Neutron capture measurements at the CERN neutron Time Of Flight (n_tof) facility. Nuclear astrophysics aspects

Transcription

1 Neutron capture measurements at the CERN neutron Time Of Flight (n_tof) facility Nuclear astrophysics aspects Paolo Maria MILAZZO (on behalf of the n_tof collaboration)

2 The n_tof Collaboration CERN Technische Universitat Wien IRMM EC-Joint Research Center, Geel IN2P3 IPN Orsay, IN2P3 -IReS-Strasbourg, CEA Saclay FZK Karlsruhe Austria Belgio Francia Germania AstroParticle Consortium (Athens, Thessaloniki, Thrace ) Grecia INFN and Dipartimento Fisica Bari, Bologna, LNL, Trieste ENEA Bologna, Università di Pavia LIP -Universitadede Coimbra, ITN Lisbona INR Dubna, IPPE Obninsk CIEMAT - Madrid, IFIC Valencia, University of Santiago de Compostela, University of Cataluna University of Basel University of Notre Dame, LNL, ORNL Italia Portogallo Russia Spagna Svizzera USA

3 Objectives Cross sections relevant for Nuclear Astrophysics (but also) Measurements of neutron cross sections for Nuclear Waste Transmutation and related Nuclear Technology purposes (and) Neutrons as probes for fundamental Nuclear Physics

4 To study Nuclear n_tof means to investigate: Stellar nucleosynthesis Stellar thermal conditions Cosmochronology

5 Elemental abundances beyond Fe ashes of stellar burning ABUNDANC CE (Si = 10 6 ) BB Fusion Fe Neutrons MASS NUMBER Elements heavier than Fe are the result of neutron capture processes Solar system abundances

6 Nucleosynthesis processes sprocess T 10 8 K - n n 10 8 neutrons/cm 3 Capture times 1 year rprocess T > 10 9 K - n n neutrons/cm 3 Exposure times few seconds β-decay lifetime of elements (few hours to some months) drives through s and r pathes ~ ½ by s-process (red giants) ~ ½ by r-process (explosive)

7 The s-process s-process path 56 Fe Through the β stability valley 58 Co d 57 Fe Ni Co Fe Cu 9.74 m 61 Ni Co a 59 Fe d 63 Cu Ni Co 1.65 h 60 Fe a 64 Cu 12.7 h 63 Ni 100 a 61 Fe 6 m 64 Ni 0.926

8 The r-process Out of the β stability valley (neutron rich nuclei) ntof(neutron cross CERN)

9 Magic nuclei act as bottleneck along s-process path (n,γ) x-sections of 90 Zr, Zr, 139 La 90,91,92,93,94 Zr origin from s-process only, show small capture cross-sections, Existing data have uncertainties larger than 10% σn constant Zr La Physics case: normalization of s-process abundances

10 the s-process branching at A=95 N=50 90,91,92,94,96 Zr: s-process branching at A=95 (observed abundance patterns in stellar grains) (n,γ) x-sections of Zr Sensitivity to neutron flux during the s-process 92 Mo 94 Mo 95 Mo 96 Mo 93 Nb 95 Nb 90 Zr 91 Zr 92 Zr 94 Zr 93 Zr 97 Mo 97 Mo s-process r-process While 93 Zr is practically stable on the time scale of the s-process, 95 Zr act as a branch point due to the competition between ß-decay and neutron capture Physics case: observed abundances ( 96 Zr/ 94 Zr) in stellar grains are sensitive to stellar neutron flux (competition between s- and r-process) 95 Zr 96 Zr

11 ntof (neutron cross CERN) How to recover infos on abundances? Analysis of interstellar grains

12 the s-process branching at A=151 s-process 152 Gd 154 Gd 151 Eu 152 Eu 153 Eu 154 Eu 150 Sm 151 Sm Sm Sm The branching ratiofor 151 Smdependson: Termodynamical conditionofthe stellar site (temperature, neutrondensity, etc ) Cross-sectionof 151 Sm(n,γ) 151 Sm usedasstellar thermometer (n,γ) x-sections of 151 Sm laboratory half-life of 93 yr reduced to t 1/2 = 3 yr at s-process site a probe for the temperature at s-process site

13 Cosmocronology BANG! Tnucleosynthesis 4.5 Gyr Now nuclear clocks 235 U / 238 U 232 Th / 238 U 187 Re / 187 Os

14 The Re/Os Cosmochronometer Os Re d W s-process s-only Os d Re d W Os Re W Os Re h W d the β-decay half-life of 187 Re is 42.3 Gyr Os Re x10 9 a W Os Re h r-only effects on the abundance of the daughter 187 Os W h Os Re h W d σn s ( 186 Os) = σn s ( 187 Os) Os d Re m r-process Os (n,γ) x-sections of Os

