Folie 1

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Lecture 2 The synthesis of the trans-iron
elements

• the s-process
• the r-process
• their astrophysical sites
• nuclear data needs

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nuclear processes
charged-particle induced reaction
mainly neutron capture reaction
during quiescent stages of stellar evolution
mainly during explosive stages of stellar
evolution
involve mainly STABLE NUCLEI
involve mainly UNSTABLE NUCLEI
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why neutron capture processes for the synthesis
of heavy elements?

• exponential abundance decrease up to Fe
• ? exponential decrease in tunnelling
• probability for charged-particle reactions
• almost constant abundances beyond Fe
• ? non-charged-particle reactions
• binding energy curve ? fusion reactions
• beyond iron are endothermic
• characteristic abundance peaks at magic
• neutron numbers
• neutron capture cross sections for heavy
• elements increasingly larger
• large neutron fluxes can be made available
• during certain stellar stages

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nucleosynthesis beyond iron
start with Fe seeds for neutron capture

• whenever an unstable species is produced one of
  the following can happen
• the unstable nucleus decays (before reacting)
• the unstable nucleus reacts (before decaying)
• the two above processes have comparable
  probabilities

if tn gtgt tb ?? unstable nucleus decays if tn ltlt
tb ?? unstable reacts if tn tb ? branchings
occur
with
mean lifetime of nucleus X against destruction by
neutron capture
mean lifetime of nucleus X against b decay
NOTE tn varies depending upon stellar conditions
(T, r) ? different processes dominate in
different environments
ALSO tb can be affected too by physical
conditions of stellar plasma!
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aside
factors influencing the b-decay lifetime of an
unstable nucleus

• both b- and b decay are hampered in the
  presence of electron or positron degeneracy
• b- and b decays may occur from excited isomeric
  states maintained in equilibrium
• with ground state by radiative transitions
• electron-capture rates are affected by
  temperature and density through population of
• the K electronic shell

example 7Be
7Be nucleus can only decay by electron capture
with a lifetime tEC 77 d
in the Sun, T 15x106 K ? kT 1.3 keV ?
low-Z nuclei almost completely ionized e.g.
binding energy of innermost K-shell electrons in
7Be Eb 0.22 keV ? if no electrons available
7Be becomes essentially STABLE!
in fact free electrons present in the plasma can
be captured for solar conditions tEC
120 d ? factor 1.6 larger than in
terrestrial laboratory
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the s-process
(from Rene Reifarth)
s-only
Zr
Y
Sr
(n,g)
Rb
p-only
Kr
Br
(b-)
Se
As
(b)
Ge
Ga
Zn
r-only
Cu
Ni
Co
Fe
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s-only, r-only and p-only isotopes help to
disentangle the individual contributions
A85
A140
A208
abundance peaks at A 85, 140, and 208 how to
explain abundance curve with the s process?
s-process abundances
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the s-process

• the process
• its astrophysical site(s)
• nuclear data needs
• (experimental equipment and techniques)

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s-process (s slow neutron capture process)
?
unstable nucleus decays before capturing another
neutron
tb ltlt tn
how many neutrons are needed?
typical lifetimes for unstable nuclei close to
the valley of b stability seconds ? years
assuming s 0.1 b _at_ E 30
keV ? v 3x108 cm/s
?
tn 10 y ? Nn 108 n/cm3
requiring
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classical approach of the s process
time dependence of abundance NA given by
Z
A
production
destruction
N
assuming

• T const
• tn gtgt tb

Maxwellian averaged cross section
with
neutron exposure
in steady state condition (so-called local
equilibrium approximation)
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small capture cross sections at neutron magic
numbers ? pronounced abundance peaks
A85
A140
A208
Rolfs Rodney Cauldrons in the Cosmos, 1988
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condition fulfilled between magic numbers of
neutrons
A140
A208
sudden drops observed at neutron magic numbers
A85
NOTE
a superposition of many neutron irradiations is
needed to correctly reproduce the abundance curve

