methods. In this respect, there is reasonable hope that the 187Re– 187Os chronometric pair could offer better prospects. The special case of 176Lu, which is a pure s-process product, is also reviewed. It is generally considered today that this radionuclide cannot be viewed as a reliable s-process chronometer. Second, we comment on the astrophysical messages that could be brought by short-lived (10 5≲ t 1/2≲10 8 yr) radionuclides that have been present in live or in fossil form in the early solar system. From an astrophysical point of view, the demonstrated early existence of live short-lived radionuclides is generally considered to provide the most sensitive radiometric probe concerning discrete nucleosynthetic events that presumably contaminated the solar system at times between about 10 5 and 10 8 yr prior to the isolation of the solar material from the general galactic material. Of course, this assumes implicitly that the radionuclides of interest have not been synthesized in the solar system itself. This is still a matter of debate, as we briefly stress. If indeed the short-lived radionuclides that have been present live in the early solar system are not of local origin, the external contaminating agents that have been envisioned are supernovae, evolved stars of the Asymptotic Giant Branch (AGB) type, or massive mass-losing stars of the Wolf–Rayet (WR) type. We comment on some aspects of the AGB or WR contamination. In the latter case, we discuss more specifically the role of rotation and of binarity on the predicted yields of 26Al, a radionuclide of special cosmochemistry and astrophysics interest. Some comments are also devoted to 146Sm and 205Pb. The former one is a short-lived p-process radionuclide that has most probably been in live form in the solar system, while the latter one is of s-process origin. It is shown to raise interesting nuclear physics and astrophysics questions, and to deserve further cosmochemical studies in order to evaluate its probability of existence in live form in the early solar system. Third, the case of extinct short-lived radioactivities carried by pre-solar grains is shortly mentioned, and some comments are made about the possible origin of these grains. Finally, a brief mention is made to γ-ray line astrophysics, which provides interesting information on live short-lived radionuclides in the present interstellar medium, and thus complements in a very important way, the study of extinct radionuclides in meteorites. This is illustrated in the case of 26Al.

Short-Lived Radionuclides in Meteorites: Constraints on Nebular Timescales for the Production of Solids

Variations in the abundances of short-lived radionuclides such as 26Al (τ1/2 ≈ 0.74 Ma) and 53Mn (τ1/2 ≈ 3.7 Ma) in meteoritic solids may be used to infer relative formation intervals of these solids in the nebula at precisions of less than 1 Ma. In a strict chronometric interpretation of the isotopic variations, whereby criteria such as spatial and temporal isotopic homogeneity and closed system isotopic evolution are met, solid formation occurred in the nebula for at least several million years. This is longer than some theoretical and astronomical estimates for the duration of the active nebula. The evidence for live 41Ca (τ1/2 ≈ 0.10 Ma) in meteoritic inclusions further indicates that the onset of solid formation occurred quite early, perhaps within a few hundred thousand years after the onset of the collapse of the sun's parent molecular cloud. Failure of the chronometric interpretation may arise for a variety of reasons, including but not limited to, the late, inhomogeneous injection of material from a nearby stellar source or the local production of short-lived radionuclides by an energetic particle irradiation, e. g., from T Tauri (X-wind) or galactic cosmic ray sources. Although some isotopic evidence exists that the criteria required for a strict chronometric interpretation are not met by each of the short-lived chronometers, there is no compelling reason to shorten the interval of solid formation in the nebula to less than 1 Ma.

Measurement of cosmogenic radionuclides in meteorites with a sensitive gamma-ray spectrometer

A large-volume HPGe gamma-ray spectrometer in a NaI(Tl) well has been set up underground at 70 m.w.e. depth for whole body counting of cosmogenic radionuclides in meteorites. The detectors are housed in a 20 cm thick lead shield with a lining of cadmium and OFHC copper. The scintillator is simultaneously operated in anticoincidence as well as in coincidence in selected energy channels to achieve low background levels (in the range of counts per day) and high specificity. In this way a large number of radionuclides such as26Al,44Ti,60Co,22Na,54Mn, and shorter-lived nuclides produced in extraterrestrial materials like meteorites and lunar rocks can be analysed. Results on Bouvante and Bereba achondrites and Dhajala and Torino chondrites are presented.

