Cimmino, Bruno (2020) Dark matter search: the Neutron Veto in the XENONnT Detector Upgrade. [Tesi di dottorato]


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Item Type: Tesi di dottorato
Resource language: English
Title: Dark matter search: the Neutron Veto in the XENONnT Detector Upgrade
Date: 12 October 2020
Number of Pages: 162
Institution: Università degli Studi di Napoli Federico II
Department: Fisica
Dottorato: Fisica
Ciclo di dottorato: 32
Coordinatore del Corso di dottorato:
Iacovacci, MicheleUNSPECIFIED
Date: 12 October 2020
Number of Pages: 162
Keywords: XENONnT Neutron Veto
Settori scientifico-disciplinari del MIUR: Area 02 - Scienze fisiche > FIS/04 - Fisica nucleare e subnucleare
Date Deposited: 14 Oct 2020 15:18
Last Modified: 28 Oct 2021 12:11

Collection description

The existence of dark matter and dark energy were hypothesized in the first decades of the last century due to many experimental evidences. Regarding the first, its existence was necessary to explain some gravitational phenomena that cannot be explained by the existence of ordinary matter only. The most accredited hypothesis is that dark matter is composed of massive particles but which interact with ordinary matter only gravitationally and through weak interaction, and are therefore called WIMP (Weak Interacting Massive Particles). The ways to detect them are of three types: production at accelerators, indirect detection and direct detection. The latter technique consists in detecting the effects that dark matter produces on ordinary matter, in particular the WIMP-nuclei scattering. As these events are very rare, it is necessary to build detectors of big mass and located them deep underground or underwater, in order to reduce all possible background events. The XENON experiment is located at the Gran Sasso National Laboratories (LNGS) under 1,4 km of rock and uses dual phase liquid (LXe) and gaseous (GXe) Xenon as detection medium. Xenon is a noble gas and, as such, has many advantages. The XENON experiment presented various upgrades. XENON1T was the last to finish the data analysis and was operational from 2016 to 2018, with an active mass of 3.2 tons of Xenon. XENON1T set the lower limit for the spin-independent WIMP-nucleon elastic scattering cross section at 4.1 10−47 cm2 at 90% of C.L. and detected some events that perhaps could be identified as Axion signals. In order to improve the lower limit for the cross section by a further order of magnitude, as well as to investigate more thoroughly the nature of the latter mentioned signals, a further upgrade, XENONnT, has been planned, which is currently under construction. The mass of the detector has been increased to 8 tons of Xenon of which 6 tons make the active part. A new apparatus was also installed, the Neutron Veto (nVeto), which uses water doped with Gadolinium as a detector medium. The nVeto will be able to detect the so-called "sneaky neutrons" that is a type of radiogenic neutrons that produce a signal identical to that of WIMPs. Gadolinium was chosen because it has a high cross-section for neutron capture with release of about 8 MeV in gamma radiation. The gammas from the neutron capture will be detected by 120 photomultipliers (PMTs) supported by a stainless steel cylindrical structure, where they are organized in 15 columns and 8 rows around the cryostat, at about 1 m distance from it. Reflector foils wrapped around the cryostat and put behind the PMT support structure, with a reflectivity of about 98%, will enclose the nVeto region, which results in a cylindrical corona of inner radius 1 m, outer radius 2 m and hight of 5 m. Before proceeding with the installation of these PMTs, many tests were carried out to verify their correct functioning. These tests have recently been completed and the Neutron Veto is being put into operation together with the entire XENONnT detector at LNGS. Various research groups are contributing to the implementation of the nVeto, including the Naples group. The Naples group is expected to implement, with the contribution of the Japanese colleagues, the Data Acquisition System for the nVeto signals; at the same time the Naples expertise on Control Systems will be available for implementing the recirculation system of the Gd-doped water. As for nVeto DAQ, a trigger-less approach will be adopted. This will facilitate the accomplishment of the task thanks to use of the software infrastructure already developed for XENON1T. The nVeto will therefore be part of a wider and unique detector from the DAQ point of view. Correspondently a new system is needed to successfully operate the recirculation of the Gd-doped water; at present just solutions characterized by low level of complexity and almost no automation, mainly monitoring, have been taken into account. The collaboration between Italian and Japanese physicists is of fundamental importance here to implement the nVeto of the XENONnT detector. The Naples group will take care of the Data Acquisition of the 120 PMTs in charge of the gamma detection. The Gadolinium technology will be implemented by the first time in Europe, being conceived for the SuperK detector in Japan. The Japanese part of the project holds the expertise on Gadolinium technology, namely the "selective molecular band-pass filter" to separate Gadolinium by both smaller and larger impurities in the filtration process. The expertises of the two groups match very well in view of the nVeto implementation. Soon, by the end of 2020, everything will be ready and XENONnT will be able to start data taking.


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