Research

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Study of core-collapse supernovae

Picture of Cassiopeia A : a core-collapse supernovae remanent, with a neutron star located in its center. Image Credit: NASA, CXC, SAO.

• Project summary:

Core-collapse supernova (CCSN) simulations are extremely sensitive to microphysics ingredients (nuclear and neutrino physics). The two main nuclear physics ingredients of these simulations are the equation of state and the electro-weak interaction rates (beta decay and electron capture).
The electron-capture process in particular governs the neutralisation of matter and determines the position of the formation of the shock wave, which ultimately leads to the explosion. The electron-capture rates depend in an important way on the composition of matter. The latter can be calculated using extended Nuclear Statistical Equilibrium (NSE) models. The NSE models depend in a crucial way on the masses of the different nuclei and are almost insensitive on the other nuclear and astrophysical inputs. In summary experiments dedicated to measure and to improve the masses and the electron-capture rates are mandatory for the study of core-collapse supernovae.
Last sensitivity studies identified the nuclei whose electron-capture rate play the most important role during the collapse phase. These nuclei are located around ⁷⁸Ni and ¹²⁸Pd, thus around shell closures (N=50 and N=82 respectively).
In November 2017, new mass measurements of neutron-rich nuclei located arounf 78Ni was realised with the JYFLTRAP Penning trap mass spectrometer at the IGISOL facility. The masses of these isotopes are relevant for the study of core-collapse supernovae as well as for nuclear structure, as the nuclei are located close to the closed Z=28 and N=50 shells. In order to produce the isotopes of interest, 10 mA, 30-MeV protons on an uranium target and a fission ion guide will be employed. The mass measurements are based mainly on the time-of-flight ion cyclotron resonance technique.

• Experimental work related to the study of core-collapse supernovae:

Conception, preparation, realization and data analysis supervision of the I220 experiment at JYFL (Jyvaskyla, Finland) on « Mass measurements in the vicinity of 78Ni to constraint core-collapse supernovae models and to study the N=50 and Z=28 shell closures evolution near the neutron dripline. », realized from 06/11/17 to 13/11/17.
Spokespersons: (experiment) : B. Bastin (GANIL) and A. Kankainen (JYFL) / (theory) : A. Fantina (GANIL) and F. Gulminelli (LPC Caen)
Collaboration: GANIL (FR), University of Jyvaskyla (FI), LPC Caen (FR), CENBG (FR) and IFIN-HH (RO)

General scheme of the I220 experiment carried out at JYFL IGISOL facility (Jyvaskyla, Finland): high-precision mass measurements of neutron-rich nuclei located around 78Ni were realized using a double-Penning trap system. Ions of interest were produced from the fission of a Uranium target induced by a proton beam of 35MeV, in a gas cell via the IGISOL technique.

• Publications related to this work:

Ph.D. thesis of Simon Giraud « Study of core-collapse supernovae », defense expected October 2019.

Study of p-nuclei

• Project summary:

Most of the observed heavy nuclei (Z > 26) have been produced in neutron-induced nucleosynthesis processes, like the slow neutron capture (s-process) and rapid neutron capture (r-process). Nevertheless, about 35 nuclei are observed in nature on the neutron-decient side of the valley of stability, from 74Se to 196Hg, which are shielded from production by n-capture processes. The nucleosynthesis process corresponding to the production of the so-called p-nuclei in stars is referred to as p-process and its nature is still under debate.
From the experimental observations (mainly meteoritic data from Anders and Grevesse) it appears that the p-nuclei constitute a small but non negligible fraction of the total abundance. The predicted abundances are, in most of the cases, in agreement within a factor 3 with the observed ones. However, there are still significant discrepancies for isotopes of Mo, Ru, Sn, La and Gd, which are systematically underproduced with respect to solar abundances.
According to the current picture, p-nuclei are supposed to be synthesized in the supernova shock front which results in a rapid increase of the temperature. The associated intense photon flux induces (gamma; n) reactions shifting the seed abundances toward the proton-rich side, until the process becomes less efficient. At this point, (gamma; p) and (gamma; alpha) reactions maintain the flow. Finally, after the shock, the temperature decreases exponentially and the proton-rich nuclei decay back to stability. (p, gamma) reactions can also contribute depending on the temperature and the proton density.
For what concerns the astrophysical production sites, the most favoured scenario considers O-Ne layers of massive stars (around 25M) during explosive or pre-explosive processes of Supernovae type II (SNII, 1.7 < T9 < 3.3). Other possible scenarios include explosive processes in Supernovae type Ia (SNIa – dwarf-giant binary systems, 1.5 < T9 < 3.7) and neutrino-driven wind from core-collapse Supernovae (p-process, 1 < T9 < 3).
We have recently measured experimentally the cross section of one of these reactions, the 72Ge(p, gamma)73As reaction, in order to put constraint on the destruction channel of the lightest p-nuclei (74Se). This cross section value is estimated with the Hauser-Feshbach statistical model and depends a lot on the input parameters used for the calculation (optical potential, level density and g-strength function). An experimental point would help to define the best set of parameters to use for the calculation.
The activation method was used. End of 2017, the irradiation of enriched 70,72,73Ge targets (>95%) with a proton beam of 2.5 MeV has been performed at the 3MV tandetron facility, located at the IFIN-HH Institute (Bucharest, Romania). The decay study was then realized in a very low background environment at the underground Slanic Salt Mine laboratory. Some discrepancies have been observed with values available in the litterature. The comparison with results from Hauser-Feshbach statistical model is ongoing.

