1. 204Tl neutron capture cross section measurement at the n_TOF facility (CERN)
A. Casanovas (UPC), C. Domingo-Pardo (IFIC), C. Guerrero (U. Sevilla), F. Calviño (UPC), A. Tarifeño-Salidivia (UPC), J. Lerendegui (U. Sevilla)
and the n_TOF Collaboration
VIII CPAN days, Zaragoza, 28/11/16

2. VIII CPAN days, Zaragoza, 28/11/16
Outline
Brief introduction to stellar nucleosynthesis and the s-process
204Tl: an interesting nuclide for the s-process
Neutron capture cross section measurement
Reaction yield analysis, and preliminary results
VIII CPAN days, Zaragoza, 28/11/16

3. Nuclear data for astrophysics: a brief introduction
Fundamental question in astrophysics: origin of the elements and elemental abundances observed
Neutron capture process
p-process (10 to 100 times less abundant)
Fusion in stars
He to Fe
Neutron capture processes
Z>Fe
Primordial
(Big Bang)
s-process: time scale 104 y; nn≈108 (slow)
r-process: time scale ~μs; nn>1020 (rapid)
VIII CPAN days, Zaragoza, 28/11/16

4. Nuclear data for astrophysics: s-process
80Br, t1/2=17 min, 92 % (b-), 8 % (b+) Y
Half of the abundances of heavy elements from Fe (Z=23) to Bi (Z=83) are produced by successive neutron capture (n,g) and beta decay reactions in the so called slow (s)-process in Low Mass (1-3 M⊙) AGB stars and Massive Stars (>8 M⊙) Sr Rb Kr Br
85Kr, t1/2=11 y Se As
79Se, t1/2=65 ky Ge Ga Zn
(n,g) Cu
(b-)
(b+)
63Ni, t1/2=100 y
VIII CPAN days, Zaragoza, 28/11/16

5. VIII CPAN days, Zaragoza, 28/11/16
S-process in stars
VIII CPAN days, Zaragoza, 28/11/16

6. Nuclear data for astrophysics: s-process
80Br, t1/2=17 min, 92 % (b-), 8 % (b+) Y
Half of the abundances of heavy elements from Fe (Z=23) to Bi (Z=83) are produced by successive neutron capture (n,g) and beta decay reactions in the so called slow (s)-process in Low Mass (1-3 M⊙) AGB stars and Massive Stars (>8 M⊙) Sr Rb Kr Br
85Kr, t1/2=11 y Se As
79Se, t1/2=65 ky Ge
Along the s-process path, some unstable nuclei with half-lives of days to years are produced; In this nuclei, capture processes compete with beta-decay processes Ga Zn
(n,g) Cu
(b-)
(b+)
63Ni, t1/2=100 y
⇒ 𝜆 𝑛 ≈ 𝜆 𝛽
VIII CPAN days, Zaragoza, 28/11/16

7. VIII CPAN days, Zaragoza, 28/11/16
Branching points
𝜆 𝑛 ≈ 𝜆 𝛽 ⇒ 𝑛 𝑛 𝜎𝑣 𝑣 𝑇 ≈ ln 2/ 𝑡 1 2
Strongly enhanced in stellar conditions, T dependent
F. Käppeler et al., Rev. Mod. Phys., Vol. 83, No. 1, January–March 2011
Lederer et al., PRL 110 (2013)
Abbondanno et al., PRL 93 (2004)
VIII CPAN days, Zaragoza, 28/11/16

8. 204Tl: An interesting branching point
204Tl is a radioactive (β-decay) isotope of thallium (Z=81), that β-decays to 204Pb
𝑡 =3,78 𝑦 ; 𝑄 𝛽 =763 𝑘𝑒𝑉
⟹ 204Tl capture cross-section affects dramatically the 205Pb/205Tl ratio
Already measured (C. Domingo-Pardo et al., Phys. Rev. C 75, (2007))
204Pb
205Pb
203Tl
204Tl
205Tl
(n,g)
During s-process
𝑡 =15 𝑀𝑦 b- EC
b- : possible under stellar conditions
After s-process
(n,g)
(n,g)
CS x2
no experimental values!
(just theoretical calculations)
VIII CPAN days, Zaragoza, 28/11/16

