Galactic Cosmic-ray Spallation Research Articles

Space-based measurement of the neutron lifetime using data from the neutron spectrometer on NASA's MESSENGER mission

We establish the feasibility of measuring the neutron lifetime via an alternative, space-based class of methods, which use neutrons generated by galactic cosmic ray spallation of planets' surfaces and atmospheres. Free neutrons decay via the weak interaction with a mean lifetime of around 880 s. This lifetime constrains the unitarity of the CKM matrix and is a key parameter for studies of Big-Bang nucleosynthesis. However, current laboratory measurements, using two independent approaches, differ by over 4$\sigma$. Using data acquired in 2007 and 2008 during flybys of Venus and Mercury by NASA's MESSENGER spacecraft, which was not designed to make this measurement, we estimate the neutron lifetime to be $780\pm60_\textrm{stat}\pm70_\textrm{syst}$ s, thereby demonstrating the viability of this new approach.

Observation of galactic cosmic ray spallation events from the SoHO mission 20-yr operation of LASCO

A shower of secondary Cosmic Ray (CR) particles is produced at high altitudes in the Earth's atmosphere, so the primordial Galactic Cosmic Rays (GCRs) are never directly measured outside the Earth magnetosphere and atmosphere. They approach the Earth and other planets in the complex pattern of rigidity's dependence, generally excluded by the magnetosphere. GCRs revealed by images of single nuclear reactions also called spallation events are described here. Such an event was seen on Nov. 29, 2015 using a unique LASCO C3 space coronagraph routine image taken during the Solar and Heliospheric Observatory (SoHO) mission observing uninterruptedly at the Lagrangian L1 point. The spallation signature of a GCR identified well outside the Earth's magnetosphere is obtained for the 1st time. The resulting image includes different diverging linear "tracks" of varying intensity, leading to a single pixel, this frame identifies the site on the silicon CCD chip of the coronagraph camera. There was no solar flare reported at that time, nor Coronal Mass Ejection (CME) and no evidence of optical debris around the spacecraft. More examples of smaller CR events have been discovered through the 20 years of continuous observations from SoHO. This is the first spallation event from a CR, recorded outside the Earth's magnetosphere. We evaluate the probable energy of these events suggesting a plausible galactic source. Key words: Sun: Galactic Cosmic Rays, Solar Energetic Particles, Heliosphere.

Gamma Radioactivity Of The January 2013 Rainfall In Ostrava / Gama Radioaktivita Deště V Lednu 2013 V Ostravě

Abstract Gamma radioactivity of an 8 mm thick ice layer formed after a modest rain on 21 January 2013 in the VŠB-Technical University of Ostrava campus was analyzed and gamma-ray emissions from primordial 40K have been found. Cosmogenic 7Be produced mainly in galactic cosmic-ray spallation processes on atmospheric nitrogen and oxygen and 226Ra, 235U and 137Cs have been also observed.

Coupled W–Os–Pt isotope systematics in IVB iron meteorites: In situ neutron dosimetry for W isotope chronology

