Fission Barrier of neutron rich Ra isotopes for r-process investigation

Project Information

FiBRa

Grant agreement ID: 101025651

DOI

10.3030/101025651

Project terminated on 28 February 2023

EC signature date 10 March 2021

Start date 1 April 2021

End date 31 March 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 166 320,00

EU contribution

€ 166 320,00

Coordinated by

KATHOLIEKE UNIVERSITEIT LEUVEN
Belgium

Periodic Reporting for period 1 - FiBRa (Fission Barrier of neutron rich Ra isotopes for r-process investigation)

Reporting period: 2021-04-01 to 2023-03-31

Summary of the context and overall objectives of the project

The FiBRa project aims at directly measuring the Fission Barrier height along the neutron-rich side of the Radium Isotopic chains.
The action moves from the fact that the region between Thallium and Radium is a poorly-known region,
which now can be accessed by exploiting the radioactive ion beam accelerated at the HIE-ISOLDE facility (CERN).
Such a region, and in particular the neutron-rich side of these elements,
can be a good bench test for the fission barrier models where they tend to return conflicting fission barrier height.
Indeed, to date, fission barrier models have been mainly developed
mainly testing them in the vicinity of the valley of stability and on the neutron-deficient side of the nuclide chart.
The action aims at performing transfer-induced fission experiments to shed light on such a region,
using radioactive ion beams from HIE-ISOLDE and the active target detector ACTAR TPC as a detection device.

Eventually, the determination of the goodness of actual fission barrier models can also shed light on the astrophysical location of the r-process.
The r-process is one of the two main nucleosynthesis mechanics for the production of elements heavier than iron.
Despite the main reaction mechanism being well established, its astrophysical location is not clear.
To date, the main two candidate sites are the Neutron Star Merger and Core Collapse SuperNovae.
In the former, which is the favorite site of the astrophysical community, the fission of nuclei around mass 230 is relevant,
the fission barrier models thus are crucial. As a consequence, new experimental data that can stress the present fission barrier models
are needed.

The objectives of the FiiBRa action can be summarized as:
1) Measurement of the Fission Barrier height in the Tl-Ra region of the nuclide chart, mainly along the neutron-rich side of the Ra isotopic chain
2) Comparison with the state-of-the-art Fission Barrier models to add new constraints that could be relevant for the astrophysical location of the r-process site
3) Eventually, the FiBRa action aims at offering a challenging learning process for the researcher, that will work in a frontier physics case and with advanced nuclear detectors, developing and boosting the researcher's career in order to provide a solid base for his future in the academic sector.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Work was conducted within 5 working packages (WP).
Two WPs (WP1 and WP2) are related to the experimental campaign at HIE-ISOLDE. WP1 covered the main training part of the project through the experimental campaign on the Fr ions (approved by the INTC Committee, IS581), while WP2 was expected to happen following WP1, containing the experimental campaign along the Radium isotopic chain. WP3, WP4, and WP5, which run in parallel to the two previous ones, deal with the dissemination of the project results, outreach activities, and the monitoring of the whole action, respectively.

The fellowship was impacted by the decision of the Actar Collaboration not to use the ACTAR TPC detector as originally planned in the IS581 proposal.
The collaboration decided to adopt as a detection device the ACTAR TPC demonstrator,
a device built during the R&D phase of the main ACTAR TPC detector, 8 times smaller than the main active target, and without the mechanical and technical features.
As a consequence, within WP1, the action moved on to more technical aspects, preparing the detector in view of the IS581 experiment, designing and building all the technical equipment needed to
perform the Fr campaign, as the gas delivery and maintenance system to be used at HIE-ISOLDE. The first part of the IS581 experiment was conducted on November 2021. However, due to the intrinsic limitations of the ACTAR demonstrator, it was not possible to acquire satisfactory data from the point of view of the active target: however, four neutron detectors were placed in the vicinity of the gas detector and used to measure the neutron distribution to deduce the fission barrier height. Following this first part, all the efforts of the researcher focused on the upgrade of the ACTAR demonstrator detector, and on the study of its response using Geant4 simulation and previously acquired data. In particular, the second part of the IS581 experiment is expected to take place by the end of 2023. The submission of the Ra proposal was postponed, in accordance with the supervisor, to the end of the IS581 experiment.

In general, the fellowship delivered: 3 presentations at international workshops organized within the EVEREST and MANALSU collaborations,
3 seminars (2 at KU Leuven, 1 at University of Florence),
1 Master thesis, with the researcher as co-supervisor, at KU Leuven with the title "Study of 209Fr transfer-induced fission in inverse kinematics with the ACTAR demonstrator",
a forthcoming paper on Physical Review C titled "Determination of fission barrier height of 210Fr via neutron measurement",
a forthcoming paper on Nuclear Instruments and Method A titled "Energy loss profile measurements using the Actar demonstrator active target".
a forthcoming contribution on "INFN-LNF Nuclear Physics Mid-term Plan"
As outreach FiBRa delivered: 1 contribution to the Falling Wall Lab - Marie Curie 2021, and 1 contribution to the ISOLDE newsletter.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The FiBRa action faced the possibility to perform transfer-induced fission experiments, in inverse kinematics,
using an active target as a detection device. This kind of measurement was only attempted once at GANIL laboratories using a cyclotron beam and the MAYA detector.
From the detector point of view, the technical issues to deliver a bunched heavy beam, with an instantaneous current of up to 10E9 pps (particle-per-second), into a gaseous detector were highlighted during the first Fr experiment. To our knowledge, this was the first time that such an approach was attempted.
Mitigation options were designed, built, and tested in August 2022 using a He beam delivered (10E12 pps)
by the Tandem accelerator available at the IMBL KU Leuven laboratory. A beam mask is now installed in the Actar demonstrator in view of the second part of the Fr campaign.

Exploiting the neutron detectors during the first part of the IS581 experiment,
it was possible to have an (indirect) estimation of the fission barrier height of the 210Fr.
By means of simulation employing a Talys calculation, a reduction of about 10-30% of the standard fission barriers was suggested,
with respect to the model present in literature (e.g. Sierk model).

On the detector side, the energy losses in the active target of the different beams were tested.
It was shown that an active target can be used as a good device to improve the actual knowledge of energy loss and stopping power,
since such detectors offer the unique possibility to measure energies below 10keV, well below the minimum energy available in the literature,

On the personal fellow side, the FiBRa action deeply developed the expertise of the researcher on active target detectors and transfer reaction physics.
As a matter of fact, he was contacted by the INFN to join as a main contributor to the "Nuclear Physics Mid Term Plan at LNF”,
reporting on the status and possibility of the active target detectors. Moreover, thanks to the expertise obtained through the fellow,
he obtained a tenure track position at the University of Florence (started on March 2023, after the early termination of the FiBRa project).

Actar demonstrator installation on November 2021 (IS581)