Superficial migration of tritium physisorbed on monocrystalline nickel (111)

Abstract

Abstract The aim of this study is the measurement of superficial migration coefficient of tritium physisorbed on monocrystalline nickel without chemisorbed sublayer. The chosen crystalline orientation was (111) because it offers the greatest concentration of adsorption sites per square centimeter. A clean surface sample is obtained by mechanical polishing, chemical etching and finally ionic bombardment by high purity argon gas. The pressure in the experimental vessel is maintained below 10 −9 torr, by liquid helium cryopumping after zeolite sorption pumping. A little spot of adsorbed tritium is produced by introduction of a finite amount of tritium gas on the clean surface of the nickel sample through a stainless steel tube. Temperatures of nickel and of the gas introduction tube are respectively regulated at 5 K and 35 K. Tritium is used as a radioactive marker and its 10 keV β-radiation is measured by a channeltron type detector which permits the localization of the deposit without acting on the surface. We observed that tritium sorbed at 5 K is quite immobile (at the time scale of our experiment). After heating up to a fixed temperature T chosen between 10 K and 20 K, the deposite profile variation in function of time is observed to determine the superficial diffusion coefficient D . For the values of T from 13 K to 20 K, D varies from 10×10 −6 to 150×10 −6 cm 2 sec −1 . A diffusion activation energy of 200 cal mole −1 is deduced from the exponential increase of the curve. A vibrational frequency can be evaluated to 3×10 12 sec −1 . The rate of desorption permits the evaluation of sorption energy at about 1800 cal mole −1 in good agreement with usual results concerning physorption of H 2 on metals.