# Neutron burning in Stars

Neutron burning in Stars

Neutron burning in Stars

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Structure and evolution

of AGB stars

M bol

T eff

The ma**in** s-process s

**in** AGB stars

H

12

C(p,γ) 13 N(β + υ) 13 C

13

C(α,n)

He-flash

neutron source 13 C(α,n)

16

dY

13

dt

dYn

dt

C

= −Y

= −

∑

x

13

Y

C

X

⋅Y

4

⋅Y

n

He

⋅ ρ ⋅ N

⋅ ρ ⋅ N

A

A

συ

συ

13

16 O

C ( α , n)

X ( n,

γ )

+ Y

+ Y

13

C

12

C

⋅Y

⋅Y

4

He

1

H

⋅ ρ ⋅ N

⋅ ρ ⋅ N

A

A

συ

συ

13

12

C(

p,

γ )

C ( α , n)

Present status on 13 C(α,n) 16 O

σ(E) =

1

E

exp( 2πη)S(E)

Previous experiments

Indicate large uncerta**in**ty

In low energy range!

Low-energy, low-background

Experiment with 4π4

BaF 2 γ-array

α-beam

Experimental Set-Up@FZ-Karlsruhe

Active shield**in**g

techniques;

Pulse shape

analysis;

Low energy reaction yield of

13 C(α,n)

16 O

Yield

Yield

/

/

mC

mC

10

10 6 6

10

10 5 5

10

10 4 4

10

10 3 3

10

10 2 2

10

10 1 1

10

10 0 0

10

10 -1 -1

Cosmic Ray &

Beam **in**duced

background

0.3

0.3

0.4

0.4

0.5

0.5

0.6

0.6

0.7

0.7

0.8

0.8

0.9

0.9

1

E 1

E lab **in**

lab **in**

MeV

MeV

S-Faktor

S-Faktor

**in**

**in**

10

10 6 6 MeV

MeV

barn

barn

6

6

5

5

4

4

3

3

2

2

1

1

Kellogg

Kellogg

Bair

Bair

Brune

Brune

Davids

Davids

Drotleff

Drotleff

diese Arbeit

diese Arbeit

0

0

0

0.2

0.2

0.4

0.4

0.6

0.6

0.8

0.8

1

1

1.2

E 1.2

E cm **in**

cm **in**

MeV

MeV

1 order of magnitude improvement **in** background reduction necessary!

proton number

Ni

Co

Fe

The s-Process

(n,γ)

(β − )

(β + )

Zn

Cu

80

Br, t 1/2 =17 m**in**, 92 % (β − ), 8 % (β + )

p-process

p-only

Ge

Ga

Se

As

Kr

Br

Sr

Rb

s-only

r-only

Zr

Y

85

Kr, t 1/2 =11 a

79

Se, t 1/2 =65 ka

r-process

64

Cu, t 1/2 =12 h, 40 % (β − ), 60 % (β + )

63

Ni, neutron t number

1/2 =100 a

f**in**ished, press left mouse button to cont**in**ue

s-process studies **in** lab

**Neutron** production by 7 Li(p,n) reaction at

1.98 MeV: **Neutron** spectrum resembles a

kT=30keV Maxwell Boltzmann spectrum!

Activation Method

MB neutron spectrum bombardment of natural sample (mg)

e.g. Krypton gas.

80

Kr(n,γ) 81 Kr(β--γ)

84

Kr(n,γ) 85 Kr(β - -γ)

86

Kr(n,γ) 87 Kr(β - -γ)

85

Kr(4.48h)

81

Kr(13.3s)

Measurement of result**in**g

γ-activity yields

cross section & reaction rate.

Cross section determ**in**ation by

activation method

( ) (

−λ⋅t

P e )

irr

= ⋅ 1−

A t

irr

P = N ⋅ f

( ) = A( t )

A t

w

⋅φ

⋅σ

n

irr

⋅e

( n,

γ )

−λ⋅t

w

σ

( n,

γ )

1

η

n

( t )

λ ⋅e

λ⋅t

c

= ⋅

⋅

λ

N

⋅

f

I

⋅φ

⋅

(

−λ⋅t

) ( )

irr

− ⋅tc

1−

e 1−

e

w

I

t

−λ⋅t

( ) ( )

( )

c

c

−λ⋅tc

t = η ⋅ A t ⋅e

⋅ dt = η ⋅ ⋅ A( t )

c

∫

0

w

1−

e

λ

N: number of target atoms/cm 2

t irr : irradiation time

t w : wait**in**g (transport) time

t c : count**in**g time

η: efficiency of detection system

f: neutron absorption factor

w

σ (n, γ) : energy averaged cross section

φ n : time **in**tegrated neutron flux

I(t c ): yield

**Neutron** sources VdG

In-beam γ-measurement for A X(n,γ) A+1 X

In beam cross section

I

γ

= φ ⋅σ ⋅ N ⋅b

⋅η

( )

n n,

γ

I γ : gamma yield

σ (n, γ) : energy averaged cross section

φ n : time **in**tegrated neutron flux

N: number of target atoms/cm 2

η: detector efficiency

b: gamma branch**in**g

Through summ**in**g effects

~90% efficiency

In large detectors

In-Beam γ-Spectra

**Neutron** **in**duced nucleosynthesis

dN

A,

Z

dt

() t

( + λ ⋅ N )

= −nn

⋅ N

A,

Z

συ − λA,

Z

⋅ N

A,

Z

+ nn

⋅ N

A−1,

Z

συ

A

A−1,

Z A,

Z −1

A,

Z −1

Approximation for s-process network for A neglect**in**g branch**in**g po**in**ts

dN

A

dt

() t

= −nn

⋅ N

A

συ + nn

⋅ N

A

A−1 συ

A−1

Cross section and abundance

dN

A

dt

() t

= −nn

⋅ N συ

A

+ nn

⋅ N συ

A

A−1 A−1

For time **in**tegrated neutron exposure

τ ≡ ∫

n n

⋅

υ ⋅dt

And relation for s-wave neutron capture συ = σ ⋅ υ

dN

A

dτ

For equilibrium:

( t)

= −N

Aσ

A

+ N

A−1σ

A−1

( t)

dN

A = 0;

N

Aσ

A

= N

A A

d

=

−1σ

−1

τ

const

s-process results

Low neutron capture

cross section at

closed shell nuclei,

N=50, 82, 126

⇒ enrichment of

closed shell nuclei,

⇒ s-process peaks!

