Junior Researcher Webinars 2022 (past)

Monday, 30/05 (16h, CET)
Measurements of 12C fusion at deep sup-barrier energies with STELLA
Jean NIPPERT (PhD Student at IPHC-Strasbourg)

Carbon burning is an important phase of the massive stars evolution. It occurs mainly through the 12C + 12C fusion reaction. This reaction is also very specific in terms of nuclear physics due to the identification of oscillation in the cross section at energies above and below the Coulomb barrier. The energy region of astrophysical interest is located deeply below the Coulomb barrier, where the cross section is dropping down to the picobarn range, making its measurement extremely challenging. The STELLA apparatus has been developed to measure such deep sub-barrier cross sections using gamma-particle coincidences.

The context of the 12C + 12C fusion reaction will be discussed in this presentation, as well as how the STELLA experiment aims to tackle the challenge of the 12C + 12C cross section measurement. In this presentation, we shall discuss the possible occurrence of fusion hindrance in 12C + 12C at deep sub-barrier energies, as observed in most systems. This phenomenon may change our understanding of massive stars evolution. 

Monday, 09/05 (16h, CET)
Experimental Study of the 30Si(p,γ)31P for understanding elemental anomalies in Globular Clusters.
Sarah HARROUZ (PhD Student at IPN-Orsay)

In collaboration with: N. de Séréville, P. Adsley, F. Hammache, R. Longland, B. Bastin, T. Faestermann, R.Hertenberger, M. La Cognata, L. Lamia, A. Meyer, S. Palmerini, R. G. Pizzone, S. Romano, A. Tumino, and H.-F. Wirth.

Globular clusters are key grounds for models of stellar evolution and early stages of the formation of galaxies. Abundance anomalies observed in the globular cluster NGC 2419, such as the enhancement of potassium and depletion of magnesium [1] can be explained in terms of an earlier generation of stars polluting the presently observed stars [2]. However, the nature and the properties of the polluting sites are still debated. The range of temperatures and densities of the polluting sites depends on the strength of a number of critical thermonuclear reaction rates. The 30Si(p,γ)31P reaction is one of the few reactions that have been identified to have an influence for elucidating the nature of polluting sites in NGC 2419 [3]. The current uncertainty on the 30Si(p,γ)31P reaction rate has a strong impact on the range of possible temperatures and densities of the polluter sites.

Hence, we investigated the 30Si(p,γ)31P reaction with the aim to reduce the associated uncertain- ties by determining the strength of resonances of astrophysical interest. In this talk I will present the study of the reaction 30Si(p,γ)31P that we performed via the one proton 30Si(3He,d)31P transfer reaction at the Maier-Leinbnitz-Laboratorium Tandem. With the high resolution Q3D magnetic spectrograph, we measured the angular distributions of the light reaction products. These angular distributions are interpreted in the DWBA (Distorted Wave Born Approximation) framework to determine the proton spectroscopic factor information needed to determine the proton partial width of the states of interest. This information was used to calculate the 30Si(p,γ)31P reaction rate. The uncertainties on the reaction rate have been significantly reduced and key remaining uncertainties have been identified.

[1] C. Iliadis et al., The Astrophysical Journal, vol. 470, p. 98, Feb. 2016.
[2] R. G. Gratton et al., The Astronomy and Astrophysics Review, vol. 20, p. 50, Feb. 2012.
[3] J. R. Dermigny and C. Iliadis, The Astrophysical Journal, vol. 848, p. 14, Oct. 2017.

Tuesday, 19/04 (16h, CET)
Investigation of isomeric states in 255Rf
Rikel CHAKMA (PostDoc at GANIL-Caen)

