Surface-alpha backgrounds for the Majorana neutrinoless double-beta decay experiment

Abstract

The Majorana Experiment will use arrays of enriched HPGe detectors to search for the neutrinoless double-beta decay of 76Ge. Such a decay, if found, would show lepton-number violation, confirm the Majorana nature of the neutrino, and help determine the effective Majorana neutrino mass. A potentially important background contribution to this and other double-beta decay experiments arises from decays of alpha-emitting isotopes in the 232Th and 238U decay chains on and near the surfaces of the detectors. An alpha particle emitted from the surface can lose energy within the dead region of a detector, depositing only a partial amount of its kinetic energy within the active region and possibly mimicking the energy signal from neutrinoless double-beta decay. Cleanliness, exposure to radon, detector design, and analysis techniques all contribute to the effect from surface alphas. Our experimental and simulation efforts to understand and mitigate surface alpha backgrounds for both n-type and p-type HPGe detectors will be presented.