U. Herpers

Cross sections for the production of residual nuclides by low- and medium-energy protons from the target elements C, N, O, Mg, Al, Si, Ca, Ti, V, Mn, Fe, Co, Ni, Cu, Sr, Y, Zr, Nb, Ba and Au

Abstract Cross sections for residual nuclide production by p-induced reactions were measured from thresholds up to 2.6 GeV using accelerators at CERN/Geneve, IPN/Orsay, KFA/Julich, LANL/Los Alamos, LNS/Saclay, PSI/Villigen, TSL/Uppsala, LUC/Louvain La Neuve. The target elements C, N, O, Mg, Al, Si, Ca, Ti, V, Mn, Fe, Co, Ni, Cu, Sr, Y, Zr, Nb, Ba and Au were investigated. Residual nuclides were measured by X- and γ-spectrometry and by Accelerator Mass Spectrometry (AMS). The measured cross sections were corrected for interfering secondary particles in experiments with primary proton energies above 200 MeV. Our consistent database covers presently ca 550 nuclear reactions and contains nearly 15000 individual cross sections of which about 10000 are reported here for the first time. They provide a basis for model calculations of the production of cosmogenic nuclides in extraterrestrial matter by solar and galactic cosmic ray protons. They are of importance for many other applications in which medium energy nuclear reactions have to be considered ranging from astrophysics over space and environmental sciences to accelerator technology and accelerator-based nuclear waste transmutation and energy amplification. The experimental data are compared with theoretical ones based on calculations using an INC/E model in form of the HETC/KFA2 code and on the hybrid model of preequilibrium reactions in form of the AREL code.

Cosmogenic nuclides and nuclear tracks in the chondrite Knyahinya

Abstract Cosmic-ray produced He, Ne, Ar as well as 10Be, 26Al, 53Mn and nuclear tracks were determined in samples from known positions within the L5 chondrite Knyahinya. Our results show that Knyahinya experienced a single-stage exposure history as a meteoroid of approximately spherical shape. The inferred preatmospheric mass was about 1300–1400 kg, corresponding to a mean radius of 45 cm. Good agreement is observed between the preatmospheric shapes derived from cosmic-ray track densities and from 22 Ne 21 Ne ratios. The exposure age of Knyahinya is 40.5 Ma. The 10 Be 21 Ne ratios are constant, although the concentrations of both nuclides increase by more than 20% from positions of lowest to highest shielding. The constancy of this ratio allows refined shielding corrections in computations of exposure ages. High 26Al activities of up to 77 dpm/kg indicate that this nuclide is more efficiently produced by low energy particles than is 21Ne.

Depth and size dependence of cosmogenic nuclide production rates in stony meteoroids

Abstract Depth profiles of the cosmogenic isotopes 3He, 20e, 21Ne, 22Ne, 10Be and 26A1 have been measured by conventional and accelerator mass spectrometry in the chondrites Madhipura, Udaipur, and Bansur. Shielding depths of the samples and meteorite sizes were derived from cosmic ray track density data and from 21Ne exposure ages. In addition, 10Be and 26A1 were measured in seven fragments of Dhajala. The measured data, together with the existing 53Mn profiles in these meteorites and with other well-investigated depth profiles of cosmogenic radionuclides and rare gas isotopes in ALHA 78084, Keyes, St. Severin, Jilin and Knyahinya, now provide an experimental data base describing the depth and size dependence of cosmogenic nuclides in ordinary chondrites for preatmospheric radii between 8.5 cm and about 100 cm. Production rates are found to change only slightly with depth in small meteorites (R ≤ 15 cm). For larger bodies (15 cm ≤ R ≤ 65 cm), the profiles show significant depth dependence, the cosmogenic production increases from the surface to the center by about 30%. The center production rates increase with meteoroid size and show a broad maximum for radii between 25 and 65 cm. The location of the maxima for different nuclides depends on the dominant energy of particles responsible for their production from the main target elements. For R ≥ 70 cm, a significant decrease of center production rates is seen for 10Be, 26A1,53Mn and 21Ne, the individual depth profiles being essentially flat with shallow transition maxima. The observed depth profiles and the dependence of the center production rates on meteoroid size are well reproduced by model calculations based on Monte Carlo calculations of the intra- and internuclear cascade of galactic protons in meteoritic matter and on experimental and theoretical excitation functions of the underlying nuclear reactions. The model calculations provide a basis for identification of meteorites with anomalous levels of radioisotopes and give information about the irradiation history of meteorites and changes in the cosmic ray intensity with time and orbital space of the meteoroid. The results of the Dhajala chondrite are discussed in this context.

