Computation Dosimetry
- Fig. 1 Cascade of secondary cosmic radiation in the atmosphere
- Fig. 2 Neutron spectra at flight altitude calculated with GEANT4
1. High Energy Particle Transport Calculation in the Atmosphere
The cosmic radiation field in the Earth’s atmosphere is a complex environment consisting of neutrons, protons, photons, electrons, positrons, pions, muons and heavy ions in energy range extending up to hundrets of GeV, which is generated via interactions of the primary cosmic rays (CR) particles with the nuclei in the atmosphere.
To complement the continuous detection of secondary neutrons from CR ( Zugspitze and Spitsbergen project) and determine the particle dose components obtained for example by persons onboard of aircrafts ( EPCARD project), we focus on transport calculations of secondary particles from CR in the atmosphere.
In the past FLUKA Monte Carlo code was used to simulate the transport of secondary cosmic particles in the atmosphere.
Recently, GEANT4 Monte Carlo code is used to calculate the spectral particle fluences of secondary CR for different altitudes between sea level and top of atmosphere at altitude of about 80 km, and for set of solar modulation and vertical cutoff rigidity parameters to cover all possible values of each variable.
Figure 2 shows, as an example, a neutron spectrum in the atmosphere calculated with GEANT4 at a typical flight altitude.
- Fig. 3 Geometry of Bonner sphere with 3He counter, green lines: neutron tracks simulated with GEANT4
2. High Energy Particle Transport Calculation - EURADOS comparison
The measurements with Bonner sphere spectrometer (BSS) ( Zugspitze and Spitsbergen project) require determination of the response as function of neutron energy, for each sphere of spectrometer. Response functions are often calculated by Monte Carlo (MC) codes. For neutron energies below 20 MeV, where evaluated cross sections are available, the response functions are rather similar whatever code is used. However, for energies above 20 MeV where every neutron transport code is based on theoretical intranuclear cascade models (INC), rather large differences are observed.
EURADOS WG11 (on “High Energy Radiation Fields”) has initiated an intercomparison of different transport codes with emphasis on calculation of high-energy response functions. The goal of this intercomparison is to decide which MC codes and INC models are most suitable to calculate low Z material (such as PE) and high-Z material (such as lead). The participants of this intercomparison exercise calculate the response of two Bonner spheres (one made of PE, one made of PE plus lead) by different MC transport codes using different INC models. The codes to be included are MCNP/LAHET, FLUKA, MCNPX, GEANT4, PHITS, MARS.
References
C. Pioch, V. Mares, W. Rühm, Influence of Bonner sphere response functions above 20 MeV on unfolded neutron spectra and doses, Radiation Measurements, (2010), in print
C. Pioch, V. Mares, E. V. Vashenyuk, Yu. V. Balabin, W. Rühm, Measurement of cosmic neutrons with Bonner sphere spectrometer and neutron monitor at 79° N, Nucl. Instr. Meth, (2010), submitted