S. Sangiorgio

First measurement of the ionization yield of nuclear recoils in liquid argon

This Letter details a measurement of the ionization yield (Q(y)) of 6.7 keV(40)Ar atoms stopping in a liquid argon detector. The Q(y) of 3.6-6.3 detected e(-)/keV, for applied electric fields in the range 240-2130 V/cm, is encouraging for the use of this detector medium to search for the signals from hypothetical dark matter particle interactions and from coherent elastic neutrino-nucleus scattering. A significant dependence of Q(y) on the applied electric field is observed and explained in the context of ion recombination.

Results from a search for the 0 neutrino beta beta-decay of Te-130

A detailed description of the CUORICINO 130 Te neutrinoless double-beta (0 νββ) decay experiment is given and recent results are reported. CUORICINO is an array of 62 tellurium oxide (TeO 2 ) bolometers with an active mass of 40.7 kg. It is cooled to ∼8-10 mK by a dilution refrigerator shielded from environmental radioactivity and energetic neutrons. It is running in the Laboratori Nazionali del Gran Sasso (LNGS) in Assergi, Italy. These data represent an exposure of 11.83 kg yr or 91 mole-years of 130 Te. No evidence for 0 νββ-decay was observed and a limit of T 0ν 1/2 ( 130 Te) ≥ 3.0 x 1024 y (90% CL) is set. This corresponds to an upper limit on the effective mass, , between 0.19 and 0.68 eV when analyzed with the many published nuclear structure calculations. In the context of these nuclear models, the values fall within the range corresponding to the claim of evidence of 0 νββ-decay by H. V. Klapdor-Kleingrothaus et al. The experiment continues to acquire data.

Composite macro-bolometers for the rejection of surface radioactive background in rare-event experiments

Abstract Experiments searching for rare events, such as neutrinoless double beta decay and interactions of dark matter candidates, require extremely low levels of background. When these experiments are performed using macro-bolometers, radioactive contamination near the surfaces is of particular concern. For a bolometric neutrinoless double beta decay experiment, it can cause counts in the spectral region where the signal is expected, while for a dark matter experiment which exploits ionization signals for particle identification, it originates an incomplete charge collection simulating a nuclear recoil. In order to control the effects of surface contamination, we developed a novel technique that uses composite macro-bolometers to identify energy depositions that occur close to the surfaces of materials immediately surrounding the detector. The composite macro-bolometer proposed and studied here consists of a main energy absorber that is thermally coupled to and entirely surrounded by thin absorbers that act as active shields. Surface energy depositions can be rejected by the analysis of simultaneous signals in the main absorber and the shields. In this paper, we describe a full thermal model and experimental results for three prototype detectors. The detectors consist of Ge, Si, or TeO2 thin absorbers as active shields, all with TeO2 crystals as main absorbers. In all cases, the surface event rejection capability is clearly demonstrated. In addition, simulations and preliminary results show that it is possible to detect energy depositions that occurred on the shields without separate readout channels for them. The energy depositions in the shields are distinguished from those in the main absorber through pulse shape discrimination. This simplification makes this technique a viable method for the rejection of surface energy depositions in next-generation bolometric double beta decay searches, such as possible extensions or upgrades of the CUORE experiment.

Low-energy (<10keV) electron ionization and recombination model for a liquid argon detector

Abstract Detailed understanding of the ionization process in noble liquid detectors is important for their use in applications such as the search for dark matter and coherent elastic neutrino-nucleus scattering. The response of noble liquid detectors to low-energy ionization events is poorly understood at this time. We describe a new simulation tool which predicts the ionization yield from electronic energy deposits ( E 10 keV ) in liquid Ar, including the dependence of the yield on the applied electric drift field. The ionization signal produced in a liquid argon detector from 37Ar beta decay and 55Fe X-rays has been calculated using the new model.

The hunt for coherent neutrino-nucleus scattering with ionization argon detectors

Coherent scattering of neutrinos on nuclei is a well-known prediction of the Standard Model that has so far eluded all experimental attempts to detect it. The enhancement in the cross-section due to the coherence nature of the interaction makes this process interesting for the construction of high-rate, kg-size detectors for monitoring of nuclear reactors. Dual-phase noble-gas detectors are the prime technology for detection of the small recoil energy produced in the neutrino coherent scattering. We are pursuing a three-phased approach to build a compact 10kg dual-phase Argon ionization detector to attempt a first-ever measurement of coherent neutrino scattering at a power reactor. We will present here an overview and current status of the program pursued at Lawrence Livermore National Laboratory.

