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Poster Summaries part B: 1 – NEXT-100; 2 – Gerda Phase II; 3 – Sources in Borexino

March 14, 2013

Here are three other short summaries of posters submitted to the conference.

1 – Francesc Monrabal: The NEXT-100 experiment

The NEXT-100 time projection chamber, currently under construction, will search for neutrinoless double beta decay (ββ0ν) using 100–150 kg of high-pressure xenon gas enriched in the 136Xe isotope to ∼90%. The detector possesses two important features for ββ0ν searches: very good energy resolution (better than 1% FWHM at the Q value of 136Xe) and event topological information for the distinction between signal and background. Furthermore, the technique can be extrapolated to the ton-scale, thus allowing the full exploration of the inverted hierarchy of neutrino masses.
The detection process is as follows. A charged particle propagating in the HPXe losses its energy through ionization and excitation of xenon atoms. The light emmited in the de-exitation of atoms (S1) provides the initial time of the event needed for the 3D reconstruction. The ionized electrons are then drifted towards the anode where they enter in a region of higher electric field producing electroluminescence (EL) light. The light produced there is 4 pi distributed. The light readed by the PMTs in the cathode is used to reconstruct the energy of the event with a resolution better than 1% FWHM at Qββ. On the other hand, a set of 1mm2 SiPMs in a rectangular grid just behind the anode is used to reconstruct the topology details of the event.
NEXT-DEMO prototype has been operating for 2 years and it has evolved according the collaboration necessities. Nowadays it is operating with the same configuration that will be used for NEXT-100: charge amplification with electroluminescence, energy measurement with PMTs and topology reconstruction with SiPMs. First results shows that energy resolution of 0.8% FWHM at Qββ is achieved. The very prelimirary topology reconstructions of Cs137 events shows clearly identification of regions with very different energy deposition along the track (image) allowing the identification of the end-point of the event.
The figure above shows the electron topology reconstruction using NEXT-DEMO SiPM tracking plane. The end-point is easily identified in the region with higher energy deposition (in red).


2 – Sabine Hemmer: Keeping the background low – The GERDA Phase II detectors

The GERmanium Detector Array (GERDA) experiment at the INFN Laboratori 
Nazionali del Gran Sasso uses high-purity germanium detectors to search
for neutrinoless double beta decay (0νββ) of Ge-76. The detectors,
isotopically enriched in Ge-76, are directly immersed in liquid Argon
that serves as coolant as well as passive and soon also active shielding
against external radiation. The first phase of the experiment started in
November 2011, using ~15 kg of coaxial germanium detectors from previous
experiments. In a second phase, additional ~20 kg of newly produced
enriched germanium detectors will be deployed. Since the sensitivity
of the GERDA experiment depends not only on the total detector mass
but also on the energy resolution and the background level, a novel
design was chosen for the Phase II detectors: Broad Energy Germanium
(BEGe) detectors.
The figure shows a schematic of a BEGe detector.

In addition to the improved energy resolution, the peculiar shape
of these detectors enhances the power to discriminate between signal
and background events using pulse shape analysis techniques.
To guarantee the extremely low background level needed for
the GERDA experiment, the production of the 30 BEGe detectors
was carefully planned and carried out. In order to limit the
generation of cosmogenically induced radioisotopes, such as
Ge-68, Co-60, and Co-58, the exposure of the germanium to
cosmic radiation was minimized. An acceptance and characterization
campaign of the newly produced detectors was carried out at the
Hades Experimental Research Of Intrinsic Crystal Appliances
(HEROICA) facility in the HADES underground laboratory located
on the premises of the Belgian Nuclear Research Centre SCK·CEN
in Mol, Belgium. Since a large number of detectors had to be
tested within a short time frame, much effort was put into the
automatization of the measurements. To this scope, the HEROICA
infrastructure comprises two static tables for measurements with
fixed radioactive sources to determine energy resolution, active
volume fraction, average deadlayer, depletion voltage, and pulse
shape performance of the detectors. Three automated mechanical
set-ups, on the other hand, allow a full surface scan of the
detectors with a collimated source, needed to detect deadlayer
and charge collection variations. All five set-ups can be operated
in parallel. Thanks to this advanced infrastructure the testing
procedure of all diodes could be completed ahead of the time schedule.
 3 – David Bravo: Sources in Borexino
Anomalous neutrino rate results have been measured in multiple experiments of different
disciplines: solar neutrino experiments that make up the “gallium anomaly”, analysis of reactor
fluxes which show a 6-7% flux deficit, cosmological models that prefer a number of neutrinos
greater than 3. All of these results, which when isolated may provide low significance, when
grouped together pose a compelling case for studying the possibility of a new kind of neutrino
which wouldn’t interact through chromodynamic or electroweak interactions (sterile). Other sets of experiments already provide strong constraints on its hypothetical mass splitting and mixing
angles. New sources of data are then needed to exclude or verify possible light (1eV2) sterile
neutrino models.
In Borexino we propose to install one or several sources close to or inside the detector (possible
configurations are shown in the figure), as was outlined in Dr Pallavicini’s talk. Using its unique
resolution and position reconstruction precision we can explore, through rate and oscillometry
studies, the phase space where sterile neutrinos may lie. Around 2015, when the current solar
program will be winding down, a ~10MCi high-purity enriched 51Cr source will be placed under
Borexino, where its electron neutrino flux can be scrutinized for oscillations into possible sterile
states. Weaker similar sources have already been used for calibrations in the GALLEX and SAGE experiments. Currently, irradiation facilities are being downselected (with 3-4 reactors favored) and activity determination strategies are being devised, since it should be measured to ~1% precision in several independent ways to reach the program’s goals. Calorimetry is one such method that Virginia Tech’s group is studying, where a continuous measurement from irradiation to deployment would be preferred. Thanks to Borexino’s size (comparable to the oscillation wavelength in the models which currently are favored) and radiopurity, it is expected to exclude a great part of the currently allowed phase space. Its discerning capabilities can be further extended to cover all of the 90% C.L. anomaly region with an internal antineutrino 144Ce-144Pr source. It could be placed in the water tank on a first ~2016 phase, or later on in the center of the detector after upgrading its systems after ~8 years of solar neutrino observations.
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