ANANNA, Chemseddine (2024) Characterization of EJ-309 organic liquid scintillators for the SHADES experiment. [Tesi di dottorato]

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Abstract

This thesis belongs to the direct experimental measurement of the astrophysically significant 22Ne(α,n)25Mg nuclear reaction framework. Improving the state-of-the-art results requires overcoming several technical challenges such as the background activity of the detectors and the low cross-section of the reaction. The present work contributes to this by performing a full characterization of EJ-309 organic liquid scintillators used in the SHADES experimental setup in preparation for the reaction measurement with the newly commissioned LUNA-MV accelerator, located at the deep-underground facility of Laboratori Nazionali del Gran Sasso (AQ), Italy. The characterization procedure starts by gain matching the 12 EJ-309 liquid scintillators of the detection array, then identifying and quantifying the internal background of each. A full study of the detectors response to γ, α, and neutrons is conducted, establishing light-output-to-energy calibration for each particle type. The internal activity of the detectors is quantified by placing the liquid scintillators under 5 cm of lead inside the underground facility in August 2021. The collected data showed the typical 238U α-activity in the detectors’ walls. Time coincidence analysis confirmed its origin, incidentally providing a new estimate of 214Po half-life. A Monte-Carlo simulation allowed explain the contamination scenario and quantify 238U/232Th de-cay chains’ contribution to the observed activity. This result is to be used after the 22Ne(α,n)25Mg measurement to exclude α-counts, indistinguishable from neutrons in the scintillators. A neutron energy calibration adapted to the 22Ne(α,n)25Mg mea-surement (i.e. optimized for low energies) is established in this work. It is a result of a series of neutron beam measurements performed at the Goethe University of Frankfurt Germany in July 2022, using a simplified array of 1 scintillator and 6 3He counters. Including the counters allowed the investigation of eÿciency improvement through active shielding using coincidence information between the two detector types, preparing the ground for the 22Ne(α,n)25Mg measurement. This work is organized such as Chapter 1 describes the 22Ne(α,n)25Mg astrophys-ical scenario, providing insights on stellar evolution and AGB stars. Following, the reaction’s state-of-the-art is reported, focusing on the experimental challenges hin-dering the measurement of its cross-section. Chapter 2 then describes the SHADES experimental setup, discussing neutron detection with EJ-309 scintillators and 3He counters. Chapter 3 reports the experimental procedure followed to gain match the scintillators and achieve a high level of particle-type separation between neutrons and γ-rays through optimized Pulse Shape Discrimination (PSD). The internal back-ground of the scintillators and the environmental background at the underground facility are investigated in Chapter 4, reporting the di˙erent means and techniques used to achieve it, noticeably the PSD optimization discussed in the previous chap-ter. Chapter 5 is dedicated to the Frankfurt measurement, discussing data acquisi-tion and analysis methodology while also presenting the PSD and calibration results. The last part of Chapter 5 is an investigation of the active shielding performance and its direct and indirect impacts on the SHADES experiment.

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