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ARGON-37, ARGON-39 AND TRITIUM IN RECENT METEORITE FALLS E. L. Fireman Smithsonian Astrophysical Observatory Cambridge, Massachusetts Most of the work on radioactivity in meteorites has been concerned with nuclides of relatively long half-lives simply because the samples available were too old for short-lived activities to be present at measur- able levels. Within the past few months, however, three freshly fallen meteorites have been obtained and studied. These are the Hamlet chon- drite (fell October 13, 1959, in Indiana), the Aroos iron (fell November 25. 1959, in the USSR) and the Bruderheim stone (fell March 4, 1960, in Alberta). Data on Ar37, Ar39 and H3 in Hamlet, Aroos, and Benton (fell in 1949; used as a blank for Ar ), as well as information on irradiated targets can now be reported; results on Bruderheim are not yet complete. The content of Ar3' (34 day half-life) provides information on the cosmic ray flux during the last fifty or so days before the meteorite struck the Earth. If one had a reliable estimate of the orbit of a freshly fallen meteorite, one could consider the meteorite to be a kind of space probe for cosmic rays. Unfortunately, visual observations of meteorite falls can do no more than supply a rough indication of the direction in which the meteorite was traveling when it collided with the Earth. However, there is one accurately determined orbit, obtained with wide-angle rotating- shutter cameras, that of the Luhy chondrite which fell in Czechoslovakia on April 25. 1959 (Cepleca 1959). This meteorite had an orbit with aphe- lion 4. 1 A. U., perhelion 0. 79 A. U. , and inclination of 10o. Its period was 3. 8 years. If this orbit is typical of Hamlet and Aroos, then Ar37 integrates the cosmic-ray flux over the region of space quite close to the Earth, H3 integrates over seven or eight orbital periods, and Ar39 (325 year half-life) would integrate over more than a hundred periods. The experimental procedures used were essentially the same as those reported previously (Fireman and DeFelice 1960). The total argon activity from the Hamlet and Aroos samples gave evidence of a complex decay curve, which was resolvable into the expected components. The data is summarized in Table 1. The errors quoted are at the 95 percent confidence level, derived from the least-squares analyses used to corre- late the raw data. Since no decline in the argon activity from the Benton meteorite was found, only Ar39 was still present in this meteorite. 28
TABLE 1 Summary of Data (Extrapolated to Time of Fall Where Necessary) Meteorite or Target Date of Fall Type Ar39 Decays /g day Ar37/Ar39 Decays /g day Hamlet 10-13-59 Chondrite 2. 3 ± 0. 230 ± 30 29 6 8. 0 ± 0. 5 2 ± Hamlet 10-13-59 Chondrite 8 ± 2 2. 0 ± 0. 5 200 ± 50 25 ± 10 Hamlet (10-13-59) Irradiated 800 ± 100 1. 2 ± 0. 3<D (500 ± 30) 24 ± 5(1) Target Chondrite x 1 0* Benton 1-16-49 Chondrite 11.8 ± 0.6 340 ± 30 29 ± 4 Aroos 11-25-59 Iron 23 ± 2 1.4 ± 0. 3 50 ± 10 2 ± 0. 5 Iron Irradiated __- 0. 8 ± 0. 1(2) -- . 20 ± 5(3) Target terrestrial iron (1) Production ratios. (2) O. A. Schaeffer and J. Zahringer (1958), Z. Naturf. 13a, 347. (3) E. L. Fireman and J. Zahringer (1957), Phys. Rev. 107, 1695. The H3/Ar3^ ratios in the Hamlet meteorite, including that from the irradiated Hamlet sample, agree quite well among themselves. (The production ratios listed in Table 1 for the irradiated targets are derived by correcting the observed activities to saturation levels, and should therefore be directly comparable with the extrapolated activity ratios in the meterorites exposed to cosmic rays only.) The Ar /Ar3' activity (or production) ratios do not agree so well, however. The higher Ar3'/Ar3^ ratio in Hamlet than that in the irradiated Hamlet target may indicate a cosmic ray flux that is higher near the Earth than a few A. U. out from the Sun. It is also possible that this anomaly may reflect pro- duction of Ar37 from Ca^ by relatively low energy reactions. The serious discrepancy between the ratio of H3/Ar3^ in Aroos and that in the iron target requires comment. A similar, but even more striking effect was observed in Sikhote Alin. A possible explanation is that H3 is lost during the passage of these iron meteorites through the atmosphere. It would not be necessary to heat the interiors of iron mete- orites very much in order to cause loss of hydrogen by diffusion. Anders: This suggestion is very unconventional, since extremely high diffusion rates seem to be required. Would it be feasible to test this idea experimentally, by irradiating an iron meteorite, heating it to a few hundred degrees for a few seconds, and looking for escaped H3? 29
Fireman: We have started to do this on several occasions, but always got sidetracked. I feel, however, that some such explanation must apply, since I believe our experimental results cannot be that far wrong for the H3 in Aroos. We have done other samples by the same method with no trouble. Certainly, the target presents no problem. Reynolds: We have some data, obtained accidentally, which supports your comment about production of Ar in low-energy reactions. In one of our pile irradiations of Richardton, the sample was placed in a large flux of fast neutrons, so that enough argon was formed for us to be able to see it on the mass spectrometer. The Ar37/Ar39 atomic ratio was about seven in that case. Apparently, half as much Ar'" as Ar was also formed, presumably from the Ca40 by (n, na). Van Allen: There is an appreciable flux of solar protons, presumably varying as the inverse square of the distance from the Sun. Could they be involved in this Ar37 excess ? Fireman: Not for production of Ar37 from Fe, but they probably con- tribute to the type of production from Ca under discussion, as well as to H production. ' Arnold: Once again the question of ablation during infall is important. If 10 cm or more of material is removed, the effects of the solar pro- tons should not be observable. If very little material is ablated, the solar protons could contribute to the Ar37. Olbert: There is another effect which should be taken into account. The cosmic-ray flux, for low energies, is affected by the eleven-year solar cycle. This could be very important for Ar37 and H3, but not, of course, for Ar39. Variations by factors of two or larger in the flux have been observed, and measurements of the type reported should be correlated with these changes. REFERENCES Cepleca, Z. D., et al (1959), Bull. Astron. Inst. Czech. 4, 10. Fireman, E. L. and DeFelice, J. (1960), Geochim. et Cosmochim. Acta 18, 183. 30