• main component (A88-209)
• weak component (Alt90)

s-process best understood nucleosynthesis
process from nuclear point of view
what about the astrophysical site?
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s-process site(s) and conditions
free neutrons are unstable ? they must be
produced in situ in principle many (a,n)
reactions can contribute in practice, one needs
suitable reaction rate abundant nuclear
species most likely candidates as neutron source
are
22Ne(a,n)25Mg
13C(a,n)16O
astrophysical site He-flashes followed by H
mixing into 12C enriched zones low-mass (1.5 - 3
Msun) TP-AGB stars T8 0.9 2.7
astrophysical site core He burning (and shell
C-burning) in massive stars (e.g. 25 solar
masses) T8 2.2 3.5
contribution to weak s-process
contribution to main s-process
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in some cases tb tn ? a branching
occurs in nucleosynthesis path
example
176Lugs ? 176Hf tb 5x1010 y
176Lum ? 176Hf tb 5.3 h
for Nn 108 cm-3 ? tn 1 y
176Lugs essentially STABLE 176Lum quickly decays
into 176Hf
from abundance determinations
(note 174Hf p-only nucleus, i.e. not affected
by s-process)
? significant amount of s-process branching from
176Lum b-decay is required ? need temperatures
T8 gt 1 to guarantee that isomeric state is
significantly populated
branching points can be used to determine

• neutron flux
• temperature
• density

about 15-20 branchings relevant to s process
in the star during the s process
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stellar enhancement of decay (stellar decay
rate/terrestrial rate) for some important
branching-point nuclei in s-process path _at_ kT
30 keV
F. Kaeppeler Prog. Part. Nucl. Phys. 43 (1999)
419 483
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experimental requirements

• neutron capture cross sections on unstable
  isotopes
• 12 A 210 and 10 kT 100 keV
• about 1/10 of s(n,g) on radioactive isotopes
  measured so far
• (n,g) rates on long-lived branching point nuclei
• mass region 79 A 204

nuclear data needs

• improved stellar models for s-process
• temperature, density and neutron flux conditions
• galactic nucleosynthesis
• cosmo-chronometry

motivations

• nuclear reactions 7Li(p,n)7Be, 3H(p,n)3He
• (Karlsruhe, Tokyo)
• (g,n) reactions from energetic electrons on
  heavy metal targets
• (ORELA _at_ Oak Ridge, Tennessee and GELINA _at_
  Geel, Belgium)
• spallation reactions by energetic particle beams
  
• (LANSCE _at_ Los Alamos, n-TOF _at_ CERN)

neutron production techniques

• prompt g detection from (n,g) reaction (TOF)
• activation technique (t½ lt 0.5 y)

(n,g) measurements
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capture cross-section measurements
Time-Of-Flight technique
applicable to all stable nuclei need pulsed
neutron source for En determination via TOF
signature for neutron capture events total
energy of g cascade to ground state
need 4p detector of high efficiency, good time
and energy resolution
?
Karlsruhe 42 individual BaF2 crystals
F. Kaeppeler Rep. Prog. Phys. 52 (1989) 945
1013
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capture cross-section measurements
activation technique
7Li(p,n)7Be (or 3H(p,n)3He) angle-integrated
spectrum closely resembles a MB distribution at
kT 25 keV (52 keV) reaction rate measured in
such spectrum gives proper stellar cross section
advantages high sensitivity ? tiny samples
are enough for (n,g)
mesurements good for RIBs high
selectivity ? samples of natural composition
can be used
limitations (n,g) capture must produce
unstable species cross
section measurements at E 25 (and
52) keV only
applications with RIA type facilities R.
Reifarth et al. NIMA 524 (2004) 215-226
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the s-process in a nutshell
Weak component
Main component
temperature 2.2 3.5x108 K 0.9x108 K neutron
density 7x105 cm-3 4x108 cm-3 neutron
source 22Ne(a,n) 13C(a,n)
22Ne(a,n) stellar site core helium
burning TP-AGB stars in massive stars

• synthesis path along valley of b-stability up to
  209Bi
• n-source 13C(a,n)16O and/or 22Ne(a,n)25Mg
• quiescent scenarios e.g. He burning (T8 1
  4 E0 30 keV)
• branching points if ?? ?n ? several paths
  possible

data needs (n,g) cross sections on unstable
nuclei along stability valley capture data at
branching points motivation s-process stellar
models physical conditions of astrophysical site
review F. Kaeppeler Prog. Part. Nucl. Phys. 43
(1999) 419 483
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the r-process

