NET achievements

SUPERHEATED LIQUID DETECTORS

The superheated liquid detectors developed by our group were upgraded for the IV^th phase of the SIMPLE dark matter search, and exploited towards novel applications in alpha particle detection. Developments of the SIMPLE bubble chamber aiming at a 10³ sensitivity improvement comprise a new hydrophone-based acoustic instrumentation and an integrated frequency-based analysis capable of discriminating signal events from acoustic background. The alpha-droplet interaction within superheated droplet detectors (SDD) was examined via computational studies, and a geometric model developed to describe the detector response. Experiments with U and Sm-doped SDDs at 5-12^oC evidenced a dependence of the event rate with the droplet sizes, in model agreement for temperatures below 8^oC. Above higher temperature, the increasing bubble population yields an acoustic sensitivity loss that, once corrected by attenuation coefficient, restored the agreement with experiment. The results suggest the viability of a SDD-based alpha spectrometer using mono-sized droplets.

DETAILED STUDIES OF AMPHOTERIC MG-DOPANT IN GAN

Mg-doped p-type GaN is a core component of solid state white lighting or blue lasers optoelectronic devices. Despite the technological maturity of GaN, there is an inherent doping limit since above a certain concentration of substitutional Mg introduced during growth, it becomes interstitial. Using the b-emission channeling radioactive technique, we obtained direct evidence for the amphoteric nature of Mg [U.Wahl et al. PRL 118 (2017) 095501]. Recent detailed studies showed that above 400°C interstitial ²⁷Mg progressively converts to substitutional Ga sites in all GaN doping types with migration activation energy estimated between 1.3 and 2.1 eV. These studies performed at the CTN EC-SLI unique setup mounted at the ISOLDE beam line provide a breakthrough understanding of the Mg behaviour in GaN suggesting that ion implantation and annealing can achieve higher p-type doping levels, as required for high power electronics. Arrhenius curves plotted as a function of temperature, for interstitial ²⁷Mg becoming substitutional at Ga sites in GaN for differently doped samples: top-left n-GaN:Si, top-right GaN (undoped), bottom-left GaN:Mg (as grown), bottom-right p-GaN:Mg.

NUCLEAR HYPERFINE INTERACTIONS ON HG HALIDES

Where early experiments (Berkeley, Bonn) in the 1970s had failed, we have successfully made the first Perturbed Angular Correlations, PAC, nuclear hyperfine interactions experiments on the 5/2+ ¹⁹⁷Hg state in free Hg-halide molecules. The use of linear molecules (HgX₂, X = I, Cl, Br) allow very precise measurements of the quadrupole moments, Q, of Hg isotopes, which are currently applied in numerous material and soft matter/bio physics experiments tracking heavy metals, binding and dynamics. Such experiments also provide precise Q-values used in the refinement of nuclear fundamental theories. The data are still requiring final fitting including centrifugal stretching, vibrational excitation, quantum effects and nuclear recoil to compare with quantum chemistry calculations for Hg halides.

X-RAY AND NEUTRON-BASED NON-INVASIVE ANALYSIS OF STONE ARTEFACTS AS A CONTRIBUTION TO UNDERSTAND MOBILITY AND INTERACTION NETWORKS IN PRE-HISTORY OF SOUTHERN IBERIAN PENINSULA

Stone idols and vases from Perdigões Chalcolithic site (Portugal) were analysed. The investigation had interdisciplinary character based on physical-chemical and archaeological knowledge. These carbonate-rich archaeological artefacts are difficult to correlate with raw materials of such heterogeneous geological sources, especially when only non-invasive analysis is possible. A novel combination of X-ray and neutron-based non-invasive analysis was implemented and used for the first time to study prehistoric stone idols and vessels, comprising prompt gamma activation analysis (PGAA) and external beam particle-induced x-ray emission for chemical composition and neutron radiography to provide structure and shape diversity. Results, especially of PGAA, were capable to distinguish between marble/limestone samples originating from different known geological sources, and matching artefacts to sources. The stone artefacts show signs of both nearby and long-distance procurement, and of unknown attribution. Different raw material provenances seem to be associated with different contexts and rituals, deepening the contrasts between funerary features in Perdigões site.

CHARACTERISATION OF AD-ATOMS AND AD-MOLECULES ON GRAPHENE

Achieving n-, p-type stable doping of graphene is being investigated for different metal dopants, particularly for Hg single atoms and the (easily made) stable linear HgO₂ molecule. Several configurations of freestanding Hg and HgO₂ molecules with respect to the graphene surface were analysed with Density Functional Theory (DFT). In parallel Perturbed Angular Correlation experiments were performed with ^199mHg obtaining the experimental hyperfine fields characterising a specific Hg–... HgO₂ ad-atom or ad-molecule configuration on the graphene layer. Among a multitude of possible configurations, the free O-Hg-O dumbbell lying in-plane with the Hg atom above the hollow site (in the centre of the C-hexagon) with oxygen bonds aligned along the C-C bonds, uniquely fit the PAC experimental results.

DEVELOPMENT OF SOFTWARE FOR X-RAY ANALYTICAL METHODS AND QUANTITATIVE INSTRUMENTAL SPECIATION METHODS FOR PARTICLE INDUCED X-RAYS EMISSION (PIXE)

L3 yields of Yb measured during Oxygen beam irradiations showed differences between O²⁺ and O³⁺ beam irradiations, for the same energy, after taking into account the collecting charge for each single spectrum, corrected to the ion charge. As shown below, at 3.0 MeV, the values obtained for O²⁺ are nearly twice the values obtained for O³⁺.

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RESEARCH ACHIEVEMENTS

RESEARCH ACHIEVEMENTS