Experiment [1] M. Di Marco, P. Peiffer, S. Schonert, LArGe: Background suppression using liquid argon scintillation for 0νββ - decay search with enriched germanium detectors, arXiv:physics/ v1 Motivation Investigation of SiPM characteristics Dark rate vs. temperature Gain vs. temperature In an experiment a six-fold segmented HPGe detector was operated together with 12 SiPMs in a light and gas tight cryostat. The dewar was filled with LAr that was certified to a purity of 4.6. Inside the dewar 15 meters of WLS fiber were spirally rolled up around the detector and connected to the SiPMs. The setup was surrounded with TPB coated VM2000 foil. For analysis 19 readout channels (6HPGe segments + 1HPGe core + 12SiPMS) were connected to an XIA data acquisition system. A LAr anti-Compton veto based on SiPMs can achieve significant background reduction capability in a GERDA like experiment. The results presented are very promising. SiPMs are appropriate detectors for LAr scintillation light detection. At cryogenic temperatures they can be operated with very low dark rate and excellent gain. To optimize this new technology the following tasks are still planned: Small mass (~1mg)  potentially little radioactivity Excellent photon det. eff. up to 60% UV sensitivity (300nm to 900nm), peak at 450nm SiPMs do not require HV (HV leads to discharges in gaseous Ar atmosphere)‏ Moderate price SiPMs are excellent detectors for an anti- Compton veto in LAr. They consist of an array of APDs that are operated in Geiger mode. Incident photons result in a Geiger discharge in the individual APDs. The signal of the SiPM is the sum of all the APD outputs. The number of incident photons can be deduced from the number of APDs that fired. Single photon counting thus becomes possible. SiPMs are commercially available instruments. Their properties make them promising devices for LAr scintillation detection on low background surrounding: The two electrons emitted in 0vββ decay deposit their energy locally. Segmented detectors can identify events with multiple site energy deposit as background. However, if singly Compton scattered gammas escape the detector, the event topology is the same as for the signal-like events. The idea behind a LAr-based anti- compton veto is to use the scintillation light of the escaped gamma to identify it as background and to increase the background recognition efficiency. At room temperature SiPMs have a dark rate of up to ~1MHz. Cooling reduces the dark rate significantly. At temperatures below 150K the dark rate is negligible. SiPMs can be operated with good gain in the temperature range from 77K to 300K. The bias voltage has to be optimized for the temperature as the breakdown voltage changes with temperature. Light detection efficiencies of SiPMs with 100, 400 and 1600 pixels. Introduction GERDA is designed to reach a background of counts/(kg*keV*yr) in its first phase and counts/(kg*keV*yr) in Phase II. However, to disentangle normal from inverted mass hierarchy a ton scale level HPGe experiment needs to achieve a background of counts/(kg*keV*yr). Thus more efficient and new background reduction techniques are necessary. It has been shown that liquid argon (LAr) scintillation light can be used as an effective veto reducing Compton background in High-purity Germanium detectors (HPGeDs) with photomultiplier tubes by a factor of 17 [1]. Due to their low mass and thus potentially very low background contribution, their high photon detection efficiency and low dark rate at cryogenic temperatures, silicon photomultipliers, SiPMs, are a promising candidate for scintillation light detection in LAr. An important goal was to demonstrate that a similar background recognition efficiency as in [1] can be achieved with SiPM's and wave length shifting (WLS) fibers. Spectrum of a SiPM. Each peak corresponds to a discrete number of APDs that fired The double escape peak (DEP) of the 208 Tl 2614keV gamma at 1593keV and the full absorption peak (FAP) of 212 Bi at 1621keV demonstrate the background identification capability of the method. In DEP events a 2614keV gamma undergoes pair production. The resulting electron-positron pair deposits its energy inside the detector locally. If both annihilation gammas escape the detector, the energy deposited in the detector is 1593keV. As the total energy is mostly deposited locally, it will be detected in a single segment. The LAr anti-Compton veto suppresses the DEP by a factor of 6.1. The neighbouring FAP is only suppressed by a factor 1.1. This proves that the scintillation light from the two gammas escaping the HPGe detector was detected and that the background identification technique works. The FAP is suppressed by the segment anti-coincidence cut. This shows that the two cuts are complementary. In the experiment a background suppression by a factor of 4.2 in the region of interest could be demonstrated. Applying both, segment anti-coincidence and anti-Compton veto, a reduction of the background by a factor of 10 could be achieved. Gammas and charged particles produce scintillation light in LAr. Right: The black line shows the absorption and the emission peak of the WLS fiber. The blue line shows the emission peak of the TPB coating. Results References Conclusion & outlook Emission spectrum of LAr scintillation light. The main emission peak of LAr scintillation light is at 128nm. The highest sensitivity of SiPMs is around 450nm. Thus the LAr scintillation light needs to be shifted and guided to the small surface of the SiPMs. This is done using TPB coated VM2000 foil and WLS fibers surrounding the HPGe detector(s). Collection of LAr scintillation light VM2000 foil is a reflector. It was coated wit TPB, a fluorescent dye that emits light in the optical range. The fibers collect this light, shift it to a wavelength of ~500nm and guide it to the SiPMs. This way the light collection efficiency of the SiPMs is increased. Improvement of the light collection efficiency Optimization of advanced design of coupling between WLS fiber and SiPM Development and optimization of TPB coating techniques Identification of radiopure light guides Simulations to optimize geometry for maximum light collection efficiency while minimizing the additional background. Background reduction of HPGe Detectors using LAr scintillation light detection with SiPMs H. Aghai, N. Becerici-Schmidt, A. Caldwell, F. Cossavella, S. Dinter, F. Faulstich, S. Hemmer, J. Janicsko, J. Liu, X. Liu, K. Kröninger, D. Lenz, B. Majorovits, C. O’Shaughnessy, J. Schubert, H. Seitz, F. Stelzer, A. Vauth, O. Volynets Max-Planck-Institut für Physik, München Fachbeirat of the MPG, Munich, October 27 th - 28 th mm Picture of a SiPM with 400 pixels Measured dark rate as a function of temperature. For T<150K the dark rate reduces to negligible levels. Gain as a function of bias measured for different temperatures 228 Th spectrum with anti-compton and segment anti- coincidence veto. A suppression by a factor of 4.2 has been achieved in the region of interest. The anti-compton veto suppresses the DEP by a factor 6.1 and the FAP by a factor 1.1. The segment anti-coincidence suppresses only the FAP.