The origin of heavy elements in the solar system

1
The origin of heavy elements in the solar system
(Pagel, Fig 6.8)
each process contribution is a mix of many events
!
2
Heavy elements in Metal Poor Halo Stars
3
A single (or a few) r-process event(s)
4
Overview heavy element nucleosynthesis
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The r-process
Temperature 1-2 GK Density 300 g/cm3 (60
neutrons !)
Seed
Proton number
Neutron number
6
show movie
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Waiting point approximation
Definition ASSUME (n,g)-(g,n) equilibrium within
isotopic chain
How good is the approximation ?
This is a valid assumption during most of the
r-process BUT freezeout is neglected
Freiburghaus et al. ApJ 516 (2999) 381 showed
agreement with dynamical models
Consequences
During (n,g)-(g,n) equilibrium abundances within
an isotopic chain are given by

• time independent
• can treat whole chain as a single nucleus in
  network
• only slow beta decays need to be calculated
  dynamically
• neutron capture rate independent
  (therefore during most of the r-process
  n-capture rates do not matter !)

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Endpoint of the r-process
r-process endedby n-induced fission
or spontaneousfission
(different pathsfor different conditions)
(Goriely Clerbaux AA 348 (1999), 798
b-delayed fission
spontaneous fission
n-induced fission
n-capture (DC)
b-
(Z,A)
(Z,A)
fission
fission
fission
(Z,A1)
fission barrier
(Z,A1)
(Z1,A)
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Consequences of fission
Fission produces AAend/2 125 nuclei
modification of abundances around A130 peak
fission products can serve as seed for the
r-process - are processed again into A250
region via r-process - fission again
fission cycling !
Note the exact endpoint of the r-process and the
degree and impact of fission are unknown
because

• Site conditions not known is n/seed ratio
  large enough to reach fission ? (or even
  large enough for fission cycling ?)
• Fission barriers highly uncertain
• Fission fragment distributions not reliably
  calculated so far (for fission from excited
  states !)

10
Role of beta delayed neutron emission
Neutron rich nuclei can emit one or more neutrons
during b-decay if SnltQb
(the more neutron rich, the lower Sn and the
higher Qb)
b-
(Z,A)
n
Sn
(Z1,A-1)
g
(Z1,A)
If some fraction of decay goes above Sn in
daughter nucleusthen some fraction Pn of the
decays will emit a neutron (in addition to e- and
n)(generally, neutron emission competes
favorably with g-decay - strong interaction !)
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during r-process none as neutrons get recaptured
quicklyduring freezeout
Effects

• modification of final abundance
• late time neutron production (those get
  recaptured)

Calculated r-process production of elements
(Kratz et al. ApJ 403 (1993) 216)
after b-decay
before b-decay
smoothing effect from b-delayed n emission !
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Cs (55)
r-processwaiting point
Xe (54)
Pn0
I (53)
Te (52)
Pn99.9
Sb (51)
Sn (50)
In (49)
Cd (48)
Ag (47)
r-process waiting point
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Summary Nuclear physics in the r-process
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The r-process path
r-process abundance distribution
r-processpath
RIA Reach
New MSU/NSCL Reach
Known
(Reach for half-life)
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National Superconducting Cyclotron Laboratory
atMichigan State University
New Coupled Cyclotron Facility experiments
since mid 2001
Ion Source86Kr beam
86Kr beam140 MeV/u
Tracking (Momentum)
TOF start
Implant beam in detectorand observe decay
86Kr hits Be target and fragments
TOF stop dE detector
Separated beamof r-processnuclei
Fast beam fragmentation facility allows event
by event particle identification
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Installation of D4 steel, Jul/2000
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First r-process experiments at new NSCL CCF
facility (June 02)

• Measure
• b-decay half-lives
• Branchings for b-delayed n-emission

• Detect
• Particle type (TOF, dE, p)
• Implantation time and location
• b-emission time and location
• neutron-b coincidences

New NSCL Neutron detectorNERO
3He n -gt t p
neutron
Fast Fragment Beam
Si Stack
(fragment. 140 MeV/u 86Kr)
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NSCL BCS Beta Counting System

• 4 cm x 4 cm active area
• 1 mm thick
• 40-strip pitch in x and y dimensions -gt1600 pixels

Si
Si
Si
BCS
b
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NERO Neutron Emission Ratio Observer
BF3 Proportional Counters
3He Proportional Counters

