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

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

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

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

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

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

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) 0225201 Abbondanno et al., PRL 93 (2004) 0161103 VIII CPAN days, Zaragoza, 28/11/16

204Tl: An interesting branching point 204Tl is a radioactive (β-decay) isotope of thallium (Z=81), that β-decays to 204Pb 𝑡 1 2 =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, 015806 (2007)) 204Pb 205Pb 203Tl 204Tl 205Tl (n,g) During s-process 𝑡 1 2 =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

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

204 𝑇𝑙+𝑛→ 205 𝑇 𝑙 ∗ → 205 𝑇𝑙+𝛾 What do we need? Neutron source 204 𝑇𝑙+𝑛→ 205 𝑇 𝑙 ∗ → 205 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

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 project @ CERN. ECAART 2010 – Athens, Sept. 13 - 17, 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

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

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

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

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

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) 2.23 2.27 keV 916 eV 789 eV ANT Work! Advanced Nuclear Technologies Research Group VIII CPAN days, Zaragoza, 28/11/16

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