9. 1. The neutron source… The n_TOF facility at CERN
n_TOF at CERN: time-of-flight facility which produces high peak intensity, high energy resolution pulsated neutron beams, ideal for capture and fission cross section experiments
𝐸 𝑛 ≅72.29 𝑡 2 / 𝐿 2
n_TOF EAR1:185 m flight path
n_TOF EAR2: 19 m flight path
Booster
1.4 GeV
Proton beam
17GeV/c
7x1012 ppp
Pb Spallation target
Neutron beam at 10º
Linac
50 MeV
PS 20GeV
VIII CPAN days, Zaragoza, 28/11/16

10. 204 𝑇𝑙+𝑛→ 205 𝑇 𝑙 ∗ → 205 𝑇𝑙+𝛾 What do we need? Neutron source
204 𝑇𝑙+𝑛→ 𝑇 𝑙 ∗ → 𝑇𝑙+𝛾
Neutron source
High enough flux
Wide neutron energy range and high energy resolution
Reaction detection
Gamma rays produced by the deexcitation of the compound nucleus
The sample
VIII CPAN days, Zaragoza, 28/11/16

11. 2. Radiative capture reaction & detection
Capture reactions are measured by detecting g- rays emitted in the de-excitation process.
At n_TOF, built two different systems, to minimize different types of background
C6D6 (deuterated liquid scintillators)
low neutron sensitivity device
Nicola Colonna, Neutron measurements for advanced nuclear energy systems: the n_TOF CERN. ECAART 2010 – Athens, Sept , 2010
Neutron beam
C12H20O4(6Li)2
Neutron beam
Total Absorption Calorimeter (TAC)
High-efficiency 4p detector (40 BaF2 scintillators with neutron shielding) 11
VIII CPAN days, Zaragoza, 28/11/16

12. VIII CPAN days, Zaragoza, 28/11/16
3. The 204Tl sample
Pellet containing 225 mg of 203Tl, 99.5% purity, produced at PSI (Switzerland) and inserted into quartz container
Seed irradiated at ILL exp. reactor 203Tl for 56 days
→ 15 mg of 204Tl produced
180 GBq of β activity plus radioactive impurities (60Co)
→ dangerous handling!
Gamma scanning to to determine 204Tl material distribution
204Tl sample was put in the beam at n_TOF EAR1 for 6 weeks (June to August 2015)
9 mg of 204Tl at measurement time
4% relative concentration
A pure 203Tl sample was also measured for normalization purposes
203Tl(n,γ)204Tl
VIII CPAN days, Zaragoza, 28/11/16

13. Capture yield analysis
Counting rate → Yield → Cross section:
𝐶( 𝐸 𝑛 )→ 𝑌 𝐸 𝑛 = 𝐶 𝐸 𝑛 −𝐵 𝐸 𝑛 𝜙 𝑛 𝐸 𝑛 × 𝜀 𝐸 𝑛 = (1−𝑒 −𝑛𝜎 ) 𝜎 𝛾 𝜎
CS is much higher at some discrete 𝐸 𝑛 → resonances
Resonance parameters are extracted using R-Matrix formalism code (SAMMY, ORNL)
VIII CPAN days, Zaragoza, 28/11/16

14. Maxwellian Averaged C.S. (MACS)
In star particles are thermalized at very high temperatures → Maxwell-Boltzmann distribution
Cross-section convoluted by Maxwellian neutron spectrum = MACS ×
= 𝜎 𝑘𝑇
VIII CPAN days, Zaragoza, 28/11/16

15. VIII CPAN days, Zaragoza, 28/11/16
Preliminary MACS: 203Tl
203Tl CS and MACS determination:
CS necessary for 204Tl sample normalization
First ever experimental measurement at low energies
Change in MACS → Possible impact on 204Tl branching point
𝐴𝑡 8 𝑘𝑒𝑉: 331−391 𝑚𝑏
VIII CPAN days, Zaragoza, 28/11/16

16. 204Tl yield analysis (in progress)
Up to now, 6 resonances + 4 strong candidates
Very challenging resonance analysis:
Low mass and low relative concentration (4% 204Tl )
→ Spectrum dominated by 203Tl resonances
3.15 keV
3.52 keV
4.83 and 4.86 keV
2.03 keV
1.6 keV (Pb-204)
keV
916 eV
789 eV
ANT
Work!
Advanced Nuclear Technologies Research Group
VIII CPAN days, Zaragoza, 28/11/16

17. ¡Gracias por su atención!
VIII CPAN days, Zaragoza, 28/11/16