Neutrons at CERN n_TOF: A window to stellar evolution and nucleosynthesis

Neutrons at CERN n_TOF: A window to stellar evolution andnucleosynthesis

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• (Astro)Physics case for neutron cross section measurements• Example of measurements performed so far at n_TOF• The present experimental plan• New opportunities with a second experimental beam-line

The n_TOF Collaboration

A Mengoni, ENEA, Bologna – IAEA, Vienna

Why neutrons in astrophysics?

e-charge : 0mass : 940 MeVhalf-life : 10 min

Now4.5 Gyr

solar-

system

formati

on

BANG!


GCE

Abundances: Anders and Grevesse, 1989

Nucleosynthesis of the Elements



Nuclear reactions:• energy generation• nucleosynthesis

condensation

ejec

tion,

exp

losi

on

interstellargas & dust

Mixing(abundance distribution)



All chemical elements beyond Ironare synthesized by neutron interactionsin stars

~ ½ by the s-process (red giants)~ ½ by the r-process (explosive)



56Fe 91.72

57Fe 2.2

58Fe 0.28

Fe

Co

60Ni 26.223

59Co100

59Fe 44.503 d

60Fe 1.5 106 a

60Co5.272 a

61Co1.65 h

61Ni 1.140

62Ni 3.634

Ni 63Ni 100 a

64Ni 0.926

58Co70.86 d

62Cu9.74 m

63Cu69.17

64Cu 12.7 h

61Fe 6 m

Cu

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56Fe 91.72

57Fe 2.2

58Fe 0.28

Fe

Co

60Ni 26.223

59Co100

59Fe 44.503 d

60Fe 1.5 106 a

60Co5.272 a

61Co1.65 h

61Ni 1.140

62Ni 3.634

Ni 63Ni 100 a

64Ni 0.926

58Co70.86 d

62Cu9.74 m

63Cu69.17

64Cu 12.7 h

61Fe 6 m

Cu

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The canonical s-process


Neutron capture cross sections

reaction rate: NA Nn<sn,g(En)·v>kT

key quantity: sn,g(En)

canonical s-process: s · NA ~ const.



weak

main

A

i iA

AAAA

NfNN

56

1

00

56

0

11 11/11 ss

ss

from phenomenological…



C Arlandini, et al.: ApJ 525 (1999) 886

• M = 2 M• Z = 0.5 Z

… to detailed stellar modeling: TP-AGB

Needed: cross sections with uncertainties between 1 and 5% for complete set of isotopes from 12C to 210Po,

including unstable samples

Status and requests

Source: F Käppeler (2009)

wide energy range high neutron flux &

high energy resolution low repetition rate

of the proton driver

457

130

518718001280

2

1

2

1

mm.

/s/s.

LL

ff

comparison with GELINA (~ same average flux at 30m)

232Th(n,g)

10 100 1000 10000 10000010-1

100

101

102

Res

pons

e (c

ount

s / n

s)Neutron Energy / eV

n-TOF 232Th (0.0041 at/b) 208Pb

GELINA 232Th (0.0016 at/b) 208Pb

source: P Rullhusen (GELINA)

Basic characteristics of experiments at n_TOF


Basic characteristics of experiments at n_TOF

wide energy range high neutron flux &

high energy resolution low repetition rate

of the proton driver low background conditions

U Abbondanno et al. (The n_TOF Collaboration)Phys. Rev. Lett. 93 (2004), 161103

&S Marrone et al. (The n_TOF Collaboration)

Phys. Rev. C 73 03604 (2006)151Sm(n,g)


0.0E+00

5.0E-09

1.0E-08

1.5E-08

2.0E-08

2.5E-08

3.0E-08

0 1 2 3 4 5 6

Temperature T8 [K]

deca

y ra

te [1

/s]

ne = 1, 3, 10, 30×1026 cm-3

• The branching at 151Sm can be used to determine the temperature during the s process.

T = 350 ± 40 Million KelvinkT = 30 keV

Sm

Eu

Gd

150Sm

151Sm 93 a

152Sm

153Sm

154Sm

151Eu

152Eu

153Eu

154Eu

155Eu

156Eu

152Gd

153Gd

154Gd

155Gd

156Gd

157Gd

Sm

Gd151

150

152

Sm NσNσ

)(λλλ

fnβ

ββ

Example: Branching point at 151Sm

source: M Heil

Measurements performedCapture151Sm204,206,207,208Pb, 209Bi232Th24,25,26Mg90,91,92,94,96Zr, 93Zr139La186,187,188Os233,234U237Np,240Pu,243Am

