Cosmogenic isotopes Radionuclides

The most famous cosmogenic radionuclide is 14C (t1/2 = 5730 a), which is produced by the interaction of cosmic ray neutrons via an (n,p) reaction with nitrogen [14N(n, p)14C], whereas the radioactive decay of 14C takes place by (3 decay to form the stable 14N isotope. 14C is the most important cosmogenic radionuclide for dating (see Section 9.7.5) in archaeology and can be analyzed using isotope sensitive accelerator mass spectrometry. Extremely small isotope ratios 14C/12C = 12 in nature can be measured by means of AMS.28... 

The atmospheric concentration of natural and bomb-produced radionuclides has been measured at ground level for several years at three locations throughout the world. The manner in which the concentration decreased suggested a half-residence time for stratospheric aerosols of 11.8 months at 46°N latitude. The annual spring concentration maximum occurred one to four months earlier at 71°N than at 46°N. Cosmogenic 7Be attained a maximum concentration before the bomb-produced radionuclides at 71° N and later than the bomb-produced isotopes at 46°N. The rate of increase toward the annual peak concentration for most radionuclides could be approximated by an exponential in which the concentration doubled every 60 days likewise, the rate of decrease from the maximum concentration could be approximated by an exponential with a half-time of about 40 days for most radionuclides except 7Be at 46°N, which shows a half-time of about 60 days. 

Note After Bierman (1994), Gosse and Phillips (2001), and Lai (1988). See Gosse and Phillips (2001) for details about radionuclide decay constants and production rate uncertainties. Isotopes in italics are those that have been used routinely in cosmogenic nuclide studies. Stable isotopes will resolve paleoaltimetry for Cenozoic and older rocks. High-latitude, sea-level rates from empirical studies only. Production mostly via neutron spallation unless noted. Recent debate on die half-life of 10Be has focused on whether this number is off by >10%. 

The present review will focus on the cosmogenic noble gas isotopes (mainly He and Ne), particularly where experimental issues are concerned. In view of the close relation and interdependence with radionuclide studies however, the latter will be included in the considerations wherever it seems appropriate, and all general information on surface exposure dating, such as the basic equations, will be given in a form applicable to radionuclides as well. For a more detailed discussion of cosmogenic radionuclides I refer to the extensive review of Gosse and Phillips (2001). 

Internal standard Sometimes standards are not available for all the cosmogenic and SN radionuclides. Therefore, many AMS facilities have embarked on elaborate programs to manufacture their own calibrated standards. In most cases, these materials have been prepared locally with the utmost care using conventional radiometric counting, mass spectrometric methods, and/or standard gravimetric dilution techniques to establish their isotopic ratios. Alternatively, some AMS laboratories have prepared standard materials by series dilutions of primary standards. 

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