René Reifarth
Recent neutron capture measurements at FZK
Outline:
• Overview of s-process nucleosynthesis
• Nuclear data needs for the s-process – where do we stand?
• Recent (n,g) cross section measurements at FZK
• The planned neutron source at University Frankfurt: FRANZ
• Summary
Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology
July 8-11 2008, Ohio University, Athens Ohio
Nucleosynthesis of the elements
p-process
Proton number
s-process
rp-process
r-process
Fusion up to iron
Neutron number
Heavy elements (A>56) are
produced by the
s-process (~50%) and the
r-process (~50%)
Beyond Iron – mainly neutron induced
Sr
Rb
proton number
(n,g)
p-only
(b-)
Se
As
(b+)
Ge
Ga
Zn
Cu
Ni
Co
Fe
Kr
Br
r-only
neutron number
s-process nucleosynthesis
Two components were identified and connected to stellar sites:
Main s-process 90<A<210
TP-AGB stars 1-3 M⊙
shell H-burning
0.9·108 K
kT=8 keV
107-108 cm-3
13C(a,n)
He-flash
3-3.5·108 K
kT=25 keV
1010-1011 cm-3
22Ne(a,n)
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-3
1011-1012 cm-3
22Ne(a,n)
The main s-process
Success of the main s-process in
TP-AGB stars of 1-3M⊙
r-residuals method:
N r  Nsolar - Ns
Arlandini et al. ApJ 525 (1999) 886
Nuclear data needs for the main s-process
Success of the main s-process is based on
• reliable neutron capture cross sections (uncertainty < 5%)
• but also: stellar enhancement factors (SEF) and
stellar b-decay rates are important

Stellar neutron capture rate
σv 
8
1
 E 

σ(E)  E  exp    dE
3/2 
π  μ kT  0
 kT 
Neutrons
12
10
8
6
4
2
50
100
150
200
Energy
keV
We need to measure (n,g) cross sections between 0.1 and 500 keV.
Nuclear data needs for the weak s-process
62Ni(n,g)63Ni
Problems:
• small cross sections
• resonance dominated
• contributions from direct capture
• propagation effects
previous:12.5 mb
new: 28.4 mb
Nassar et al.
Phys. Rev. Lett. 94, 092504 (2005)
Michael Heil
Nuclear Physics for Astrophysics, Dresden, March 2007
Branchings in the s-process path
Classical analysis:
fβ 
λβ
λβ + λ n

