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

pocket

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