Download PPT - GSI

The role of fission in the
r-process nucleosynthesis
Needed input
Aleksandra Kelić and Karl-Heinz Schmidt
GSI-Darmstadt, Germany
http://www.gsi.de/charms/
Overview
•
Characteristics of the astrophysical r-process
•
Signatures of fission in the r-process
•
Relevant fission characteristics and their uncertainties
•
Benchmark of fission saddle-point masses*
•
GSI model on nuclide distribution in fission*
•
Conclusions
*) Attempts to improve the fission input of r-process calculations.
Nucleosynthesis
Only the r-process leads to the heaviest nuclei (beyond
209Bi).
Identifying r-process nuclei
Truran 1973
176Yb, 186W, 187Re
can only be produced by r-process.
Nuclear abundances
r-process
s-process
Cameron 1982
Characteristic differences in s- and r-process abundances.
Role of fission in the r-process
TransU elements ? 1)
r-process endpoint ? 2)
Fission cycling ?
3, 4)
1) Cowan et al, Phys. Rep. 208 (1991) 267
2) Panov et al., NPA 747 (2005) 633
3) Seeger et al, APJ 11 Suppl. (1965) S121
4) Rauscher et al, APJ 429 (1994) 49
Cameron 2003
Difficulties near A = 120
r-process abundances
compared with model
calculations (no fission).
Calculation with shell
quenching (no fission).
Chen et al. 1995
Are the higher yields around A = 120 an indications for fission cycling?
Relevant features of fission
Fission competition in de-excitation of excited nuclei
n
f
γ
Daughter
nucleus
Fission occurs
Most important input:
• after neutron capture
• Height of fission barriers
• after beta decay
• Fragment distributions
Saddle-point masses
Experimental method
• Experimental sources:
Energy-dependent fission
probabilities
• Extraction of barrier
parameters:
Requires assumptions on level
densities
Resulting uncertainties:
about 0.5 to 1 MeV
Gavron et al., PRC13 (1976) 2374
Fission barriers - Experimental information
Uncertainty:
≈ 0.5 MeV
Available data on fission barriers, Z ≥ 80 (RIPL-2 library)
Far away from r-process path!
Complexity of potential energy on the fission path
Influence of nuclear structure (shell corrections, pairing, ...)
Higher-order deformations are important (mass asymmetry, ...)
LDM
LDM+Shell
Studied models
1.) Droplet model (DM) [Myers 1977], which is a basis of often used results of the
Howard-Möller fission-barrier calculations [Howard&Möller 1980]
2.) Finite-range liquid drop model (FRLDM) [Sierk 1986, Möller et al 1995]
3.) Thomas-Fermi model (TF) [Myers&Swiatecki 1996, 1999]
4.) Extended Thomas-Fermi model (ETF) [Mamdouh et al. 2001]
W.D. Myers, „Droplet Model of Atomic Nuclei“, 1977 IFI/Plenum
W.M. Howard and P. Möller, ADNDT 25 (1980) 219.
A. Sierk, PRC33 (1986) 2039.
P. Möller et al, ADNDT 59 (1995) 185.
W.D. Myers and W.J. Swiatecki, NPA 601( 1996) 141
W.D. Myers and W.J. Swiatecki, PRC 60 (1999) 0 14606-1
A. Mamdouh et al, NPA 679 (2001) 337
Diverging theoretical predictions
Theories reproduce measured barriers but diverge far from stability
Neutron-induced fission
rates for U isotopes
Kelić and Schmidt, PLB 643 (2006)
Panov et al., NPA 747 (2005)
Idea: Refined analysis of isotopic trend
Predictions of theoretical models are examined by means of a detailed
analysis of the isotopic trends of saddle-point masses.
exp
macro
macro
U sad  E exp

