Prototipo di un rivelatore per la misura di flusso di neutroni di alta energia

In questo lavoro di tesi si è studiato il prototipo di un rivelatore per la misura del flusso di neutroni di alta energia a n_TOF (CERN). La determinazione del flusso incidente sul bersaglio d’interesse è fondamentale per la misura delle sezioni d’urto indotte da neutroni. In dettaglio si è realizzato e caratterizzato un Proton Recoil Telescope da affiancare a una camera a fissione nella misura della sezione d’urto di fissione del 235U per energie cinetiche dei neutroni tra 20 MeV e 1 GeV, in programma per il 2018. In primo luogo mi sono occupata delle simulazioni Monte Carlo per lo sviluppo e l’ottimizzazione del rivelatore: scelta del tipo rivelatore, geometria, accoppiamento ottico e risoluzione energetica. Successivamente ho partecipato alle misure per la determinazione delle proprietà del rivelatore presso la facility n_TOF al CERN tramite l’utilizzo delle sorgenti gamma e presso i Laboratori Nazionali del Sud con un fascio di protoni monoenergetici da 62 MeV. Infine, ho preso parte al primo test per verificare la risposta del rivelatore ad un fascio di neutroni incidenti su un bersaglio di Polietilene per studiare lo scattering elastico n-p. L’analisi dei dati raccolti nelle varie campagne di misure chiude questo lavoro di tesi

Measurement of the 235 U(n,f) cross section relative to the neutron-proton elastic scattering up to 500 MeV at n_TOF

Neutron cross-section standards are fundamental ingredients for both measurements and evaluations of neutron-induced reaction cross sections. This is the case of 235U(n,f) cross section: one of the most important standard cross sections at thermal neutron energy and between 0.15 MeV and 200 MeV. Above 200 MeV this reaction plays an important role for several applications, from biological effectiveness, via nuclear astrophysics, to nuclear technology, as well as for fundamental nuclear physics. However, no measurement exists for energies above 200 MeV. This led to a request for new absolute measurements of 235U(n,f) cross section, in order to extend the precision and possibly to establish it as a standard up to 1 GeV. The n_TOF facility at CERN offers the possibility to study such reaction thanks to the wide neutron energy spectrum available in its experimental area, from thermal to 1 GeV. A dedicated measurement campaign was carried out to provide accurate and precise cross-section data of the 235U(n,f) reaction in the energy region from 10 MeV to 500 MeV. The experimental setup consisted of two chambers to detect the 235U fission events, while the neutron flux was simultaneously measured by exploiting the neutron-proton scattering process using three Proton Recoil Telescopes. In this PhD thesis the development, implementation and characterisation of the two telescopes under the responsibility of the INFN are discussed, as well as the analysis of the data acquired during the campaign. In this measurement, the n_TOF neutron flux in the energy interval between 10 and 500 MeV was measured for the first time. In addition, the analysis of one of the two chambers dedicated to the measurement of fission events is presented in detail. From these data, the 235U(n,f) cross section was determined and represents, at this time, the unique measure of this kind

fully solution processed conductive films based on colloidal copper selenide nanosheets for flexible electronics

A novel colloidal synthesis of copper selenide nanosheets (NSs) with lateral dimensions of up to 3 μm is developed. This material is used for the fabrication of flexible conductive films prepared via simple drop-casting of the NS dispersions without any additional treatment. The electrical performance of these coatings is benchmarked against copper selenide spherical nanocrystals (SNCs) in order to demonstrate the advantage of 2D morphology of the NSs for flexible electronics. In this contest, Cu2−xSe SNC films exhibit higher conductivity but lower reproducibility due to the formation of cracks leading to discontinuous films. Furthermore, the electrical properties of the films deposited on different flexible substrates following their bending, stretching and folding are studied. A comparison of Cu2−xSe SNC and CuSe NS films reveals an increased stability of the CuSe NS films under mechanical stress applied to the samples and their improved long-term stability in air