Determination of Trace Elements and Cosmogenic Radionuclides in Meteorites by Neutron Activation Analysis and Gamma Spectrometry

In the meteorites trace elements have been determined by instrumental and radiochemical N.A.A. and the cosmogenic radioactivity of 22Na, 26Al, 48V, and 54Mn are measured by low-level gamma spectrometry in the two meteorites Hohenlangenbeck and Trebbin fallen November 1985 and March 1988 onto the territory of the GDR. The contents of Rb and Cs in the meteorite Hohenlangenbeck are relatively high indicating a low condensation temperature. Becuase of the small activity ratio of 22Na/26Al a lower particle flux in its orbit during the last five years is assumed.

Radiochemical separation techniques for the determination of long-lived radionuclides in meteorites by means of accelerator-mass-spectrometry

The record of the interaction of cosmic ray particles with meteoroids in space is preserved in these objects in radioactive and stable nuclides. In order to determine the low concentrations of long-lived radionuclides such as10Be (T1/2=1.6×106a),26Al (T1/2=7.2×105a),36Cl (T1/2=3.0×105a),41Ca (T1/2=1.0×105a),53Mn (T1/2=3.8×106a) and59Ni (T1/2=7.5×104a) in extraterrestrial material by means of accelerator-mass-spectrometry and neutron activation techniques, radiochemical separations with high decontamination factors of interfering isotopes have been developed. Detection limits are achieved down to 1×10−14 for isotopic ratio measurements. For absolute analytical techniques they could be lowered to 10−14g/g. The separation methods applied include ion exchange, solvent extraction as well as classical precipitation methods. The successful application of the procedure to problems in meteorite research is given by example.

On the production of cosmogenic nuclides in meteorites by primary galactic particles: Cross sections and model calculations

Cross sections for the proton‐induced production of radionuclides (40≦A≦60) from natural titanium, iron, and nickel were measured for proton energies from 80 to 200 MeV. To critically survey the theoretical predictions of unknown excitation functions, the experimental data were compared with calculations according to Blann's hybrid model of nuclear reactions and to the semiempirical formulas of Rudstam and of Silberberg and Tsao. Then, based mainly on the new excitation functions, the production rates of cosmogenic radionuclides in meteorites by primary galactic protons were calculated. Since the interaction of primary solar and galactic particles with meteorites can be described with accuracy, the model calculations allow for a quantitative estimate of the action of secondary particles in meteorites. A method of completely describing the production of cosmogenic nuclides in meteorites that takes into account all sources of production is proposed.

Solar cosmic-ray-produced radionuclides in meteorites

Model calculations were performed for the production of cosmogenic nuclides by solar protons in meteorites. Depth-dependent proton spectra are derived via energy loss calculations for a simple irradiation geometry (vertical incidence on a half plane). In a second step, solar proton fluxes in arbitrarily shaped meteorites can be derived from thesespectra by numerical methods or, for ellipsoidal geometry, by closed form expressions. Based on these proton spectra, depth profiles of cosmogenic radionuclides in meteorites were evaluated by a system of excitation functions for the production of radionuclides (44⩽A⩽59)from target elements (22⩽Z⩽28). Moreover, production rates for 26Al and 22Na were calculated. The new excitation functions were also used to derive production ratesfor 26Al, 53Mn and 59Ni in cosmic dust. Our data differ considerably from earlier predictions and indicate that the interactions of solar protons in meteorites should be of greater importance than supposed earlier. Exemplarily, 53Mn is used for a detailed discussion of the role of various target elements, the influence of meteoritic shape, and the contributions by solar and galactic cosmic rays. Theoretical production rates for 53Mn are compared with depth profiles measured in St. Severin and Madhipura. Evidence is supported that for a successful interpretation of cosmic ray interaction especially in small meteorites and/or in surface samples with minor ablation it is necessary to consider interactions of both solar and galactic cosmic rays.

