Theoretical predictions as well as experiments performed at storage rings have shown that the lifetimes of β-radionuclides can change significantly as a function of the ionization state. In this paper we describe an innovative approach, based on the use of a compact plasma trap to emulate selected stellar-like conditions. It has been proposed within the PANDORA project (Plasmas for Astrophysics, Nuclear Decay Observation and Radiation for Archaeometry) with the aim to measure, for the first time in plasma, nuclear β-decay rates of radionuclides involved in nuclear-astrophysics processes. To achieve this task, a compact magnetic plasma trap has been designed to reach the needed plasma densities, temperatures, and charge-states distributions. A multi-diagnostic setup will monitor, on-line, the plasma parameters, which will be correlated with the decay rate of the radionuclides. The latter will be measured through the detection of the γ-rays emitted by the excited daughter nuclei following the β-decay. An array of 14 HPGe detectors placed around the trap will be used to detect the emitted γ-rays. For the first experimental campaign three isotopes, 176Lu, 134Cs, and 94Nb, were selected as possible physics cases. The newly designed plasma trap will also represent a tool of choice to measure the plasma opacities in a broad spectrum of plasma conditions, experimentally poorly known but that have a great impact on the energy transport and spectroscopic observations of many astrophysical objects. Status and perspectives of the project will be highlighted in the paper.

/pdf/a-novel-approach-to-b-decay-pandora-a-new-experimental-setup-1kn5wdd4.pdf

A Novel Approach to β-Decay: PANDORA, a New Experimental Setup for Future in-Plasma Measurements

Neutron captures produce the vast majority of abundances of elements heavier than iron in the Universe. Beyond the classical slow ( s) and rapid ( r) processes, there is observational evidence for neutron-capture processes that operate at neutron densities in between, at different distances from the valley of β stability. Here, we review the main properties of the s process within the general context of neutron-capture processes and the nuclear physics input required to investigate it. We describe massive stars and asymptotic giant branch stars as the s-process astrophysical sites and discuss the related physical uncertainties. We also present current observational evidence for the s process and beyond, which ranges from stellar spectroscopic observations to laboratory analysis of meteorites. 

The s Process and Beyond

The main aim of Plasmas for Astrophysics Nuclear Decays Observation and Radiation for Archaeometry (PANDORA) project is to build a compact and flexible magnetic plasma trap where plasma reaches a density n e ∼ 1011–1013 cm−3, and a temperature, in units of kT, kT e ∼ 0.1–30 keV in order to measure, for the first time, nuclear β-decay rates in stellar-like conditions. One of the most important aspects of an ECR Ion Source (ECRIS) is its magnetic system. In this paper, the numerical design of the PANDORA magnetic system is presented and validated by using the commercial simulators OPERA and CST Studio Suite, showing an excellent agreement between each other in terms of axial and radial field profiles. In conjunction to the magnetic system design, the overall injection system, including the microwave lines for plasma heating and the isotopes injection schemes with a focus on the developments of the oven for solid elements, has been conceived and will be discussed.

An Innovative Superconducting Magnetic Trap for Probing β-decay in Plasmas

In the frame of the PANDORA project, a new experimental approach aims at measuring in-plasma β-decay rate as a function of thermodynamical conditions of the environment, namely a laboratory magnetized plasma able to mimic some stellar-like conditions. The decay rates (expected to change dramatically as a function of the ionization state) will be measured as a function of the charge state distribution of the in-plasma ions. The new experimental approach aims at correlating the plasma environment and the decay rate. This can be performed by simultaneously identifying and discriminating—through an innovative multi-diagnostic system working synergically with a γ-ray detection system —the photons emitted by the plasma and γ-rays emitted after the isotope β-decay. In this study, the numerical simulations supporting the design of the γ-ray detector array, including a statistical significance study to check the feasibility of measuring the in-plasma decay rates, are presented. Geant4 simulations focused on the design of the array of γ-ray detectors and the evaluation of total efficiency depending on the detector type and the optimal displacement of detectors around the trap (including collimation systems and shielding). The simulation results showed that, due to technical limitations in the number of apertures that can be created in the magnetic trap, the best compromise is to use 14 HPGe (70% of relative efficiency) detectors surrounding the magnetic trap. The HPGe detectors were chosen for their excellent energy resolution (0.2% @ 1 MeV), since the harsh environment (the background is represented by the intense plasma self-emission) strongly affects the signal-to-background ratio. Once determined the total photopeak efficiency (0.1–0.2%), the sensitivity of the PANDORA experiment was checked in a “virtual experimental run,” by exploring the measurability of isotope decay rates for the first three physical cases of PANDORA: 176Lu, 134Cs and 94Nb. The preliminary results demonstrated the feasibility of the measurement in terms of the signal-to-background ratio and significance that it is possible to reach. The results indicated that experimental run durations could take from several days to 3 months, depending on the isotope under investigation, thus shedding new light on the role of weak interactions in stellar nucleosynthesis.

