1. Nuclear Astrophysics at the n_TOF facility at CERN
M. Barbagallo, INFN Bari and CERN, on behalf of the n_TOF Collaboration
101o Congresso Nazionale della Societa’ Italiana di Fisica, Roma, Settembre 2015

2. Outline Stellar Nucleosynthesis The facility The experimental program
151Sm(n,g) and 63Ni(n,g)
Conclusions
M. Barbagallo, Nuclear Astrophysics at the n_TOF facility at CERN, SIF 2015, Roma Settembre 2015

3. The n_TOF Collaboration
CERN
Technische Universitat Wien Austria
IRMM EC-Joint Research Center, Geel Belgium
IN2P3-Orsay, CEA-Saclay France
KIT – Karlsruhe, Goethe University, Frankfurt Germany
Univ. of Athens, Ioannina, Demokritos Greece
INFN Bari, Bologna, LNL, LNS, Trieste, ENEA – Bologna Italy
Tokyo Institute of Technology, JAEA Japan
ITN Lisbon Portugal
Charles Univ. (Prague) Czech Republic
Univ. of Lodz Poland
IFIN – Bucarest Rumania
INR – Dubna *, IPPE – Obninsk * Russian Fed.
CIEMAT, Univ. of Valencia, Santiago de Compostela,
University of Cataluna, Sevilla Spain
University of Basel, PSI Switzerland
Univ. of Manchester, Univ. of York UK
~ 30 Institutions >100 Researchers
+ Newcomers
Hig haccuracy measurements of neutron induced reaction cross-sections
(n,f), (n,cp), (n,g)
M. Barbagallo, Nuclear Astrophysics at the n_TOF facility at CERN, SIF 2015, Roma Settembre 2015

4. Stellar Nucleosynthesis
s-process (Red Giants)
r-process (Supernovae)
Neutron beams
Radioactive beam facilities
s-process (slow process):
Capture times long relative to decay time
Involves mostly stable isotopes
Nn = 108 n/cm3 , kT = 0.3 – 300 keV
r-process (rapid process):
Capture times short relative to decay times
Produces unstable isotopes (neutron-rich)
Nn = n/cm3
M. Barbagallo, Nuclear Astrophysics at the n_TOF facility at CERN, SIF 2015, Roma Settembre 2015

5. capture rate: ln = Nn<s(n,g)·v>kT
s-process
s-process nucleosynthesis: neutron captures and successive b-decays
capture rate: ln = Nn<s(n,g)·v>kT
Along the b-stability valley
The abundance of elements in the Universe depends on:
thermodinamic conditions in stars (temperture and neutron density)
neutron capture cross-sections
62Cu
9.74 m
63Cu
69.2%
64Cu
12.7 h
60Ni
26.2%
61Ni
1.14%
62Ni
3.63%
63Ni
100 y
64Ni
0.93%
58Co
70.86 d
59Co
100%
60Co
5.272 y
61Co
1.65 h
s(n,g) is a key quantity
s-process
56Fe
91.7%
57Fe
2.2%
58Fe
0.28%
59Fe
44.5 d
60Fe
1.5·106 y
61Fe
6 m
Need of new and accurate neutron capture cross-sections:
refine models of stellar nucleosynthesis in the Universe;
obtain information on the stellar environment and evolution
M. Barbagallo, Nuclear Astrophysics at the n_TOF facility at CERN, SIF 2015, Roma Settembre 2015

6. Uncertainties in Nuclear Data
Huge amount of data collected on many isotopes, mostly stable. Main features of s-process now well understood.
However, cross-section uncertainties in some cases remain high, in particular if compared with progresses in:
observations of abundances (i.e. in meteorite grains)
models of stellar evolution
Bao et al. ADNDT 76 (2000)
For three classes of nuclei data are lacking or need substantial improvements:
1. Nuclei with low cross-section, in particular neutron magic nuclei (s-process bottleneck)
N=50 86Kr, 87Rb, 88Sr, 90Zr
N=82 138Ba, 139La, 140Ce
2. Isotopes unavailable in large amount, such as rare or expensive isotopes:
186,187Os, 180W, etc…
3. Radioactive branching isotopes (“stellar thermometers”):
79Se, 85Kr, 151Sm, 163Ho, 204Tl, 205Pb
M. Barbagallo, Nuclear Astrophysics at the n_TOF facility at CERN, SIF 2015, Roma Settembre 2015

7. The n_TOF facility Neutron Time Of Flight
The advange of n_TOF are a direct consequence of the characteristics of the PS proton beam: high energy, high peak current, low duty cycle.
Neutron Time Of Flight
See “Commissioning of the n_TOF second experimental area at CERN”, today at 12.00, Aula Amaldi
EAR2
20 m
EAR1
20 GeV/c protons
Spallation
Target
185 m
M. Barbagallo, Nuclear Astrophysics at the n_TOF facility at CERN, SIF 2015, Roma Settembre 2015