Fission233,234,235,236,238U232Th209Bi237Np241,243Am, 245Cm


s-process branching, temperature, densitytermination of the s-process, nucleosynthesis of Lead

neutron source for the s-process: 22Ne(a,n)25Mg

massive stars, pre-solar grains

nuclear cosmo-chronometer

Capture measurementsMo, Ru, Pd stable isotopes

Fe, and Ni (all stable isotopes)

A≈150 (isotopes varii)

79Se, Se (stable isotopes)

r-process residuals calculationisotopic patterns in SiC grains

s-process nucleosynthesis in massive stars

s-process branching pointslong-lived fission products

s-process branching (T and r for the weak component)

Other measurements147Sm(n,a), 67Zn(n,a), 99Ru(n,a)58Ni(n,p), other (n,lcp)

p-process studies

n_TOF-Ph2

(*) Approved by the RB (P208/n_TOF13)

Planned measurements


>>

n_TOF-Ph2

n_TOF target

NewExperimentalArea (EAR-2)

The second n_TOF beam line & EAR-2

EAR-1 (at 185 m)

~ 20 m

Flight-path length : ~20 mat 90° respect to p-beam directionexpected neutron flux enhancement: ~ 100drastic reduction of the t0 flash

Improvements (ex: 151Sm case)

sample mass / 3 s/bkgd=1

use BaF2 TAC e x 10 use D2O F30 x 5 use 20 m flight path F30 x

100

consequences for sample mass

50 mg 5 mg 1 mg 10 mg

boosts sensitivity by a factor of 5000 !

problems of sample production and safety issues relaxed

EAR-2: Optimized sensitivity



Possible measurements at EAR-2

• 79Se• 90Sr• 126Sn• 147Pm• 135Cs

all s-process branching points + they are important fission fragments


ConclusionNeutron cross sections are key quantities for studying stellar evolution and nucleosynthesis. n_TOF offers the best conditions to obtain these nuclear physics quantities with the required accuracy

The n_TOF Collaboration is carrying on an extensive plan to measure cross sections relevant for nuclear astrophysics, in particular for s-process nucleosynthesis studies

Opportunities for obtaining new data for presently inaccessiblenuclei (using extremely low quantities of material) will be open with the implementation of a second beam-line and a new experimental area (EAR-2)


The end

n_TOF-Ph2

Motivations:

• Study of the weak s-process component (nucleosynthesis up to A ~ 90)

• Contribution of massive stars (core He-burning

phase) to the s-process nucleosynthesis.

• s-process efficiency due to bottleneck cross sections (Example: 62Ni)

In addition:

Fe and Ni are the most important structural materials for nuclear technologies. Results of previous measurements at n_TOF show that capture rates for light and intermediate-mass isotopes need to be revised.

Capture studies: Fe, Ni, Zn, and Se<<back

n_TOF-Ph2

32

30

34

38 40 42 44 46 48 50

The 79Se case

• s-process branching: neutron density & temperature conditions for the weak component.

• t1/2 < 6.5 x 104 yr

Capture studies: Fe, Ni, Zn, and Se<<back

Observational uncertainties

Cross section uncertainties

Bao et al. ADNDT 76 (2000)

• Stellar model • s-abundance calculation• r-abundance (CS22892-

052)

See:Tavaglio et al., APJ521 (1999)Arlandini et al., APJ525 (1999) s-only

isotopes

Need for more than one r-process or need for improved s- abundances and cross sections?

In general:

<<back

Source: F Herwig

<<

AGB stars

n_TOF experiments & publications

Capture151Sm204,206,207,208Pb, 209Bi232Th24,25,26Mg90,91,92,94,96Zr, 93Zr139La186,187,188Os233,234U237Np,240Pu,243Am

Fission233,234,235,236,238U232Th209Bi237Np241,243Am, 245Cm

In summary:

Facility & Experimental setup : 11 papersMeasurements : 10 papers

(1 PRL, 9 PRC)Conference Proceedings : 51Total number of documents on db : 150


All data (published) can be found in the EXFOR database >>

http://nds121.iaea.org/alberto/n_TOF/wiki/index.php/Documentation:Papers

151Sm

204,206,207,208Pb, 209Bi

24,25,26Mg

90,91,92,94,96Zr, 93Zr, 139La

186,187,188Os

Measurements performed so far

The n_TOF Collaboration www.cern.ch/n_TOF

s-process branching, temperature, density

termination of the s-process, nucleosynthesis of Lead

neutron source for the s-process: 22Ne(a,n)25Mg

massive stars, pre-solar grains

nuclear cosmo-chronometer

Neutrons at CERN n_TOF: A window to stellar evolution and nucleosynthesis

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