Resumen de High resolution 80se(n,γ) cross section measurement at cern n_tof and development of the novel i-ted detection system
The slow neutron capture (s-) process is responsible for the formation of half of the elements heavier than iron in the universe. Despite the long time scale of this process, the long half-life of some unstable isotopes throughout the s-process reaction flow creates branching points that lead to a split of the nucleosynthesis path. 79Se (t1/2 = 3.27 x 10^5 y) represents one of the most relevant and debated s-branching nuclei for two main reasons. On the one hand, the existence of low-energy excited states in 79Se, whose population can vary with the temperature of the stellar medium, makes the local abundance pattern around this branching especially sensitive to the thermal conditions. On the other hand, the observed abundances of the s-only krypton isotopes (80,82Kr) are very well-known from meteoric data. Thus, by comparing these abundances with those predicted by stellar models, information about the thermal conditions of the stellar media in which the s-process occurs can be obtained. To this aim, state-of-the-art hydrodynamic stellar models need experimental input data on the neutron capture cross section of the isotopes involved in the branching over a broad thermal energy range. The latter statement is certainly true for the unstable 79Se and its closest neighboring nuclei, 78,80Se. However, neutron capture measurements on radioactive nuclei are very challenging and indeed, thus far, there is no experimental data on the 79Se(n,gamma) reaction. Also, previous experimental data on 80Se was rather limited in terms of resolution and completeness.
In this context, the present work has contributed in two different fronts with the aim of shedding light on to the 79Se s-process branching.
The first part of this work describes the neutron capture cross section measurement of 80Se at CERN n_TOF, with very high energy resolution and covering the full stellar energy range of interest for the first time. The previous measurement on 80Se(n,gamma) suffers from a very limited energy resolution and a short neutron-energy range. These drawbacks have been remarkably improved in this work by means of a high-resolution time of flight (ToF) measurement employing a high purity 80Se sample of 3.8~g of mass. The use of C6D6D total energy detectors in combination with the Pulse-Height Weighting Technique (PHWT), have allowed us to obtain a capture yield with high accuracy and covering the entire energy range of astrophysical interest between 1 eV and 100 keV. One hundred and thirteen resonances have been analyzed by means of the R-matrix formalism, ninety-eight of them for the first time. The impact is sizable, being the MACS at kT = 8 keV 36% smaller than the value recommended in KADoNiS. The statistical uncertainty affecting this new MACS has been reduced from 10% down to 1%. The achieved systematic accuracy between 3.2% and 5.7% is comparable to the uncertainties of the isotopic abundances of the s-only Kr-isotopes, which is the requirement of hydrodynamic stellar models to deliver accurate results.
The second main contribution of this work to the study of the 79Se branching consisted of the first developments towards a novel detection system, called i-TED, for measuring (n,gamma) cross sections with enhanced signal-to-background ratio. This new detection system will be applied for the first time in the measurement of the 79Se(n,gamma) cross-section at CERN n_TOF in 2022. The i-TED imaging capable Total Energy Detector exploits the Compton imaging technique to select mainly the gamma-rays generated in the sample by neutrons captured therein, while rejecting contaminant gamma-rays coming from stray neutrons captured in the surroundings. In order to technically implement this concept, i-TED consists of two detection planes operating in time coincidence, in which the position, energy and time of the gamma-ray interactions are registered. A first demonstrator called i-TED5.3, with three position sensitive detectors (PSDs), has been developed and characterized in this thesis work and the first experimental proof of concept has been carried out. In i-TED5.3, one PSD is placed in the scatter plane while the remaining two are arranged in a vertical configuration within the absorber layer. Each PSD consists of a monolithic LaCl3(Ce) scintillation crystal optically coupled to a silicon photomultiplier, which is connected to an ASIC-based readout system manufactured by PETsys Electronics. A complete characterization of this prototype yielded position resolutions ranging between 1 mm and 2 mm FWHM, and energy resolutions of 6% and 7% FWHM at 661 keV for the singles and coincidence deposited energy spectra, respectively. Finally, a first experimental proof of concept experiment carried out at CERN n_TOF with i-TED5.3 allowed us to technically validate the system for ToF experiments, and demonstrate the background rejection capabilities. A background reduction by up to a factor of 3.8 was achieved after comparing the 56Fe(n,gamma) neutron energy spectra measured with the i-TED5.3 demonstrator and state-of-the-art C6D6D detectors. Further improvements undertaken outside of the scope of this thesis work comprise the assembly and characterization of an array of 4 i-TED detectors, each one comprising 5 PSDs, and the use of artificial intelligence and machine-learning techniques for enhancing further the background rejection capability and overall system performance.