Tungsten isotope compositions of magmatic iron meteorites yield ages of differentiation that are within ±2Ma of the formation of CAIs, with the exception of IVB irons that plot to systematically less radiogenic compositions yielding erroneously old ages. Secondary neutron capture due to galactic cosmic ray (GCR) irradiation is known to lower the ε182W of iron meteorites, adequate correction of which requires a measure of neutron dosage which has not been available, thus far. The W, Os and Pt isotope systematics of 12 of the 13 known IVB iron meteorites were determined by MC-ICP-MS (W, Os, Pt) and TIMS (Os). On the same dissolutions that yield precise ε182W, stable Os and Pt isotopes were determined as in situ neutron dosimeters for empirical correction of the ubiquitous cosmic-ray induced burn-out of 182W in iron meteorites. The W isotope data reveal a main cluster with ε182W of ∼−3.6, but a much larger range than observed in previous studies including irons (Weaver Mountains and Warburton Range) that show essentially no cosmogenic effect on their ε182W. The IVB data exhibits resolvable negative anomalies in ε189Os (−0.6ε) and complementary ε190Os anomalies (+0.4ε) in Tlacotepec due to neutron capture on 189Os which has approximately the same neutron capture cross section as 182W, and captures neutrons to produce 190Os. The least irradiated IVB iron, Warburton Range, has ε189Os and ε190Os identical to terrestrial values. Similarly, Pt isotopes, which are presented as ε192Pt, ε194Pt and ε196Pt range from +4.4ε to +53ε, +1.54ε to −0.32ε and +0.73ε to −0.20ε, respectively, also identify Tlacotepec and Dumont as the most GCR-damaged samples. In W–Os and W–Pt isotope space, the correlated isotope data back-project toward a 0-epsilon value of ε192Pt, ε189Os and ε190Os from which a pre-GCR irradiation ε182W of −3.42±0.09 (2σ) is derived. This pre-GCR irradiation ε182W is within uncertainty of the currently accepted CAI initial ε182W. The Pt and Os isotope correlations in the IVB irons are in good agreement with a nuclear model for spherical irons undergoing GCR spallation, although this model over-predicts the change of ε182W by ∼2×, indicating a need for better W neutron capture cross section determinations. A nucleosynthetic effect in ε184W in these irons of −0.14±0.08 is confirmed, consistent with the presence of Mo and Ru isotope anomalies in IVB irons. The lack of a non-GCR Os isotope anomaly in these irons requires more complex explanations for the production of W, Ru and Mo anomalies than nebular heterogeneity in the distribution of s-process to r-process nuclides.

INTERSTELLAR EXPOSURE AGES OF LARGE PRESOLAR SiC GRAINS FROM THE MURCHISON METEORITE

ABSTRACT We report the Li isotopic compositions of nine large, greater than 5 μm, presolar SiC grains from the Murchison (CM) meteorite. Most of the SiC grains analyzed are isotopically similar in C and Si and morphologically distinct from other presolar SiC grains. Their 7Li/6Li ratios range from ∼9.3 to the solar value (∼12). The 6Li enrichments observed in the grains can only arise from galactic cosmic ray spallation off C atoms during travel through the interstellar medium (ISM) and before incorporation into the meteorite parent body. Using appropriate production rates, we calculate recoil-loss-corrected individual exposure ages of eight grains with 6Li excesses that range from 40 Myr to about 1 Gyr. The long exposure ages (>500 Myr) are consistent with calculations of grain survival lifetimes in the ISM, while the shorter ages (<100 Myr) of two grains are more consistent with previous cosmogenic noble gas studies. Although the sample size is only eight grains, there appears to be little evidence for clustering of exposure ages and no obvious correlation between exposure age and grain size.

Lithium Isotopic Abundances in Metal‐poor Halo Stars

Very high-quality spectra of 24 metal-poor halo dwarfs and subgiants have been acquired with ESO's VLT/UVES for the purpose of determining Li isotopic abundances. The derived 1D, non-LTE 7Li abundances from the LiI 670.8nm line reveal a pronounced dependence on metallicity but with negligible scatter around this trend. Very good agreement is found between the abundances from the LiI 670.8nm line and the LiI 610.4nm line. The estimated primordial 7Li abundance is $7Li/H = 1.1-1.5 x 10^-10, which is a factor of three to four lower than predicted from standard Big Bang nucleosynthesis with the baryon density inferred from the cosmic microwave background. Interestingly, 6Li is detected in nine of our 24 stars at the >2sigma significance level. Our observations suggest the existence of a 6Li plateau at the level of log 6Li = 0.8; however, taking into account predictions for 6Li destruction during the pre-main sequence evolution tilts the plateau such that the 6Li abundances apparently increase with metallicity. Our most noteworthy result is the detection of 6Li in the very metal-poor star LP815-43. Such a high 6Li abundance during these early Galactic epochs is very difficult to achieve by Galactic cosmic ray spallation and alpha-fusion reactions. It is concluded that both Li isotopes have a pre-Galactic origin. Possible 6Li production channels include proto-galactic shocks and late-decaying or annihilating supersymmetric particles during the era of Big Bang nucleosynthesis. The presence of 6Li limits the possible degree of stellar 7Li depletion and thus sharpens the discrepancy with standard Big Bang nucleosynthesis.

