Transcript No Slide Title

Studies of r-process nuclei with fast radioactive beams
Fernando Montes
National Science Superconducting Cyclotron
Joint Institute for Nuclear Astrophysics
Supernova 2002bo in NGC 3190
Outline
• Motivation: Origin of the elements heavier than iron
• Signatures of different nucleosynthesis processes in the solar
system and in the abundances of metal-poor stars
Supernova 1997bs in M66
• Nuclear properties required for an understanding of the r-process
• R-process experiments at the NSCL
• Conclusions
Creation of the elements
Nucleosynthesis is a gradual, still ongoing process:
Dense clouds
Big Bang
M~104..6 Mo
108 y
Condensation
Star formation
Interstellar
medium
H, He
continuous
enrichment,
increasing
metallicity
M > 0.7Mo
Life of a star
Nucleosynthesis:
Stable burning
106..10 y
Dust mixing
Remnants
(White dwarf,
neutron star, black hole)
Nucleosynthesis
Nucleosynthesis:
Explosive burning
Death of a star
(Supernova, planetary nebula)
Creation of the elements: nucleosynthesis
Most of the heavy elements (Z>30)
are formed in neutron capture
processes, either the slow (s)
or rapid (r) process
p process
Mass known
Half-life known
nothing known
r process
rp process
np process
Light element primary process
LEPP
stellar burning
s process
protons
Big Bang
Cosmic Rays
neutrons
Contribution of different processes
Contribution of the diff. processes to the solar abundances
Ba: s-process
Eu: r-process
s-process:
Astrophysical model
Ba
Eu
p-process:
Astrophysical model
r-process:
Abundance of
enriched-r-process
star
LEPP = solar-s-p-r
Metal-poor star abundances
underabundant
agreement stars and solar
Metallicity
(amount of iron) ~ time
“Solar r”
e
Very metal-poor stars
are enriched by just a
few nucleosynthesis
events
R-process + LEPP
F. Montes
Nuclear Astrophysics
R-process basics
Element formation beyond iron involving rapid neutron
capture and radioactive decay
High neutron density
b-decay
Seed
Waiting point
(n,g)-(g-n) equilibrium
Y(Z,A+1)
G(Z,A+1) Sn(Z,A+1)/kT
~ nnT-3/2
e
G(Z,A)
Y(Z,A)
Waiting point approximation
F. Montes
Nuclear Astrophysics
Nuclear physics in the r-process
b-delayed n-emission
branchings
(final abundances)
Fission rates and distributions:
• n-induced
• spontaneous
• b-delayed
b-decay half-lives
(progenitor abundances,
process speed)
n-capture rates
Smoothing progenitor
abundances during freezeout
n-physics ?
Seed production
rates
Masses:
• Sn location of the path
• Qb, Sn theoretical b-decay properties,
n-capture rates
r-process beams at the NSCL Coupled Cyclotron Facility
r-process
beam
Delta E
Future: low energy beams1-2 MeV/u
Tracking
(=Momentum)
TOF
Primary beam
100-140 MeV/u
Be target
Experimental station
Fast beams from
fragmentation with
Coupled Cyclotrons
F. Montes
Nuclear Astrophysics
Implantation station:
The Beta Counting System (BCS)
•Implantation DSSD:
x-y position (pixel), time
•Decay DSSD:background)
Fit (mother, daughter, granddaughter,
x-y position (pixel), time
T
1/2
Veto light particles
from A1900
105Zr
4 x Si PIN
DSSD (40×40)
6 x SSSD (16)
Ge
Silicon PIN Stack
Beta calorimetry
F. Montes
Nuclear Astrophysics
Implantation station:
The Neutron Emission Ratio Observer (NERO)
3He
Proportional BF3 Proportional
Counters
Counters
Polyethylene
Moderator
Boron Carbide
Shielding
G. Lorusso, J.Pereira et al., PoS NIC-IX (2007)
F. Montes
Nuclear Astrophysics
Implantation station:
The Neutron Emission Ratio Observer (NERO)
Nuclei with b-decay
Nuclei with b-decay AND neutron(s)
Pn-values
Measurement of neutron in “delayed”
coincidence with b-decay
F. Montes
Nuclear Astrophysics
Implantation station:
The Segmented Germanium Array (SeGA)
16 SeGA detectors around the BCS
Efficiency ~7.5% at 1 MeV
W.Mueller et al., NIMA 466, 492 (2001)
F. Montes
Nuclear Astrophysics
Implantation station:
The Segmented Germanium Array (SeGA)
b-delayed gamma spectroscopy of daughter
F. Montes
Nuclear Astrophysics
Astrophysics motivated experiments
Known before
NSCL reach
120Rh
107Zr
78Ni
Critical region
NSCL Experiments done
69Fe
F. Montes
• P. Hosmer, P. Santi, H. Schatz et al.
• F. Montes, H. Schatz et al.
• B. Tomlin, P.Mantica, B.Walters et al.
• J. Pereira, K.-L.Kratz, A. Woehr et al.
• M. Matos, A. Estrade et al.
Nuclear Astrophysics
I) b-decay half-live of 78Ni50 waiting point
Predicted 78Ni T1/2: 460 ms
1.E+02
+100
Abundance (A.U.)
