Speaker
Description
Fission reactions investigated in inverse kinematic at relativistic energies have proven
to be a powerful technique to get access to a large variety of non stable heavy nuclei
[1] but also to fully identify, for the first time, in atomic and mass number both fission
fragments [2]. These systematic investigations made possible to establish the role of
proton shells in the asymmetric fission modes present in actinides and pre-actinides [3]
but also to identify a new compact symmetric fission mode from the fission yields
evolution in thorium isotopes [4].
Until now, inverse kinematic fission was mostly induced by the excitation of the giant-
dipole resonance (GDR) in electromagnetic interactions using targets with large atomic
number. The advantage of Coulex is the large fission cross sections, being the only
drawback that the range in excitation energy covered is limited to the GDR width and
no energy selection possibility. To go a step further, we propose now to induce fission
using quasi-free scattering (QFS) of protons (p,2p).
QFS of protons on heavy nuclei induces fission through particle-hole excitations that
can range from few to ten’s of MeV. Moreover, the measurement of the momenta of the
two scattered protons made it possible to determine the missing energy in the reaction
and then, the excitation energy of the fissioning compound nucleus.
The realization of such an experiment requires a complex setup providing the full
isotopic identification of both fission fragments, and at the same time a precise
measurement of the momenta of the two outgoing protons. This was realized at the
R3B experiment at GSI/FAIR (see Fig.1).
At R3B, a liquid hydrogen target is used to induced (p,2p) QFS on the incoming heavy
projectiles accelerated at relativistic energies (~ 500A MeV). Fission fragments
produced in those reactions fly forward and are identified in atomic number by
measuring their energy loss in a twin ionization chamber and their time-of-flight
between a start plastic scintillator and a stop large-area wall of plastic scintillators. The
mass numbers of the fragments is obtained by tracking their trajectories in the large-
acceptance super-conducting magnet GLAD. In order to achieve a resolution in charge
of around 0.4 units (FWHM) (see Fig. 2) and in mass of around 0.6 units, the energy
loss was measured with an accuracy of 1%, time-of-flight with a resolution of 40 ps
(FWHM) and the trajectories with a resolution of around 200 m (FWHM).
The missing energy in the reaction is determined using a silicon tracker and the
CALIFA calorimeter surrounding the target to measure the momenta of the two
protons. The measurement of these momenta with a resolution around 1% allows us to
reconstruct the excitation energy of the fissioning compound nucleus with a resolution
of around 3 MeV.
In this work we will present the first results of an experiment we run last April 2021 to
investigate the dumping of shell effects in the fission yields. We will also outline the
future program at R3B to investigate fission at the new FAIR facility.