Design study of a HPGe detector array for β-decay investigation in laboratory ECR plasmas

We present the status of nucleosynthesis beyond Sr, using up-to-date nuclear inputs for both the slow (s-process) and rapid (r-process) scenarios of neutron captures. It is now widely accepted that at least a crucial part of the r-process distribution is linked to neutron star merger (NSM) events. However, so far, we have found only a single direct observation of such a link, the kilonova GW170817. Its fast evolution could not provide strict constraints on the nucleosynthesis details, and in any case, there remain uncertainties in the local r-process abundance patterns, which are independent of the specific astrophysical site, being rooted in nuclear physics. We, therefore, estimate the contributions from the r-process to solar system (S.S.) abundances by adopting the largely site-independent waiting-point concept through a superposition of neutron density components normalized to the r-abundance peaks. Nuclear physics inputs for such calculations are understood only for the trans-Fe nuclei; hence, we restrict our computations to the Sr–Pr region. We then estimate the s-process contributions to that atomic mass range from recent models of asymptotic giant branch stars, for which uncertainties are known to be dominated by nuclear effects. The outcomes from the two independent approaches are then critically analyzed. Despite the remaining problems from both sides, they reveal a surprisingly good agreement, with limited local discrepancies. These few cases are then discussed. New measurements in ionized plasmas are suggested as a source of improvement, with emphasis on β-decays from unstable Cs isotopes. For heavier nuclei, difficulties grow as r-process progenitors lie far off experimental reach and poorly known branchings affect s-processing. This primarily concerns nuclei that are significantly long-lived in the laboratory and have uncertain decay rates in stars, e.g., L u 176 and R e 187 . New measurements are urgently needed for them, too.

/pdf/production-of-solar-abundances-for-nuclei-beyond-sr-the-s-1nvypcdg.pdf

Production of solar abundances for nuclei beyond Sr: The s- and r-process perspectives

Presolar Grain Isotopic Ratios as Constraints to Nuclear and Stellar Parameters of Asymptotic Giant Branch Star Nucleosynthesis

The design of a microwave antenna sustaining a high-energy-content plasma in Electron Cyclotron Resonance Ion Sources (ECRISs) is, under many aspects, similar to the design of a conventional antenna but presenting also peculiarities because of the antenna lying in a cavity filled by an anisotropic plasma. The plasma chamber and microwave injection system design plays a critical role in the development of future ECRISs. In this paper, we present the numerical study of an unconventionally shaped plasma cavity, in which its geometry is inspired by the typical star-shaped ECR plasma, determined by the electrons trajectories as they move under the influence of the plasma-confining magnetic field. The cavity has been designed by using CST Studio Suite with the aim to maximize the on-axis electric field, thus increasing the wave-to-plasma absorption. As a second step, an innovative microwave injection system based on side-coupled slotted waveguides is presented. This new launching scheme allows an uniform power distribution inside the plasma cavity which could lead to an increase of ion source performances in terms of charge states and extracted currents when compared to the conventional axial microwave launch scheme. Finally, the use of both the “plasma-shaped” cavity and the microwave side coupled scheme could make the overall setup more compact.

Design and Analysis of Slotted Waveguide Antenna Radiating in a “Plasma-Shaped” Cavity of an ECR Ion Source

A quantitative analysis of X-ray emission from an electron cyclotron resonance (ECR) plasma was performed to probe the spatial properties of electrons having energy for effective ionisation. A series of measurements were taken by INFN-LNS and ATOMKI, capturing spatially and spectrally resolved X-ray maps as well as volumetric emissions from argon plasma. Comparing the former with model generated maps (involving space-resolved phenomenological electron energy distribution function and geometrical efficiency calculated using ray-tracing Monte Carlo (MC) routine) furnished information on structural aspects of the plasma. Similarly, fitting a model composed of bremsstrahlung and fluorescence to the volumetric X-ray spectrum provided valuable insight into the density and temperature of confined and lost electrons. The latter can be fed back to existing electron kinetics models for simulating more relevant energies, consequently improving theoretical X-ray maps and establishing the method as an excellent indirect diagnostic tool for warm electrons, required for both fundamental and applied research in ECR plasmas.

Probing Electron Properties in ECR Plasmas Using X-ray Bremsstrahlung and Fluorescence Emission

In the context of the INFN project PANDORA_Gr3 (Plasma for Astrophysics, Nuclear Decays Observation and Radiation for Archaeometry) and of multi-messenger astronomy, we propose a feasibility study for in-laboratory plasma's opacity investigation, in an environment resembling thermodynamic conditions typical of the ejecta of compact binary mergers containing at least a neutron star. We aim to advance knowledge on the physics of kilonovae, the electromagnetic transients following a merger, which are relevant for the study of the origin of heavy nuclei in the Universe produced via r-process nucleosynthesis. In this paper, we present preliminary results of numerical simulations for some physics cases considered in the light of a possible experimental setup for future in-laboratory opacity spectroscopic measurements.

In-plasma study of opacity relevant for compact binary ejecta

Recent models for evolved Low Mass Stars (with $M \lesssim 3M_\odot$), undergoing the AGB phase assume that magnetic flux-tube buoyancy drives the formation of $^{13}$C reservoirs in He-rich layers. We illustrate their crucial properties, showing how the low abundance of $^{13}$C generated below the convective envelope hampers the formation of primary $^{14}$N and the ensuing synthesis of intermediate-mass nuclei, like $^{19}$F and $^{22}$Ne. In the mentioned models, their production is therefore of a purely secondary nature. Shortage of primary $^{22}$Ne has also important effects in reducing the neutron density. Another property concerns AGB winds, which are likely to preserve C-rich subcomponents, isolated by magnetic tension, even when the envelope composition is O-rich. Conditions for the formation of C-rich compounds are therefore found in stages earlier than previously envisaged. These issues, together with the uncertainties related to several nuclear physics quantities, are discussed in the light of the isotopic admixtures of s-process elements in presolar SiC grains of stellar origin, which provide important and precise constraints to the otherwise uncertain parameters. By comparing nucleosynthesis results with measured SiC data, it is argued that such a detailed series of constraints indicates the need for new measurements of weak interaction rates in ionized plasmas, as well as of neutron-capture cross sections, especially near the N = 50 and N = 82 neutron magic numbers. Nontheless, the peculiarity of our models allows us to achieve fits to the presolar grain data of a quality so far never obtained in previously published attempts.

Presolar grain isotopic ratios as constraints to nuclear and stellar parameters of AGB nucleosynthesis

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