Core/Shell CdSe/CdS bone‐shaped nanocrystals with a thick and anisotropic shell as optical emitters

Colloidal core/shell nanocrystals are key materials for optoelectronics, enabling control over essential properties via precise engineering of the shape, thickness, and crystal structure of their shell. Here, the growth protocol for CdS branched nanocrystals is applied on CdSe nanoplatelet seeds and bone-shaped heterostructures are obtained with a highly anisotropic shell. Surprisingly, the nanoplatelets withstand the high growth temperature of 350 degrees C and structures with a CdSe nanoplatelet core that is overcoated by a shell of cubic CdS are obtained, on top of which tetrahedral CdS structures with hexagonal lattice are formed. These complex core/shell nanocrystals show a band-edge emission around 657 nm with a photoluminescence quantum yield of approximate to 42% in solution, which is also retained in thin films. Interestingly, the nanocrystals manifest simultaneous red and green emission and the relatively long wavelength of the green emission indicates charge recombination at the cubic/hexagonal interface of the CdS shell. The nanocrystal films show amplified spontaneous emission, random lasing, and distributed feedback lasing when the material is deposited on suitable gratings. This work stimulates the design and fabrication of more exotic core/shell heterostructures where charge carrier delocalization, dipole moment, and other optical and electrical properties can be engineered

NEAR: A New Station to Study Neutron-Induced Reactions of Astrophysical Interest at CERN-n_TOF

We present NEAR, a new experimental area at the CERN-n_TOF facility and a possible setup for cross section measurements of interest to nuclear astrophysics. This was recently realized with the aim of performing spectral-averaged neutron-capture cross section measurements by means of the activation technique. The recently commissioned NEAR station at n_TOF is now ready for the physics program, which includes a preliminary benchmark of the proposed idea. Based on the results obtained by dedicated Monte Carlo simulations and calculation, a suitable filtering of the neutron beam is expected to enable measurements of Maxwellian Averaged Cross Section (MACS) at different temperatures. To validate the feasibility of these studies we plan to start the measurement campaign by irradiating several isotopes whose MACS at different temperatures have recently been or are planned to be determined with high accuracy at n_TOF, as a function of energy in the two time-of-flight measurement stations. For instance, the physical cases of 88Sr(n,γ ), 89Y(n,γ ), 94Zr(n,γ ) and 64Ni(n,γ ) are discussed. As the neutron capture on 89Y produces a pure β -decay emitter, we plan to test the possibility to perform activation measurements on such class of isotopes as well. The expected results of these measurements would open the way to challenging measurements of MACS by the activation technique at n_TOF, for rare and/or exotic isotopes of interest for nuclear astrophysic

Preparing Phase 4 of the n_TOF/CERN facility

After CERN's Long Shutdown 2, the n_TOF facility infrastructure was largely upgraded. The biggest change is the installation of a new lead spallation target, the performance of which needs to be carefully examined. During Summer 2021, the facility's two flight paths were characterised in terms of neutron beam energy distribution, profile and resolution. In this work, the characterisation of the facility is described and the first results are given

First Results of the 140^{140}Ce(n,γ)141^{141}Ce Cross-Section Measurement at n_TOF

An accurate measurement of the $^{140}$Ce(n,γ) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance in stars. The measurement was motivated by the significant difference between the cerium abundance measured in globular clusters and the value predicted by theoretical stellar models. This discrepancy can be ascribed to an overestimation of the $^{140}$Ce capture cross-section due to a lack of accurate nuclear data. For this measurement, we used a sample of cerium oxide enriched in $^{140}$Ce to 99.4%. The experimental apparatus consisted of four deuterated benzene liquid scintillator detectors, which allowed us to overcome the difficulties present in the previous measurements, thanks to their very low neutron sensitivity. The accurate analysis of the p-wave resonances and the calculation of their average parameters are fundamental to improve the evaluation of the $^{140}$Ce Maxwellian-averaged cross-section

