Contribution of Neutron Beta Decay to Radiation Belt Pumping from High Altitude Nuclear Explosion

In 1962, several satellites were lost following high altitude nuclear tests by the United States and the Soviet Union. These satellite failures were caused by energetic electrons injected into the earth's radiation belts from the beta decay of bomb produced fission fragments and neutrons. It has been 40 years since the last high altitude nuclear test; there are now many more satellites in orbit, and it is important to understand their vulnerability to radiation belt pumping from nuclear explosions at high altitude or in space. This report presents the results of a calculation of the contribution of neutron beta decay to artificial belt pumping. For most high altitude nuclear explosions, neutrons are expected to make a smaller contribution than fission products to the total trapped electron inventory, and their contribution is usually neglected. However, the neutron contribution may dominate in cases where the fission product contribution is suppressed due to the altitude or geomagnetic latitude of the nuclear explosion, and for regions of the radiation belts with field lines far from the detonation point. In any case, an accurate model of belt pumping from high altitude nuclear explosions, and a self-consistent explanation of the 1962 data, require inclusion of the neutron contribution. One recent analysis of satellite measurements of electron flux from the 1962 tests found that a better fit to the data is obtained if the neutron contribution to the trapped electron inventory was larger than that of the fission products [l]. Belt pumping from high altitude nuclear explosions is a complicated process. Fission fragments are dispersed as part of the ionized bomb debris, which is constrained and guided by the earth's magnetic field. Those fission products that beta decay before being lost to the earth's atmosphere can contribute trapped energetic electrons to the earth's radiation belts. There has been a large effort to develop computer models for the contribution of fission products to belt pumping (e.g., the SNRTACS code system [2]). It is a daunting task because multiple, difficult-to-model plasma processes must be included, and existing belt pumping models cannot reproduce the trapped electron numbers, spectra, and spatial distributions observed following the 1962 tests. Neutrons are not affected by the earth's magnetic field or by plasma electric and magnetic fields, so their contribution to belt pumping can in principle be calculated more accurately than that of fission products. Those neutrons emitted in an upward direction from a high altitude nuclear explosion travel in straight lines. They beta decay and contribute a trapped energetic electron to the earth's radiation belts at a rate determined by the 900-second neutron lifetime (which is much longer than the {approx}2-second flight time of a fast neutron through the radiation belts). Neutrons emitted in a downward direction will scatter in the atmosphere. Most of them will diffuse out of the atmosphere and travel up through the earth's radiation belts, where their probability of injecting a trapped electron is now greater because of their lower velocity. This albedo effect increases the amount of belt pumping from neutron beta decay by several fold. In the present work, we use Monte Carlo neutron transport techniques to account for the albedo effect of the earth's atmosphere and obtain accurate values for the magnitude and distribution of trapped electrons from neutron beta decay.