Surface channel effect on γ-ray response functions of coaxial germanium detectors

A surface channel formed in a coaxial germanium detector degrades the pulse-height of output signals. The pulse-height degradation gives rise to an enhancement of low pulse-height events in γ-ray response functions at the cost of the full-energy peak. This phenomenon was closely examined for a twin coaxial-germanium detector. It was found that imperfect charge collection is primarily caused by p-type surface channels and supplemented by the n+-i and i-p+ boundaries, producing low-energy events both in the valley between the full-energy peak and the Compton edge and on the hill below the Compton edge. It is shown that experimental response functions of the twin Ge can be reproduced with high precision by incorporating the imperfect charge collection in Monte Carlo simulations in a phenomenological way.