• Experimental work related to the study of p-nuclei:

• Publications related to this work:

– Master thesis of L. Al Ayoubi « Study of the 72Ge(p,gamma)73As reaction involved in the nucleosynthesis of p-nuclei », defended September 2018.
– Master thesis of J. Balsamelli « Étude des processus de nucléosynthèse stellaire dans les étoiles massives », defended June 2015.

Study of classical novae

Artistic view of a classical nova. Credit: David Hardy, www.astroart.org.

• Project summary:

• Experimental work related to the study of classical novae:

• Publications related to this work:

– « Search for a new broad resonance in ¹⁹Ne relevant for the study of novae stellar explosions. », F. Boulay, B. Bastin, F. de Oliveira Santos, T. Davinson, A. Lemasson, J.C. Dalouzy, P. Ujic, J. Mrazek, A. M. Sanchez−Benitez, E. Traykov, D. Ramos et al., Physical Review Letters in preparation.
– « Measurement of ¹⁹Ne spectroscopic properties via a new method of inelastic scattering to study novae. », F. Boulay, B. Bastin, F. de Oliveira Santos, T. Davinson, A. Lemasson, P. Ujic, J. Mrazek, A. M. Sánchez-Benítez, E. Traykov, D. Ramos et al., Journal of Physics 940 (2018) 012003.
– Ph.D. thesis of Florent Boulay « Deformation and mixing of co-existing shapes in the neutron-deficient polonium isotopes », defended March 2015. Available online.

Shape coexistence in neutron-deficient Polonium isotopes

• Project summary:

In the region around Z=82 shell closure with neutron number around the midshell between N=82 and N=126, shape coexistence occurs at low excitation energy. This phenomenon is well-established in the neutron-decient polonium isotopes. The proton-pair excitations across the magic Z=82 along with the strong proton-neutron interaction in the vicinity of the neutron midshell are considered as a driving mechanism for shape coexistence in this region. The strong perturbation of the level systematics in the very light Po isotopes is also interpreted as arising from the interaction between regular and intruder structures.

• Experimental work related to the study of the evolution of shape coexistence in neutron-deficient Polonium isotopes:

Conception, preparation, realization and partial analysis and student supervision of the IS479 experiment at CERN-ISOLDE (Geneva, Swizerland) on« Shape coexistence measurements in even-even neutron-deficient Polonium isotopes by Coulomb excitation using REX-ISOLDE and the Ge MINIBALL array », realized from 07/09/09 to 09/09/09 and from 13/09/12 to 19/09/12
Spokesperson: B. Bastin (KU Leuven).
Collaboration: KU Leuven (BE), SCK•CEN (BE), GANIL (FR), University of Liverpool (UK), University of Warsaw (PL), University of Jyväskylä (FI), Helsinki Institute of Physics (FI), Technische Universität Darmstadt (DE), CENBG (FR), Technische Universität München (DE), Universität zu Köln (DE), CERN-ISOLDE (CH), The University of Manchester (UK), Université de Genève(CH), University of Lund (SE), Universidad de Huelva (ES), Université Libre de Bruxelles (BE), Ghent University (BE), CEA-IRFU (FR), Ludwig-Maximilians-Universität-München (DE), University of the West of Scotland (UK), SUPA (UK) and The University of York (UK).

