The neutron source for the weak component of the s-process: latest experimental results
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Transcript of The neutron source for the weak component of the s-process: latest experimental results
The neutron source for the weakcomponent of the s-process:latest experimental results
Claudio UgaldeUniversity of North Carolina at Chapel Hilland Triangle Universities Nuclear Laboratory
OUTLINE
●Synthesis of nuclei beyond iron in stars: the s-process●The main and weak components of the s-process●The 22Ne(,n)25Mg as a neutron source●The current status of the reaction rate●The 22Ne(6Li,d) experiment and results●Conclusions
The S-PROCESS
The slow neutron capture process (s-process) is responsible
for the synthesis of most nuclei heavier than iron.
● The s-process involves neutron captures with the
emission of gamma radiation (n,).● The captures occur at a SLOW rate compared to the beta
decay rate.
(n,)
-
FAST!
Slooow
Stable
Unstable
Z
N
●Therefore, the s-process follows the path of ●beta stable nuclei.
Charged-particle reactions synthesize nuclei in the low-mass region
of the B/A curve by exoergic processes up to the iron-like nuclei,
where the nucleon binding energy has a maximum.
Beyond iron, nuclear processes become endoergic. The result is an
abundance peak around A=58.
The Coulomb barrier hinders charged
particle reactions at these high Z, but ...
( n,)
Neutron captures are favored for N>30.
In a nutshell, the s-process is a series of neutron captures along the
valley of stability that requires iron-like nuclei as a seed.
But where do neutrons come from?
THE SITES OF THE S-PROCESS
AGB stars
(6 > MSun > 0.8)
NGC 6543, HST
Massive stars
(M > 13 MSun)
Betelgeuse, HST
AGB starsKarakas, Ph.D. Thesis 2003
M3, NOAO
Convective
envelope H burningHe burning
C-O core
Neutron source in AGB stars
H envelope
Convective
He intershell
PulseH mixing
C-O core
mixing
()13C (,n)16O12C(p,)13N
12C(,)16O
but ...
14N(n,p)14C
For nuclei with A>90 the phenomenological N ( = cross section,
N = s-only nuclei abundance) curve describes the data fairly well.
However, for 60<A<90 TWO contributions (each with
its own neutron exposure are needed.
The s-process abundance pattern has contributions from
two components:
a) Main component (A>90)
AGB stars, 13C(,n)16O
b) Weak component (60<A<90)
Massive stars
Betelgeuse, HST
The weak component of the s-process
It has been proposed that the site of the weak component of
the s-process is stars with M>13Msun.
The weak component helps to constrain the contribution of
the main component to the nucleosynthesis of nuclei with
60<A<90.
It also depends very strongly on the initial metallicity of the
star, so it may be used to study the role of massive stars
is the early phase of the chemical evolution of the galaxy.
It was proposed in 1968 by Peters (ApJ 154, 224) that the main
neutron source triggering the s-process in massive stars is
the 22Ne(,n)25Mg reaction.
14N()18F()18O()22Ne
or14C()18O()22Ne
but ...
25Mg(n)26Mg
22Ne()26Mg
22Ne(n)25Mg
The chain proceeds as follows:First, the CNO cycle (main mechanism of hydrogen burning in massive stars) enriches the core of the massive star with 14N.
Red giant in hydra supercluster
The rates for 22Ne()26Mg and 22Ne(,n)25Mg
Both reactions are in competition with each other at low temperatures.
It is also possible to obtain isotopic abundances from analyzing
presolar grains.
There is very limited experimental and theoretical information about
possible natural parity resonances in 26Mg in the energy of relevance to
neutron production for the s-process.
Both rates carry considerable uncertainties.
Both reactions are important producers of the magnesium isotopes
(25Mg and 26Mg).
However, at least we are lucky in that Mg is one of the few elements for
which we can obtain isotopic information from stellar spectroscopy.
From Karakas et al., astro-ph/0601645
The rate for 22Ne(,n)25Mg
In the temperature range between 0.3 and 0.5 GK the
rate is dominated by the Ex=11.328 MeV resonance,
measured by Jaeger et al, 2001.
For T< 0.3 GK the rate is dominated by the threshold
states (still unmeasured). The largest uncertainty in the
rate is associated with this low temperature range
(~1 order of magnitude).
The uncertainty depends mainly on the spectroscopic
-strengths of the threshold resonances.
NGC4526
The rate for 22Ne()26Mg
Most of the subneutron threshold information used to
evaluate the rate comes from a 22Ne(6Li,d)26Mg experiment
at Notre Dame. The deuteron spectra resolution came out
to be 120 keV.
