Aleksandra Kelic, Maria Valentina Ricciardi, KarlHeinz Schmidt

1
Recent improvements in the GSI fission model Task
11, subtask 3

• Aleksandra Kelic, Maria Valentina Ricciardi,
  Karl-Heinz Schmidt
• GSI Darmstadt

http//www.gsi.de/charms/
2
Motivation

• RIB production (fragmentation method, ISOL
  method),
• Spallation sources and ADS

Data measured at FRS
Ricciardi et al, PRC 73 (2006) 014607 Bernas
et al., NPA 765 (2006) 197 Armbruster et al.,
PRL 93 (2004) 212701 Taïeb et al., NPA 724
(2003) 413 Bernas et al., NPA 725 (2003) 213
www.gsi.de/charms/data.htm
Challenge - need for consistent global
description of fission and evaporation
3
What do we need?
Fission competition in de-excitation of excited
nuclei

• Fission barriers
• Fragment distributions
• Level densities
• Nuclear viscosity
• Particle-emission widths

4
Mass and charge division in fission
5
Experimental information - High energy
In cases when shell effects can be disregarded,
the fission-fragment mass distribution is
Gaussian ?
Data measured at GSI T. Enqvist et al, NPA 2001
(see www.gsi.de/charms/)
6
Experimental information - Low energy

• Particle-induced fission of long-lived targets
  and spontaneous fission
• Available information
• - A(E) in most cases
• - A and Z distributions of light fission group
  only in the thermal-neutron induced fission on
  the stable targets
• EM fission of secondary beams at GSI
• Available information
• - Z distributions at "one" energy

7
Experimental information - Low energy
More than 70 secondary beams studied from Z85
to Z92
Schmidt et al., NPA 665 (2000) 221
8
Macroscopic-microscopic approach
- Transition from single-humped to double-humped
explained by macroscopic and microscopic
properties of the potential-energy landscape near
outer saddle.
Macroscopic part property of CN Microscopic
part properties of fragments
Maruhn and Greiner, Z. Phys. 251 (1972) 431,
PRL 32 (1974) 548 Pashkevich, NPA 477 (1988) 1
9
Basic assumptions

• Macroscopic part
• Macroscopic potential is property of fissioning
  system ( f(ZCN2/ACN))
• Potential near saddle from exp. mass
  distributions at high E (Rusanov)

cA is the curvature of the potential at the
elongation where the decision on the A
distribution is made. cA f(Z2/A) ? Rusanov
Rusanov et al, Phys. At. Nucl. 60 (1997) 683
10
Basic assumptions

• Microscopic part
• Microscopic corrections are properties of
  fragments ( f(Nf,Zf))
• Assumptions based on shell-model calculations
  (Maruhn Greiner, Pashkevich)
• Shells near outer saddle "resemble" shells of
  final fragments (but weaker)
• Properties of shells from exp. nuclide
  distributions at low E

A ? 140
A ? 132
Calculations done by Pashkevich
11
Basic assumptions

• Dynamics
• Approximations based on Langevin calculations (P.
  Nadtochy)
• t (mass asymmetry) gtgt t (saddle scission)
  decision near outer saddle
• t (N/Z) ltlt t (saddle scission) decision near
  scission
• Population of available states with statistical
  weight (near saddle or scission)

Mass of nascent fragments
N/Z of nascent fragments
12
Macroscopic-microscopic approach

• Fit parameters
• Curvatures, strengths and positions of two
  microscopic contributions as free parameters
• These 6 parameters are deduced from the
  experimental fragment distributions and kept
  fixed for all systems and energies.

• For each fission fragment we get
• Mass
• Nuclear charge
• Kinetic energy
• Excitation energy
• Number of emitted particles

13
ABLA - evaporation/fission model

• Evaporation stage
• - Extended Weisskopf approach with extension to
  IMFs
• - Particle decay widths
• - inverse cross sections based on nuclear
  potential
• - thermal expansion of source
• - angular momentum in particle emission
• - g-emission at energies close to the particle
  threshold (A. Ignatyuk)
• Fission
• - Fission decay width
• - analytical time-dependent approach (B. Jurado)
• - double-humped structure in fission barriers
• - symmetry classes in low-energy fission
• - Particle emission on different stages of the
  fission process

14
Comparison with data
15
ABLA
Test of the fission part ? Fission probability
235Np ? Data (A. Gavron et al., PRC13 (1976)
2374) ? ABLA
Test of the evaporation part ? 56Fe (1 A GeV)
1H ? Data (C. Villagrasa et al, P. Napolitani
et al) ? INCL4ABLA
16
Fission of secondary beams after the EM excitation
Black - experiment (Schmidt et al, NPA 665
(2000)) Red - calculations
With the same parameter set for all nuclei!
17
Neutron-induced fission of 238U for En 1.2 to
5.8 MeV
Data - F. Vives et al, Nucl. Phys. A662 (2000)
63 Lines - ABLA calculations
18
More complex scenario
238Up at 1 A GeV