A mechanism for gas loss from in-core ionisation chambers

A mechanism is presented for gas loss from in-core ionization chambers. During in-core testing of miniature d.c. fission chambers it was observed that sensitivity decreased much faster than could be explained by burnup of the fissile material. The rate of sensitivity loss slowed with time and some of the sensitivity reappeared during reactor shutdown. The mechanism invoked to explain this phenomenon argues that fission fragments from the coating of a d.c. fission chamber can collide with filling gas molecules and transfer kinetic energy to them. If this energy is sufficiently high the molecule or atom of interest may be driven deeply into an electrode and retained for long periods. Loss of gas and hence of nuclear sensitivity could occur. THe terms ''hard'' and ''soft'' pumping were invoked to distinguish between molecules to which were imparted high and low energies respectively. That is, atoms which strike the surface with hundreds of eV will not desorb while those colliding with lower energies will desorb at close to room temperature restoring partial sensitivity. Calculations in support of the theory are presented. (O.T.)