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Paper text:
the goal of the germanium detector array ( gerda ) @xcite is to search for neutrinoless double beta decays of @xmath2ge .
bare germanium detectors ( hpge ) , isotopic enriched in germanium @xmath2ge , will be operated in liquid argon ( lar ) .
the cryogenic fluid serves simultaneously as a cooling medium and as a shield against external radiation .
r&d is carried out to use the scintillation light of lar to tag and discriminate backgrounds .
the concept and the proof of principle were first reported in @xcite .
the signature for @xmath3 decay of @xmath2ge is a point - like energy deposition with @xmath4mev inside a hpge diode .
background events come mainly from radioactive decays and muon induced interactions .
these events deposit typically only a part of their energy inside a hpge crystal while the residual energy is dissipated in the adjacent shielding material . detecting
the scintillation light of lar would allow to discriminate these events .
the work presented here is an r&d project within the framework of the gerda experiment .
the experimental setup used for the measurements is displayed in fig .
[ fig : setup ] .
it is located in the underground laboratory of the mpik heidelberg with an overburden of 15  mwe .
a bare hpge crystal ( 5.1  cm diameter , 3.5  cm height , 0.39  kg ) is submerged in a dewar ( 29  cm diameter , 90  cm height ) filled with lar .
a wavelength shifting and reflecting foil defines the active volume of 19  kg lar .
the shifted scintillation photons are detected with one 8  etl 9357 pmt immersed in lar .
the dewar system is enclosed in a gas tight stainless steel tank to prevent quenching from oxygen or water traces .
low activity calibration sources can be inserted up to 8  mm from the hpge crystal via a hermetically closed stainless steel tube .
the daq is triggered by the hpge diode .
the hpge and pmt  signals are then recorded event - by - event and stored for the off - line analysis on disk .
hpge signals are discarded in the analysis if a simultaneous scintillation signal has been recorded .
an analysis threshold at the single photo electron level was applied . a photo electron ( pe )
yield of about 410 pe / mev was observed during these measurements .
the measurement were performed from october to december 2005 using various gamma sources ( @xmath5cs , @xmath6co , @xmath7th , @xmath8ra ) , alternated with periods of background measurements .
given the limited space available in this proceedings , we present only the results achieved with the @xmath7th  gamma source .
the source consists of a natural thorium metal wire thus containing @xmath9th and its progeny @xmath10tl . the measured energy spectrum is displayed in fig .  [ fig : prop ] .
the line histograms correspond to spectra without background subtraction and the filled histograms after background subtraction .
the bottom plot shows a zoom in the region of interest for 0@xmath1 .    th  energy spectrum with ( red ) and without ( blue ) lar anti - coincidence ( c.f .
text).,title="fig : " ] th  energy spectrum with ( red ) and without ( blue ) lar anti - coincidence ( c.f . text).,title="fig : " ]    the survival probability @xmath11 is defined as the number of counts in a given energy region after applying the lar anti - coincidence cut divided by the number of counts in the same region in the non - vetoed spectrum and the suppression factor is defined as @xmath12 .
the background spectra have been measured separately and subtracted prior to forming the ratio .
for single gamma decays as for example for the 662  kev @xmath5cs  line , the full energy ( fe ) peak is not suppressed after applying the lar anti - coincidence cut , since no energy deposition occurs in the lar .
the measured value for @xmath5cs  is @xmath13 .
an important background source for 0@xmath1  is the photons emitted in the decay of @xmath10tl , a progeny of @xmath7th . as the @xmath10tl2615  kev gamma is part of a gamma cascade in the deexcitation of @xmath10tl , the fe peak is suppressed in case that a second gamma deposits energy inside the lar .
@xmath14 for the 2615  kev line is @xmath15 .
@xmath10tl  compton events which deposit an energy close to @xmath16  kev inside the crystal are vetoed with @xmath17 , or in other words , the compton continuum is suppressed by a factor 17 .
the experimental data are compared with monte - carlo simulations using the mage  geant4 framework @xcite developed jointly by the majorana  and gerda  collaborations . the spectral shape and peak - to - compton ratio measured with @xmath5cs , @xmath7th  and
@xmath8ra  are well reproduced within typically 5% or better .
[ fig : realmc ] shows the simulated @xmath7thspectrum .
the background spectrum ( gray ) measured without source is added to the mc spectrum describing the source .
note that the @xmath7th  source is not in secular equilibrium , thus the poor agreement at low energies .
the survival probability after applying the lar veto cut at 2039  kev obtained with mage  mc simulation corresponds to @xmath18 .
a possible origin of the higher mc value may be related to the simplified geometrical description of the crystal holder which had small lar dead volumes not included in the simulation .
the experimental data show that the detection of lar scintillation photons is a powerful method to suppress backgrounds with negligible loss of 0@xmath1  signals . in the setup with an active lar mass of 19  kg
we observed a background suppression of the @xmath10tlcompton continuum at 2039  kev by a factor of 17 .
the suppression factor is limited by gammas escaping from the small lar volume .
mage  mc simulations reproduce the energy spectra as well as the suppression factors .
an ultra - low background prototype setup is presently under construction at lngs ( gerda - large ) .
the purpose of the device is to study the novel suppression method at ultra - low backgrounds with an active lar mass of approximately 1  ton .
the instrument will be used to study the background of gerda  phase  i detector assemblies prior to their operation in gerda .
applications of the method as an anti - compton spectrometer for trace analysis is envisioned .
measurements with a bare p - type high purity germanium diode ( hpge ) submerged in a 19  kg liquid argon ( lar ) scintillation detector at mpik - heidelberg are reported . 
 the liquid argon  germanium system ( large ) is operated as a 4@xmath0 anti - compton spectrometer to suppress backgrounds in the hpge . 
 this r&d is carried out in the framework of the gerda  experiment which searches for 0@xmath1  decays with hpge detectors enriched in @xmath2ge . 
 the goal of this work is to develop a novel method to discriminate backgrounds in 0@xmath1  search which would ultimately allow to investigate the effective neutrino mass free of background events down to the inverse mass hierarchy scale . 
 other applications in low - background counting are expected .