1. René Reifarth GSI Darmstadt/University of Frankfurt
Experiments close to stability contributing to our understanding of the r-process
René Reifarth
GSI Darmstadt/University of Frankfurt
Presentation Title Slide Notes
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The first EURISOL UG topical meeting –
The formation and structure of r-process nuclei, between N=50 and 82
(including 78Ni and 132Sn areas)
Catania, Italy, December, 9-11, 2009

2. Outline
Some words about astrophysics – r- vs. s-process
(n,g) experiments – DANCE, FRANZ
(p,g) experiments – GSI
Conclusions

3. Solar Abundance Distribution
big bang nucleosysnthesis
fusion of charged particles
thermal equilibrium
neutron induced reactions
H, He
C, O Fe Ba Pb s r ?

4. How well do we know the (solar) s-abundance?
A = Ar + As (+Aothers)
Uniqueness of r-process?
Several components?
What IS the solar r-abundance?
How well do we know the (solar) s-abundance?

5. Neutron-induced …

6. The s-process neutron number proton number Br Se As Ge Ga Zn Cu (n,g)Ni Co Fe

7. s-process – the stellar site
Two components were identified and connected to stellar sites:
Main s-process 90<A<210
Weak s-process A<90
massive stars > 8 M⊙
core He-burning shell C-burning
3-3.5·108 K ~1·109 K
kT=25 keV kT=90 keV
106 cm cm-3
22Ne(a,n)
TP-AGB stars 1-3 M⊙
shell H-burning He-flash
0.9·108 K ·108 K
kT=8 keV kT=25 keV
cm cm-3
13C(a,n) Ne(a,n)

8. (n,g) experiments with unstable isotopes and fundamental stellar physics evaluations
Branch Isotope
Half-Life
Facility
Observable
Stellar Physics
Comment
151Sm
93 yr
FZK, n_TOF,
DANCE
152Gd in solar distribution
151Eu/153Eu ratio hyperfine line split
Timescale of hot Helium-shell flash
s-process in very old stars
done
134Cs
2 yr
DANCE?,
Future facilities?
Ba isotope ratios from presolar grains
Sets 12C abundance of He-shell flash
current uncertainty: ± 30%
135Cs
2 Myr
n_TOF,
Ba isotope ratios
Amount of rotation
± 10%
(activation)
95Zr
64 d
Future facilities
96Zr/94Zr ratio presolar grains
Temperature at bottom of He-shell flash region
Current uncertainty: mb

9. What’s needed? Reaction rates Half-lives Neutron induced (1-200 keV)
Charged particles
Half-lives

10. Connection on the stellar modeling side
Better understanding of stars helps to better understand potential r-process sites
How does the hydrodynamics in stars work?
What are the convection times under different conditions?

11. Nuclear data needs for the weak s-process
Problems:
small cross sections
resonance dominated
contributions from direct capture
propagation effects

12. The s-process around 63Ni
s-process nucleosynthesis in the region between iron and tin with the important branching at 63Ni

13. Detector for Advanced Neutron Capture Experiments
spallation source
thermal keV
20 m flight path
3 105 n/s/cm2/decade
neutrons:
collimated
neutrons
beam
160 BaF2 crystals
4 different shapes
Ri=17 cm, Ra=32 cm
7 cm 6LiH inside
eg  90 %
ecasc  98 %
g-Detector:
sample
t1/2 > 1 year
m ~ 1 mg
34 cm
»Nuclear Astrophysics with Neutron Facilities and LANL and RIA«

14. New high-resolution campaign has been performed at n_TOF
62Ni(n,g) at DANCE
(stable)
A. M. ALPIZAR-VICENTE et al., PRC 77, (2008)
New high-resolution campaign has been performed at n_TOF

15. Activation Method 62Ni(n,g)63Ni reaction detected via
copper Au
62Ni(n,g)63Ni reaction detected via
63Ni/62Ni ratio, AMS (t1/2=100 years)
proton beam
neutron cone
lithium
62Ni
Determination of neutron flux via
197Au(n,g)198Au
62Ni(n,g)63Ni
Neutron source:
7Li(p,n)7Be
previous:12.5 mb
new: 28.4 mb
Nassar et al. PRL. 94, (2005)

16. Neutron spectrum kT = 25 keV Emax = 110 keV
Quasi-Maxwellian averaged distribution:

17. 63Ni(n,g) performed at DANCE
t1/2 = 100 years
No experimental data exist so far (only transmission measurements)

18. The Frankfurt neutron source at the Stern-Gerlach-Zentrum (FRANZ)
Neutron beam
for activation
neutron flux:
1012 s-1
2 mA proton beam
250 kHz
< 1ns pulse width
neutron flux: 107 s-1 cm-2
Design by U. Ratzinger,
A. Schempp, O. Meusel and
P. C. Chau

19. Schematic TOF spectrum
En (keV)
200
128
80 cm flight path
10 cm inner detector radius
Emax = 200 keV
Emin = 1 keV
prompt flash
other reactions
(n,g)
on sample
130
160
TOF (ns)

20. Charged particle induced
Indirect Methods – Coulomb Dissociation
New approach for direct measurements
Inverse kinematics & storage ring
Proof of principle at GSI

21. p-process supernovae large networks (g,n), (g,p), (g,a)
(p,g), (a,g), (n,g)
natPb(AMo,A-1Mo+n) with LAND A[92-94,100] – performed, analysis in
progress
natPb(27P,26Si+p) with LAND – performed, to be analyzed
some activation measurements on stable isotopes
see also talk by K. Boretzky

22. Reaction Studies at the ESR
Measurements of (p,g) or (α,g) rates in the Gamow window of the p-process in inverse kinematics.
ESR
Gas jet
Particle detectors
Advantages:
Applicable to radioactive nuclei
Detection of ions via in-ring particle detectors (low background, high efficiency)
Knowledge of line intensities of product nucleus not necessary
Applicable for gases

23. Reaction Studies at the ESR
First pilot experiment performed with stable beams:
96Ru(p,g)97Rh
Measurements performed at 9, 10, 11 AMeV
5·106 particles per spill
Target density 1·1013 atoms/cm2
Luminosity 2.5·1025
Cross section 2 mbarn -> ~180 counts/h
GAMOW
Range of pilot experiment

24. Preliminary result @ 11 MeV – upper limit
Ignore (p,n) component –
resulting in an upper limit
for (p,g)
σ(p,g) < 4.0 mb
Non-smoker: 3.5 mb

25. Summary
The main s-process as a nucleosynthesis process is well understood and established
Current research uses the s-process as a link between abundance observations and stellar models
A better Understanding of the s-process contributes to our understanding of the r-process
There will always be the need for more neutrons – maybe direct (n,g) measurments will be possible in the future for the r-process freeze-out phase
Extreme lack of data outside the valley of stability
EURISOL is a good place to harvest isotopes for direct (n,g) experiments
Dedicated (small) additional facility like FRANZ could be very successful
Storage ring experiments will open a new era in charged-particle measurements, proof of principle was successful