Peter Dragovitsch

Production of stable and radioactive nuclides in thick stony targets (R = 15 and 25 cm) isotropically irradiated with 600 MeV protons and simulation of the production of cosmogenic nuclides in meteorites

Abstract Two artificial meteoroids made out of gabbro with a density of 3 g cm−3 with radii of 15 and 25 cm were isotropically irradiated with 600 MeV protons in order to simulate the production in meteoroids of cosmogonie nuclides by galactic cosmic ray protons. The depth dependent production of a wide range of radionuclides from target elements O, Mg, Al, Si, Ti, Fe, Co, Ni, Cu, Ba, Lu, and Au was measured. Furthermore, the production of He and Ne isotopes from Al, Mg, Si as well as from degassed meteoritic material was determined. Together with earlier results on an artificial meteoroid with a radius of 5 cm, and with data derived from thin-target experiments, the depth dependence of production rates is investigated for radii from 0 to 75 g cm−2. 60Co from Co shows the strongest size dependence; the center production rates differ by a factor of 100 for radii of 5 and 25 cm. Other low-energy products, like 58Co from Co and 24Na produced from Al, increase only up to a factor of 3.5 over the entire range of radii. For extreme high-energy products, in contrast, the center production rates decrease by up to a factor of 10. The observed depth profiles show a wide varity of shapes. Low-energy products have pronounced maxima in the center, high-energy products exhibit strong decreases from surface to center and, in between, essentially flat profiles are seen as well as such with a transition maximum. The spectra of primary protons and of secondary protons and neutrons in the artificial meteoroids were calculated using Monte Carlo techniques. The fluxes of secondary protons and neutrons depend strongly on depth and size, the spectral shapes being different for protons and neutrons. Calculating also the nucleon spectra which result from irradiation with real GCR p-spectra, the differences between simulation experiments and cosmic irradiation conditions are quantitatively described. On the basis of all these spectra and of thin-target excitation functions, production rates were calculated and compared with the experimental ones. The theoretical depth profiles allow to distinguish the different contributions of primary and secondary particles and to unravel the various production modes of cosmogenic nuclides in meteoroids. Our investigation shows that it is possible to model the production of residual nuclides in artificial meteoroids with excellent accuracy by thin-target calculations, provided that reliable thin-target excitation functions are at hand.

Proton-induced spallation at 600 MeV

In the course of a systematic investigation of proton-induced reactions up to p-energies of 3000 MeV, the target elements O, Mg, Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Rh, Ba, Lu and Au were irradiated with 600-MeV protons at the CERN synchrocyclotron. A consistent set of more than 200 thin-target cross-sections for the production of radionuclides and stable He, Ne and Xe isotopes has so far been measured. Here, 199 cross-sections for the production of radionuclides are presented. On the basis of the new data the quality of existing semi-empirical equations for the calculation of spallation cross-sections is discussed. In a more physical approach, the production of residual nuclides in the course of the intranuclear cascade was calculated using Monte Carlo techniques and compared with experimental cross-sections.

AMS measurements of thin-target cross sections for the production of 10Be and 26 Al by high-energy protons

Abstract In order to determine thin-target cross sections for the proton-induced prduction of residual nuclides from cosmochemically relevant target elements (Z≤28), irradiation experiments were carried out with energies between 100 and 2600 MeV at various accelerators. For p-energies below 200 MeV the stacked-foil technique was used, while for higher energies individual thin targets were irradiated at 600, 800, 1200 and 2600 MeV. Using accelerator mass spectrometry, cross sections for the production of Be from O, Mg, Al, Si, Mn, Fe and Ni and of 26Al from Al, Mn, Fe and Ni were measured. The experimental data are compared with earlier literature data and the present status of the respective excitation functions is discussed. Theoretical cross sections for the reactions investigated were derived by Monte Carlo calculations on the basis of an intranuclear-cascade/evaporation (INC/E) model using the HERMES code system. For energies below 200 MeV, hybrid model calculations were performed using the code Alice Livermore 87. The calculated and experimental data are compared and the different production modes of 10Be and 26Al are discussed.

Production of cosmogenic nuclides in meteoroids: simulation experiments and modelling

The production of cosmogenic nuclides in meteoroids by galactic protons was simulated in a series of thick-target experiments at the 600-MeV proton beam of the CERN synchrocyclotron. Using a special technique, isotropic irradiation of three artificial meteoroids made out of diorite and gabbro with radii of 5, 15 and 25 cm was performed. Results obtained for the production of 22Na, 44Ti and 10Be from Ti and Fe are reported. The experimental data were compared with model calculations based on Monte Carlo calculations of the high-energy transport of primary and secondary particles and on experimental and theoretical thin-target excitation functions. A method for translating the results obtained for the monoenergetic 600-MeV irradiations to real irradiation in space by a complex p-spectrum is presented.

Cosmogenic nuclides in eucrites

Abstract The cosmogenic nuclides 10 Be and 26 Al were measured in a number of selected Antarctic and non-Antarctic eucrites by accelerator mass spectrometry. Whenever available, different samples from the same meteorite were analyzed, in order to get information on depth-dependent variations of activities of cosmogenic radionuclides. For 26 Al, measurements by gamma-gammacoincidence techniques were also done. For the Antarctic meteorites stable He-, Ne- and Ar-isotopes were determined in the same samples by conventional mass spectrometry. A detailed mineralogical study of the meteorites analyzed allowed a classification or reclassification as monomict and polymict eucrites. In meteorite falls 10 Be and 26 Al are in saturation. Some 10 Be and most 26 Al data in Antarctic eucrites are lower than those in falls. For 26 Al this is attributed to the long terrestrial residence times of Antarctic meteorites. For 10 Be the lower concentrations are unlikely to be due to the decay during terrestrial residence. The experimental data are discussed together with rare gas measurements in the context of model calculations of the depth- and size-dependent production of cosmogenic nuclides in eucrites. In the case of four Antarctic Allan Hills eucrites a possible pairing is derived on the basis of cosmogenic radionuclides, stable rare gas isotopes and mineralogical similarity.

Proton-Induced Spallation Between 600 and 2600 MeV

In the course of a systematic investigation of proton-induced reactions fifteen target elements ranging from O to Au were irradiated with protons having energies of 600, 800, 1200, 1600 and 2600 MeV. Thin-target cross sections for the production of residual radionuclides were measured by X- and gamma-spectrometry and by accelerator mass spectrometry (AMS). In this work, we deal exemplarily with the production of 10Be, 22Na and 26Al from target elements (25≤Z≤28). These data are essential for an understanding of the production of cosmogenic nuclides by the interaction of galactic cosmic ray protons with matter. The new cross sections are compared with literature data and discussed in the context of Monte Carlo calculations describing the production cross sections by an intranuclear cascade evaporation model.