Boron Benchmarks for the Galactic Disk

Sixteen Population I solar-type dwarfs have been selected to ascertain the baseline B abundance in the Galactic disk for a range of a factor of 4 in metallicity: from [Fe/H] of -0.5 to +0.1. All the stars selected are undepleted in Be, which ensures that they have also retained their full initial abundance of B. Evaluation of the trend of B with Fe provides a means to study the evolution of B in the Galactic disk. We observed 16 bright stars around the B I 2497 A line, using the STIS echelle spectrograph on HST. New observations of Li and Be in some stars were made, and previous abundance studies of Li and Be in these stars were reevaluated using revised parameters and a modified spectral synthesis code for consistency with the B measurements. Abundances of B were calculated by spectrum synthesis with the revised MOOG code, which accounts for the increased opacity in the UV due to metals; the LTE B abundances were then corrected for non-LTE effects. Four additional stars with undepleted Be have HST B observations, which increase our sample to 20. For these disk stars there is a shallow slope for B versus Fe and Be versus Fe, such that as Fe increases by a factor of 4, B and Be increase by 1.7 times. The slope for BLTE versus Fe is 0.31 ± 0.09, for BNLTE versus Fe 0.40 ± 0.12, and for Be versus Fe 0.38 ± 0.14. We have estimated the effect of additional UV opacity from Mg and find that an increase of 0.3 dex in Mg results in a higher B abundances by 0.1 dex for all the disk stars. Individual stars are not consistently above (or below) the mean in both B and Be, implying that the star-to-star differences are not due to variations in the elemental content of the natal clouds. We find that the trend of B abundance with [Fe/H] is consistent with the general trend observed in halo stars. If we connect the halo and disk stars, then an increase in the Fe abundance by 103 is accompanied by increases of 100 times in B and 550 times in Be. However, fitting two separate relations for the disk and the halo stars results in a somewhat steeper slope for Be for the halo stars (1.08 ± 0.07) relative to the disk stars (0.38 ± 0.14). This is the case for B also in LTE, with Bhalo (0.90 ± 0.07) versus Bdisk (0.32 ± 0.12). However, the NLTE B abundance increases more slowly for halo stars than the Be abundance does; since this is not predicted by light-element synthesis or depletion, we suggest that a full NLTE analysis would be preferable to making the (small) corrections to the LTE abundances. Some of the lowest metallicity stars are thought to have only upper limits on the B abundance; if that is the case, the NLTE B slope is steeper, nearing 1.0. The abundance of B in the disk stars is observed to be a factor of ~15 more than the abundance of Be in these stars, a result consistent with the predictions of Galactic cosmic-ray (GCR) spallation, B/Be = 15 ± 5. The upper envelope for Li versus Fe yields Li/B and Li/Be ratios that, when coupled with models and predictions, indicate that 20%-45% of Li might be produced by GCRs. While there is no evidence to support the production of B by neutrino spallation, we cannot rule it out.

An Ultra–high‐Resolution Survey of the Interstellar7Li/6Li Isotope Ratio in the Solar Neighborhood

In an effort to probe the extent of variations in the interstellar 7Li/6Li ratio seen previously, ultra-high-resolution (R ~ 360,000), high signal-to-noise spectra of stars in the Perseus OB2 and Scorpius OB2 associations were obtained. These measurements confirm our earlier findings of an interstellar 7Li/6Li ratio of about 2 toward o Per, the value predicted from models of Galactic cosmic-ray spallation reactions. Observations of other nearby stars yield limits consistent with the isotopic ratio of ~12 seen in carbonaceous chondrite meteorites. If this ratio originally represented the gas toward o Per, then to decrease the original isotope ratio to its current value an order of magnitude increase in the Li abundance is expected, but it is not seen. The elemental K/Li ratio is not unusual, although Li and K are formed via different nucleosynthetic pathways. Several proposals to account for the low 7Li/6Li ratio were considered, but none seems satisfactory. Analysis of the Li and K abundances from our survey highlighted two sight lines where depletion effects are prevalent. There is evidence for enhanced depletion toward X Per, since both abundances are lower by a factor of 4 when compared to other sight lines. Moreover, a smaller Li/H abundance is observed toward 20 Aql, but the K/H abundance is normal, suggesting enhanced Li depletion (relative to K) in this direction. Our results suggest that the 7Li/6Li ratio has not changed significantly during the last 4.5 billion years and that a ratio of ~12 represents most gas in the solar neighborhood. In addition, there appears to be a constant stellar contribution of 7Li, indicating that one or two processes dominate its production in the Galaxy.