Heavy and super-heavy nuclei are of general interest to study the tug of war between electromagnetic and nuclear forces. Determining the nature and sequence of states in these nuclei is crucial in understanding the different interactions and the dynamics at play, but yet remains challenging both theoretically and experimentally. A viable experimental approach is to search for and study low-lying metastable states since their decay properties depend on the available orbitals near the Fermi surface. Around the N = 152 single-particle gap, isomeric states in 254Rf[1], 256Rf[2, 3] and 257Rf[4] have been studied in detail. Thus, it seemed logical to look for isomeric states in 255Rf. In this isotope, two high-K isomers at excitation energies around 0.9-1.45 MeV, spins ≥ 17/2h (suggested tentatively) and half-lives T = 38+12μs and T = 15+6μs have been observed[5]. Another isomer around 135 keV 1/2 −7 1/2 −4 with a half-life of 50 ± 17μs has also been observed in an alpha decay study of 259Sg to 255Rf and was inferred to be a spin isomer from the systematic of N = 151 isotones[6]. We have carried out an ex- periment to populate isomeric states of 255Rf by the fusion-evaporation reaction 50Ti(207Pb,2n)255Rf. The beam 50Ti of 300 pnA was delivered by the U400 accelerator at FLNR, Dubna. The evaporation residues were separated from the background using SHELS[7], which were then implanted into the focal plane detector of the GABRIELA multidetector array[8, 9]. Using GABRIELA, it is possible to make time and position correlations between the implanted nuclei and their subsequent decays. Experimental decay spectra were interpreted using Geant4[10] simulations. New results from the Geant4 assisted spectroscopy on the isomeric decay properties of 255Rf will be discussed.

References :
[1] H. M. David et al., PRL 115, 132502 (2015)
[2] H. B. Jeppesen et al. Phys. Rev. C 79, 031303(R) (2009)
[3] A. P. Robinson et al. Phys. Rev. C 83, 064311 (2011)
[4] J. Rissanen et al. Phys. Rev. C 88, 044313, (2013)
[5] P. Mosat et. al., Phys. Rev. C, 101, 034310, (2020)
[6] S. Antalic et al., Eur. Phys. J. A, 51, 41 (2015)
[7] A.G. Popeko et al., Nucl. Instr. and Meth. B 376, 140-143, (2016)
[8] R. Chakma et al., Eur. Phys. J. A 56, 245,(2020)
[9] K. Hauschild et al., Nucl. Instr. and Meth. A 560, 388 (2006)
[10] S. Agostinelli et al., Nucl. Instr. Meth. A 506, 250-303,(2003)

Monday, 21/03 (16h, CET)
Tidal Disruption Events: what can we learn from them?
Martina TOSCANI (PostDoc at L2IT-Toulouse)

Tidal Disruption Events (TDEs) are extremely powerful phenomena that take place when a star, passing by a massive black hole (BH), is stripped away due to the tides induced by the BH. After the disruption, roughly half of the star is expected to circularize around the BH and form an accretion disc.

These events are a treasure trove of astrophysical information, that can be extracted (i) from their electromagnetic radiation produced during the disruption and the accretion process, (ii) from the astrophysical neutrinos generated within the circularization phase and (iii) from gravitational waves (GWs) produced along the entire process.

In this talk, I will focus on the GW radiation from these events, in particular illustrating the results of Toscani et al. 2020 and Toscani et al. 2021. I will show how, thanks to the study of this emission, it is possible to extract information about the BH distribution through the universe, BH spin, stellar dynamics and, possibly, also the internal structure of the star.

Monday, 07/03 (16h, CET)
NeAb-initio description of the monopole resonance in light- and medium-mass nuclei∗
Andrea PORRO (PhD student at CEA-Saclay)

Giant monopole resonances have a long-standing theoretical importance in nuclear structure. The interest resides notably in the so-called breathing mode that has been established as a standard observable to constrain the nuclear incompressibility [1]. The Random Phase Approximation (RPA) within the frame of phenomenological Energy Density Functionals (EDF) has become the standard tool to address (monopole) giant resonances and extensive studies, mostly in doubly-closed-shell systems, have been performed throughout the years, including via the use of so-called sum rules [2]. A proper study of collective excitations in the ab-initio context is, however, missing.

In this perspective, the first systematic ab-initio predictions of (giant) monopole resonances will be presented [3]. Ab-initio Quasiparticle-RPA (QRPA) [4] and Projected Generator Coordinate Method (P-GCM) [5] calculations of monopole resonances are compared in light- and mid-mass closed- and open- shell nuclei, which allows in particular to investigate the role of superfluidity from an ab-initio standpoint. Sum rules are also employed within both many-body schemes to characterize the fragmentation of the monopole strength. The study further focuses on the dependence of the results on the starting nuclear Hamiltonian derived within the frame of chiral effective field theory.

