INFN Gran Sasso National Laboratory Research Articles

A new hybrid gadolinium nanoparticles-loaded polymeric material for neutron detection in rare event searches

Experiments aimed at direct searches for WIMP dark matter require highly effective reduction of backgrounds and control of any residual radioactive contamination. In particular, neutrons interacting with atomic nuclei represent an important class of backgrounds due to the expected similarity of a WIMP-nucleon interaction, so that such experiments often feature a dedicated neutron detector surrounding the active target volume. In the context of the development of DarkSide-20k detector at INFN Gran Sasso National Laboratory (LNGS), several R&D projects were conceived and developed for the creation of a new hybrid material rich in both hydrogen and gadolinium nuclei to be employed as an essential element of the neutron detector. Thanks to its very high cross-section for neutron capture, gadolinium is one of the most widely used elements in neutron detectors, while the hydrogen-rich material is instrumental in efficiently moderating the neutrons. In this paper results from one of the R&Ds are presented. In this effort the new hybrid material was obtained as a poly(methyl methacrylate) (PMMA) matrix, loaded with gadolinium oxide in the form of nanoparticles. We describe its realization, including all phases of design, purification, construction, characterization, and determination of mechanical properties of the new material.

Simulation-based design study for the passive shielding of the COSINUS dark matter experiment

The COSINUS (Cryogenic Observatory for SIgnatures seen in Next-generation Underground Searches) experiment aims at the detection of dark matter-induced recoils in sodium iodide (NaI) crystals operated as scintillating cryogenic calorimeters. The detection of both scintillation light and phonons allows performing an event-by-event signal to background discrimination, thus enhancing the sensitivity of the experiment. The choice of using NaI crystals is motivated by the goal of probing the long-standing DAMA/LIBRA results using the same target material. The construction of the experimental facility is foreseen to start by 2021 at the INFN Gran Sasso National Laboratory (LNGS) in Italy. It consists of a cryostat housing the target crystals shielded from the external radioactivity by a water tank acting, at the same time, as an active veto against cosmic ray-induced events. Taking into account both environmental radioactivity and intrinsic contamination of materials used for cryostat, shielding and infrastructure, we performed a careful background budget estimation. The goal is to evaluate the number of events that could mimic or interfere with signal detection while optimising the geometry of the experimental setup. In this paper we present the results of the detailed Monte Carlo simulations we performed, together with the final design of the setup that minimises the residual amount of background particles reaching the detector volume.

Underground Radiobiology: A Perspective at Gran Sasso National Laboratory.

Scientific community and institutions (e. g., ICRP) consider that the Linear No-Threshold (LNT) model, which extrapolates stochastic risk at low dose/low dose rate from the risk at moderate/high doses, provides a prudent basis for practical purposes of radiological protection. However, biological low dose/dose rate responses that challenge the LNT model have been highlighted and important dowels came from radiobiology studies conducted in Deep Underground Laboratories (DULs). These extreme ultra-low radiation environments are ideal locations to conduct below-background radiobiology experiments, interesting from basic and applied science. The INFN Gran Sasso National Laboratory (LNGS) (Italy) is the site where most of the underground radiobiological data has been collected so far and where the first in vivo underground experiment was carried out using Drosophila melanogaster as model organism. Presently, many DULs around the world have implemented dedicated programs, meetings and proposals. The general message coming from studies conducted in DULs using protozoan, bacteria, mammalian cells and organisms (flies, worms, fishes) is that environmental radiation may trigger biological mechanisms that can increase the capability to cope against stress. However, several issues are still open, among them: the role of the quality of the radiation spectrum in modulating the biological response, the dependence on the biological endpoint and on the model system considered, the overall effect at organism level (detrimental or beneficial). At LNGS, we recently launched the RENOIR experiment aimed at improving knowledge on the environmental radiation spectrum and to investigate the specific role of the gamma component on the biological response of Drosophila melanogaster.

Accurate GPS-based timestamp facility for Gran Sasso National Laboratory

A new system to assign accurate timestamps to events recorded by experiments running underground has been designed, installed and tested at INFN Gran Sasso National Laboratory, Italy. This facility is based on a Master unit installed on surface and receiving time information from a GPS receiver, and Slave units placed underground which get data packet from the Master via optical fiber and assign the timestamps. The system is able to provide a time accuracy of 15 ns (1σ) and precise reference frequencies to the experiments. It is now part of the infrastructure of the Laboratory for all the experimental activities which need accurate timestamps.

