Thin-target excitation functions: a powerful tool for optimising yield, specific activity and radionuclidic purity of accelerator-produced radionuclides

Abstract

In accelerator production of radionuclides, thin-target yield yE~ vs. projectile energy, at the end of an instantaneous bombardment (EOIB), is the slope at the "origin" of the growth curve of the activity of a radionuclide per unit beam current vs. irradiation time, for a target in which the energy loss is negligible in respect to projectile energy. Conversely, thick-target yield Y(E,AE) is a function of particle energy E (MeV) onto the target and energy loss AE (MeV) into target itself, obtained experimentally or by integration of thin-target excitation function y(E). Some local maxima on Y(E,AE) curves are present in many cases. As a relevant conclusion, use of target thickness larger than the "effective" value is unsuitable, due to larger specific power deposited by the beam in target material, instead of cooling system. Moreover, increasing amount of target material leads to a lower value of specific activity, due to larger amounts and volumes of chemicals and equipment. Y(E,AE) curves and their maxima permit calculating optimal irradiation conditions to produce radionuclides with maximum yield, specific activity and radionuclidic purity. In order to join the advantages of accurate knowledge of thin-target excitation functions, very selective radiochemical separations were optimised, without intentional addition of an isotopic carrier.