Thermic desorption of carrier free radionuclides of rare earths from polycrystalline tantalum surfaces

The mean desorption time tau (sec) of carrier free isotopes of some lanthanides, Y, Sc, Hf and Zr on polycrystalline gascontaminated tantalum surfaces has been measured. Under this conditions (surface coverage theta = 0) in the temperature range 1350 < T < 2250 K we have obtained an increasing of the mean life time tau in the following line: Yb < Nd < Sc, Ce, Pm, La, Tm, < Gd < Lu and Y << Hf and Zr. The desorption energy q increases from q = 4.2 eV per atom (Yb) to q = 6.4 eV per atom (Lu and Y). The logarithm of the preexponential factor lg tau0 in all cases of Rare Earth (RE)-desorption was lg tau0 = -13. The mean life time for carrier free Hf and Zr-desorption was lg tau = -4.5 + exp(3.3(eV)/T) in the temperature range 2450 < T < 2800 K. The high preexponential factor indicates another mechanism of desorption of Hf/Zr and those of the RE. Due to the large differences of the mean adsorption time between the RE it is possible to realize in some cases an on-line thermochromatographic separation of short lived isotopes of RE in ISOL-facilities. (author)