The underground muon flux with 24 years of data of the LVD detector

The Large Volume Detector (LVD), in the INFN Gran Sasso National Laboratory (Italy), has been taking data since June 1992. The experiment, 1 kton of liquid scintillator at the equivalent depth of 3600 m w.e., has been mainly designed to observe low energy neutrinos from the core collapse of a supernova but allows also the measure of the atmospheric muon flux underground as well as the induced neutron production.In this work we summarize the results of the analysis of the global LVD data set, 5.6 107 muons in a livetime of 8402 days collected during 24 years of continuos operations since 1994 up to 2017. The present measurement represents an unprecedented collection obtained by a unique experiment in a fixed location. The modulation in time of the flux and its correlation with the the effective temperature in the upper atmosphere are here discussed.

Ultra-sensitive γ-ray spectroscopy set-up for investigating primordial lithium problem

To precisely determine BBN 6Li production, the cross-section of the nuclear reaction 2H(α, γ)6Li must be directly measured within the astrophysical energy range of 30–400keV. This measure requires an ultra-low γ-ray background in the experimental set-up. We have realized the conditions matching these very strict requirements at LUNA, the deep underground accelerator laboratory active in the INFN Gran Sasso National Laboratory (LNGS), Italy: the γ-ray spectrometer background has been reduced down to reach unmatched low levels, comparable to the good ones experienced in dedicated off-line underground ultra low γ counting rate. We present and discuss the γ-ray background reduction reached in the HpGe spectrometer, where most of the remaining γ-ray background seen in the spectra are coming from the energetic deuterons scattered in the gas target by the α beam. Thanks to the low neutron environmental background at LUNA, the effect of this weak flux of 2–3MeV neutrons on HpGe detectors has been studied in details and the results are presented and discussed.

IMPLICATION FOR THE CORE-COLLAPSE SUPERNOVA RATE FROM 21 YEARS OF DATA OF THE LARGE VOLUME DETECTOR

The Large Volume Detector (LVD) has been continuously taking data since 1992 at the INFN Gran Sasso National Laboratory. LVD is sensitive to neutrino bursts from gravitational stellar collapses with full detection probability over the Galaxy. We have searched for neutrino bursts in LVD data taken in 7335 days of operation. No evidence of neutrino signals has been found between June 1992 and December 2013. The 90% C.L. upper limit on the rate of core-collapse and failed supernova explosions out to distances of 25 kpc is found to be 0.114/y.

Light output response of the LVD liquid scintillator to neutron-induced nuclear recoils

The organic liquid scintillator used in the LVD experiment (INFN Gran Sasso National Laboratory) has been exposed to an Am-Be neutron source to measure the light response function for neutron energies in the region from about 4 to 11 MeV.A full Monte Carlo simulation, incorporating the detector response, is used to generate neutron scattering spectra which are matched to the observed ones to determine the quenching factors.The obtained light output response is well described by the semi-empirical Birks model.The results, consistent with those obtained by other authors using similar hydrocarbonate scintillators, can be of interest for fast and high-energy neutron spectroscopy that could be performed with this detector.

Neutrino bursts from gravitational stellar collapses with LVD

The main goal of the Large Volume Detector (LVD), in the INFN Gran Sasso National Laboratory, Italy, is the study of neutrino bursts from gravitational stellar collapses in our Galaxy or in the Magellanic Clouds. Both the detector and the data analysis procedure have been optimized for this purpose. No neutrino burst candidates were selected over 6013 days of observation. As a result, the 90% c.l. upper limit to the rate of gravitational stellar collapses in our Galaxy is 0.14y−1.

Uranium, radium and tritium groundwater monitoring at INFN-Gran Sasso National Laboratory, Italy

Uranium groundwater anomalies, which were observed in cataclastic rocks crossing the underground Gran Sasso National Laboratory before the L’Aquila earthquake (April 6th, 2009), have been studied versus radium and tritium contents. The radionuclide analysis supports the role of endogenic fluid dynamics for uranium content in groundwater rather than percolation processes, due to meteoric events occurring above the water table of the Gran Sasso aquifer. The uranium anomalies represent a key geochemical signal of a progressive increase of deep fluids fluxes at middle-lower crustal levels associated with the geodynamics of the earthquake. Moreover, the uranium represents a more precise strain-meter than radon as its presence can be modulated during the preparation phase of the earthquake, and only successively released by microfracturing during the main shock and aftershocks.

