  Neutrinoless double-beta decay is practically the only way to establish the
Majorana nature of the neutrino mass and its decay rate provides a probe of an
effective neutrino mass. Double beta experiments are long-running underground
experiments with specific challenges concerning the background reduction and
the long term stability. These problems are addressed in this work for the
Heidelberg-Moscow (HdM), GENIUS Test Facility (TF) and GERDA experiments. The
HdM experiment collected data with enriched 76Ge high purity (HPGe) detectors
from 1990 to 2003. An improved analysis of HdM data is presented, exploiting
new calibration and spectral shape measurements with the HdM detectors.
GENIUS-TF was a test-facility that verified the feasibility of using bare
germanium detectors in liquid nitrogen. The first year results of this
experiment are discussed. The GERDA experiment has been designed to further
increase the sensitivity by operating bare germanium detectors in a high purity
cryogenic liquid, which simultaneously serves as a shielding against background
and as a cooling media. In the preparatory stage of GERDA, an external
background gamma flux measurement was done at the experimental site in the Hall
A of the Gran Sasso laboratory. The characterization of the enriched detectors
from the HdM and IGEX experiments was performed in the underground detector
laboratory for the GERDA collaboration. Long term stability measurements of a
bare HPGe detector in liquid argon were carried out. Based on these
measurements, the first lower limit on the half-life of neutrinoless double
electron capture of 36Ar was established to be 1.85*10^18 years at 68% C.L.