Exp. 78Ni T1/2 = 110 -60 ms
Observed Solar Abundances
Model Calculation: Half-Lives from
Moeller, et al. 97
Same but with present 78Ni Result
1.E+01
1.E+00
1.E-01
1.E-02
70
170
120
220
Mass (A)
P. Hosmer et al. PRL 94, 112501 (2005)
Half-live of ONE single waiting-point nucleus:
 Speeding up the r-process clock
 Increase matter flow through 78Ni bottle-neck
 Excess of heavy nuclei (cosmochronometry)
II) “Gross” nuclear structure around
120Rh
45 from b-decay properties
Inferring (tentative) nuclear deformations with QRPA model calculations
F. Montes et al., PRC73, 35801 (2006)
•120Rh Pn value direct input in
r-process calculations
•Half-lives and Pn-values
sensitive to nuclear structure
• Over-predictions for Ru and
Pd isotopes: larger Q-values
or problems in the GT strength
• Need microscopic
calculations beyond QRPA
F. Montes
Nuclear Astrophysics
II) Probing the strength of N=82 shellclosure from b-delayed g-spectroscopy
B.Walters, B.Tomlin et al., PRC70 034414 (2004)
• No evidence of shell-quenching when
approaching shell closure in Pd isotopes up
to N=74
• Need more E(2+) data at 74<N<82
• R-process abundances at A~115 are
directly affected by the strength of shell
closure
• Experimental evidence is mixed: 130Cd
E(2+) does not show evidence of quenching
F. Montes
Nuclear Astrophysics
III) b-decay properties of Zr isotopes
beyond mid-shell N=66
Half-life (ms)
10000
1000
Zr literature
Zr preliminary
QRPA Def.
QRPA Spher.
100
J.Pereira et al., in preparation
10
62
63
64
65
66
67
68
N
•Possible double-magic Z=40, N=70: Effects from spherical shape of 110Zr70
observable at 66<N<70?
•Shorter half-life of (potential) waiting-point 107Zr affect predicted r-process
abundances at A~110
•Mean-field model calculations predict N=82 shell-quenching accompanied
by a new harmonic oscillator shell at N=70
Nuclear Physics
Same “astrophysical model”, different nuclear physics …
• Theoretical models
are in the majority of
cases within a factor of
3 from observed
abundance
• Models agree within a
factor of 3-4 except for
In (Z=49) and Lu (Z=71)
Montes et al. AIP Conf. Proc.,
947, 364 (2007).
This “agreement” however is not good enough to
calculate LEPP isotopic abundances
F. Montes
Nuclear Astrophysics
Light element primary process (LEPP)
LEPP = solar-s-p-r
If it involves high neutron densities
peak should be here
If it involves low neutron densities
peak should be here instead
F. Montes
Nuclear Astrophysics
Reach for future r-process experiments
with new facilities (ISF, FAIR, RIBF…)
Almost all b-decay half-lives of r-process nuclei at N=82
and N=126 will be reachable with ISF
Known before
NSCL reach
107Zr
78Ni
NSCL Experiments done
Future Facility Reach
(here ISF)
Conclusions
•Despite many years of intensive effort, the r-process site and the
astrophysical conditions continues to be an open question. New
LEPP process complicates the situation
•Besides being direct r-process inputs, beta-decay properties of
exotic nuclei turned out to be an effective probe for nuclear structure
studies of exotic nuclei
•R-process experimental campaigns at NSCL provide beta-decay
properties of r-process nuclei and comparisons with theoretical
calculations will improve astrophysical r-process calculations
•New facilities will largely extend the r-process regions accessible
(FAIR, ISF). Meanwhile, new observations (SEGUE) and new
measurements of exotic n-rich nuclei are highly necessary
F. Montes
Nuclear Astrophysics
More metal-poor stars
Qian & Wasserburg Phys. Rep 442, 237 (2007); Montes et al. ApJ 671 (2007)
Slope indicates
ratio of light/heavy
[Y/Eu]
[La/Eu]
Z=39
Z=57
Some stars have
light elements
at solar level
[Sm/Eu]
Z=47
[Ag/Eu]
Light elements
at high enrichment fairly robust
and subsolar
Solar r
Z=62
[Eu/Fe]
Multiple nucleosynthesis
processes in the early universe
F. Montes
[Eu/Fe]
Heavy r-pattern
robust and
agrees with
solar
Metal poor star =
r-process
+
Light element primary process
Nuclear Astrophysics
Summary features of fast beams from fragmentation
Fast beams from fragmentation complement other techniques and
they have these particular features :
• High selectivity even with mixed (“cocktail”) beams because due to its high
energy, relevant particle properties can be detected (TOF, energy losses …)
• Fast beam – negligible decay losses (~100 nanoseconds..)
• Production of broad range of rare isotope beams with a
single primary beam
Typical beam energies: 50-1000 MeV/nucleon
Typical new rare isotope beams can be produced within ~ 1h
F. Montes
Nuclear Astrophysics
Nuclear physics behind everything…
number fraction
0
10
-1
10
-2
10
-3
10
-4
10
-5
10
-6
10
-7
10
-8
10
-9
10
-10
10
-11
10
-12
10
-13
10
Gap
B,Be,Li
a-nuclei
12C,16O,20Ne,24Mg, …. 40Ca,44Ti
r-process peaks (nuclear shell closures)
s-process peaks (nuclear shell closures)
U,Th
Fe peak
(width !)
Au Pb
0
50
F. Montes
100
150
mass
Massnumber
number
200
Nuclear Astrophysics
250