The classical s-process model

Correlation between observed and calculated

σ·N A vs A curve reflects two neutron exposure

events with different neutron flux exposure:

weak s-process: τ w =4mb -1 ,

ma**in** s-process: τ m =0.3mb -1

Indication for s-process branch**in**gs

s-process branch**in**g po**in**ts

Branch**in**g po**in**t analysis

provides important **in**formation

on s-process conditions

n-flux, density, temperature

r-process shielded

branch**in**g po**in**t

148

Pm branch**in**g po**in**t is reflected **in** the observed

abundances of the s-isotopes 148 Sm and 150 Sm.

β-unstable

f

β

=

λ

β

+

λ

n

β

n

συ

≈

σ

σ

148

150

Sm

Sm

N

N

148

150

Sm

Sm

≈

0.9

Accurate analysis of the neutron density depends on:

• accuracy In the cross section measurements (20%)

• stellar decay rate of 148 Pm

(not necessarily identical with laboratory decay rates

– e-capture, thermally **in**duced decays …

**Neutron** flux: ~(4.1±0.6)·10 8 cm -3

f

β

=

λ

β

+

n

λ

n

β

συ

148

Pm

≈

σ

σ

148

150

Sm

Sm

N

N

148

150

Sm

Sm

≈

0.9

n

n

=

1−

f

β

f

β

⋅

λ

συ

β

148

Pm

=

1−

f

β

f

β

⋅

υ ⋅

σ

1

148

Pm

⋅

t

1/ 2

ln 2

(

148

Pm)

Additional s-process parameters

deduced from branch**in**g po**in**t analysis

Temperature dependent t 1/2

Density dependent (e-capture) t 1/2

Temperature dependent t 1/2

through isomer population

End-po**in**ts of s-process Pb

First tests at n-ToF neutron source

204,206,207,208

Pb(n,γ) 205,207,208,209 Pb, 209 Bi(n,γ) 210 Bi

Po

84

Bi

83

Pb

82

Tl

81

Hg

80

s-process

feed**in**g

204

203

205 206 207 208 209

β- β- β- β-

204

EC

205

β- β-

206

202 203 203 204 205

α

α

210 211

209

207

β-

β-

210m

210 g.s.

α

β-

211

r-process contributions

Old star abundance distribution

po**in**ts to r-process orig**in** of Pb

Z

N

122 123 124 125 126 127 128

n-capture on stable

Pb isotopes needed!

CERN accelerator Complex

n_TOF

L**in**ac(s): up to 50 MeV PSB: up to 1 GeV PS: up to 24 GeV

alberto.mengoni@cern.ch

CERN accelerator Complex: n_TOF

1 pulse/2.5 s

alberto.mengoni@cern.ch

alberto.mengoni@cern.ch

CERN n_TOF Facility

• Design: the n_TOF Target

• The Tunnel

movie by V Vlachoudis

Pb target 80x80x60 cm 3

In water (3cm) filled Al conta**in**er

Production 300 n/p

Shield**in**g required for γ,µ,π absorption

**Neutron** flux distribution @ n-Tof

**Neutron** energy

E

n

∆E

E

n

= 0.5⋅m

n

=

2

⋅

L

Time of flight technique

2

−10

⎛ L ⎞ 0.5⋅

mn

0.5⋅939.59MeV

7.23⋅10

[ s] ⋅ L[ cm]

⋅⎜

⎟ t = ⋅ L =

⋅ L =

2

⎝ t ⎠ En

En[ MeV ] ⋅c

En[ MeV ]

( ∆L) 2 + ( υ ⋅ ∆t) 2

n

Energy resolution

n

For 180 m flight path

a 10keV neutron has

a flight time of t=41µs

∆E n /E n ≈10 -3

180 m flight path

Conversion example

Conversion of time of flight

spectrum **in**to the neutron

energy spectrum for the

n capture reaction 151 Sm(n,γ)

counts

n-capture on Pb - first results

208

Pb(n,γ) 209 Pb ToF

spectrum simulation

204

Pb(n,γ) 205 Pb

1/v lav

resonances

neutron-energy [MeV]

neutron-energy [MeV]

Pb analysis with r-Matrix

204

Pb

In general, good agreement with previous parameters. But,

only 4 resonances below 7 keV have complete

resonance parameters **in** literature

Detail of Pb analysis

204

Pb

Analysis still underway… but results **in**dicate that previously

adopted resonance parameters need improvement!

Conversion example

Conversion of time of flight

spectrum **in**to the neutron

energy spectrum for the

n capture reaction 151 Sm(n,γ)

Multitude of open questions!

• impact of threshold cluster states **in** He **burn ing**

• low energy contributions to neutron sources

• neutron capture on light nuclei – neutron poison

• neutron capture on long-lived radioactive nuclei

for branch**in**g po**in**t analysis

• end-po**in**t of s-process (n-capture on Pb, Bi isotopes)

The accuracy of stellar s-process abundance distribution

limits the accuracy of the predicted r-process abundance

distribution and the identification of the r-process site!