15 The n_tof CERN Spallation of high-energy proton beam on a lead target (~360 neutrons/proton) 7x GeV/c from the PS accelerator (6 ns time resolution) 0.8 Hz maximum repetition rate

16 The n_tof CERN The Pbspallationtarget, 80x80x60cm 3, is water cooled (moderator) Base of Flight ~200 m Two collimators, three walls for shielding, a deflecting magnet Very high instantaneousflux 10 5 n/cm 2 /pulse Wide energy range 1eV< E n < 250 MeV Goodenergy resolution E/E 10-4 (finoa100 kev) Low repetition rate 1 pulse/2.4 s (0.8 Hz) Low background 10-5 (1particle/cm 2 /pulse)

17 n_tof features broad neutron energy range (1 ev < E n < 250 MeV) high instantaneous flux (10 5 n/cm 2 /bunch) excellent energy resolution ( E/E 10-4 up to 100 kev low neutron sensitivity low backgrounds ( particle/cm 2 /bunch) Effects on measurements neutron capture cross sections in a wide energy range (1 ev 1 MeV) small capture cross sections small sample quantities (isotopically enriched samples) radioactive samples (low intrinsic background) resonance dominated cross sections accurate cross section measurements even for large σ el /σ capture

18 The experimental area Flux monitor Setup for fission Setup for capture ntof(neutron cross CERN)

19 What do we have to detect? ntof(neutron cross CERN)

20 Experimental set-up (1): liquid scintillators Beam monitor C 6 D 6 detector Samples C 6 D 6 detector neutrons Specifically designed low neutron sensitivity C 6 D 6 liquid scintillators NIM A496 (2003) 425

21 Experimental set-up (2): the total absorber calorimeter (TAC) 10 B loaded Carbon Fibre Capsules C 12 H 20 O 4 ( 6 Li) 2 Neutron Absorber 42 BaF 2 crystals, 15 cm in length High efficiency to γ-rays from capture events Good discrimination from the background Discrimination between γ-rays cascades from capture and => ideal for capture measurements on fissile samples and/or samples available in small quantities sample

22 Data Analysis Main steps Energy Calibration ( 137 Cs, 60 Co,Pu/C) Noise rejection by suitable choice of cuts and threshold Efficiency correction PHWT Background estimation Flux normalization

23 Experimental results 1 Zr-92 data Resonance Fit 2 SAMMY fit of 92 Zr Estimated resonance parameters E n, Γ γ, Γ n 3 4

24 From resonance parameters to Capture Strength = g Γ n Γ γ /Γ tot S Maxwellian Averaged Cross Section (MACS)

25 Experimental results 151 Sm ntof(neutron cross CERN) Maxwellian averaged cross-section experimentally determined for the first time Capture Yield Eu 149Sm 149Sm 152Sm 151Eu n_tof-data Kirouac and Eiland n_tof-fit 151Eu Neutron Energy (ev) Measured for the first time at a time-of-flight facility Resonance analysis with SAMMY code ( 500 resonances, mostly new) s-process in AGB stars produces 77% of 152 Gd, 23% from p process Maxwellian averaged (n,γ) cross section of the 151 Sm and previous calculation (symbol)

26 Experimental results 90 Zr S ntof 14% lower than previous data PRC 77 (2008) Weaker kernels reflects in lower MACS 90 Zr paolo.milazzo@ts.infn.it ntof(neutron cross CERN)

27 Experimental results 90,91,92,93,94 Zr ntof(neutron cross CERN)

28 Experimental results 186,187 Os PRC (2010) to be printed 187 Os c = 187 σ (186) Os σ (187) 186 Os σ (186) = 0.42 ± 0.02 σ (187)

29 Experimental results 186,187 Os The use of Re/Os abundance pair as a clock address few complications: The β-decay half-life of 187 Re is strongly dependent on temperature The stellar (n, γ) cross section of 187 Os is influenced by low-lying excited levels (strong population of 1 st state at 9.8 kev, competition by inelastic channels) Branching(s) at 185 W and/or at 186 Re Destruction of 187 Re in later stars (Astration) The chemical evolution of the galaxy was not uniform Re and Os abundance uncertainties Cosmological way 13.7 ± 0.2 Gyr based on the Hubble time definition ( expansion age ) Astronomical way 14. ± 2. Gyr based on observations of globular clusters Nuclear way 14.9 ± 2. (*) Gyr based on abundances & decay properties of long-lived radioactive species Ages (*) 0.4 Gyr uncertainty due to x-sections with SEF 10.4 Gyr no SEF 8.5 Gyr

30 Conclusion Neutron cross sections are key quantities for studying stellar evolution and nucleosynthesis. n_tofoffers the best conditions to obtain these nuclear physics quantities with the required accuracy The n_tofcollaboration is carrying on an extensive plan to measure cross sections relevant for nuclear astrophysics, in particular for s-process nucleosynthesis studies Opportunities for obtaining new data for presently inaccessible nuclei (using extremely low quantities of material) will be open.