D. Fink

41Ca: past, present and future☆

Abstract Accelerator mass spectrometry measurements of 41Ca have come of age. The ability to routinely obtain currents of CaH3− of 5 μA and backgrounds of less than 10−15, and even 5×10−16 at times, makes possible measurements with a precision of 5% in an hour. Studies of 41Ca in a wide variety of extraterrestrial materials addressing several very different problems, including the temporal constancy of galactic and solar cosmic rays, the determination of terrestrial ages of meteorites, and their pre-atmospheric size are now in progress. However, the goal of employing 41Ca to date bones still remains elusive. The major experimental problem is the production of currents of sufficient intensity. But more fundamental, it seems likely that the radiocalcium dating model is seriously flawed. In this short review, we summarize the technical developments that have led to a successful technique to measure 41Ca, and discuss the more significant applications of 41Ca both on the Earth and above.

In situ10Be-26Al exposure ages at Meteor Crater, Arizona

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

41Ca: Measurement by accelerator mass spectrometry and applications

We describe in detail methods and techniques we have developed to measure 41Ca40Ca ratios to a sensitivity of 6 × 10−16 us our FN tandem accelerator. The major steps involved were (1) adoption of 41CaH3− negative ions to reduce 41K (41K/40Ca ~ 5 10−13); (2) production of typically 5 μA of 40CaH3−ions from calcium hydride in a high-intensity source; (3) refinement of a technique to efficiently convert CaO samples, as small as 15 mg, into CaH2; (4) implementation of a high resolution velocity selector to remove interferring 40Ca and 42Ca ions and (5) use of a multi-anode gas ionization detector to provide excellent discrimination of 41Ca from the remaining background ions. n nThe system performs admirably, enabling measurements with ± 5% reproducibility on extraterrestrial samples, such as meteorites and lunar rocks. In addition, we have also carried out the first measurements, without pre-enrichment, of 41Ca in terrestrial rocks and bones. From these measurements, we conclude that radiocalcium dating of bones will most likely not develop into a viable dating technique. The 41Ca concentrations in modern bones are not only disappointingly low and quite close to our sensitivity limit, but show a marked variability. On the other hand, the study of 41Ca in extraterrestrial samples offers much promise as a means of determining terrestrial ages and pre-atmospheric sizes of meteorites, and of investigating the average solar cosmic ray flux during the past 300 000 years.

A CO2 negative ion source for 14C dating

Abstract A negative ion source has been developed capable of producing 20 to 30 μA of 12 C − ions from CO 2 gas with an ionization efficiency of close to 10%. Memory effects have been shown to be low (comparable with sputter sources using solid targets) and, with reasonable care, it is possible to date samples with ages up to at least 76000 years. A novel feature of the source is that the flow of CO 2 is metered into the source, not by a needle valve, but rather by controlling the temperature of a reservoir containing calcium carbonate. The reservoir may contain natural materials, such as limestone, dolomite, seashell, foraminifera, etc., or artificial carbonates prepared by absorbing sample CO 2 in heated calcium oxide. Since the total sample can be used very efficiently it is feasible that a 1 or 2% measurement can be made on a modern sample containing as little as 10 μg of carbon.

Exposure histories of the lunar meteorites: MAC88104, MAC88105, Y791197, and Y86032

Four lunar meteorites, MacAlpine Hills (MAC) 88104, MacAlpine Hills 88105, Yamato (Y) 791197, and Yamato 86032 were analyzed for the cosmogenic radionuclides 10Be, 26Al, 36Cl, and 41Ca. From these and published data, histories of exposure to cosmic rays were modelled in terms of two-stage irradiations each with a long first stage on the Moon lasting a time T2π > 5 Ma at a burial depth d2π[gcm2] followed by a second stage in space, i.e., the transit time between the Moon and the Earth, lasting a time T4π [Ma] in a body of typical meteoroidal size. The terrestrial age Tt [Ma]gives the time elapsed between meteorite fall and recovery in Antarctica. The following sets of parameters were obtained: MAC88104/5, 390 ≤ d2π ≤ 500, 0.04 ≤ T4π ≤ 0.11, 0.10 ≤ Tt ≤ 0.19; Y791197, d2π 1000, T4π = 10 ± 2, 0.08 < Tt < 0.12. From the number and exposure histories of lunar meteorites we infer a production rate on the order of 5 Ma−1 and an arrival rate worldwide of about 3 × 106 meteorites Ma−1. These results suggest that each impact event large enough to produce lunar meteorites sends a large number of them to the Earth.