Performance of a MICROMEGAS-based TPC in a high-energy neutron beam

Abstract The MICROMEGAS (MICRO-MEsh GAseous Structure) charge amplification structure has found wide use in many detection applications, especially as a gain stage for the charge readout of Time Projection Chambers (TPCs). Here we report on the behavior of a MICROMEGAS TPC when operated in a high-energy (up to 800 MeV ) neutron beam. It is found that neutron-induced reactions can cause discharges in some drift gas mixtures that are stable in the absence of the neutron beam. The discharges result from recoil ions close to the MICROMEGAS that deposit high specific ionization density and have a limited diffusion time. For a binary drift gas, increasing the percentage of the molecular component (quench gas) relative to the noble component and operating at lower pressures generally improves stability.

The NIFFTE project

The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) is a double-sided Time Projection Chamber (TPC) with micromegas readout designed to measure the energy-dependent neutron-induced fission cross sections of the major and minor actinides with unprecedented accuracy. The NIFFTE project addresses the challenge of minimizing major sources of systematic uncertainties from previous fission chamber measurements such as: target and beam non-uniformities, misidentification of alpha and light charged particles as fission fragments, and uncertainties inherent to the reference standards used. In-beam tests of the NIFFTE TPC at the Los Alamos Neutron Science Center (LANSCE) started in 2010 and have continued in 2011, 2012 and 2013. An overview of the NIFFTE TPC status and performance at LANSCE will be presented.

Measurement of the nuclear ionization quench factor in a dual-phase argon detector

Coherent neutrino-nuclear scattering (CNS) has long been predicted by the standard model [1], but has yet to be observed. One of the most promising approaches that could result in CNS detection is the use of a dual-phase argon detector [2], For this demanding measurement it is essential that the nuclear ionization quench factor be known, which is not the case for liquid argon at typical CN. S energies — We have built the Gamma or Neutron Argon Recoils Resulting in Liquid Ionization (G/NARRLI) detector for the purpose of measuring the nuclear ionization quench factor at CNS energies. We plan to use neutron scatter recoils to measure the nuclear ionization quench factor at ∼8 keV, while using nuclear resonance fluorescence (NRF) recoils to measure the nuclear ionization quench factor between −∼100 eV and ∼ 4.5 keV. In making these measurements, we will map the regime of reactor neutrino CNS recoil in Ar and validate Monte Carlo models for calculating the nuclear quench factor [2].

A “must” for a double beta decay experiment: the reduction of the background

The main goal of the next generation neutrinoless double beta decay (0νββ) experiments will be the investigation of the inverse hierarchy region of the neutrino mass pattern. One of the major challenges for these experiments is to greatly reduce the background in order to reach the required sensitivity. In this work we report some results obtained with a new technique that, exploiting the dynamic of the heat flow in the detector, gives us information on the particle impact point providing a new means for an active background discrimination.

Innovations in low-temperature calorimeters: surface sensitive bolometers for background rejection and capacitive bolometers for higher energy resolution

In this contribution, we describe two innovations of the structure of large mass bolometers, proposed by the cryogenic group of the Insubria University (Como) and developed in collaboration with the Firenze group. First, up to now, low temperature calorimeters do not have any sort of spatial resolution. This means that it is not possible to reject events coming from the material that faces the detectors (holder, refrigerators shields, ...). In order to cope this problem, we developed a new kind of composite bolometers able to discriminate, by means of active ultra-pure semiconductor shields, external surface events from those coming from the absorber bulk. A second innovation that we discuss here concerns the temperature sensors. Presently, neutron transmutation doped Ge thermistors are the most common kind of phonon sensors. Unfortunately, this kind of readout dissipates power on the detector because of the thermistor biasing and also introduces a Johnson noise term. To improve energy resolution we studied and test the application of capacitive sensors that in principle could allow us to achieve a better signal-to-noise ratio. Modeling, simulations and first encouraging measurements on surface sensitive bolometers will be discussed along with preliminary results on capacitive sensors.