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Fragmentation cross sections: measurement, systematics and model developement

Doktorsavhandling, 2007

Man's attitude towards space exploration has been mainly limited by the health risk related to the exposure to the space radiation environment. Some components of the space field can be characterized by such a high flux to kill a man if directly exposed and in general, all space radiation constitutes a hazard for astronauts engaged in long-term missions. The organizations involved in space exploration programs have therefore given high priority to the problem of radioprotection in space. On one hand, appropriate shielding has been designed and developed to decrease the dose delivered to crew members; on the other hand, efforts have been made to characterize accurately the amount of dose absorbed during a space mission and therefore the biological hazard. One of the issues for estimating the risk is the knowledge of the radiation field both outside and inside the space vessel. While the external environment is fairly well known, the inner field is constantly modified by the presence of the spacecraft hull, the internal materials and even the human body: all these materials interact with the incoming particles and in particular with the heavy ions, which can be fragmented into lighter ions. Nuclear fragmentation of heavy ions in matter has therefore been investigated both from an experimental and a theoretical point of view: fragmentation cross sections have been measured to phenomenologically characterize the process and theoretical and semiempirical models have been developed to predict unmeasured cross sections for any type of incoming radiation field.
This thesis is focused on the nuclear fragmentation process and on the observables that can characterize it, namely fragmentation cross sections. Part of the work deals with the data analysis necessary to obtain the cross sections, the study of their systematics and the development of a semiempirical model which can predict them. Experimental data were analyzed to obtain fluences as well as projectile total and partial charge-changing cross sections for several heavy-ion projectiles, at energies ranging from a few hundred MeV/nucleon up to a few GeV/nucleon, interacting with both elemental targets and compounds. Two new quantities, namely event-track- and even-dose-averaged LET, have also been defined and used to characterize the beam quality. The systematics of all measured projectile partial charge-changing cross sections has then been investigated with particular attention towards the validity of weak and strong factorization. Finally, weak factorization has been used as a pivotal element for the development of a new semiempirical model, GNAC---Generator of Numerical Approximations for charge-changing Cross sections: the comparison between GNAC's predictions and the measurements showed that this model can be considered as a valid tool for the calculation of projectile partial charge-changing cross sections.

projectile total charge-changing cross sections

weak factorization

strong factorization

projectile partial charge-changing cross sections

heavy-ions

space radioprotection

nuclear fragmentation