Results of a 3-ton experiment with a Gd loaded liquid scintillator target performed in the frame of LVD at LNGS

In the frame of the LVD experimental program in the INFN Gran Sasso National Laboratory (LNGS), we performed a 3-ton Gd experiment, by loading (up to 0.1% in weight) the LVD organic liquid scintillator with a Gd organic salt developed and produced as the result of a joint INFN/INR research activity. Performances, feasibility and preliminary results on stability have been presented.

Doping the 1 kton Large Volume Detector with Gd

The Large Volume Detector (LVD) in the INFN Gran Sasso National Laboratory (LNGS), Italy, is a ν observatory which has been monitoring the Galaxy since June 1992 to study neutrinos from core collapse supernovae. The experiment in the present configuration is made by 840 scintillator detectors, for a total active mass of 1000 tons. The detector sensitivity to neutrino bursts due to a core collapse supernova has been already discussed in term of maximum detectable distance. In this paper we evaluate the improvements that LVD could obtain if all its active scintillator mass was doped with a small amount (0.14% in weight) of Gadolinium. We simulated neutron captures following ν̄e inverse beta decay reactions in one LVD counter (1.2 ton) with Gd doped liquid scintillator obtaining an efficiency for the detection of this process of ηn|Gd = 80% and a mean capture time τ = 25μs, in good agreement with the results obtained by the measures. This implies a gain of a factor ∼ 20 in the signal to noise ratio for neutron capture detection with respect to the undoped liquid scintillator. We discuss how the captures of neutrons from rock radioactivity on Gd modify the background conditions of the detector and we calculate the curves expressing the sensitivity to a ν̄e burst from core collapse supernovae depending on the distance of the collapsing star. It results that doping the 1 kton Large Volume Detector with Gd would assure a 90% detection efficiency at the distance of the Large Magellanic Cloud (50 kpc), an achievement which is equivalent to that obtained by doubling the number of counters in LVD.

Neutrino bursts from gravitational stellar collapses with LVD

The main goal of the Large Volume Detector (LVD), in the INFN Gran Sasso National Laboratory (Italy), is the study of neutrino bursts from gravitational stellar col- lapses in the Milky Way. Both the detector and the data anal- ysis procedure have been actually optimized for this purpose. Moreover the modularity of the apparatus allows to obtain a duty cycle that is very close to 100%, so that the experiment is continuously monitoring the Galaxy. The search for Supernova neutrino signal is performed online, within fixed duration time windows (20 s), and of- fline with variable duration time windows from few ms up to 200 s. In both cases, LVD is able to disentangle a cluster of neutrino signals from the background fluctuations, and its sensitivity extends to the whole Galaxy. No candidates have been detected during almost 18 years of observation (6013 days of lifetime): the resulting 90% c.l. upper limit to the rate of gravitational stellar collapses in the Galaxy is 0.14 events/year. Detector performances, search method and data results are here reported.

Long-term study of low energy counting rate with the Large Volume Detector

The Large Volume Detector (LVD), located in the hall A of the INFN Gran Sasso National Laboratory, is a ν observatory mainly designed to study low energy neutrinos from the gravitational collapse of galactic objects. The experiment, which is continuosuly taking data and consists of 840 scintillator counters, can monitor the local radioactivity background at energies < 0.5 MeV.In this paper we study the long-term behavior of this low-energy counting rate since 1997. The Fourier analysis of the period 1997-2002 shows the existence of a seasonal modulation with the maximum occurring at the end of August. The same modulation is found by analyzing data collected by a Radon-meter placed inside the detector: this shows that changes in the concentration of Radon in the experimental hall are responsible for the measured variations of the counting rate in LVD.

