Cosmic-ray-produced hydrogen 3 and helium 3 in stony meteorites

Abstract

Houston, Tex•s 77001Cosmic radiation interacts with meteorites in space to produce spallation nuclei, both stable and radioactive. By comparing the stable and the radioactive nuclides produced by cosmic rays, a radiation age can be calculated. The pair of nuclides that has been used most often for dating stony meteorites is I-P and I-IeS; they give a radiation age T• according to T• = e dNH,/dt(1)•vhere e, the ratio of production, is given by-•(H •)+ •(H•) = i + •(H•)/•(H •) •nd where NH• is the number of Hes •to• per gr•m • the s•mple •nd dNH•/d• is the tritium dec•y r•te per gr•m in the s•me s•mple •t the time of f•ll. •(Hes) •nd •(Hs) •re the effective cro• sections of direct Hes •nd Hs production in meteoritic matter by the cosmic r•di•tion. Most authors Mve •dopted equ•l direct production r•tes of He • •nd H • on the b•sis of cross sections determined in iron by Schae•et and Z•hKnger [1958]; hence, e = 0.5 [cf. Begemann e• al., 1959]. The principal weakness of this choice is tMt stony meteorites contain relatively little iron; in f•ct, they contain on the •ver•ge only 11 •t. %. The most •bundant elements •re oxygen (•54 •t. %), silicon (•16 •t. %), and m•gnesium (•15 •t. %). The e•rly investigators were well •w•re of this distribution, but they Md no He d•t• •v•il•ble for O, Si, •nd Mg. Both Schae•et and ZShMnget [1958] •nd Goebel and Schmidlin [1960] suggested that direct He • production in stony meteorites may be substantially gre•ter tMn the direct production of H•; therefore, e would be •ller than 0.5. However, these suggestions have not been used in later publications. More recently, Goebel e• •. [1964] Mve determined H •, He •, •nd He' cross sections in several elements, among them Si and Mg. Although these results have definite implications for the production of tritium and helium isotopes in stony meteorites, no adjustment of the adopted value of e seemed necessary. This question cannot be firmly settled, however, until direct measurements of He cross sections on the most abundant element (oxygen) become available.Meanwhile, it is of considerable interest to see whether a reasonable estimate of e in oxygen is possible, using certain systematics in the data of Goebel et al. [1964] and others. We think that it is possible to make this estimate by plotting H•/He • versus the ratio of neutron to proton in the elements. The pertinent facts are as follows.Although tritium production has been studied in a number of targets and at several bombarding energies, there have been few measurements of the production of He isotopes by high-energy particles. The most complete investigation was made by Goebel et al. [1964], who measured H •, He •, and He' production cross sections in targets ranging from Mg to Bi at proton energies of 0.6, 2.2, and 25 bev. The authors found that the H 8 yield decreases for elements of lower Z but the He' yield increases somewhat or remains constant. Accordingly, the H•/He • yield ratio generally decreases for elements of lower nuclear charge. Closer inspection of the •tata reveals th...

Differentiated meteorites

“…The isotope of Pb in irons was studied by Oversby [1970]. Cosmogenk radionuclides in irons were measured by Fireman and Goebel [1970], Fisher [19676, c], Kruger andHey mann [1968], andMcCorkell et al [1968].…”