Evolution of Li, Be, and B in the Galaxy

In this paper we study the production of Li, Be, and B nuclei by Galactic cosmic-ray spallation processes. We include three kinds of processes: (1) spallation by light cosmic rays impinging on interstellar CNO nuclei (direct processes), (2) spallation by CNO cosmic-ray nuclei impinging on interstellar p and 4He (inverse processes), and (3) α-α fusion reactions. The latter dominate the production of 6,7Li. We calculate production rates for a closed box Galactic model, verifying the quadratic dependence of the Be and B abundances for low values of Z. These are quite general results and are known to disagree with observations. We then show that the multizone, multipopulation model that we used previously for other aspects of Galactic evolution produces quite good agreement with the linear trend observed at low metallicities without fine-tuning. We argue that reported discrepancies between theory and observations do not represent a nucleosynthetic problem but instead are the consequences of inaccurate treatments of Galactic evolution.

7Li in Metal-Poor Stars: The Spread of the Li Plateau

A highly homogeneous study of 23 halo field dwarf stars has achieved a Li abundance accuracy of 0.033 dex per star. The work shows that the intrinsic spread of the Li abundances of these stars at a given metallicity is < 0.02 dex, and consistent with zero. That is, the Spite Li plateau for halo field dwarfs is incredibly thin. The thinness rules out depletion by more than 0.1 dex by a rotational-induced extra-mixing mechanism. Despite the thinness of the plateau, an increase of Li with [Fe/H] is seen, interpreted as evidence of Galactic chemical evolution (GCE) of Li, primarily due to Galactic cosmic ray (GCR) spallation reactions in the era of halo formation. The rate of Li evolution is concordant with: (1) observations of spallative 6Li in halo dwarfs; (2) GCE models; and (3) data on Li in higher metallicity halo stars. New data have also revealed four new ultra-Li-deficient halo dwarfs, doubling the number known. Based on their propensity to cluster at the halo main sequence turnoff and also to exist redward of the turnoff, we hypothesise that they are the products of binary mergers that ultimately will become blue stragglers. We explain their low Li abundances by normal pre-main-sequence (and possibly main-sequence) destruction in the low mass stars prior to their merging. If this explanation is correct, then such stars need no longer be considered an embarrassment to the existence of negligible Li destruction in the majority of field halo dwarfs.

Concluding Remarks II

The symposium has shown that our subject is well and very alive. It is progressing rapidly, thanks to a large amount of new observational data, obtained in particular by a young generation of competent astronomers. It is encouraging, incidentally, to note the large fraction of women in this generation. When I started in this field, some forty years ago, the feminine contribution was much smaller. I plan to review the state of the situation for each of the three nucleosynthesis processes responsible for the light elements: Big Bang Nucleosynthesis (BBN), galactic cosmic ray spallation of interstellar nuclei (GCR) and stellar nucleosynthesis. I will point out their successes, their riddles and possible avenues by which these riddles could be solved. I plan first to review the observations.

The Revival of Galactic Cosmic‐Ray Nucleosynthesis?

