On the Production of Cosmogenic Nuclides in Extraterrestrial Bodies by Galactic Cosmic Rays
Abstract
In the interplanetary space, meteoroids are continuously irradiated by highenergy ray particles. These particles have enough energy to penetrate into and interact with matter in the solar system. Some of these interactions leave reaction products that persist for long period of time. These reaction products can be used both to determine the nature and behavior of cosmic rays in the past and to study the history of the target. The accurate modeling of the reaction products is necessary for the interpretation of measured values. In this paper we present the results of the simulation of production rates of cosmogenic nuclides in lunar and meteoritic samples. The presented calculations use the system of coupled Monte Carlo codes KASKADA [1]. These treat different physical phenomena that have to be considered in the accurate computer simulation of radiation transport and interaction. The simulation of the interaction of an incoming highenergy 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 cutoff energies are reached or the geometry is left. Production rate P(sub)j of cosmogenic nuclide j at depth d in an irradiated body was calculated with the equation, which appears below in the hard copy. Where N is the number of atoms for target element i per kg material in the sample, sigma is the cross section for the production of nuclide j from target element i by particle k, and J is flux of primary and secondary particles of type k with energy E(sub)k. In the case of meteoroids the irradiated body was divided into concentric shells. The lunar surface was simulated with a large cylinder, which was also divided into smaller cylindrical sublayers. The corresponding fluxes were calculated within each sublayer for both geometries. In recent years a series of studies of different aspects of production and transport processes have been carried out for a variety of targets and shielding conditions [2]. All these calculations showed the importance of detailed simulation of neutron and proton fluxes for accurate calculation of production rates of cosmogenic nuclides. In our present calculations, we have used spectrum of the shape given by [3], normalized to unit integral flux above 10 MeV. One of goals of our calculations is the determination of the average GCR spectrum. Our earlier calculations [1] and results of work [4] allowed us to use linear dependence of production rates on the integral flux. The average fluxes of GCR particles were determined by fitting the measured depth profiles with calculated ones. In the lunar case, on the base of simulation of ^26Al and ^53Mn production in Apollo 15 drill core samples we obtained best fit for integral flux 4.85 part.cm^2s^1 that corresponds to the primary GCR proton spectrum with modulation parameter 467 MeV. Simulation of the production of the same nuclides in Knyahinya and St.Severin led to the initial integral particle flux 5.29 part.cm^2s^1 that is equivalent to the spectrum with modulation parameter 419 MeV. Our results are in fairly good agreement with [4], however the increase of primary GCR particle flux with distance from the Sun is a few percent higher than measured by Pioneer 10 and 11 [5]. References: [1] Masarik J. et al. (1991) J. Phys. G. Nucl. Part. Phys., 17, S493S504. [2] Masarik J. et al. (1992) Meteoritics, 27, 209210. [3] Castagnoli G. and Lal D. (1980) Radiocarbon, 22, 133159. [3] Michel R. et al. (1991) Meteoritics, 26, 221242. [5] McKibben R. B. (1987) Rev. Geophys., 25, 711722.
 Publication:

Meteoritics
 Pub Date:
 July 1993
 Bibcode:
 1993Metic..28R.336C
 Keywords:

 APOLLO 15 DRILL CORE;
 COSMIC RAYS;
 COSMOGENIC NUCLIDES;
 KNYAHINYA;
 LUNAR ROCKS;
 METEORITES;
 METEOROIDS;
 ST. SEVERIN