41Ca in iron falls, Grant and Estherville: production rates and related exposure age calculations

Recent technical developments in accelerator mass spectrometry have enabled us to measure routinely and with a precision of 5–7% the41Ca (1/2 = 104 ky) contents of extraterrestrial samples weighing approximately 100 mg. In essence, these advances have elevated41Ca to the role of a new and potent cosmogenic radioisotope with wide-ranging applications. We present here the results from the first phase of our41Ca cosmogenic studies program, aimed at establishing baseline concentrations and trends in selected meteorites and the use of41Ca in estimating exposure ages and pre-atmospheric meteorite radii. n nThe average41Ca saturation activity recorded in four small iron falls is 24 ± 1 dpm/kg. This result, together with measurements at the center and surface of the large iron Grant, indicates that production of41Ca from spallation on iron is weakly dependent on shielding to depths as large as 250 g/cm2. We estimate the41K—41Ca exposure age of Grant to be 330 ± 50 My, and an upper limit to its terrestrial age of 43 ± 15 ky. A comparison of the41Ca contents of stony and metallic material separated from the mesosiderite Estherville identifies low-energy neutron capture on native Ca as a second important channel of production. We find that the41Ca signal in the stone phase from three meteorites correlates with their size, and that the inferred low-energy neutron fluxes vary by a factor of at least 20.

Exposure history of the lunar meteorite, Elephant Moraine 87521

Abstract We report the noble gas concentrations and the 26Al, 10Be, 36Cl, and 41Ca activities of the Antarctic lunar meteorite Elephant Moraine 87521. Although the actual exposure history of the meteorite may have been more complex, the following model history accounts satisfactorily for the cosmogenic nuclide data: A first stage of lunar irradiation for ~1 Ma at a depth of 1–5 g/cm2 followed, not necessarily directly, by a second one for 26 Ma at ~565 g/cm2; launch from the Moon less than 0.1 Ma ago; and arrival on Earth 15–50 ka ago. The small concentration of trapped gases shows that except for some material that may have been introduced at the moment of launch, EET 87521 spent K Ar age in the range 3.0–3.4 Ga, which is typical for lunar mare basalts.

Exposure histories of lunar meteorites: ALHA81005, MAC88104, MAC88105, and Y791197

The cosmogenic radionuclides {sup 41}Ca, {sup 36}Cl, {sup 26}Al, and {sup 10}Be in the Allan Hills 81005, MacAlpine Hills 88104, MacAlpine Hills 88105,and Yamato 791197 meteorites were measured by accelerator mass spectrometry (AMS). {sup 53}Mn in Allan Hills 81005 and Yamato 791197 was measured by activation. These four lunar meteorites experienced similar histories. They were ejected from near the surface of the Moon ranging in depth down to 400 g/cm{sup 2} and had very short transition times (less than 0.1 Ma) from the Moon to the Earth. A comparison of the cosmogenic nuclide concentrations in MacAlpine Hills 88104 and MacAlpine Hills 88105 clearly indicates that they are a pair from the same fall.

Application of accelerator mass spectrometry in aluminum metabolism studies

Abstract The recent recognition that aluminum causes toxicity in uremie patients and may be associated with Alzheimers disease has stimulated many studies of its biochemical effects. However, such studies were hampered by the lack of a suitable tracer. In a novel experiment, we have applied the new technique of accelerator mass spectrometry to investigate aluminum kinetics in rats, using as a marker the long-lived isotope 26Al. We present the first aluminum kinetic model for a biological system. The results clearly demonstrate the advantage this technique holds for isotope tracer studies in animals as well as in humans.

Production of 41Ca and K, Sc and V short-lived isotopes by the irradiation of Ti with 35 to 150 MeV protons: applications to solar cosmic ray studies

In order to accurately model 41Ca production from solar cosmic ray (SCR) proton interactions on titanium at the lunar surface, we have measured the natTi(p,3pχn)41Ca excitation function for Ep = 35−150 MeV. The irradiated Ti foils were initially analyzed for their induced γ-activity to identify short-lived products and subsequently measured for their 41Ca content by accelerator mass spectrometry. Excitation curves, over the above energy range, were also obtained for 42,43K, 44g,44m,46,47,48Sc and 48V results for 41Ca are compared to both semi-empirical and model-based nuclear spallation calculations. We also used it to determine the magnitude and energy spectrum of the SCR flux from the measured Ca production profile in the Ti-rich lunar basalt, 74275. The results for K, Sc and V radionuclides were compared to previous measurements and agree favourably at low energies (E < 50 MeV) but are larger by ∼10–30% for higher energies.