σ  N Z+1

σ  N Z+1,A +1
λ n  n n  σv
A
Z+1
(n,g)
A+1
Z+1
(b-)
A-1
(n,g)
(b-)
A
(n,g)
A+1
Branchings can be used to determine
• neutron density
• temperature
Every branch point is a sensitive
• mass density
test of the stellar model!
• convection time scales
in the interior of stars
Experimental challenge: Measure (n,g) of unstable isotopes
Connection between stellar model and experiment
Classical s-process Modern
s-process
models
(AGB stars)
60Fe
Classical s-process
new n-facility
DANCE @ LANL
DANCE @ LANL
(FRANZ)
Activation technique at kT=25 keV
Neutron production via 7Li(p,n) reaction at a
proton energy of 1991 keV.
Induced activity can be measured after
irradiation with HPGe detectors.
• Only possible when product nucleus is radioactive
• High sensitivity -> small sample masses or small cross sections
• Use of natural samples possible, no enriched sample necessary
• Direct capture component included
Results - neutron capture cross sections
Isotope
58Fe
Bao et al. @ kT=30keV
in mbarn
12.1 ± 2
MACS @ kT=30 keV
in mbarn
13.0 ± 0.6
60Fe
5.3 ± 2 (Rauscher 2000)
9.0 ± 3.0
59Co
38 ± 4
38.7 ± 2
64Ni
8.7 ± 0.9
94 ± 10
7.2 ± 0.4
54.2 ± 2
79Br
41 ± 5
627 ± 42
28.7 ± 2
626 ± 19
81Br
313 ± 16
241 ± 9
87Rb
15.5 ± 1.5
16.1 ± 2.0
63Cu
65Cu
Motivation –
60Fe
in the universe
Detection of g-ray lines from interstellar 60Fe with SPI (INTEGRAL)
Deep-Sea Manganese Crust
Eg = 1173 and 1333 keV
60Fe/26Al
= 0.11 ± 0.03
ongoing production in massive stars
and
distribution by subsequent supernovae
tests stellar model and SN rate
Harris et al, A&A 433 (2005) L49
Motivation –
60Fe
on earth
• can be found in deep sea manganese crusts
• Gives hints about a nearby supernova
• 2.8 Ma ago
Knie et al, PRL 93 (2004) 171103
Production of
60Fe
• Weak s-process component in massive stars
• During He-core and C-shell burning
• 60Fe(n,g) cross section needed – no experimental
data available yet (estimates: 1-10 mb)
Sample
• 7.8 1015 atoms 60Fe (0.78 µg) (t1/2 = 1.5(3) Ma)
• Retrieved from proton-irradiated copper beam
stop (PSI)
• carrier: natFe, C
• active impurities:
– 55Fe (t1/2 = 2.7 y)
– 60Co (ingrowth)
• 6 mm diameter
activation only (presently) feasible method
g-detection
70 mm
2 Ge-Clovers, face to face
Efficiency @ 1115 keV:
sample
single crystal:
etot
=
epeak =
11 %
1.1 %
addback:
epeak
15 %
=
61Fe
decay
6 min
61Fe
1325
27 %
298
1205
38 %
1205
1205 keV (single)
298 & 1027 keV (coincidence)
1027
1027
61Co
g-rays used for 61Fe detection:
Single spectra
1205 keV (single)
Coincidences: 298 & 1027 keV
- almost no background
- significantly reduced counts
61Fe
Sample background only
1333 keV
(60Co)
Neutron poisons
Neutron capture on light elements.
Most important neutron poisons are:
16O(n,g), 12C(n,g), 23Na(n,g), 25Mg(n,g)…
23Na(n,g)24Na
Measurement of
activation method
Important during C-burning:
12C(12C,p)23Na
kT (keV)
25
Bao et al.
(mbarn)
2.2 ± 0.2
with
This work
(mbarn)
1.8 ± 0.1
20% lower cross section measured!
16O(n,g)17O
* 0.9
The Frankfurt neutron source at the Stern-GerlachZentrum (FRANZ)
Neutron beam
for activation
neutron flux:
1·1012 s-1
2 mA proton beam
250 kHz
< 1ns pulse width
neutron flux: 4·107 s-1 cm-2
Design by Prof. Ratzinger,
Prof. Schempp, O. Meusel
and P. C. Chau
Comparison with other neutron sources
The Frankfurt neutron source will provide the highest neutron flux in the
astrophysically relevant keV region (1 – 500 keV) worldwide.
Facility
Neutron flux at
sample position*
[cm-2 s-1]
Repetition
rate
[Hz]
Flight
path
[m]
Pulse
width
[ns]
Neutron
energy range
[keV]
Frankfurt
4·107
250000
0.4
<1
1-200
FZ Karlsruhe
1·104
250000
0.8
0.7
1-200
DANCE at Los Alamos
5·105
20
20
250
th -105
n_TOF at CERN
5·104
0.4
185
6
th -106
GELINA at Geel
5·104
800
30
1
th -105
ORELA at Oak Ridge
2·104
525
40
8
th -104
*Integrated flux between 1 keV and 100 keV
Michael Heil
Nuclear Physics for Astrophysics, Dresden, March 2007
Experimental program at FRANZ
The Frankfurt neutron source will provide the highest neutron flux in the
astrophysically relevant keV region (1 – 500 keV) worldwide.
Factor of 1000 higher than at FZK!!!
Neutron capture measurements of small cross sections:
• Big Bang nucleosynthesis: 1H(n,g)
• Neutron poisons for the s-process: 12C(n,g), 16O(n,g), 22Ne(n,g).
• ToF measurements of medium mass nuclei for the weak s-process.
Neutron capture measurements with small sample masses:
• Radio-isotopes for g-ray astronomy 59Fe(n,g) and 60Fe(n,g)
• Branch point nuclei, e.g. 85Kr(n,g), 95Zr(n,g), 147Pm(n,g),
154Eu(n,g), 155Eu(n,g), 153Gd(n,g), 185W(n,g)
Double neutron capture
X-1
X
X+1
t1/2 ~ 1..100 d
• produce the sample “on the fly”
• 1012 n/s/cm2 @ 25 keV ~ 5 103 n/cm3
59Fe(n,g)
at FRANZ (t1/2=45 d)
• activate 58Fe, wait for 2nd neutron capture
• measure 60Fe/58Fe ratio via AMS
Summary
• We have a good understanding of the main s-process.
To do: - measure neutron capture cross section of branch points nuclei if possible
- improve theoretical predictions
• Weak s-process is less understood.
To do: Measure neutron capture cross sections of light and medium mass nuclei
with sufficient accuracy.
• Neutron capture cross sections of neutron poisons are also important for both,
the main and the weak s-process.
• Cross section measurements of neutron producing reactions,
e.g. 13C(a,n) and 22Ne(a,n) are also important.
• New facilities like FRANZ will open a completely new area of cross section
measurements, not only for astrophysics.
Thanks to
M.Heil, F. Käppeler
E. Uberseder, I. Dillmann
R. Plag, F. Voss, S. Walter
C. Domingo Pardo
R. Gallino
M. Pignatari
U. Ratzinger
O. Meusel
E. Schempp
L.P. Chau
D. Schumann
J. Görres, M. Wiescher
U. of Notre Dame
Background due to
elastic scattering
• Old measurements possibly suffer
from underestimation of background
from scattered neutrons.
PM
C6D6
neutrons
Michael Heil
Nuclear Physics for Astrophysics, Dresden, March 2007
MASS COORDINATE (M)
The s-process in
Asymptotic Giant Branch stars (Red
Giants)
0.68
convective envelope
H - burning
He –
intershell
0.67
13C(a,n)
22Ne(a,n)
C-O - core
0.66
200
35000
200
TIME (a)
He - burning
35000
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