This is a presentation about the last results obtained with the R3B setup for the study of fission reactions

1. José Luis Rodríguez-Sánchez
University of Coruña & USC
15th
June 2023
Complete kinematics studies of fission reactions induced
by quasi-free nucleon scattering collisions
238
U
Proton
target
237
Pa*

2. 2
Outline
J.L. Rodríguez-Sánchez
●
Introduction
●
Nuclear fission studies performed at GSI in complete kinematics to
investigate the fission dynamics and the nuclear structure at large
deformations
●
Nuclear fission studies with a new approach based on quasi-free (p,2p)
scattering collisions
●
Conclusions and perspectives

3. 3
Nuclear fission
J.L. Rodríguez-Sánchez
Complex decay process A diffusion process along the nuclear potential landscape based
on transport approaches like
Langevin equation
Fokker-Planck equation
d ν
dt
=−β ν−
1
m
∂
∂ x
U (x)+
F '(t)
m
∂W
∂ t
=
[−∂
∂ x
ν+ ∂
∂ ν (βν−
1
m
∂
∂ x
U (x))+
βκT
m
∂2
∂ ν
2 ]W
The main inputs are the potential U(x) and the dissipation parameter β
Main observables: Fission probabilities & particle emission
Understanding of nuclear fission still represents an important challenge in nuclear physics

4. 4
Nuclear fission
J.L. Rodríguez-Sánchez
Complex decay process A diffusion process along the nuclear potential landscape based
on transport approaches like
Langevin equation
Fokker-Planck equation
d ν
dt
=−β ν−
1
m
∂
∂ x
U (x)+
F '(t)
m
∂W
∂ t
=
[−∂
∂ x
ν+ ∂
∂ ν (βν−
1
m
∂
∂ x
U (x))+
βκT
m
∂2
∂ ν
2 ]W
The main inputs are the potential U(x) and the dissipation parameter β
Main observables: Fission probabilities & particle emission
Understanding of nuclear fission still represents an important challenge in nuclear physics
At low excitation energies the dynamics is governed by the
macroscopic and microscopic components of the nuclear
potential leading to shell effects in the mass asymmetry parameter
Main observables
Charge and mass distributions of the fission fragments
Symmetric
Asymmetric
Randrup et al., PRC 88, 064606 (2013)

5. 5
Reaction mechanisms to investigate nuclear fission
J.L. Rodríguez-Sánchez
Complete kinematics measurements of fission products (fission fragments, light particles
and γ-rays) in inverse kinematics by using state-of-the-art detectors
Spallation-induced fission reactions
• Liquid hydrogen targets
• High excitation energies > 60 MeV
• Fission dynamics
Coulomb-induced fission reactions
• Solid targets made of U and Pb
• Low excitation energies ~ 11 MeV
• Isospin dependence of fission fragment charge and
mass distributions
Quasi-free (p,2p) scattering reactions
• Liquid hydrogen targets
• Isospin and temperature dependencies of the fission
fragment charge and mass distributions
• Fission barriers
We also need to measure a lot of fission observables and their correlations with high precision

6. 6
Complete kinematics measurements at GSI
J.L. Rodríguez-Sánchez
J.L. Rodríguez-Sánchez et al.,
Phys. Rev. C 91, 064616 (2015)
E. Pellereau et al.,
Phys. Rev. C 95, 054603 (2017)
Twin-MUSIC
● Charge resolution 0.44 units
● Magnetic field ~2 Tm
● Position resolution 250 µm
● ToF resolution 40 ps
Full identification in A, Z of both fission fragments and some light-charged particles
Beam energies of 500-700 MeV/u

7. 7
Complete kinematics measurements at GSI
J.L. Rodríguez-Sánchez
208
Pb+p at 500A MeV
σ = 0.19
J.L. Rodríguez-Sánchez et al., Phys. Rev. C 91, 064616 (2015)

8. 8
Nuclear dynamics at high excitation energies
J.L. Rodríguez-Sánchez
Constraint of the nuclear viscosity coefficient and nuclear density parametrization
J.L. Rodríguez-Sánchez et al., Phys. Rev. C 90, 064606 (2014) Phys. Rev. C 92, 044612 (2015)
Phys. Rev. C 91, 064616 (2015) Phys. Rev. C 94, 034605 (2016)
208
Pb+p at 500A MeV
E*

9. 9
Nuclear dynamics at high excitation energies
J.L. Rodríguez-Sánchez
J.L. Rodriguez-Sánchez et al., Phys. Rev. C 94, 061601(R) (2016) M. Feijoo, PhD thesis, University of Santiago de Compostela (2021)
236
U(650A MeV)+Al
Constraint of the nuclear viscosity coefficient at large deformations (saddle-scission)
Using the average N/Z of fission fragments as a complementary observable of neutron evaporation
High excitation energies → larger neutron evaporation → smaller <N/Z> for fission fragments
Excitation energies