• the process
• its astrophysical site(s)
• nuclear data needs

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r-process abundances Nr can be obtained as the
difference between solar abundances Nsolar and
calculated s-process abundances Ns
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constraints from elemental abundances
Ultra Metal Poor giant halo stars give info on
early nucleosynthesis in Galaxy
example
CS22892-052red giant located in galactic
haloFe/H -3.0 Dy/Fe 1.7
recallX/Ylog(X/Y)-log(X/Y)solar
weak r-process
main r-process

• for A 130 solar-like abundances even for stars
  originating from very different
• regions of Galactic halo
• ? main r-process independent of astrophysical
  site
• for A 130 under-abundances
• ? weak r-process

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r-process (r rapid neutron capture process)
?
unstable nucleus reacts before capturing decay
tn ltlt tb
termination point fission of very heavy nuclei
waiting points
Rolfs Rodney Cauldrons in the Cosmos, 1988
typical lifetimes for unstable nuclei far from
the valley of b stability 10-6 10-2 s
tn 10-4 s ? Nn 1020 n/cm3
requiring
explosive scenarios needed to account for such
high neutron fluxes
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classical approach of the r process
waiting point approximation

• assume
• (n,g) ? (g,n) equilibrium within isotopic chain,
  and
• b-flow equilibrium
• b-decay of nuclei from each Z-chain to (Z1)
  is equal to the flow from (Z1) to (Z2)

the nucleus with maximum abundance is each
isotopic chain must wait for the longer b-decay
time scales
good approximation for parameter studies, BUT
steady-flow approximation is not always valid
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classical approach of the r process

• abundance ratios of neighbouring isotopes only
  depends on Nn, T and Sn

A
A1
Z
N
neutron capture rates do not play relevant role
in most of r-process
note

• abundance flow from neighbouring isotopic chains
  is governed by b decays

define total abundance in each isotopic chain
Z1
Z
A
N
need nuclear masses (Sn) and lifetimes (tb)
together with environment conditions (Nn, T)
late neutron captures can modify final abundance
distribution mainly in region A gt 140
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timescale of r process
summing up time spent at waiting points t 0.5
10 s
courtesy H. Schatz
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at present very little is known for neutron-rich
nuclei very far away from b stability ? must rely
on theoretical calculations
NuPECC Long Range Plan 2004
_at_ ISOLDE b-decays for 30 neutron-rich nuclei
have been determined including N82 waiting
points 130Cd 129Ag GSI 70 new masses
determined recently in region N50 82
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time-dependent r-process calculations
T 1.35x109 K
a full fit to the solar r-process
abundances requires a superposition of different
stellar conditions (not necessarily different
sites)
Pfeiffer et al. Nucl. Phys. A 693 (2001) 282
324
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time-dependent r-process calculations
Sn values from four different mass models
constant astrophysical parameters t½ for 129Ag
and 130Cd calculated according to respective mass
values
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astrophysical site(s) for the r process
actual site(s) still unknown

• however, possible candidates for r-process site
• type II supernovae
• merging neutron stars
• neutrino driven wind of proto-neutron star
• He shell of exploding massive star merging
  neutron stars
• others?...

type II supernova
merging neutron stars
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the r-process in a nutshell
temperature 1-2x109 K timescale
seconds neutron density 1020-1024 cm-3 neutron
source unknown stellar site type II
supernovae? neutron star mergers?

• synthesis path far from valley of b-stability
• synthesis of n-rich nuclei
• waiting points ?? ltlt ?n at closed shells ?
  abundance peaks (after fn ? 0)

data needs neutron separation energies Sn
(model dependent) nuclear masses far away from
stability b-decay lifetimes for neutron rich
nuclei neutron capture cross sections on key
isotopes motivation synthesis of heavy elements
up to Th, U, Pu r-process path(s) abundance
pattern conditions for waiting point
approximation
review Pfeiffer et al. Nucl. Phys. A 693
(2001) 282 324
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next-generation Radioactive Ion Beam
facilities will allow us to greatly improve our
knowledge of nuclear properties at extreme
conditions and will help our understanding of
nuclear processes in equally extreme
astrophysical scenarios