Because of the roughly linear correlation between Be/H and Fe/H in low metallicity halo stars, it has been argued that a ``primary'' component in the nucleosynthesis of Be must be present in addition to the ``secondary'' component from standard Galactic cosmic ray nucleosynthesis. In this paper we critically re-evaluate the evidence for the primary versus secondary character of Li, Be, and B evolution, analyzing both in the observations and in Galactic chemical evolution models. While it appears that [Be/H] versus [Fe/H] has a logarithmic slope near 1, it is rather the Be-O trend that directly arises from the physics of spallation production. Using new abundances for oxygen in halo stars based on UV OH lines, we find that the Be-O slope has a large uncertainty due to systematic effects, rendering it difficult to distinguish from the data between the secondary slope of 2 and the primary slope of 1. The possible difference between the Be-Fe and Be-O slopes is a consequence of the variation in O/Fe versus Fe: recent data suggests a negative slope rather than zero (i.e., Fe $\propto$ O) as is often assumed. In addition to a phenomenological analysis of Be and B evolution, we have also examined the predicted LiBeB, O, and Fe trends in Galactic chemical evolution models which include outflow. Based on our results, it is possible that a good fit to the LiBeB evolution requires only traditional the Galactic cosmic ray spallation, and the (primary) neutrino-process contribution to B11. We thus suggest that these two processes might be sufficient to explain Li6, Be, and B evolution in the Galaxy, without the need for an additional primary source of Be and B.

Constraints on Galactic Center Activity: A Search for Enhanced Galactic Center Lithium and Boron

The abundances of lithium and boron provide important information about big bang nucleosynthesis, Galactic chemical evolution, stellar evolution, and cosmic-ray spallation reactions. We conducted the -rst search for the ground-state hyper-ne-structure transitions of Li I F 2E1; 803 MHz) and B I (2S 1@2 ; F 2E1; 732 MHz). We used the 43 m NRAO radio telescope to search for enhanced Galactic (2P 1@2 ; center (GC) Li and B expected from models of Galactic activity. We did not detect Li I or B I and obtained upper limits of N(Li I) 1.9 ) 1016 cm~2, (Li/H) 3.9 ) 10~8, N(B I) 2.2 ) 1018 cm~2, and (B/H) 9.2 ) 10~6 for the dense 20 km s~1 Sgr A molecular cloud where our largest sources of uncer- tainties are Li I/Li, B I/B, and N(H). Our observations imply (Li/H) GC 22 (Li/H) disk , (Li/H) GC 39 For a simple (Li/H) diskvspallation , (B/H) GC 1.2 ) 104 (B/H) disk , (B/H) GC 1.5 ) 104 (B/H) diskvspallation . model combining mass loss from AGB stars (only for Li), spallation reactions, and SN l-nucleosynthesis, we estimate (13 times enhancement) and (10 times (Li/H) GC 1.3 ) 10~8 (B/H) GC 7.4 ) 10~9 enhancement). If Li is primarily produced via spallation reactions from a cosmic-ray proton Nux / p (t) with the same energy and trapping as in the disk, then (/ Comparing our / p (t)dt) GC 13(/ / p (t)dt) disk . results to AGN models, we conclude that the GC has not had an extended period of AGN activity containing a large cosmic-ray Nux ergs s~1 for 108 yr), a large low-energy cosmic-ray Nux (L CR 1044 (less than 100 times the disk Nux), or a large c-ray Nux ergs s~1 for 109 yr). Furthermore, (L c 1042 since any Galactic deuterium production will signi-cantly enhance the abundances of Li and B, our results imply that there are no sources of D in the GC or Galaxy. Therefore, all the Galactic D orig- inated from the infall of primordial matter with the current D/H reduced by astration and mixing. Subject headings: Galaxy: center E Galaxy: evolution E ISM: abundances E radio lines: general E radio lines: ISM