10. 10
Nuclear dynamics at high excitation energies
J.L. Rodríguez-Sánchez
Feijoo et al (2021)
Rodriguez et al (2016)

11. 11
Nuclear structure at large deformations
J.L. Rodríguez-Sánchez
Andreyev et al., PRL 105, 252502 (2010)
Summary of measured fission yield distributions

12. 12
Nuclear structure at large deformations
J.L. Rodríguez-Sánchez
Summary of measured fission yield distributions
➔ Recent experiments: Charge and mass distributions for 141 isotopes
Schmidt et al., NPA 693, 169 (2001)

13. 13
Nuclear structure at large deformations
J.L. Rodríguez-Sánchez
A. Chatillon et al., Phys. Rev. C 99, 054628 (2019)
J.-F. Martin et al., Phys. Rev. C 104, 044602 (2021)
A. Chatillon et al., Phys. Rev. C 106, 024618 (2022)
• Clear observation of the transition from symmetric to
asymmetric fission in the charge distribution of Th isotopes
Nuclear charge (z)

14. 14
Nuclear structure at large deformations
J.L. Rodríguez-Sánchez
A. Chatillon et al., Phys. Rev. C 99, 054628 (2019)
J.-F. Martin et al., Phys. Rev. C 104, 044602 (2021)
A. Chatillon et al., Phys. Rev. C 106, 024618 (2022)
• Evidence of a new symmetric fission mode
• Clear observation of the transition from symmetric to
asymmetric fission in the charge distribution of Th isotopes
A. Chatillon et al., Phys. Rev. Lett. 102, 202502 (2020)
Nuclear charge (z)
Nuclear charge (z)

15. 15
Fission experiments at the GSI-FAIR facility
J.L. Rodríguez-Sánchez
Fragmentation reactions
Super-FRS
SIS-100
Pre-separator (RIBs)
Main separator
Targets for the
production of RIBs
Experimental area
Beam intensity (pps) 10⁹ 10¹²
Transmission for U 55 % 98 %
Beam energy (GeV) 1.1 1.5
GSI FAIR

16. 16
Fission experiments at the GSI-FAIR facility
J.L. Rodríguez-Sánchez
The proton-pair kinematics provides access to the
excitation energy of the compound nucleus by using
energy and momentum conservations
Missing mass Rest mass
The advantages of (p,2p) reactions and the inverse kinematics technique are combined to
induce fission and measure the E* of the compound nuclei
T. Noro et al., Prog. Theor. Exp. Phys. 093D02 (2020)
6 4 2 0
Excitation energy (MeV)

17. 17
First (p,2p)-fission experiment at GSI (March 2021)
J.L. Rodríguez-Sánchez
● CALIFA and Si-tracker: Proton momenta and γ-rays
Energy res. protons(gamma) 1%(5% at 1 MeV), Position res. 70µm
● MUSIC, ToF wall and MWPC detectors: Fission fragments
∆Z~0.37 , ToF~40ps, Position res. 200µm (FWHM)
● NeuLand: Neutron multiplicities (max. 10 neutrons)
● Magnetic field: 4.8 T
Beam of 238
U at 560 MeV/u
experimental setup for complete kinematics measurements
GLAD
ToF
LH2
CALorimeter for In-Flight detection of γ-rays
and high energy charged pArticles
He gas

18. 18
R3B
First (p,2p)-fission experiment at GSI (March 2021)
Charge identification of fission fragments & fissioning systems
J.L. Rodríguez-Sánchez
238
U+p at 560A MeV
● A. Graña, PhD student working on the data analysis
Pa isotopes

19. 19
Charge and mass distributions: Evolution with E*
J.L. Rodríguez-Sánchez
● A. Graña, PhD student working on the data analysis
Preliminary
First results for 238
U(p,2p)237
Pa* (E* ~ 0 and 60 MeV)→ Fission fragments
✔ Distributions for different excitation energies: E1 < E2 < E3

20. 20
Conclusions and perspectives
J.L. Rodríguez-Sánchez
First fission experiments based on complete kinematics measurements of fission
fragments have been performed at GSI providing new fission observables to study the
dynamics and nuclear structure at large deformation
Spallation induced fission
●
Constraint of viscosity and density parameters at high excitation energies
Coulex-induced fission
●
Discovery of a new fission mode, measurement of charge and mass distributions for 141 isotopes
The GSI-FAIR facility will give us new opportunities to extend this investigation to exotic
neutron-rich nuclei involved in the r-process nucleosynthesis by using quasi-free (p,2p)
scattering reactions to induce fission
✔
Charge and mass distributions and their evolutions with the excitation energy
✔
Fission barrier heights as a function of isospin
Ongoing developments for future experiments (2027 and beyond)
✔
Recoil tracker based on pixel silicon detectors to reconstruct the (p,2p) excitation energy
✔
Fission fragment tracking detectors in vacuum to improve the mass resolution

21. 21
J.L. Rodríguez-Sánchez
Thanks to all the collaborators

22. 22
J.L. Rodríguez-Sánchez