Interpretation of cosmogenic nuclides in meteorites on the basis of accelerator experiments and physical model calculations R MICHEL and S NEUMANN Center for Radiation Protection and Radioecology, University Hannover, Am Kleinen Felde 30, D-30167 Hannover, Germany e-mail: [email protected], uni-hannover.de Cosmogenic nuclides in extraterrestrial matter provide a wealth of information on the exposure and collision histories of small objects in space and on the history of the solar and galactic cosmic radiation. The interpretation of the observed abundances of cosmogenic nuclides requires detailed and accurate knowledge of their production rates. Accelerator experiments provide a quantitative basis and the ground truth for modeling cosmogenic nuclide production by measurements of the relevant cross sections and by realistic simulations of the interaction of galactic protons with meteoroids under completely controlled conditions, respectively. We ' review the establishment of physical model calculations of cosmogenic nuclide production in extraterrestrial matter on the basis of such accelerator experiments and exemplify this approach by presenting new experimental and theoretical results for the cosmogenic nuclide aaTi. The model calculations describe all aspects of cosmogenic nuclide production and allow the determination of long-term solar and galactic cosmic ray spectra and a consistent interpretation of cosmogenic nuclides in extraterrestrial matter. 1. Introduction energies are a few GeV/n. GCR spectra are modulated by interaction of GCR particles with the solar mag- In the solar system, one observes two types of natural netic field and thus depend on the solar activity. medium- and high-energy corpuscular radiation, the Typical SCR and GCR proton spectra at 1 A.U. are solar cosmic radiation (SCR) and the galactic cosmic shown in figure 1. For both, SCR and GCR spectra, radiation (GCR). SCR particles are emitted during suitable mathematical parameterizations exist. energetic solar events from the sun and consist on the For SCR particles, it is convenient to describe the average of 98% protons and 2% a-particles (Goswami differential flux density OJ/OR as function of the rigi- et al 1988). Their spectral distributions and intensities dity R. R is the relativistic momentum of the particle vary from event to event, typical energies going up to over its charge. This means for SCR protons a few hundred MeV/n. GCR particles come from out- OJ,,scR side the solar system. They are injected into the inter- OR - J0,sCR(47r, Ep > 10 MeV)-exp(-R/R0) stellar medium by supernova explosions and are (1) accelerated stochastically by complicated processes (Bryant et al 1992) and occasionally attain extreme with a characteristic rigidity, R0 [MV], and a 4r- energies up to 1021 eV. GCR consists of 87% protons, integral flux density of protons with energies Ep above 12% s-particles and 1% heavier ions which show 10MeV, J0,sca (47r, Ep > 10 MeV) [am -2 s-l]. similar energy spectra when compared as function of The energy-differential flux densities of GCR energy per nucleon (Alsmiller et al 1972). Mean GCR spectra OJcca/OE can be parameterized according to Keywords. Cosmogenic nuclides; meteorites; simulation experiments; cross sections; modeling of production rates. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 107, No. 4, December 1998, pp. 441-457 Printed in India 441 442 R Michel and S Neumann ,1_0 +2 called cosmogenic nuclides have found manifold appli- cations. On Earth, they are produced by interactions with the atmosphere and these cosmogenic nuclides are incorporated in the large scale environmental SCR protons processes. There they act as natural tracers for use in 10 +o [~,J0] (years) archaeology, hydrology, glaciology, climatology, and other environmental sciences (Finkel and Surer 1993). [75,200] (1954-1964) ,.7 ;:>r"'~ Cosmogenic nuclides produced in situ in the earth's - [70,110] (1954-1986) crust have a high potential for studying processes like "7 erosion, deglaciation (Lal 1986; Lal et al 1987) and to [50,55] (1977-1986) I0-2 M [MeV] date individual events e.g. impacts of extraterrestrial (year) objects (Nishiizumi et al 1991b). Investigations of cosmogenic nuclides in extrater- 300 / restrial matter allow us to study the history of the -(1977) matter exposed to SCR and GCR particles as well as / the dependence on time of the spectral distribution of 9OO SCR and GCR over time scales of millions of years GCRprotons (1969) (Geiss et al 1962; Lal 1972). Cosmogenic nuclides in extraterrestrial matter provide a past record which can- 1 0 -6 not be retrieved by any other means. Already these very first reviews demonstrated the scientific potential of cosmogenic nuclides in extraterrestrial matter. The 10 *o 10 *2 10 +4 Table 1. List of