Results on 24,25,26 Mg(n, g ) and proposals Lisboa, 13-15 December 2011

Results on 24,25,26Mg(n,)

and proposals

Lisboa, 13-15 December 2011

Outline

• Introduction / Motivation• Results on 24,25,26Mg(n,)• Astrophysical implications

– s-process– Constraints on 22Ne(,n)25Mg

• Proposal for future measurements– 25Mg(n,) and 25Mg(n,tot)– 25Mg(n,) CHALLENGE!

Introduction

The s-process and Mg stable isotopes

Introduction


“Main component”22Ne(,n)25Mg is a neutron source in AGB stars: 1Msun < M < 3Msun

– kT=8 keV and kT=25 keV

Introduction


“Main component”22Ne(,n)25Mg is a neutron source in AGB stars: 1Msun < M < 3Msun


“Weak component”22Ne(,n)25Mg is the main neutron source in massive stars: M > 10 – 12Msun


Other important reactions:22Ne(,)26Mg, 25Mg(n,)26Mg, 26Al (n,p)26Mg

Motivations1. (24),25,26Mg are the most important neutron poisons due to neutron

capture on Mg stable isotopes in competition with neutron capture on 56Fe that is the basic s-process seed for the production of heavy isotopes.

2. Several attempts to determine the rate for the reaction 22Ne(,n)25Mg either through direct 22Ne(,n)25Mg measurement or indirectly, via 26Mg(,n)25Mg or charged particle transfer reactions. In both cases the cross-section is very small in the energy range of interest.

1. The main uncertainty of the reaction rate comes from the poorly known property of the states in 26Mg. Information can come from neutron measurements (knowledge of J for the 26Mg states).

3. The production of 26Al in the cosmos the main production mechanism is affected by uncertainties of several cross sections, in particular 24Mg(n,)

Resonance shape analysis

Transmission ORELA Capture n_TOF

MACS of 24Mg

KADoNisThis work

(Resonance + DRC)

(kT=5 keV)= 0.11 mb

(kT=30 keV)= 3.3±0.4 mb

(kT=80 keV)=2.7 mb

(kT=5 keV)= (0.17+0.04)±0.04 mb

(kT=30 keV)= (3.7+0.1)±0.2 mb

(kT=80 keV)=(2.6+0.2)±0.2 mb

Uncertainty reduced to 5%

p-wave Direct Radiative Capture (DRC) included

MACS of 25Mg

KADoNisThis work

(Resonance + DRC)

(kT=5 keV)= 4.8 mb

(kT=30 keV)= 6.4±0.4 mb

(kT=80 keV)=4.4 mb

(kT=5 keV)= (4.9+0.03)±0.6 mb

(kT=30 keV)= (4.0+0.1)±0.6 mb

(kT=80 keV)=(1.8+0.1)±0.3 mb

Uncertainty INCREASED to 15 %


MACS of 26Mg

KADoNisThis work

(Resonance + DRC)

(kT=5 keV)= 0.1 mb

(kT=30 keV)= 0.126±0.009 mb

(kT=80 keV)=0.226 mb

(kT=5 keV)= (0.067+0.02)±0.001 mb

(kT=30 keV)= (0.09+0.05)±0.01 mb

(kT=80 keV)=(0.29+0.08)±0.04 mb

Activation


MACS of 26Mg

MACS of 26Mg

Normalization of the DRC calculations

Activation Experiment

MACS of 26Mg

TOFExperiment

Check on the mass of the sample

Astrophysical implication

s-process abundancesReduced poisoning effect.

