The Solar System Boron Abundance: a Measurement of Boron Concentration in Carbonaceous Chondritic Meteorites

Boron concentrations were determined for six carbonaceous chondritic meteorites using the reaction 11B(d,p)12B. The results imply a solar system B/H ratio of (2 ± 1) x 10-9. Although this ratio is much lower than that determined from previous meteoritic measurements, it remains significantly higher than the B/H ratios determined from the solar photosphere and other astrophysical environments. Light element abundance ratios obtained from both meteoritic and photospheric data are compared with calculated values. It is concluded that two contributions are probably necessary to account for the observed ratios. Lithium, beryllium and boron nuclei produced according to the standard galactic cosmic ray model are expected to contribute significantly to the observed abundances. However, a component arising from low-energy spallation of CNO nuclei also appears necessary. Several possibilities are considered for the origin of these low-energy particles. However, the data and calculations are too uncertain to permit any firm conclusions.</p

The solar system boron abundance

The concentrations of B in seven carbonaceous chondritic meteorites have been determined by the use of two different analytical techniques. The data correspond to a B/H ratio of about 10^(-9) compared to the value of 10^(-8) previously proposed by Cameron, Colgate, and Grossman. However, the meteoritic abundance remains at least a factor of 2-10 higher than various estimates of the solar photosphere abundance. We conclude that both meteoritic and photospheric B and Be abundances must be considered in comparisons with nucleosynthesis calculations. Using our revised B abundances and assuming ^7Li was synthesized in the big bang, we find that the residual ^6Li/1°B, ^9Be/^(10)B, and ^(11)B/^(10)B abundance ratios are well matched by the production rates for bombardment of a CNO mixture of solar proportions by protons and a-particles with a kinetic energy per nucleon spectrum of the form E^(-1.8)

Surface sticking probabilities for sputtered atoms of ^(93)Nb and ^(103)Rh

The sticking probabilities for sputtered atoms of ^(93)Nb and ^(103)Rh incident on Al_2O_3 surfaces have been measured using the backscattering of MeV heavy ions. In the circumstance where the collecting surface has become thickly covered, the sticking probabilities integrated over the energy distribution of sputtered atoms are 0.97 ± 0.01 and 0.95 ± 0.01 for ^(93)Nb and ^(103)Rh, respectively. In the limit of negligible areal coverage of the collector. the accuracy is less; in this case the sticking probabilities are 0.97^+_(0.08) and 0.95^(+0.05)_(0.08)

Isotopic composition of boron secondary ions as a function of ion-beam fluence

We have examined the effects of target mass on sputtered material using 100 keV argon and neon to sputter an elemental target comprising the two naturally occurring isotopes of boron. At low beam fluences (≈ 1 × 10^(15)ions/cm^2) a light-isotope secondary enhancement is observed relative to steady-state secondary ion yields collected at higher beam fluences. The enhancement (46.1%o for Ne^+ irradiation and 51.8%o for Ar^+ irradiation) is large compared to the predictions of analytical theories and is independent of variations in surface potential, chemical effects, and surface impurities. This effect is consistent with an explanation based on an energy and momentum asymmetry in the collision cascade

Boron concentrations in carbonaceous chondrites

We have analyzed B in carbonaceous chondrites in order to clarify a factor of 100 difference between the solar system B abundance derived from the solar photosphere and that inferred from previous meteorite data. Consistent results were obtained from two instrumental methods for B analysis: (a) counting of the high energy betas from ^(12)B produced by the ^(11)B(d,p) reaction, and (b) measurement of particle track densities from ^(10)B(n,α)^7Li in a plastic track detector affixed to a homogenized meteorite sample. Contamination is a major problem in B analyses, but extensive testing showed that our results were not seriously affected. Our B concentrations are typically 1–2 ppm and are a factor of 2–6 lower than previous carbonaceous chondrite measurements. Our data for the Cl chondrites Ivuna and Orgueil would indicate a solar system B/Si atomic abundance ratio of 58 × 10^(−6), but this is still a factor of 2–10 higher than the photospheric estimates. It may be that B is depleted in the sun by thermonuclear processes; however, the similarity of photospheric and meteoritic Be abundances is a problem for this point of view. Alternatively, B may be enhanced in carbonaceous chondrites, but this would make B a cosmochemically unique element. A mm-sized (Fe,Mn,Mg)CO_3 crystal from Orgueil shows no B enrichment. We find ^(10)B ≤ 10^(16) atoms/g in two Allende fine-grained inclusions suggesting that B is not a refractory element under solar nebula conditions. This ^(10)B limit, when taken as a limit on ^(10)Be when the inclusion formed, puts constraints on the possibility of a solar system synthesis of ^(26)Al. For a proton spectrum of E^(−a), a must be ≥ 3 if a solar gas is irradiated or a ≥1.5 if dust of solar composition is irradiated

Sticking probabilities for sputtered Ag and Au atoms incident on oxidized aluminum surfaces

We have measured the sticking probabilities for sputtered Ag and Au atoms incident on oxidized aluminum surfaces as a function of the areal density of deposited atoms. For Ag sputtered by 200 keV Ar ions, we found a sticking probability at zero coverage of k(0) = 0.46 ± 0.20. For very high coverages, we found k(n > 10^(17)at./cm^2) = 0.80 ± 0.20. For Au sputtered by 200 keV Ar ions, w found k(0) = 0.92 ± 0.08 and k(n > 10^(17)at./cm^2) = 0.80 ± 0.03. For Au sputtered by 200 keV Xe ions, values of k(0) = 0.98 ± and k(n > 10^(17)at./cm^2) = 0.89 ± 0.03 were obtained. In all cases, the sticking probability varied noticeably for densities between o and twenty monolayers. For Au, a maximum value near 1 was observed in this region while for Ag, the sticking probability exhibited a minimum of approximately 35%. Some possible explanations for these results are offered and the implications for collection-type sputtering experiments are discussed