M

(
M

E
)
f
GS
GS
f
Experimental
saddle-point
mass
Macroscopic
saddle-point
mass
Usad  Experimental
minus macroscopic
saddle-point mass
(should be shell
correction at saddle)
Nature of shell corrections
What do we know about saddle-point shell-correction energy?
1. Shell corrections have local character
2. Shell-correction energy at SP should be small (topographic theorem:
e.g Myers and Swiatecki PRC 60; Siwek-Wilczynska and Skwira, PRC 72)
SCE
1-2 MeV
Neutron
number
If a model is realistic  Slope of Usad as function of N should be ~ 0
Any general trend would indicate shortcomings of the model.
The topographic theorem
Detailed and quantitative
investigation of the
topographic properties of
the potential-energy
landscape (A. Karpov et al.,
to be published) confirms
the validity of the
topographic theorem to
about 0.5 MeV!
238U
Topographic theorem: Shell corrections alter the saddle-point
mass "only little". (Myers and Swiatecki PRC60, 1999)
Example for uranium
Usad as a function of a neutron number
A realistic macroscopic model should give almost a zero slope!
Results
Slopes of δUsad as a function of the neutron excess
 The most realistic predictions are expected from the TF model and
the FRLD model
 Further efforts needed for the saddle-point mass predictions of the
droplet model and the extended Thomas-Fermi model
Kelić and Schmidt, PLB 643 (2006)
Mass and charge division in fission
- Available experimental information
- Model descriptions
- GSI model
Experimental information - high energy
In cases when shell effects can be disregarded (high E*), the fissionfragment mass distribution is Gaussian.
Second derivative
of potential in mass
asymmetry
deduced from
fission-fragment
mass distributions.
σA2 ~ T/(d2V/dη2)
← Mulgin et al. 1998
Width of mass distribution is empirically well established. (M. G. Itkis, A.Ya.
Rusanov et al., Sov. J. Part. Nucl. 19 (1988) 301 and Phys. At. Nucl. 60 (1997) 773)
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 stable targets
• EM fission of secondary beams
at GSI
Available information:
- Z distributions at energy of GDR (E*≈12 MeV)
Experimental information – low energy
Experimental survey at GSI by use of secondary beams
K.-H. Schmidt et al., NPA 665 (2000) 221
Models on fission-fragment distributions
 Empirical systematics on A or Z distributions –
Not suited for extrapolations
Theoretical models - Way to go, not always precise enough and
still very time consuming
Encouraging progress in a full microscopic description of fission:
H. Goutte et al., PRC 71 (2005)  Time-dependent HF calculations with GCM:
 Semi-empirical models – Our choice: Theory-guided systematics
Macroscopic-microscopic approach
Measured element yields
Potential-energy landscape (Pashkevich)
K.-H. Schmidt et al., NPA 665 (2000) 221
Close relation between potential energy and yields. Role of dynamics?
Most relevant features of the fission process
Basic ideas of our macro-micro fission approach
(Inspired by Smirenkin, Maruhn, Mosel, Pashkevich, Rusanov, Itkis, ...)
Dynamical features:
Approximations based on Langevin calculations (P. Nadtochy)
τ (mass asymmetry) >> τ (saddle-scission): Mass frozen near saddle
τ (N/Z) << τ (saddle-scission) : Final N/Z decided near scission
Statistical features:
Population of available states with statistical weight (near saddle or scission)
Macroscopic potential:
Macroscopic potential is property of fissioning system ( ≈ f(ZCN2/ACN))
Potential near saddle from exp. mass distributions at high E* (Rusanov)
Microscopic potential:
Microscopic corrections are properties of fragments (= f(Nf,Zf)). (Mosel)
-> Shells near outer saddle "resemble" shells of final fragments.
Properties of shells from exp. nuclide distributions at low E*. (Itkis)
Main shells are N = 82, Z = 50, N ≈ 90 (Responsible for St. I and St. II)
(Wilkins et al.)
Shells of fragments
Two-centre shell-model calculation by A. Karpov, 2007 (private communication)
Test case: multi-modal fission around
226Th
- 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*
(deduced from data)
208Pb
N=82
238U
N≈90
* Maruhn and Greiner, Z. Phys. 251 (1972) 431, PRL 32 (1974) 548; Pashkevich, NPA 477 (1988) 1;
Neutron-induced fission of
238U
for En = 1.2 to 5.8 MeV
Data - F. Vives et al, Nucl. Phys. A662 (2000) 63;
Aleksandra Kelić (GSI)
Lines - Model calculations
NPA3 – Dresden, 30.03.2007
Comparison with EM data
Fission of secondary beams after the EM excitation:
black - experiment
red - calculations
92U
91Pa
142
140
90Th
138
89Ac
131
132
133
134
135
136
137
139
141
Comparison with data - spontaneous fission
Experiment
Calculations
(experimental
resolution not included)
Application to astrophysics
Usually one assumes:
a) symmetric split: AF1 = AF2
b) 132Sn shell plays a role: AF1 = 132, AF2 = ACN - 132
But! Deformed shell around A≈140 (N≈90) plays an important role!
Predicted mass distributions:
260U
276Fm
300U
A. Kelic et al., PLB 616 (2005) 48
A new experimental approach to fission
Electron-ion collider ELISE of FAIR project.
(Rare-isotope beams + tagged photons)
Aim: Precise fission data over large N/Z range.
Conclusions
- Important role of fission in the astrophysical r-process
End point in production of heavy masses, U-Th chronometer.
Modified abundances by fission cycling.
- Needed input for astrophysical network calculations
Fission barriers.
Mass and charge division in fission.
- Benchmark of theoretical saddle-point masses
Investigation of the topographic theorem.
Validation of Thomas-Fermi model and FRLDM model.
- Development of a semi-empirical model for mass and charge division
in fission
Statistical macroscopic-microscopic approach.
with schematic dynamical features and empirical input.
Allows for robust extrapolations.
- Planned net-work calculations with improved input (Langanke et al)
- Extended data base by new experimental installations
Additional slides
Comparison with data
nth +
Mass distribution
235U
(Lang et al.)
Charge distribution
Z
Needed input
Basic ideas of our macroscopic-microscopic fission approach
(Inspired by Smirenkin, Maruhn, Pashkevich, Rusanov, Itkis, ...)
Macroscopic:
Potential near saddle from exp. mass distributions at high E* (Rusanov)
Macroscopic potential is property of fissioning system ( ≈ f(ZCN2/ACN))
The figure shows the second derivative of
the mass-asymmetry dependent potential, deduced
from the widths of the mass distributions within
the statistical model compared to different LD
model predictions.
Figure from Rusanov et al. (1997)
Ternary fission
Ternary fission  less than 1% of a binary fission
Open symbols experiment
Full symbols theory
Rubchenya and Yavshits, Z. Phys. A 329 (1988) 217
Applications in astrophysics - first step
Mass and charge distributions in neutrino-induced fission of
r-process progenitors

Phys. Lett. B616 (2005) 48