Lower MACS of 25Mg higher neutron density

KADoNiS• Present work

Abundance distribution of the weak s-process

Small impact on AGB abundances


Constraints for the 22Ne(,n)25Mg reaction

Element Spin/parity

22Ne 0+

4He 0+

€

rJ =

r I +

r i +

r l

€

rJ = 0 +

r l

Only natural-parity (0+, 1-, 2+, 3-, 4+, …) states in 26Mg can participate in the 22Ne(,n)25Mg reaction

MeV



Element Spin/parity

25Mg 5/2+

n 1/2+

€

rJ =

r I +

r i +

r l

€

rJ = 2 +

r l

s-wave J= 2+, 3+

p-wave J= 1-, 2-, 3-, 4-

d-wave J= 0+, 1+, 2+, 3+, 4+, 5+

States in 26Mg populated by 25Mg(n,) reaction

€

rJ = 3+

r l

MeV

MeV


22Ne(,n)25Mg

-particle energyLab.


22Ne(,n)25Mg

-particle energyLab.

T= (0.1-1.0)×109K


22Ne(,n)25Mg

0.5 0.6 0.9 1


22Ne(,n)25Mg

Ex (MeV) 26Mg* energy CM

0.5 0.6 0.9 1

11.037 11.207 11.376


22Ne(,n)25Mg

Ex (MeV) 26Mg* energy CM

0.5 0.6 0.9 1

11.037 11.207 11.376

En (keV)

25Mg(n,)22Ne, 25Mg(n,)26Mg, 25Mg+nNeutron energy Lab.

0 30 117 294


22Ne(,n)25Mg

0.5 0.6 0.9 1

En (keV)0 20 234

?

25Mg(n,)22Ne



25Mg(n,)26Mg



25Mg(n,)26Mg



25Mg(n,)26Mg

R. Longland et al., Phys. Rev. C 80, 055803, 2009“Nuclear resonance

fluorescence”

Before the 62.727-keV resonance was thought to be 1-



25Mg(n,)26Mg

?

J uncertain

NO constraint for 22Ne(,n)25Mg



25Mg(n,)26Mg

The reaction rate of the neutron source can be calculated:



25Mg(n,)26Mg

The reaction rate of the neutron source can be calculated:

•ER from (n,)• No information from neutron spectroscopy different values assumed



Upper limit for

Lower limit for

With respect to recommended value from NACRE

Conclusions

• Resonance parameters improved:– thermal cross section– doubtful resonances– corrected previous results

• MACS and related Astrophysical implication

• Constraints on the s-process neutron source 22Ne(,n)25Mg

Future measurements

Capture and transmission experiment on

ENRICHED 25Mg

(metallic!) sample:• Improve uncertainties• Determine J

24Mg

25Mg(n,)22Ne

The 25Mg(n,)22Ne (Q-value=480 keV) cross-section is linked to the 22Ne(n)25Mg

€

(a,b )

σ (b,a )

=mbmB EbB (2Jb +1)(2JB +1)

mamA EaA (2Ja +1)(2JA +1)

a A B b

Energy region of interest:0 < En < 300 keV

25Mg(n,)22Ne

Energy region of interest:0 < En < 300 keV

Reaction Q-value (MeV)25Mg(n,a)22Ne 0.4825Mg(n,p)25Na -2.525Mg(n,D)24Na -9.825Mg(n,t)23Na -10.5

25Mg(n,)22NeRange of 480-keV -particle few µm

The corresponding areal density of a 1-µm thick 25Mg sample is 4.2×10-6 atoms/barn.

25Mg(n,)22Ne

n = 4.2×10-6 atoms/barn = 1 mb = 50% (using a thin magnesium layer

and a 2 a particle detector)

P = 80%

the resulting counting rate, as a function of the neutron fluence is:

CR=2×10-8 ×

For instance in

• EAR-1, EAR-1 = 7.1×104 neutrons are delivered per proton burst in the 10-100 keV energy range

• EAR-2, EAR-2 ≈ 1.8×106

1 signal / 25 bunches 30 signals / hour

€

CR = nσεPα φ

Detector

• Diamond

• MicroMegas

• Silicon

• Compensated ionization chamber

• …

Conclusion

GOOD IDEAS are WELCOME

Cristian MassimiDipartimento di Fisica

[email protected]

www.unibo.it

Neutron sensitivity

Neutron sensitivity

Documents

Results on 24,25,26 Mg(n, g ) and proposals Lisboa, 13-15 December 2011