Galactic Cosmic Rays and the Evolution of Light Elements

We reevaluate the contribution of Galactic cosmic ray (GCR) spallation to the abundances of the Li, Be, and B elements at various metallicities. We calculate absolute yields, as was formerly done by Reeves and collaborators in their pioneering studies. We discuss all the relevant parameters (spectrum shape and flux, escape length, cross sections, and spectral index) in the light of updated cosmic-ray data. We assume a dependence of the flux (t) on the supernova rate, (t) ∝ (dNSN/dt)x, and we introduce the calculated production rates of LiBeB in a standard model of chemical evolution of the Galaxy. We find that the light elements are overproduced by GCR spallation with respect to their solar abundances unless (1) the cosmic-ray injection spectrum strongly flattens out at low energies or (2) the dependence of the cosmic-ray flux on the supernova rate is weak, i.e., x ≤ 1. We argue that constraint (1) should be expected in nonlinear shock acceleration, and is also supported by the relatively weak ionization rate of the interstellar medium; constraint (2) could result from the fact that supernovae occur in associations in superbubbles, where cosmic-ray fluxes of different supernovae do not add linearly. We argue that even with substantial modifications, GCRs are not able to account for the BeB abundances at low metallicites. We then discuss the new mechanisms proposed to account for these abundances. When incorporating a new mechanism of spallation, the above constraints are more demanding. Finally, we propose a solution to explain all the absolute abundances of 6Li,9Be,10B, and 11B, in which Galactic cosmic ray spallation and spallation by fast C and O nuclei originating from stellar winds and/or massive star explosions each produce about half of the solar/meteoritic abundances; neutrino-driven spallation in Type II supernovae contributes to 15% of the 11B meteoritic abundance.

Boron in Lithium‐ and Beryllium‐deficient F Stars

The Goddard High Resolution Spectrograph (GHRS) has been used with the Hubble Space Telescope (HST) to observe the B I region at 2497 A in nine F and G dwarfs of approximately solar metallicity. The stars were selected because they have a variety of Li and Be deficiencies. Most of the nine stars were newly observed at high spectral resolution and high signal-to-noise ratios at the Keck I 10 m telescope, the Canada-France-Hawaii 3.6 m telescope, and the University of Hawaii 2.2 m telescope at 3131 A for Be II and 6708 A for Li I. With spectrum synthesis we have determined the abundances of B in our nine program stars and in five other stars from the HST archive. The stellar parameters we have used have been determined in a self-consistent way for the program stars and the archive stars. Spectrum synthesis has also been used to determine the Li and Be abundances or upper limits. Corrections to the B and Li abundances due to non-LTE effects have been applied. The stars originate from the region on the ZAMS of the Li (and Be) dip. In spite of large deficiencies in Li and Be, we find a striking uniformity in the B abundances, i.e., there is no B dip. In all cases the Li deficiency is greater than the Be deficiency. For the coolest and most evolved star in our sample, ζ Her A, the B abundance is 0.6 dex lower than the mean for the other stars. This star also has the largest Be deficiency (more than a factor of 80) and the largest Li deficiency (more than a factor of 600). These data, together with other studies of the Li dip, argue strongly against diffusion and mass loss and in favor of slow mixing as the cause of the Li and Be dip and the absence of a B dip. Six stars with [Fe/H] from -0.75 to +0.15 have Be abundances ranging from the maximum of the sample to a factor of 4 below the maximum, yet these stars have a B/Be ratio that is constant to within ±0.10 dex and that is close to the predictions of Galactic cosmic-ray spallation of 10-15. The Be range for four stars with solar metallicity is still a factor of 2, and yet the B/Be ratio is constant to within ±0.03 dex. These results imply that the Galactic cosmic-ray production of B and Be is not uniform relative to the production of elements such as Fe by stellar nucleosynthesis.

The boron-to-beryllium ratio in halo stars - A signature of cosmic-ray nucleosynthesis in the early Galaxy

We discuss Galactic cosmic-ray (GCR) spallation production of Li, Be, and B in the early Galaxy with particular attention to the uncertainties in the predictions of this model. The observed correlation between the Be abundance and the metallicity in metal-poor Population II stars requires that Be was synthesized in the early Galaxy. We show that the observations and such Population II GCR synthesis of Be are quantitatively consistent with the big bang nucleosynthesis production of Li-7. We find that there is a nearly model independent lower bound to B/Be of about 7 for GCR synthesis. Recent measurements of B/Be about 10 in HD 140283 are in excellent agreement with the predictions of Population II GCR nucleosynthesis. Measurements of the boron abundance in additional metal-poor halo stars is a key diagnostic of the GCR spallation mechanism. We also show that Population II GCR synthesis can produce amounts of Li-6 which may be observed in the hottest halo stars.