• Publications related to this work:

– « Deformation and mixing of coexisting shapes in neutron−deficient polonium isotopes. », N. Kesteloot, B. Bastin, L.P. Gaffney, K. Wrzosek−Lipska et al., Physical Review C 92 (2015) 054301.
– Ph.D. thesis of Nele Kesteloot « Deformation and mixing of co-existing shapes in the neutron-deficient polonium isotopes », defended March 2015. Available online.
– Master thesis of Nele Kesteloot « Coulomb excitation of 200Po studied at REX-ISOLDE with the miniball gamma spectrometer « , defended 2010. Available online.

Evolution of the N=28 shell closure far from stability

• Project summary:

For a few years now, a loss of magicity in neutron-rich nuclei near the neutron drip-line at N=28 has been suggested and observed. Deformation in these nuclei has been observed. The deformation was explained in S isotopes as being due to a moderate reduction of the N=28 shell closure together with a proton induced collectivity originating from the near degeneracy of the proton d3/2 and s1/2 orbitals. As a consequence, the observed deformation seems to result from a subtle interplay between neutron and proton excitations. Since the proton configuration in the Si isotopes is expected to be more stable due to the Z=14 sub-shell gap, 42Si was considered as a key nucleus in order to distinguish the different effects responsible for the structural changes observed at N=28. Even if it is at the limits of our technical possibilities, an in-beam gamma-spectroscopy experiment using two-step fragmentation and one or several nucleons knockout reaction mechanisms was performed at GANIL. The measurement of the energy of the first excited state in 42Si, combined with the observation of 38,40Si and the spectroscopy of 41,43P, has given evidence for the loss of magicity at N=28 far from stability. Modifications of the effective interaction used in modern shell model calculations have been completed following this investigation, increasing its predictive character. This study conforms the role of the tensor force and the density dependence of the spin-orbit interaction in the collapse of the N=28 shell closure.

• Work from my Ph.D. thesis (2004-2007) 🔗

Preparation, realization and analysis of the e287c experiment at GANIL (Caen, France) on“Evolution of the N=28 shell gap in the Si isotopes”, realized from 10/11/04 to 16/12/04.
Spokesperson: S. Grévy (GANIL).
Collaboration: GANIL (FR), LPC Caen (FR), Institute of Nuclear Research (HU), IPN Orsay), Nuclear Physics Institute (CZ), IFIN-HH (RO), CEA-SPhN (FR), Universität Bonn (DE), University of Paisley (UK), FLNR (RU), University of Surrey (UK), IReS and Universidad Autónoma de Madrid (ES).

General scheme of the e287c experiment carried out at the GANIL facility (Caen, France): the fragmentation of a primary beam of 48Ca on a thick C/Ta target produced a radioactive beam of 44S. This secondary beam impigned on a Be target, producing in-flight a set of nuclei including the 42Si nucleus of interest. Prompt gamma-ray emission from 42Si was detected around the Be reaction target using the « Chateau de Cristal » multi-detectors setup (74 BaF2 scintillators). 42Si nuclei identification was realized via the SPEG spectrometer.

• Publications related to this work:

– Ph.D thesis « Study of neutron-rich nuclei structure around the N=28 shell closure using the in-beam gamma spectroscopy technique », defended October 5th 2007. Available online.
– « Collapse of the N=28 shell closure in 42Si. » B. Bastin, S. Grévy, D. Sohler, O. Sorlin, Zs. Dombrádi et al., Physical Review Letters 99 (2007) 022503.
– « Spectroscopy of 39,41Si and the border of the N = 28 island of inversion. » D. Sohler, S. Grévy*, Zs. Dombrádi, O. Sorlin, L. Gaudefroy, B. Bastin et al. Physics Letters B 703 (2011) 417.
– « In-beam spectroscopic studies of 44S nucleus. » L. Caceres, D. Sohler, S. Grévy*, O. Sorlin, Zs. Dombradi, B. Bastin et al., Physical Review C 85 (2012) 024311.