DEPENDENCE OF THE PROPAGATION OF ULTRA-HEAVY COSMIC-RAY NUCLEI ON FIRST IONIZATION-POTENTIAL

Over 2000 Ultra Heavy (Z greater than or equal to 65) cosmic ray ions with energies over 3 GeV/n have been recorded in the UHCRE, of which a total of 205 have already been located, measured and positively ident ified as Ultra Heavy (UH) ions by our group. The histogram of UH eleme ntal abundances in the Earth's neighbourhood is obtained and the UH co smic ray source abundances may be determined by means of an appropiate propagation model. This model describes the travel of UH ions from th eir sources to near the Earth, through the insterstellar medium (ISM). In our case, a dynamical Leaky Box (DLB) model has been used for prop agation studies. Due to the nature of the transport equations correspo nding to this model, it is necessary to assume given source abundances , perform the transport calculation to near the Earth, and to compare the result with the experimental measurements. A 'trial and error' pro cedure is applied until the source abundances giving the best agreemen t between the calculated and measured abundances near the Earth are fo und. Among all variables on which the transport equation depend, this work focuses on the effect of the First Ionization Potencial (FIP) on the propagation process. Although a better agreement between propagate d and measured abundances is found when a correction for the effect of FIP is used for lighter cosmic ray nuclei, it has been found that the UHCRE experimental results are better reproduced when no FIP correcti on is assumed.