The largest uncertainty in the rate comes from the spin-parity
values of the Ecm=330 keV resonance (Ex=10.95 MeV).
Unluckily, this is the most important resonance in the rate.
Possible contributions to the rate may also come from the
Ecm=538 keV, 568 keV, and 711 keV resonances.
Cross over region
To give an idea on how the current situation is
for 22Ne()26Mg below the neutron threshold...
10.646 10.650 10.682 10.693 10.707 10.719 10.726 10.746 10.767 10.806 10.824 10.881 10.893 10.915 10.927 10.945 10.978 10.998 11.012 11.048 11.0840
1
2Included in rate
Spin-parity known?
Resonances reported by Endt 1990 below the neutron threshold
A lot of experimental work is urgent!
What is needed
b) Determine the quantum numbers of 26Mg states around the
neutron threshold. Of special interest is the state at 10.95 MeV.
a) Resolve states in 26Mg below the neutron threshold by improving
the energy resolution of previous experiments.
A plausible solution would be to study the 22Ne(6Li,2H)26Mg
transfer reaction at lab energies where the direct reaction
mechanism is dominant (say 30-40 MeV) and populate states
in 26Mg.
The experiment
The 6Li beam could be accelerated without problem
by a Tandem and the target could be prepared by implanting22Ne on a thin carbon foil. The reaction products can then be
analyzed with a split pole spectrometer positioned at several
angles.
NGC 6543, HST
The excitation energy could be reconstructed from the energy of
the deuterons detected at the focal plane, the reaction kinematics,
and the energy losses in the target.
On the other hand, we shall try to obtain the spins of 26Mg states
by measuring angular distributions moving the spectrometer to
different angles and then analyzing in terms of DWBA.
The experiment (continued)
Target preparation with the Eaton ion implanter at North Carolina
• Produces stable beams from 20 keV to 200 keV with a mass resolution m/m ~ 0.01.
• Beam currents can be obtained at hundreds of A
22Ne targets
•40g/cm2 12C-enriched foils•Implanted on both sides, two energies each•Dose ~ 20 mC per target•Targets are very, very fragile. Substrates can withstand up to 400 nA of 22Ne beam
The Wright Nuclear Structure Laboratory floorplan
ESTU-1 Tandem Van de Graaff Accelerator at Yale
Vmax= 22.5 MV
Enge split-pole spectrometer
Bmax ~ 14-15 kG max = 12.8 msr
Focal plane detector
Position resolution ~ 1mm
Gas filled (isobutane @150 Torr)
E (cathode), E (Plastic scintillator), position (FW and BW)
22Ne(6Li,d)26Mg
6Li beam, @ 30 MeV
BEnge = 13.0 kG
Engeo
Focal plane coincident with the front wire.
Enge = 1.5 msr
Particles enter the detector at 45o relative to the wires
E~80 keV (as opposed to ~120 keV in Giesen et al.1994)
Red giant in hydra supercluster
22Ne implanted on 12Cdeuteron spectrum
12C, deuteron spectrum
16O
16O - 6.05, 6.13 MeV16O – 6.92, 7.12 MeV
16O
26Mg – 10.95 ?, 10.82 MeV
Target content analysis
6Li beam, @ 30 MeV
BEnge = 7.7 kG
Engeo
Elastic scattering experiment
Enge = 1.5 msr
12C substrate, 6Li spectrum
22Ne-implanted, 6Li spectrum
12C
22Ne
27Al 35Cl?
56Fe16O
12C
16O27Al
35Cl?
56Fe
Offline focus
Shapira et al., NIM 129(1975),123
Focal plane
Trajectory
Solving for x and y
S = 3.5 cm
before focus
after (S/H=2)
Old situation
New situation
Conclusions
We observed the 10.82 MeV state in 26Mg; it is likely tohave natural parity, thus would contribute significantlyto the rate of the 22Ne()26Mg reaction.
Both the 22Ne(,n)25Mg and 22Ne()26Mg reactions hold large uncertainties at temperatures of relevance to the s-process.
We failed to measure the spin and parity of the 10.95 MeV state in 26Mg. We’ll try next time.
Thank you!
Eta CarinaeUniversity of Colorado & NASA
North Carolina
Art ChampagneStephen DaigleChristian IliadisJoseph NewtonEliza Osenbaugh
Yale
Jason ClarkCatherine DeibelAnuj ParikhPeter ParkerChris Wrede