First Results of the 140^{140}Ce(n,γ)141^{141}Ce Cross-Section Measurement at n_TOF

An accurate measurement of the $^{140}$Ce(n,γ) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance in stars. The measurement was motivated by the significant difference between the cerium abundance measured in globular clusters and the value predicted by theoretical stellar models. This discrepancy can be ascribed to an overestimation of the $^{140}$Ce capture cross-section due to a lack of accurate nuclear data. For this measurement, we used a sample of cerium oxide enriched in $^{140}$Ce to 99.4%. The experimental apparatus consisted of four deuterated benzene liquid scintillator detectors, which allowed us to overcome the difficulties present in the previous measurements, thanks to their very low neutron sensitivity. The accurate analysis of the p-wave resonances and the calculation of their average parameters are fundamental to improve the evaluation of the $^{140}$Ce Maxwellian-averaged cross-section

Genetic mechanisms of critical illness in COVID-19.

Host-mediated lung inflammation is present1, and drives mortality2, in the critical illness caused by coronavirus disease 2019 (COVID-19). Host genetic variants associated with critical illness may identify mechanistic targets for therapeutic development3. Here we report the results of the GenOMICC (Genetics Of Mortality In Critical Care) genome-wide association study in 2,244 critically ill patients with COVID-19 from 208 UK intensive care units. We have identified and replicated the following new genome-wide significant associations: on chromosome 12q24.13 (rs10735079, P = 1.65 × 10-8) in a gene cluster that encodes antiviral restriction enzyme activators (OAS1, OAS2 and OAS3); on chromosome 19p13.2 (rs74956615, P = 2.3 × 10-8) near the gene that encodes tyrosine kinase 2 (TYK2); on chromosome 19p13.3 (rs2109069, P = 3.98 ×  10-12) within the gene that encodes dipeptidyl peptidase 9 (DPP9); and on chromosome 21q22.1 (rs2236757, P = 4.99 × 10-8) in the interferon receptor gene IFNAR2. We identified potential targets for repurposing of licensed medications: using Mendelian randomization, we found evidence that low expression of IFNAR2, or high expression of TYK2, are associated with life-threatening disease; and transcriptome-wide association in lung tissue revealed that high expression of the monocyte-macrophage chemotactic receptor CCR2 is associated with severe COVID-19. Our results identify robust genetic signals relating to key host antiviral defence mechanisms and mediators of inflammatory organ damage in COVID-19. Both mechanisms may be amenable to targeted treatment with existing drugs. However, large-scale randomized clinical trials will be essential before any change to clinical practice

S-2-hydroxyglutarate regulates CD8+ T-lymphocyte fate.

R-2-hydroxyglutarate accumulates to millimolar levels in cancer cells with gain-of-function isocitrate dehydrogenase 1/2 mutations. These levels of R-2-hydroxyglutarate affect 2-oxoglutarate-dependent dioxygenases. Both metabolite enantiomers, R- and S-2-hydroxyglutarate, are detectible in healthy individuals, yet their physiological function remains elusive. Here we show that 2-hydroxyglutarate accumulates in mouse CD8+ T cells in response to T-cell receptor triggering, and accumulates to millimolar levels in physiological oxygen conditions through a hypoxia-inducible factor 1-alpha (HIF-1α)-dependent mechanism. S-2-hydroxyglutarate predominates over R-2-hydroxyglutarate in activated T cells, and we demonstrate alterations in markers of CD8+ T-cell differentiation in response to this metabolite. Modulation of histone and DNA demethylation, as well as HIF-1α stability, mediate these effects. S-2-hydroxyglutarate treatment greatly enhances the in vivo proliferation, persistence and anti-tumour capacity of adoptively transferred CD8+ T cells. Thus, S-2-hydroxyglutarate acts as an immunometabolite that links environmental context, through a metabolic-epigenetic axis, to immune fate and function