Beyond the Barrier: Transfer-Induced Fission with Radioactive Ion Beams at the ISOLDE Solenoidal Spectrometer
Doctoral thesis, 2026
The height of the fission barrier is a nuclear property that governs how readily a nucleus splits: it determines spontaneous-fission half-lives and fission probabilities, hence how likely fission is compared to other decay modes. In the astrophysical r-process, responsible for the synthesis of the heaviest elements found in Nature, successive neutron captures and beta-decays drive nuclei towards ever larger masses and charges until they become fissile. Fission then splits them into two lighter nuclei, which might resume capturing neutrons and participate in the r-process again.
The fission barrier of r-process nuclei determines at which mass and nuclear charge fission starts to limit nucleosynthesis. Relevant nuclei are, however, inaccessible to conventional fission measurements, which require the fissioning species as a target. Extending fission studies towards the neutron-rich region therefore needs a different experimental strategy. This thesis presents a novel approach to determine fission barriers in inverse kinematics.
Heavy radioactive ion beams provided by the ISOLDE facility at CERN impinge on a deuterated plastic target; the (d,p) reaction populates states in the fissioning nucleus above the barrier, and the barrier height is extracted from the fission probability measured as a function of excitation energy. Performing the measurement inside a solenoidal spectrometer optimises the excitation-energy resolution, mitigating the kinematic compression and kinematic shift that limit other inverse-kinematics measurements. Protons, fission fragments and gamma radiation are detected in coincidence by detectors mounted inside the 2 T field of a former MRI magnet.
Commissioned with a 22Ne beam, the approach was then applied to a 232U beam. The reported results provide the first measurement of
the 232U(d,pf) reaction, with fission fragments resolved in charge and a possible indication of the 233U barrier in the reconstructed excitation-energy spectrum.
The fission barrier of r-process nuclei determines at which mass and nuclear charge fission starts to limit nucleosynthesis. Relevant nuclei are, however, inaccessible to conventional fission measurements, which require the fissioning species as a target. Extending fission studies towards the neutron-rich region therefore needs a different experimental strategy. This thesis presents a novel approach to determine fission barriers in inverse kinematics.
Heavy radioactive ion beams provided by the ISOLDE facility at CERN impinge on a deuterated plastic target; the (d,p) reaction populates states in the fissioning nucleus above the barrier, and the barrier height is extracted from the fission probability measured as a function of excitation energy. Performing the measurement inside a solenoidal spectrometer optimises the excitation-energy resolution, mitigating the kinematic compression and kinematic shift that limit other inverse-kinematics measurements. Protons, fission fragments and gamma radiation are detected in coincidence by detectors mounted inside the 2 T field of a former MRI magnet.
Commissioned with a 22Ne beam, the approach was then applied to a 232U beam. The reported results provide the first measurement of
the 232U(d,pf) reaction, with fission fragments resolved in charge and a possible indication of the 233U barrier in the reconstructed excitation-energy spectrum.
Heavy Elements
CERN
ISS.
ISOLDE
Inverse Kinematics
Fission
r-process
Radioactive Beams
Actinides
PJ-salen, Kemigården 1, Chalmers
Opponent: Associate Professor Ali Al-Adili, Uppsala University, Sweden
Author
Maria Vittoria Managlia
Subatomic, High Energy and Plasma Physics 1
Where do the heaviest elements in the Universe come from? Gold, platinum,
uranium are not made on Earth, but forged in some of the most violent events in
the cosmos, such as the collisions of neutron stars. There, atomic nuclei
capture neutrons and grow heavier and heavier, until they become so unstable
that they split in two parts. This splitting, called nuclear fission, sets a limit on
how heavy the elements around us can be, and shapes which ones survive to become
part of planets, and of us.
To understand that limit, we need to measure how easily these exotic nuclei
break apart, that is, how high the energy barrier is that holds each one
together. But many of the nuclei that matter most exist for only a fraction of a
second and cannot be easily studied in the usual way, because they cannot be
held as a target.
This thesis develops a new experimental method to reach them, using radioactive
beams produced at CERN and a magnetic spectrometer that catches the fragments
and particles emerging from each fission event. It is a first step towards
recreating, in the laboratory, the physics that is responsible for the creation of the heaviest elements in the
Universe.
uranium are not made on Earth, but forged in some of the most violent events in
the cosmos, such as the collisions of neutron stars. There, atomic nuclei
capture neutrons and grow heavier and heavier, until they become so unstable
that they split in two parts. This splitting, called nuclear fission, sets a limit on
how heavy the elements around us can be, and shapes which ones survive to become
part of planets, and of us.
To understand that limit, we need to measure how easily these exotic nuclei
break apart, that is, how high the energy barrier is that holds each one
together. But many of the nuclei that matter most exist for only a fraction of a
second and cannot be easily studied in the usual way, because they cannot be
held as a target.
This thesis develops a new experimental method to reach them, using radioactive
beams produced at CERN and a magnetic spectrometer that catches the fragments
and particles emerging from each fission event. It is a first step towards
recreating, in the laboratory, the physics that is responsible for the creation of the heaviest elements in the
Universe.
Creation of heavy elements in neutron-star mergers
Knut and Alice Wallenberg Foundation (2020.0076), 2021-01-01 -- 2025-12-31.
Roots
Basic sciences
Subject Categories (SSIF 2025)
Other Physics Topics
Subatomic Physics
DOI
10.63959/chalmers.dt/5924
ISBN
978-91-8103-467-7
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5924
Publisher
Chalmers
PJ-salen, Kemigården 1, Chalmers
Opponent: Associate Professor Ali Al-Adili, Uppsala University, Sweden