Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.209c&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 209
Other
Scientific paper
Stable and radioactive nuclides produced by the interaction of cosmic ray particles with matter can provide valuable information about variation of cosmic rays. The accurate modeling of the depth and size dependence of the production processes is a necessary prerequisite for the physical interpretation of measured activities. The production rates of cosmogenic nuclides depend on the type and the flux of the primary particles, on the bulk chemical composition of target material, on the size of the irradiated body, and on the shielding depth of a sample in it. The bulk chemical composition of the meteorite influences production rates describing the production from particular elements and differential shape and integral value of particle fluxes. Both the production and transport processes are dependent on Z and A of the meteorite. The dependence on primary particle fluxes is given by the intensity, spectral shape, and isotopic composition of primary fluxes. In this paper we deal with the dependence of production and transport phenomena on the size and shape of the meteorite. The presented calculations use the system of coupled Monte Carlo codes KASKADA (Masarik et. al., 1986). These treat different physical phenomena that have to be considered in the accurate computer simulation of radiation transport and interaction. In general the simulation of the interaction of an incoming high-energy particle is started by a choice of primary particle coordinates and direction relative to the target. The incident particle is followed to its first collision with a target nucleus, in which the production of secondaries is performed using the intranuclear cascade evaporation model. Then the histories of individual secondary particles are followed one after the other until the predefined cut-off energies are reached or the geometry is left. In recent years a series of calculations of production rates has been carried out for different targets. These calculations showed the importance of the above-mentioned factors on the final depth profiles of production rates. Some general features can be extracted on the basis of these calculations. The first very important fact is the difference in spectral shape and flux densities between protons and neutrons. The shape of nucleon spectra changes only very slowly with meteorite radius. All neutron spectra are characteristically steep, decreasing with energy. For smaller radii, neutron spectra are flatter. Proton spectra are characterized by broad maxima around 100 MeV, and the influence of Coulomb stopping is observed for low energies. The difference between proton and neutron spectra is more appreciable for their integral fluxes as a function of depth and size. Primary proton flux strongly decreases with depth and with radius of the meteorite, and as a secondary proton flux increases only slightly for radii smaller than 70 cm the total proton flux shows decrease with depth. The behavior of secondary neutron flux is opposite--continuous increase of integral flux is visible over the entire depth for meteorites with radii smaller than 70 cm. Steepest is the increase in the depth below 30 cm. The same conclusions are valid for the dependence of center activity on meteorite radius. The ratio of total fluxes of neutrons to protons increases with depth for given radii and the center activity increases with the increase of meteorite radius. These conclusions are in accordance with calculations of Michel et al., (1991) and confirm the importance of detailed simulation of neutron and proton fluxes for accurate calculation of production rates of cosmogenic nuclides in meteorites. References: Masarik J., Emrich P., Povinec P. and Tokar S. (1986) Nucl. Instr. Meth. Phys. Res. B 17, 483-489. Michel R., Dragovitsch P., Cloth P., Dagge G. and Filges D. (1991) Meteoritics 26, 221-242.
Chochula P.
Masarik Jozef
Povinec P. P.
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