CNR*2011 - PRAHA 19/09/2011

G. Boutoux1, B. Jurado1, V. Méot2, O. Roig2, C. Théroine2 , M. Aïche1, A. Bail2 , G. Barreau1, E. Bauge2, A. Blanc2 , J.T. Burke9 , N. Capellan1,7, P. Chau2 ,

I. Companis1, S.Czajkowski1, J.M. Daugas2, X. Derkx5 , L. Gaudefroy2, F. Gunsing4, B. Haas1, G. Kessedjian1,7, I. Matea6, L. Mathieu1, P. Morel2, N. Pillet2, M.G Porquet8,

P. Romain2, K.-H. Schmidt1, O. Sérot3 , J. Taieb2, L. Tassan-Got6, I. Tsekhanovich1

1CENBG Bordeaux, CNRS/IN2P3, Université Bordeaux 1

2CEA – DAM – DIF 3CEA – Cadarache , DEN/DER/SPRC/LEPh

4CEA – Saclay , DSM/DAPNIA/SPhN 5GANIL, CNRS/CEA

6IPN Orsay, CNRS/IN2P3 7LPSC Grenoble, CNRS/IN2P3 8CSNSM Orsay, CRNS/IN2P3

9Lawrence Livermore National Laboratory, California, USA

Nucleosynthesis or origin of the nuclei in the Universe

- Minor-actinide incineration

- 232Th/233U cycle

σ(n,f) and σ(n,γ) needed in the energy range

1 keV < En < 10 MeV

Nuclear energy

Stables nuclei

Known nuclei

Production of nuclei above 56Fe in the explosion of massive stars

Very difficult to measure – Short-lived nuclei!

Need of σ(n,γ)

AX

neutron

A+1X* Compound Nucleus

Y

Light charged-particle (p, d, t , 3He, α) ejectile

NEUTRON-INDUCED REACTION

SURROGATE REACTIONS

fission

γ

neutron

radiative capture

Entr

ance

ch

ann

el

FOR

MA

TIO

N

Exit

ch

ann

el

DEC

AY

( )CN

n En

( *)CNP E

x

( , ) ( )n

A En

=

--------Bohr’s hypothesis------

Optical Model CALCULATIONS

EXPERIMENTAL decay

probability

*

n nE S E

Spin-parity mismatch? The validity of the surrogate method need to be discussed…

Jπ(n)

Jπ(s)

( *) ( *, ) ( *, )CN

form

J

P E P E J G E J

Probability that the CN is formed in the state {E*,Jπ}

Branching ratios for the decay channel χ

( *) ( *, ) ( *, )CN

form

J

P E P E J G E J

DWBA angular momentum distribution calculation

B.B. Back et al., PRC, 1974

θ=90° 240Pu

very difficult to model!

0

0

5

5

<l>≈4ħ <l>≈1ħ

J.P. Delaroche et al., 2002

l

En=1MeV

s-wave neutron l=1/2ħ

TALYS angular momentum distribution calculation

(2l+

1)σ

(l)

Valid at excitation energies where most of the decay is dominated by the level density!

CN

con

tin

nu

m

Strong mixing of Jπ states

Discrete levels high Jπ selectivity

E*

Measurements that test the validity of the surrogate method are needed!

( *) ( *, ) ( *, )CN

form

J

P E P E J G E J

( *, ) ( *)G E J G E

Weisskopf-Ewing hypothesis

243Am(3He,αf)242Am* 243Am(3He,tf)243Cm*

G. Kessedjian et al., Phys. Lett. B, Volume 692, Issue 5, 2010

Nice agreement even at low neutron energy!

SURROGATE METHOD APPLIED TO FISSION

T1/2=163 d

(T1/2=7370 y) 243 Am

3He

T1/2=432 y

α t

176Lu

175Lu

174Lu

173Yb

174Yb

3He

(3He,p)

(3He,d) (3He,t)

(3He,α)

175Lu+n

174Lu+n 173Lu+n (t1/2=1.37y)

172Yb+n

Ejectile

TARGET 174Yb

3He beam Eprojectile=24MeV

γ

4 scintillators C6D6

*( *)

( )( *) ( *)

ej

ej

N EP E

N E E

Capture probability:

Residual energy E[Ch]

E

[Ch

]

p d t

3He

SURROGATE METHOD APPLIED TO CAPTURE

RESULTS FOR 174Yb(3He,p)176Lu AS SURROGATE FOR 175Lu(n,γ)

( *) ( *, ) ( *, )CN

form

J

P E P E J G E J

2

220.5

2

J J

e

Gaussian independant of E* Branching ratios calculated with TALYS

Two free parameters: <J> and σ

FIT

<l>=7 ħ σ=2.3 ħ

Sn

174Yb(3He,p)176Lu*

Sn=6.27MeV

n

E*

γ

7/2+ 175Lu

7- 0 176Lu*

11/2+ 9/2+

J=3-4

NEUTRON-INDUCED REACTION CASE

(n,γ) competes with (n,n)

<l>=1/2ħ is exactly the l carried away by the neutron in

the compound elastic (n,n) reaction dominant decay!

Pγ decreases very quickly!

Sn=6.27MeV

n

E*

γ

7/2+ 175Lu

7- 0 176Lu*

11/2+ 9/2+

J=7

(3He,p) reaction CASE

higher spins populated

neutron emission forbidden

radiative capture is the only way of deexcitation!

Sn=6.27MeV

n’

E*

γ

7/2+ 175Lu

7- 0 176Lu*

11/2+ - 251 keV 9/2+ - 113 keV

J=7

the first excited states of the residual nucleus after neutron emission may also be forbidden…

15/2+ - 595 keV

The neutron emission channel does not verify the Weisskopf-Ewing approximation in the vicinity of Sn

situation is reduced as one moves to heavier nuclei (with higher level densities)

RESULTS FOR 174Yb(3He,α)173Yb AS SURROGATE FOR 172Yb(n,γ)

( *) ( *, ) ( *, )CN

form

J

P E P E J G E J

2

220.5

2

J J

e

Gaussian independant of E* Branching ratios calculated with TALYS

Two free parameters: <J> and σ

FIT

<l>=4 ħ σ=3.2 ħ

Sn

4+

6+

CONCLUSION

Radiative capture more sensitive

than fission

Excitation energies in the vicinity of Sn

Pγ decreases very quickly! Small variation in absolute several factors in relative

Pγ measured in surrogate experiments over-estimated by several factors

Determination of l distributions populated in (3He,p) and (3He,α)

Higher spins populated in transfer reactions

2012: - (p,dγ) reaction in the rare-earth region - (d,pf)/(d,pγ) reactions in the actinide region

PERSPECTIVES: - Use more realistic spin distributions. - Compare our results with FRESCO (I. Thompson et al.)

The neutron emission channel does not verify the Weisskopf-Ewing approximation in the vicinity of Sn.

Sn=5.53MeV

E*

7/2+

241Am

242Am*

11/2+ 9/2+

WHY DOES IT WORK FOR FISSION?

II

A

B

I

BfA=6.32MeV

243Am(3He,αf)242Am*

J≈4

Sn=5.53MeV

E*

7/2+

241Am

242Am*

11/2+ 9/2+

WHY DOES IT WORK FOR FISSION?

II

A

B

I

BfA=6.32MeV

fission n’

11/2- 158 keV 9/2- 93 keV 7/2- 41 keV

J≈4

243Am(3He,αf)242Am*