Astrophysical 7Li as a product of Big Bang nucleosynthesis and galactic cosmic-ray spallation

RECENTLY measured abundances of beryllium1–4 and boron5 in a number of hot population II halo stars are orders of magnitude above the predicted abundances of those elements from standard Big Bang nucleosynthesis6. Be and B do not, however, show a plateau of constant abundance over a wide range of low metallicities and high temperatures, as is the case for 7Li (refs 7–15). The implication is that the 7Li abundance is largely primordial, whereas the Be and B abundances are due to galactic cosmic ray (GCR) spallation reactions16–22 on top of a much smaller Big Bang component23. But GCR spallation should also produce 7Li. As a consistency check on the combination of Big Bang nucleosynthesis and GCR spallation, we use the Be and B data to subtract from the measured 7Li abundance an estimate of the amount generated by GCR spallation21,22 for each star in the sample, and then add to this baseline an estimate of the metallicity-dependent augmentation of 7Li, due to spallation. The slightly reduced primordial 7Li abundance is still consistent with Big Bang nucleosynthesis, and a single GCR spallation model can fit the Be, B and corrected 7Li abundances for all the stars in the sample.

The case for a martian origin of the shergottites, II. Trapped and indigenous gas components in EETA 79001 glass

Analysis of nitrogen and light noble gases in a large sample of glass (lithology C) from the antarctic shergottite EETA 79001 yields a minimum δ 15N > +300‰ for the isotopic composition of nitrogen trapped in the glass. The new data fall on the mixing line through the martian atmospheric composition defined by δ 15N vs. 40Ar/ 14N for two smaller samples analyzed previously. The results from all three samples are consistent with a two-component nitrogen system in which ∼ 84 ppb of trapped martian atmospheric N is mixed in variable proportions with another, more thermally labile N component during stepped heating. This second component, which appears to be indigenous to the glass rather than adsorbed from air and is present in amounts that vary by more than a factor of 3 from sample to sample, may represent volatiles from the martian interior. Data from crystalline phases of several SNC meteorites indicate that the indigenous gas may have δ 15N < −35‰ and 36Ar/ 14N≲ 3 × 10 −6 , similar to the enstatite chondrites. Neon compositions in EETA 79001 glass samples suggest an earth-like value of ∼ 10.1 ± 0.7 for the unknown 20Ne/ 22Ne ratio in the martian atmosphere. The nitrogen-argon correlation systematics yield trapped 40Ar/ 36Ar= 2260 ± 200 , within error of the Viking value. There is evidence that 36Ar/ 38Ar in the martian atmosphere is 4.1 ± 0.2, strikingly different from terrestrial or typical chondritic ratios near 5.3. Attribution of this low value to excess 38Ar generated over martian history by galactic cosmic-ray (GCR) spallation of surface materials would be difficult for a number of reasons, among them the excessive GCR fluences required and the absence of a corresponding 21Ne excess.

Neon in gas-rich samples of the carbonaceous chondrites Mokoia, Murchison, and Cold Bokkeveld

High-resolution stepwise-release patterns of neon extracted from gas-rich whole-rock samples of the carbonaceous chondrite meteorites Murchison, Cold Bokkeveld, and Mokoia are characterized by multiple release peaks and complex isotopic variations. Mokoia contains abundant ancient solar wind neon with 20Ne/ 22Ne = 13.7, identical in composition to the modern solar wind. For Murchison and Cold Bokkeveld, 20Ne/ 22Ne ratios of low-temperature release fractions are about 11 and 12.4, respectively. In higher-temperature fractions 20Ne/ 22Ne of trapped neon migrates toward neon-E, reaching a minimum of 6.9 for Murchison. Features in the Murchison thermal release and isotopic variation patterns can be associated with the release of neon from the gas-rich trace phases chromite/carbon and “Q”. The characteristic high-temperature release of neon-E is closely related to the release of neon from “Q” in Murchison. For simplicity, in this paper we consider a model in which neon in the early solar system consisted originally of two components, one component in the gas phase and the second trapped in dust grains. Neon present in solar system reservoirs today is then described as a mixture of galactic cosmic ray spallation neon and two primary trapped components, neon with ( 20Ne/ 22Ne) t = 13.7 and neon-E. The composition of neon in the gas phase is determined from the composition of neon in the sun. Intermediate neon compositions observed in condensed materials such as the earth and meteorites require mechanisms such as gravitational collection, ion implantation or adsorption to combine gas-phase neon with dust-phase neon. The present detailed release patterns show no clear evidence of preferred intermediate compositions such as neon-A. The apparently uniform composition of neon in gas-rich chromite/carbon residues may represent a special mixture of gas-phase and dust-phase neon, but requires detailed investigation. The composition of neon in “Q” is variable and different from the composition in chromite/carbon. It is suggested that the total neon composition in a solar system object will be determined by the relative proportions of various gas-rich trace phases accreted, plus the amounts incorporated of galactic cosmic ray spallation neon and pure gas-phase neon such as the solar wind.