Production of residual nuclei by proton-induced reactions on C, N, O, Mg, AI and Si

Cross sections for the production of residual nuclides by p-induced reactions are the basic nuclear quantities for an accurate modelling of the interaction of solar cosmic protons with matter. In a series of irradiation experiments at the cyclotron of the Svedberg Laboratory/University of Uppsala sixteen different target elements were investigated for proton energies up to 100 MeV in order to determine such thin-target excitation functions. Residual nuclides were measured by gamma-spectrometry and, in the case of10Be and26Al, by accelerator mass spectrometry. Here, we report results for the light target elements C, N, O, Mg, Al, and Si including also new cross sections for the production of10Be and26Al from aluminum for p-energies between 100 MeV and 200 MeV. The latter were derived from targets irradiated earlier at the IPN Orsay. The new experimental data are compared with earlier work and analyzed in the framework of the hybrid model of preequilibrium reactions investigating several options with respect to the choice of nuclear masses including also corrections for shell effects of nuclear masses and level densities.

SCANDAL - a facility for elastic neutron scattering studies in the 50-130 MeV range

A facility for detection of scattered neutrons in the energy interval 50 130 MeV, SCAttered Nucleon Detection AssembLy (SCANDAL), has recently been installed at the 20-180 MeV neutron beam facility of The Svedberg Laboratory, Uppsala. It is primarily intended for studies of elastic neutron scattering, but can be used for (n,p) and (n,d) reaction experiments as well. The performance of the spectrometer is illustrated in measurements of the (n,p) and (n,n) reactions on H-1 and C-12. In addition, the neutron beam facility is described in some detail

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.

Simulation and modelling of the interaction of galactic protons with stony meteoroids

Abstract Stable and radioactive products of cosmic ray interactions with extraterrestrial matter provide a record of the history of small bodies in the solar system and of the cosmic radiation itself. A basis for an improved understanding of these cosmogenic nuclides was laid out in a recent experiment at Laboratoire National Saturne, by which an artificial stony meteoroid was irradiated with 1.6 GeV protons in order to simulate the exposure of meteoroids to galactic cosmic ray particles. Results for the production of 10Be, 22Na and 26Al are given. The modelling of the experimental data by a thin-target approach is described. The impact of results and analysis of the simulation experiment on model calculations of cosmogenic nuclide production rates in stony meteoroids is discussed in detail, taking 10Be production rates and their relations to those of other cosmogenic nuclides as an example.

Cross sections for the proton-induced production of He and Ne isotopes from magnesium, aluminum, and silicon

Abstract We measured integral thin target cross sections for the proton-induced production of the rare gas isotopes 3 He, 4 He, 20 Ne, 21 Ne and 22 Ne from Mg, Al, and Si from the respective reaction threshold up to 1.6 GeV. These target elements were chosen since they account for more than 50% and 95% of the cosmogenic He and Ne production in extraterrestrial matter, respectively. In order to minimize the influences of secondary particles on the production of residual nuclides a so-called “mini-stack”-approach was used instead of the well known “stacked-foil-technique” for all irradiation experiments with proton energies above 200 MeV. With this new data base a complete and consistent set of excitation functions for the proton-induced production of He and Ne isotopes is established for all target elements relevant for deciphering the cosmic ray record in extraterrestrial matter.

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.

Study on the solid solubility of transition metals in high-purity silicon by instrumental neutron activation analysis and anticompton-spectrometry

The solid solubility of the 3d metals Cr, Mn, Fe, Co and Ni in high-purity silicon was studied by INAA and electron paramagnetic resonance techniques in the temperature range of ≊600°C–1250°C. The solubility increases with atomic number from Cr to Mn and more distinctly from Co to Ni. For Mn, Fe and Co the solubilities are nearly the same within the experimental errors. An enthalpy of formation (ΔH) of about 2.8 eV was determined for Cr, Mn, Fe and Co, whereas for Ni 1.7 eV was derived. Relatively large diffusion coefficients were estimated from the rather short times, in which saturation of the solid solution was reached. From these observations and from the results of the EPR measurements it is concluded that the 3d metals occupy predominantly interstitial sites in the silicon lattice in thermal equilibrium.