Records of ancient cosmic radiation in extraterrestrial rocks

Recent results on cosmic ray interactions in lunar samples and meteorites resulting in production of stable and radionuclides, particle tracks and thermoluminescence are reviewed. A critical examination of26A1 depth profiles in lunar rocks and soil cores, together with particle track data, enables us to determine the long term average fluxes of energetic solar protons (>10 MeV) which can be represented by (J s ,R o )=(125, 125). The lunar rock data indicate that this flux has remained constant for 5×105 to 2×106 years. Production rates of stable and radionuclides produced by galactic cosmic rays is given as a function of size and depth of the meteoroid. Radionuclide (53Mn,26Al) depth profiles in meteorite cores, whose preatmospheric depths are deduced from track density profiles are used to develop a general procedure for calculating isotope production rates as a function of meteoroid size. Based on the track density and22Ne/21Ne production rates, a criterion is developed to identify meteorites with multiple exposure history.22Ne/21Ne ratio <1.06 is usually indicative of deep shielded exposure. An examination of the available data suggests that the frequency of meteorites with multiple exposure history is high, at least 15% for LL, 27% for L and 31% for H chondrites. The epi-thermal and the thermal neutron density profiles in different meteorites are deduced from60Co and track density data in Dhajala, Kirin and Allende chondrites. The data show that the production profile depends sensitively on the size and the chemical composition of the meteoroid. Cosmic ray-induced thermoluminescence in meteorites of known preatmospheric sizes has been measured which indicates that its production profile is nearly flat and insensitive to the size of the meteoroid. Some new possibilities in studying cosmic ray implanted radionuclides in meteorites and lunar samples using resonance ionisation spectroscopy are discussed.

Records of ancient cosmic radiation in extraterrestrial rocks

Recent results on cosmic ray interactions in lunar samples and meteorites resulting in production of stable and radionuclides, particle tracks and thermoluminescence are reviewed. A critical examination of26A1 depth profiles in lunar rocks and soil cores, together with particle track data, enables us to determine the long term average fluxes of energetic solar protons (>10 MeV) which can be represented by (J s,R o)=(125, 125). The lunar rock data indicate that this flux has remained constant for 5×105 to 2×106 years.Production rates of stable and radionuclides produced by galactic cosmic rays is given as a function of size and depth of the meteoroid. Radionuclide (53Mn,28Al) depth profiles in meteorite cores, whose preatmospheric depths are deduced from track density profiles are used to develop a general procedure for calculating isotope production rates as a function of meteoroid size. Based on the track density and22Ne/21Ne production rates, a criterion is developed to identify meteorites with multiple exposure history.22Ne/21Ne ratio <1·06 is usually indicative of deep shielded exposure. An examination of the available data suggests that the frequency of meteorites with multiple exposure history is high, at least 15% for LL, 27% for L and 31% for H chondrites. The epi-thermal and the thermal neutron density profiles in different meteorites are deduced from60Co and track density data in Dhajala, Kirin and Allende chondrites. The data show that the production profile depends sensitively on the size and the chemical composition of the meteoroid.Cosmic ray-induced thermoluminescence in meteorites of known preatmospheric sizes has been measured which indicates that its production profile is nearly flat and insensitive to the size of the meteoroid.Some new possibilities in studying cosmic ray implanted radionuclides in meteorites and lunar samples using resonance ionisation spectroscopy are discussed.

Decay Intervals of Extinct Radionuclides in Meteorites and in the Earth.

Decay Intervals of Extinct Radionuclides in Meteorites and in the Earth.