8. The experimental program so far
Phase 1 ( )
Phase 2 ( )
Phase 3
(2014-today)
Capture
151Sm
232Th
204,206,207,208Pb, 209Bi
24,25,26Mg
90,91,92,94,96Zr, 93Zr
186,187,188Os
233,234U
237Np,240Pu,243Am
Fission
233,234,235,236,238U
232Th, 209Bi, 237Np
241,243Am, 245Cm
Capture
25Mg, 88Sr
58,60,62Ni,63Ni
54,56,57Fe
236,238U, 241Am
Fission
240,242Pu
235U(n,/f)
232Th, 234U
237Np (FF ang.distr.)
(n,)
33S,59Ni
Capture
171Pm EAR2
Ge EAR1
242Pu EAR1
171Tm EAR1
203,204Tl EAR2
Fission
240Pu EAR2
237Np EAR1&2
Reactions (n,cp)
7Be(n,a) EAR2
33S(n,a) EAR2
M. Barbagallo, Nuclear Astrophysics at the n_TOF facility at CERN, SIF 2015, Roma Settembre 2015

9. Measurements relevant for Astrophysics
The combination of excellent resolution, unique brightness and low background has allowed to collect high-accuracy data, in some cases for the first time ever.
Cross sections relevant for Nuclear Astrophysics
branching point isotopes
151Sm, 63Ni, 147Pm, 171Tm, 203Tl
abundancies in presolar grains
91,92, 93,94,96Zr
magic nuclei and end-point
139La, 90Zr, 204,206,207,208Pb,209Bi
seeds isotopes
54,56,57Fe, 58,60,62Ni
isotopes of special interest
186,187,188Os (cosmocronometer), 24,25,26Mg (neutron poison), 7Be (CLiP)
7Be(n,a) and 7Be(n,p) cross section measurement for the Cosmological Lithium Problem at the n_TOF facility
Today at 11.45, aula Amaldi
M. Barbagallo, Nuclear Astrophysics at the n_TOF facility at CERN, SIF 2015, Roma Settembre 2015

10. Capture setups at n_TOF
Capture reactions are measured by detecting g-rays emitted in the de-excitation process.
At n_TOF, two detection systems are used, for different purposes.
Total Absorption Calorimeter (TAC)
High-efficiency 4p detector (40 BaF2 scintillators)
Mostly used for fissile isotopes (actinides)
C6D6 (deuterated liquid scintillators)
Low neutron sensitivity device
Total energy method (+Pulse Height Weighting Technicque)
Neutron beam
Neutron beam
C12H20O4(6Li)2
By combining the results, it is possible to reduce systematic uncertainties and to reach accuracy as low as few percent.
M. Barbagallo, Nuclear Astrophysics at the n_TOF facility at CERN, SIF 2015, Roma Settembre 2015

11. Branching point isotopes: the 151Sm case
151Sm (T1/2=90 y) causes a branching of the s-process nucleosynthesis
152Gd
154Gd
151Eu
152Eu
153Eu
154Eu
s-Process
150Sm
151Sm
152Sm
153Sm
The branching ratio for 151Sm depends on:
Termodynamical condition of the stellar site (temperature, neutron density, etc…)
Cross-section of 151Sm(n,g)
151Sm used as stellar thermometer !!
M. Barbagallo, Nuclear Astrophysics at the n_TOF facility at CERN, SIF 2015, Roma Settembre 2015

12. Branching point isotopes: the 151Sm case
First measurement ever of the Maxwellian-Averaged Cross Section (MACS) of 151Sm (5% accuracy).
Results quite different from model predictions
Background
1000
1500
2000
2500
3000
3500
1970
1975
1980
1985
1990
1995
2005
Year
MACS [mb]
n_TOF
Models
n_TOF results provide confirmation of the model of Thermal Pulsing AGB stars
M. Barbagallo, Nuclear Astrophysics at the n_TOF facility at CERN, SIF 2015, Roma Settembre 2015

13. Branching point isotopes: the 63Ni case
63Ni (t1/2=100 y) represents the first branching point in the s- process, and determines the abundance of 63,65Cu
62Ni sample (1g) irradiated in thermal reactor (1984 and 1992), leading to enrichment in 63Ni of ~13 % (131 mg)
In 2011 ~15.4 mg 63Cu in the sample (from 63Ni decay).
After chemical separation at PSI, 63Cu contamination <0.01 mg
First high-resolution measurement of 63Ni(n,g) in the astrophysical energy range.
M. Barbagallo, Nuclear Astrophysics at the n_TOF facility at CERN, SIF 2015, Roma Settembre 2015

14. Conclusions
There is need of accurate new data on neutron cross-section both for nuclear astrophysics (and advanced nuclear technology, nuclear medicine, fundamental physics)
Since 2001, has provided an important contribution to the field, with an intense activity on capture and n-charged particle measurements.
Several results of interest for stellar nucleosynthesis (Sm, Os, Zr, Ni, Fe, etc…).
To date, high resolution measurements performed in EAR1 in optimal conditions (borated water moderator, Class-A experimental area, etc…).
A second new (and more intense flux) experimental area is now available. EAR2 opens new perspectives for frontier measurements on short-lived radionuclides.
A rich experimental program related to nuclear astrophysics is foreseen both in EAR1 and EAR2.
M. Barbagallo, Nuclear Astrophysics at the n_TOF facility at CERN, SIF 2015, Roma Settembre 2015