Search for neutrino bursts from stellar collapses into neutron stars or black holes with the LVD experiment at Gran Sasso

The Large Volume Detector (LVD), in the INFN Gran Sasso National Laboratory (Italy), is a 1 kt liquid scintillator detector whose major purpose is monitoring the Galaxy to study neutrino bursts from gravitational stellar collapses. The experiment has been taking data, under different configurations, since 1992, reaching in 2001 its present and final configuration.The LVD performances from the point of view of the on-line identification of a neutrino burst are presented in this study together with the results of the updated search for neutrinos from core collapse supernovae.The possibility to observe weak neutrino signals expected from neutron stars collapsing into black holes on the basis of a model prediction which deals with bursts durations of few milliseconds is discussed. The behavior of the detector background over a wide range of time windows, from I ms to 200 s, has been analyzed, and the detector sensitivitv to these kind of events is discussed.

First CNGS events detected by LVD

The CERN Neutrino to Gran Sasso (CNGS) project aims to produce a high energy, wide band νμ beam at CERN and send it toward the INFN Gran Sasso National Laboratory (LNGS), 732 km away. Its main goal is the observation of the ντ appearance, through neutrino flavour oscillation. The beam started its operation in August 2006 for about 12 days: a total amount of 7.6×1017 protons were delivered to the target. The LVD detector, installed in hall A of the LNGS and mainly dedicated to the study of supernova neutrinos, was fully operating during the whole CNGS running time. A total number of 569 events were detected in coincidence with the beam spill time. This is in good agreement with the expected number of events from Monte Carlo simulations.

On-line recognition of supernova neutrino bursts in the LVD

In this paper we show the capabilities of the Large Volume Detector (INFN Gran Sasso National Laboratory) to identify a neutrino burst associated with a supernova explosion, in the absence of an “external trigger”, e.g., an optical observation. We describe how the detector trigger and event selection have been optimized for this purpose, and we detail the algorithm used for the on-line burst recognition. The on-line sensitivity of the detector is defined and discussed in terms of supernova distance and ν ¯ e intensity at the source.

GERDA. Germanium Detector Array. Search for Neutrino-less ββ Decay of 76Ge

The GERDA experiment, being assembled in the INFN Gran Sasso National Laboratory, will search for neutrino-less double-beta decay of 76 Ge at a background level of 10 −3 kg −1 keV −1 yr −1 , two orders of magnitude lower than the best present value. In phase-I existing diodes used by the Heidelberg-Moscow and IGEX experiments will be immersed naked in hyperpure liquid Ar, for a sensitive mass of about 15 kg. In phase-II about 20 k of new, segmented, detectors will be added. In phase-I the Klapdor et al. evidence will be tested, in phase-II the degenerate mass region will be explored with an exposure of about 100 kg yr.

Study of the effect of neutrino oscillations on the supernova neutrino signal in the LVD detector

The LVD detector, located in the INFN Gran Sasso National Laboratory (Italy), studies supernova neutrinos through the interactions with protons and carbon nuclei in the liquid scintillator and interactions with the iron nuclei of the support structure. We investigate the effect of neutrino oscillations in the signal expected in the LVD detector. The MSW effect has been studied in detail for neutrinos travelling through the collapsing star and the Earth. We show that the expected number of events and their energy spectrum are sensitive to the oscillation parameters, in particular to the mass hierarchy and the value of θ 13 , presently unknown. Finally we discuss the astrophysical uncertainties, showing their importance and comparing it with the effect of neutrino oscillations on the expected signal. We do not discuss in this paper the determination of oscillation parameters from the results of the observations; this task seems to be very difficult until astrophysical uncertainties are significantly reduced.

Background reduction in neutrinoless double beta decay experiments using segmented detectors—A Monte Carlo study for the GERDA setup

The identification of gamma radiation is essential for a new generation of double beta decay experiments. The GERmanium Detector Array, GERDA, located at the INFN Gran Sasso National Laboratory (LNGS) in Italy, uses germanium, enriched in Ge 76 , as source and detector, and aims at a background level of less than 10 - 3 counts / ( kg keV y ) in the region of the Q β β -value. For the first time highly segmented detectors will be installed in a double beta decay experiment. A detailed GEANT4 Monte Carlo study was performed to evaluate the background reduction achievable by anti-coincidence cuts between crystals and segments. Within the overall geometry of GERDA, the segmentation scheme considered here provides around an order of magnitude of extra background reduction.