Differential Neutron Flux in Atmosphere at Various Geo- physical Conditions

The intensity and composition of cosmic radiation at atmosphere altitudes up to 30 km o.s.l. are modulated by solar activity and strongly depend on geophysical characteristics and atmosphere parameters. An accurate evaluation of secondary neutron energy spectra may constitute a valuable contribution to a better knowledge of the primary cosmic ray intensity. Moreover, because of neutron high Relative Biological Effectiveness (RBE), a correct assessment of the neutron component allows an improved estimate of the risk associated to a human exposure to ionizing radiation arising from cosmic rays. In addition, neutron intensity and energy distribution is strictly correlated to atmospheric composition. In this work the results of different experiments, concerning neutron integral and differential measurements at various altitudes and latitudes, are presented. Introduction The radiation environment around the Earth is due to the interaction of primary galactic cosmic rays (GCR) with nuclei constituting the atmosphere. Primary particles, entering into the upper layers of the atmosphere mainly interact with Oxygen and Nitrogen nuclei and produce a secondary shower, consisting of different particles such as protons, neutrons and mesons. The secondary particle production is balanced by absorption in air and leads to a variation of ionizing particle flux with altitude. The secondary shower characteristics depends on geophysical coordinates, solar activity and atmosphere composition. Recently the possible role of cosmic ray intensity variation in the present global warming has also been considered. In fact observations by satellite suggest that cosmic rays play an important role in the climate, showing a correlation between cosmic ray pp. 4291–4294 c ©2003 by Universal Academy Press, Inc. 4292 intensity and the fraction of the Earth covered by clouds. Among the components of the secondary shower, a special attention should be payed to neutron field distribution, because the neutron fluence strongly depends on atmospheric composition and characteristics, and, neutrons give an important contribution to the total dose in human exposure to cosmic radiation environment. The results of different experiments concerning neutron spectra at various altitudes and latitudes are presented (Matterhorn, 46◦N, 3480 m o.s.l., Chacaltaya 16◦S, 5230 m o.s.l., Alitalia Flights 10500m o.s.l., ASI (Space Italian Agency) Transmediterranean Balloon flights, 15000, 28000, 30000 m). 1. Experimental methods Experimental techniques. The complexity of the experimental neutron spectra and dose evaluation – wide energy range and dependence on many parameters – requires the development of appropriate techniques. A complete experimental system, based on passive detectors, is realized in two energy ranges: A. 100 keV-100 GeV (extended), B. 10 keV 20 MeV (limited). A. The extended energy range detector system is constituted by passive detectors with different energy thresholds and responses: Bubble Dosimeter BD100R (100keV-20MeV), Polycarbonate detector, foils (1MeV-150MeV), Polycarbonate detector bottles (1MeV-150MeV), Fission detector 209Bi layers (100MeV-hundreds of GeV)[3]. The experimental data have been processed with a special version of the unfolding code BUNTO [4]. B. The BDS[1] spectrometer with an adapted version of the BUNTO unfolding code is used like limited energy range detector system; it is constituted by six types of bubble detectors, which differ in energy thresholds and responses. Because the extended system requires long exposure time to get reasonable statistic in the high energy region, in several cases, where a first information is needed in shorter time, the limited system, integrated by MC simulation, can be useful. Monte Carlo simulations. Two MC codes (GEANT 3.2 and FLUKA) have been used to simulate the hadronic cascade and the interaction between primary protons and atmosphere at various altitudes and latitudes. In figure 1. (left), the secondary radiation separated into the main components is represented, in terms of ambient dose equivalent rate H* vs altitude (Northern latitude 6.0 GV, minimum solar activity 465 MV). In figure 1. (right), the relative importance of the neutron component to respect to the total dose is clear. In figure 2. the results of various experiments, collected in North and South hemisphere and at different altitudes, are presented in terms of integral neutron fluence rate vs. altitude. Taking into account the differences between experimental conditions and simulation parameters, the comparison between the experimental and simulated vertical profiles shows an overall agreement.