A. G. Cocco

Oxygen contamination in liquid Argon: combined effects on ionization electron charge and scintillation light

A dedicated test of the effects of Oxygen contamination in liquid Argon has been performed at the INFN-Gran Sasso Laboratory (LNGS, Italy) within the WArP R&D program. Two detectors have been used: the WArP2.3 lt prototype and a small (0.7 lt) dedicated detector, coupled with a system for the injection of controlled amounts of gaseous Oxygen. O2 contamination in LAr leads to depletion of both the free electron charge (via attachment process) and the scintillation light (via quenching and absorption mechanisms) available for ionization signal detection. Purpose of the test with the 0.7 lt detector was to detect the reduction of the long-lived component lifetime of the Argon scintillation light emission and of the overall light yield at increasing O2 concentration. Data from the WArP prototype were used for determining the behavior of both the ionization electron lifetime and the scintillation long-lived component lifetime at decreasing O2 concentration by the purification process activated in closed loop during the acquisition run. The electron lifetime measurements allowed to infer the O2 content of the Argon and correlate it with the long-lived scintillation lifetime data. The effects of Oxygen contamination on the scintillation light have been thus extensively measured over a wide range of O2 concentration, spanning from ~ 10−3 ppm up to ~ 10 ppm.

Demonstration and comparison of photomultiplier tubes at liquid Argon temperature

Liquified noble gases are widely used as a target in direct Dark Matter searches. Signals from scintillation in the liquid, following energy deposition from the recoil nuclei scattered by Dark Matter particles (e.g. WIMPs), should be recorded down to very low energies by photosensors suitably designed to operate at cryogenic temperatures. Liquid Argon based detectors for Dark Matter searches currently implement photo multiplier tubes for signal read-out. In the last few years PMTs with photocathodes operating down to liquid Argon temperatures (87 K) have been specially developed with increasing Quantum Efficiency characteristics. The most recent of these, Hamamatsu Photonics Mod. R11065 with peak QE up to about 35%, has been extensively tested within the R&D program of the WArP Collaboration. During these testes the Hamamatsu PMTs showed superb performance and allowed obtaining a light yield around 7 phel/keVee in a Liquid Argon detector with a photocathodic coverage in the 12% range, sufficient for detection of events down to few keVee of energy deposition. This shows that this new type of PMT is suited for experimental applications, in particular for new direct Dark Matter searches with LAr-based experiments.

The WArP experiment

Cryogenic noble liquid detectors are presently considered one of the best options for WIMP Dark Matter searches, especially when extensions to multi ton scale sensitive masses are foreseen. The WArP experiment is the first one that exploits the unique characteristics of liquid Argon to make a highly sensitive search for WIMP Dark Matter candidates. In 2008, a double phase detector has been assembled in the Gran Sasso National Laboratory with 140 kg sensitive mass and a discovery potential in the range of 5 × 10−45 cm2 in the spin-independent WIMP-nucleon cross-section. In addition to standard neutrons and gamma-rays passive shields, WArP implements an 8 ton liquid Argon active shield with 4π coverage. The detector was commissioned and put into operation during the first half of 2009 for a first technical run. This first run lasted about three months and then it was stopped for some detector repairs and modifications in the summer of 2009. A second run was started at the beginning of 2010. Detector design, construction and assembly are described, together with the results of the technical run and the very first results of the 2010 run.

Probing low energy neutrino backgrounds with neutrino capture on beta decaying nuclei

In this paper we investigate the possibility to detect Cosmological Relic Neutrinos, the oldest (after the Cosmological Microwave Background) particles produced after the Big Bang. In this paper we make a short overview of the methods proposed so far and we propose a new method that allows the CRN detection based on beta decaying target nuclei. The most important features of this process is that it does not require any minimum energy in order the neutrino interacts with nucleus. A detailed calculation of the cross section of the neutrino interaction on beta decaying nuclei is shown. The quoted value of the cross section times the neutrino velocity is of the order of 10-42cm2 ? c.

Low energy antineutrino detection using neutrino capture on electron capture decaying nuclei

In this paper we present a study of the interaction of a low energy electron antineutrino on nuclei that undergo electron capture. We show that the two corresponding crossed reactions have a sizable cross section and are both suitable for detection of low energy antineutrino. However, only in the case where very specific conditions on the Q value of the decay are met or significant improvements on the performances of ion storage rings are achieved, these reactions could be exploited in the future to address the long standing problem of a direct detection of cosmological neutrino background.

The GAP-TPC

Several experiments have been conducted worldwide, with the goal of observing low-energy nuclear recoils induced by WIMPs scattering off target nuclei in ultra-sensitive, low-background detectors. In the last few decades noble liquid detectors designed to search for dark matter in the form of WIMPs have been extremely successful in improving their sensitivities and setting the best limits. One of the crucial problems to be faced for the development of large size (multi ton-scale) liquid argon experiments is the lack of reliable and low background cryogenic PMTs: their intrinsic radioactivity, cost, and borderline performance at 87 K rule them out as a possible candidate for photosensors. We propose a brand new concept of liquid argon-based detector for direct dark matter search: the Geiger-mode Avalanche Photodiode Time Projection Chamber (GAP-TPC) optimized in terms of residual radioactivity of the photosensors, energy and spatial resolution, light and charge collection efficiency

Performance of a SensL-30035-16P silicon photomultiplier array at liquid argon temperature

Next generation multi-ton scale noble liquid experiments have the unique opportunity to discover dark matter particles at the TeV scale, reaching the sensitivity of 10−48 cm2 in the WIMP nucleon scattering cross-section. A prerequisite will be the reduction of radiogenic background sources to negligible levels. This is only possible if ultra-pure high efficiency photosensors are available for the scintillation light readout. Current experiments (e.g. Xenon, LUX, Darkside, ArDM) use cryogenic PMTs as photosensors. An attractive alternative is represented by silicon photomultiplier arrays (SiPM arrays), which show unrivalled performances in single photon detection. This paper reports on the performance of the SensL-ArrayB-30035-16P SiPM array and a custom made cryogenic front-end board at the liquid argon temperature. Its performance at VOV=3.5 V, where the PDE is maximal, are very promising in terms of SPE resolution (about 8%), dark rate (about 250 Hz) and correlated pulses (30%).

Capturing Relic Neutrinos with beta-decaying nuclei

We summarize a novel approach which has been recently proposed for direct detection of low energy neutrino backgrounds such as the cosmological relic neutrinos, exploiting neutrino/antineutrino capture on nuclei that spontaneously undergo β decay.

Relic neutrino detection using neutrino capture on beta decaying nuclei

In this paper we present a study of the interaction of low energy electron antineutrino on nuclei that undergo electronic capture. This completes the investigation on the possibility to detect very low energy neutrino and antineutrino using unstable nuclei that has been recently addressed by the study of neutrino capture on β± decaying nuclei. Experimental challenges toward the detection of Cosmological Relic Neutrino Background by the use of these reactions are discussed.

Cosmological relic neutrino detection using neutrino capture on beta decaying nuclei

We study the interaction of low energy neutrino on nuclei that spontaneously undergo beta decay showing that the product of the cross section times neutrino velocity takes values as high as 10−42 cm2 c for some specific nuclei that decay via allowed transitions. The absence of energy threshold and the value of the cross section single out these processes as a promising though very demanding approach for future experiments aimed at a direct detection of low energy neutrino backgrounds such as the Cosmological Relic Neutrino.