Numerical simulations with the MCNPX and LAHET Code Systems compared with direct measurement of neutron fluxes in terrestrial environments K.J. Kim a,b, * , I.G. Graham c , J. Masarik d , R.C. Reedy e a Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea b NSF Arizona AMS Laboratory, University of Arizona, Tucson, AZ 85721, USA c GNS Science, 1 Fairway Drive, Avalon, Lower Hutt, New Zealand d Nuclear Physics, Komensky University, SK-842 15 Bratislava, Slovakia e Institute of Meteoritics, University of New Mexico, Albuquerque NM 87131, USA Available online 4 March 2007 Abstract Neutron fluxes as a function of geomagnetic latitude were investigated using both the MCNPX and LAHET Code Systems (LCS) and compared with Southern Hemisphere measurements made during 1998–2001. Preliminary calculations for neutron fluxes near sea level at various geomagnetic latitudes are consistent with the measured values using a cosmic-ray flux slightly lower than the effective proton flux associated with nuclide production in lunar samples over thousands to millions of years. This lower neutron flux could be due to the strong solar modulation effect during the measurement period. Ó 2007 Elsevier B.V. All rights reserved. PACS: 96.50.S; 96.50.Sf; 92.40.Cy; 92.60.Kc Keywords: Neutron flux; MCNPX; LAHET Code System; Cosmic-ray intensities; Geomagnetic latitudes 1. Introduction The importance of understanding the cosmic-ray flux in the Earth’s atmosphere and surface has increased because of dating using cosmic-ray-produced (cosmogenic) nuc- lides. The production rate of a cosmogenic nuclide in the Earth’s atmosphere or at the Earth’s surface is a function of the particle (i.e. proton and neutron) fluxes, the cross sections for the nuclear reactions, and the concentrations of the target atoms in the atmosphere or surface. The galactic cosmic-rays (GCR) are mostly protons with about 13% of alpha particles and 1% of heavier nuclei. The aver- age GCR energy is several GeV, and the relevant energies for GCR proton production of cosmogenic nuclides are between 0.1 and 10 GeV. GCR protons penetrate up to several meters of matter and produce numerous secondary particles via spallation processes [1]. The secondary neu- tron fluxes dominate the production of cosmogenic nuc- lides in both the atmosphere and on the Earth’s surface. Both measurements of the terrestrial fluxes of cosmic-ray particles and calculations using numerical simulation codes are needed to better predict the production rates of cosmo- genic nuclides as a function of time and location. To numerically simulate cosmic-ray interactions with the Earth, we used the Monte Carlo N Particle eXtended (MCNPX) and the LAHET Code System. The LAHET code system has been used to study cosmic-ray interactions in the Earth’s atmosphere and in extraterrestrial matter by Masarik and Reedy [2,3] and have been followed by similar studies using the MCNPX code by Kim and Reedy [4,5]. 0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.03.002 * Corresponding author. Address: Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea. Tel.: +82 42 868 3669; fax: +82 42 868 3393. E-mail address: kjkim@kigam.re.kr (K.J. Kim). www.elsevier.com/locate/nimb Nuclear Instruments and Methods in Physics Research B 259 (2007) 637–641 NIM B Beam Interactions with Materials & Atoms