The subject of natural tritium is reviewed from the inception of the search for this isotope in 1932 until the present. Three sources of natural tritium are considered: production in the atmosphere by galactic cosmic rays, production in the atmosphere by solar flare accelerated particles, and accretion from the sun. A recalculation of the cosmic‐ray production rate utilizing experimental data for the last solar cycle yields a worldwide average of 0.20 ± .09 triton/cm² sec during solar minimum and 0.16 ± .09 triton/cm² sec during solar maximum. Production of tritium by interaction of solar flare accelerated particles with the atmosphere is found to be less than 3% of the production by galactic cosmic rays. Calculations of the tritium decay rate from material balance, utilizing measured tritium concentrations of rain and ocean water corrected for synthetic tritium, are consistent within the limits of error with the production rate by galactic cosmic rays. The best estimate of the pre‐bomb inventory corresponds to a decay rate of 0.5 ± .3 triton/cm² sec as compared with the estimated production rate of 0.19 ± .09 triton/cm² sec. In view of the large errors it is not possible to determine whether appreciable amounts of tritium are accreted from the sun.
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...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.