• Specifications
• 60 counters total
• (16 3He , 44 BF3)
• 60 cm x 60 cm x 80 cm
• polyethylene block
• Extensive exterior
• shielding
• 43 total neutron
• efficiency (MCNP)

Polyethylene Moderator
Boron Carbide Shielding
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June 2002 Data preliminary results
Mainz K.-L. Kratz, B. Pfeiffer PNNL P.
Reeder Maryland/ANL W.B. Walters, A. Woehr Notre
Dame J. Goerres, M. Wiescher NSCL P. Hosmer, R.
Clement, A. Estrade, P.F. Mantica, F. Montes, C.
Morton, M. Ouellette, P. Santi, A. Stolz
r-processpath
77Ni
74Co
71Fe
Gated b-decay time curve(implant-decay time
differences)
Energy loss
Time of flight
Fast RIBs

• cocktail beams
• no inflight decay losses
• measure with low rates (gt1/day)

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Neutron Data
Nuclei with decay detected
With neutron in addition

420
420
Nn
370
370
DE (arb units)
DE (arb units)
320
320
76Ni
76Ni
73Co
73Co
270
270
220
220
350
400
450
500
550
350
400
450
500
550
TOF (arb units)
TOF (arb units)
neutron detection efficiency
(neutrons seen/neutrons emitted)
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Overview of common r process models

• Site independent models
• nn, T, t parametrization (neutron density,
  temperature, irradiation time)
• S, Ye, t parametrization (Entropy, electron
  fraction, expansion timescale)
• Core collapse supernovae
• Neutrino wind
• Jets
• Explosive helium burning
• Neutron star mergers

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Site independent approach
Goal Use abundance observations as general
constraints on r-process conditions
BUT need nuclear physics to do it
nn, T, t parametrization
(see Prof. K.-L. Kratz transparencies)
obtain r-process conditionsneeded
for which the right N50 andN82 isotopes are
waiting points(A80 and 130 respectively)
often in waiting point approximation
Kratz et al. ApJ403(1993)216
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S, Ye, t parametrization

• Consider a blob of matter with entropy S,
  electron abundance Ye in NSE
• Expand adiabatically with expansion timescale t
• Calculate abundances - what will happen
• NSE
• QSE (2 clusters p,n,a and heavy nuclei)
• a-rich freezeout (for higher S) (3a and aan
  reactions slowly move matter from p,n,a
  clusterto heavier nuclei once a heavy nucleus
  is created it rapidly captures a-particlesas a
  result large amounts of A90-100 nuclei are
  producewhich serve as seed for the r-process
• r-process phase initially n,g g,n
  equilibriumlater freezeout

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Evolution of equilibria
cross most abundant nucleuscolors degree of
equilibrium with that nucleus
(difference in chemical potential)
from Brad Meyers website
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Results forneutron to seed ratios(Meyer
Brown ApJS112(1997)199)

• n/seed is higher for
• lower Ye (more neutrons)
• higher entropy (more light particles, less
  heavy nuclei less seeds) (or low
  density low 3a rate slow seed assembly)
• faster expansion (less time to assemble
  seeds)

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r-process in Supernovae ?
Most favored scenario for high entropy
Neutrino heated wind evaporating from proto
neutron star in core collapse
ne neutrino sphere (nep ? ne weak opacity
because only few
protons present)
ne neutrino sphere (ven ? pe strong opacity
because many neutrons
present)
protoneutron star(n-rich)
weak interactions regulate n/p ratio
faster as ne come from deeperand are therefore
hotter !
nep ? ne
nen ? pe-
therefore matter is drivenneutron rich
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Results for Supernova r-process
Takahashi, Witti, Janka AA 286(1994)857(for
latest treatment of this scenario see Thompson,
Burrows, Meyer ApJ 562 (2001) 887)
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r-process in neutron star mergers ?
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Ejection of matter in NS-mergers
Rosswog et al. AA 341 (1999) 499
Destiny of Matterred ejected blue
tailsgreen disk black black hole
(here, neutron stars areco-rotating tidally
locked)
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r-process in NS-mergers
large neutron/seed ratios, fission cycling !
But Ye free parameter
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Summary theoretical scenarios
both seem to be viable scenarios (but discussion
ongoing)
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r- and s-process elements in stars with varying
metallicity
(Burris et al. ApJ 544 (2000) 302)
s-process
r-process
age