Monopole resonance represents, thus, the first step towards the investigation of higher multipolarities. Eventually, the mid-term goal to establish P-GCM as a new method to study resonances in the light- and medium-mass region of the nuclide chart will be discussed: interpretation and analysis of resonance data in lighter nuclei is a very demanding task on which ab-initio P-GCM could shed new promising light.

[1]  J. P. Blaizot, D. Gogny, and B. Grammaticos, “Nuclear compressibility and monopole resonances,” Nuclear Physics A, vol. 265, pp. 315–336, July 1976.
[2]  O. Bohigas, A. Lane, and J. Martorell, “Sum rules for nuclear collective excitations,” Physics Reports, vol. 51, pp. 267–316, Apr. 1979.
[3]  A. Porro, M. Frosini, T. Duguet, V. Som ́a, Y. Beaujeault-Taudi`ere, J.-P. Ebran, and R. Roth To be published, 2021.
[4]  Y. Beaujeault-Taudi`ere and J.-P. Ebran To be published, 2021.
[5]  M. Frosini, T. Duguet, J.-P. Ebran, R. Roth, V. Som ́a, and A. Tichai, “Application of Projected Generator Coordinate Method to ab-initio description of Ne isotopes,” To be published, 2021.

Monday, 14/02 (16h, CET)
New results on the decay spectroscopy of 254No with GABRIELA@SHELS 
Margaux FORGE (PhD Student at IPHC – Strasbourg)

The structure of the 254No nucleus has been studied for more than 20 years: the last publications on the decay spectroscopy are from four experiments performed at LBNL [1], GSI [2], JYFL [3] and ANL [4]. Four decay schemes featuring two isomers have been published. These are interpreted as 2 and 4qp states. Unfortunately, while the authors agree on the excitation energy and decay scheme of the 2-qp ∼260 ms 8- isomer, they disagree on the configuration assignment for both the 8- and the shorter-lived ∼180 μs 4-qp isomer and differ considerably on the excitation energy and decay scheme of the 4qp isomer. These discrepancies have triggered new experiments worldwide, for example the combined prompt and decay spectroscopy performed using SAGE and GREAT [5]. We report here on an experiment performed with the GABRIELA [6] array, at the focal plane of the SHELS [7] separator at the FLNR, Dubna.

The 254No nucleus was produced using the cold fusion-evaporation reaction 48Ca(208Pb, 2n)254No. The first part of this talk will present the experimental setup and the analysis techniques used to reveal the electromagnetic decay of the known isomers in 254No. The second part will focus on the new results obtained. Due to the combination of a higher transmission of the separator (as compared to VASSILISSA [8]) and an increased efficiency of the upgraded GABRIELA array [9], more than 1 million 254No nuclei were implanted in the focal plane detector enabling the electromagnetic decay of the short and long-lived isomers to be studied in more detail. In particular, the internal conversion electron spectrum observed in decay of the 8- isomer has revealed the presence of a strong transition, possibly E0, suggesting low-lying shape coexistence in this nucleus as predicted in [10]. Finally, the gamma spectrum obtained from the decay of the short-lived isomer has revealed new peaks that will be attempted to be placed in the new decay scheme. The spectroscopic information extracted from alpha, gamma and electron correlations will be presented and discussed in terms of the likely underlying single-particle structure.

References :
[1] R.M Clark et al., Phys. Lett. B 690, 19 (2010).
[2] F.P. Hessberger et al., Eur. Phys. J. A 43, 55 (2010).
[3] R.-D. Herzberg et al., Nature 442, 896 (2006).
[4] S.K. Tandel et al., Phys. Rev. Lett. 97, 082502 (2006).
[5] A. Ward PhD, University of Liverpool (2016).
[6] K. Hauschild et al., Nucl. Instr. Methods A 560, 388-394 (2006).
[7] A.G. Popeko et al., NIM B 376, 140-143 (2016).
[8] O.N. Malyshev et al., Nucl. Instrum. Methods Phys. Res. A 440, 86 (2000).
[9] R. Chakma et al., Eur. Phys. J. A 56, 245 (2020).
[10] J.-P. Delaroche et al., Nucl. Phys. A 771, 103-168 (2006).