The origin of heavy elements in the solar system

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The origin of heavy elements in the solar system
(Pagel, Fig 6.8)
each process contribution is a mix of many events
!
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Heavy elements in Metal Poor Halo Stars
3
A single (or a few) r-process event(s)
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Overview heavy element nucleosynthesis
process conditions timescale site
s-process(n-capture, ...) T 0.1 GKtn 1-1000 yr, nn107-8/cm3 102 yrand 105-6 yrs Massive stars (weak)Low mass AGB stars (main)
r-process(n-capture, ...) T1-2 GKtn ms, nn1024 /cm3 lt 1s Type II Supernovae ?Neutron Star Mergers ?
p-process((g,n), ...) T2-3 GK 1s Type II Supernovae
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The r-process
Temperature 1-2 GK Density 300 g/cm3 (60
neutrons !)
Seed
Proton number
Neutron number
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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 !)

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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
Quantity Effect
Sn neutron separation energy path
T1/2 b-decay half-lives abundance pattern timescale
Pn b-delayed n-emission branchings final abundance pattern
fission (branchingsand products) endpoint abundance pattern? degree of fission cycling
G partition functions path (very weakly)
NAltsvgt neutron capture rates final abundance pattern during freezeout ? conditions for waiting point approximation
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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)