Li7andBe7production in theα+αreaction

Differential cross sections for the $^{4}\mathrm{He}(\ensuremath{\alpha},p)$ reaction leading to the ground and 478-keV states of $^{7}\mathrm{Li}$ have been measured at 11 energies between 39.0 and 49.5 MeV and used to determine integral cross sections for $^{7}\mathrm{Li}$ production in $^{4}\mathrm{He}(\ensuremath{\alpha},p)$. Comparison of the ground state cross sections with $^{7}\mathrm{Li}(p,\ensuremath{\alpha})$ data using the principle of detailed balance indicates good agreement with the most recent $^{7}\mathrm{Li}(p,\ensuremath{\alpha})$ measurements but not with some older results. Cross sections for $^{4}\mathrm{He}(\ensuremath{\alpha},d)^{6}\mathrm{Li}$ (ground state), extracted at three energies, are consistent with the most recent $^{6}\mathrm{Li}(d,\ensuremath{\alpha})$ measurements. Cross sections for $^{7}\mathrm{Be}$ production via $^{4}\mathrm{He}(\ensuremath{\alpha},n)^{7}\mathrm{Be}$ were determined at nine energies between 39.4 and 47.4 MeV by direct collection of the $^{7}\mathrm{Be}$ and detection of the $\ensuremath{\gamma}$ rays following its decay. These cross sections are essentially identical to the $^{7}\mathrm{Li}$ production cross sections above 43 MeV but are smaller below this energy because of threshold effects. The $^{4}\mathrm{He}(\ensuremath{\alpha},p)$ and $^{4}\mathrm{He}(\ensuremath{\alpha},n)$ reactions are involved in several mechanisms which have been proposed to explain the universal abundance of $^{7}\mathrm{Li}$, and calculations of these processes have assumed cross sections larger than our data. We therefore confirm earlier conclusions that $^{7}\mathrm{Li}$ is unlikely to be produced by galactic cosmic-ray spallation alone, but the status of the other mechanisms remains unclear. Peaks in the $^{4}\mathrm{He}(\ensuremath{\alpha},p)$ and $^{4}\mathrm{He}(\ensuremath{\alpha},n)$ excitation functions are discussed in terms of resonances in $^{8}\mathrm{Be}$. Parameters determined by Kumar and Barker in a fit to $^{7}\mathrm{Li}(p,\ensuremath{\alpha})$ data are inadequate to explain our $^{4}\mathrm{He}(\ensuremath{\alpha},p)^{7}\mathrm{Li}$ (478 keV) cross sections.NUCLEAR REACTIONS $^{4}\mathrm{He}(\ensuremath{\alpha},p)$, $E=39.0\ensuremath{-}49.5$ MeV; measured $\ensuremath{\sigma}(E;\ensuremath{\theta},{E}_{p})$, $\ensuremath{\sigma}(E)$. $^{4}\mathrm{He}(\ensuremath{\alpha},n)$, $E=39.4\ensuremath{-}47.4$ MeV; measured $\ensuremath{\sigma}(E)$. $^{4}\mathrm{He}(\ensuremath{\alpha},d)$, $E=46.7\ensuremath{-}49.5$ MeV; measured $\ensuremath{\sigma}(E;\ensuremath{\theta})$, $\ensuremath{\sigma}(E)$. Astrophysical implications for $^{7}\mathrm{Li}$ production considered. Discussion of $^{8}\mathrm{Be}$ levels in $R$-matrix formalism.