cosmogenic nuclides in meteorites with half- lives above 0.5 month and their target elements. ENERGY [MeV] Nuclide Half-life Main target elements Figure 1. Free space GCR proton spectra at 1 A.U. for times of a quiet (1977) and an active sun (1969) and average SCR 48V 0.0438 a Ti, Fe, Ni proton spectra for three periods from 1954 to 1986. SCR spectra 51Cr 0.0759 a Fe, Ni can be parameterized for individual flares as well as for time- 3TAr 0.096 a Ca, Ti, Fe, Ni averaged spectra by exponentially falling rigidity spectra 7Be 0.146 a C,O, Mg, A1, Si, S*, P*, Ca, Ti, Fe, Ni (McGuire and yon Rosenvinge 1984) characterized by a char- 5SCo 0.194 a Fe~, Ni acteristic rigidity, R0 [MV], and a 41r integral flux density of 56Co 0.213a Fe, Ni protons with energies above 10MeV, J0(41r, E >10MeV) 46Sc 0.230 a Ti, Fe, Ni [cm-2 s-l]. Rigidities of the observed flare spectra show a much 57Co 0.743 a Fe, Ni broader range (20MV - 150MV) than shown here for solar 54Mn 0.855 a Fe, Ni, Mn cycle averaged SCR spectra (Shea and Smart 1990). According 22Na 2.6 a Mg, A1, Si, Ca, Ti, Fe, Ni to Castagnoli and Lal (1980), GCR spectra can be character- 55Fe 2.7a Mn, Fe, Ni ized by only one parameter M [MeV] which describes the 6~ " 5.26 a CoS, Ni modulation by the solar magnetic field of GCR particles when aH 12.3a C, O, Mg, A1, Si, S*, P*, Ca, Ti, Fe, Ni entering the solar system. M is equivalent to the energy a GCR 44Ti 59.2 a Ti, Fe, Ni particle looses when penetrating into the solar system to a 328i 133 a Ca, Ti, Fe, Ni given heliocentric distance. 39Ar 269 a Ca, Ti, Fe, Ni 14C 5.73ka O, Mg, A1, Si, S*, P*, Ca, Ti, Fe, Ni 59Ni 75 ka Feg, Ni Castagnoli and Lal (1980) using a modulation para- 41Ca 103ka Ca $, Ti, Fe, Ni meter M [MeV] which represents the energy loss which 81Kr 210ka Rb, Sr, Y, Zr a particle undergoes when it enters the solar system 36C1 300ka C1$, Ca, Ti, Fe, Ni 26A1 716ka Mg, A1, Si, S*, P*, Ca, Ti, Fe, Ni and propagates to a certain heliocentric distance. For ~~ 1.5 Ma Ni GCR protons one obtains 1~ 1.51 Ma C, O, Mg, A1, Si, S*, P*, Ca, Ti, Fe, Ni 53Mn 3.7 Ma Fe, Ni OJp,GCR Ep.(Ep+2mp . c2) (Ep + x + M) -2"65 129I 15.7Ma Te$, Ba, I~EE OEp ( Ep + M) . ( Ep + 2mp . c 2 + M) 4~ 1.28 Ga Ca, Ti, Fe, Ni He stable C, O, Mg, A1, Si, S*, P*, Ca, Ti, Fe, Ni (2) Ne stable Na, Mg, A1, Si, S*, P*, Ca, Ti, Fe, Ni with x = 780- exp(-2.5 104Ep) and c = 1.244- 106 Ar stable C1$, Ca, Fe, Ni Kr stable Br$, Rb, Sr, Y, Zr cm -2 s -1 MeV -1. For heavier GCR particles analogous Xe stable Te, I S, Ba $, REE parameterizations exist. In the solar system, SCR and GCR interact with Note: Special relevance of target elements (exclusively or also) with respect to production of cosmogenic nRclides by neutron cosmic dust, meteoroids, asteroids, comets and lunar capture ($) or by a-induced reactions (~) are indicated. and planetary surfaces, thereby producing a wide range Elements marked with (*) are only relevant in iron meteorites of stable and radioactive nuclides (table 1). These so- or the metal phase of stony irons. Cosmogenic nuclides in meteorites 443 exploitation of this potential, however, suffered for a sophisticated chemical separation schemes (Vogt and long time from large experimental and theoretical Herpers 1988), AMS measurements can be performed uncertainties due to limited analytical techniques as on many of these nuclides simultaneously from one well as due to crude and empirical modeling. individual sample. AMS is sensitive enough to even Shortly after the discovery of the first cosmogenic allow for the determination of cosmogenic nuclides in radionuclide (3H) in meteoritic material (Begemann individual grains of cosmic dust (Nishiizumi et a11991a) et al 1957; Fireman and Schwaxzer 1957), the whole and to investigate in situ produced cosmogenic nuclides variety of cosmogenic radio nuclides measurable in in terrestrial surface samples (Lal 1986, Lal et al 1987); meteorites became evident (Ehmann and Kohman see (Finkel and Suter 1993) for further references. 1958; Shedlovsky 1960; Arnold et al 1961). These Today, the advanced measuring techniques allow for investigations used radiochemical techniques to the determination of the whole suite of stable and extract the whole suite of cosmogenic radionuclides long-lived cosmogenic nuclides in a single sample of from hundreds of grams or even kilograms of meteo- 500rag combining rare gas mass spectrometry and ritic materials to allow for the detection of their decay AMS by consortia studies by different laboratories. with the measuring techniques available at that time. With respect to the short-lived radionuclides with Stable cosmogenic nuclides in extraterrestrial matter half-lives between a few days and several decades, were observed as positive isotopic abundance anomalies measuring techniques also have improved.