Title: The origin of heavy elements in the solar system
1The origin of heavy elements in the solar system
(Pagel, Fig 6.8)
each process contribution is a mix of many events
!
2Abundance pattern Finger print of the
r-process ?
Solar abundance of the elements
Abundance (Si 106)
Element number (Z)
But sun formed 10 billion years after big bang
many stars contributed to elements
? This could be an accidental combination of many
different fingerprints ? ? Find a star that is
much older than the sun to find fingerprint of
single event
3Heavy elements in Metal Poor Halo Stars
4A single (or a few) r-process event(s)
5New Observations r-process elements from single
r-process eventsin 3 very metal poor stars
6Overview 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
Light Element Primary Process (LEPP) ? ? (maybe s-process like?) ? (long if s-process) ?
7The r-process
Temperature 1-2 GK Density 300 g/cm3 (60
neutrons !)
Seed
Proton number
Neutron number
8Waiting 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 !)
9Pt
Xe
Ni
10Nuclear physics in the r-process
- Fission rates and distributions
- n-induced
- spontaneous
- b-delayed
b-delayed n-emissionbranchings(final abundances)
b-decay half-lives(abundances and process speed)
- n-capture rates
- in slow freezeout
- maybe in a weak r-process ?
n-phyiscs ?
Seed productionrates (aaa,aan, a2n, ..)
Masses (Sn)(location of the path)
11Sensitivity of r-process to astro and nuclear
physics
Sensitivity to astrophysics
Sensitivity to nuclear physics
Hot bubble
ETFSI-Q masses
Classical model
ETFSI-1 masses
Same nuclear physics
Same r-process model
Abundance
Freiburghaus et al. 1999
Mass number
Mass number
Contains information about
But convoluted with nuclear physics
- masses (set path)
- T1/2, Pn (Y T1/2(prog), key
waiting points set timescale) - n-capture rates
- fission barriers and fragments
- n-density, T, time (fission signatures)
- freezeout
- neutrino presence
- which model is correct
12Shell quenching effect on masses/r-process
Cd
Pd
Ru
Mo
Zr
S2n (MeV)
ETFSI-1
Neutron number
13Shell quenching effect on masses/r-process
Cd
Pd
Ru
Mo
Zr
S2n (MeV)
ETFSI-1
Neutron number
14Endpoint 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)
15Consequences 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 !)
16Role 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 !)
17during 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 !
18Cs (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
19Summary 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
20National 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
21NSCL Coupled Cyclotron Facility
W. Benenson (NSCL) and B. Richards (WKAR)
22Installation of D4 steel, Jul/2000
23First 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)
24NSCL 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
25NERO 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
26NERO Assembly
27Nero efficiency
28Particle Identification
r-process nuclei
Energy loss in Si (Z)
- Fast RIB from fragmentation
- no decay losses
- any beam can be produced
- multiple measurements in one
- high sensitivity
Time of flight (m/q)
29Decay data
time (ms)
time (ms)
time (ms)
Fast radioactive beams
- No decay losses
- Rates as low as 1/day useful !
- Mixed beam experiments easy
30Results for the main goal 78Ni (14 neutrons
added to stable Ni)
Decay of 78Ni major bottle-neck for synthesis
of heavy elements in the r-process
Managed to create 11 of the doubly magic 78Ni
nuclei in 5 days
Time between arrival and decays
Result for half-life 110 100-60 ms Compare to
theoretical estimate used470 ms
StatisticalAnalysis
time (ms)
- Acceleration of the entire r-process
- Models need to be adjusted to explain observed
abundance distribution
31Neutron 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)
32Results (Hosmer et al.)
DFCQRPA Borzov et al. 2005, QRPA Moller
et al. 2003, Shell model Lisetzky Brown
2005
T1/2 (s)
A
A
33Impact of 78Ni half-life on r-process models
- need to readjust r-process model parameters
- Can obtain Experimental constraints for r-process
models from observations and solid nuclear
physics - remainig discrepancies nuclear physics ?
Environment ? Neutrinos ? Need more data
34NSCL and future facilities reach
Bright future for experiments and observations?
Experimental test of r-process models is
within reach? Vision r-process as precision
probe
35Towards an experimental nuclear physics basis for
the r-process
Final isotopes, for which gt90 of progenitors in
the r-process path can be reachedexperimentally
for at least a half-life measurement
Solar r-
? These abundances can be compared with
observations to test r-process models
36Collaboration
78Ni Collaboration
Mainz O. Arndt K.-L. Kratz B. Pfeiffer
MSU P. Hosmer R.R.C. Clement A. Estrade P.F.
Mantica F. Montes C. Morton W.F. Mueller E.
Pellegrini P. Santi H. Schatz M. Steiner A.
Stolz B.E. Tomlin M. Ouellette
Pacific Northwest Natl. Lab. P. Reeder
Notre Dame A. Aprahamian A. Woehr
Maryland W.B. Walters
37Overview 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
38Site 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
39S, 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
40Evolution of equilibria
cross most abundant nucleuscolors degree of
equilibrium with that nucleus
(difference in chemical potential)
from Brad Meyers website
41Results 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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43How does the r-process work ? Neutron capture !
44r-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
45Results 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)
46other problem the a effect
Recall equilibrium of nucleons in neutrino wind
nep ? ne
Maintains a slight neutron excess
nen ? pe-
What happens when a-particles form, leaving a mix
of a-particles and neutrons ?
47r-process in neutron star mergers ?
48Ejection 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)
49r-process in NS-mergers
large neutron/seed ratios, fission cycling !
But Ye free parameter
50Summary theoretical scenarios
NS-mergers Supernovae
Frequency(per yr and Galaxy) 1e-5 - 1e-4 2.2e-2
Ejected r-process mass(solar masses) 4e-3 4e-2 1e-6 1e-5
Summary less frequent but more ejection more frequent and less ejection
51What does galactic chemical evolution
observations tell us ?
Argast et al. AA 416 (2004) 997
NS mergers
Supernovae
Model star averagewith error
observations
AverageISM
Dots model stars
52r- and s-process elements in stars with varying
metallicity
(Burris et al. ApJ 544 (2000) 302)
s-process
r-process
age
53Multiple r-processes
Star to star stability of all elements (for very
r-rich stars)
Star to star scatter of light vs heavyfor all
stars Fe/Hlt-2.5, no s-process
(J.J. Cowan)
(Honda et al. 2004)
- Additional light element primary process (LEPP)
exists (Travaglio et al. 2004 , Montes et al.
2006 to be published) - ? It contributes to solar r-process residual
abundances
54Honda et al. 2006
Ivans et al. 2006
55Honda et al. 2006
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57? Disentangling by isotope?
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