Neutron beams and techniques for the measurement of

Neutron beams and techniques for the measurement of neutron capture cross sections J. Lerendegui-Marco, C. Guerrero and J. M. Quesada VIII CPAN DAYS, Zaragoza, 28 th-30 th November 2016

Neutron Induced Reactions ? Neutron radiative capture Elastic/Inelastic scattering g Fission g g g Many others: (n, ch), knock-out, spallation, …. . 2

Interest for neutron capture measurements § Nuclear Astrophysics: s-process Nucleosynthesis of elements A>60 • Site: He-intershell in AGB stars (red giant) • Branching point: β- decay competes with (n, g) Suff. long t 1/2 || Suff. High Fn (n, g) b- decay Z (n, g) N (n, g) b- Branching point s-process Stable 171 Tm (n , γ) σ(n, γ) input from nuclear data + ratio of chemical abundances bounds the temperature and neutron flux in stellar models 3

171 Tm (n, γ): Energy ranges and beams Ec Maxwell Boltzmann A +n Xdistribution @ KB. T=30 ke. V Sn p(7 Li, n)7 Be A+1 X D-T) or En sg Ec n@ o o s F O T d n a MACS ANOS-CNA HISP MACS: Maxwellian Averaged Cross Section Nuclear reactor Activation→ σth (i. e TRIGA, BRR) (single value) NEUTRON CROSS SECTION MEASUREMENTS: Pulsed white n-beam How to produce of neutrons in each energy range Time-of-flight (i. e. n_TOF@CERN, etc. ) → pointwise s(n, g) How to measure neutron energy and detect the reaction products “quasi-Mawellian” n-beam Activation→ MACS (i. e. FZK, Li. T, FRANZ, etc. ) (single value)4 4

171 Tm (n, γ): a European trip towards the Interest for neutron thermal, resonance and MACS cross sections capture measurements 3) Activation: thermal reactor TRIGA Mainz 2) Separation and samples BRR (n, g) PSI (n, g) 6) Activation: b- decay thermal reactor 1) Irradiation of CERN ILL Er-170 seed bb. Stable (n, g) 4) TOF + white n beam: Pointwise cross section Branching point s-process U. Sevilla 5) Activation: Coordination and analysis Quasi-stellar spectra 5 Li. T

Preparation of radioactive targets Production via (n, g) or (n, g)+b- at the ILL research reactor [Contact: Ulli Koester] Neutron flux: 1. 5 x 1015 n/cm 2/s Irradiation time: 55 days ----------------------------------------171 Tm: 170 Er(n, g)171 Er(b-, 7. 5 h)171 Tm (enrichment 1. 8%) ~3. 6 mg of 171 Tm (1. 9 y) [1. 3 1019 atoms] Do not exist in nature Small masses produced Activity challenge to handle and measure Chemical separation and targets @ PSI [Contact: Stephan Heinitz] Assembly mounted 147 Pm deposit (20 mm diameter) Aluminum (7 mm) backing PCB frame (50 mm diameter) Mylar (5 mm) 6

171 Tm (n, γ): MACS measurement @ Li. T 2 x 1010 n/s/m. A 2 1 -2 m. A ~2 k. W Stellar Experimental Liquid Lithium Target (@SARAF, Israel) 7

171 Tm (n, γ): Setup @ Li. T facility ste ns (~ o r t u ne ~12 mm 1 -2 m. A 197 Au-front monitor 25 mm ~1. 935 Me. V protons ti ribu t s i d r lla 22 mm Liquid lithium: 7 Li(p, n) e. V) k 2 4 on @ Precise beam intensity and energy distribution depend on distance and angle coverage 197 Au-back monitor Case 1. 197 Au(n, g)198 Au -> b- decay with t 1/2=2. 69 d -> Eg = 412 Case 2. 171 Tm(n, g)172 Tm NORMALIZATION -> b- decay with t 1/2=2. 65 d -> Eg = 1093, 1387, 1529, 1608 8

171 Tm (n, γ): Activated nuclei to MACS Measured quantity Correction for the shape of the flux and Tm 171 eval. x-section shape Correction for the shape of the flux of the Au ref. MACS 9

MACS results for 171 Tm s. MACS (171 Tm, k. T=30 ke. V) = 183 (27*) mb x 0, 6 *Preliminary 15% uncertainty This work (183± 27 mb) Significant reduction of the x-section following the trend of the n_TOF data 25% error bars for theoretical estimates 10

Thermal Cross-section measurement § Thermal cross-section: § Maxwellian spectra @ KT= 25 me. V § Prompt Gamma Activation Analysis using the n_TOF targets MACS and thermal provide just two points: TOF measurement needed for pointwise cross-section Budapest Research Reactor 11

n_TOF: Time-of-flight technique BEAM LINE EAR 1 PS Proton pulses (20 Ge. V/c) σ = 7 ns Pb Spallation Me. V-Ge. V Neutrons L= 184 m t 0 Flux (Φ) (neutrons/cm 2) Neutrons (me. V to Ge. V) γ t Reaction products nat (atoms/cm 2) Transmission Scattering 5 cm water moderator Time-of-Flight to En relation (non-rel. ): 12

171 Tm (n, γ) measurement @ n_TOF EAR 1 using Total Energy Detectors Geant 4 model of the setup Nov-Dec 2014 C 6 D 6 Scintillators PULSE HEIGHT WEIGHTING TECHNIQUE: Accurate simulations required 13

171 Tm (n, γ) @ n_TOF in brief Counts -> Pointwise cross section Challenge: Sample activity Resonance analysis Counts vs En PHWT c 2 (Gn vs. Gg) 14

171 Tm (n, γ): Extrapolating from the resonances Average resonance parameters Level Spacing: D 0 RRR: Directly measured (27 new resonances!) Significant reduction of x -section wrt systematic studies URR: Extrapolated with these avg. parameters Neutron strenght: S 0 Avg. Radiative width: <Gg> 15

Summary and conclusions § Radiative neutron capture cross sections are relevant for many applications § s-process: (n, g) cross-section certain isotopes key input for stellar evolution models § Tm-171 s-process branching point: Radioactive Challenging production & measurement § Different neutron beams and techniques: Complementary energy ranges § § § n_TOF : White neutron beam produced via spallation protons in lead + partial moderation Time-of-flight technique: Neutron energy from its time of flight (source <-> sample) Pointwise cross section Resonances and average parameters for URR § § Li. T @ SARAF : Quasi-stellar spectra ( Li-7(p, n)Be-7 reaction) MACS measurement via activation Decay of the produced Tm-172 nuclei § § TRIGA (analysis ongoing) and BRR (near future): Maxwellian spectra at thermal Measurement via activation and PGAA § Preliminary results indicate a significant reduction of x-section compared to models 16

THANKS FOR YOUR ATTENTION! 16